【正文】 , on the other hand, is usually constructed of bituminous material such that they remain in contact with the underlying material even when minor irregularities occur. Flexible pavements usually consist of a bituminous surface underlaid with a layer of granular material and a layer of a suitable mixture of coarse and fine materials. Coarse aggregates Fine aggregates Traffic loads are transferred by the wearing surface to the underlying supporting materials through the interlocking of aggregates, the frictionaleffect of the granular materials, and the cohesion of the fine materials. Flexible pavements are further divided into three subgroups: high type, intermediate type, and low type. Hightype pavements have wearing surfaces that adequately support the expected traffic load without visible distress due to fatigue and are not susceptible to weather conditions. Intermediatetype pavements have wearing surfaces that range from surface treated to those with qualities just below that of hightype pavements. Lowtype pavements are used mainly for lowcost roads and have wearing surfaces that range from untreated to loose natural materials to surfacetreated earth. ? The ponents of a flexible pavement include the subgradeor prepared roadbed, the subbase, basecourse, and the surface course (). ? Upper surface course Middle surface course Lower surface course The performance of the pavement depends on the satisfactory performance of each ponent, which requires proper evaluation of the properties of each ponent separately. ? The subgrade is usually the natural material located along the horizontal alignment of the pavement and serves as the foundation of the pavement structure. ? The subgrademay also consist of a layer of selected borrow materials, well pacted to prescribedspecifications. ? Compacting plant Compaction device Compactness It may be necessary to treat the subgrade material to achieve certain strength properties required for the type of pavement being constructed. —2— Located immediately above the subgrade, the subbase ponent consists of a superior quality to that which generally is used for subgrade construction. The requirements for subbase materials are usually given in terms of the gradation, plastic characteristics, and strength. When the quality of the subgrade material meets the requirements of the subbase material, the subbase ponent may be omitted. In cases where suitable subbase material is not readily available ,the available material can be treated with other materials to achieve the necessary properties. This process of treating soils to improve their engineering properties is know as stabilization. ? The base course lies immediately above the subbase. It is placed immediately above the subgrade if a subbase course is not used. ? This course usually consists of granular materials such as crushed stone, crushed or uncrushed. The specifications for base course materials usually include stricter requirements than those for subbase materials, particularly with respect to their plasticity, gradation, and strength. Materials that do not have the required properties can be used as base materials if they are properly stabilized with Portland cement, asphalt, or lime . In some cases, highquality base course materials may also be treated with asphalt or Portland cement to improve the stiffness characteristics of heavyduty pavementsThe surface course is the upper course of the road pavement and is constructed immediately above the base course. The surface course in flexible pavement usually consists of a mixture of mineral aggregates and asphaltic materials. It should be capable of withstanding high tire pressures, resisting the abrasive forces due to traffic, providing a skidresistant driving surface, and preventing the peration of surface water into the underlying layers. ? The thickness of the wearing surface can vary from 3 in. to more than 6 in.（ inch，英寸， ） , depending on the expected traffic on the pavement. It was shown that the quality of the surface course of a flexible pavement depends on the mix design of the asphalt concrete used. ? Rigid highway pavements usually are constructed to carry heavy traffic loads, although they have been used for residential and local roads. Properly designed and constructed rigid pavements have long service lives and usually are less expensive to maintain than the flexible pavements. ? The Portland cement concrete monly used for rigid pavements consists of Portland cement, coarse aggregate, fine aggregate, and water. Steel reinforcing rods may or may not be used, depending on the type of pavement being constructed. Rigid highway pavements be divided into three general type: plain concrete pavements, simply reinforced concrete pavements, and continuously reinforced concrete pavement. The definition of each pavement type is related to the amount of reinforcement used. Plain concrete pavement has no temperature steel or dowels for load transfer. —3— However, steel tie bars are often used to provi