1、英文翻译 Flexible pavement design Generally speaking,pavements(and bases) may be divided into two broad classifications or tipes:rigid and flexible. As commonly used in the United States,the term “rigid pavement”is applied to wearing surfaces constructed of Portland-cement concrete. A pavement construct
2、ed of concrete is assumed to possess considerable flexural strength that will permit it to act as a beam and allow it to bridge over minor irregularities which may occor in the base or subgrade on which it rests；hence the term “rigid”.Similarity,a concrete base that supports a brick or block layer m
3、ight be described as “rigid”. All other types of pavement have traditionally been classed as “flexible”.A commonly used definition is that “a flexible pavement is a structure that maintains contact with and distributes loads to the subgrade and depends on aggregate interlock,particlefriction,and coh
4、esion for stability”.Thus,the classical flexible pavement include primarily those pavement that are composed of a series of granular layers topped by a relatively thin high-quality bituminous wearing surface .Typically,the highest-quatily materials are at or near the surface.It should be pointed out
5、 that certain pavementsthat have an asphalt surface may behave more like the classical “rigid”pavement,for example, pavement that have very thick asphalt surface or that have base courses composed of aggregate treated with asphalt,cement, or lime-fly ash. However,for convenience of presentation,thes
6、e pavements will be considered to be in the flexible class. The structure of flexible pavement is composed of a “wearing surface”, base, subbase(not always used), and subgrade . The wearing surface and the base often comprise two or more layers that are somewhat different in composition and that are
7、 put down in separate construction operations.On many heavy-duty pavements,asubbase of select material is often placed between the base and subgrade.the wearing surface may range in thickness from less than 1 in. in the case of a bituminous surface used for low-cost, light-traffic loads to 6 in. or
8、more of alphalt concrete used for heavily traveled routes. The wearing surface must be capable of withstanding the wear and abrasive effects of moving vehicles and must possess sufficient stability to prevent it from shoving and rutting under traffic loads. In addition,it serves a useful purpose in
9、preventing the entrance of excessive quantities of surface water into the base subgrade from directly above. The base is a layer (or layers) of very high stability and density. Its principle purpose is to distribute or “spread” the stresses created by wheel loads acting on the wearing surface so tha
10、t the stresses transmitted to the subgrade will not be sufficiently great to result in excessive deformation or displacement of that foundation layer. The base must also be of such character that it is not damaged by capillary water and/or frost action. Locally available materials are extensively us
11、ed for base construction, and materials preferred for this type of construction vary wwidely in different sections of the country. For example, the base may be composed of gravel or crushed rock or it may bae a granular material treated with asphalt,cement,or lime-fly ash stabilizing agents. Asubbas
12、e of granular material or stabilized material may be used in areas where frost action is severe, in locations where the subgrade soil is extremely weak. It may also be used , in the interests of economy ,in locations where suitable subbase material are cheap than base materials of higher quality. Th
13、e subgrade is the foundation layer, the structure that must eventually support all the loads which come onto the pavement. In some cases this layer will simply be the natural earth surface. In other or more usual instances it will be compacted soil existing in a cut section or the upper layer of an
14、embankment section. In the fundamental concept of the action of flexible pavement,the combined thickness of subbase (if used), base, and wearing surface must be great enough to reduce the stresses occuring in the subgrade to values that are not sufficiently great to cause excessive distortion or dis
15、placement of the subgrade soil layer. The principle factors entering into the problem of the thickness design of flexible pavement are: (1) Traffic loading. (2) Climate or environment. (3) Material characteristics. A number of other elements must also be considered in order to arrive at a final thic
16、kness design. This include cost, construction, maintenance,an design period. Thus, the students should realize that the design process is complex, and it is highly unlikely that any extremely simple method of approach will prove entirely successful under all conditions. Protection of the subgrade fr
17、om the loading imposed by traffic is one of the primariy functions of a pavement structure. The designer must privide a pavement that can withstand a large number of repeated applications of variable-magnitude loading. The magnitude of maximum loading is commonly controlled by legal load limits. Tra
18、ffic surveys and loadometer studies are often used to establish the relative magnitude and occurrence of the various loading to which a pavement is subjected. Prediction or estimation of the total traffic that will use a pavement during its design ife is a very difficult but obviously important task
19、. The climate or environment in which a flexible pavement is to be established has an important influence on the behavior and performance of the various in the pavement and subgrade. Probablly the two climate factors of major significance are temperature and moisture. The magnitude of temperature an
20、d its fluctuations affect the properties of certain materials. For example, high temperatures cause asphaltic concrete to lose stability whereas at low temperatures asphaltic concrete becomes very hard and stiff. Low temperature and temperature fluctuations are also associated with frost heave and f
21、reeze-thaw damage. Granular materials, if not properly graded, can experience frost heave. Likewise, the subgrade can exhibit extensive loss in strength if it becomes frozen. Certain stabilized materials (lime, cement, and lime-fly ash treated) can suffer substantial damage if a large number of freeze-thaw cycles occur in the material. Moisture also has an important influence on the behavior and performance of
