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It is necessary to phase the bridge construction periods so that the bridges are completed ahead of the paving operations and the existing roads diverted over or through them. The carriageway （pavement） paving operation then begins at the top layer of earth—the formation—being accurately trimmed to a 2 inch （51 mm） tolerance by scrapers or a grader. A grader is a wheeled machine which has a steel blade mounted horizontally between its four wheels. This blade can be accurately raised， lowered or tilted by the driver to cut a precise surface.
A sub-base layer of gravel or crushed rock， generally 12 inch （305 mm） thick， is then spread over the formation to increase its load bearing capacity. Alternatively， when the earth is suitable and imported materials are difficult to obtain， it may be possible to mix a quantity of dry cement into the top layer of earth， which is then damped with water to cause it to harden. This is called soil stabilization.
The final layers of the road are then built， either of concrete or of tar-bitumen black-top materials.
The bitumen and stone are heated and mixed together in a site mixing plant， and brought hot，by truck， to the laying point. The material is then tipped into a paver which spreads it evenly to a thickness of 2.5 inch （64 mm） and to an accurate level. This layer is compacted by road rollers to give a firm surface. The accuracy of each successive layer until the wearing course （ usually of asphalt） provides the accuracy of the finished road. Bitumen coated stone chippings are spread over the top surface and rolled into it while they are still hot.
If the final surface is to be concrete， then this will consist of a concrete slab approximately 10 inch （254 mm） thick. The actual thickness will depend upon whether the concrete is reinforced or not. Joints will be incorporated in this slab at about 15 ft （4.6 m） intervals to enable expansion and contraction of the concrete to take place.
Conventionally， the concrete is laid between temporary steel road forms， which support the edge of the concrete slab， by a concrete train. This consists of a series of machines which run on rails supported on the road forms. The forms and thus the rail are accurately laid to level well ahead of the train and provide the level control for the finished road surface. The first machine in the train is a placer spreader which puts the concrete， transported by truck from the concrete mixing plant，between the road forms. The concrete is then compacted and trimmed to true level by successive machines. To provide a skid resistant surface， the wet concrete is then lightly brushed or otherwise grooved to a shallow depth.
Recently slip form machines have been developed， and by using these machines to form the concrete slab， it is possible to eliminate the lengthy process of accurately laying out road forms.These slip form pavers incorporate traveling side forms within the body of the machine. The degree of vibration compacting the concrete is much greater than with the conventional train so that after the moving forms—approximately 15 ft （5 m） long—have slipped past， the fresh concrete is able to stand up without further support. The surface level of the finished concrete is formed by the same machine which is controlled， both for level and direction， by means of electronic or hydraulic sensor controls which follow string lines placed at each side of the machine along the carriageway. With this paver， it is possible to achieve up to 6 ft （1.8 m） per minute.
phase v. 使……按计划进行
carriageway n. 路面
tolerance n. 公差
grader n. 平路机，推土机
tilt v. 倾斜
soil stabilization 稳定土
tip v. 倒，倾泻
paver n. 摊铺机
chippings n. 屑
slab n. 厚板
joint n. 连接杆
incorporate v. 结合
interval n. 间隔
expansion n. 膨胀
contraction n. 收缩
conventionally ad. 通常
placer spreader 铺料机
brush v. 刷
groove v. 扫
vibration n. 振动
approximately ad. 大约