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Facts
There are over 3.95 million public centerline road miles (8.21 million lane
miles1) in the U.S. and of this, 2.50 million
miles (or 63 percent) are paved (FHWA, 2001). About 70 percent of
Washington State roads are paved (WSDOT, 2001). This section contains
several quick pavement facts that serve as a good basic background for the rest
of the Guide.
Pavement Purpose
Typically, pavements are built for three main purposes:
- Load support. Pavement material is generally stiffer than the
material upon which it is placed, thus it assists the in situ material in
resisting loads without excessive deformation or cracking.
- Smoothness. Pavement material can be placed and maintained much
smoother than in situ material. This helps improve ride comfort and
reduce vehicle operating costs.
- Drainage. Pavement material and geometric design can effect quick
and efficient drainage thus eliminating moisture problems such as mud and ponding (puddles).
Pavement Categories
Pavements can be broken down into two broad categories:
- Flexible pavements. These are asphalt pavements (sometimes
called bituminous pavements), which may or may not incorporate underlying
layers of stabilized or unstabilized granular materials on a prepared
subgrade. These types of pavements
are called "flexible" since the total pavement structure bends (or
flexes) to accommodate traffic loads. Flexible pavements comprise about
93 percent of
U.S. paved roads (about 87 percent of Washington State roads).
- Rigid pavements. These are portland cement concrete
pavements, which may or may not incorporate underlying layers of stabilized or
unstabilized granular materials. Since PCC is quite stiff,
rigid pavements do not flex appreciably to accommodate traffic
loads. Rigid pavements comprise 7 percent of U.S. paved
roads (about 13 percent of Washington State roads).
Hot Mix Asphalt (HMA) Defined
Hot mix asphalt (HMA) is a bituminous concrete made principally from asphalt
binder and aggregate. It is distinguished from other bituminous products
by its constituent materials (asphalt and aggregate), mixture design methods and
elevated mixing temperature (thus the term "hot mix"). Although it is
known by many different names such as hot mix, asphalt concrete (AC or ACP),
asphalt, blacktop or bitumen, this Guide makes a conscious effort to
consistently refer to this material as HMA. Other types of asphalt-based
pavement surfaces discussed in this Guide such as
fog seals,
slurry seals and
BSTs
are not HMA but are nonetheless important to the pavement industry.
More information
Tar vs. Asphalt
Tar and
asphalt are two different materials. Tar, often called "coal tar" is a
byproduct of the destructive distillation of coal to form coke. Coal is of
plant origin and was formed in swamps similar to present-day peat bogs and in
lagoons, probably partly from plants growing in the area and partly from plant
material carried in by water and wind (The Columbia Electronic Encyclopedia,
1999). Asphalt is a petroleum residue left over from the distillation of
crude oil. Crude oil is the result of incompletely decayed ancient plant
and animal remains. The carbon chains within tar are, in general, slightly shorter than those in asphalt. In pavement applications, tar is
generally more brittle and less elastic than asphalt causing it to crack under
typical loading and environmental conditions. Tar will, however, not
dissolve as easily in the presence of lighter petroleum distillation products
such as gasoline and kerosene. While asphalt is used throughout the pavement
industry, tar is generally limited to sealant use.
Road Use
U.S. and State roads are being used at an ever increasing rate. Although
U.S. road centerline miles have only increased by about 10 percent since 1960,
U.S. registered vehicles have increased by almost 300 percent and vehicle miles
traveled have increased by more than 350 percent over that same time. In
sum, our road network, which has not significantly expanded since 1960, is now
carrying over 3.5 times the number of vehicles. Moreover, truck (the most damaging type of vehicle) vehicle miles traveled (VMT) is increasing at an even
faster rate than automobile VMT. A typical tractor-semi trailer combination
averages 100 - 200 miles/day in the U.S. for a total of
35,000 - 70,000 miles/year, which is substantially more than the typical passenger
vehicle (USDOT, 2000). Thus, pavement loading is growing at an even faster
rate than traffic.
Figure 1: Growth of Vehicle
Miles Traveled, Registered Vehicles and
Statute Miles of Roadway in the U.S. Since 1960 (from FHWA, 2000)
Note 1: A centerline mile is based on a
road's physical length regardless of the number of lanes. A lane mile is
based on the total length of lanes. For instance a 1 mile road with four
lanes would constitute 4 lane miles.
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