Asphalt Field Level I

Asphalt Field Level I
2ND EDITION
Table of Contents
Preface
Preface .................................................................................................................... 2
Asphalt Plant Certification Information .................................................................... 3
About this Guide ...................................................................................................... 4
Introduction to Asphalt ............................................................................................. 5
Chapter 1: Project Communication and Workzone Safety

Project Organization and Communication ............................................................... 8
Project Communication ........................................................................................... 10
Project Safety .......................................................................................................... 11
Chapter One Knowledge Check .............................................................................. 12
Chapter 2: Surface Preparation

Introduction .............................................................................................................. 14
Drainage Considerations ......................................................................................... 14
Subgrade/Base Preparation for New Pavements .................................................... 17
Preparation of Existing Surfaces for AC Overlays ................................................... 18
Milling to Prepare Existing Surfaces ....................................................................... 21
Tacking ................................................................................................................... 25
Chapter Two Knowledge Check .............................................................................. 31
Chapter 3: Mix Delivery

Introduction .............................................................................................................. 34
Mix Delivery Equipment ........................................................................................... 34
Mix Delivery Considerations .................................................................................... 37
Chapter Three Knowledge Check ........................................................................... 43
Chapter 4: Equipment and Laydown Operations

Introduction .............................................................................................................. 46
Paver Equipment ..................................................................................................... 47
Laydown .................................................................................................................. 58
Constructing Transverse and Longitudinal Joints .................................................... 62
Chapter Four Knowledge Check ............................................................................ 74
Chapter 5: MAT and Joint Compaction
AC Compaction ....................................................................................................... 78
Factors Affecting Compaction.................................................................................. 79
Compaction Variables Outside of Operator Control ................................................ 80
Compaction Variables Under Operator Control ....................................................... 84
Roller Types............................................................................................................. 86
Compacting the Mat ................................................................................................ 90
Compacting the Joint ............................................................................................... 99
Chapter Five Knowledge Check .............................................................................. 105
Chapter 6: Mat Problems: Causes, Solutions and Effect on Pavement Performance

Mat Problems .......................................................................................................... 108
Common Mat Problems and Causes....................................................................... 109
Surface Defects: Shorts Waves, Long Waves and Washboarding .......................... 111
Segregation ............................................................................................................. 114
Tearing (Streaks) ..................................................................................................... 118
Checking.................................................................................................................. 120
Non-Uniform Texture ............................................................................................... 124
Screed Marks .......................................................................................................... 125
Screed Responsiveness .......................................................................................... 127
Poor Precompaction ................................................................................................ 128
Surface (Auger) Shadows ....................................................................................... 129
Joint Problems: Poor Joints ..................................................................................... 131
Joint Problems: Joint Irregularities .......................................................................... 133
Roller Marks ............................................................................................................ 135
Shoving and Rutting ................................................................................................ 137
Bleeding and Fat Spots ........................................................................................... 139
Poor Mix Compaction .............................................................................................. 141
Chapter Six Knowledge Check ................................................................................ 142
Appendices
A: Study Questions & Answers ................................................................................ 145
B: Acknowledgements ............................................................................................. 155
C: Glossary .............................................................................................................. 157
Preface
IMPORTANT
This manual is a practical guide and not to be used as a source for VDOT
specifications.
Page 1
Preface
Preface
This workbook covers asphalt paving, including transportation of materials, surface preparation, laydown,
compaction and quality assurance processes. It is aimed at the field personnel who are responsible for
these operations – both contractor personnel who do the work and department personnel who oversee
and inspect the work.
The course will provide a working knowledge of the techniques and processes used to place asphalt
concrete materials, including the surface preparation, hauling and discharging, paving, and compacting
procedures. Each step in the placement of asphalt concrete has a role in the overall pavement performance.
Additionally, the course attendees will have an understanding of improper handling of materials and the
adverse affect to their properties and ultimately the behavior in the finished pavement. We feel having
such information will give the technician confidence to make proper day-today decisions and will ensure
that good quality control is maintained.
It is hoped that the guide will promote a common understanding of the processes involved and thereby
result in improved asphalt concrete placement.
Page 2
Preface
Asphalt Field Certification Information

Course Certification Components
Asphalt • Prerequisites: None
Field Level I
Certification • Written Exam
- Before taking the exam you must show photo identification

and sign the Technician Certification Application.
- You are allowed 1 hour to complete the exam.
- You must pass with a minimum score of 70%. If you fail, you
have one opportunity for re-testing.
Passing exam score results in an Asphalt Field Level I Certification.

Asphalt • Prerequisites: Certified in Asphalt Field Level I
Field Level II
Certification • Written Exam
- Before taking the exam you must show photo identification

and sign the Technician Certification Application.
- You are allowed 3 hours to complete the exam.
- You must pass with a minimum score of 70%. If you fail, you
have one opportunity for re-testing.
Passing exam score results in an Asphalt Field Level II Certification.
The exam must be completed by December 31 of the calendar year in which the class is taken.
Initial certification will be effective for five (5) years from the calendar year in which you obtained
certification.
If a first attempt at the course’s final exam results in a failing grade, the student may take the exam
again. If the second attempt also results in a failing grade, the student will be required to attend the
classroom course before being allowed to try again.
Your test results and card may be found at the VDOT University website listed below:
https://virginiadot.csod.com/client/virginiadot/default.aspx
If you are enrolled in this class or hold any VDOT Materials certifications, do not register as a new
user when you go to this site.
If you do not know your logon or password, call (804) 328-3158.
Page 3
Preface
About This Guide

Various visual cues are used to symbolize several consistent concepts. Below you will see the visual
cues as well as a description of each.
DEFINITION
A definition will be provided. Describes and/or defines terminology.
TOOLS/EQUIPMENT/TESTS
Describes what tools, equipment and tests are required to complete the job safely
and with the highest level of quality.
WORK PROCEDURE (BEST PRACTICES / AWARENESS)

Describes a condition where equipment and procedure integrity is at risk. This
is used to alert personnel to operating procedures and practices, which if not
observed, could result in severe damage to the machine, its components or the
end product.
BEST PRACTICE
Describes a best practice to be utilized when possible.
IMPORTANT
Highlights a step in the procedure which is either unusual or very particular to this
procedure. May also indicate awareness (additional information) or a cautionary
concern in the procedure.
INSPECTION LIST AND MEASUREMENTS
Describes inspection and Quality Assurance practices.
SAFETY WARNING
Describes a condition where personal safety may be at risk. This is used to alert
personnel to operating procedures & practices which, if not observed, may result
in personal injury.
Page 4
Preface
Introduction to Asphalt
The modern use of asphalt for road and street construction began in the late 1800s, and grew rapidly with
the emerging automobile industry. Since that time, asphalt technology has made giant strides such that
today the equipment and techniques used to build asphalt pavement structures are highly sophisticated.
One rule that has remained constant throughout asphalt’s long history in construction is that an asphalt
concrete layer is only as good as the materials and workmanship that go into it. No amount of sophisticated
equipment can make up for use of poor quality materials, poor sampling and testing techniques or poor
construction practices.
Asphalt concrete is composed of three basic components - asphalt binder, mineral aggregate and air
voids. Aggregates are generally classified into two groups - coarse and fine, and normally constitutes 90
to 96 percent by weight of the total mixture. Asphalt binders are classified by various grading systems
and normally constitutes 4 to 10 percent of the total mixture. Another very important but often overlooked
component of an asphalt mix is air voids. In this course, only asphalt binder, aggregates and other additives
are discussed.
There are many different types of asphalts and many different types of aggregates. Consequently, it
is possible to make different kinds of asphalt pavements. Among the most common types of asphalt
pavements are:
• Asphalt Concrete (dense-graded hot or warm mix asphalt)
• Open-graded Asphalt or Porous Friction Course
• Ultra-Thin Bonded Wearing Course
• Asphalt Surface Treatments
• Emulsified Asphalt Mixes (cold mixes)
• Permeable Asphalt Drainage Course
• Others, SMA, In-Place Recycled Mixes
This program primarily addresses asphalt concrete (AC) produced at conventional plant temperatures
or through a warm mix technology. AC is a paving material that consists of asphalt binder and mineral
aggregate with appropriate air voids. The asphalt binder, either an asphalt cement or a modified asphalt
cement, acts as a binding agent to glue aggregate particles into a dense mass and to waterproof the
mixture. When bound together, the mineral aggregate acts as a stone framework to impart strength
and toughness to the system. The performance of the mixture is affected both by the properties of the
individual components and the combined reaction in the system.
Page 5
Preface
Page 6
1 Project Communication and Workzone
Safety
Learning Objectives:
Upon completion of this chapter, you should be able to:
Describe the purpose of :
• project documents,
• pre-construction and pre-paving conferences, and
• cooperative communication on the job
Know the different types of communication that are used prior to and during the course
of project work.
Demonstrate acute awareness of the importance of safety in and around the job site.
Page 7
Project Organization and Communication

The most essential part of project planning and organization
is communication. Effective communication is vital to all
elements of project organization:
• The project documents are written instructions that must

describe the requirements clearly and in detail.
• The preconstruction conference initiates verbal.

communication between the representatives of the
department and contractor personnel; it sets the tone for
both the working relationship and direct communications
during project execution.
• Ongoing communication between the contractor and the

department is essential to performing high-quality work.
• Project records make it possible to track events should doing so become necessary.
• Safety on the job cannot be maintained if communication between parties is inadequate.
Page 8
Project Documents
Project documents illustrate and describe work to be done under the contract. Project documents include:
Standard Drawings Calculated dimensions, unless obviously incorrect, will

govern over scaled dimensions
Standard specifications Directions, provisions, and requirements for performing

the work illustrated and described in the plans. The items
in the standard specifications relate to or illustrate the
method and manner of performing the work or describe
the qualities and quantities of materials and labor to be
furnished under the contract
Special or supplemental specifications Approved additions and revisions to the standard

specifications
Plans Drawings that show the location, character, dimensions,

and details of the work to be done
Special provisions Additions or revisions to the standard or supplemental

specifications that are applicable only to an individual
project
Special provision copied notes (SPCN) A document that sets forth specifications or requirements,
usually limited in scope, for a particular project item or
issue and as a modification to a Special Provision or
Specification. Note: a contract that has a special provision
copied note will follow the requirements described in that
copied note even if it is in conflict with the standard
specifications referenced by the contract.
If there is a discrepancy between the instructions and specifications in any of the contract documents,
a definite hierarchy exists among the above major types of documents.
The order of priority, from lowest to highest is:
This order of priority corresponds to the documents specific applicability to a project or contract.
Page 9
Plans and specifications need to be accurate and

complete and should leave little room for assumptions or
later reinterpretation. In addition, plans and specifications
need to define the responsibilities of both department
and contractor.
Project Communication
The quality of the work completed and the safety of those
performing and inspecting the construction are directly related
to the quality of the communication between the department
and the contractor. It is important that the individuals in
daily charge of the project for both the department and the
contractor meet periodically, on both a formal and an informal
basis, to discuss the progress and q uality of the work done to
date and the schedule for future work.
Preconstruction Conference The preconstruction conference should be held before work on

a project begins. The meeting should bring agreement on the
proposed schedule, methods to be used to complete the project
on schedule with a minimum of delays and change orders,
material sources, plant production rates, haul distances and
routes, paving widths and speed, and type and operation of
compaction equipment. Additionally the role of each person
associated with the project should be discussed and clarified.
One of the most important items to be addressed at the
preconstruction conference is job safety.
Formal Meetings Key personnel from both the department and the contractor
should be present at these meetings. The meetings should discuss;
the quantity of work completed, test results obtained, what has
yet to be accomplished, the schedule for the coming weeks,
and changes in personnel, equipment, construction methods used
and mix design. Written minutes containing those in attendance
should be completed and distributed as quickly as possible.
Informal Meetings Daily informal meetings provide a forum for the exchange of
information, they can be held at the job site, asphalt plant or at the
paver. The purpose of informal meetings is twofold. First,
occurrences the day before such as work completed, test results,
and any problem areas, should be discussed and resolved.
Page 10
Second, the discussion should address what is expected to happen

during the next several days – an update on the information
exchanged at the last formal meeting.
Written Communication Much information can be communicated in oral form, but

discussion of important information should be followed up in
written form, particularly when conditions on the project change
substantially. If an occurrence is important enough to be
remembered later on, it is important enough to be written down
immediately after it happens so the information will be accurate and
complete.
Project Safety
Working around an asphalt paving site can be
hazardous. Communication is one of the keys
to a safe work environment. Every individual
involved in the project should know what is
expected and how to perform the assigned
tasks. F o r e x a m p l e , proper training in the
operation of a piece of equipment is essential
for its operators. Retraining is necessary
at frequent intervals because the longer a
person continues to perform the same task,
day after day, the more likely he or she is
to do things by habit and ignore surrounding
events.

Safety talks are a good way to start the day. People
need to be reminded that they are operating in a
potentially dangerous environment at both the plant
and the laydown site, and daily talks are one way
of meeting this need. Further, if an unsafe work
practice is noticed, corrective action should be taken
immediately, even if the paving operation must be
shut down until the unsafe practice has changed.
Page 11
Chapter 1: Project Management and Workzone Safety
Chapter One Knowledge Check

1 In terms of contracts, which of the following is the highest priority
a. Standard Specifications
b. Special Provisions
c. Special Provisions Copied Notes
d. Plans
2. _____________ are approved additions and revisions to the standard specifications.

a. Special or supplemental specifications
b. Standard Drawings
c. Supplemental Specifications
d. Special provisions
3. Daily informal meetings held at a project site or plant location that provide a forum for the exchange of
information are called:
a. Preconstruction conferences
b. Formal team meetings
c. Plant conferences
d. Informal meetings
4. The ______________ should be held before work on a project begins to ensure agreement on the
proposed schedule, methods to be used to complete the project on schedule, change orders and
other critical project information.
a. Preconstruction conference
b. Opening ceremonies
c. Project kickoff
d. Formal meetings
5. If an unsafe work practice is noticed on the job site or in a plant:

a. Corrective action should be taken immediately even if the paving operation must be shut down
until the unsafe practice is changed
b. Corrective action should be taken as soon as operations have been completed for the day to
avoid delaying the day’s work
c. Project workers should look the other way
d. Teammates should assist to help get the unsafe work finished quicker
6. ___________ is one of the keys to a safe work environment:

a. State of the art safety equipment
b. Proper training
c. Communication
d. Both A and B
Page 12
2 Surface Preparation
List the steps involved in preparing bases and existing pavements for overlays
Understand how base properties impact paving decisions
Recognize drainage considerations in creating lasting pavement structures
Understand reasons for milling
Identify impacts of milling equipment speed and teeth condition on quality
Recognize impacts of mill depth on quality
Understand reasons for tacking
Identify materials used for tacking in Virginia
Recognize impacts of poor or no tack on quality
Page 13
Introduction
The performance of AC under traffic is directly related to the condition of the surface on which the pavement
layers are placed. Before pavement is placed, the surface to be paved must be adequately prepared by
addressing any drainage issues and then preparing the area accordingly. This preparation is essential to
long-term pavement performance, as pavements constructed without adequate surface preparation may
not meet smoothness specifications, may not bond to the existing pavement (in the case of overlays) or
may fail because of inadequate subgrade support causing the life of the roadway to be significantly reduced.
Drainage Considerations
Proper drainage is an important part of the process of ensuring a long-lasting, high-quality pavement.
Water is the enemy of all good pavement structures as moisture in the subgrade and aggregate base
layers can weaken the subbase and/or base materials, causing them to lose their structural stability. The
loss of structural stability can then lead to failure of the overlying asphalt layers. If water is able to enter the
asphalt pavement itself either due to poor density or other factors, it will strip away the binder that provides
the stone-to-stone bond. Aggregate particles that lose their asphalt coating will no longer be able to hold
together and the asphalt mix falls apart, a phenomenon called raveling on the surface and stripping within
the pavement structure.
Moisture sources are typically rainwater, runoff and high groundwater. These sources are prevented from
entering the pavement structure or accumulating in the subgrade through surface drainage and subsurface
drainage. Usually, it is more cost effective and less risky to prevent moisture entry and accumulation using
surface drainage than to remove moisture using subsurface drainage. There are three primary means
used to prevent surface drainage: correcting drainage problems before overlay occurs, assuring for proper
crown and slope of surface to be paved, and allowing for a curb and gutter system to carry waterflow away.
1. Correcting Drainage Problems Before Overlay

Keep roadside drainage systems clear of vegetation and other
foreign material to allow rainfall to be quickly carried away from
the pavement structure.
2. Proper Crown and Slope

Avoid creating “birdbaths” or low spots on the
asphalt surface. This prevents water from quickly
moving to drainage systems such as curb and
gutter or roadside ditches. Give pavement an
effective crown and an adequate slope to assure
rapid transit of water off of the pavement surface
Page 14
3. Curb and Gutter Elevations

Gutter systems that are higher than the adjoining asphalt surface serve little purpose. Water is
allowed to collect on the asphalt surface and at the gutter joint instead of being conveyed away
from the pavement to drop inlets or other parts of the drainage system. Significant damage can
result from failures to properly match the asphalt pavement to the curb and gutter system.
Pavement Restoration and Patching

Removal and replacement should be carried out on all existing pavement areas where severe load-related
distress has occurred. Subgrade distortion should be repaired by undercutting and replacement with
suitable backfill material. Proper sub-surface drainage should be installed as necessary. New granular
base course material, stabilized base course layers, or AC mix should be placed in order to bring the
strength of the pavement structure in each failed area to the same level as the surrounding good pavement
layers. Localized failed areas should be patched properly.
BEST PRACTICE
Each distressed area should be cut back to sound pavement and squared up,
with the sides as vertical as possible, the loose material and water in the hole
removed, a tack coat applied to the sides and bottom of the hole, the mix place
in the hole, and the new material adequately compacted, preferably with a roller.
If the pothole is deeper than 4 inches, the mix should be placed in more than one
layer and each layer compacted properly.
Crack Sealing/Filling
Badly cracked pavement sections, especially those with
pattern cracking (e.g., map or alligator), must be patched
or removed. If the cracks are narrow, less than 1/8 inch
wide, they do not need to be sealed as it is doubtful that the
crack- sealing material will actually enter the crack instead
of pooling on the pavement surface. If wider cracks (up to
1.5” wide) are present, they should be blown out with air
and cleaned of debris prior to sealing. The crack-sealing
material should be inserted when the cracks are clean and
dry. If that pavement structure contains a great number of
cracks, consideration should be given to applying surface treatment instead of filling individual cracks.
In all, pavement repair should be extensive enough to provide an existing pavement with adequate
structural support. Pavement management techniques should provide for overlays before an existing
pavement has lost most or all of its structural support capability.
Page 15
Surface preparation generally takes one of two forms:
1. The first involves preparing the subgrade and granular base course for new pavement, which can
involve such activities as subgrade stabilization, over-excavation of poor subgrade, or applying a
prime coat or compacting the subgrade.
2. The second form involves preparing an existing pavement surface for overlay which might include
such activities as replacing localized areas of extreme damage, applying a leveling course, milling,
or applying a tack coat to ensure a bond between the existing pavement surface and the new
asphalt overlay. Specific actions for each method depend upon the pavement type and purpose,
environmental conditions, subgrade conditions, local experience and specifications.
Page 16
Subgrade/Base Preparation for New Pavements

Anything that can be done to increase the load-bearing capacity of the subgrade soil will most likely
improve pavement load-bearing capacity and therefore pavement strength and performance. Greater
subgrade structural capacity can also result in thinner and more economical pavement structures. Care
should be taken to ensure that the finished subgrade meets the elevation, grade and slope requirements
specified in the contract plans.
Subgrade Soil
Before the asphalt pavement is placed on the subgrade soil, the subgrade material should be checked
to verify that it meets all applicable specifications for moisture content, density, structural support, grade
and uniformity. Once it is determined to be ready for paving and before paving is allowed to commence,
it should also be checked to ensure a firm foundation that will be able to support the weight of the haul
traffic. If distortion of the subgrade soil occurs during the paving operation, placement of the mix should be
stopped until the condition of the soil can be corrected.
Granular Base Course

If the asphalt layer is to be constructed directly on a new or existing
untreated granular base layer, that base material should also
meet all the specifications for moisture content, density, structural
strength, grade and uniformity. Proof rolling should be done on the
base layer to test the granular base material, and both the amount
of deflection of the base and amount of indentation of the truck
wheels in the granular base course material should be noted. If
the base material is stable and dry and does not deflect and indent

significantly under the wheels of a loaded tandem-axle truck,

placement of the prime coat or the new asphalt mix can be allowed
to begin. If the condition of the granular material is not satisfactory, however, the base course should be
reworked or stabilized until it is in the proper condition for overlaying.
Prime Coat
The graded subgrade or the top granular base layer is prepared with a prime coat: a sprayed application
of a cutback or asphalt emulsion applied to the surface of untreated subgrade or base layers. The prime
coat helps to:
• Fill the surface voids and protect the subbase from absorbing excess moisture during rain before paving.
• Stabilize the fines and preserve the subbase material by binding together any dust on the surface of
the granular base layer.
• Promote bonding to the subsequent pavement layers.
• Prevent slippage of thin overlying pavement layers.
Page 17
A prime coat is most needed when the soil is a silty clay or other clay material because the prime coat
material cannot be absorbed into that subgrade material. The use of a prime coat on sandy subgrade
soils, however, is questionable. If the sandy material displaces excessively under the wheels of the haul
trucks, it should be stabilized with some type of binding material before paving to achieve the required
load-bearing properties. In such cases, the application of a prime coat will generally not be enough to hold
the sandy soil in place during paving operations. A prime coat should not be used as a substitute for proper
preparation of the subgrade soil.
There is no requirement to place a prime coat unless specified in the contract. Prime coats are only
required by specification under stabilized open graded drainage layers, unless otherwise noted on the
plan. Some contracts require the use of prime coats on secondary roads and subdivision streets.
IMPORTANT
A prime coat is not required on aggregate subbase or base layers unless it is

specified in the contract documents.
Preparation of Existing Surfaces for AC Overlays

Overlays (either structural or non-structural) make up a large portion of the roadway paving done today.
The degree of surface preparation for an overlay is dependent on the condition and type of the existing
pavement. Generally, the existing pavement should be structurally sound, level, clean and capable of
bonding to the overlay. To meet these prerequisites, the existing pavement is usually patched, leveled (by
milling, pre-leveling or both), cleaned and then tacked.
Patching
Generally, pavement overlays are used to restore
surface course characteristics such as smoothness,
friction and aesthetics, or to add structural support
to an existing pavement. However, even a structural
overlay needs to be placed on a structurally sound
base. If an existing pavement is cracked or provides
inadequate structural support, these defects will be
reflected through even the best-constructed overlay
and cause premature pavement failure in the form of
cracks and deformations. To maximize an overlay’s
useful life, failed sections of the existing pavements
should be patched or replaced and existing pavement
cracks should be filled.
Page 18
AC over AC
The degree of preparation needed for an existing asphalt pavement depends on the condition of that
surface. At a minimum, failed areas should be removed and replaced; potholes properly patched, cracks
cleaned out and sealed and ruts filled in or removed by cold milling.
AC over Concrete Pavement

When AC is placed over a Portland Cement Concrete (PCC) pavement, the PCC surface should likewise
be properly prepared. Any severely distressed areas in the concrete slabs should be cut out, removed,
and replaced with either PCC or AC using full-depth slab repair techniques. Corrective work should also
be completed on the underlying sub-base or subgrade material. Any severely spalled areas at joints
should be repaired using partial-depth slab replacement methods, AC or PCC may be used for partial-
depth repairs. Rocking slabs should be stabilized. Depending on the condition of the PCC pavement,
procedures such as crack and seat, break and seat, or rubblization of the existing pavement can be used
before the overlay is placed, particularly if the slabs are rocking under traffic loading. Consideration can
also be given to the use of a crack-relief layer between the existing PCC pavement and the new overlay.
For joints that are poorly sealed, the old seal

material – particularly rubberized, should be
removed and the joints cleaned. When dry, the
joints should be resealed with appropriate joint-
sealing material. Care should be taken not to overfill
the joints, particularly in cool weather when they
are open wide. In all cases, as with crack sealant,
the final level of the joint-sealing material should
be below the top of the surrounding pavement
surface. Once the patching and resealing have
been accomplished, the surface of the PCC
pavement should be cleaned completely using
mechanical brooms and air blowing or water
flushing, or both, where needed.
Scratch and Leveling

A common practice in the past has been to place a leveling course on the existing pavement surface to
improve the rideability and uniformity of the pavement structure. This leveling course (sometimes called
a wedge course or a scratch course) is designed to fill in the low spots on the pavement surface and is
accomplished using the floating screed on the paver with more AC being placed in the low spots than on
the high spots in the existing pavement surface (areas with thicker mix typically compact more than areas
with thinner mix).
Page 19
When a leveling course is placed, the AC placed in the low areas (in the wheel paths if the pavement is
rutted) will be thicker than the mix placed over the high points in the surface (between the wheel paths).
The thicker mix will compact more under the rollers, particularly if a pneumatic tire roller is used, than will
the mix that is thinner. Thus, low spots will still exist in the wheel paths where the mix has been compacted
to a different degree (and thus a different air void content) than the mix between the wheel paths.
This problem, termed differential compaction, requires that multiple courses be constructed over a
pavement surface that is badly out of shape before a smooth surface can be obtained. As the mix passes
from under the paver screed, it is in loose condition. Compaction by the rollers reduces the thickness
of the newly placed layer. The rule of thumb is that conventional mixes will compact approximately ¼
inch per 1 inch of compacted thickness. Thus to achieve a
compacted course 1 inch thick, about 1¼ inch of mix would
have to be placed by the paver. Similarly, approximately
3¾ inches of mix would need to pass from under the paver
screed to construct a layer with a compacted thickness of 3
inches. With gap graded mixes (i.e., Stone Matrix Asphalt,
or, “SMA”) and open graded mixes (i.e., porous friction
course and open graded drainage layer), the amount of
compaction is much less – approximately 1/8 inch per 1
inch compacted thickness.
BEST PRACTICE
A rule of thumb is that one layer after compaction will remove approximately 80
percent of a low spot. Two layers, each being compacted separately, will remove
approximately 95 percent of a low spot.
Page 20
Milling to Prepare Existing Surfaces

Pavement milling (also called cold planing), is the process of removing at least part of the surface of a
paved area such as a road, bridge, or parking lot to prepare for a new overlay. Milling removes anywhere
from just enough thickness to level and smooth the surface to a full depth removal. There are a number of
different reasons for milling a paved area instead of simply repaving over the existing surface:
1. Removal of deteriorating pavement layers
When an overlay is placed onto a deteriorating pavement surface, faster deterioration may occur if the
road continues to be subjected to traffic. Some of the issues that milling can help to resolve include:
• Raveling: Aggregate becoming separated from the binder and loose on the road.
• Bleeding: The binder (asphalt) coming up to the surface of the road.
• Rutting: Formation of low spots in pavement along the direction of travel usually in the wheel path.
• Cracking: Cracks in asphalt layers that are often caused by repeated traffic loadings, sub-base
failure, or poor drainage conditions.
• Shoving: A washboard like effect transverse to the direction of travel.
• Ride quality: Uneven road surface such as swells, bumps, sags, or depressions.
2. Elevations, clearances and drainage
When overlaying an existing pavement, you raise the entire surface by the thickness of the new layer,
creating additional work to raise the surrounding structures like guardrails, shoulders and utilities. By
milling the existing pavement, then inlaying new pavement, you can avoid modification of other structures
on the job site. Properly performing this step also preserves clearances for bridges and overpasses and
can maintain or correct roadway water run-off and drainage.
3. Additional weight
Structures like bridges, overpasses and on-ramps are

subject to increased stress from the weight of additional
pavement. When you first mill and then inlay asphalt mix,
you maintain the approximate weight that the structure
was originally designed to withstand without needing to
reduce weight limits for transport.
Page 21
4. Complicated leveling courses

Milling eliminates the need for complicated leveling courses to fill existing pavement depressions. Leveling
course quantities make it difficult to accurately estimate asphalt mix tonnage.
5. Consistent pavement density

Leveling course thicknesses are usually small, precluding the use of nuclear gauge density testing.
Adequate mix density is therefore difficult to achieve and measure. Overlays also create inconsistent
pavement densities because of aggregate segregation at the outside edge of mats. When you mill, you
contain the mix, preventing aggregate segregation and density inconsistencies.
6. Reclaimed Asphalt Pavement

Reclaimed Asphalt Pavement, also known as RAP, is the term given to the materials generated when
asphalt pavements are removed for reconstruction or resurfacing. Harvested RAP from milling operations
provides fiscal advantages as well as environmental benefits, as reclaimed asphalt is worth the virgin
materials that it replaces. Anytime RAP is used for a paving project, fewer natural resources are ultimately
consumed, leading to more sustainable pavements, in line with maintaining asphalt as America’s most
recycled product.
The Milling Machine
The basic components of a milling machine are:
• A cutting drum to mill the existing pavement
• A vacuum to collect the milled particles
• A conveyance system to transport the milled particles to a dump truck for hauling
Page 22
Tracks
Tracks Cutter Drum
Cutter Drum Conveyor
Conveyor Belts Chassis
Chassis
Milling Machine Cutting

CuttingDrum
Drum Milling
MillingMachine
MachineCutting Teeth
Cutting Teeth
Equipped with automatic grade and slope controls like those used on an asphalt paver, the milling machine
is capable of producing a level surface in one pass over the existing surface.
Page 23
When milling a surface, it is important

to watch for “scabbing”, where the
milling machine leaves patches of
uneven pavement. These areas may
pop loose under traffic and take part
of the new overlay with it. Scabbing
can be minimized by reducing the
travel speed of the milling machine or
adjusting the depth of milling.
A pavement surface that has been

milled is typically very dusty and dirty,
requiring a good cleaning before
any overlay is placed. If the milled
surface is properly cleaned, its texture
can enhance the bond between the
new and old asphalt mat, and may
reduce the possibility of slippage of
the overlay on the existing surface.
Methods of cleaning the surface can
include sweeping, vacuuming,
brooming, flushing with water or
allowing traffic back on the surface
to help clean the dust and fine material off the surface. When sweeping, more than one pass is typically
needed to remove all the dirt and dust. If the milled surface is washed, the pavement must be allowed to
dry prior to paving.
Page 24
Tacking
Proper tack coat application is also critical to long-term

pavement performance. The purpose of a tack coat is to
ensure an adequate bond between the existing pavement
surface and the new asphalt overlay. If a good bond is not
formed between the existing surface and the new overlay,
slippage and delamination may occur.
IMPORTANT
Remove dried, caked mud and other foreign material. Spilled asphalt should be
shoveled or picked up (especially if placing a thin overlay).
The tack coat material is normally an asphalt emulsion - a performance grade asphalt suspended in
water using acid and surfactant (soap). Emulsions are used because they effectively reduce asphalt
viscosity for lower temperature uses such as the tack coat application. Emulsions are typically either
anionic (asphalt droplets are negatively charged) or cationic (asphalt droplets are positively charged).
What Types of Emulsions Are Used?

The time of year determines what type of tacking material is used for department projects: conventional
or trackless. In essence, a conventional emulsion is one that takes longer to break and set, while a
trackless emulsion has a shorter set time allowing for faster access for construction traffic. The table
below shows what types of emulsions are used depending on the time of year.
Conventional tack coat materials: May be used prior to May and after September
• CQS – 1h
• CRS – 1h
• CSS – 1h
Note: Conventional tacks used must be listed

on VDOT Approved Materials List 50.1A
Trackless tack coat materials May be used year-round, but MUST be used from
May through September
Page 25
The tack coat is applied by a pressure distributor with

nozzles on the distributor fully open and functioning,
and turned at the same angle to the spray bar
(approximately 300º degrees). The spray bar should
be at the proper height above the pavement surface
to provide for a double or triple lap of the liquid asphalt
material. The result will be the proper amount of overlap
between the nozzles and a uniform application of the
tack coat to the road surface. The tack coat material
should be heated to the proper temperature so that
it is fluid enough to be sprayed uniformly from the
nozzles instead of coming out in strings. It is also important to follow the manufacturer’s recommendation
for storage and handling of the emulsion so the tack material is properly maintained prior to being sprayed.
This typically involves keeping the emulsion heated and not storing for long periods of time.
Application Rate Versus Residual Rate

Uniformity of application and a proper application rate are keys to achieving a successful tack coat. If the
correct amount of tack coat is sprayed on the surface, minimal portions of the existing surface may still be
visible through the tack coat; in most cases all of the existing pavement surface will be covered. In general,
proper tack coat application (0.05 to 0.10 gal/yd2) will leave a residual asphalt cement content of
approximately 0.04 to 0.06 gal/yd2 on the roadway. The amount of residual tack coat needed will depend
on the condition of the pavement surface and the tack coat material used. An open-textured surface
requires more tack coat than a surface that is tight or dense, and a dry, aged surface requires more tack
coat than a surface that is “fat” or flushed.
It is essential to differentiate between the residual tack coat rate (the amount of asphalt cement remaining
on the pavement surface after the water has evaporated) and the application rate (the amount
of emulsion sprayed from the distributor). Most asphalt emulsions contain 60-65o percent residual asphalt
cement and 35-40 percent water, plus a small amount of emulsifying agent. For ease of calculation, it can
be assumed that an asphalt emulsion is approximately two-thirds asphalt cement and one-third water. The
amount of asphalt cement left on the pavement surface after the water has evaporated from the emulsion
is the most important factor in obtaining a bond between the existing pavement surface and the new
overlay. If asphalt cement instead of an asphalt emulsion is used as the tack coat material, the residual
amount of asphalt on the pavement surface should be the same as the applied amount. Thus, if 0.04 gal/
yd2 of residual binder material is desired, the application rate from the distributor should also be 0.04 gal/
yd2.
Page 26
If the amount of water in an asphalt emulsion is not taken into account when determining the application
rate from the distributor, the correct degree of adhesion may not be achieved. Too little tack coat will not
provide sufficient bond between the old and new pavement layers. On the other hand, too much tack coat
may contribute to slippage of the overlay on the existing pavement surface and bleeding of the tack coat
material through a thin overlay.
Breaking and Setting Time

When an asphalt emulsion is applied as a tack coat, it is brown in color because it contains both asphalt
cement and water. After a very short period of time, the emulsion will “break” – change color from brown
to black – and the water will begin to evaporate. The rate of evaporation will depend on the type and
grade of the emulsion used, the application rate, the temperature of the existing pavement surface, and
environmental conditions. Once all the water is gone, the emulsion is said to have “set.” The rate of set
depends on the same conditions that control the rate of break of the emulsion. Under most circumstances,
a conventional emulsion will set in 1-2 hours. For non-tracking emulsions, the set time may be as short as
a few minutes.
New AC can usually be placed on top of an unset tack coat and even over an unbroken tack coat emulsion
with no detrimental effect on pavement performance/ the bond will still be formed. While it is believed that
the asphalt emulsion can be properly paved over before being fully set, and even before being broken it
is also important that the tack coat material remain on the pavement surface to create the bond between
the layers. If the tack coat material is not set and a significant amount of haul truck traffic runs over the
unset material, much of the tack coat may be picked up by the truck tires and tracked down the roadway.
Thus, either the tack coat should be allowed to set before haul truck traffic is permitted to run over it,
or the amount of truck traffic should be minimized. It is desirable to place the tack ahead of the paving
train as much as possible while still within the limits of the workzone, and placed so both passenger and
construction traffic can be kept off of the tack coat.
Page 27
IMPORTANT
Proper application and ensuring the correct amount of tack coat material applied
are essential to successful bonding between an existing pavement surface and a
new overlay.
Tacking Joints
It is often a challenge to achieve proper density at both transverse and longitudinal joints. Additional
tack must applied to vertical joint faces as well as underneath the joint area to aid in obtaining density
requirements. Loose material should be removed before this area is tacked. (VDOT Road and Bridge
Specification Section 315.05(b)1)
Page 28
Tacking of Joints After Milling
Tack is to be applied to the vertical face with a hand wand or spray bar at the rate of 0.2 gal/yd2. The
application should be heavy enough that slight puddling occurs at the base of the vertical face. At the
joints, the tack is applied shall be 2 feet in width with 4-6 inches protruding beyond the first pass. Tack for
the adjacent pass shall completely cover the vertical face of the mat edge so that slight puddling of asphalt
occurs at the joints and extends a minimum of 1 foot into the lane to be paved.
Tacking of Joints During Asphalt Placement

Tack is applied under the proposed first lift 18 inches from the joint edge with a 6 inch overlap onto the
second lift area. Tack is then applied on the second course to the vertical face and within 12 inches of the
Page 29
joint area. A longitudinal joint should be tacked using a hand wand or spray bar at the rate of 0.2gal/yd2.
There should be slight puddling at the joints and extend extend a minimum of 1 foot into the lane to be
paved.
Page 30
Chapter Two Knowledge Check

1. A prime coat is required in which of the following situations:
a. When paving with SMA
b. When the contractor wants to make a really good impression on the project manager
c. When required by the contract
d. When the prime coat can be absorbed completely within 24 hours
2. The purpose of a tack coat is to ensure a bond between the existing pavement surface and the new
asphalt overlay.
a. True
b. False
3. What could cause poor bonding on an asphalt overlay on a milled pavement?

a. Breezy, cloudy day with 700F temperature
b. Multiple sweepings
c. Dirt, dust or scabbing left on the milled surface
d. Air-blowing the milled surface before sweeping
4. Material taken off the roadway when milling may not be used again.
a. True
b. False
5. Methods to clean a milled surface may include:

a. Air blowing the surface
b. Dampening/flushing the surface with water
c. Sweeping with a mechanical broom
d. All of the above
e. A and C only
6. Conventional tack coat materials should be used between the months of May and September.
a. True
b. False
Page 31
7. The ______________________________ is the amount of asphalt cement remaining on the pavement

surface after the water has evaporated:
a. Application rate
b. Breaking and setting rate
c. Residual tack coat rate
d. Emulsification Rate
8. The _________________________ is the amount of asphalt binder remaining on the pavement

surface after the water has evaporated, which is typically between _______________________gal/
yd2.
a. Asphalt yield; 0.10 to 0.15
b. Application rate; 0.05 to 0.10
c. Residual rate; 0.04 to 0.06
d. Applied or actual rate; 0.04 to 0.06
Page 32
3 Mix Delivery
Identify factors that impact mix delivery
Understand proper approaches to load asphalt mix to minimize segregation
Recall Virginia requirements for hauling asphalt mix
Understand proper ways to transfer asphalt mix to the paver
Page 33
Introduction
The mix delivery process involves all equipment and steps required to convey AC from a production facility
to a paving site, including mix delivery planning and preparation, truck loading, hauling to the paving
site, unloading the mix, and truck return to the production facility. The goal of mix delivery is to transport
AC to the paving job site that conforms to the specified mix design. Transport practices used during the
process, however, can have a profound effect on mix characteristics such as temperature at the paving
site, aggregate and/or temperature segregation of the mix and mat quality. As a result, it is very important
to follow strict guidelines any time that AC is being moved from production facility to paving site. If properly
managed, however, the delivery can successfully move AC from the production facility to the paving site
with little or no change in mix characteristics.
Mix Delivery Equipment

The trucks and equipment used in the mix delivery process play a key role in ensuring successful delivery
from production facility to paving site with the desired result of little or no change made to the mix being
transported. In general, there is one type of AC transport truck used in Virginia - the end dump truck. In
rare instances, however, a live bottom dump is also sometimes used.
End Dump Truck
An end dump truck delivers the mix directly into the

hopper of the paver. When the bed is raised, it should
not be in contact with the hopper and should not press
down on or ride on the paver.
End dump trucks unload their payload by raising the

front end and letting the payload slide down the bottom
of the bed and out the back through the tailgate. Contact
between the truck bed and the paver can be a problem,
however, when large semi-tractor trailer units are used
as haul vehicles - particularly when the truck bed is
extended to its highest point. When the weight of the
truck is being carried by the paver, care must be taken
because the laydown machine may lose its ability to
operate properly. Semi-trailer units typically haul more
than single units and carry the maximum legal gross
load. The payload depends on the number of axles,
local regulations, and the empty weight of the vehicle.
Page 34
End dump trucks are the most popular transport vehicle type because they are plentiful, maneuverable
and versatile. Some important considerations to remember when using end dump trucks are:
BEST PRACTICE
1. When the truck bed is raised, it should not contact the paver. Bed
contact with the paver may affect the screed tow point elevation, which
can affect mat smoothness.
2. The truck bed should be raised slightly before the tailgate lock is
opened. This allows the AC to slide back against the tailgate, which will
cause it to flood into the paver hopper when the tailgate is opened. AC
that trickles into the paver hopper is more susceptible to aggregate segre-
gation.
3. Truck-paver contact should be established by allowing the paver to

move forward into a stationary truck. This ensures that the truck does
not bump the paver too hard and cause the paver to lurch to a sudden
stop, which could cause a rough spot in the mat.
4. Once the paver and truck are in contact, they should remain in con-
tact. This ensures that no AC is accidentally spilled in front of the paver
because of a gap between the truck and paver. Usually the truck driver will
apply the truck’s brakes hard enough to offer some resistance to the paver
but light enough so as not to cause the paver tracks to slip from excessive
resistance. Most pavers can also be coupled to an unloading truck using
truck hitches located on or near the push rollers.
Live Bottom (Conveyor) Truck

A live bottom truck, also known as a flo-boy,
employs a conveyor belt or slat conveyor in
the bottom of the truck bed to discharge the
mix. This eliminates the need to raise the bed,
and deposits the mix directly into the hopper of
the paver as does an end dump truck.
Horizontal discharge units can also carry the

legal load limit. The payload is dependent
on the same factors as semi-trailers with the
advantage being that the unit can discharge
directly into the paver.
Live bottom trucks are more expensive to use and maintain because of the conveyor system, but they also
can reduce segregation problems because the AC is moved in a large mass, and can eliminate potential
truck bed – paver contact because the bed is not raised during discharge.
Page 35
SAFETY WARNING
All trucks must have a back-up alarm, be equipped with rotating beacons and
have a camera system or use a spotter near the paver.
Both the end dump truck and the live bottom truck are capable of adequately delivering AC from a
production facility to a paving site; however, certain situations such as the ones listed below may make
one truck type advantageous over another.
Situation Possible Truck Type Reason
Better maneuverability because

it has no trailer and is smaller
Paving on congested city streets End Dump
than a bottom dump or live
bottom truck.
Live bottom trucks deliver

Paving using a mix highly
Live Bottom the AC by conveyor, which
vulnerable to segregation
minimizes segregation.
Paving on rural highways Either
Material Transfer Vehicle (MTV)

A Material Transfer Vehicle (MTV) is basically
a surge bin on wheels. Asphalt mix is deposited
into the hopper on the front of the MTV and is
then transported to a conveyor which delivers
the mix to the extended hopper on the paver.
The MTV also allows the paver to be operated
almost continuously when a continuous supply
of mix is available from the asphalt plant. This
provides for a smoother mat behind the paver
screed by permitting the paver operator to keep
the material in front of the screed constant. The
equipment also eliminates the problem of the haul trucks bumping the paver, and truck drivers holding the
brakes on the truck when being pushed by the paver. Because of the weight of this piece of equipment
when full of mix, it must be determined ahead of time that the pavement over which this machinery will be
running can support the weight of the device without being over stressed and damaged.
Page 36
Mix Delivery Considerations

Ideally, the goal of mix delivery should be to maintain mix characteristics between the production facility
and the paving site. The following mix transport considerations are essential to maintaining ideal AC
characteristics between the production facility and the paving site.
1. Mix Delivery Planning and Preparation

The delivery of mix should match the speed and production of the paving train so that the paver does not
have to stop and wait for the mix to arrive and so that numerous trucks are not backed up at the project site
waiting to deliver mix. Rain, equipment breakdown, and traffic delays all can ruin a well-intentioned paving
plan. It is wise to have an alternative plan in place and spare equipment available if needed.
2. Loading at the Production Facility

Loading at the production facility involves transferring AC from the truck storage silo or batcher (if
transferring from a batch plant) to the transport truck. Potential issues to consider during this part of the
transport process are truck bed cleanliness and lubrication and aggregate segregation. Two potential
issues can arise at the time of transfer:
BEST PRACTICE
Pre-Loading Inspection of the Truck Bed:

The bed of the haul truck should be free of all deleterious materials before the
mix is placed in it. The bed should be reasonably smooth and free from any
major indentations or depressions where the truck bed release agent and the AC
can accumulate.
A. Truck bed cleanliness and lubrication: Truck beds

should be clean and lubricated prior to receiving
a mix load to prevent the introduction of foreign
substances into the AC and to prevent the AC from
sticking to the truck bed. A non-petroleum based
release agent, such as lime water or soapy water,
should be used for lubrication. Petroleum based
products, such as diesel fuel, should not be used
because of environmental issues and because they
tend to break down the asphalt binder. The release
agent should be sprayed over the sides and bottom of the truck bed, using the minimum quantity
necessary to cover the surface area of the bed without runoff. Any excess liquid in the bed should
be drained off the truck prior to loading the asphalt mix. After the mix is loaded, the driver should
again “wash off” the back of the truck to prevent further buildup of materials
Page 37
IMPORTANT
Never use diesel fuel as a release agent for the truck bed. Diesel fuel can
cause changes in the properties of the asphalt mix material with which it comes in
contact. Diesel fuel can also contribute to environmental problems as it evaporates
or if it soaks into the ground.
B. Aggregate segregation: AC should be discharged directly into the truck bed to minimize the
changes for segregation. Dropping AC from the storage silo or batcher (for batch plants) in one
large mass creates a single pile of AC in the truck bed. Large-sized aggregate tends to roll off
this pile and collect around the base. Dropping AC in several smaller masses (three is typical) at
different points in the truck bed will largely prevent the collection of large aggregate in one area and
thus minimize aggregate segregation.
3. Load Distribution
Most haul trucks need to be loaded with the load slightly forward in the truck bed to comply with axle
weights and load distribution regulations.
4. Loading Sequence
A main goal of the truck-loading operation is to get the vehicle filled with mix and on its way to the paver
as quickly as possible. This objective must be balanced, however, with the need to minimize segregation
of the mix.
Some mixes are more prone to segregation than others and special care must be taken to ensure the
mix load is as uniform as possible. Other mixes are not prone to segregation, therefore conventional load
out procedures work fine. The loading plan should be discussed and agreed upon at the pre-construction
conference.
IMPORTANT
There is no one correct way to load a truck. The method will depend on the mix
type, truck type, and a host of other factors. Use the method that will minimize the
potential for segregation to occur during the loading.
Page 38
5. Topping Off
There is a tendency for the plant operator to fill the truck to its legal capacity to reduce haul costs, particularly
if the truck is sitting on a scale under the silo. Although this is good economic practice, it should not be
accomplished by dribbling mix into the truck from the silo after most the mix is already in the truck. The
discharge of small amounts of mix from the silo greatly increases the chance for segregation of the mix.
IMPORTANT
Truck overloading is illegal, dangerous, and can cause extreme stresses in

pavements. Modern electronic weigh out equipment and printed tickets have
helped to eliminate this.
6. Place a tarp over the truck bed.

A tarp over the truck bed provides additional insulation, protects the
AC from rain and decreases heat loss. In Virginia, every haul truck
must be equipped with a waterproof, tear-resistant (non-mesh) tarp
to protect the mix in case of inclement weather. If there is any water
on the tarp when the truck is ready to discharge mix into the paver
hopper, the water must be removed from the tarp by raising the bed
of the truck and letting the water run off before the truck backs into
the hopper.
SAFETY WARNING
It is most desirable to use tarps that can be extended by mechanical means over
the bed of the truck without the driver having to climb up on the sides of the ve-
hicle to unroll the tarp.
7. Hauling to the Paving Site

Once loaded, ticketed, and tarped, the haul truck should
proceed immediately to the jobsite. Staggering the
asphalt deliveries to the project helps to reduce the time
trucks must wait before dumping the asphalt into the
paver, but not be spaced so far apart that the paver must
stop and wait for deliveries. Truck transport affects AC
characteristics mostly through cooling during transport.
AC is usually loaded into a truck at a uniform temperature
between 250ºF to 350ºF. During transport, however, the
heat is transferred to the surrounding environment by
Page 39
convection and radiation, and the AC surface temperature drops. This cooler AC surface insulates the
interior mass, causing the transported AC to develop a cool thin crust on the surface that surrounds
a much hotter core. Things such as air temperature, rain, wind and length of haul can also affect the
characteristics and temperature of this crust. Some measures that can be taken to minimize AC cooling
during transport are:
• Minimize haul distance. This can be accomplished by choosing an AC production facility as close
as possible to the paving site. Closer production facilities mean shorter haul times and result in less
AC cooling during transport. Unfortunately, many paving locations may not be near any existing
production facilities and economics may prohibit the use of a mobile production facility.
• Insulate truck beds. This can decrease AC heat loss during transport. Insulation as simple as a
sheet of plywood has been used on the exterior of the truck bed.
IMPORTANT
If chunks of asphalt mix do not affect the quality of the mat behind the paver, the cool
surface crust that forms on top of the mix during delivery will not be detrimental to
the long-term performance of the mix.
8. Waiting on Site
Haul trucks should park in designated areas and attempt to minimize tracking of tack coats. Trucks should
stay far enough ahead of the paving train as to not interfere with operations, but close enough to back to
the paver to keep the operation moving. Communication is the key. Drivers should be informed of the
paving plan prior to coming on the job site.
9. Backing
When an end dump or a live bottom truck is used to deliver mix
to the paver, the truck driver should back the truck up to the
laydown machine but stop just short of the push rollers on the
front of the paver. It is very important that the truck be centered
before the paver contacts it. Off-center positioning can push

the paver out of alignment, create problems as the mix floods
the hopper, and cause the truck bed to contact and damage
the hopper wings.
Page 40
SAFETY WARNING
Project personnel tend to tune out back-up alarms and are vulnerable while walk-
ing in the driver’s blind spot. This is the area between the tailgate and the paver
hopper. Many accidents that could be easily avoided occur in this area.
10. Truck Hitches/Contact/Configuration

Once the truck has come to a halt and the driver has released the brakes on the vehicle, the paver operator
should pick up the stopped truck. The key to this process is that the paver picks up the truck instead of the
truck backing into and bumping the paver. As much as possible, the paver should maintain a constant
speed throughout the paving operation. Use of the proper procedure will reduce the incidence of screed
marks and roughness in the mat.
11. Unloading the Mix

AC should be unloaded quickly when it arrives at the paving site. This will minimize mix cooling before it
is placed. Also, before AC is loaded into the paver, the inspector and/or foreman should be certain it is
the correct mix. Occasionally, paving jobs require several different mix designs (i.e., one for the leveling
course and one for the wearing course) and these mixes should not be interchanged.
If an end dump truck is used and the mix being delivered tends to segregate, the bed of the truck should be
raised a short distance to allow the mix in the bed to slide against the tailgate before the tailgate is opened.
Once the tailgate is opened, the mix is discharged from the truck in a mass and floods the hopper of the
Material Transfer Vehicle (MTV), reducing the possibility of segregation behind the paver screed. The
same procedure should be employed, if possible, when a live bottom truck is used to transport the mix.
12. Steering
While paving, the driver must focus not only on the dump person but also on the area in front of the truck.
The driver is responsible for steering within the “pull” of the paver, and keeping a slight pressure on the
brakes to maintain contact with the paver and rollers.
13. Braking
Too much braking force from the haul truck may cause the paver to slip and affect the mat.
14. Pulling Away and Clean Up

Once the final mix has exited the truck bed, the bed should
be lowered so the apron can clear the hopper guards. The
truck should immediately depart from the area to let the next
truck back up, and head to a pre-determined cleanup station
where a “wash down”, a cleanup of the truck’s bed can
be performed to prevent buildup of materials. A common
Page 41
practice is to leave the truck gate up, speed up and stop quickly to bang the tailgate to expel any remaining
mix. This practice, known as “banging the gate”, should be discouraged. Remember also that banging
the gate and other site delivery practices after unloading material can result in small piles of mix being
dropped on the roadway. These must be removed immediately before they are rolled over by other
incoming trucks, thus negatively impacting the quality of the finished roadway.
BEST PRACTICE
On Site Clean Up
Clean up locations need to be determined in advance because wash water
should not be discharged into catch basins, road ditches or environmentally sen-
sitive areas
Mix Delivery actions such as reducing transport time, insulating truck beds or tarping trucks can decrease
AC surface cooling rate. Additionally, since most of the AC mass is still at or near its original temperature
at loading, mixing the crust and interior mass together at the paving site (“remixing”) will produce a uniform
mix near the original temperature at loading. The key factor here is the workability of the mix and its ability
to break down enough to pass through the paver screed.
Page 42
Chapter Three Knowledge Check

1. In Virginia, haul trucks are required to be equipped with tarps:
a. Only during inclement weather
b. Only when the roadway temperature goes below 400F
c. All the time
d. Only when the supervisor has issued the order to do so
2. When using diesel fuel as a release agent, the residue must be dumped:
a. In a container listed on VDOT’s approved waste removal list
b. Onto the ground in a well-drained area
c. Diesel fuel should never be used as a release agent
d. Only at a VDOT residency
3. Some mixes are more prone to segregation than others and special care must be taken to ensure that
the mix load is as uniform as possible.
a. True
b. False
4. Hauling equipment (end dump or live bottom trucks) used to deliver mix to the paver should back up
until:
a. A very slight bump is felt by the truck driver
b. The truck comes into contact with the paver
c. There is contact with the transverse joint
d. The truck stops just short of paver contact
5. A material transfer vehicle can improve asphalt placement by:

a. Helping to reduce segregation
b. Allowing for more continuous supply of mix
c. Eliminating trucks bumping into the paver
d. All of the above
6. Banging the gate of the delivery truck is highly recommended because it helps to ensure that none of
the mix is wasted by being left in the truck.
a. True
b. False
Page 43
7. The truck transportation for mix delivery should be planned in a way that:
a. Minimizes the number of trucks used
b. Matches the laydown rate by the paver and plant production rate as closely as possible
c. Overloads each truck to deliver as much asphalt as possible
d. All of the above
8. Proper application and ensuring the correct amount of tack coat material applied are essential to
successful bonding between an existing pavement surface and a new overlay.
a. True
b. False
Page 44
4 Equipment and Laydown Operations
Identify the main parts of a paver
Define the screed and it’s purpose
Describe grade control processes and technologies
Recall the factors that impact final mat thickness, smoothness and quality
Page 45
Introduction
Mix placement and compaction are the two most important elements in AC pavement construction and
overlay. Mix placement involves any equipment or procedures used to place the delivered AC on the de-
sired surface at the desired thickness, while compaction is the process through which proper density is
achieved. Mix placement can involve complicated asphalt paver operations or simple manual shoveling.
In 1934, Barber-Greene introduced the Model 79 asphalt laydown machine, a self-propelled formless
laydown machine with a floating screed. Since then, the basic concept of the asphalt paver has remained
relatively unchanged: AC is loaded in the front, carried to the rear by a set of flight feeders (conveyor belts),
spread out by a set of augers, then leveled and compacted by a screed. This set of functions can be divid-
ed into two main systems: the tractor and the screed. The most commonly used types of pavers include
rubber tire pavers, and track pavers which run on steel tracks with rubber pads.
Page 46
Paver Equipment
Tractor (Material Feed System)
The tractor contains the material feed system, which accepts the AC at the front of the paver, moves it
to the rear and spreads it out to the desired width in preparation for screed leveling and compaction.
The basic tractor components are:
1. Hopper
2. Push Roller and Truck Hitch
3. Tow Points
4. Side Arms
5. Conveyor
6. Auger
Hopper
The hopper is used as a temporary storage area for AC delivered by the transport vehicle so that the
paver can accept more material than is immediately needed and can use the volume in the hopper to
compensate for fluctuating material demands (created by such things as paving over irregular grades,
utility access openings or irregular intersection shapes, etc.) Hopper sides (or “wings”) can be tilted up (or
“folded”) to force material to the middle where it is carried to the rear by the conveyor system. Hoppers
can also be fit with inserts to allow them to carry more AC; however, these inserts are typically used in
conjunction with an MTV.
Page 47
Hopper with wings folded up. Example of a hopper insert
IMPORTANT
The hopper must be wide enough to allow the body of the haul truck to fit inside
of it. In addition, particularly for smaller pavers, the hopper must be low enough to
permit the truck bed to be raised without the bed placing excessive weight on the
front of the hopper.
At the back of the hopper is a set of flow gates, one over each of the two slat conveyors. These are used
to regulate the amount of mix that can be delivered by the conveyors to the augers. The gates move
vertically, either by manual manipulation or mechanically. The flow gates should be adjusted to provide a
uniform head of material (at a level of or just above the center of the auger shaft) in front of the screed.
Push Roller and Truck Hitch

The paver hopper is used to receive and temporarily hold the asphalt mix from the haul vehicle or the
pickup machine.
Push roller and truck hitch. Truck hitch engaged.
Page 48
Tow Points
The screed unit is attached to the tractor at only one point on each side of the paver. This point is called
the tow point or the pull point by the different paver manufacturers. The tow point is really a pin-type
connection that allows the leveling arms (also called side arms or pull arms) of the screed to rotate or pivot
around the point. This pin connection reduces the transmission of movement between the tractor unit and
the screed unit.
Conveyor
At the bottom of the paver hopper is a set of
slat conveyors whose purpose is to carry the
AC from the hopper, under the chassis and
engine, then to the machine’s augers. The
slat conveyors are on either side of the paver,
operating independently of each other. The
amount of AC carried back by the conveyors is
regulated by either variable speed conveyors
and augers or flow gates, which can be raised
or lowered by an automatic feed control system
or the operator.
Auger
The auger receives AC from the conveyor and spreads it out
evenly over the width to be paved. There is one auger for each
side of the paver and they can be operated independently.
Some pavers allow the augers to be operated in reverse
direction so that one can be operated forward and the other
in reverse to send all the received AC to one side of the paver.
The auger gearbox can either be located in the middle or on
the outside edge of each auger. If an inadequate amount of
AC is distributed under a middle-located gearbox the result
can be a thin longitudinal strip of mat aligned with the gearbox
that exhibits lower densities from aggregate segregation and/
or temperature differentials.
Page 49
Example of Gearbox Streak
IMPORTANT
Operation of the tractor, and specifically the material feed system, can have significant
effects on overall construction quality and thus long-term pavement performance.
Although there are many detailed operational concerns, the two broad statements below
encompass most of the detailed concerns:
1. AC must be delivered to maintain a relatively constant head of material in front

of the screed. This involves maintaining a minimum amount of AC in the hopper,
regulating AC feed rate by controlling conveyor/auger speed and flow gate openings
(if present), and maintaining a constant paving speed. As the next section will
discuss, a fluctuating AC head in front of the screed will affect the screed angle of
attack and produce bumps and waves in the finished mat.
2. The hopper should never be allowed to empty during paving. This results in
the leftover cold, large aggregate in the hopper sliding onto the conveyor in a
concentrated mass and then being placed on the mat without mixing with any hot or
fine aggregate. This can produce aggregate segregation or temperature differentials,
which will cause isolated low mat densities. If there are no transport vehicles
immediately available to refill the hopper it is better to stop the paving machine than
to continue operating and empty the hopper .
If the tractor is set and operating properly, the slat conveyor and augers on each side of the paver will rare-
ly shut off. This continuous action of the conveyors and augers is accomplished by setting the proper po-
sition for the hopper
flow gates and de-
termining the correct
speed setting for the
slat and screw con-
veyors. The primary
key to the placement
of a smooth pave-
ment layer is the use
Page 50
of the material feed system to keep the head (level) of material in front of the screed constant, primarily by
keeping the slat conveyor and augers running as close to 100 percent of the time as possible.
Material Feed Sensor

For the automatic feed control system to function properly, the feed sensor control arm should be located
as close to the outside end of the augers as possible. If rigid paver screed extensions are used, as dis-
cussed later, the control arm should be mounted beyond the end of the augers, just inside the end gate
on the paver screed.
For this system, a feed control sensor (a type of limit switch) is used to determine the amount of mix in
the auger chamber. If the volume of mix available in front of the screed falls below the desired amount,
the feed control sensor will move enough to engage the slat conveyor and auger system, pulling more mix
back to the screed area. As the material is distributed in front of the screed, the feed control sensor will
rise and disengage the feed system.
This action will maintain the pre-selected head of mate-

rial in front of the screed. This sequence repeats itself,
continuously maintaining consistent head of mix as long
as material is available in the hopper. On some pavers, a
variable-speed (potentiometer- type) feed system is used to
control the amount of material in front of the screed. Instead
of an on-off system, the speed of the delivery system is in-
creased when more mix is needed and the speed of the slat
conveyor and auger system (on each side of the paver) is
decreased when the head of material in front of the screed
is too great.
In addition to these limit switch-type and variable-speed

systems, both ultrasound and infrared sensing devices can
be used to determine the amount of mix in the auger cham-
ber. These two types of systems operate on the same basis
as the limit switch system; measuring the amount of mix in front of the screed and controlling the slat con-
veyor and auger feed system to maintain a constant head of mix at the screed.
Page 51
Screed
The most critical feature of the paver is the self-leveling screed unit, which determines the profile (i.e.,
cross slope, thickness and texture) of the AC being placed. The screed takes the head of AC from the
material delivery system, strikes it off at the correct thickness and provides initial mat compaction. The
following is a photograph depicting screed components and descriptions of their use:
1. Screed Plate
2. Screed Angle (Angle of Attack)
3. Strike-off plate
4. Screed Arm
5. Tow Point
6. Pivot Point
7. Depth Crank
Screed Plate
The screed plate is the flat bottom portion of the screed assembly that flattens and compresses the AC. It
is the only portion of the screed that develops the initial texture of the mat and can be adjusted. The center
of the leading edge of a rigid screed has grooves cut into it to allow the screed to be flexed or warped.
Screed Angle (Angle of Attack)

The screed angle is the angle the screed makes with the ground surface.
Strike-off Plate
The strike-off plate, also known as a prestrike-off plate, is the vertical plate just above the leading
edge of the screed that controls the feed of the asphalt mix under the paver screed, thereby regulating
the amount of mix that reaches the nose of the screed plate. It also helps to reduce wear and tear on the
leading edge of the screed. When the strike-off is attached to the front of the screed, its position becomes
important relative to the ability of the screed to handle the asphalt mix properly. If the strike-off is set too
high, extra material will be fed under the screed. This action will cause the screed to rise. The resulting
increase in the mat thickness will be overcome by manually reducing the angle of attack of the screed,
using the thickness-control cranks. This, in turn, will cause the screed to pivot around its hinge point and
ride on its nose. Rapid wear of the nose plate will result. In addition, the screed will settle when the paver
is stopped between truckloads of mix because the weight of the screed is carried only on the front part of
the screed.
Page 52
When the strike-off is set too low, the thickness of the lift will be
reduced because of the lack of mix being fed under the screed.
To maintain the proper thickness, the angle of attack of the screed
must be altered, causing the screed to ride on its tail. This increas-
es the wear on the back of the screed and causes the screed to
settle whenever the paver is stopped because of the concentra-
tion of weight of the screed on a smaller surface area.
Screed Arm (Tow Arm)

The screed is attached to the leveling or tow arms on each side of the paver through a hinge or tow point.
Tow Point
The tow point is the point at which the screed arm is attached to the tractor unit. This allows the screed
to use the tractor as a leveling device, much like a ski.
Pivot Point
The pivot point is located at the center of the wheelbase of the tractor and allows the screed to use the
tractor as a leveling device, much like a ski.
Screed Depth Crank

The speed depth crank is the manual control device used to
set screed angle and ultimately, mat thickness.
Page 53
IMPORTANT
INSPECTION AND MEASUREMENTS
• Verify head of material at the auger – should be maintained at center of auger shaft
for consistent mat thickness
• Augers should be turning slowly and consistently approximately 100% of the time
• Verifying material thickness behind screed by sticking the mat
• Placement of straightedge behind screed, parallel to the screed to verify correct

cross- slope
• Minimizing folding of hopper wings to every third or fourth load
• Uniform mat appearance behind the screed to minimize streaking
Other components included but not shown in the example photograph:
1. Crown Control
The screed on the paver can be angled at its center to provide for positive or negative crown. The
amount of crown that can be introduced into the screed varies. The adjustment of the crown is typically
done using a turnbuckle device to flex the bottom of the screed and impart the desired degree of crown.
Normally the lead crown setting is 1/32 to 3/16 inch greater than the tail crown position, with 1/8 inch being
the average difference in the crown settings.
2. Screed Vibrator
Device located within the screed used to increase the screed’s compactive effort. Screed compaction
depends upon screed weight, vibration frequency and vibration amplitude.
3. Screed Extensions
Fixed or adjustable additions to the screed to make it longer. Basic screed widths are between 2.4 m (8
ft.) and 3.0 m (10 ft.). However, often it is economical to use wider screeds or adjustable width screeds.
Therefore, several manufacturers offer rigid extensions that can be attached to a basic screed or hydrauli-
cally extendable screeds that can be adjusted on the fly.
4. End Plate (End Gate)

An end plate (or end gate or edger plate) is attached to
the end of the screed to restrict the outward movement
of the mix around the end of the screed. In typical op-
erating mode, however, the end plate is positioned tight
to the surface being paved to retain the mix and control
the width of material being placed.
Page 54
5. Cut Off Shoes

Cut-off shoes can be used, if necessary, to reduce the width of mix placed to a width that is less than
the basic main screed width. Typically, the cutoff shoes come in widths of 1 - 2 feet, and are adjustable in
increments of 1½ - 3 inches, depending on the manufacturer.
6. Thickness Control Screws

The thickness-control mechanism, usually either a crank or a
handle, allows the screed to be moved or rotated around the pivot
point. As the mix passes under the screed plate, the screed floats on
the mix, determining the mat thickness and the texture of the material
as well as providing the initial compaction of the asphalt mix. For a
constant position of the tow point (the tractor unit running on a level
surface and without automatic screed controls), altering the setting of
the thickness- control devices change the attitude (angle of attack) of
the screed and changes the forces acting on the screed. This in turn causes the screed to move up to,
or down to, a new elevation as the paver moves forward, and thus alters the thickness of the mat being
placed. The reaction of the screed to changes in position of the thickness-control settings, however, is not
instantaneous.
7. Heating Systems
The screed is equipped with heaters or burners, the primary
purpose of which is to increase the temperature of a cold bottom
screed plate to approximately 300°F. It is necessary for the screed to
be at the same temperature as the asphalt material passing under it
in order to assure that the mix does not stick to the screed plate and
tear, providing a rough texture to the mat. A properly heated screed,
particularly at the start of the day’s paving operations or after any
extended shutdown of the laydown process, provides for a more uniform mat surface texture. The screed
heaters cannot be used to increase the temperature of the mix being placed because the amount of time
that the mix is under the screed is much too short to accomplish any temperature rise in the mix.
8. Screed Extensions and End Plates
A. Hydraulically Extendible
Most paver manufacturers have developed hydraulically extendable paver screeds that trail the
primary or basic screed on the paver. One make of pavers, however, is equipped with a power
extendable screed that places the extendable portion of the screed in front of the main screed. For
all hydraulically extendable screeds, it is very important that the angle of attack for the extendable
screeds is the same as the basic screed. If the extensions on the extendable portion of the screed
Page 55
are not properly aligned with the main screed, a

longitudinal mark or ridge will occur in the surface of
the mix at the junction between the two screeds. In
addition to the longitudinal mark, a mismatch in the
elevation between the two screeds can also result
in a possible difference in surface texture in the mix.
Finally, the lack of proper alignment between the
two screeds can cause a difference in the degree
of compaction that is obtained in the mix under the
extendable screed. In addition, proper placement of
the material feed sensor is important to avoid segregation problem
B. Rigid Screed Extension

When the basic width of the paver screed (8
feet for small pavers and 10 feet for the larger
machines) needs to be changed to accommodate
increased paving widths, rigid screed extensions
can be employed. These extensions come in
several widths, usually 6 in., 1 ft., 2 ft., 3 ft. and
5 ft. sections. Further, it is very important that the
extension be set at the same elevation and angle
as the basic screed to prevent the presence of a transition line or ridge at the intersection of the
main screed and the extension or between different sections of extension. Whenever a rigid screed
extension is employed on the basic paver screed, auger extensions and the accompanying auger
tunnel extensions should also be added. The length of all the auger and tunnel extensions should, in
general, be the same length as the added screed extensions to allow room between the end of the
auger and the end plate of the screed. Typically, the distance between the end of the auger extension
and the end plate should be about 18 inches.
C. Hydraulic Strike-Off
The hydraulic strike-off is an option for most
screeds and allows the screed to be extended for
brief periods to form turnouts, ramps, etc. Either
a strike off or mini screed approximately 6 inches is
available to impart initial texture and compaction to
the mix that passes under it. However, this texture
and compactive effort are normally different from
Page 56
that which develops under the screed. In addition, the mix will need to be left higher than the screed
placed mix. The use of hydraulic strike-offs to place mainline paving instead of adding rigid extensions
is not.
Pre-Compaction Systems
Early pavers were equipped with tamper bars that were located on the leading edge of the paver screed.
These tamper bars were used to tuck the asphalt mix under the screed and to provide some degree of
initial compaction to the mix as it passed under the screed. The tamper bar system was replaced by the
more efficient vibratory screed system.
Two factors within the screed itself also contribute to the degree of compaction. The first is the frequency
of vibration and the second is the amplitude of the compactive effort. The frequency of vibration is
controlled by the rotary speed of the vibrator shaft. Increasing the revolutions per minute of the shaft
will increase the frequency of the vibration. The applied amplitude is determined by the location of the
eccentric weights that are located on the shaft. The position of the eccentric weights can be altered to
increase or decrease the amount of compactive effort applied to the mix by the screed. In general, the
vibrators should be used near the maximum possible frequency. On screeds where it is possible to change
the amplitude of the applied vibrational force, the amplitude setting selected is related to the thickness of
the mat being placed; lower amplitude for thinner lifts and higher amplitude for thicker lifts.
The amount of density obtained by the paver screed is also a function of the speed of the paver. The
faster the paver moves, the less time the screed sits over any particular point in the new mat, and, thus,
the amount of compactive effort applied by the screed decreases. For asphalt concrete mixes, it can
be expected that approximately 70 to 80 percent of the theoretical maximum density of the mix will be
realized in the mix when it passes out from under the paver screed. A few of the most recent pavers (and
many pavers used in other countries) are equipped with combination screeds--both tamper bars and a
vibratory screed.
Page 57
Laydown
The process known as laydown is the portion of the AC paving process where the AC is actually placed or
“laid down” by the paving machine. There are, of course, many considerations to take into account when
placing AC including materials, weather, crew knowledge, individual experience and alternating screed
forces.
Screed Forces
Since the paver screed is free floating, it can be impacted at any time by any one of six basic forces which
when left undisturbed, result in an equilibrium screed angle and elevation that determines mat thickness.
Adjusting paver speed, material feed rate or tow point elevation will change these forces and result in a
new equilibrium screed angle and elevation and eventually a new mat thickness. Although the screed
angle can be adjusted manually to change mat thickness, excessive adjustments will result in a wavy,
unsmooth mat. In addition to grade, screeds can also control mat slope and crown to provide almost com-
plete control over mat elevation at any location. Following is a list of the six basic forces that act, helping
to determine its position and angle
1. Towing force. This is provided by the tractor and

exerted at the tow point. Thus, towing force is con-
5
trolled by paver speed. 1
2
2. Force from the AC head resisting the towing 3
force. This is provided by the AC in front of the
screed and is controlled by the material feed rate 6 4
and AC characteristics.
3. Weight of the screed acting vertically downward. This is obviously controlled by screed weight.
4. Resistive upward vertical force from the material being compacted under the screed. This is
also a function of AC characteristics and screed weight.
5. Additional downward force applied by the screed’s tamping bars or vibrators. This is controlled
by vibratory amplitude and frequency or tamping bar force.
6. Frictional force between the screed and the AC under the screed. This is controlled by AC and
screed characteristics.
Page 58
Factors Affecting Mat Thickness and Smoothness

Since mat thickness needs to be closely controlled, pavers have controls to manually set screed angle
rather than rely on a natural equilibrium to determine mat thickness. In typical paving operations, the
screed angle is adjusted as needed to control mat thickness. Following is a brief overview of how speed,
material feed rate and tow point elevation affect screed angle, screed height and therefore mat
thickness.
1. Speed
Paver speed affects mat thickness by changing the screed angle. If a paver speeds up and all other forces
on the screed remain constant, the screed angle decreases to restore equilibrium, which decreases mat
thickness. Similarly, as paver speed decreases, screed angle increases, which increases mat thickness.
2. Material Feed Rate

The amount of AC in front of the screed (the material “head”) can also affect screed angle and thus mat
thickness. If the material head increases (either due to an increase in material feed rate or a reduction in
paver speed), screed angle will increase to restore equilibrium, which increases mat thickness. Similarly,
if the material head decreases (either due to a decrease in material feed rate or an increase in paver
speed), screed angle will decrease to restore equilibrium, which decreases mat thickness. Therefore,
in order to maintain a constant mat thickness for a change in paver speed or material head in front of
the screed, the natural equilibrium of forces on the
screed cannot be relied upon and the screed angle
must be manually adjusted using a thickness control
screw or depth crank. Screed angle adjustments do
not immediately change mat thickness, but rather
require a finite amount of time and tow distance
to take effect. The following picture shows that it
typically takes five tow lengths (the length between
the tow point and the screed) after a desired level is
input for a screed to arrive at the new level. Because of this screed reaction time, a screed operator who
constantly adjusts screed level to produce a desired mat thickness will actually produce an excessively
wavy, unsmooth pavement.
3. Tow Point Elevation

Tow point elevation will also affect screed angle and thus mat thickness. As a rule-of-thumb, a 25 mm (1-
inch) movement in tow point elevation translates to about a 3 mm (0.125 inch) movement in the screed’s
leading edge. Without automatic screed control, tow point elevation will change as tractor elevation
changes. Tractor elevation typically changes due to roughness in the surface over which it drives. As the
tow point rises in elevation, the screed angle increases, resulting in a thicker mat. Similarly, as the tow
Page 59
point lowers in elevation, the screed angle decreases, resulting in a thinner mat. Locating the screed tow
point near the middle of the tractor significantly reduces the transmission of small elevation changes in
the front and rear of the tractor to the screed. Moreover, because the screed elevation responds slowly to
changes in screed angle, the paver naturally places a thinner mat over high points in the existing surface
and a thicker mat over low points in the existing surface.
IMPORTANT
The interaction of paver speed, material feed rate and tow point elevation determine
the screed position without the need for direct manual input. This is why screeds
are sometimes referred to as “floating” screeds.
Automatic Screed Control

As discussed previously, the screed angle can be manipulated manually to control mat thickness but tow
point elevation is not practical to manually control. Therefore, pavers usually operate using an automatic
screed control, which controls tow point elevation using a reference other than the tractor body. The
following references, also known as “grade reference systems”, assist in controlling AC pavement grade:
1. Erected stringline.
This consists of stringline erected to specified elevations that are independent of existing ground elevation.
Most often this is done using a survey crew and a detailed elevation/grade plan. Although the stringline
method provides the correct elevation (to within surveying and erecting tolerances), stringlines are fragile
and easily broken, knocked over or inadvertently misaligned. Lasers can be used to overcome the difficulties
associated with stringlines because they do not require any fragile material near the pavement construction
area. Lasers can establish multiple elevation or grade planes even in dusty or high-electronic and light-
noise areas and are therefore sometimes used to construct near-constant elevation airport runways. The
laser method becomes quite complicated, however, when frequent pavement grade changes are required.
2. Mobile reference.
This consists of a reference system that travels with the paver such as a long beam or tube attached to the
paver (called a “contact” device since it actually touches the road) or an ultrasonic device (called a “non-
contact” device since it relies on ultrasonic pulses and not physical contact to determine road elevation).
The mobile reference system averages the effect of deviations in the existing pavement surface over a
distance greater that the wheelbase of the tractor unit. Minimum ski length for a contact device is normally
about 7.5 m (25 ft.) with a length being on the order of 12 to 18 m (40 to 60 ft.).
3. Joint matching shoe.

This usually consists of a small shoe or ski attached to the paver that slides on an existing surface (such
as a curb) near the paver. Ultra-sonic sensors accomplish the same task without touching the existing
Page 60
surface by using sound pulses to determine

elevation. This type of grade control results in the
paver duplicating the reference surface on which
the shoe or ski is placed or ultra-sonic sensor is
aimed.
In addition to grade control, the screed can also be set to control pavement slope and/or crown. A
slope controller uses a slope sensor mounted on a transverse beam attached to the screed to determine
screed slope, then adjusts screed slope to the desired amount. Generally, one side of the screed is set up
to control grade and the opposite side is set up to control slope based on that grade. The usual practice is
to run grade control on the side of the screed nearest the pavement centerline and run slope control on the
screed side nearest the pavement edge because it is easier to match the centerline joint if grade control
is used on that side of the paver. Screed crown (the elevation of the middle in relation to the edges) can
also be controlled. Typically screeds offer separate front and rear crown controls. If crown control is used,
the front control is usually set to a slightly more severe crown than the rear control to allow for easier
passage of AC under the screed.
BEST PRACTICE
In order to achieve the most consistent thickness and smoothest possible sur-
face, pavers attempt to maintain a constant speed, use automatic feed controls
to maintain a consistent head of material in front of the paver, and use automatic
screed control to maintain a consistent tow point.
Page 61
Constructing Transverse and Longitudinal Joints

All pavements have one potential internal weakness that cause more problems than any other – joints.
When constructed properly, however, a joint area will perform as well as the remainder of the asphalt
pavement. It is difficult if not impossible to construct a pavement without any joints; therefore, the real trick
is to make as few joints as possible and to construct them carefully and properly. The appearance and
quality of the joints determines the overall appearance and quality of the finished mat.
Purpose of Transverse Joints

When the placement of the asphalt mix is to be suspended for a period of time, it is necessary to construct
a transverse joint across the pavement being placed. This is accomplished in one of several ways,
depending primarily on whether traffic is to travel over the asphalt mix between the time the paving is
stopped and it is started again.
If traffic is not going to pass over the end of the paving, a vertical butt joint can be constructed. If traffic will
be permitted to travel over the transverse joint, a tapered joint will be necessary.
End of Paving
It is very important that the paver be run in normal fashion right up to the point at which the transverse joint
is constructed. This means that the head of material carried in front of the screed should be as consistent
as possible at the location of the joint. This requirement permits the forces acting on the screed to be
constant and maintains the angle of attack for the paver screed. The result of such a paving operation is
a uniform mat thickness through the joint area.
It is a common but incorrect practice, however, to empty out the paver hopper whenever a transverse joint
is to be built. It is a much better practice to locate the transverse joint at the point where the amount of
material in front of the screed is normal.
Butt Joint
For a butt joint, a vertical face is constructed by hand methods across the width being paved. This
operation consists of raking, shoveling, and then removing the mix that is located downstream of the
selected joint location. The asphalt mix that is in place upstream of the joint is not touched in any manner.
The mix that is removed from the downstream side of the joint is then recycled or discarded. Compaction
of the mix on the upstream side of the joint is accomplished in normal fashion. It is necessary, however, for
the rollers to compact the mix immediately adjacent to the joint. For this to be done properly, runoff boards
must be placed next to the joint.
Page 62
Tapered Joints
If traffic is to be carried over the transverse joint, it is necessary to build a tapered joint. For this type of
joint, as for the butt joint, it is proper for the paver operator to keep the head of material in front of the paver
screed as consistent as possible up to the point that the joint is to be built. This process assures that the
thickness of the mix being placed is uniform up to the joint. There is more opportunity for this to be done
in practice with tapered joint construction than with butt joint construction because the mix left in the paver
hopper can be used to build the taper.
At the point of the transverse joint, the asphalt mix downstream of the joint is temporarily pushed aside.
A vertical edge is formed at the upstream face of the mix. If the tapered joint is to be only temporary,
treated paper or other similar material is then placed downstream of the joint directly on the existing
pavement surface. This paper is used because the asphalt mix will not stick to it. The length of the paper
is dependent on the thickness of the course just placed but is typically about 3 or 4 feet long and the width
of the lane being paved.
Once the paver is in place, the asphalt mix is shoveled

back over the paper and a ramp is formed in this mix
with a lute or rake. Any asphalt mix that is not used to
construct the ramp or taper is discarded. If the joint is
to be left in place permanently, the taper is constructed
in the same manner except that the paper is not used.
Constructing a temporary tapered joint using sand or
dirt as the bond-breaking medium is not an acceptable
paving practice.
A third type of tapered joint is the non-formed, sawed joint. For this type of joint, the paver operator keeps
the paver operating normally until there is no more mix in the hopper or in the auger chamber. At the point
where the mix becomes non-uniform across the width of the lane being paved, a ramp is constructed with
the “left-over” mix. No vertical face is formed, and the mix is merely tapered from the proper layer thickness
to the level of the adjacent existing pavement. Any mix not needed to make the ramp is removed.
One advantage of the tapered joint is the fact that the compaction equipment can run over the transverse
joint and down the ramp without rounding the joint. Because the rollers can pass over the end of the mat
easily, the compaction of the mix upstream of the joint is usually superior to the mix adjacent to the butt
type joint. A second advantage is that generally there is less mix to shovel from the joint, because some of
the extra mix is used to make the ramp or taper. The disadvantage of this kind of joint is that this mix must
eventually be removed before paving commences downstream of the transverse joint.
Page 63
Tying In
In areas where the new hot-mix asphalt layer abuts
an existing structure, such as a bridge deck, it is often
necessary to place the mix adjacent to the joint by hand.
The mix that is needed to complete the joint is deposited
in the area to be paved either by the paver or by being
dumped from a haul truck. In order to avoid overworking
the mix and possibly causing segregation, the mix should
be placed as close as feasible to its final location. The mix
is then spread by hand methods, normally using rakes or lutes.
In addition, the mix must be “left high” in order to allow for the compression of the material by the compaction
equipment. Because the mix is being placed by hand, it will not be as dense as it would be if it were laid
by the paver. Thus, the ¼ in/1 inch rule of thumb usually used will not be valid for hand-spread mix. To
permit proper compaction of the mix and have the mix end up at the proper elevation to match the adjacent
structure, the level of the mix should be approximately 3/8 inch higher than the surrounding pavement for
each 1 inch of compacted layer thickness.
Note: The loose to compacted thickness ratio varies by mix and is determined by experience.
The hand work area must be rolled by the compaction equipment as soon as possible after the mix is in
the proper location. Because of the time necessary to place the mix, rolling will be delayed and the mix
will be cooling during the placement process. In order to achieve the required density, extra rolling may
be needed.
Start of Paving
1. Squaring Up
A straightedge should be used to determine the condition of the transverse joint before paving begins. If
the mix upstream of the joint is level, the location of the transverse joint is fine. If the straightedge indicates
that the previously placed mix is not level, the location of the transverse joint should be moved to a point
where the proper thickness and smoothness of the pavement layer exists. The mix downstream of the
new joint location should be removed and recycled.
2. Removal of Material
If a tapered joint has been constructed at the transverse joint, the mix in the ramp or taper must be removed
before the paving can be started. For a taper built with treated paper, there is no bond between the mix
Page 64
in the ramp and the underlying pavement. The paver and the mix are readily removed and returned for
recycling. A vertical face is left at the upstream edge of the joint.
For a taper constructed with the board and a ramp of asphalt mix, the material downstream of the board
will be partially bonded to the existing pavement surface. A front-end loader typically is used to pry up the
mix in the taper. This can be very difficult to do, depending on the amount of traffic that has passed over
the transverse joint and the environmental conditions at the site. Once the mix has been removed, the
board is then removed, exposing the vertical face of the joint.
If a non-formed tapered transverse joint is used, it is necessary

first to saw a transverse joint in the asphalt mat. It can be placed
far enough back from the taper to assure that the thickness of
the layer is constant. Once the joint is cut completely through
the asphalt mat, a front-end loader is used to pry up the mix
that is downstream of the saw cut.
As with the tapered joint that uses the board, one disadvantage
of this type of joint is that it is often very difficult to remove
the mix downstream of the saw cut from the existing roadway.
As an alternative, a cold-milling machine can be used both to
form the vertical edge of the transverse joint and to remove the
mix in the taper.
3. Cleaning and Tacking
After the existing joint materials are removed it is important

to thoroughly clean the area exposed and properly tack the
surfaces. It requires little work and is good practice to heavily
tack the vertical edge of the joint such that slight puddling
occurs at its base before paving resumes.
4. Raking/Luting the Joint

If the transverse joint is constructed properly up to this point,
the amount of raking that needs to be done is minimal. If the
paver screed starts out on blocks and if the head of material
against the screed is constant, the thickness of the mat downstream of the joint will be correct. Very little
mix, if any, will need to be brushed back from the joint. There is never any reason to rake a transverse
joint excessively.
It is considered good practice to shovel up and recycle any coarse aggregate that remains after luting
a surface course. The appearance of the final surface is very important and should be as uniform as
Page 65
possible. Passing the rollers over the edge of the transverse joint, without having any boards beyond
the edge to support the weight of the rollers, will cause rounding of the edge of the joint. This latter type
of joint construction results in two problems. First, the rounding of the edge of the butt joint prevents the
construction of a proper vertical joint when paving is restarted. Second, the amount of compactive effort
applied to the asphalt to the joint is typically not adequate.
5. Initial Straightedge Check

Before the material on the downstream side of the joint is compacted, it should be determined if the joint is
smooth by running a straightedge across the joint. The straightedge should
rest on the uncompacted mat and extend over the already compacted mix.
The distance between the bottom of the straightedge and the top of the
compacted mat should be equal to the amount of rolldown that will occur
during the compaction process.
6. Final Straightedge Check

The straightedge should be used again to check the level of the joint once
the compaction process has been completed.
Transition into Bridges & Under Bridges (ACOT1)

No pavement overlay shall decrease the vertical clearance under a bridge. In situations where the
pavement under the overpass cannot be milled in direct proportion to the overlay, the new pavement is to
be tied down to the existing pavement under the overpass 75 feet from the outer edge of the overpass.
Page 66
Resuming of Paving Operations

When paving is ready to resume, the following procedure is used to form a suitable transverse joint.
1. The taper of material is removed.
2. A 10ft. straightedge is used to check the longitudinal grade of the mat. Because the paver was running
out of material as it placed the last few feet of mat, it is possible that those last few feet taper slightly
(ramp down) from the specified level of the mat. If this is the case, a new transverse edge must be cut
to full pavement depth behind the point where the ramping down begins.
3. The vertical face of the mat is tack-coated.
4. The paver is backed up to the edge of the mat and the screed rested on the cold mat surface.
5. The screed is heated while it rests on the mat. This provides some heat to the material at the edge of
the mat.
6. The heated screed is raised and at least 3 shims or starting blocks as thick as the difference between
the uncompacted and compacted mat are positioned under it. The starting blocks should extend the
full length of the screed, front to back.
7. The truck with the first load of AC is backed carefully to the hopper. During discharge of the mix from
the truck bed to the paver, it is essential that the truck not bump the paver, and cause it to move.
8. The paver starts forward in a low gear.
9. Once the paver has moved away, excess AC is cleaned off the surface of the mat and the evenness
of the joint is checked with a straightedge.
10. If a joint is satisfactory, a 6-inches width of the AC is rolled transversely and the joint checked for
smoothness. If the joint is satisfactory, transverse rolling is continued in 6 to 12-inch-wide increments
until the entire width of the roller is on the new AC. If the straightedge shows an uneven joint, the sur-
face of the new mat must be scarified while still warm and workable. Scarification is done, preferably
with a tined lute. Excess material can then be removed or additional material added, and the joint
rolled and rechecked.
Page 67
Constructing Longitudinal Joints
The longitudinal joint occurs when one lane of asphalt mix is constructed adjacent to a previously placed
lane of mix. The first pull of a paver almost always leaves at least one unsupported longitudinal edge in
the mat. This joint will be a potential weakness in the finished pavement and must be handled carefully to
minimize or avoid this potential.
When referring to longitudinal joints, you will often see the following terms and/or conditions:
A previously paved lane. Mix temperature is at or near ambient

Cold lane/mat
temperature and the lane can support traffic loads.
Hot lane/mat The lane being currently placed. AC is at placement temperature.
Joint overlap The width that the hot lane overlaps the cold lane.
That portion of the cold lane that is at a significantly lower density

than the rest of the cold lane. This area is typically at the outer edge
Low density area
of the cold lane taper in the joint overlap area and can have low
density
The material beyond the anticipated final mat thickness. As a newly

Extra material for
placed mat is compacted, its thickness decreases and it becomes
compression more dense.
Page 68
Staggered Joints
When placing multiple layers of HMA,
longitudinal joints should be staggered so a
single vertical joint does not run the depth of
the pavement.
Conventional Unconfined Longitudinal

Joint
A conventional unconfined longitudinal joint
is most common. The edge of the material is
extruded through the screed and restrained
by the edge plate. The material is compacted
from the top down and is unconfined on one
edge. Therefore, there is a tendency for the
edge of the mat to have a lower density than
say the middle of the mat which has adequate
mix confinement all around. All longitudinal joints should be constructed as cleanly and straight as pos-
sible. This joint matching process has a much higher success rate than a joint that isn’t cleaned and me-
anders side to side.
Page 69
Compaction of Unsupported Joints

Longitudinal joints can be compacted in a variety of ways.
During the initial construction of an unsupported or wedge
joint, the roller should hang the edge of the roller drum ap-
proximately 6 inches over the unsupported edge to provide
some confinement. On thicker lifts, another technique is to
hold the first pass of the roller approximately 1 foot inside
of the outside edge, then compact the strip of mix in a sub-
sequent pass. Compaction will be covered in greater detail
in a later chapter.
Preparing a Cold Joint

The edge of the longitudinal joint may contain loose materials after being exposed to traffic and should be
swept before a new mat is placed along side. The cold joint should then be tacked as described later in
this chapter.
Paving a Longitudinal Joint

One of the keys to the construction of a good longitudinal
joint between lanes of asphalt mix is the amount of overlap
between the new mat and the previously placed mat. This
amount of overlap provides just enough material on top
of the joint to allow for proper compaction without having
extra mix, which must be pushed back from the joint by
a raker. Overlap a sufficient amount to produce a tight
joint and allow for the steering tolerance of the paver. This distance varies, but 1 to 1½ inches has worked
successfully in many cases. The end gate should be in full contact with surface and leave a tight edge.
It is important that there be enough HMA material placed by the paver on the hot mat side of the joint
construction such that when fully compacted, it is as high or slightly higher than the adjoining mat (cold
side). Should the second lift placed end up slightly higher than the adjoining mat after compaction, the
chances are good that acceptable joint density has been obtained.
One major problem with longitudinal joint construction is an excessive amount of overlap of the paver
screed over the previously placed mat. (This may be caused, in part, by a ragged or wavy longitudinal
edge on the first pass. Use of a string to guide the paver operator as the first lane is placed will usually
reduce this problem greatly.) Because this extra asphalt mix cannot be pushed into the compacted mat,
the material is raked or luted onto the new mat. If the longitudinal edge of the first lane is straight and if
the correct amount of overlap is used, the amount of raking that must be done will be minimal. Excessive
luting and raking can also be a safety issue if the cold lane is under traffic.
Page 70
Joint Construction Devices

Several different devices or techniques have been used with varying degrees of success to aid in the con-
struction of longitudinal joints. These devices are:
• Notched wedge joint. An extended joint taper placed on the first paved lane that helps reduce joint
air voids. An attachment on the paver screed forms the mat edge into a tapered section. Notches on
either end of the taper eliminate the extremely thin taper extremities which might otherwise cause poor
compaction. The notches are at least as deep as the nominal maximum aggregate size of the mix and
the taper is usually spread out over about 1 foot. The hot lane overlaps the cold lane notch by about
0.5 to 1 inch and is bumped back to the notch to ensure enough material at the notch for adequate
compaction. The notched wedge joint also provides a safe ramp for traffic transition between the cold
lane and the yet unpaved portions of the hot lane.
• Cutting wheel. 10-inch diameter cutting wheel mounted on an intermediate roller or a motor grader
that cuts 1-2 inches of the unconfined, low density edge of the initial lane after compaction, while the
mix is still plastic. This technique cuts away and discards the high air void portion of a typical longitudi-
nal joint.
• Joint maker. A boot-like device about 3 inches wide which is attached to the side of the screed at the
corner during construction. The device forces extra material at the joint through an extrusion process
prior to the screed. A kicker plate is also furnished which is attached to the side of the paver to lute back
the overlapped AC mix.
Page 71
• Edge restraining device. A 3-inches wide hydraulically powered wheel mounted on a roller that, when
operated is positioned alongside the roller drum that pinches the unconfined edge of the first lane to-
wards the drum providing lateral resistance during the first roller pass.
Joint Adhesion
Sometimes longitudinal joints can fail because the hot and cold sides fail to adequately bond with one
another. In Virginia, the specified technique to increase the likelihood of bonding is:
1. Coat the cold side with a heavy tack coat. A tack coat
applied to the cold side before paving the hot side will
assist in bonding. The cold side vertical face and at least
12 to 18 inches of the surface to be overlaid is tacked with
tack puddling along the edge. specified by VDOT.
Joint Overlap and Luting

If the amount of overlap of the new mix on the old lane is 1
to 1½ inches, any raking that is done should be used merely
to “bump” the joint, pushing the mix off of the old lane and
onto the new mat directly over the joint. If the adjacent lane is
overloaded too far and too much mix is deposited on the old
mat, the excess material should be pulled away from the new
mat instead of being pushed onto the new mix. The mix should not rebroadcast across the new lane. The
excess mix should be picked up and recycled.
Excellent longitudinal joints can be constructed without raking the joint. If the new mix has been properly
overlapped on the previously placed mat, raking of the longitudinal joint can be eliminated. It is recommended
that raking of this joint be deleted if proper overlap and compaction can be obtained.
A trimmed joint is sometimes used. This joint is constructed by removing all freshly placed material that
has overlapped the rolled lane. This is best done by trimming the joint immediately behind the paver with
a square-ended shovel. In this way, the operator can tell where the edge of the cold joint is and gauge his
cutoff line accordingly.
If the lanes are placed simultaneously with two pavers moving in echelon, the loose depths of the mats
should match exactly, with no overlap for a hot joint. The joints of a freshly paved mat are usually compacted
before the rest of the paved width.
Page 72
Location of Longitudinal Joints

Overlapping of successive courses rather than stacking the joint directly on top of the joint below helps
to prevents cracking and separation along the longitudinal joint. The locations of joints must be planned
such that the joint in the final layer of pavement is located, where possible, between designated travel
lanes of the final traffic pattern. This will assure that the joint is not located in the wheel path of a lane.
When possible, the joint should not be in the same location as the final pavement marking. Future main-
tenance operations to the joint such as crack sealing could obscure the marking and make placement of
new marking difficult.
Page 73
Chapter Four Knowledge Check

1. ____________________ involves any equipment or procedures used to place the delivered AC on
the de¬sired surface at the desired thickness, while ___________________ is the process through
which proper density is achieved.
a. Asphalt laydown; compaction
b. Mix placement; compaction
c. Manual laydown; crown control
d. None of the above
2. The paver consists of two primary parts: the tractor unit and the screed.
a. True
b. False
3. ____________________________ regulate the amount of mix that can be delivered to the augers.
a. Hopper wings
b. Conveyor flow gates
c. Conveyor belt thickness
d. Size of the hopper
4. The screed heater is important because it prevents the mix from sticking to the screed and causing
tearing in the mix.
a. True
b. False
5. When changing trucks during paving, it is best if the paver can be stopped briefly to allow the transfer
to be accomplished more easily.
a. True
b. False
6. When the strike-off device is set too low, which of the following problems can occur:
a. The screed’s angle of attack must be altered
b. Not enough mix is allowed under the screed
c. Excessive wear on the back of the screed
d. All of the above
e. All of the above, except C
Page 74
7. The interaction of paver speed, material feed rate and tow point elevation determine the screed position
without the need for direct manual input.
a. True
b. False
8. Which of the following factors can impact mat thickness and smoothness?
a. Paver speed
b. Material feed rate
c. Tow point elevation
d. All of the above
e. None of the above
Page 75
Page 76
5 MAT and Joint Compaction
Understand the importance of compaction efforts
Recognize paving influences outside of operators control (environmental factors and mix
properties to achieve compaction)
Learn paving influences under operators control (roller speed, number of roller passes, rolling
zone, and roller pattern)
Learn the best practices to compact a confined and unconfined longitudinal joint
Learn the best approach to compact a transverse joint
Page 77
AC Compaction
The single most important factor that affects the ultimate performance of asphalt concrete layers is
compaction. Compaction is the final stage of AC paving operations where a given volume of asphalt
concrete is pressed into a smaller volume by pressing asphalt coated aggregate particles closer together,
thereby reducing the air voids (space) in the mix and increasing the density (weight to volume ratio) of the
mixture. Proper compaction of a mix can have a critical impact on the outcome of a pavement in many
ways, including decreased stiffness and strength and reduced fatigue.
Even if an asphalt mixture has desirable mix design characteristics, it will still perform poorly under
traffic conditions if the mix is not compacted to the proper density level. The need for a pavement to be
compacted to the required density is better understood when the effect of air, water, and traffic on an
undercompacted layer(s) is realized. The voids in an undercompacted mix tend to be interconnected,
creating open channels and permitting the intrusion of air and water throughout the pavement. Air and
water carry oxygen, which in turn accelerates the oxidation of the asphalt binder in the mix causing it
to become brittle. Consequently, the pavement itself will fail as it can no longer withstand the repeated
deflections due to traffic loading. The internal presence of water at freezing temperatures can also cause
an early failure in the AC due to expansion and contraction of the freezing and thawing water.
Page 78
Factors Affecting Compaction

AC compaction is influenced by a number of important factors. Some are related to the environment or
mix and structural design, while others fall under contractor or department control during the construction
process.
Below is a list of some of the factors affecting compaction:
Compaction Variables
Compaction Variables Outside of Operator Control
Under Operator Control
Environmental Factors Mix Property Factors Construction Process

Factors
Temperature Aggregate
Rollers
• Air and base (ground) • Gradation
temperature • Size • Type
• Wind speed • Shape • Number
• Solar flux • Fractured faces • Speed and timing
• Volume • Number of passes
• Lift thickness
Asphalt Binder
Other
• Chemical properties
• Physical properties • AC production temperature
• Amount • Haul time and distance
• Laydown Site Conditions
• Layer Thickness
Page 79
Compaction Variables Outside of Operator Control

Environmental Factors
Environmental factors, namely temperature, are determined by when and where paving occurs. These
factors are outside the operator’s control of the process. Paving operations may have some float time,
which allows a limited choice of “when”, but paving location is determined by road location so there is
essentially no choice of “where”. Three variables have been found to influence the rate of cooling (and
therefore the possibility of obtaining a required level of density) of a layer of asphalt placed on top of
another existing layer of the same type of material. These variables are:
1. Air and Base Temperature

A portion of the heat in the asphalt layer is lost to the air. All other factors Temperature being equal, an
increase in the ambient air temperature decreases the rate of cooling of the mix. Heat in the mix is also lost
to the layer on which the new material is placed. There usually is more rapid cooling of the mix downward
into the base than upward into the air.
Base temperature is actually more important than air temperature in determining the time available for
compaction. It is often assumed that air and base temperature are the same. This is not necessarily
true, particularly in cool weather. A moist base layer significantly increases the cooling rate of the new
overlaying asphalt layer. Heat is lost from the mix to the moisture, turning water into steam and increasing
the rate of heat transfer.
2. Wind Speed
A thin layer of mix will cool more quickly in a strong wind than when there is little or no wind. Wind has a
greater effect at the surface of the mix than within the mix, and can cause the surface to cool so rapidly
that a crust will form.
3. Solar Flux
The amount of radiant energy available from the sun (solar flux) is a function of many variables, including
the position of the sun above the horizon, the distance above sea level ofthe paving project, the amount
of turbidity in the air, and the degree of cloud cover. A mix will cool more slowly on a sunny day compared
with a cloudy one. The amount of solar flux is more important in its effect on base temperature than its
effect on mix temperature.
AC temperature has a direct effect on the viscosity of the asphalt cement binder and thus compaction. As
AC temperature decreases, its asphalt cement binder becomes more viscous and resistant to deformation,
which results in a smaller reduction in air voids for a given compactive effort. As the mix cools, the asphalt
binder eventually becomes stiff enough to effectively prevent any further reduction in air voids regardless
of the applied compactive effort. The temperature, at which this occurs, is commonly referred to as
cessation temperature. In some literature, it is reported to be about 175°F for dense-graded AC. Below
Page 80
cessation temperature rollers can still be operated on the mat to improve smoothness and surface texture
but further compaction will generally not occur. Conversely, if the binder is too fluid and the aggregate
structure is weak (e.g., at high temperatures), roller loads will simply displace, or “shove” the mat rather
than compact it. In general, the combination of asphalt cement binder and aggregate needs to be viscous
enough to allow compaction but stiff enough to prevent excessive shoving.
Mat temperature then, is crucial to both the actual amount of air void reduction for a given compactive
effort, and the overall time available for compaction. If the initial temperature and cool-down rate are
known, the temperature of the mat at any time after laydown can be calculated. Based on this calculation
rolling equipment and patterns can be employed to:
• Take maximum advantage of available roller compactive effort. Rollers can be used where the
mat is most receptive to compaction and avoided where the mat is susceptible to excessive shoving.
• Ensure the mat is compacted to the desired air void content before cessation temperature is
reached. This can be done by calculating the time it takes the mat to cool from initial temperature
to cessation temperature. All compaction must be accomplished within this “time available for
compaction.”
Research work in the early 1970’s determined the time available for compaction of various asphalt
concrete mixes. The time available for compaction was defined as the time, in minutes; it took for a mix to
cool from laydown temperature to a minimum compaction temperature. Laydown temperature is the mix
temperature when the paver screed passes over the mix. Minimum compaction temperature for this study
was set at 175°F. Below this temperature, it was found that the internal friction and cohesion of the mix
increases to the point that little density gain is achieved with the application of additional compactive effort.
Page 81
Mix Property Factors

Mix property factors for aggregates and binder used can also affect compaction. They do so by affecting
(1) the ease with which aggregate will rearrange under roller loads and (2) the viscosity of the binder at
any given temperature. In general, aggregate effects on compaction can be broken down by aggregate
size. Five properties of the coarse aggregate particles used in an asphalt mixture that can affect the ability
to obtain the proper level of density are:
Gradation affects the way aggregate interlocks and thus the ease with which
1. Gradation
aggregate can be rearranged under roller loads.
Mixes with larger aggregate sizes tend to have larger sized voids, which are
more likely to be interconnected. Therefore, at the same air void level, mixes
2. Aggregate Size
having larger aggregate sizes have more potential for high density than mixes
of smaller aggregate sizes.
The shape of aggregate will affect the workability of the mix. Aggregates
3. Aggregate Shape
come in many shapes, including rounded, irregular, angular, flaky, etc.
An angular, rough, or broken surface of an aggregate particle created by

4. Fractured Faces
crushing, or by other means.
5. Volume
There is a simple statement that defines mix properties and the level of density: As the crushed content
of the coarse aggregate increases, as the maximum size of the aggregate increases, and as
the hardness of the aggregate (granite compared with limestone, for example) increases, the
compactive effort needed to obtain a specific level of density also increases.
Angular particles offer more resistance to manipulation than do rounded aggregate particles. How well the
mix will be compactive is also affected by the shape of the aggregate. A cubical or block-shaped aggregate
needs a greater degree of manipulation than a rounded particle shape before achieving a given density
level.
A continuously graded (dense-graded) aggregate, from coarse to fine, may be easier to compact than
a mixture with any other aggregate gradation. A harsh mix typically requires a significant increase in
compactive effort to obtain the desired level of density. An over sanded or finely graded mix, on the other
hand, tends to be extremely workable. It still might be difficult to achieve density on such a mix, however,
because of the inherent tender nature of such an over sanded mix. A mix designed with a high dust content
will generally be more difficult to compact than a mix designed with a lower dust content.
Page 82
The grade and amount of asphalt binder used in the mix affects the ability to densify the mix. A binder
that is higher in viscosity or lower in penetration will generally provide for a stiffer mix at a given mix
temperature and therefore require a greater compactive effort. The degree of hardening that occurs in
the binder during the manufacture of the mix affects the compactibility of that material. The binder content
of the mix influences its compactibility. In general, a mix with too little binder may be stiff and require an
increase in compactive effort, whereas a mix with too much binder may shove under the rollers.
Three properties of the asphalt binder that can affect the ability to obtain the proper level of density are:
Aggregate surface chemistry can determine how well an asphalt cement bind-
1. Chemical
er will adhere to an aggregate surface. Poor adherence, commonly referred to
Properties as stripping, can cause premature structural failure.
Aggregate physical properties are the most readily apparent aggregate
2. Physical properties and they also have the most direct effect on how an aggregate
Properties performs as either a pavement material constituent or by itself as a base or
subbase material.
The amount of mineral materials such as sand, gravel and crushed stone that
3. Amount
are used with a binding medium to form compound materials
Page 83
Compaction Variables Under Operator Control

Construction Process Factors
Construction process factors are the most controllable and adaptable of all the factors affecting
compaction. Although some factors like haul distance/time, AC production temperature, lift thickness and
type/number of rollers may be somewhat predetermined, other factors associated with roller timing, speed,
pattern and number of passes can be manipulated as necessary to produce an adequately compacted
mat.
The roller operator is in control of more variables when using a vibratory roller and thus should be well
educated in the proper selection and interaction of the variables. In addition to the roller speed, location on
the layer being compacted and number of passes made, both the nominal amplitude and the frequency of
the vibratory impact, can be varied. Roller speed and vibratory frequency are combined to determine the
impact spacing. Further, for double-drum vibratory rollers, the operator can vibrate either one or both rolls.
Care should be taken when operating a vibratory roller in areas where buildings are nearby. In addition,
the use of vibration on the roller when underground utilities and drainage structures are directly under the
pavement layer being compacted needs to be considered carefully.
The primary compaction variables for all types of rollers that can be controlled during the rolling process
include roller type, number of rollers present, speed and timing and number of passes
There are three basic pieces of equipment available for AC compaction: (1) the
1. Type
paver screed, (2) the steel wheel roller and (3) the pneumatic tire roller.
Having more rollers on the project allows for more roller passes to occur
2. Number
before the mix cools.
Rollers moving slower provides more compactive effort than the same roller
3. Speed and Timing
moving at a higher speed.
A roller pass is defined as the entire roller moving over one point in the mat at
4. Number of Passes
one time.
Page 84
Other Miscellaneous Variables
Along with the primary variables related to rollers, other variables impact compaction as well. Some can
be controlled by the contractor but others are a function of the project.
1. AC Production The temperature that the mix is at coming out of the plant factors into the mix
Temperature temperature at compaction
2. Haul Time & The greater the time that the mix is in the truck as it is being delivered to the
Distance project site, the greater the temperature loss will be.
Several factors at the laydown site directly affect the ability of the compaction
equipment to gain the required level of pavement density. The relationship
between lift thickness and nominal aggregate size in the mix is another variable
that affects the amount of density that can be obtained. The uniformity of the
3. Laydown Site
lift thickness is another factor to be considered. It is easier to obtain a required
Conditions
level of density in an asphalt layer that has a constant thickness compared with a
course that varies in depth. Asphalt leveling courses that, by their very nature and
purpose are non-uniform in thickness, are often difficult to densify to a given air
void content uniformly, especially when placed over a rutted or wavy road.
Layer thickness is probably the single most important variable in the rate of
cooling of asphalt mixtures, especially for thin lifts. It is very difficult to obtain the
desired density on thin lifts of mix in cool weather because of the rapid loss in
temperature in the mix.
4. Layer
Thickness For example, for a mix laydown temperature of 250°F and a base of 40°F, a 1
inch thick mat will cool from that 250°F temperature to the 175°F compaction
cutoff point in less than 4 minutes. For a 2-inch-thick layer, under the same mix
and base temperature conditions, it will take about 10 minutes for the material to
cool to 175°F.
Asphalt mixes are usually produced at temperatures between250°F and

Temperature 350°F. Depending on environmental conditions and the length of
5. Mix Laydown haul, the mixture can lose between 5°F and 25°F from the plant to the paver.
Temperature As the temperature of the AC being placed is increased, the time available for
compaction is greater. The effect of mat laydown is more significant at lesser mat
thickness and lower base temperatures.
Page 85
Roller Types
Compaction aims at producing a mat of specific density (target density) and smoothness. Although the
compaction process appears rather simple and straightforward, it is, in reality, a procedure requiring
skill and knowledge on the part of the roller operator and the technician. Both must have a thorough
understanding of the mechanics of compaction and the factors that affect the compaction effort.
Compaction is done by any of several types of compactors, or rollers - vehicles which, by their weight or
by exertion of dynamic force, compact the pavement mat by driving over it in a specific pattern. The two
basic pieces of equipment used for AC compaction in addition to the screed are:
1. Static Steel Wheel Roller
2. Vibratory/Static Steel Wheel Roller
3. Pneumatic or Rubber Tired Roller
4. Combination Rubber Tired and Steel Wheel Roller
Steel Wheel Rollers

Steel wheel rollers are self-propelled compaction
devices that use steel drums to compress the underlying
AC. They can have one, two or even three drums,
although tandem (2 drum) rollers are most often used.
The drums can be either static or vibratory. Steel wheel
rollers normally range in weight from 3 to 14 tons and
have compression drums or rolls that vary in diameter
from approx. 40 inches to more than 60 inches. The
gross weight of the roller can usually be altered by
adding ballast to the roller, but this adjustment cannot be made while the roller is operating, and is not
normally changed during the term of a paving project.
Effective weight or contact pressure, in terms of pounds per square inch of contact area, is the key variable
for this type of equipment and is dependent on the depth of the penetration of the rolls into the mix. The
greater the depth of penetration, the greater the contact area and then, the less the contact pressure.
In addition to their own weight, some steel wheel rollers can be ballasted with either sand or water to
increase their weight and thus, compactive effort. Although this ballasting is a fairly simple process it is
usually done before rolling operations start and rarely during rolling operations.
The table below illustrates how the roller contact pressure increases as the mix’s internal strength
increases during the compaction process. Look at a typical ton roller. As the roller makes its first pass, the
contact pressure may range from 48 to 36 psi as the drum sinks in 1/2 to 3/4 of an inch, depending on
the initial stiffness of the mix. As additional passes are made, the mix becomes stiffer as the aggregate is
Page 86
packed together and the air voids are reduced. The drum eventually “walks out” of the mix and the contact
pressure becomes extremely high. At 1/16 inch penetration, the contact pressure reaches 132 psi (A 266
percent increase over the original contact pressure at 3/4 inch penetration).
Roller Contact Pressure at Penetration Depths In Pounds per Square Inch
1/16 1/8 3/16 1/4 1/2 3/4

Static Roller Type
in. in. in. in. in. in.
Two Axle Tandem
270 p.l.i. – Drive
190 p.l.i. – Guide
8.5 ton (less ballast) 96 64 53 45 33 26
12 ton (less ballast) 132 88 74 63 46 36
Rollers with large-diameter rolls have lower drawbar pull (rolling resistance) because they do not tend to
penetrate as far into the mix as does a roller with smaller-diameter rolls. Once the size and weight of a
static steel wheel roller is selected, the variables under the control of the roller operator are the speed of
the roller, the position of the roller on the mat in relation to the paver, operation with the drive wheel toward
the paver, and the number of passes made with the roller.
Pneumatic (Rubber Tire or Traffic) Roller

The pneumatic tire roller is a self-propelled
compaction device that uses pneumatic tires
to compact the underlying AC. Pneumatic tire
rollers employ a set of smooth (no tread) tires on
each axle; typically, four on one axle and five on
the other. The tires on the front axle are aligned
with the gaps between tires on the rear axle to
give complete and uniform compaction coverage
over the width of the roller. Compactive effort
is controlled by varying tire pressure. Most
pneumatic rollers are operated in the intermediate roller position, behind a vibratory or static steel wheel
breakdown roller and in front of a static steel wheel finish roller. These rollers are sometimes used, however,
for initial rolling of the mix as well as occasionally for finish rolling.
Page 87
Vibratory Roller
Vibratory rollers come in a variety of
configurations. Single-drum vibratory
rollers are manufactured with both a rigid
frame and an articulated frame. Double-
drum vibratory rollers come in rigid-frame,
single-articulated-frame, and double-
articulated-frame models. These rollers can
be operated in any one of three modes:
static (with the vibrator off), with one drum
vibrating and one drum static, and with both
drums vibrating.
These rollers thus have two types of compactive force that is applied to the AC:
1. Static weight
2. Dynamic (impact) force.
The compactive effort derived from the static weight of the rollers is caused by the weight of the roller and
frame. The compactive effort derived from the dynamic (impact) force is produced by a rotating eccentric
weight located inside the drum (or drums). As the eccentric weight rotates about the shaft inside the drum,
a dynamic force is produced. Changing the eccentric moment arm or adjusting the eccentric mass has a
directly proportional effect on the dynamic force.
The elements of comparison for dynamic component of a vibratory roller are the magnitude of the
centrifugal force, its vibrating frequency, the nominal amplitude, and the ratio of the vibrating and non-
vibrating masses acting on the drum.
DEFINITION
The nominal amplitude is defined as equal to the weight of the drum divided by
the eccentric movement of the rotating weight and is a function of the weight of the
drum and the location of the eccentrics.
Normal values of nominal amplitude range from 0.01 to 0.04 inches. Some rollers can operate at only one
fixed amplitude. Other rollers have “high” and “low” amplitude positions. Still other rollers have 3 amplitude
settings.
As the layer thickness increases, it is often advantageous to increase the nominal amplitude applied to
the asphalt mix. Unless “high” amplitude is needed to achieve a particular density level, the vibratory roller
should be operated in “low” amplitude.
Page 88
The frequency of vibration is the number of complete cycles that the eccentrics rotate per minute. Simply
stated, the faster the rotation of the eccentrics, the greater the frequency of vibration. Some vibratory
rollers can operate at only one frequency or have a very limited selection of frequencies. Other vibratory
rollers can alter the frequency of the applied load between 1600 and 3000 vibrations/minute.
IMPORTANT
Frequencies below 2000 vibrations/minute are not normally acceptable to compact

asphalt mixtures.
The spacing of the impacts of the applied force is a function of the frequency of the vibration and the
travel speed of the roller. A smaller impact spacing (a greater number of impacts per foot) is thus usually
preferred.
A recent survey of major roller manufacturers recommended the following impact spacing. The impact
spacing should be in the range of 10-12 impacts per foot (1.2 - 1.0 in. between impacts) to ensure
the highest efficiency of the vibratory rollers and reduce the possibility of leaving ripples in the finished
pavement.
BEST PRACTICE
In general, the vibratory roller should be operated at as high a frequency as possible.

The use of the highest possible frequency of vibration increases the number of
impacts per foot at a given roller speed.
How do Rollers Compact?

Essentially, each piece of roller equipment compacts the AC by two principal means:
1. By applying its weight to the AC surface and compressing the material underneath the ground
contact area. Since this compression will be greater for longer periods of contact, lower equipment
speeds will produce more compression. Obviously, higher equipment weight will also increase
compression.
2. By creating a shear stress between the compressed material underneath the ground contact
area and the adjacent uncompressed material. When combined with equipment speed, this
produces a shear rate. Lowering equipment speed can decrease the shear rate, which increases the
shearing stress. Higher shearing stresses are more capable of rearranging aggregate into more dense
configurations.
Page 89
Compacting the Mat

Compaction of the pavement material must begin immediately after the material is spread, struck off,
shaped to the required width, depth, cross-section, and edge irregularities adjusted. The mix must be
compacted to the required degree of compaction for the type of mixture being placed. Compaction must
be carried out in such a manner as to obtain uniform density over the entire section. Perform compaction
rolling at the maximum temperature at which the mix will support the rollers without moving horizontally.
Complete the compaction (including both breakdown and intermediate rolling) prior to the mixture cooling
below a workable temperature. Perform finish rolling to remove roller marks resulting from the compaction
rolling operations.
Most asphalt mixtures compact quite readily if spread and rolled at temperatures that assure proper
asphalt viscosity. Rolling should start as soon as possible after the material has been spread by the paver,
but should be done with care to prevent unduly roughening the surface.
A mix that is relatively stable at high temperatures as it leaves the spreader is compacted by the vertical
movement of the aggregate particles under the roller. On any paving mixture, the roller wheel must settle
into the mix until the area of contact between the wheel and mix multiplied by the resistance of the mix is
equal to the weight on the roller wheel. If the AC is quite firm, the roller will not cause any horizontal mix
displacement.
Horizontal displacement results from apparent crawling of the mix ahead of the roller and the forming
of ridges on either side of the roller path. If there is no horizontal displacement, there will be virtually no
crawl or ridges along the edge of the roller path.
Horizontal displacement also results in a rough and uneven surface, thus defeating the intentions of
careful grade control and good screed operation of the asphalt paver. Horizontal movement of the mix
often occurs due to the breakdown roller being operated too fast.
Mix temperature is a principal factor affecting compaction. Compaction can only occur while the asphalt
binder is fluid enough to act as a lubricant. When it cools enough to act as an adhesive, further compaction
is extremely difficult to achieve. The best time to roll an asphalt mixture is when its resistance to compaction
is the least, while at the same time it is capable of supporting the roller without excessive shoving.
The best rolling temperature is influenced by the internal particle friction of the aggregates, the gradation
of the mix, and the viscosity of the asphalt. Therefore, it can change if any of these factors change. The
critical mix temperature in an asphalt concrete paving project is the temperature at the time of compaction.
This should determine the temperature at which the mixture is produced at the plant. It is best to be able
to compact the mix as quickly as possible after being spread, which means that it’s best for the mixing
temperature and the compacting temperature to be reasonably close to the same.
Page 90
There are several variables associated with rollers that can be adjusted from job to job. These variables
are:
• The sequence and number of rollers
• Roller speed
• The number of roller passes over a given area of the mat
• The location at which each roller works
• The pattern that each roller uses
Not all these variables are infinitely adjustable, but by adjusting a combination of them, a rolling plan can
be developed that will optimize mat compaction.
AC compaction is typically accomplished by a sequential train of compaction equipment. This allows each
piece of equipment to be used only in its most advantageous situation resulting in a higher quality mat
(both in density and in smoothness) than could be produced with just a single method of compaction. A
typical compaction train consists of the following (in order of use):
1. Screed The screed is the first device used to compact the mat and may be operated
in the vibratory mode.
2. Breakdown Roller The breakdown roller is the first roller behind the screed and therefore,
generally effects the most density gain of any roller in the sequence.
Breakdown rollers can be of any type but are most often vibratory steel
wheel and sometimes pneumatic tire.
3. Intermediate Roller The intermediate roller is used behind the breakdown roller if additional
compaction is needed. Pneumatic tire rollers are sometimes used
as intermediate rollers because they provide a different type of compaction
(kneading action) than a breakdown steel wheel vibratory roller. This can
help further compact the mat or at the very least, rearrange the aggregate
within the mat to make it receptive to further compaction.
4. Finish Roller The finish roller is last in the sequence and is used to provide a smooth mat
surface. Although the finish roller does apply compactive effort, by the time
it comes in contact with the mat, the mat may have cooled below cessation
temperature. Static steel wheel rollers are almost always used as finishing
rollers because they can produce the smoothest surface of any roller type.
5. Traffic After the rollers have compacted the mat to the desired density and
produced the desired smoothness, the new pavement is opened to traffic.
Traffic loading will provide further compaction in the wheel paths of a
finished mat. For instance, a mat compacted to eight percent air voids and
Page 91
then opened to heavy traffic (e.g., an interstate freeway) may further

compact to about three to five percent air voids in the wheel paths over
time.
Each position in the roller train (breakdown, intermediate and finish) may be performed by one roller or
several rollers in parallel. For instance, a large paving project may use two vibratory steel wheel rollers for
breakdown rolling, one pneumatic tire roller for intermediate rolling and two static steel wheel rollers for
finish rolling. The determination of the best rolling sequence and the number of rollers is generally made
on a case by case basis and depends upon the desired final air voids, available rollers and their operating
parameters, rolling patterns, mix properties, and environmental conditions.
Prior to beginning paving operations, the Materials Section or Inspector must inspect the Contractor’s
compaction equipment to see that it meets all requirements of the Specifications and is in good working
order. If the equipment meets Specifications and is in satisfactory operating condition a statement shall
be entered in the Technician’s Daily Diary. Before any of the rollers are used on a project they should be
checked to see that they are in good mechanical condition and to assure their compliance with any project
specifications that apply to the project.
During rolling, roller wheels are kept moist with only enough water to avoid picking up material.
IMPORTANT
Fuel oil should not be used to moisten roller wheels since it will damage the mix.
Phases of Rolling
Rolling freshly placed asphalt mix is generally done in the following order:
1. Transverse joints
2. Longitudinal joints (when adjoining a previously placed lane)
3. Initial or breakdown rolling
4. Second or intermediate rolling
5. Finish rolling
Page 92
BEST PRACTICE
Compacting Transverse Joints
Good practice is to make an initial breakdown pass and then check the joint with
a straightedge. In case the new mat is too high, the mix can still be reworked
and then recompacted. It is important to understand that the first pass will still
leave the mat slightly high, but experience will determine if it is too high. The
goal is to produce a transverse joint that rides as smooth as the rest of the mat.
Transverse Rolling
Ideally, it is best to compact transverse joints in the transverse direction.
However, it is not always possible. If rolling in the transverse direction, run-off
boards the thickness of the mat may be needed to keep the roller level. This
drum should overlap the new mat with most of the drum on the cold mat and
pinch the joint. Some contractors use multiple passes, each pass placing more
of the drum on the hot mat. Again, the method that produces a dense, level joint
most efficiently is the one to use.
Longitudinal Rolling
If a transverse joint is to be compacted in the longitudinal direction, it is import-
ant the first passes of the roller be made at very slow speeds. Higher speeds
may force the new mix away from the joint creating a void or weakness. Some
only roll in with one drum on the new mat and then reverse back. Angling the
drum while entering the new mix can be effective also.
There is no one right way to roll a transverse joint; the method that
works most efficiently and consistently is the one to use.
Page 93
There are three basic phases in the compaction process of asphalt pavements: 1) Breakdown,
2) Intermediate, and 3) Finish phases. Each of these are described in detail below.
Breakdown Breakdown rolling is best accomplished with steel-wheeled rollers. Either static-weight
or vibratory tandem rollers may be used. Vibratory rollers may be used in the vibratory
mode on all mixes, except that on the final wearing surface the thickness must be 1
inch or greater before use is permitted. The weight of the roller used for breakdown
rolling depends to a large degree upon the temperature, thickness, and stability of the
mix being placed. Generally, a roller weighing from 8 to 12 tons is used for this operation.
It is important to start the rolling operation on the low side of the spread, which is
usually the outside of the lane being paved, and progress toward the high side. The
reason for this is that asphalt mixtures, when hot, tend to migrate towards the low side
of the spread under the action of the roller. If rolling is started on the high side, this
migration is much more pronounced than if rolling progresses from the low side. When
adjoining lanes are placed, the same rolling procedure should be followed, by only
after compaction of fresh mix at the longitudinal joint with a 6 to 8 in of roller width.
Intermediate Second or intermediate rolling should closely follow breakdown rolling while the
asphalt mix is still plastic and at a temperature that is still well above the minimum
temperature at which compaction can be achieved, preferably 225° - 250°F.
Pneumatic-tired, steel-wheeled static and vibratory rollers may be used for intermediate
rolling. When using pneumatic rollers, keeping the tires hot is the most effective means
of preventing pickup. Applying a small amount of non-foaming detergent or water
soluble oil on the wetting mat of a pneumatic-tired roller at the beginning of rolling
operations helps prevent asphalt from sticking to the tires until they warm up. Pneumatic-
tired rollers have several advantages:
• They provide a more uniform degree of compaction than steel-wheeled rollers
• They improve the seal near the surface, thus decreasing the permeability of the
layer; and
• They orient the aggregate particles for greatest stability, as high pressure truck
tires do after using the asphalt surface for some time.
Tire contact pressures should be as high as possible without causing displacement

of mix that cannot be remedied in the final rolling. Pneumatic-tired rolling should be
continuous after breakdown rolling until all of the mix placed has been thoroughly
compacted. At least three passes should be made. Turning of pneumatic-tired rollers
on the paving mix should not be permitted unless it can be done without causing
undue displacement.
Vibratory tandem rollers-of proper static weight, vibration frequency and amplitude
are used to provide required densities with fewer roller passes than static-weight
tandem or pneumatic-tired rollers (or combinations of the two). As mentioned previously,
the vibratory roller may be used in the vibratory mode, at any time (subject to
Page 94
Specification requirements) on all pavement layers during the breakdown and i

ntermediate phases of rolling.
Finish Finish rolling is done primarily for the improvement of the surface. It should be
accomplished with steel-wheeled, static-weight tandems or non-vibrating vibratory
tandems while the material is still warm enough for removal of roller marks. Only
enough passes should be made to remove the roller marks and smooth the mat. Finish
roller operators should be cautioned about over rolling the mat since it can decrease
the mix density. Vibratory rollers operated in the vibratory mode are not permitted as
finish rollers.
NOTE: Some SUPERPAVE™ mixes may exhibit a “tender zone” or lateral movement during compaction
when the mix has cooled to the 250° - 210°F range. Because of this possibility, it is very important to
obtain as much density as possible during the breakdown rolling phase. The use of vibratory rollers,
more passes, additional rollers, compacting at a hotter temperature, adjusting the mix to get more “fines”
in it, making certain that silicone has been added to the asphalt binder for surface mixes, excessive
moisture in the aggregate, etc., are some things that may need to be considered. Some success has also
been achieved by using rubber tired rollers during this “tender zone”. If none of these solve the lateral
movement problem, it may be necessary to hold back the intermediate and finish rollers until the mix has
cooled below this “tender zone” temperature range and then continue rolling.
Rollers should move at a slow but uniform speed with the drive roller or wheels nearest the paver. The
speed should not exceed 3 mph for steel-wheeled breakdown rollers or 5 mph for pneumatic-tired rollers.
Rollers must be kept in good condition, capable of being reversed without backlash. The line of rolling
should not be suddenly changed or the direction of rolling suddenly reversed, thereby displacing the mix.
Any pronounced change in direction should be made on stable material.
If rolling causes material displacement, the affected areas are loosened at once with lutes or rakes and
restored to their original grade with loose material before being re-rolled. Heavy equipment, including
rollers, should not be permitted to stand on the finished surface before it has thoroughly cooled or set.
When paving in echelon, 2 or 3 inches of the edge that the second paver is following are left unrolled
when the joint between the lanes is rolled. Edges should not be exposed for more than 15 minutes
without being rolled. Particular attention must be given to the construction of transverse and longitudinal
joints in all courses.
All final wearing surfaces except open-graded asphalt friction course shall be compacted using a minimum
of 2 steel wheel tandem rollers. Steel wheel tandem vibratory rollers which have been specifically designed
for the compaction of asphalt pavements may be used. Vibratory rollers, operating in the vibratory mode,
may generally be used on all pavement layers 1 inch or greater in thickness during the breakdown and
intermediate phases of rolling. Operation in the vibratory mode will not be permitted during the finish
rolling phase on any mix type or pavement course or when the layer thickness is less than 1 inch.
Page 95
Vibratory rollers must have variable frequency and amplitude capability. The rollers must be equipped
with controls which automatically disengage the vibration mechanism before the roller stops when being
used in the vibratory mode. Vibratory rollers used on asphalt mixtures should normally be operated at
high frequencies and low amplitudes and specifically designed for asphalt compaction.
Rolling of open-graded asphalt friction course will consist of one coverage with a tandem steel wheel
roller weighing a maximum of 10 tons with additional rolling limited to one coverage where necessary
to remove roller marks. Excessive rolling should not be allowed inasmuch as this leads to possible
breakdown of the aggregate, thereby reducing the drainage capacity of the friction course layer. Vibratory
rollers may be used on friction course provided they are operated in the static mode.
On all other mixtures, the number and weight of rollers shall be sufficient to compact the mixture to the
required density while it is still hot and in a workable condition. Vibratory rollers may be used, as specified
in above paragraphs, provided satisfactory results are obtained, excessive displacement or crushing
of the aggregate does not occur, and no vibratory roller marks (indentations) remain in the finished
surface. The Engineer may prohibit or restrict the use of vibratory rollers where damage to the underlying
pavement structures, drainage structures, utilities, adjoining structures, or the pavement itself is likely to
occur or is evident.
The use of a pneumatic rubber tired roller is optional for compaction purposes on all mixes except for
surface layers where it is required. Some Project Special Provisions may require additional use of a rubber
tired roller, therefore, it is essential that the technician review all contract Project Special Provisions for
this possible requirement.
While it is the Contractor’s responsibility to determine roller requirements based on contract specifications,
the technician is an essential part of this determination. The exact number of passes that will be required
to obtain adequate density is initially unknown and this is due to some uncertainty about the mixtures rate
of cooling, among other things. These uncertainties are cleared up by careful observation, measuring,
and testing during the early stages of the paving operation.
Page 96
Roller Speed
Rollers are slow; for the fastest, operating speeds may reach about 7 mph. In order to provide complete
and uniform mat compaction, rollers should be operated at a slow, constant speed. Operating at high
speeds will reduce compactive effort while varying roller speed can cause non-uniform compaction.
The following table provides an indication of the range of roller speed for three different types of
rollers and three different operating positions:
Type of Roller Breakdown Intermediate Finish
Static Steel Wheel 2.0 – 3.5 mph 2.5 – 4.0 mph 3.0 – 5.0 mph
Pneumatic 2.0 – 3.5 mph 2.5 – 4.0 mph 4.0 – 7.0 mph
Vibratory Steel Wheel 2.0 – 3.0 mph 2.5 – 3.5 mph not used
Roller compactive effort comes in two forms:
1. Material compression under the ground contact area and
2. Shear stress between the compressed area and adjacent uncompressed areas
Compactive effort is significantly improved at slower roller speeds. Roller speed will also be governed by
the lateral displacement or tenderness of the asphalt mix. If the mixture moves excessively under the
rollers, the speed of the compaction equipment should be reduced. In addition, for vibratory compactors,
roller speed also affects the impact spacing. Roller speed is usually established by the speed of the paver.
Varying the speed of the compaction equipment merely causes variations in density. “Slow and steady” is
the key to proper compaction.
Operating at lower speeds allows the roller to remain in contact with a particular mat location longer
than it would at higher speeds. This results in more compression per roller pass and therefore increases
compactive effort. Speed also affects the magnitude of shear stress developed. Lower speeds result in the
shearing force between compressed and uncompressed areas being applied for a longer period of time for
a particular area (giving a lower shear rate), which results in a higher shear stress. The higher the shear
stress, the better able it is to rearrange aggregate into a denser configuration. Therefore, as roller speed
decreases, shear stress increases and compactive effort increases.
Page 97
Because speed affects compactive effort, varying roller speed will vary compactive effort resulting in
uneven compaction. Varying roller speed typically occurs when operators are not closely monitoring their
speed or when they speed up to roll an area more quickly so that they can catch up to the paver. If the mat
is being laid down at a faster rate than it can be rolled, the solution should not be to speed up the rollers
but rather should involve one of the following options (TRB, 2000):
1. Slow down the paver. This may involve adjusting production and material delivery rate as well.
2. Use more rollers. Adding rollers can increase the number of roller passes in a given time without
reducing the compactive effort per pass.
3. Use larger, wider rollers. Wider rollers allow greater coverage per pass.
Finally, rollers should not be stopped on a fresh mat because they can cause large indentations that are
difficult, if not impossible, to remove.
Roller speed directly affects compactive effort. The best compactive effort and most uniform densities are
achieved by slow, consistent roller speeds. If rollers cannot keep up with the pace of the paving operation,
they should not be operated at higher speeds because this reduces compactive effort. Rather, the paving
operation should be slowed or more/larger rollers should be used.
Number of Roller Passes
Generally, it takes more than one roller pass over a particular area to achieve satisfactory compaction. A
roller pass over a specific mat area is defined as one complete trip over the area in question by the entire
roller. This means that if the roller uses two steel drums, both drums must travel over the area in question
to make “one pass”. In general, earlier passes over hotter AC will increase density (decrease air voids)
more than later passes over cooler AC.
WORK PROCEDURE (BEST PRACTICES / AWARENESS)

In addition to keeping accurate detailed records and observing that the operation
is performed safely, the Technician must also be sure that compaction is done
properly and that the finished pavement meets all specifications. To achieve this,
the technician must understand the compaction procedure and the equipment
involved. The Technician must acquire samples of the compacted mat or take
readings with special instruments to determine mix density and smoothness.
Page 98
Compacting the Joint

The compaction process, which must be closely controlled to achieve a quality pavement, is the last
chance you will have to make or break a job. The best pavement design, the latest mix design, and the
most sophisticated AC facility do not stand a chance unless you compact the mix (while it is hot) to the
specified density. Compaction determines the ultimate performance of the mix and pavement. Although
simple in concept, it is often very difficult to achieve in the field because of a number of variables that
affect the ability of the rollers to densify the mix. You must understand what these variables are, and how
to control them in order to ensure that you can achieve density under a variety of conditions.
As previously mentioned, if the level of the new, uncompacted mix for a longitudinal or transverse joint is
even with or below the level of the compacted mix in the adjacent lane, the compaction equipment will not
be able to densify the mix along the joint properly. Whether the first pass of the roller is on the cold side of
the joint or on the hot side of the joint, part of the weight of the roller will be supported on the previously
compacted mat. This means that the compaction equipment will bridge over the mix in the longitudinal
joint, leaving it essentially uncompacted or only partially compacted. (Use of an intermediate pneumatic
tire roller instead of a steel wheel roller-static or vibratory can reduce this problem.)
The level of the mix at the longitudinal joint must be above the elevation of the compacted mix, by an
amount equal to approximately ¼ inch for each 1 inch of compacted pavement, if proper compaction of
the mix at the joint is to be accomplished.

Page 99
Compacting a Transverse Joint
Ideally, a transverse joint should be compacted transversely.

If the rolling is done transversely, wood boards must
be used to support the roller as it moves beyond the
longitudinal edge of the pavement. If the roller cannot
compact the joint in the transverse direction because of
site restrictions (adjacent guardrail or steep side slope, for
example) or traffic in the next lane, the transverse joint will
have to be rolled in the longitudinal direction.
The initial (breakdown) rolling should be accomplished,

however, as quickly as possible after the paver has moved
off of the joint. The roller should pass slowly and completely
over the joint before the machine is reversed. If the joint
has been constructed properly, the compaction process
is no different from the application of ordinary compactive
effort on any other part of the asphalt mixture.
Edge Definitions

Page 100
Rolling Unconfined or Unsupported Edge - First Paver Pass
This method is usually best for achieving maximum density at the unsupported edge.
• Roll from the outside edge toward the middle.
• The preferred approach shown below is done by overlapping the outside edge with the drum by
about 6 inches to obtain some confinement.

Edge of drum outside of unsupported edge - Preferred approach

Shown below is an alternate method of rolling the first pass on an unsupported edge. This method can
cause cracking near the edge and lateral mix movement at the unsupported edge.

Edge of drum inside unsupported edge
Page 101
Here is another alternate method for rolling an uncompacted edge. This method can cause lateral mix
movement at the unsupported edge.

Edge of drum on the unsupported edge
Mat Compaction - Subsequent Pass
The mat should be rolled from the unconfined/outside edge to the longitudinal joint (number of passes to
cover the mat depends on the roller widths).
Page 102
Confined Edge Compaction
Adjacent Lane Open to Traffic

The last pass across the mat pinches the longitudinal joint.
typically 1/16” - 1/4”
Page 103
Confined Edge Compaction - Alternate Method
Adjacent Lane Open to Traffic
The next to the last pass across the mat leaves 6” uncompacted at the joint. The last pass pinches the
longitudinal joint.
Confined Edge Compaction
This shows the first pass

Page 104
Chapter Five Knowledge Check

1. Where should longitudinal joints be located?
a. In between travel lanes at pavement markings
b. Approximately 6 inches or more offset from the joint in the layer below
c. Between the wheel paths
d. All of the above
2. When forming a longitudinal joint, it is best Not to overlap the cold joint with the end gate.
a. True
b. False
3. Environmental factors that can impact compaction include:

a. Air and ground temperature
b. Wind speed
c. Daylight savings time
d. All of the above
e. A and B only
4. Normal values of nominal amplitude for a vibratory roller range from:

a. 0.01 to 0.04 inches
b. 0.02 to 0.04 inches
c. 0.04 to 0.08 inches
5. When a mat is being compacted, what is the minimum overlap between roller passes?
a. 1 foot
b. 6 inches
c. ½ roller width
d. No overlap is needed
6. In general, the vibratory roller should be operated at as high a frequency as possible. The use of the
highest possible frequency of vibration increases the number of impacts per foot at a given roller
speed.
a. True
b. False
Page 105
7. There are several variables associated with rollers that can be adjusted from job to job. These
variables are:
a. The sequence and number of rollers
b. Roller speed
c. The number of roller passes over a given area of the mat
d. The pattern that each roller uses
e. All of the above
8. The _______________________ is the first device used to compact the mat and may be operated in
the vibratory mode:
a. Breakdown roller
b. Intermediate roller
c. Screed
d. Finish roller
9. There is only one right way to roll a transverse joint.

a. True
b. False
10. The three basic phases in the compaction process are:

a. Breakdown; midpoint; finish
b. Beginning; middle; end
c. Breakdown; intermediate; finish
Page 106
6 Mat Problems: Causes, Solutions and Effect
on Pavement Performance
Identify common MAT problems in Virginia
Identify if the MAT problem is material, equipment or construction practice related
Recognize the causes for corrective action
Page 107
Chapter 6: Mat Problems
Mat Problems
Mat problems can be defined as defects that occur in the asphalt mixture during or soon after the laydown
and compaction operations have been completed.
These problems fall into two primary categories:
1. Equipment-related problems and
2. Mixture-related problems
A familiarity with common causes of the more typical mat problems can help improve construction quality.
In this chapter, the following mat problems are reviewed and a description of each problem is presented,
including its causes, solutions and effects on long-term pavement performance.
Page 108
Common Mat Problems and Causes

IMPORTANT
Please note that only some of the that problems that occur at the time of the asphalt
mix production, laydown and compaction are addressed in this book. Several
other deficiencies can occur with time and traffic loading on an asphalt pavement
structure once construction has been completed.
Improper Joint Overlap
#
Sitting Long Period Between Loads
# #
Grade Reference Inadequate
# # # #
Grade Control Wand Bouncing on Reference
# # #
Grade Control Hunting (Sensitivity too High)
Grade Control Mounted Incorrectly
#
Vibrators Running Too Slow
# #
#
Screed Extensions Installed Incorrectly
#
Screed Starting Blocks Too Short
# #
Incorrect Nulling of Screed
Kicker Screws Worn Out or Mounted Incorrectly # #
Feeder Gates Set Incorrectly
# #
#
Running Hopper Empty Between Loads #
Moldboard on Strike-off Too Low
# #
Cold Screed
#
#
Screed Plates Not Tight

#
# # #
#
Screed Plates Worn Out or Warped
#
#
#
Screed Riding in Lift Cylinders

# #
# # #
# #
#
#
Excessive Play in Screed Mechanical Connection # #
#
Overcorrection of Thickness Control Screws

#
Too Little Lead Crown in Screed

#
Too Much Lead Crown in Screed

#
Finisher Speed too Fast

#
# ##
# # #
Feeder Screws Overloaded

# #
# #
#
Fluctuating Head of Material

CAUSE
Transvers Cracking (Checking)

Tearing of Mat – Center Streak
Wavy Surface – Long Waves
RELATED TO THE PAVER
Bleeding or Fat Spots in Mat

Wavy Surface- Short Waves
Mat Texture – Non-Uniform

Tearing of Mat – Full Width
Screed Not Responding to
Mat Shoving Under Roller

Tearing of Mat - Outside
Poor Longitudinal Joint

Poor Transverse Joint
Poor Mix Compaction

Poor Pre-compaction
Auger Shadows
Screed Marks
Roller Marks
PROBLEM
Page 109
CAUSE
OTHER PROBLEMS TO BE
Page 110
INVESTIGATGED
Moisture in Mix
Mix Segregation
Truck Holding Brakes

Cold Mix Temperature
Trucks Bumping Finisher

Improper Roller Operation
Improper Base Preparation

Parking of Roller on Hot Mat
Improper Asphalt Mix Design
Variation of Mix Temperature
Improper Aggregate Mix Design
Reversing or too Rapidly Turning Rollers
Improper Mat Thickness for Max. Agg. Size

PROBLEM
Wavy Surface- Short Waves (Ripples) X X X X X X

Wavy Surface – Long Waves X X X X X X X
Tearing of Mat – Full Width X X X X X X X
Tearing of Mat – Center Streak X
Tearing of Mat - Outside Streaks X
Mat Texture – Non-Uniform X X X X X X X
Screed Marks X X
Screed Not Responding to Correction X X X
Auger Shadows X X
Poor Pre-compaction X X X
Poor Longitudinal Joint X X
Poor Transverse Joint X X
Transverse Cracking (Checking) X X X X X X
Mat Shoving Under Roller X X X X X X X
Bleeding or Fat Spots in Mat X X X X
Roller Marks X X X X X
Poor Mix Compaction X X X X X X X X X
Surface Defects: Shorts Waves, Long Waves and Washboarding

An asphalt surface can have two types of waves:
short and long. Short waves, also sometimes
called rippers or auger shadows, are generally
one to three (1-3) feet apart, with one and one
half to two (1.5-2) feet being the most common
separation.
Long waves are considerably farther apart. The

distance between them may correspond to the
distance between truckloads of mix. Long waves
may also be associated with the reversal points of
the compaction equipment, particularly on thick- lift
construction or when the AC being placed is tender and moving longitudinally under the compaction
equipment.
An additional surface pavement defect is washboarding. Washboarding is basically toughness built into
the pavement surface during the compaction operation. Because it affects the degree of density obtained
during the compaction process, this type of defect can significantly reduce the long-term durability of
the pavement layer. In addition, washboarding contributes to a rough ride for the vehicles using the
pavement. The distance between the waves is generally very small, typically less than three or four (3
or 4) inches.
Causes
Causes can be roughly categorized by the type of surface wave:
Washboarding: Improper operation of a vibratory roller. Generally, excessive roller speeds, low vibratory
frequencies or excessive vibratory amplitudes for a given pavement thickness can lead to washboarding.
Short waves: Causes can be numerous but most relate to items that can cause frequent or periodic
changes in the screed angle of attack and corresponding changes in mat thickness. Likely causes include:
1. Fluctuation of material head in front of the paver screed: As the material head changes, the
screed angle and elevation will change. This fluctuating head can be caused by excessive auger
starts and stops or an improperly set material feed rate. Fluctuations may cause either short waves
or non-uniform texture.
2. Screed in poor mechanical condition: Screeds with excessive play in their controls may fluctuate
slightly around the equilibrium screed angle.
Page 111
3. Unresponsive screed: Extremely high paver speeds, in excess of 25 m (83 ft) per minute for thin lifts
or more than 15 m (50 ft) per minute for thick lifts (more than 62.5 mm (2.5 inches) thick), can cause
to screed to be relatively unresponsive.
4. Improper mounting, operation or sensitivity of the automatic screed control system: If the
screed control system is improperly mounted, operated, or it is too sensitive it could supply a fluctuating
grade reference causing the screed to fluctuate in response.
5. Truck driver applying the breaks too hard while being pushed by the paver: If break application
is too much the paver tracks or wheels may slip, which will cause a change in material head in front
of the screed.
6. Improper mix design: Mixtures whose stiffness is extremely sensitive to temperature of composition
may compact differentially. Tender mixes may shove and displace as they are compacted.
Long waves: Causes can be numerous and involve truck, paver and roller operation. Likely causes
include:
1. Fluctuation of material head in front of the paver screed: If the problem as described under “short
waves” occurs over a longer time interval it can result in long waves.
2. Screed in poor mechanical condition: Screeds with excessive play in their controls may fluctuate
slightly around the equilibrium screed angle.
3. Improper mounting or operation of the automatic screed control system: If the grade control
system is improperly mounted or operated it could supply a slowly changing grade reference causing
the screed angle to slowly change in response. If a stringline is used, sagging between support posts
can cause long waves.
4. Improper mix design: Mixtures whose stiffness is extremely sensitive to temperature of composition
may compact differentially. Tender mixes may shove and displace as they are compacted.
5. Emptying the paver hopper between truck loads: This can be caused by improperly adjusted
hopper flow gates or poor operating practices. As the material is emptied and there is none to replace
it in front of the screed, the material head in front of the screed will decrease.
6. Compaction equipment operation: Direction reversals, especially on lifts that are quite thick in
relation to nominal maximum aggregate size, can leave bow waves in place at the point of direction
reversal. Abrupt roller turns can also cause mix shoving.
7. Segregation
Page 112
8. Temperature differentials
Solutions
Surface waves can only be eliminated by preventing their formation.
• Short waves: Because there are so many different causes, solutions vary widely. Most importantly,
the amount of mix in front of the screed should be kept constant, which implies that slat conveyors
and augers should be run at a near constant rate and as close to 100% of the time as possible.
Material flow to should be controlled by the hopper flow gates as much as possible. Screed control
problems can be identified by turning off the grade control device and determining whether or not the
short waves continue. Most other solutions involve strict and careful compliance with established best
practices.
• Long waves: Same solutions as short waves.
• Washboarding: Decreasing vibratory amplitude, increasing frequency or slowing roller speed will
likely eliminate washboarding.
Effects on Long-Term Pavement Performance
Long-term pavement performance is affected by surface waves, both short and long, in two ways.
1. The waves reduce the smoothness of the pavement, which lowers the pavement condition rating
or the present serviceability index of the roadway. The structural performance of the pavement
will be changed, however, only if the waves are severe enough to increase the dynamic or impact
loading of the pavement under heavy truck traffic.
2. Short waves and the factors that cause them can affect pavement density levels. A tender mix is
generally more difficult to compact properly than is a stable mix; the result may be a decrease in
density and a corresponding increase in air void content.
Page 113
Segregation
Segregation is the separation of the coarse aggregate from the rest of the mix in an AC mix. Segregation
results from mishandling the mix at any of several points during the mix production, hauling, and placing
operations. When segregation occurs in a paving project, it is likely to lead to forms of long-term pavement
distress such as wavy surface and poor compaction. It can occur s the mix is delivered from the asphalt
plant to a surge silo, as the mix is deposited into the haul truck from the silo, and as the mix is discharged
from the truck into the paver hopper.
Segregation that is evident behind the paver screed generally

takes one of three forms:
1. It may consist of areas of coarse aggregate (rock pockets)

that occur randomly across the length and width of the layer.
2. It may occur at a transverse location across the width of the

lane (truckload- to-truckload segregation).
3. It may occur along one side of the paver width (longitudinal

or side- to-side segregation).
Causes
The cause of segregation behind the paver is directly related to the type of segregation involved. Rock
pockets are generally caused by improper handling of the aggregate in the stockpiles, cold- feed bins or
storage of the AC at the asphalt plant. They seldom occur when a batch plant is used to produce the mix
(without a silo), because the screens and hot bins in the plant recombine any segregated material before it
is fed into the pugmill. Further, the pugmill blends all the aggregates together and normally eliminates any
segregation that might have occurred previously. If a silo is used on a batch plant, however the mix may
segregate for all the same reasons that affect a mix produced in a drum-mix plant and passed through a
surge or storage silo.
Rock pockets and random segregation are occasionally found on the roadway when the mix was
manufactured in a drum-mix plant. If the loader operator places a bucketful of segregated aggregate
in a cold-feed bin, that material can pass through the drum, surge silo, haul truck and paver without
being completely mixed in with the other aggregate. This I because the drum-mix plant operates on a
continuous-flow instead of a batch basis. If the aggregate in the cold-feed bins is segregated, that material
will show up on the roadway in a random pattern both transversely and longitudinally.
Some mixes are more prone to segregation than others. Asphalt mixes that have large maximum size
coarse aggregate (1in. or greater), have low asphalt cement content, or are gap-graded will tend to
segregate more readily when handled than a dense-graded mix containing optimum asphalt content and
a smaller maximum-size coarse aggregate.
Page 114
Segregation that occurs on one side of the paver (side-to-side segregation) when a batch plant without a
silo is used to produce the mix is normally caused by improper loading of the haul truck from the pugmill.
If the mix is not loaded in the center of the width of the truck bed, the coarse aggregate particles in the mix
may roll to one side of the truck and accumulate along that side. When the mix is delivered to the paver
hopper, the segregated mix will be placed on the roadway along the same side, and the segregation will
appear as a longitudinal streak on one side of the paver only.
Segregation that occurs on one side of the paver when a batch plant with a silo or a drum-mix plant is
used to produce the mix is typically caused by improper loading of the mix into the surge silo. As the mix
is deposited into the silo from the conveying device (slat conveyors, belt conveyor, or bucket elevator),
the mix is thrown to one side of the silo, and the coarse aggregate particles are separated from the finer
materials. When the silo is emptied, the coarse aggregate is deposited on only one side of the truck. This
segregated material then passes through the paver and is seen on one side of the mix after laydown.
Further, as with a batch plant, if the truck is not loaded in the center of its width under the silo, rolling of
the coarse aggregate particles may occur, and longitudinal segregation will then appear on one side of
the new mat.
Truckload-to-truckload segregation has many potential causes. The most common is improper loading of
the haul truck from the silo. If mix is placed in the truck bed in one drop from the silo, the coarse aggregate
particles in the mix have a tendency to run to both the front of the bed and the back tailgate. This rolling
of the coarse aggregate is exacerbated if the plant operator continuously opens and closes the silo gates
near the end of the truck-loading procedure to ensure that the full weight of mix is placed on the truck.
Some believe that truckload-to-truckload segregation can also be caused by improper discharge of the
mix into the silo. Mix that is dribbled into the silo from the conveying device is said to be susceptible to
segregation inside the silo. Even if this occurs, the mix that is segregated in the silo will appear only as
random rock pockets in the layer behind the paver, instead of in a systematic manner between truckloads
of mix delivered to the paver. Thus, it is doubtful that any segregation of the mix that occurs during the
continuous process of loading the silo will appear on the roadway in a discontinuous pattern – only at the
beginning or the end, or both, of a truckload of mix.
Temperature segregation of the mix has also been shown to be a problem. The mix cools more quickly
near the edge, bottom, and top of the truck during haul. This cooler material is not always remixed with
the hotter HMA, leading to temperature segregation during the laydown operation. The result can be more
variability in density during construction and a nonuniform surface. This problem can be monitored by
infrared technology.
Page 115
Solutions
The solution to each type of segregation is related to its cause. For random rock pockets that appear
intermittently in the mat, the method of stockpiling the coarse aggregate at the asphalt plant and the
charging of that material into the cold-feed bins by the front- end loader should be checked to ensure that
proper aggregate handling techniques are used. Further, all points in the mix-production system at which
coarse aggregate particles might accumulate should be inspected to determine whether the flow of the
coarse and fine aggregate pieces is uneven. A batcher should be used at the top of the silo to direct the
mix into the center of that piece of equipment.
For longitudinal (side-to-side) segregation, the loading of the haul truck from the batch plant pugmill or
from the silo at either the batch or drum-ix plant should be monitored to ensure that the mix is being
delivered into the center of the width of the vehicle. When a drum-mix plant is used to manufacture the
mix and the segregation plant is used to manufacture the mix and the segregation always appears on one
side of the paver, several trucks should be loaded at the silo while facing in the opposite direction from
their normal loading procedure. When the mix is passed through the paver, the longitudinal segregation
should change sides – go from one side of the paver lane to the other. If the transverse position of the
longitudinal segregation does change (and it should), the solution to the side-to-side segregation problem
must take place at the top of the silo. The mix deposited into the silo from the conveying device must be
directed into the center of the silo instead of to one side, so that the coarse aggregate particles in the mix
are not thrown to only one side of the silo. IF the transverse position of the longitudinal segregation does
not change, the segregation is probably caused by a paver problem.
Most truckload-to-truckload segregation can be reduced significantly by using multiple drops of mix to
load the haul trucks. If a tandem-axle truck is being loaded, at least three different drops of mix should
be made – into the front of the truck near the front bulk – head, into the back of the truck near the tailgate
and into the center of the truck bed between the first and second drops. If a larger truck is used, additional
drops of mix should be made – the first into the front of the truck bed and the second near the tailgate.
One of the main solutions for truckload-to- truckload segregation is to minimize the distance the coarse
aggregate particles can roll. This is accomplished by making multiple drops of mix into the truck.
The plant operator should be prohibited from topping off the load of mix at the end of the loading process.
Each time the silo gates are opened and a little bit of mix is dribbled into the truck, the coarse aggregate
particles will tend to separate from the finer material. This problem can be eliminated only be preventing
it from occurring.
If segregation does take place during the loading of the truck and there is an accumulation of coarse
aggregate particles at the tailgate of the truck, at the front of the bed, or both, the amount of segregation
that appears on the roadway can usually be reduced by proper unloading of the haul truck at the paver.
Page 116
First, the truck bed should be raised a short distance, before the tailgate of the truck is opened, so that
the mix can shift in the bed and slide against the tailgate. This procedure surrounds any coarse particles
that have rolled to the tailgate are with non- segregated mix. Instead of only the coarse aggregate being
deposited first into the paver hopper, a mass of mix is discharged when the truck tailgate is opened,
flooding the hopper with mix and typically incorporating the segregated coarse aggregate into that mass
of AC mix.
The operation of the paver can also increase or reduce the amount of segregation that occurs behind
the screed. If the paver hopper is emptied of mix, if the slat conveyors are visible, and if the wings of the
hopper are dumped after each truckload of mix, any coarse aggregate particles that have collected at the
tailgate of the next truckload of mix will be deposited into the bottom of the hopper and then carried directly
back to the empty auger chamber in front of the screed. This segregated material will appear behind the
screed as soon as the paver moves forward. This transverse segregation, therefore, does not really occur
at the end of the truckload, but rather at the beginning of the next truckload of mix.
Segregation can be reduced by keeping the hopper full of mix between truckloads. The mass of mix
that floods the hopper from the haul truck will be blended with the mix already in the paver hopper. Any
segregated material will be further incorporated in the mix that is pulled back to the augers by the slat
conveyors and passed under the paver screed. The amount of truckload-to- truckload segregation can be
decreased significantly, but not always eliminated completely, by good paver operating techniques. The
problem should really be solved during the truck-loading procedure.
The use of MTV’s has also shown some benefit in reducing segregation. The MTV remixes the HMA, and
this reduces aggregate segregation, as well as differential temperatures within the mix (also known as
temperature segregation).

Segregation can affect pavement durability directly by increasing the air void content of the mix in the
segregated areas and increasing the potential for moisture damage. Further, the segregated locations are
very susceptible to raveling and, if bad enough, to total disintegration under traffic. Segregation, whether
in the form of rock pockets, longitudinal (side-to-side) segregation, or transverse (truckload-to-truckload)
segregation, is extremely detrimental to the long-term performance of the pavement.
Page 117
Tearing (Streaks)
There are three general types of mat tearing or pulling of the asphalt mix under the screed of the paver.
The three types are defined by the location of the tear marks in the mat:
1. In the center of the lane
2. On the outside edges, and
3. Across the full lane width
Causes
A gearbox streak, often mistaken as being a type of segregation (due to the fact that the rougher texture
or makes the surface appear more open or segregated), can sometimes be seen in the surface of the
mat, directly behind the center of the main screed. This streak is typically six to eight (6-8) inches wide
and is normally caused by a lack of asphalt mix being pushed under the auger gearbox located in front of
the center of the screed (essentially, less mix is being passed under the screed at the auger gearbox than
under the screed on either side of the gearbox). The lack of mix may be the result of improper flow gate
settings – not enough mix being fed back to the screed but is more likely to be caused by missing, worn,
or improperly set reverse augers or auger paddles (located adjacent to the gearbox) that are used to force
mix underneath the gearbox. Gearbox streaks are more prevalent with harsher mixes – those containing
larger-size aggregate, more crushed aggregate, or lesser amounts of asphalt.
A centerline streak can also be caused by improper setting of the crown on the main paver screed.
The appearance of streaks behind the screed is caused primarily by an improper relationship between
the crows at the leading (front) and trailing (back) edges of the screed. A tearing or open texture about a
meter (several feet) wide in the center of the mat may be caused by a lack of lead crown in the screed.
Conversely, a tearing or open texture along both outside edges of the asphalt mixture is normally caused
by an excess of lead crown in the screed. For most mixes the lead crown of the screed should be set
slightly higher, approximately 1/8 inch, than the tail crown. A proper relationship between lead and tail
crowns will result in a uniform texture of the mat across its full width.
Page 118
Edge streaks can be caused by improper flow gate settings or incorrect installation of the screed
extensions.
Partial-width tearing can result from a cold screed plate if the screed has not been uniformly preheated
before paving begins.
Full-width tearing of the mat can be attributed to several factors. One factor is warped or worn screed
plates. Another is the forward speed of the power being too high for a particular mix. The use of a mixture
with aggregate that is large compared with the mat thickness being laid can also be responsible for full-
width tearing of the mat. A good rule of thumb for the relationship between the maximum aggregate size in
the mix and the minimum compacted course thickness is that the depth of the compacted layer should be
at least twice the largest coarse aggregate particle size or three times the nominal maximum aggregate
size. Thus, a mix containing a maximum aggregate size of three quarters (3/4) of an inch should be placed
at least one and a half (1 1/2) inch thick. Lastly, cold mix temperatures, particularly when combined with a
cold paver screed, can significantly affect the amount of tearing that occurs.
Solutions
A gearbox streak can usually be eliminated only by changing the amount of mix being forced under the
screed at the auger gearbox. This change is made by installing reverse paddles or reverse augers on each
side of the gearbox in order to push more mix under the gearbox. If the paver is already equipped with
such devices, they should be checked to see whether they are worn and need to be replaced.
Constant center or outside edge mat tearing can usually be eliminated by adjusting the relationship
between the lead and tail crowns on the paver screed. If this change does not solve the problem, the
setting of the paver flow gates should be modified. Full- width tearing can be eliminated by increasing the
mix temperature, preheating the screed properly before paving starts, replacing warped or worn screed
pates, or increasing the lift thickness.

Tearing of the mat affects long-term pavement performance by causing changes in density in those areas
where the tearing has occurred. Torn areas may appear segregated and are usually deficient in mix
quantity. Pavement performance will be reduced in relation to the degree to which the tearing reduces the
density and increases the air void content of the mat. In addition, the torn areas will be more susceptible
to raveling and to the effects of moisture (stripping).
Page 119
Checking
Checking can be defined as short transverse cracks, usually one to three (1-3) inches in length and one
to three (1-3) inches apart, which occur in the surface of the AC mat at some time during the compaction
process. The checks are not visible immediately behind the paver screed. Rarely does checking occur
during the first or second pass of the compaction equipment over the mat. If checking is going to occur, it
will normally take place after the mix has cooled to a temperature of less than 240˚F and additional passes
of vibratory or static steel wheel rollers (or both) are made over the mat. Checking does not usually occur
when the mix is compacted with a pneumatic tire roller.
Most AC mixtures do not check at all during compaction, whereas others exhibit tender characteristics and
check readily. As checking becomes severe, the cracks become longer and are spaced closer together.
The cracks do not extend completely through the depth of the course, but are only three eighths (3/8) to
one half (1/2) inch deep.
Causes
A mix that checks during compaction is a tender mix. The mix shoves or moves in front of the drums on
either vibratory or static steel wheel rollers. Checks or cracks are formed when a bow wave occurs in front
of the roller drums as the mix moves longitudinally before the roller reaches that location.
Checking may be caused by two primary factors: (a) excessive deflection of the pavement structure under
the compaction equipment and (b) one or more deficiencies in the asphalt mix design. A mix that checks is
not internally table enough – does not have enough internal strength at elevated temperatures – to support
the weight of the compaction equipment during the rolling process.
When a yielding foundation is the cause of the checking problem, the underlying pavement on which the
new AC layer is being placed is weak and yields under the movement of the compaction equipment. The
weight of the rollers causes the layers in the pavement structure to move, shove, and bend excessively,
placing the new mix in tension at its surface. The check marks are then formed when the surface of the
new AC is pulled apart as the pavement structure deflects during the rolling operation. The checks should
appear in the new mix surface only at locations where there is movement of the pavement structure under
the compaction equipment. If the paver passes over a soft spot in the underlying structure, for example,
the checking should occur only where the soft spot exists.
A more common cause of checking is one or more deficiencies in the AC mixture: (a) an excess of fluids
in the mix – too much asphalt cement or too much moisture in the mix, or both; (b) hump in the sand
gradation curve – too much midsize sand material and too little fine sand material and (c) a lack of room
in the aggregate gradation for the asphalt cement (low VMA).
An excess of fluids in the AC mix makes the mix tender and allows it to be displaced easily under the
Page 120
applied compaction effort of the rollers. The mix will be tender if the binder content is too high for the
gradation and characteristics of the aggregate used, particularly if the mix has a low VMA content. If the
mix contains too much moisture because the aggregate was not completely dried when passing through
the batch plant drier or drum mixer (parallel flow or counter flow), the excess moisture will act as asphalt
cement at elevated temperatures and over lubricate the mix. The moisture remaining in the aggregate
pores will prevent the binder material from entering those pores in the aggregate in effect leaving more
binder material between the aggregate particles instead of partly inside the aggregate.
Solutions
If checking is caused by the presence of a yielding foundation underneath the new AC layer, the solution
is to repair and properly prepare the existing pavement structure before the new AC layer is placed. Soft
spots should be removed and replaced. All areas of excessive deflection should be removed and replaced
or stabilized. Uniform support is need in the underlying pavement structure if the new pavement layers are
to perform adequately.
If checking is caused by a deficiency in the mix design – an excess of fluids in the mix or a problem with
the gradation of the fine aggregate or the VMA content of the mix – the long-term solution is to change the
mix properties. Those changes must be made at the asphalt plant and cannot be made at the paving site.
If the mix contains an excess of fluids – either asphalt cement or moisture – the binder content should be
reduced or the aggregate properly dried to remove all of the moisture. In some cases, the production rate
of the plant will have to be reduced for the moisture to be completely removed from the aggregate. In other
cases, plant operating conditions may need to be changed (e.g., flights, drum angle).
IF checking is caused by the gradation of the fine aggregate incorporated into the mix, the gradation
should be changed. It may be necessary to increase or decrease the amount of fine aggregate used, add
a small amount of fine aggregate with a different gradation, increase the angularity of the fine aggregate,
or use a completely different material from a different source. If checking is caused by a lack of VMA in the
AC mix, changes need to be made to increase the VMA.
Checking is often thought to result from the mix being too hot. This is only partially correct; the mix is
too hot at some temperatures to support the weight of the compaction equipment because the mix lacks
internal strength and stability. If the mix were properly designed, it would not be too hot to be compacted
at any temperature below about 300˚F. most checking occurs when the mix temperature is decreasing
from about 240˚F down to about 190˚F; rarely does checking occur when the mix temperature is above
approximately 240˚F or below approximately 190˚F.
In the short term, changes in both the rolling zone and the type of rollers used to densify the mix can
be made to reduce the amount of checking that occurs. If the mix is tender because of excess fluids, a
Page 121
problem with the fine aggregate gradation, or lack of BMA, it may be possible to densify the mix properly
at an elevated temperature without causing the checking.
A mix that checks is tender, but this mix can usually be compacted satisfactorily at high temperatures
– above 250˚F. The required level of density can generally be obtained if enough roller passes can be
applied to the mix before it cools to the point at which the checking begins. This can be done by suing
two breakdown rollers instead of one – using two rollers operating in echelon (side by side) instead of
using a breakdown roller followed by an intermediate roller. The two breakdown rollers each apply their
compactive effort to one side of the newly placed lane. Many passes are made over each point in the
pavement surface before the mix begins to check. Once checking starts the rolling process is suspended.
If compaction operations are attempted when the mix is moving, shoving, and checking under the action
of vibratory or static steel wheel rollers, the mix will decompact rather than compact. Rolling should not
be carried out with steel wheel rollers when the mix is tender and checking. Most tender mixes will remain
tender until the surface of the mix cools to a temperature of approximately 190˚F. At this temperature, the
mix has cooled sufficiently so that the viscosity of the asphalt binder has increased to the point where the
mix can again support the weight of the compaction equipment. Static steel wheel rollers can then be used
to achieve the final density in the mix and remove any roller marks in the pavement surface.
When a tender mix is in the middle temperature range, between about 240˚F and 190˚F, rolling should not
be attempted as discussed above, with either vibratory or static steel wheel rollers. A pneumatic tire roller,
however, can be used in this temperature zone since the rubber tires on this roller will typically not shove
the mix and a bow wave will not form in front of the tires. The tender mix will densify, instead of check,
under the compactive effort of the pneumatic tire roller. Finish rolling using a static steel wheel roller can
be completed once the mix has cooled to a temperature below about 190˚F.
In most cases, when checking occurs in the mix, the roller operators tend to back off the mix and allow it
to cool. This is the wrong approach to the problem. Delaying the compaction permits the mix to cool and
stiffen but most often does not then allow enough time for the mix to achieve the required level of density.
With a tender mix, it may not be possible to accomplish both objectives (no checking an adequate density)
at the same time if the mix is allowed to cool before rolling operations if the mix is allowed to cool before
rolling operations are stated. It is much better to compact the mix as much as possible before checking
starts, stay off the mix in the middle temperature zone when checking is most likely to occur, and then
finish roll the mix once it has cooled enough to support the weight of the final roller.
If the mix delivered to the paver is too hot – above 325˚F – it should be allowed to cool after laydown before
the compaction process is started. Improper rolling techniques should be corrected. The surface of the
underlying pavement should be clean and properly tack coated before placement of the new mix begins.
Page 122
None of the solutions to the checking problem will work in all cases. Each mix will have its own compaction
characteristics. For some extremely tender mixes, checking may occur at a wider range of temperatures,
from as high as 270˚F down to as low as 170˚F. As noted, mixes that lack internal stability will generally
check under steel wheel rollers (operated in either the vibratory or static modes) and thus these mixes
should be redesigned.

Although checks extend only a short distance down from the surface, they are highly detrimental to long-
term performance because the tender mix characteristics affect the level of density obtained. If the rollers
are kept back from the paver in an attempt to decrease the amount of checking that occurs, the level of
density obtained by the compaction equipment will normally be reduced significantly. Thus, the air void
content of the AC mat will increase. A mix that contains checks will therefore lack density and have a
greatly reduced pavement life under traffic.
Page 123
Non-Uniform Texture
Nonuniform mat texture can be described as differences in the appearance of the mix, both transversely
and longitudinally, as it is placed and compacted. Normally, minor differences in surface texture will be
apparent because of differences in the alignment of the larger coarse aggregate particles as the mix
passes out from beneath the paver screed. In addition, a mix with a higher fine aggregate (sand) content
will have a more uniform surface texture than a mix containing a larger percentage of coarse aggregate.
Causes
Many factors related to the operation of the asphalt paver affect the uniformity of the surface texture of the
mix. A variable amount of mix against the screed, caused by overloading the augers or running the hopper
empty between truckloads, can cause variations in the amount of mix tucked under the screed and
thus produce a nonuniform texture. Improper screed maintenance, including worn or loose screed plates
or screed extensions incorrectly installed, as well as low screed vibratory frequency, may alter the mat
texture and cause nonuniformity. In addition, a low mix temperature, caused either by plant problems or by
the paver sitting too long between truckloads of mix, can be a factor in uneven mat texture, especially if the
paver screed is also cold. The tearing that results when the compacted layer thickness is less than twice
the dimension of the largest aggregate particles (as discussed above) is still another contributing factor.
A soft or yielding base under the course being constructed may cause the new layer to have a variable
surface texture. Moreover, segregation of the mix caused by poor mix design or improper handling of
the mix during mixing, loading, hauling, unloading, or placing operations can contribute to a nonuniform
surface texture. The variability of the texture will be affected as well by any factors that cause nonuniformity
in the mix, such as deviations in aggregate gradation, asphalt content, or mix temperature.
Solutions
The solutions for nonuniform surface texture are as varied as the causes. Paver operation, particularly
with regard to the need for a constant head of material in front of the screed, should be monitored closely.
The paver and screed should both be well maintained and in good operating condition. The compacted
thickness of the mat being placed should be designed so that it is at least twice the size of the largest
coarse aggregate particles incorporated into the mix. Finally, a mix that is tender, variable in aggregate
gradation or asphalt content or easily segregated should be modified to increase its stiffness and improve
its properties before it is produced at the plant and delivered to the paver for laydown.

Non-uniform surface texture is usually associated with nonuniform density. The same compactive effort
will generally achieve lower density in areas in which the coarse aggregate has been dragged by the paver
screed or segregation of the mix has occurred, as compared with areas having uniform surface texture.
As density decreases and air void content increases, the durability and serviceability of the asphalt mat
decrease markedly.
Page 124
Screed Marks
Screed marks are transverse indentations in the surface of the asphalt mat. They occur when the paver
stops between truckloads of mix. Depending on the mixture being placed, some screed marks are barely
noticeable, whereas others are very distinct and deep. Screed marks can also occur in the longitudinal
direction when rigid or hydraulic extensions are used and the elevation of the extension is not the same
as that of the main screed.
Causes
There are several causes of transverse screed marks.
One is excessive play in the mechanical connections
on the screed. Such marks also result when the screed
is set up incorrectly and rides heavily on its rear end. If
the asphalt mix is tender and if the paver is equipped
with a very heavy screed, such as hydraulic extensions
with additional rigid extensions attached, the screed
will tend to settle into the mix and leave marks. If any
of these causes are involved, the screed marks will be
visible each time the paver stops.
Another cause is the haul truck bumping into the paver when preparing to discharge the mix or the truck
driver holding the brakes on the truck when the paver starts to push the truck. In these cases, the screed
marks will appear only when the truck – paver interchange is improper.
Longitudinal screed marks are caused by improper setting of the screed extensions relative to the main
screed. When extensions are used, their vertical position and angle of attack must be the same as those
of the main screed. If rigid extensions are set at the wrong elevation, a longitudinal mark will occur at the
point where the different screed sections are joined. If hydraulic extensions are used, two longitudinal
marks may occur – one at the end of the main screed and one at the inside edge of the extension on each
side of the machine.
Solutions
If the transverse screed marks are a result of the mechanical condition or improper setup of the paver
screed, the screed should be repaired. If the marks are caused by the truck bumping into the paver, the
laydown operation should be altered so that the paver picks up the haul truck instead of the truck backing
into the paver. In addition, once the paver has established contact with the truck, the truck driver should
apply only enough pressure to the brakes to keep the truck in contact with the paver.
In some cases, particularly if the mix is very tender, screed marks can be eliminated by not stopping
the paver between truckloads of mix. This can be accomplished by using a window elevator or material
Page 125
transfer vehicle to deliver mix to the paver hopper. If dump trucks are used to haul the mix, however, it is
generally better to stop the paver between truckloads of material (stopping and restarting the paver as
quickly as possible) instead of allowing the paver operator to run the hopper dry, reduce the head of mix in
front of the paver screed, and increase the opportunity for truckload-to-truckload segregation.
To achieve uniform surface texture, the elevation and angle of attack of the screed extensions must
be matched to those of the main screed. Longitudinal screed marks caused by improperly setting the
elevation of the extensions can be eliminated by correcting the position of each extension relative to that
of the main screed. Adjustments to both the vertical position and the angle of attack of the extensions may
be needed. These adjustments should be made whenever hydraulic or rigid extensions are used.

Transverse screed marks generally are not detrimental to the durability of the mat. They may, however,
affect the ride by creating a bump whenever the marks cannot be completely rolled out by the compaction
equipment. In many cases, the screed marks have less of an effect on the performance of the mix than
does the slowdown and startup of the paver when the operator attempts to keep it moving as the empty
truck pulls away and the loaded truck backs into the hopper.
Longitudinal screed marks indicate that the level of the mix under the screed extensions is different
from that under the main screed. If the screed marks are severe, differential compaction may occur across
the mark or “joint”, with the compaction equipment initially riding on the higher mat. The marks can leave
a ridge in the mix if they cannot be completely rolled out.
Page 126
Screed Responsiveness
As the thickness control cranks on the screed are changed, the screed’s angle of attack increases or
decreases. As the paver moves forward to place the mix, the screed moves up or down to the new
equilibrium point for the newly set mat thickness. When the screed fails to respond to changes in the
setting of the thickness control cranks, the operator is unable to alter the depth of the layer being placed.
The paver also loses its inherent ability, through the principle of the floating screed, to provide the self-
leveling action needed to place a smooth asphalt mat.
Causes
An extremely high paver speed -more than eighty-three (83) feet per minute for thick lifts or more than
fifty (50) feet per minute for layers more than two and a half (2.5) inches thick- may cause a lack of
responsiveness of the screed. The mechanical condition of the screed affects its ability to react. The
screed riding on its lift cylinders or loose connections on the thickness control cranks will cause the screed
to be unresponsive. If automatic grade controls are used, an incorrect sensor location will render the
screed unable to react to input signals from the grade sensors.
If the maximum aggregate size used in the mix is too great compared with the depth of mix being placed,
the screed will ride on or drag the largest aggregate pieces. As a result, the screed will be unable to
change its angle and will thus be unresponsive to changes in the thickness control settings. Variations in
mix temperature will also cause the screed to be unresponsive to changes in the angle of attack because
the mix stiffness variations themselves will cause the screed to continually seek new equilibrium levels for
the forces acting on it.
Solutions
The paver and screed must be in good operating condition. The sensor for automatic grade controls must
not be located either at the tow points or behind the pivot points of the screed; rather, it should be located
in the area between one-third (1/3) and two-thirds (2/3) of the length of the leveling arms. If the mix texture
is uniform (indicating a proper relationship between course thickness and maximum aggregate size), the
screed will be able to respond to changes in the thickness control settings.

An unresponsive screed causes a rough asphalt mat. The screed is unable to react to manual changes
in the thickness settings. It also loses its ability to self-level on an existing pavement surface because it
cannot reduce the thickness of mix placed over the high points in that surface and increase the thickness
placed in the low areas. Thus, the rideability of the course being placed can be affected significantly if the
paver screen is unresponsive.
Page 127
Poor Precompaction
A modern asphalt paver is normally equipped with a vibratory screed. This type of screed allows the mix
to be partially compacted as it passes beneath the screed. Depending on such variables as forward paver
speed, layer thickness, mix temperature, and ambient environmental conditions, the density of the asphalt
mixture measured behind the screed before compaction is usually in the range of seventy to eighty (70-80)
percent of the theoretical maximum density (a voidless mix).
A few pavers are equipped with combination screeds, which have both tamper bars and vibrators. At slow
paver speeds, the combination screed typically achieves greater compaction of the mix than is obtained
with the vibratory screed alone. At paver speeds greater than twenty five (25) feet per minute, however, the
increased compactive effort achieved with the tamper bar is typically lost, and the degree of compaction
obtained is similar to that achieved with a simple vibratory screed.
Causes
The amount of precompaction achieved with the screed decreases as the paver speed increases.
Precompaction generally increases slightly as the frequency of the screed vibration increases. It decreases
significantly, however, if the screed is riding on the screed lift cylinders, thereby limiting the available
compactive effort. The level of precompaction obtained is also limited if the mat is too thin for the maximum
aggregate size used in the mix (less than twice the largest size aggregate). If the mix being placed is too
cold, or if the base on which the new layer is being laid is soft and yielding.
Solutions
Decreasing the paver speed and increasing the frequency of vibration of the screed should, within limits,
increase the level of precompaction achieved during the laydown operation. It is also possible on some
pavers to increase the amplitude of the vibration in order to increase the impact force of the screed on
the mix. Proper maintenance of the screed helps as well in obtaining a uniform compactive effort from the
screed.

As long as the required density level is obtained using conventional rollers behind the paver, the level of
precompaction accomplished by the screed will not affect the long-term performance of the AC layer. It
may be possible, however, to reduce the number of roller passes needed to meet the density and air void
content criteria if the amount of precompaction obtained by the screed is higher. In addition, increased
precompaction density can reduce the amount of differential compaction that occurs in low spots and
rutted areas.
Page 128
Surface (Auger) Shadows

Surface (auger) shadows are dark areas that appear
in the surface of an AC mix. In most cases, the
shadows cannot be seen until sometime after the
pavement has been used by traffic and some of the
asphalt cement film has been worn off the exposed
aggregate particles by the vehicle tires. Surface
shadows are seen most easily when the sun is low on
the horizon and the pavement is viewed when looking
toward the sun. The shadows are also visible when
the pavement surface is damp or when the surface is
viewed from the shoulder of the roadway at night and
vehicle headlights are shining on the surface.
In severe cases, surface shadows may be visible immediately behind the screed during the laydown
operation. Even in this latter case, the shadows will disappear when the mix is being compacted by
the rollers, only to be visible again later under the conditions described above. The shadows may be
completely across the lane width being placed, or they may be only partially across the width. The extent
of the shadows depends on how the paver is operated, particularly the portion of on to off time of the
augers on each side of the machine.
Causes
Surface shadows are caused primarily by overloading of the augers on the paver. If the head of material
in the auger chamber is large enough to “bury” the augers, the screed will react to the variable forces
acting on it. The spacing between the shadows will normally correspond to the starting of the augers when
operated in a stop/start manner. Whenever the amount of mix in front of the screed is at or above the top
of the augers, the shadows will be formed and seen later in the pavement.
On most pavers, it is possible to adjust the distance between the screed and the tractor unit. This is
accomplished by unbolting connections on the leveling or tow arms of the paver and moving the tractor
forward (or backward) while the screed remains stationary on the pavement surface. Depending on the
make and model of the paver, there is typically a four (4) inch length of slide for the screed connection. The
severity of surface shadows may increase with the screed in the back position – when more mix is being
carried in the auger chamber and the augers are being overloaded.
The shadows are thought to be the result of a slight increase in mix density caused by the restarting of the
augers and the subsequent forcing of additional mix under the screed. There is no difference in surface
texture associated with the location of the surface shadows; they can be seen only from an angle. Their
Page 129
intensity often increases when a tender

mix is being laid.
Solutions
The AC mixture carried in the auger
chamber should be maintained at a
level near the center of the auger shaft.
This means the flow gates should be set
so that the augers operate as close to
one hundred (100) percent of the time
as possible and stopping and starting of
the augers is minimized. In no case should the top of the augers be completely covered with mix. Further,
the location of the screed should be set as far forward as possible so that the amount of material in
the auger chamber is reduced and the head of material in front of the screed is kept to a minimum. The
screed should not be set in the back position unless a large-stone mix one in which the maximum size of
the aggregate is more than one and a half (1 1/2) inch is being placed.

Surface shadows are not necessarily detrimental to the performance of the mix, except for a minor effect
on rideability. The difference in the density of the mix in areas with and between shadows is generally
not great enough to be determined accurately. The main concern with surface shadows is the visual
appearance of the mix to vehicle drivers.
Page 130
Joint Problems: Poor Joints

Poor transverse joints are associated either with
a bump at the joint, a dip in the pavement surface
several feet beyond the joint, or both. Poor
longitudinal joints between passes of the paver are
usually characterized by a difference in elevation
between the two lanes, by a raveling of the asphalt
mix at the joint, or both. The area adjacent to the
longitudinal joint is usually depressed below the
level of the surrounding pavement surface.
Causes
Joint problems are caused by poor construction of the joint, inadequate compaction of mix placed along
the joint, improper start-up procedures when paving resumes after a stoppage, or improper construction
and removal of tapers.
Solutions
One key to a good transverse joint is to construct the joint at the end of the paving day at a location in the
mat where the layer thickness is constant. This means the compacted thickness of the mat at the end of
the paver run is the same as that of the previously placed mat.
At the start of paving the following day, the paver screed should be placed on blocks on the cold side of
the transverse joint. The thickness of the blocks should be related to the depth of the course being laid –
approximately one quarter (1/4) inch thick for each one (1) inch of compacted layer thickness. The front
edge of the paver screed should then be placed directly over the vertical face of the joint. Once the paver
pulls away from the joint, the right amount of mix should be in the right place, and only minimal ranking,
if any, normally needs to be done. The mix at the joint should then be compacted as quickly as possible.
For longitudinal joint construction, it is extremely important to compact the edge of the first lane properly.
Doing so requires that the vibratory or static steel wheel roller hang out over the unsupported edge of the
mat by about six (6) inches. This practice provides the most compactive effort along the unconfined edge
without causing undue lateral displacement of the mix along the edge of the pavement.
When placing the second (adjacent) pavement lane, the end plate on the paver screed should overlap
the first lane by one to one and a half (1-1 1/2) inches. Minimal raking, if any, should be done on the mix
placed over the first lane. The rollers – vibratory, pneumatic tire and static steel wheel – should operate
on the hot side of the joint and extend over the joint on the cold side by approximately six (6) inches. The
same number of roller passes should be made over the longitudinal joint as over each point in the interior
of the AC mat.
Page 131
Effects on Long-Term Pavement

Performance
A poor transverse joint will not affect
pavement performance to any
significant degree if proper density
levels are obtained by the compaction equipment. A poor ride will usually be the only negative result.
An improperly constructed longitudinal joint, however, can seriously decrease the serviceability of the
pavement structure. A poorly placed and compacted joint will ravel and cause one side of the joint to be
lower than the other. If the density level is too low, the whole pavement layer thickness at the longitudinal
joint may eat away under the action of traffic. A poor joint will also be porous, allowing water to enter the
underlying pavement courses.
Page 132
Joint Problems: Joint Irregularities

A longitudinal joint is the interface between two adjacent and parallel HMA mats. Improperly constructed
longitudinal joints can cause premature deterioration of multilane HMA pavements. These distresses,
caused by relatively low density (high air voids) and surface irregularity at the joints, can largely be avoided
through proper construction techniques and equipment.
Causes
Usually a well-constructed joint will be about 1 - 2 percent less dense than the rest of the lane away from
the joint; however, a poorly constructed joint can have significantly lower density - on the order of 5 - 10
percent. Low joint density is common since the edge of the lane first paved (cold lane) is unconfined. As
the roller passes over, this unconfined edge tends to deform laterally rather than compact. The subsequent
lane (hot lane) is confined by the cold lane and therefore tends to be denser. Typically, the hot side of the
joint is about 2 - 4 percent denser than the cold side. Furthermore, the overall joint does not typically meet
minimum density requirements established for the mat as a whole. Many agencies that specify minimum
densities (maximum air void contents) for AC construction specifically have different density targets for
the mat and the joint.
Additionally, longitudinal joints can also have surface irregularities that cause them to look coarse, open-
graded or segregated. This can happen because:
• The joint contains segregated material. Because longitudinal joints occur at the edge of the paver
screed and auger system their constituent material can come from material pushed out beyond the
end of the auger and/or screed. Because it has been moved more and moved beyond the end of the
auger, this material has a higher likelihood of being segregated.
• Handwork. Typically, AC from the hot lane that overlaps the cold lane beyond its taper is luted back
onto the hot side of the joint. This handwork usually results in a coarse surface texture and can, in
extreme instances, result in segregation.
Solutions
The solution to good longitudinal joint construction involves several aspects of the paving operation. First,
pavers can be operated in a way that either eliminates the longitudinal joint altogether or at least maximizes
the likelihood of adequate material placement at the joint. Second, several different joint construction
devices can be fitted onto the paver screed to assist in material placement and joint compaction. Third,
several techniques are available to better prepare the cold side of the longitudinal joint to adhere to the
hot side. Finally, several different roller techniques can be used to increase material compaction at the
joint. In most cases, a combination of several different techniques is required to construct a high quality
longitudinal joint.
Page 133

If no construction precautions are taken, the longitudinal joint of a multi-lane paving project will often
be significantly less dense than much of the pavement and contain surface irregularities that may or
may not be caused by segregation. This low-density issue can lead to numerous distresses, including
decreased stiffness, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and
moisture damage.
Page 134
Roller Marks
During the compaction process –
whether vibratory static steel wheel
or pneumatic tire rollers are used –
longitudinal creases or marks are left
in the surface of the mix. Once the mix
has cooled to a temperature range of
160˚F-140˚F, these marks are typically
removed by the finish roller. Roller
marks are indentations that remain in
the surface of the mix after the rolling
has been completed. Roller marks
may also exist in the asphalt surface
when any roller is parked on the hot
mat for a period of time or when a
vibratory roller is vibrated in place. Particularly when used in the breakdown position, pneumatic tire
rollers can leave visible longitudinal marks that can still be seen after the finish rolling has been completed.
Vibratory washboard marks may be visible if that roller is operated at an improper vibratory amplitude,
frequency setting, or speed.
Causes
Roller marks can be an indication that the proper number of roller passes has not been made over the mix.
If the compaction process is halted before the required amount of rolling has been completed or if the mix
cools before the compaction process has been finished, the longitudinal marks or creases made by the
rolling process will remain in the surface of the mix.
Roller marks left in an asphalt layer also may indicate a tender mix. The roller operator will normally
be unable to remove all the marks left by the compaction equipment if the mix is tender or unstable. A
tender mix usually will not support the weight of the finish roller until it has cooled to the point at which the
viscosity of the asphalt cement has increased enough to stiffen the mix. By the time the mix has decreased
in temperature to this point, however, the required level of density can generally no longer be achieved
because the mix has lost its workability. For this reason, the roller marks or indentations left during the
breakdown and intermediate roller passes usually cannot be removed during the finish rolling process. All
of the asphalt cement, aggregate, and mix properties that contribute to the formation of a tender mix, as
discussed above, also contribute to the inability of the finish roller to eliminate roller marks.
Solutions
If the cause of roller marks is inadequate compaction, additional roller passes should be made with
the breakdown, intermediate, or finish rollers to properly densify the mix. The solutions for inadequate
Page 135
compaction related to mix design deficiencies all involve changes to the mix design and to the production of
the mix at the asphalt plant. Asphalt cement quality and content, aggregate properties and characteristics,
and mix temperature all play a significant role in the workability and stability of the asphalt material under
the compaction equipment. Roller marks normally cannot be removed from a tender mix until the mix
temperature has decreased to a relatively low level – usually less than 160˚F.
Sometimes it is possible, depending on environmental conditions and the properties of the mix, to remove
roller marks left in the mix by using a pneumatic tire roller. If the surface of the mix is hot enough
(140˚F) or more, several passes with a pneumatic tire roller can be made to “iron out” the surface of the
pavement. Finally, roughness or washboarding caused by incorrect operation of a vibratory roller should
be eliminated by using proper operating techniques with this equipment.

Roller marks are normally an indication that the proper level of compaction has not been achieved. In
terms of ultimate paving durability, the air void content or density of the mix is the single most important
characteristic that governs the performance of the asphalt mixture under traffic. If the air void content of
a dense-graded mix is high – the density is too low – the pavement generally will not perform well under
traffic.
Page 136
Shoving and Rutting

Shoving of an AC layer is displacement of the mixture in a longitudinal direction. Such displacement may
take place during the compaction operation or late under traffic. In most cases, shoving during construction
is accompanied by a large bow wave in front of the breakdown roller, particularly if that roller is a vibratory
or static steel wheel machine. Shoving may also occur in conjunction with mix checking if the mix is tender
enough as a result of faulty aggregate gradation or excess fluids (asphalt binder or moisture) content.
Finally, mat or mix shoving can occur at the reversal point of the rollers, especially at the location closest
to the paver.
Rutting shows displacement of the mixture in both vertical and transverse direction. Rutting occurs when
heavy traffic passes over an unstable mix. In a few cases, the rutting is purely vertical (consolidation
rutting). In this situation, the mix was not adequately compacted at the time of construction, and the traffic
loads are essentially finishing the compaction process. The most common form of rutting is transverse
distortion – the mix distorts or shoves transversely as a result of lateral flow of the mix under applied traffic
loads.
Causes
Shoving and rutting are due primarily to an
unstable AC mixture. This instability can
be caused by the same variables that are
responsible for checking – an excess of
fluids (asphalt binder or moisture) in the
mix, a hump in the fine aggregate grading
curve, or the properties of the aggregate
and the asphalt cement. A mix that has a
high Marshall or Hveem stability may still
distort longitudinally under the compaction
equipment and later both longitudinally and
transversely under traffic. Shoving and rutting can be highly prevalent when a sand mix is placed in a thick
layer (more than one and a half (1 1/2) inch at a high temperature (more than 280˚F). Further, thicker lifts
in proportion to the maximum-size aggregate used in the mix will tend to shove more than thinner lifts
with the same aggregate size and grading. Improper roller operation, particularly sudden reversal of the
roller, can also contribute to the shoving of the mix during construction. If a vibratory roller is run at too
great a speed and the impact spacing is too far apart, the mat may develop a washboard effect, where the
peak-to-peak distance is equivalent to impact spacing. Washboarding or shoving is more likely to occur
at normal frequencies but at high speeds where the impact force is greater. If a pneumatic tire roller with
high tire pressure is used for breakdown compaction, a tender mix may shove laterally under the tires.
Shoving can occur under any roller that is operated improperly.
Page 137
Another possible cause of shoving is an

excess of tack coat material that may be
pulled into the mix. In a similar manner,
excess asphalt from a bleeding underlying
surface or from joint filler material can be
pulled into the mix and increase its fluidity and
tenderness. Shoving may occur as well when
the underlying surface is dusty or dirty – a
slippage failure.
Solutions
The solution to a mix that shoves under the
compaction equipment is to increase its
internal stability. This can be accomplished by reducing the fluids content (asphalt or moisture or both)
of the mix, but only after determining the effect of a change in asphalt binder content on the mechanical
properties of the mix. The internal friction can be increased by lowering the mix temperature. Alternatively,
the internal friction among the aggregate particles can be increased by changing the aggregate gradation
or increasing the amount of angular (crushed) particles in the mix.
The compaction process for a tender mix should be changed, as discussed above under checking, to obtain
sufficient density at the time of construction. An increase in the density achieved during the construction
process will generally reduce the amount of shoving and rutting that may occur later under applied traffic.
Sand mixes, because of their inherent tender nature, should be placed in several thin layers instead of one
thick layer when used as base or binder courses.
The compaction equipment should be operated properly so as to reduce the opportunity to displace the
mix during the rolling operation. Further, if the underlying pavement surface is dirty, it should be cleaned
and a proper tack coat applied.

Mats that tend to shove under the compaction equipment are basically unstable. These mixtures will
usually continue to distort under traffic, both longitudinally and laterally. Shoving of the AC mixture during
construction is a strong indication that the pavement will rut later and not perform properly under traffic.
Page 138
Bleeding and Fat Spots

Bleeding of an asphalt mixture occurs when the asphalt cement flows to the top of the mix surface under
the action of traffic loading. Bleeding is often seen as two flushed longitudinal streaks in the wheel paths
of the roadway. Fat spots in an asphalt mixture are isolated areas where asphalt cement has come to the
surface of the mix during the laydown and compaction operation or later under traffic. These spots can
occur erratically and irregularly, or they may be numerous and in a fairly regular pattern.

Causes
Fat spots are caused primarily by excessive moisture in
the mix. The problem is more common with mixtures that
contain a high percentage of fine aggregate (oversanded
mixes) and those that contain aggregates with a high
porosity. If all the moisture in the coarse and fine aggregate
is not removed during the drying and nixing operation at
the asphalt plant, the moisture vapor will force asphalt
cement to the surface of the mix behind the paver as the

moisture escapes from the mix and evaporates.

Fat spots occur more frequently when aggregate stockpiles are wet or when the moisture content varies
in different portions of the stockpiles. Fat spots sometimes occur in areas where petroleum products, such
as oil and diesel fuel, were spilled onto the pavement surface prior to overlay or have contaminated the
mix. In addition, fat spots can be associated with segregated areas in the mix. If the mix deposited on the
roadway by the paver is segregated, areas in which excess asphalt cement is present in the mix can result
in free binder material on the top of the layer being placed.

The causes of bleeding normally fall into two
categories. The first is an excess of fluids in the
asphalt mixture – either asphalt cement or moisture
or both. Under traffic, the extra moisture and asphalt
cement will be pulled to the surface by the passage
of vehicle tires. This bleeding phenomenon usually
occurs on new mix and during hot weather when the
viscosity of the asphalt cement is at its lowest level.

Typically, the bleeding occurs shortly after traffic is
allowed to travel over the fresh mix – while there is
still some moisture in the mix and while the viscosity
of the asphalt cement binder is still relatively low.
Page 139
Bleeding may also be associated with a lack of adequate space in the mix for the asphalt cement. If the
VMA content and air void content of the mix do not provide enough room for the binder material, bleeding
can occur as the mix is densified by traffic, both shortly after construction and later. The traffic compaction
process will decrease the air void content of the mix and may, in turn, squeeze some of the asphalt cement
out of the mix. The “extra” asphalt will appear as a longitudinal streak or fat spot throughout the length of
each wheelpath.
One additional possible cause of bleeding is the condition of the pavement layer on which the new mix
is placed If the underlying pavement is experiencing bleeding with free binder on the surface, then this
binder may migrate into the void spaces of the new AC mix and lead to bleeding.
Solutions
Variations in the asphalt mix temperature behind the paver indicate that the moisture content of the mix
may also be variable. Where moisture has evaporated, the temperature is lower. It is important, therefore,
that the aggregate used in the mix be relatively dry and that the moisture content of the mix upon discharge
from the asphalt plant be as low as possible, but not more than half of one (0.5) percent. Extra care needs
to be taken in drying when producing mixtures that incorporate highly absorptive aggregate.
Bleeding problems caused by excess asphalt cement in the mix can most easily be solved by reducing
the asphalt content, consistent with other properties of the mix, such as air voids, VMA, and strength
or stability. Bleeding problems that occur in conjunction with pavement rutting usually can be solved,
however, only by a complete redesign of the asphalt mixture, with emphasis on proper air void content
and VMA criteria.

Occasional fat spots in the mix should not affect the ultimate durability of the pavement to a significant
degree. A large number of fat spots or bleeding in the wheelpaths does affect pavement performance,
however, because of variable asphalt and air void content in different parts of the mix. In addition, other
mix problems, such as shoving, rutting and loss of skid resistance, may occur in a mix that contains many
fat areas or bleeding in the wheelpaths. The design of the asphalt mixture, the operation of the asphalt
plant (more complete removal of moisture), or both should be checked to ensure that the mix produced
will provide adequate pavement performance under vehicular loading.
Page 140
Poor Mix Compaction

The AC mixture should be compacted so that the in-place air voids are at an acceptable level. If the air
voids are above 7% to 8%, the mix will be permeable to air and water and will not have the required
durability. If the initial compaction results in air voids of approximately 4%, replace or lower, the mix may
become unstable under traffic after additional densification; the result will be shoving and rutting of the
mixture. Most mixes require a significant level of compaction to reach the desired 7 to 8% or less air voids.
Causes
When the mix is too stiff or too tender, compaction is difficult. The primary cause of poor compaction is low
design mix density (high design air voids). Other causes include inadequate underlying support, improper
type and weight of rollers, improper tire pressure in rubber tire rollers, improper rolling procedure, improper
mix design, mix segregation, moisture in the mix, variation in mix temperature and low mix temperature.
Solutions
Solutions to compaction problems include taking the necessary steps to ensure adequate support,
producing an acceptable mixture and using satisfactory laydown and rolling techniques. When support is
inadequate, the compaction requirements may have to be relaxed, or the mix may have to be redesigned
to allow for satisfactory compaction.
When asphalt content is too high, the mix may compact too easily, resulting in low air voids (which leads to
rutting). When the asphalt content is too low, the mix may be stiff and difficult to compact to the specified
density. A satisfactory mix design will produce a mix with optimum asphalt content that can be compacted
with reasonable effort to required density
Good laydown and rolling techniques are necessary for good compaction. Density can normally be
increased by reducing the speed of the paver or rollers. Density can also be increased by increasing the
weight and number of rollers. The compaction process must be adjusted to produce optimum density.

When the compaction is inadequate -more than 7% to 8%, replace air voids- the mix will be permeable to
air and water. Water can flow through the AC and reduce the strength of the underlying base course. The
high voids also result in excessive oxidation of the HMA, which leads to raveling, cracking and general
deterioration of the AC over a period of time.
When the air voids are excessively low after compaction (less than 4 percent) the mix is likely to rut and
shove under traffic. The low voids are the result not of too much compaction, but of an unsatisfactory
mixture.
Page 141
Chapter Six Knowledge Check

1. Mat problems fall into the following primary categories:
a. Equipment related problems
b. Environmental problems
c. Mixture related problems
d. Both A and C
2. Mat problems that occur at the time of asphalt mix production are due only to laydown and compaction
a. True
b. False
3. ____________ is caused by the improper operation of a vibratory roller:

a. Segregation
b. Streaking
c. Washboarding
d. Bubbling of the mix
4. The most serious problem associated with asphalt placement is:

a. Surface bleeding (fat spots)
b. Material Segregation
c. Shoving or rutting
d. Poor pre-compaction
5. MAT Tearing or pulling of the asphalt mix under the screed of the paver is most often observed
a. In the center of the lane
b. On the outside edges of the lane
c. Across the full lane width
d. Any one of the above
Page 142
6. _______________ is defined as short transverse cracks which occur in the surface of the AC mat at
some time during the compaction process:
a. Checking
b. Screed marks
c. Shoving or rutting
d. Poor mix compaction
7. Which of the following does NOT cause problems with mat texture or uniformity?
a. Fluctuating head of material
b. Screed plate being set too low
c. Screed plate being worn out
d. Running hopper empty between loads
e. Screed extensions being installed incorrectly
8. Non-uniform surface texture is usually associated with _________________.

a. Banging the gate when unloading the mix at a job site
b. Unbalanced aggregate
c. Excessive cooling of the mix
d. Non-uniform density
9. _______________ normally occurs when heavy traffic passes over an unstable or poorly compacted
mix.
a. Surface waves
b. Mat tearing
c. Delamination
d. Rutting
e. Pumping
10. Roller Marks are normally an indication that the proper level of compaction has not been achieved.
a. True
b. False
Page 143
Page 144
B Acknowledgements
Page 157
Appendix B: Acknowledgements
Acknowledgements
• Transportation Research Board (TRB). (2000). Hot-Mix Asphalt Paving Handbook 2000.
Transportation Research Board, National Research Council. Washington, D.C.
• Roberts, F.L., Kandhal, P.S., Brown, E.R., Lee, D.Y., and Kennedy, T.W. (1996). Hot Mix Asphalt
Materials, Mixture Design, and Construction. National Asphalt Pavement Association Research
and Education Foundation. Lanham, MD.
• Kandhal, P.S. and Mallick, R.B. (1996). A Study of Longitudinal Joint Construction Techniques
in AC Pavements (Interim Report - Colorado Project). NCAT Report No. 96-03. http://www.
eng.auburn.edu/center/ncat/reports/rep96-3.pdf
• Kandhal, P.S. and Mallick, R.B. (1997). Longitudinal Joint Construction Techniques for
Asphalt Pavements. NCAT Report No. 97-04. http://www.eng.auburn.edu/center/ncat/reports/
rep97-4.pdf
• Kandhal, P.S.; Ramirez, T.L. and Ingram, P.M. (2002). Evaluation of Eight Longitudinal Joint
Construction Techniques for Asphalt Pavements in Pennsylvania. NCAT Report No. 02-03.
http://www.eng.auburn.edu/center/ncat/reports/rep02-03.pdf
Page 158
C Glossary
Page 159
Appendix C: Glossary
Aggregate Segregation (3) - Segregation is a lack of homogeneity in the hot mix asphalt constituents
of the in-place mat of such a magnitude that there is a reasonable expectation of accelerated pavement
distress(es). Stroup-Gardiner and Brown (2000)
Anionic (2) - An asphalt emulsion containing asphalt droplets that are negatively charged.
Application Rate (Tack Coat) (2) - The amount of emulsion sprayed from the distributor
Asphalt Emulsion (2) - A performance grade asphalt suspended in water using acid and surfactant
(soap).
Asphalt Laydown Machine (4) - A self-propelled formless laydown machine with a floating screed, used
Automatic Screed Control (4) - Controls tow point elevation using a reference other than the tractor
body. Also known as a “grade reference system”.
Banging the Gate (3) - A common practice that occurs during mix unloading where the Auger (4) -
Receives HMA from the conveyor and spreads it out evenly over the width to be paved. Driver of a delivery
truck leaves the end gate of the dump truck up, speeds up after dumping and stops quickly to “bang” the
tailgate and expel any remaining mix left in the truck. This is a practice that should be discouraged.
Bleeding (2) - The binder (asphalt) coming up to the surface of the road Cationic (2) - An asphalt emulsion
containing asphalt droplets that are positively charged.
Bleeding (6) - Occurs when the asphalt cement flows to the top of a mix surface under the action of traffic
loading.
Checking (6) - Short transverse cracks, usually one to three (1-3) inches in length and one to three (1-3)
inches apart, which occur in the surface of the AC mat at some time during the compaction process.
Cold Planning (2) - The process of removing at least part of the surface of a paved area such as a road,
bridge, or parking lot to prepare for a new overlay
Compaction (4) - The process through which proper density is achieved.
Conventional Compaction (5) - The process of compressing a given volume of asphalt concrete into
a smaller volume, accomplished by pressing asphalt coated aggregate particles closer together thereby
reducing the air voids in the mix and increasing the density of the mixture.
Conveyor (4) - Mechanism that carries the HMA from the hopper, under the chassis and engine, then to
the machine’s augers.
Coverage - A coverage is defined as the roller moving over the entire width of the mat one time
Page 160
Cracking (2) - Cracks in asphalt layers that are often caused by repeated traffic loadings, sub-base
failure, or poor drainage conditions
Density - The density of a material is simply the weight of the material that occupies a certain volume of
space.
Cessation Temperature (5) -
Static Weight (5) -
Dynamic (Impact) Force (5) -
Depth Crank (4) - The manual control device used to set screed angle and ultimately, mat thickness.
Differential Compaction (2) - Requires that multiple courses be constructed over a pavement surface
that is badly out of shape before a smooth surface can be obtained.
Emulsion (2) - An emulsion that takes longer to break and set
End Dump Truck (3) - The primary type of truck used to transport AC mix from production facility to paving
site.
Fat spots (6) - Isolated areas where asphalt cement has come to the surface of the mix during the
laydown and compaction operation or later under traffic.
Flo-Boy (3) - Employs a conveyor belt or slat conveyor in the bottom of the truck bed to discharge AC mix
during the Mix Delivery process. Also known as a Live Bottom Truck.
Gearbox Streak (6) - If an inadequate amount of HMA is distributed under a middle-located gearbox the
result can be a thin longitudinal strip of mat aligned with the gearbox that exhibits lower densities from
aggregate segregation and/or temperature differentials
Grade Reference System (4) - Controls tow point elevation using a reference other than the tractor body.
Also known as a “Automatic Screed Control”.
Hopper (4) - Used as a temporary storage area for HMA delivered by the transport vehicle so that the
paver can accept more material than is immediately needed and can use the volume in the hopper to
compensate for fluctuating material demands
Leveling Course (2) - A layer of an asphalt-aggregate mixture of variable thickness, used to eliminate
irregularities in contour of an existing surface, prior to the placement of a superimposed layer. Also known
as a Wedge Course or Scratch Course
Page 161
Live Bottom Truck (3) - Employs a conveyor belt or slat conveyor in the bottom of the truck bed to
discharge AC mix during the Mix Delivery process. Also known as a Flo-Boy.
Longitudinal Joint (4) - The interface between two adjacent and parallel AC mats.
Material Feed System (4) - Component of a paver tractor that accepts the HMA at the front of the paver,
moves it to the rear and spreads it out to the desired width in preparation for screed leveling and compaction.
Material Transfer Vehicle (MTV) (3) - Vehicle used to assist the paver in accepting HMA.
Milling (2) - The process of removing at least part of the surface of a paved area such as a road, bridge,
or parking lot to prepare for a new overlay
Mix Placement (4) - involves any equipment or procedures used to place the delivered AC on the desired
surface at the desired thickness.
Non-Uniform Mat Texture (6) - Differences in the appearance of the mix, both transversely and
longitudinally, as it is placed and compacted.
Pass - A pass is defined as the entire roller moving over one point in the mat one time.
Paver Hopper (3) - The hopper is used as a temporary storage area for AC delivered by the transport
vehicle.
Proof Rolling (2) - A physical compaction test performed with a loaded rear tandem axle dump truck
containing a recommended gross weight of 50,000. The loaded dump truck will roll over the soil subgrade
where the proposed concrete curb or asphalt concrete is to be placed. As the proof roll truck rolls over
areas being tested at two to five miles per hour and the soil subgrade does not yield or pump, it is
considered passing and suitable for curbing and asphalt placement.
Push Roller (4) - The portion of the paver that contacts the transport vehicle
RAP (Reclaimed Asphalt Pavement) (2) - The term given to the materials generated when asphalt
pavements are removed for reconstruction or resurfacing.
Raveling (2) - Aggregate becoming separated from the binder and loose on the road
Reclaimed Asphalt Pavement (RAP) (2) - The term given to the materials generated when asphalt
pavements are removed for reconstruction or resurfacing.
Release Agent (3) - A chemical used in the mix delivery process to help prevent the AC mix
Residual Rate (Tack Coat) (2) - The amount of asphalt cement remaining on the pavement surface after
the water has evaporated.
Page 162
Ride Quality (2) - Uneven road surface such as swells, bumps, sags, or depressions
Roller Marks (6) - Indentations that remain in the surface of the mix after the rolling has been completed
Rutting (2) - Formation of low spots in pavement along the direction of travel usually in the wheel path
Rutting (6) - Shows displacement of the mixture in both vertical and transverse direction.
Scabbing (2) - Surface condition that occurs when a milling machine leaves patches of uneven pavement
Scratch Course (2) - A layer of an asphalt-aggregate mixture of variable thickness, used to eliminate
as a Wedge Course or Leveling Course
Screed (4) - The device on a paver designed to flatten the asphalt below it.
Screed Angle (4) - The angle the screed makes with the ground surface. Also known as “Angle of “Attack”.
Screed Arms (4) - Long beams that attach the screed to the tractor unit.
Screed Crown (4) - The elevation of the middle in relation to the edges
Screed Extensions (4) - Fixed or adjustable additions to the screed to make it longer. Basic screed
widths are between 2.4 m (8 ft.) and 3.0 m (10 ft.).
Screed Heater (4) - Heaters used to preheat the screed to HMA temperature.
Screed Marks (6) - Transverse indentations in the surface of the asphalt mat.
Screed Plate (4) - The flat bottom portion of the screed assembly that flattens and compresses the HMA.
Screed Tow Point (3) - Tow point elevation will affect screed angle and thus mat thickness.
Screed Vibrator (4) - Device located within the screed used to increase the screed’s compactive effort.
Screed compaction depends upon screed weight, vibration frequency and vibration amplitude.
Segregation (6) - The non-uniform distribution of coarse and fine aggregate components within the
asphalt mixture.
Shoving (2) - A washboard like effect transverse to the direction of travel
Shoving (6) - Displacement of the mixture in an AC layer in a longitudinal direction.
Strike-off Plate (4) - The vertical plate just above the leading edge of the screed used to strike off excess
HMA and protect the screed’s leading edge from excessive wear.
Surface (Auger) Shadows (6) - Dark areas that appear in the surface of an AC mix.
Page 163
Tack Coat (2) - Ensures an adequate bond between an existing pavement surface and new asphalt
overlay.
Tow Point (4) - Point at which the screed arm is attached to the tractor unit.
Trackless Emulsion (2) - An emulsion that has a shorter set time allowing for faster access for construction
traffic
Trimmed Joint (4) - A joint that is constructed by removing all freshly placed material that has overlapped
the rolled lane.
Truck Hitch (4) - Holds the transport vehicle in contact with the paver
Washdown (3) - The cleanup of a mix delivery truck’s chute and discharge hopper after unloading material
at the paving site to prevent buildup of materials
Wash Off (3) - Applicable to mixing trucks, the driver should “wash off” the back of the mixer once mix is
loaded for delivery to the paving site to prevent buildup of materials.
Washout (3) - Cleaning of a mix delivery truck’s drum/bed once back at the production facility at the end
of a workday to prevent material buildup.
Wedge Course (2) - A layer of an asphalt-aggregate mixture of variable thickness, used to eliminate
as a Leveling Course or Scratch Course
Page 164

Asphalt Field Level I - No Append A Answers - PD

Uploaded by

Copyright:

Available Formats

Asphalt Field Level I - No Append A Answers - PD

Uploaded by

Document Information

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Asphalt Field Level I - No Append A Answers - PD

Uploaded by

Copyright:

Available Formats

What are some of the considerations for surface preparation before paving?

What are some of the considerations for surface preparation before paving?

What factors affect compaction of asphalt?

What factors affect compaction of asphalt?

Chapter 1: Project Communication and Workzone Safety

Chapter 2: Surface Preparation

Chapter 3: Mix Delivery

Chapter 4: Equipment and Laydown Operations

Chapter 6: Mat Problems: Causes, Solutions and Effect on Pavement Performance

Asphalt Field Certification Information

- Before taking the exam you must show photo identification

- You are allowed 1 hour to complete the exam.

Passing exam score results in an Asphalt Field Level I Certification.

- Before taking the exam you must show photo identification

- You are allowed 3 hours to complete the exam.

Passing exam score results in an Asphalt Field Level II Certification.

If you do not know your logon or password, call (804) 328-3158.

About This Guide

A definition will be provided. Describes and/or defines terminology.

WORK PROCEDURE (BEST PRACTICES / AWARENESS)

Describes a best practice to be utilized when possible.

INSPECTION LIST AND MEASUREMENTS

Describes inspection and Quality Assurance practices.

• Asphalt Concrete (dense-graded hot or warm mix asphalt)

• Open-graded Asphalt or Porous Friction Course

• Ultra-Thin Bonded Wearing Course

• Asphalt Surface Treatments

• Emulsified Asphalt Mixes (cold mixes)

• Permeable Asphalt Drainage Course

• Others, SMA, In-Place Recycled Mixes

Upon completion of this chapter, you should be able to:

Describe the purpose of :

• pre-construction and pre-paving conferences, and

• cooperative communication on the job

Project Organization and Communication

• The project documents are written instructions that must

• The preconstruction conference initiates verbal.

• Ongoing communication between the contractor and the

• Safety on the job cannot be maintained if communication between parties is inadequate.

Standard Drawings Calculated dimensions, unless obviously incorrect, will

Standard specifications Directions, provisions, and requirements for performing

Special or supplemental specifications Approved additions and revisions to the standard

Plans Drawings that show the location, character, dimensions,

Special provisions Additions or revisions to the standard or supplemental

The order of priority, from lowest to highest is:

Plans and specifications need to be accurate and

Preconstruction Conference The preconstruction conference should be held before work on

Second, the discussion should address what is expected to happen

Written Communication Much information can be communicated in oral form, but

Chapter One Knowledge Check

2. _____________ are approved additions and revisions to the standard specifications.

5. If an unsafe work practice is noticed on the job site or in a plant:

6. ___________ is one of the keys to a safe work environment:

Upon completion of this chapter, you should be able to:

Understand how base properties impact paving decisions

Recognize drainage considerations in creating lasting pavement structures

Understand reasons for milling

Identify impacts of milling equipment speed and teeth condition on quality