2 History
In its most general sense, a road is an open, generally
public way for the passage of vehicles, people, and animals. The earliest
human road builders predate recorded history by thousands of years. With
the advent of modern man, road building - the purposeful construction of general
public ways - became a common sign of an advancing civilization. Covering
these roads with a hard smooth surface (pavement) helped make them durable and
able to withstand traffic and the environment. Some of the oldest
paved roads still in existence were built by the Roman Empire.
By in large, Roman roads (see Figure 1.1) were constructed during the
Republican times - the oldest road, Via Appia, dates back to 312 B.C. (Amergence
Interactive, 2001). At its height, the Roman road network 
consisted of
over 100,000 km (62,000 miles) of roads, which is about equal to the length of
the U.S. interstate system. By law, all of the public
was entitled to use Roman roads, but the maintenance of the roadway was the
responsibility of the inhabitants of the district through which the road ran
(which, in general, is the way the U.S. views roads today). As the Roman Empire declined
and was split in two in 395 A.D., its road network declined as well.
However, the superior quality and structure of its pavements have allowed many
Roman roads to survive to this day.
A typical Roman road structure (see Figure 1.2), as seen in the United
Kingdom, consisted of four basic layers (Collins and Hart, 1936):
- Summa Crusta (surfacing). Smooth, polygonal blocks embedded in
the 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.
Figure 1.2: Roman Pavement
Structure Near Radstock, England
(after Collins and Hart, 1936)
As
can be seen, Roman pavements were quite thick - on the order of almost 0.9 m.
Roman road construction was not inexpensive. Updated construction
estimates of the Appian Way are about $2,000,000 per km (updated estimates
following Rose, 1935 and Leger, 1875).
The first insight into today's modern pavements can be seen in the pavements
of Thomas Telford (born 1757). Teleford served his apprenticeship as a building mason
(Smiles, 1904) and 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 (Smiles, 1904), thus turning his attention
more to roads. Telford attempted, where possible, to build roads on
relatively flat grades (no more than a 1 in 30 slope) in order to reduce the number of
horses needed to haul cargo. Telford's pavement section was about 350 to
450 mm (14 to 18 inches) in depth and generally specified three layers. The
bottom layer was comprised of large stones 100 mm (4 inches) wide and 75 to 180
mm (3 to 7 inches) in depth (Collins and Hart, 1936). It is this specific
layer which makes the Telford design unique (Baker, 1903). On top of this
were placed two layers of stones of 65 mm (2.5 inches) maximum size (about 150
to 250 mm (6 to 9 inches) total thickness) followed by a
wearing course of
gravel about 40 mm (1.6 inches) thick (see Figure 1.3). It was
estimated that this system would support a load corresponding to about 88 N/mm (500 lb
per in. of width).
Figure 1.3: Typical Telford Road
(after Collins and Hart, 1936)
Macadam pavements introduced the use of angular aggregates. John MacAdam (born 1756 and sometimes spelled "Macadam") observed that most of
the paved U.K. roads in early the 1800s were composed of rounded gravel (Smiles,
1904). 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 with a maximum size of 75 mm (3 inches)) in two layers for
a total depth of about 200 mm (8 inches) (Gillette, 1906). On top of this,
the wearing course was placed (about 50 mm thick with a maximum aggregate size
of 25 mm) (Collins and Hart, 1936). Macadam's reason for the 25 mm (1
inch) 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 (10 inches)
(refer to Figure 1.5). MacAdam was quoted as saying "no stone larger than
will enter a man's mouth should go into a road" (Gillette, 1906). The
largest permissible load for this type of design has been estimated to be 158 N/mm
(900 lb per in. width). In 1815, Macadam was appointed "surveyor-general"
of the Bristol roads and was then able to use his design on numerous projects.
It proved successful enough that the term "macadamized" became a term for this
type of pavement design and construction. The term "macadam" is also used
to indicate "broken stone" pavement (Baker, 1903). By 1850, about 2,200 km
(1,367 miles) 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
(Gillette, 1906). The increased use of these fines resulted in the more
traditional dense graded base materials. The first macadam
pavement in the U.S. was constructed in Maryland in 1823.
Figure 1.5: Typical Macadam Road
(after Collins and Hart, 1936)
Up through the time of Macadam
pavements, bituminous binders had not been used. Although Roman roads used
basic lime cements to hold their large stones together, roads of the late 1700s
and early 1800s did not use a binder material and usually relied on aggregate
interlock to provide cohesion. Bituminous binding materials and surface
layers began to show up in pavements in the early 1800s.
A tar macadam road consists of a
basic macadam road with a tar-bound surface. It appears that the first tar
macadam pavement was placed outside of Nottingham (Lincoln Road) in 1848
(Hubbard, 1910; Collins and Hart, 1936). At that time, such pavements were
considered suitable only for light traffic (i.e., 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 earlier efforts to
recycle waste materials into a pavement!
Soon after the Nottingham
project, tar macadam projects were built in Paris (1854) and Knoxville,
Tennessee (1866) (Hubbard, 1910). In 1871 Washington, D.C. extensively
used a "tar concrete"
for road construction. Sulfuric acid was used as a hardening agent and various
materials such as sawdust, ashes, etc. were used in the mixture (Hubbard,
1910). Over a seven-year period, 630,000 square meters (156 acres) 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 (Hubbard, 1910). 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
2.00 mm (No. 10) sieve. The wearing course was about 50 mm (2 inches) thick.
Hot tar paving products have not been used in the U.S. for many years.
As a side note, the term
"Tarmac"
was a proprietary product in the U.K. in the early 1900s (Hubbard, 1910).
Actually it was a plant mixed material, but was applied to the road surface
"cold." Tarmac consisted of crushed blast furnace slag coated with tar, pitch,
portland cement and a resin. Today the term "tarmac" is generic and
generally refers to airport pavements (however, inappropriately).
Road mixes, at the time often
known as "retread", "oil processed", "surface mix" or "mixed-in-place" roads,
refer to the mechanical mixing of asphalt and aggregate directly on the road bed
to form a thin 25 - 100 mm (1 - 4 inch) wearing course. Typically, the
construction process was as follows (Urquhart, 1934):
-
Place, grade and compact the
aggregate road bed.
-
Place the asphalt binder.
-
Mix the asphalt binder and
aggregate together using a tractor-pulled disk or harrow, windrow the mixed
material in the center of the road, turn it, then redistributed across the road
and smooth it.
-
Compact the resultant wearing
course until no movement is discernible under the roller wheels.
-
After a few weeks to several
months, spread a cover coat of fine aggregate over the surface and apply a seal
coat.
These pavements were not true
hot mix asphalt pavements because the asphalt was often applied as an
emulsion
and the mixing was done directly on the road.
The first pavements made from
true hot mix asphalt (HMA) were called sheet asphalt pavements. The HMA
layers in this pavement were premixed and laid hot. Baker (1903) describes
this pavement system as:
-
A wearing course 40 to 50 mm
(1.6 to 2 inches) thick composed of asphalt cement and sand.
-
A binder course about 40 mm
(1.6 inches) thick composed of broken stone and asphalt cement.
-
A base layer of hydraulic
cement concrete or pavement rubble (old granite blocks, bricks, etc.).
Generally, this layer was 100 mm (4 inches) thick for "light" traffic and
150 mm (6 inches) thick for "heavy" traffic (Baker, 1903).
Sheet asphalt became popular during the mid-1800s with the first
ones being built on the Palais Royal and on the Rue St. Honore in Paris in 1858
(Abraham, 1929). The first such pavement placed in the U.S. was in Newark,
New Jersey, in 1870. Sheet asphalt pavements are no longer built today.
The final steps towards modern
HMA were taken by Frederick J. Warren. In 1901 and 1903, Warren was issued patents for an early
HMA paving material and
process, which he called "bitulithic". A typical bitulithic mix contained
about 6 percent "bituminous cement" and graded aggregate proportioned for low
air voids. The concept was to produce a mix which could use a more "fluid"
binder than was used for sheet asphalt. Warren received eight patents in
1903. A review of the associated claims reveals that Warren, in effect,
patented HMA, the asphalt binder, the construction of HMA surfaced streets and
roads, and the overlay of "old" streets. A rather complete set of patents.
In 1910 in Topeka, Kansas, a court ruling found that
asphalt concrete mixes containing 12.5 mm (0.5 inch) maximum size aggregate did
not infringe on Warren's patent (Steele and Himmelman, 1986). Thus, HMA mixes
thereafter became oriented to the smaller maximum aggregate sizes. A typical
"Topeka mix" consisted of 30 percent graded crushed rock or gravel (all passing
12.5 mm (0.5 inch) sieve, about 58 to 62 percent sand (material passing 2.00 mm
(No. 10) and retained on 0.075 mm (No. 200) sieve) and 8 to 12 percent filler
(material passing 0.075 mm (No. 200) sieve). This mixture required 7.5 to 9.5
percent asphalt cement.
In 1910, Edwin C.
Wallace, a retired employee of Warren Brothers, invented Warrenite-Bitulithic. It consisted of
an
approximately 25 mm (1 inch) thick layer of "Finely divided mineral matter
coated with bitumen rolled into a lower layer of large stone, small stone, stone
dust and bitumen" (Wallace, 1910). This was basically a sheet asphalt
wearing course over hot, uncompacted bitulithic.
By adding the thin wearing course, the large aggregate of the Bitulithic mixes
were not exposed directly to heavy, steel rimmed wheels that could cracked the
aggregate and result in mix degradation. By 1920, Warren's
original patents had expired in the U.S. (Oglesby and Hewes, 1962) but the
legacy of the Topeka mix lived on as reflected by the U.S. tendency towards
finer mixes.
2.5 The Rise of Portland Cement Concrete (ACPA, 2001)
Although portland cement has been around since 1824 (when Joseph Aspdin, a
Leeds mason took out a patent on a hydraulic cement that he coined "Portland"
cement) it was not directly used in roadway pavements until the late 1800s.
2.5.1 The Original PCC Pavement
Portland cement concrete (PCC) was essentially
invented in 1824.
In 1889, George W. Bartholomew proposed building the first PCC pavement in
Bellefontaine, Ohio. Bartholomew was convinced that his "artificial stone"
(the term "concrete" had not come into use yet) was a suitable
substitute for the brick and cobblestone of the day. In order to convince the city
of Bellefontaine to allow him to build his PCC pavement, Bartholomew agreed to
donate all the materials and post a $5,000 bond guaranteeing the pavement's
performance for five years. In 1891, the first truly rigid pavement was mixed on
site and placed in 5 ft. square forms. In order to match the performance
and appearance of the standard cobblestone pavements of the day, Bartholomew
scored 100 mm (4 inch) squares into the PCC surface to give better footing for
horses (a practice continued to this day, although not for horses anymore).
By 1914, portland cement had been used to pave 2,348 miles of roadway.
2.5.2 Innovations in Performance
By the 1930s a number of states started using
de-icing salts to remove ice and snow from pavements. About the same time,
surface scaling developed on many pavements in northern climates. Research
by the Portland Cement Association (PCA) and several state highway departments
found that freeze and thaw cycles, accelerated by the use of de-icing salts,
were causing the problem. Further research lead to the development of
air entrained PCC that
was largely freeze-thaw resistant.
During the 1920's and 1930's, PCC pavement was
usually constructed directly on the underlying soil. This practice was
satisfactory until the 1930s when highway truck traffic increased to the point
where pumping
distresses began to appear on roadways carrying heavy truck traffic.
Research into this phenomenon resulted in recommendation of a non-pumping layer
called a subbase
(although now this layer is often referred to as the
base layer) be
placed under the PCC slabs. Gravel, crushed stone, and slag were commonly used
as subbase material. In the late 1940's, California began using
cement-treated granular bases
under concrete pavements. This practice quickly spread to other states.
2.5.3 Innovations in Construction
In 1946, two Iowa highway engineers, James W. Johnson
and Bert Myers, conceptualized the
slip form paver. In 1949, the Iowa Highway Department constructed the
first slipformed roadway, a 3 m (9 ft.) wide, 150 mm (6 inch) thick section of
county road. By placing two lanes side-by-side, a typical 6 m (18 ft.)
wide county road could be built. The paver attached to a ready mix
concrete truck, which would discharge its load into the paver, then pull the
paver forward. In 1955, Quad City Construction Company developed an
improved, self-propelled, track-mounted slipform paver capable of placing 8 m
(24 ft.) wide slabs up to 250 mm (10 inches) thick. In just a few years,
several equipment manufacturers were marketing slipform pavers capable of
placing concrete up to four lanes wide.
During the same period,
central mixing
replaced on-site mixing on most paving jobs. Evaluations by several
agencies showed that central-mixed concrete could be hauled from the mixer to
the slipform paver in
non-agitating dump
trucks with no loss in
workability or quality.
It was also during the late 1940's and early 1950's that paving contractors
began experimenting with
sawed concrete joints.
Previously, joints were formed in the plastic concrete with jointing tools.
These hand-formed joints often created a rough ride. After early attempts in
Kansas and California, sawing was used on several projects in 1951, and soon
became a standard construction method.
Road and pavement building
has often been used as a benchmark of a civilizations advancement. The
quality and strength of many of the ancient roads has helped them survive to
this very day. The Via Appia in Rome is now over 2,300 years old and is
still used today. As the use of slave labor declined, smaller more
economical roads, such as Telford and Macadam roads, began to arise.
Around the beginning of the 19th century, binding agents began to be used to
assist aggregate cohesion and improve the durability of roads. By the end
beginning of the 20th century, the two principal pavement types, flexible and
rigid, had taken on many of their modern qualities and were being built
throughout the U.S.
