| |
Asphalt Pavement History
Hot mix asphalt (HMA) pavements have existed in their present form, as a mixture of angular
aggregates and
asphalt binder, since the beginning of
the 20th century. However, HMA pavement can trace its roots
back to ancient Roman roads and beyond.
The first recorded use of asphalt by humans was by the Sumerians around 3,000
B.C. Statues from that time period used asphalt as a binding substance for
inlaying various shells, precious stones and pearls. Other common ancient
asphalt uses were preservation (for mummies), waterproofing (pitch on ship
hulls), and cementing (used to join together bricks in Babylonia). Around
1500 A.D., the Incas of Peru were using a composition similar to modern
bituminous macadam to pave parts of their highway system. In fact, asphalt
is mentioned several times in the Book of Genesis (Baird, 2002).
In more modern times, asphalt paving uses first began with foot paths in the
1830s and then progressed to actual asphalt roadways in the 1850s. The
first asphalt roadways in the U.S. appeared in the early 1870s (Abraham, 1929).
The oldest Roman road still in use today, Via Appia (Figure 1), dates back to
312 B.C. At its height, the Roman road network consisted of
over 62,000 miles of roads. 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
(the same basic system used in the U.S. today). Although Roman roads
did not use asphalt as a binder, they did often use lime grout and other natural pozzolans as binders. Figure 2 shows a typical Roman road structure.
 |
 |
| Figure 1: Roman Road Surface |
Figure 2: Roman Road Structure
(after Collins and Hart, 1936) |
Skipping forward several thousand years, Telford
pavements begin to show likeness to today's modern HMA pavements.
Thomas Telford (born 1757) served his apprenticeship as a building mason
(Smiles, 1904). 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 (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 14 to 18 inches in depth as shown in Figure 3. Telford pavements did not use any binding medium to hold the stones
together.
Figure 3: Typical Telford Road
(after Collins and Hart, 1936)
Macadam pavements introduced the use of angular aggregates (see Figure 4). John McAdam
(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) onto
which he placed angular aggregate (hand-broken, maximum size 3 inches)
in two layers for a total depth of about 8 inches (Gillette, 1906).
On top of this, the
wearing course was placed (about 2 inches thick with a maximum aggregate size of
1 inch) (Collins and Hart, 1936). Macadam, who did not use any binding medium
to hold the stones together, realized that the layers of broken stone would eventually
become bound together by
fines generated by traffic. The first macadam
pavement in the U.S. was constructed in Maryland in 1823.
Figure 5: Typical Macadam Road
(after Collins and Hart, 1936)
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!
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).
Sheet asphalt placed on a
concrete base (foundation) became popular during the mid-1800s with the first
one 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. Generally, the concrete layer
was 4 inches thick for "light" traffic and 6 inches thick for
"heavy" traffic (Baker, 1903). The final thickness was based on the weight of
the traffic, the strength of the concrete and the soil support.
HMA pavement began to take
on its modern form around the beginning of the 20th century when Frederick J.
Warren was issued patents for a "hot mix" asphalt 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.
In 1910 in Topeka, Kansas, a court ruling stated that
HMA mixes containing 0.5 inch
maximum size aggregate did
not infringe on Warren's patent (Steele and Himmelman, 1986). Thus, most
U.S. hot mix asphalt (HMA) thereafter became oriented to the smaller maximum aggregate sizes. A typical
"Topeka mix" consisted of 30 percent graded crushed rock or gravel (all passing
the 0.5 inch sieve), about 58 to 62 percent sand (material passing the No. 10
sieve and retained on the No. 200 sieve), 8 to 12 percent filler
(material passing the No. 200 sieve). This mixture required 7.5 to 9.5
percent asphalt cement. 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.
|
|
