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. Nearly 94% of the paved roads in the U.S. are paved with asphalt surfacing (Asphalt Institute 2014), which is a testament to the enduring value of asphalt roads. An extensive list of AsphaltFacts can be found at the Asphalt.Facts website.
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). An important and growing “drainage” application is the use of porous asphalt pavements to help with surface water treatment and to mimic natural stormwater infiltration without extensive buried drainage systems and/ or detention ponds.
Asphalt pavements are also used in a variety speciality applications, such as environmental liners or landfill/ hazardous waste site “caps”.
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 paving grade asphalt binder and crushed 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. Paving Asphalt vs. Roofing Asphalt
Tar, paving grade asphalt and roofing asphalt are entirely different and unique materials. Tar, often called “coal tar”, is a by-product 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. Coal tar use is restricted and regulated in its applications due to health concerns that have arisen over the last 20 years. On roadways or other paved surfaces, tar was generally limited to sealant use or specialty applications in the past but it has now been almost entirely displaced by specially formulated asphalt.
Asphalt, in contrast to tar, 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. In pavement applications, tar was never widely used as it is more brittle and less elastic than asphalt, causing it to crack under typical loading and environmental conditions. Paving grade asphalt has been used since the early 1900’s throughout the world for roadway surfacing and it has been proven to be an inert, safe product in roadway paving applications.
Paving grade asphalt is refined specifically for roadway paving applications and is generally never heated about about 350 degrees F. Roofing grade asphalt is a much “harder” product and it must be applied at much higher temperatures. Roofing grade asphalt is never used in roadway paving applications except in very minor amounts when roofing shingles are recycled.
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.