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Bituminous mixes (some times called asphalt mixes) are used in the surface layer of road and airfield pavements. The mix is composed usually of aggregate and asphalt cements. Some types of bituminous mixes are also used in base coarse. The design of asphalt paving mix, as with the design of other engineering materials is largely a matter of selecting and proportioning constituent materials to obtain the desired properties in the finished pavement structure.
Fig: Asphalt Mixes for Pavements
The desirable properties of Asphalt mixes are:
1. Resistance to permanent deformation: The mix should not distort or be displaced when subjected to traffic loads. The resistance to permanent deformation is more important at high temperatures.
2. Fatigue resistance: the mix should not crack when subjected to repeated loads over a period of time.
3. Resistance to low temperature cracking.This mix property is important in cold regions.
4. Durability:the mix should contain sufficient asphalt cement to ensure an adequate film thickness around the aggregate particles. The compacted mix should not have very high air voids, which accelerates the aging process.
5. Resistance to moisture-induced damage.
6. Skid resistance.
7. Workability:the mix must be capable of being placed and compacted with reasonable effort.
8. Low noise and good drainage properties:If the mix is to be used for the surface (wearing) layer of the pavement structure.
Marshall stability and Hveem stabilometer tests are largely used for the routine testing. Criteria for the suitable mix design have been specified by the Asphalt Institute.