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ASPHALT CEMENT
Asphalt is produced in a variety of types and grades ranging from hard-brittle solids to near water-thin liquids. The semi-solid form known as asphalt cement is the basic material used in Asphalt Concrete pavements. Liquid asphalt is produced when asphalt cement is blended or “cut back” with petroleum distillates or emulsified with water and an emulsifying agent. Liquid asphalt products may be produced for various uses and applications.
ASPHALT CONCRETE
Asphalt Concrete is known by many different names: hot mix asphalt, plant mix, bituminous mix, bituminous concrete, and many others. It is a combination of two primary ingredients - aggregates and asphalt cement. The aggregates total 90 to 95 percent of the total mixture by weight. They are mixed with 5 to 10 percent asphalt cement to form Asphalt Concrete.
The aggregates and asphalt are combined in an efficient manufacturing plant capable of producing specified materials. Plant equipment includes: cold bins for storage of graded aggregate; a dryer for drying and heating aggregates to the required mixing temperature; a pug mill for combining the graded, heated aggregate and liquid asphalt cement according to specified mix formulas; and tanks for storing the liquid asphalt.
Asphalt Concrete is transported by truck to the paving site where it is spread to a uniform thickness with a mechanical paving or finishing machine. Then the material is compacted to the required degree by heavy, self-propelled rollers, producing a smooth, well-compacted pavement course.
The paving or finishing machine places the Asphalt Concrete at temperatures above 225 F. The material should be compacted before the mix temperature falls below 175 F to achieve adequate density.
COLD MIX ASPHALT CONCRETE
Cold mix Asphalt Concrete, or cold placed mixture, is generally a mix made with emulsified or cutback asphalt. Emulsified asphalts may be anionic or cationic MS or SS grades. Aggregate material may be anything from a dense-graded crushed aggregate to a granular soil having a relatively high percentage of dust. At the time of mixing, the aggregate may either be damp, air-dried, or artificially heated and dried. Mixing methods may be performed either in the roadway, on the side of the roadway, or in a stationary mixing facility.
The resultant mixtures usually are spread and compacted at atmospheric temperatures. Cold mix asphalt may be used for surface, base, or sub base courses if the pavement is properly designed.
Cold mix surface courses are suitable for light and medium traffic; however, they normally require a seal coat or hot Asphalt Concrete overlay as surface protection. When used in the base or sub base, they may be suitable for all types of traffic.
Bituminous Treated Aggregate Base
Bituminous treated aggregate base is one type of cold mix Asphalt Concrete. It can consist of processing gravels; crushed stones; or blends of gravel, sand, and crushed stone materials each stabilized with a specified percentage of asphalt.
Job mix formulas are not required. These mixtures are placed as a base course and stabilized-shoulder surfacing, although other uses may be assigned by special design. All designs should provide for a seal coat or surface course to provide protection from traffic abrasion and weathering.
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