中英文资料texture is desired, this may not be available in some localities. Dense mixtures are obtained by using aggregates that are either naturally or artificially “well graded”. This means that the fine aggregate serves to fill the voids in the coarser aggregates. In addition to affecting density and therefore strength characteristics, the grading also influences workability. When an excess of co

arse aggregate is used, the mix becomes harsh and hard to work. When an excess of mineral filler is used, the mixes become gummy and difficult to manage. The asphalt cement in the flexible pavement is used to bind the aggregate particles together and to waterproof the pavements. Obtaining the proper asphalt content is extremely important and bears a significant influence on all the items marking a successful pavement. A chief objective of all the design methods which have been developed is to arrive at the best asphalt content for a particular combination of aggregates. 3 . Mix design principles Certain fundamental principles underlie the design procedures that have been developed. Before these procedures can be properly studied or applied, some consideration of these principles is necessary. Asphalt pavements are composed of aggregates, asphalt cement, and voids. Considering the aggregate alone, all the space between particles is void space. The volume of aggregate voids depends on grading and can vary widely. When the asphalt cement is added, a portion of these aggregate voids is filled and a final air-void volume is retained. The retention of this air-void volume is very important to the characteristics of the mixture. The term air-void volume is used, since these voids are weightless and are usually expressed as a percentage of the total volume of the compacted mixture. An asphalt pavement carries the applied load by particle friction and interlock. If the particles are pushed apart for any reason , then the pavement stability is destroyed. This factor indicates that certainly no more asphalt should be added than the aggregate voids can readily hold. However ,asphalt cement is susceptible to volume change and the pavement is subject to further compaction under use. If the pavement has no air voids when placed, or if it loses them under traffic, then the expanding asphalt will overflow in a condition known as bleeding. The loss of asphalt cement through bleeding weakens the pavement and also reduces surface friction, making the roadway hazardous.第 4 页 共 12 页