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Asphalt-Concrete Pavement Mixtures
Asphalt occurs naturally in crude oil deposits as an organic material in a liquid or a semi-solid form. It consists of bitumen, a black substance composed of polycyclic aromatic hydrocarbons; and it is highly viscous and sticky. Engineers recognize three categories of mixtures --- dense-graded, open-graded and gap-graded. The aggregate gradation sizes determine the composition of the mixtures. In addition to hot mix asphalt, other classifications include warm mix asphalt concrete (WMA), cold mix, cut-back, mastic and natural asphalt concrete.
Hot Mix Asphalt
At the asphalt plant, HMA is mixed at temperatures around 300 degrees F, and highway workers must perform all the paving and compaction operations while the asphalt is still pliable and hot enough for manipulation. Laying pavement is often done during the hotter months of the year but not during the winter when cold temperatures cool it down too fast for optimal compaction to drive out excess air from the voids in the mix.
Other Asphalt Mixes
Warm mix asphalt (WMA) offers several benefits to its use which include reduced fuel requirements for production, a long workability time for paving and compaction, the acceptance of recycled asphalt pavement into its composition and improved durability. Aggregate gradation can be designed into the mix to adapt each mix to the particular environmental conditions it will meet on-site. Cold mix refers to an emulsion of asphalt that is less viscous and easy to manipulate. Primarily it is used for patching pot-holes. Cut-back contains kerosene mixed with the aggregate that makes it very easy to manipulate and compact. It hardens as the kerosene evaporates. Mastic asphalt concrete is laid down in sheets after it has cooked or matured for six to eight hours, and is often used as a roofing material or footpath applications which do not receive heavy traffic.
Flexible Pavement
A flexible pavement is so named because it yields elastically to traffic loads. It must be capable of absorbing some fatigue stress. Its structure is a layered pavement with a thin surface of HMA over several aggregate base courses. Each layered course is increasingly strong in order to distribute the load and protect those layers beneath it from shearing forces. The top level surface offers protection from erosion, heat and traffic wear.