Natural asphalts range from a relatively soft material to a hard black material found in the veins of a rock formation. The two main sources for the soft material are Trinidad Lake asphalt from Pitch Lake (also known as Trinidad Lake) in Trinidad and Bermudez asphalt from Venezuela. The Bermudez deposit covers 1,100 acres and is estimated to contain about 6 million tons of asphalt. During the last twenty years of the nineteenth century, these natural asphalts were used to construct miles of streets and roads. In the 1880s, Washington, D.C., led the world in the number of miles of asphalt streets in the world.
Of the number of sources of hard asphalt, the most commercially viable today is the Gilsonite deposit in Utah. There are also some sources of rock asphalt, which is rock that has been impregnated with asphalt. The largest deposits of this material are kyRock, which was produced in Kentucky during the 1950s and 1960s, and the White Mines in southwestern Texas. Mining of these rock deposits is similar to that of on aggregate quarry. The processed rock asphalt has been used by many DOTs with a small amount of cutback or emulsified asphalts as a cold mix for maintenance patching.
Petroleum asphalt are produced by refining crude petroleum. The primary process used is distillation. The crud petroleum is heated in a large furnace to about 650˚F (345˚C) and is distilled. The lighter products (gasoline and naphtha) boil off at the lowest temperatures and the heavy gas oils at the higher temperatures. The heavy residual remaining after this process is completed is a residuum, which is further refined into the various grades of asphalt cement.
Three types of asphalt used in asphalt pavement construction are:
• Asphalt cements
•Emulsified asphalts
•Cutback asphalts
As previously discussed, asphalt cements are obtained from the distillation of crude petroleum. At room temperature, they are a semi-solid, very viscous material. At high temperatures, they are liquid. Asphalt cement is primarily used in the production of hot mix asphalt (HMA) for use in road construction. HMA is used as the surfacing material for approximately 96 percent of the road system in the United States. Several grades of asphalt cement are used, and they are primarily graded according to their consistency. The three grading systems in common use throughout the word for the purchase of asphalt cements are the penetration grading system, the viscosity (both original viscosity and aged residue), and the Superpave system.
In the last two decades, the use of modified asphalt binders has seen increased use. Modified asphalt binders may be produced in a number of ways including polymer and chemical modification, although polymer modification is the most prevalent.
Emulsified asphalts are a mixture of asphalt cement, water, and an emulsifying agent. Asphalt will not dissolve in water, so hot asphalt cement and water containing the emulsifying agent (such as soap) are passed under high pressure through a colloid mill to produce small droplets of asphalt cement, which are suspended in water. The emulsifying agent imparts an electrical charge on the surface of the droplets, which causes the droplets to repel one another. This prevents the globules from coalescing. Emulsified asphalts are liquid at room temperature. Emulsified asphalts are specified in ASTM D 977 and ASTM D 2397 (AASHTO M 140 and M2). They can be divided into two categories: anionic (electro-positively charged asphalt droplets). And cationic (electro-positively charged asphalt droplets). They are further categorized according to their setting rate-rapid, medium, and slow-setting grades.
Cutback asphalts are liquid asphalts that are manufactured by mixing asphalt cement with a petroleum solvent, such as gasoline or naphtha (which produces rapid-curing cutbacks), kerosene (which produces medium-curing asphalts), or diesel oil (which produces slow-curing asphalts). They are primarily used for maintenance in the manufacture of patching materials.
(Source: Asphalt handbook)