外文翻譯--公路和機(jī)場路面設(shè)計-其他專業(yè)-全文預(yù)覽
【正文】 thickness for the pavement structure at relatively low cost. On a weak subgrade, it also serves as a useful working platform for constructing the base course. subbase course may be omitted if the subgrade soil satisfies the requirements specified for subbase material. Prepared Subgrade Most natural soils forming the roadbed for pavement construction require some form of preparation or treatment. The top layer of a specified depth is usually pacted to achieve a desired density. The depth of paction and the pacted density required depend on the type of soil and magnitudes of wheel loads and tire pressures. For highway construction, paction to 100% modified AASHTO density covering a thickness of 12 in. (300 mm) below the formation level is monly done. Compaction depth of up to 24 in. (600 mm) may be required for heavily trafficked pavements. For example, in the case of cohesive subgrade, the Asphalt Institute [1991] 5 requires a minimum of 95% of AASHTO T180 (Method D) density for the top 12 in. (300 mm) and a minimum of 90% for all fill areas below the top 12 in. (300 mm). For cohesionless subgrade, the corresponding paction requirements are 100 and 95%, respectively. Due to the higher wheel loads and tire pressures of aircraft, many stringent paction requirements are found in airport pavement construction. In some instances it may be economical to treat or stabilize poor subgrade materials and reduce the total required pavement thickness. Portland cement, lime, and bitumen have all been used successfully for this purpose. The choice of the method of stabilization depends on the soil properties, improvement expected, and cost of construction. Rigid Pavement Rigid pavements constructed of portland cement concrete are mostly found in heavytraffic highways and airport pavements. To allow for expansion, contraction, warping, or breaks in construction of the concrete slabs, joints are provided in concrete pavements. The joint spacing, which determines the length of individual slab panels, depends on the use of steel reinforcements in the slab. The jointed plain concrete pavemen (JPCP), requiring no steel reinforcements and thus the least expensive to construct, is a popular form of construction. Depending on the thickness of the slab, typical joint spacings for plain concrete pavements are between 10 and 20 ft (3 and 6 m). For slabs with joint spacing greater than 6 m, steel reinforcements have to be provided for crack control, giving rise to the use of jointed reinforced concrete pavements (JRCP) and continuously reinforced concrete pavements (CRCP). Continuously reinforced concrete pavements usually contain higher than % steel reinforcement to eliminate the need to provide joints other than construction and expansion joints. The base course for rigid pavement, sometimes called subbase, is often provided to prevent pumping (ejection of foundation material through cracks or joints resulting from vertical movement of slabs under traffic). The base course material must provide good drainage and be resistant to the erosive action of water. When dowel bars are not provided in
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