Structural Design of Pavements

CE 6409

Md. Minhajul Islam Khan

History of Pavement
The history of pavement engineering gives us an idea about the roads of ancient times. In this
section we will see in detail about Ancient roads, Roman roads, British roads, French roads etc.,
and then into the first 150 years of asphalt and Portland cement concrete pavement.

Ancient Roads
The most primitive mode of transport was by foot. These human pathways would have been
developed for specific purposes leading to camp sites, food, streams for drinking water etc.

The invention of wheel in Mesopotamian civilization led to the development of animal drawn
vehicles. To provide adequate strength to carry the wheels, the new ways tended to follow the
sunny drier side of a path. After the invention of wheel, animal drawn vehicles were developed
and the need for hard surface road emerged.

Traces of such hard roads were obtained from various ancient civilization dated as old as 3500
BC. The earliest authentic record of road was found from Assyrian empire constructed about
1900 BC.

Roman roads
Roads in Rome were constructed in a large scale and it radiated in many directions helping them
in military operations. Thus they are considered to be pioneers in road construction.

It is suggested that the Romans took up the practice of a military road system from the
Carthaginians. It is estimated that the Romans built about 87,000 km of roads within their empire
(about equal to the length of the U.S. Interstate system). Apparently, there is no record of
“traditional” roads in the U.K. prior to the Romans.

The oldest Roman road still in use today, Via Appia (Figure 1), dates back to 312 B.C.
 Figure 1

Since the primary purpose of these roads was for foot soldiers, the roads were straight, but
virtually without regard to grade. They generated high noise levels, were rough and labor
intensive.

The Roman roads design generally consisted of four layers (top to bottom) as follows:
 Summa Crusta (surfacing): Smooth, polygonal blocks bedded in underlying layer.
 Nucleus: A kind of base layer composed of gravel and sand with lime cement.
 Rudus: The third layer was composed of rubble masonry and smaller stones also set in
lime mortar.
 Statumen: Two or three courses of flat stones set in lime mortar.

The total thickness was as much as 0.9 m and road widths of 4.3 m or less.

An illustration of Roman pavement structure near Radstock, England, is shown as Figure 2.

Figure 2
Roman road construction was not inexpensive. Updated construction estimates of the Appian
Way in Italy are about $2,000,000 per km.

French roads
The significant contributions were given by Tresaguet in 1764 and a typical cross section of this
road is given in Figure 3. He developed a cheaper method of construction than the lavish and
locally unsuccessful revival of Roman practice.

The pavement used 200 mm pieces of quarried stone of a more compact form and shaped such
that they had at least one side which was placed on a compact formation.

Smaller pieces of broken stones were then compacted into the spaces between larger stones to
provide a level surface. Finally the running layer was made with a layer of 25 mm sized broken
stone.

All this structure was placed in a trench in order to keep the running surface level with the
surrounding country side. This created major drainage problems which were counteracted by
making the surface as impervious as possible, cambering the surface and providing deep side
ditches.

Figure 3

British roads
The British government also gave importance to road construction.

Telford
Thomas Telford (born 1757) served his apprenticeship as a building mason. Because of this, he
extended his masonry knowledge to bridge building. During lean times, he carved grave-stones
and other ornamental work (about 1780). Eventually, Telford became the “Surveyor of Public
Works” for the county of Salop, thus turning his attention more to roads.
Telford attempted, where possible, to build roads on relatively flat grades (no more than 1 in 30)
in order to reduce the number of horses needed to haul cargo.
Further, the pavement section was about 350 to 450 mm in depth and generally specified in three
layers.

The bottom layer was comprised of large stones (100 mm wide and 75 to 180 mm in depth). It is
this specific layer which makes the Telford design unique.

On top of this were placed two layers of stones of 65 mm maximum size (about 150 to 250 mm
total thickness) followed by a wearing course of gravel about 40 mm thick (Figure 4).

It was estimated that this system would support a load corresponding to 88 N/mm (500 lb per in.
of width).

Figure 4

Macadam
The British engineer John Macadam introduced what can be considered as the first scientific
road construction method. John Macadam (born 1756) observed that most of the “paved” U.K.
roads in early 1800s were composed of rounded gravel.

He knew that angular aggregate over a well-compacted subgrade would perform substantially
better.

He used a sloped subgrade surface to improve drainage (unlike Telford who used a flat subgrade
surface) on which he placed angular aggregate (hand-broken, maximum size 75 mm) in two
layers for a total depth of about 200 mm.

On top of this, the wearing course was placed (about 50 mm thick with a maximum aggregate
size of 25 mm). Macadam’s reason for the 25 mm maximum aggregate size was to provide a
“smooth” ride for wagon wheels. Thus, the total depth of a typical Macadam pavement was
about 250 mm (Figure 5. An interesting quote attributed to Macadam about allowable maximum
aggregate sizes was that “no stone larger than will enter a man’s mouth should go into a road”.
The largest permissible load for this type of design was estimated to be 158 N/mm (900 lb per in.
width).

Figure 5

By 1850, about 2,200 km of macadam type pavements were in use in the urban areas of the UK.
Macadam realized that the layers of broken stone would eventually become “bound” together by
fines generated by traffic.

With the introduction of the rock crusher, large mounds of stone dust and screenings were
generated. This resulted in use of such fines resulting in the more traditional dense graded base
materials which in turn produced pavement thicknesses as thin as 100 to 150 mm. The first
macadam pavement in the U.S. was constructed in Maryland in 1823.
Early Thickness Trends

Figure 6

Thus, we have seen pavement structures decrease from about 0.9 m (3 feet) for Roman designs to
350 to 450 mm for Telford designs, to about 250 mm for Macadam designs, to 100 mm at about
the turn of the century (Figure 6).

Up to the early 1900s, the design emphasis was placed on the use of fixed standards occasionally
modified for local soil conditions. Further, the need for more durable pavements was mandated
by the changing vehicle fleet. The following partial quote by L. W. Page, Director of U. S.
Office of Public Roads (contained in a 1907 report) illustrates the problem: “The existence of
our macadam roads depends upon the retention of the road-dust formed by the wearing of the
surface. But the action of rubber-tire motor-cars moving at high speed soon strips the macadam
road of all fine material, the result being that the road soon disintegrates”.

Early Bituminous Pavement

It appears that the first tar macadam pavement was placed outside of Nottingham (Lincoln Road)
in 1848.

At that time, such pavements were considered suitable only for light traffic (not for urban
streets). Coal tar (the binder) had been available in the U.K. from about 1800 as a residue from
coal-gas lighting. Possibly this was one of the earliest efforts to recycle waste materials into a
pavement.

After the Nottingham project, tar macadam projects were built in Paris (1854) and Knoxville,
Tennessee (1866).
In 1871 in Washington, D.C., a “tar concrete” was extensively used. Sulfuric acid was used as a
hardening agent and various materials such as sawdust, ashes, etc. were used in the mixture.

Over a seven-year period, 630,000 m2 were placed. In part, due to lack of attention in specifying
the tar, most of these streets failed within a few years of construction. This resulted in tar being
discredited, thereby boosting the asphalt industry.

However, some of these tar-bound surface courses in Washington, D.C., survived substantially
longer, about 30 years. For these mixes, the tar binder constituted about 6 percent by weight of
the total mix (air voids of about 17 percent). Further, the aggregate was crushed with about 20
percent passing the No. 10 sieve. The wearing course was about 50 mm thick.

Sheet Asphalt
Sheet asphalt placed on a concrete base (foundation) became popular during the mid-1800s with
the first such pavement of this type being built in Paris in 1858. The first such pavement placed
in the U.S. was in Newark, New Jersey, in 1870.

Baker describes this pavement system as:

(1) A wearing course 40 to 50 mm thick composed of asphalt cement and sand

Bitulithic Pavements
In 1901 and 1903, Frederick J. Warren was issued patents for the early “hot mix” paving
materials.

A typical mix contained about 6 percent “bituminous cement” and graded aggregate
proportioned for low air voids.

Essentially, the maximum aggregate size was 75 mm ranging down to dust. The concept was to
produce a mix which could use a more “fluid” binder than used for sheet asphalt.

This material became known as “Bitulithic.” More specifically, Warren was issued eight U.S.
patents in 1903.
Early Portland Cement Concrete Pavements
Portland cement concrete (PCC) was not used as a pavement wearing course much until after
about 1910 ; however, it was regularly used as a “stiff” base to support other wearing courses
such as wooden blocks, bricks, cobble stones, etc.

One likely reason for this was the lack of a consistent specification for the early cements. Tillson
in 1900 summarized over 109 separate specifications on Portland cement fineness.

PCC hand mixing was still common in 1900 which undoubtedly restricted productivity and
accurate proportioning. By 1900 (as reported by Tillson), it was common to volumetrically
proportion PCC as a 1:2:4 or 1:2:5 (cement : sand : coarse aggregate).

PCC was first used as a base for other wearing courses in London in 1872 and in New York in
1888 (base for stone surfacing).

The first use of PCC as a wearing course was in Edinburgh, U.K., in 1872 and Grenoble, France,
in 1876; however, one source stated that the first PCC pavement was placed in Inverness,
Scotland, in 1865.

The first PCC pavement in the U.S. was constructed in 1891 in Bellefontaine, Ohio. This
pavement was only 3.0 m wide and 67.1 m long (probably what we would call a “test section”
today). In 1909, in Wayne County, Michigan, a PCC highway system was constructed.

References
1. https://pavementinteractive.org
2. https://www.asphaltwa.com