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1、毕业论文（外文翻译）（2012届）学院名称 土木与水利工程学院 专 业 （班 级） 土木工程七班 姓 名 （学 号） 李 小 润（20083650） 指 导 教 师 扈 惠 敏 系（教研室）负责人 方 诗 圣 PavementHighway pavements are divided into two main categories: rigitand flexible. The wearing surfaceof a rigid pavement is usually constructed of Portland cement concrete such that it acts like
2、 a beam over any irregularities in the underlying supporting material. The wearing surface of flexible pavements, 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 usu
3、ally 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 aggregatesTraffic loads are transferred by the wearing surface to the underlying supporting materials through the interlocking of ag
4、gregates, 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. High-type pavements have wearing surfaces that adequately support the expected traffic load withou
5、t visible distress due to fatigue and are not susceptible to weather conditions. Intermediate-type pavements have wearing surfaces that range from surface treated to those with qualities just below that of high-type pavements. Low-type pavements are used mainly for low-cost roads and have wearing su
6、rfaces that range from untreated to loose natural materials to surface-treated earth. The components of a flexible pavement include the subgradeor prepared roadbed, the subbase, basecourse, and the surface course (Fig.11.1). Upper surface courseMiddle surface course Lower surface course The performa
7、nce of the pavement depends on the satisfactory performance of each component, which requires proper evaluation of the properties of each component separately. The subgrade is usually the natural material located along the horizontal alignment of the pavement and serves as the foundation of the pave
8、ment structure. The subgrademay also consist of a layer of selected borrow materials, well compacted 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
9、being constructed.Located immediately above the subgrade, the subbase component 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 th
10、e quality of the subgrade material meets the requirements of the subbase material, the subbase component 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
11、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 uncru
12、shed. 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 sta
13、bilized with Portland cement, asphalt, or lime .In some cases, high-quality base course materials may also be treated with asphalt or Portland cement to improve the stiffness characteristics of heavy-duty pavementsThe surface course is the upper course of the road pavement and is constructed immedia
14、tely 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 skid-resistant driving surface, and pre
15、venting the penetration 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，2.54cm） , depending on the expected traffic on the pavement. It was shown that the quality of the surface course of a flexible pavement depends on
16、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 maintai
17、n than the flexible pavements. The Portland cement concrete commonly 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 divide
18、d 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.
19、However, steel tie bars are often used to provide a hingeeffect at longitudinal joints and to prevent the opening of these joints. Plain concrete pavements are used mainly on low-volume highways or when cement-stabilized soils are used as subbase. Joints are placed at relatively shorter distances (1
20、0 to 20 ft) than with the othertypes of concrete pavements to reduce the amount of cracking. In some case, the transverse joints of plain concrete pavements are skewed about 4 to 5 ft in plan, such that only one wheel of a vehicle passes through the joint at a time. This helps to provide a smoother
21、ride.Simply reinforced concrete pavements have dowels for the transfer of traffic loads across joints, with these joints spaced at larger distances, ranging from 30 to 100 ft. Temperature steel is used throughout the slab, with the amount dependent on the length of the slab. Tie bars are also common
22、ly used in longitudinal joints.h/2h/234cm510cm填缝料传力杆横向施工缝构造涂沥青h/2h/234cm510cm填缝料10cm拉杆平缝加拉杆型Continuously reinforced concrete pavements have no transverse joints, except construction joints or expansion joints when they are necessary at specific positions, such as at bridges. These pavements have a r
23、elatively high percentage of steel, with the minimum usually at 0.6 percent of the cross section of the slab. They also contain tie bars across the longitudinal joints.Bituminous Surface CoursesThe bituminous surface course has to provide resistance to the effects of repeated loading by tyres and to
24、 the effects of the environment. In addition, it must offer adequate skid resistance in wet weather as well as comfortable vehicle ride. It must also be resistant to rutting and to cracking. It is also desirable that surface course is impermeable, except in the case of porous asphalt.Hot rolled asph
25、alt (HRA) is a gapgraded material with less coarse aggregate. In fact it is essentially a bitumen/fine aggregate/filler mortar into which some coarse aggregate is placed. The mechanical propertiesare dominated by those of the mortar. This material has been extensively used as the wearing course on m
26、ajor road in the UK, though its use has recently declined as new materials have been introduced. It provides a durablelayer with good resistance to cracking and one which is relatively easy to compact. The coarse aggregate content is low (typically 30%) which results in the compacted mixture having
27、a smooth surface. Accordingly, the skid resistance is inadequate and precoated chippings are rolled into the surface at the time of laying to correct this deficiency.In Scotland, HRA wearing course remains the preferred wearing course on trunk roads including motorway but， since 1999 thin surfacings
28、 have been the preferred option in England and Wales. Since 1999 in Northern Ireland, HRA wearing course and thin surfacings are the preferred permitted options. Porous asphalt (PA) is a uniformly graded material which is designed to provide large air voids so that water can drain to the verges with
29、in the layer thickness. If the wearing course is to be effective, the basecourse below must be waterproof and the PA must have the ability to retain its open textured properties with time. Thick binder films are required to resist water damage and ageing of the binder. In use, this material minimize
30、s vehicle spray, provides a quiet ride and lower rolling resistance to traffic than dense mixtures. It is often specified for environmental reasons but stone mastic asphalt (SMA) and special thin surfacings are generally favoured in current UK practice. There have been high profile instances where a
31、 PA wearing course has failed early in its life. The Highways Agency does not recommend the use of a PA at traffic levels above 6000 commercial vehicles per day. Asphaltic concrete and dense bitumen macadam (DBM) are continuously graded mixtures similar in principle to the DBMs used in roadbases and
32、 basecourses but with smaller maximum particle sizes. Asphaltic concrete tends to have a slightlydenser grading and is used for road surfaces throughout the world with the excepting of the UK. It is more difficult to meet UK skid resistance Standards with DBMs than HRA, SMA or PA. This problem can b
33、e resolves by providing a separate surface treatment but doing so generally makes DBM economically unattractive. Stone mastic asphalt (SMA) material was pioneeredin Germany and Scandinavia and is now widely used in the UK. SMA has a coarse, aggregrate skeleton, like PA, but the voids are filled with
34、 a fine aggregate/filler /bitumen mortar. In mixtures using penetration 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
35、 fibres. SMA is a gap-graded material with good resistance to rutting and high durability. modified bitumen SBS SBR PEEVA 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 pro
36、vide 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 us
37、e 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 incorporat
38、e 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 all
39、ows 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 compared with respect to their mechanical properties and durability character
40、istics by reference to Fig.12.1. This shows, in principle, how low stone content HRA, asphaltic concrete, SMA and PA mixtures mobilize resistance to loading by traffic.Asphaltic concrete (Fig.12.1a) presents something of a compromise when well designed, since the dense aggregate grading can offer go
41、od 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 towa
42、rds good rutting resistance and away from good crack resistance.HRA (Fig.12.1b) 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 ru
43、tting.SMA and PA are shown in the same diagram ( Fig.c) 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.
44、 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.Drainage for Road and Airports Provision of adequate drainage is important facto
45、r 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
46、 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
47、 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
48、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.
