There are over 4.05 million public centerline road miles (8.52 million lane miles) in the U.S. and of this, 2.50 million miles (or 63 percent) are paved (FHWA 2009). About 70 percent of Washington State roads are paved.
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 place material (called “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).
Pavements can be broken down into three broad categories:
- Unpaved roads. These are roads that have no paved surface on them (e.g., dirt roads, gravel roads). Unpaved roads comprise about 33% of all roads.
- 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 95 percent of U.S. paved roads. This equates to about 63% of all roads (paved and unpaved).
- 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 5 percent of U.S. paved roads. This equates to about 4% of all roads (paved and unpaved).
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.
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. 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.
U.S. and State roads are being used at an ever increasing rate. Although U.S. road centerline miles have only increased by about 13 percent from 1960 to 2008, U.S. registered vehicles have increased by almost 300 percent and vehicle miles traveled have increased by more than 400 percent over that same time. In sum, our road network, which has not significantly expanded since 1960, is now carrying over 4 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.
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.