【文章內容簡介】 ny and Scandinavia and is now widely used in the UK. SMA has a coarse, aggregrate skeleton, like PA, but the voids are filled with a fine aggregate/ filler /bitumen mortar. ? In mixtures using peration grade bitumen , fibres are added to hold the bitumen within the mixture (to prevent “binder drainage”). Bitumen ? oil bitumen( earth oil) ? natural bitumen ? Tar Where a polymer modified bitumen is used, there is generally no need for fibres. SMA is a gapgraded material with good resistance to rutting and high durability. modified bitumen ? SBS ? SBR ? PE\EVA ? It differs from HRA in that the mortar is designed to just fill the voids in the coarse aggregate whereas, in HRA, coarse aggregate is introduced into the mortar and does not provide a continous stone matrix. The higher stone content HRAs ,however, are rather similar to SMA but are not wide used as wearing courses in the UK, being preferred for roadbase and basecourse construction. A variety of thin and what were called ultra thin surfacings (nowadays, the tendency is to use the term ?thin surfacings? for both thin and ultra thin surfacings ) have been introduced in recent years, principally as a result of development work concentrated in France. These materials vary in their detailed constituents but usually have an aggregate grading similar to SMA and often incorporate a polymer modified bitumen. They may be used over a high stiffness roadbase and basecourse or used for resurfacing of existing pavements. For heavy duty pavements (i .e those designed to have a useful life of forty years), the maintenance philosophy is one of minimum lane occupancy, which only allows time for replacement of the wearing course to these ?long life? pavement structures. The new generation of thin surfacings allows this to be conveniently achieved. The various generic mixture types described above can be pared with respect to their mechanical properties and durability characteristics by reference to . This shows, in principle, how low stone content HRA, asphaltic concrete, SMA and PA mixtures mobilize resistance to loading by traffic. Asphaltic concrete ()) presents something of a promise when well designed, since the dense aggregate grading can offer good resistance to the shear stresses which cause rutting, while an adequate binder content will provide reasonable resistance to the tensile stresses which cause cracking. In general, the role of the aggregate dominates. DBMs tend to have less dense gradings and properties which, therefore, tend towards good rutting resistance and —6— away from good crack resistance. HRA ()) offers particularly good resistance to cracking through the binder rich mortar between the coarse aggregate particles. This also provides good durability but the lack of coarse aggregate content inhibits resistance to rutting. SMA and PA are shown in the same diagram ( )) to emphasis the dominant role the coarse aggregate. In both case, well coated stone is used. In PA, the void space remains available for drainage of water, whilst in SMA, the space is occupied by a fine aggregate/ filler/ bitumen/ fibre mortar. Both materials offer good rutting resistance through the coarse aggregate content. The tensile strength of PA is low whilst that of SMA is probably adequate but little mechanical testing data have been reported to date. —7— Drainage for Road and Airports ? Provision of adequate drainage is important factor in the location and geometric design of road and airports. Drainage facilities on any highway, street and airport should adequately provide for the flow of water away from the surface of the pavement to properly designed channels. Inadequate drainage will eventually result in serious damage to the structure. ? In addition, traffic may be slowed by accumulated water on the pavement, and accidents may occur as a result of hydroplaning and loss of visibility from splash and spray. The importance of adequate drainage is recognized in the amount of highway construction dollars allocated to drainage facilities. About 25 percent of highway construction dollars are spent for erosion control and drainage structures, such as culverts, bridges, channels, and ditches. ? Highway Drainage Structures ? One of the main concerns of the highway engineer is to provide an adequate size structure, such that the waterway opening is sufficiently large to discharge the expected flow of water. Inadequately sized structures can result in water impounding, which may lead to failure of the adjacent sections of the highway due to embankments being submerged in water for long periods. ? The two general categories of drainage structures are major and minor. Major structures are those with clear spans greater than 20 feet, whereas minor structures are those with clear spans of 20 feet or less . ? Major structures are usually large bridges, although multiplespan culverts may also be included in this class. Minor structu