Construction Materials and Testing

Chapter 5

Concrete

PREPARED BY: MA. THEREZA R. VICHO 53

Chapter 5
Concrete

Introduction

This module mainly introduces the composing materials, the major technical functions and the
factors influencing the performance of ordinary concrete, specifically discusses the methods
to design the mix proportion of ordinary concrete, reveals the quality control and strength
evaluation of concrete, and simply exhibits other kinds of concrete.

Aggregate was discussed in the previous module so we will be giving more emphasis on
cemetitious materials in this module.

Specific Objectives
At the end of the lesson, the students should be able to:
▪ Know the different classifications of concrete and its characteristics
▪ Know the different kinds of cementitious materials
▪ Know the preparation and curing of concrete test specimens

Duration
Chapter/Lesson 5: Concrete = 4 hours
(3 hours discussion; 1hr
assessment)

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LESSON PROPER

1. CONCRETE
Concrete is a kind of man-made stone which is made by mixing gel materials, granular
coarse-fine aggregate and water (if necessary, a certain amount of additive and mineral
materials are added) in a proper ratio evenly, and then getting solidified and hardened.
It is one of the main building materials in projects. The one mostly used in construction
projects is the cement concrete made by mixing gel materials, aggregate (sand and
stone), and water, which should get through hardening process

1.1. CLASSIFICATION OF CONCRETE

1.1.1. By cementing materials, there are:
▪ cement concrete,
▪ gypsum concrete,
▪ asphalt concrete and
▪ polymer concrete.

1.1.2. By apparent density, there are:

▪ heavy concrete ( po > 2500kg/m3 ), ordinary concrete ( po from 1900 kg /
m3 to 2500 kg / m3 ),
▪ light concrete ( p,, from 600 kg / m3 to 1900 kg / m3 ), and
▪ super-light concrete ( po c 600kg / m3 ).

The apparent density of concrete depends on the aggregate varieties and its
own density. Many properties of concrete are connected with apparent
density.

1.1.3. By performance and application, there are:

▪ structural concrete,
▪ hydraulic concrete,
▪ ornamental concrete, and
▪ special concrete (heat-resistant, acid-resistant, alkali-resistant, and anti-
radiation concrete and so on).

1.1.4. By construction methods, there are:

▪ pump concrete,
▪ sprayed concrete,
▪ vibrating-compacting concrete, c
▪ centrifugal concrete and so on.

1.1.5. By mixtures, there are:

▪ fly ash concrete,
▪ silica fume concrete,
▪ fine blast furnace slag concrete,
▪ fiber concrete, and others.

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 1.2. CHARACTERISTICS OF CONCRETE
1.2.1. Convenient for use: the new mixtures have good plasticity that can be cast into
components and structures in various shapes and sizes.
1.2.2. Cheap: raw materials are abundant and available. More than 80% of them are
sand and stone whose resources are rich, energy consumption is low, according
with the economic principle.
1.2.3. High-strength and durable: the strength of ordinary concrete is 20 - 55MPa with
good durability.
1.2.4. Easy to be adjusted: the concrete with different functions can be made just by
changing the varieties and quantities of composing materials to meet various
demands of projects; steel bar can be added to concrete to improve its
strength, and this kind of concrete is a composite material (reinforced concrete)
which can improve its low tensile and bending strength in order to meet the
needs of various structural engineering.
1.2.5. Environment-friendly: concrete can make full use of industrial wastes, such as
slag, fly ash and others to reduce environmental pollution.
Its major shortcomings are high dead weight, low tensile strength, brittle and
easy to crack.

2. COMPONENTS OF ORDINARY CONCRETE

The basic components of ordinary concrete are

cement, water, sand, and stones. Generally, the
amount of sand and stone accounts for above
80% of the total volume, functioning as frame, so
they are respectively called as fine aggregate and
coarse aggregate. Mixed with water, cement
becomes cement paste, and cement mortar not
only wraps the surface of particles and fills their
gaps, but also wraps stones and fills their gaps,
then concrete coming into being. Cement paste
can function as greasing before hardening, which
renders concrete mixture with good mobility; after
hardening, aggregates stick together and form a
hard entity, known as man-made stone-concrete.

2.1. CEMENTITIOUS MATERIAL OR CEMENT

Cement is the most important component for concrete and relatively expensive. In
the preparation of concrete, the choice of cement varieties and strength grades are
directly related with the durability and economy, of concrete.

The Choice of Cement Varieties

When concrete is prepared, the rational choice should be made in light of the
properties of cement varieties, according to the properties of the project, parts,
construction conditions, environment and so on.

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 The Choice of Cement Strength Grades
The cement strength grades are corresponding to the design strength grades of
concrete. The standard strength grade of ordinary cement should be 1.5-2.0 times as
big as that of concrete. If the cement strength is too high or too low, the cement
content in concrete will be too small or too large that will have a negative impact on
the technical performance and the economic effect of concrete.

2.2. AGGREGATE
The aggregates used for ordinary concrete can be divided into two types by their
sizes: the fine, and coarse aggregates. For more discussion, please refer to Module 3.

3. TYPES OF CEMENT

3.1. PORTLAND CEMENT

Portland cement is made from four basic compounds, tricalcium silicate (C3S),
dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite
(C4AF). The cements used in Minnesota are made either from limestone and clay,
limestone and shale, or limestone and slag. The manufacturing process known as the
dry process is the most widely used at present. This consists of grinding the individual
raw materials and feeding at controlled amounts into a rotary kiln and burning until
they fuse into small lumps or balls called clinkers. In the wet process, a slurry of the
blend is fed into the rotary kiln. The clinkers are cooled and then ground in two
operations. Between the first and the final grind, a quantity of gypsum (usually 2 to 3%
by mass (weight) of cement) is added to regulate the setting properties of the
cement.

3.2. BLENDED CEMENT

These blended cements are composed of one of five classes of hydraulic cement for
general and special applications, using slag, fly ash or other pozzolan with portland
cement, or portland cement clinker with slag.

3.3. GROUND GRANULATED BLAST FURNACE SLAG (GGBFS)

In the blast furnace, magnetic iron ore (Fe3O4) and haematic iron ore (Fe2O3) are
fed along with limestone into a high temperature chamber containing coke. Coke is
partially oxidized to carbon monoxide, which reduces the ores to iron. The other
product that floats over the molten iron due to its relative lightness is called slag. Slag
is composed of calcium oxide (CaO), silica (SiO2) and alumina (Al203). Slag is
pulverized into a fine powder called ground granulated blast furnace slag and is used
in this form as a cementitious component of concrete.

3.4. FLY ASH

Fly ash is the most widely used pozzolan in concrete. It is a fine residue resembling
cement that is a by-product of burning coal in an electric power generating plant.
Depending on the chemical consistency of the coal source, the material is identified
as Class C (self-cementing) or Class F (non-cementing) fly ash.

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4. REINFORCED CONCRETE
Reinforced concrete is a composite material. This means that it is made up of different
constituent materials with very different properties that complement each other. In the
case of reinforced concrete, the component materials are almost always concrete and
steel. The steel is the reinforcement. Other reinforcement, such as glass fibre or
polypropylene, is used for specialised applications. Concrete is strong in compression. Steel
is strong in tension and compression, but in compression a steel bar that is thin enough to
be economic will buckle. A simple reinforced concrete structure therefore uses steel in
tension, and concrete in compression.

5. CONCRETE ADMIXTURE
Concrete admixture refers to the substance mixed in concrete according to different
requirements to improve the performance of concrete. The mixing amount is generally no
more than 5% of the cement mass (except special cases). Based on the main functions,
admixtures, mainly contain water-reducing agent, air-entraining agent, hardening
accelerator, set retarder, flash setting agent, expanding agent, antifreeze agent, rust-
resistant agent and others.

5.1. Water-reducing Agent

Water-reducing agent refers to the admixture used for reducing water consumption
and strengthening functions when the slump degrees of mixtures are basically the
same. Based on performances and functions, water-reducing admixtures can be
divided into: ordinary water-reducer, effective water-reducer, hardening water-
reducer, retarder water-reducer, and air entraining water-reducer.

5.2. Air entraining Mixture

Air entraining admixture refers to the admixture that entrains a large number of
uniform, stable and closed tiny bubbles in the process of mixing concrete to reduce
the segregation of concrete mixture, improve the workability, and also enhance anti-
freeze ability and durability of concrete. It is a kind of surfactant, too. It has influences
on concrete as follows:
1) It can improve the workability of concrete mixtures. The closed bubbles are like
balls that can reduce the friction among cement particles to improve the
mobility. Meanwhile, the bubble film can play a role of water conservation.
2) It can enhance impermeability and frost resistance. The closed stomata
entrained by air entraining admixture can effectively cut off the capillary
porosity ducts and reduce pores caused by segregation to enhance
impermeability. Meanwhile, the closed pores entrained can be an effective
buffer for the expansion caused by water freeze to improve frost resistance.
3) It can reduce strength. If the air content in concrete increases by 1%, its
compressive strength will decrease by 4%-6%. Thus, the adding amount of air
entraining admixture should be appropriate.

5.3. Hardening Accelerator

Hardening accelerator refers to the admixture that can accelerate the development
of early strength of concrete. Generally, hardening accelerator can be divided into
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 inorganic (chloride, sulfate, etc.), organic (triethanolamine, tri-isopropanolamine, and
sodium acetate, etc.) and inorganic-organic compound, the three categories.

It can accelerate the hydration and the hardening of cement, improve early strength,
and shorten conservation cycle so as to enhance the turnover rate of templates and
sites and speed up the construction process. It is especially used in winter construction
(whose minimum temperature is not less than -5C) and emergency repair works.

5.4. Set Retarder

Set retarder refers to the admixture that can delay the setting time of concrete mixing
materials, and have no bad impact on the development of concrete’s latter strength.
The most common ones are calcium lignosulfonate and molasses. And the retardant
effect of molasses is better.

Set retarder is appropriately used in the projects that need to delay time, such as high
temperature or long transport distance, to prevent the lose caused by the early slump
of concrete mixtures; and also for the layer pouring concrete, set retarder is often
added to prevent cold joint. In addition, set retarder can be added into mass
concrete to extend the heat-releasing time.

5.5. Flash Setting Admixture

Flash setting admixture refers to the admixture that can promote the rapid hardening
of concrete. The concrete added by flash setting admixture can let the gypsum mixed
in cement lose its retardant function to make concrete harden quickly.

5.6. Expansion Agent

Expansion agent is the admixture that can make concrete produce shrinkage
compensating or micro-expansion. The mixture of expansion agent has little influence
on the mechanical properties of concrete and can raise the frost resistance of
concrete above P30, enhancing the crack resistance significantly.

5.7. Anti-freeze
Anti-freeze refers to the admixture that can reduce the liquid freezing point of water
and the concrete mixtures to protect concrete against freeze under the
corresponding negative temperature and achieve the expected effect under the
regulated conditions. Anti-freeze admixtures usually include the following several
ones:
1) Sodium nitrite and calcium nitrite, which can reduce freezing point, accelerate
hardening, and resist corrosion, with the general mixing amount of 1%-8%.
2) Sodium chloride and calcium chloride, which can reduce freezing point but will
corrode steel bars, with the general mixing amount of 0.5%-1 .O%.
3) Potassium carbonate, urea and others. In practical projects, the anti-freezers
are usually complex, and meanwhile they can resist freeze, accelerate
hardening, and reduce water. Sometimes the anti-freezing effect can be
enhanced greatly by adding air entraining agents.

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 5.8. Rust-resistant Agent
Rust-resistant agent is the admixture that can retard the corrosion to steel bars in
concrete or other embedded metal, also called corrosion inhibitor. The common
agent is sodium nitrite. Some admixtures contain chloride salt which will corrode steel
bars (thus, it is necessary to control the content of chloride ions), so the adding of rust-
resistant agent can retard corrosion to steel bars for the sake of protection.

6. CONCRETE SAMPLE PREPARATION

6.1. Placing
Concrete is placed in the molds using a trowel in three layers of approximately equal
depth and is remixed in the mixing pan with a shovel to prevent segregation during
the molding of specimens. The trowel is moved around the top edge of the mold as
the concrete is discharged in order to ensure a symmetrical distribution of the
concrete and to minimize segregation of coarse aggregate within the mold.

6.2. Consolidation
Compaction is the removal of air from fresh concrete. Proper compaction results in
concrete with an increased density which is stronger and more durable. If the slump is
greater than 25 mm (1 in.), consolidation may be by rodding or vibration. Agency
specifications may dictate when rodding or vibration will be used.

6.2.1. Vibration

When the slump is 25 mm (1 in.) or less, consolidate the sample by internal vibration.

Procedure – Making Cylinders – Internal Vibration

1) Fill the mold in two layers.
2) Insert the vibrator at the required number of different points for each layer
(two points for 150 mm (6 in.) diameter cylinders; one point for 100 mm (4 in.)
diameter cylinders). When vibrating the bottom layer, do not let the vibrator
touch the bottom or sides of the mold. When vibrating the top layer, the
vibrator shall penetrate into the underlying layer approximately 25 mm (1 in.)
3) Remove the vibrator slowly, so that no large air pockets are left in the
material.
Note: Continue vibration only long enough to achieve proper consolidation
of the concrete. Over vibration may cause segregation and loss of
appreciable quantities of intentionally entrained air.

4) After vibrating each layer, tap the sides of each mold 10 to 15 times with the
mallet (reusable steel molds) or lightly with the open hand (single-use light-
gauge molds).
5) Strike off the surface of the molds with tamping rod or straightedge and
begin initial curing.

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 Procedure – Making Flexural Beams – Vibration
1) Fill the mold to overflowing in one layer.
2) Consolidate the concrete by inserting the vibrator vertically along the
centerline at intervals not exceeding 150 mm (6 in.). Take care to not over-
vibrate, and withdraw the vibrator slowly to avoid large voids. Do not
contact the bottom or sides of the mold with the vibrator.
3) After vibrating, strike the mold 10 to 15 times with the mallet.
4) Strike off to a flat surface using a float or trowel and begin initial curing.

6.2.2. Roding (Compaction)

Concrete is placed in the mold, in three layers of approximately equal volume. Each
layer is compacted with 25 strokes with the rounded end of the rod (as specified by
ASTM standards). The strokes are distributed uniformly over the cross section of the mold
and for each upper layer; the rod is allowed to penetrate through the layer being
rodded and into the layer below approximately 1 in. (25 mm).

Procedure – Making Cylinders – Rodding

1) For the standard 150 mm (6 in.) by 300 mm (12 in.) specimen, fill each mold
in three approximately equal layers, moving the scoop or trowel around the
perimeter of the mold to evenly distribute the concrete. For the 100 mm (4
in.) by 200 mm (8 in.) specimen, fill the mold in two layers. When filling the
final layer, slightly overfill the mold.
2) Consolidate each layer with 25 strokes of the appropriate tamping rod, using
the rounded end. Distribute strokes evenly over the cross section of the
concrete. Rod the first layer throughout its depth without forcibly hitting the
bottom. For subsequent layers, rod the layer throughout its depth
penetrating approximately 25 mm (1 in.) into the underlying layer.
3) After rodding each layer, tap the sides of each mold 10 to 15 times with the
mallet (reusable steel molds) or lightly with the open hand (single-use light-
gauge molds).
4) Strike off the surface of the molds with tamping rod or straightedge and
begin initial curing.
Note: Floating or troweling is permitted instead of striking off with rod or
straightedge.

Procedure – Making Flexural Beams – Rodding

1) Fill the mold in two approximately equal layers with the second layer slightly
overfilling the mold.
2) Consolidate each layer with the tamping rod once for every 1300 mm2 (2in2)
using the rounded end. Rod each layer throughout its depth, taking care to
not forcibly strike the bottom of the mold when compacting the first layer.
Rod the second layer throughout its depth, penetrating approximately 25
mm (1”) into the lower layer.
3) After rodding each layer, strike the mold 10 to 15 times with the mallet and
spade along the sides and end using a trowel.
4) Strike off to a flat surface using a float or trowel and begin initial curing.
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6.3. Curing
Curing means to cover the concrete with a layer of water, so it stays moist. By keeping
concrete moist, the bond between the paste and the aggregates gets stronger.
Concrete doesn't harden properly if it is left to dry out. Curing is done just after finishing
the concrete surface, as soon as it will not be damaged. The longer concrete is cured,
the closer it will be to its best possible strength and durability. Concrete that is cured
sufficiently is less likely to crack.

The specimens are removed from the molds 24 hours after casting. Specimens are
placed immediately in water after removal from the molds to prevent loss of moisture
from specimens.

Procedure – Initial Curing

▪ When moving cylinder specimens made with single use molds support the bottom
of the mold with trowel, hand, or other device.
▪ For initial curing of cylinders, there are two methods, use of which depends on the
agency. In both methods, the curing place must be firm, within ¼ in. of a level
surface, and free from vibrations or other disturbances.
▪ Maintain initial curing temperature of 16 to 27 C (60 to 80°F) or 20 to 26 C (68 to
78 F) for concrete with strength of 40 Mpa (6000 psi) or more.
▪ Prevent loss of moisture.

Method 1 – Initial cure in a temperature controlled chest-type curing box

1) Finish the cylinder using the tamping rod, straightedge, float, or trowel. The finished
surface shall be flat with no projections or depressions greater than 3.2 mm (1/8
in.).
2) Place the mold in the curing box. When lifting light-gauge molds be careful to
avoid distortion (support the bottom, avoid squeezing the sides).
3) Place the lid on the mold to prevent moisture loss.
4) Mark the necessary identification data on the cylinder mold and lid.

Method 2 – Initial cure by burying in earth or by using a curing box over the cylinder
▪ Note: This procedure may not be the preferred method of initial curing due to
problems in maintaining the required range of temperature.

1) Move the cylinder with excess concrete to the initial curing location.
2) Mark the necessary identification data on the cylinder mold and lid.Place the
cylinder on level sand or earth, or on a board, and pile sand or earth around the
cylinder to within 50 mm (2 in.) of the top.
3) Finish the cylinder using the tamping rod, straightedge, float, or trowel. Use a sawing
motion across the top of the mold. The finished surface shall be flat with no
projections or depressions greater than 3.2 mm (1/8 in.).
4) If required by the agency, place a cover plate on top of the cylinder and leave it
in place for the duration of the curing period, or place the lid on the mold to
prevent moisture loss.

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 Procedure – Transporting Specimens
▪ After 24 to 48 hours of initial curing, the specimens will be transported to the
laboratory for a final cure. Specimen identity will be noted along with the date and
time the specimen was made and the maximum and minimum temperatures
registered during the initial cure.
▪ While in transport, specimens shall be protected from jarring, extreme changes in
temperature, freezing, or moisture loss.
▪ Cylinders shall be secured so that the axis is vertical.
▪ Transportation time shall not exceed 4 hours.

Final Curing
▪ Upon receiving cylinders at the laboratory, remove the cylinder from the mold and
apply the appropriate identification.
▪ For all specimens (cylinders or beams), final curing must be started within 30 minutes
of mold removal. Temperature shall be maintained at 23 2 C (73
▪ ±3°F). Free moisture must be present on the surfaces of the specimens during the
entire curing period. Curing may be accomplished in a moist room or water tank
conforming to AASHTO M 201.
▪ For cylinders, during the final 3 hours prior to testing the temperature requirement
may be waived, but free moisture must be maintained on specimen surfaces at all
times until tested.
▪ Final curing of beams must include immersion in lime-saturated water for at least 20
hours prior to testing.

6.4. Cylinders Capping

Capping a concrete cylinder means placing a smooth uniform cap/layer at the end
of a concrete cylinder to provide for a uniform load distribution when testing. Since
the concrete sample will contain voids and aggregate particles at the upper surface
that is left open, it is necessary to prepare a smooth uniform surface for the testing
machine to press against.

Plaster of Paris (Gypsum) is used as capping material nowadays. Capping of all the
concrete cylinders is carried out carefully with the help of capping machine for
concrete cylinders, as shown in the figure.

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References/Additional Resources/Readings

1. Zhang, Haimei. 2011. Building materials in civil engineering. Woodhead Publishing Limited
and Science Press.
2. Concrete Manual. Retrieved from:
https://www.dot.state.mn.us/materials/manuals/concrete/Chapter1.pdf
3. What is concrete? https://www.youtube.com/watch?v=UOHURuAf5iY
4. How it works – Concrete: https://www.youtube.com/watch?v=ue0v3Ypl-0c
5. Concrete Sample Preparation: https://www.youtube.com/watch?v=ShIPt36TEQo
6. ASTM Standard Practice for Making and Curing Test Specimens:
https://www.youtube.com/watch?v=tFpxBLkjtfA

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