第五章沥青路面施工.ppt

* The tow point can be raised or lowered as necessary. Hydraulic cylinders move the position of the tow point. The cylinders can adjusted manually or automatically with a grade and slope control system. (More on those later). The tow point is typically centered when paving begins, and then is adjusted as necessary. * The proper angle of attack with the screed is where the nose is slightly above the tail. The wear on the screed will be uniform if the angle of attack is adjusted properly. * The operation of the sensor should be checked regularly. When the paver is placing mix, the sensor indicator (either lights or a meter) should indicate movement. The tow point should also be moving in response to changes in elevation. Any changes should be smooth. The screed should not be constantly moving up and down – if it is the sensitivity may be set too high. * The third type of grade reference is the joint matching shoe. The photo on the left shows a contacting type ski 1ft (0.3 m) ski that rides directly on the adjacent surface or curb. The photo on the right shows an non-contacting ultrasonic sensor. The advantage of the non-contacting sensor is that it is more reliable and less likely to be affected by loose rocks or workers. For base or binder layers, the mobile reference will provide a longer reference. For surface courses, the joint shoe will provide a matching elevation across the longitudinal joint, but the mobile reference is still better paving practice. * This is a shot of the freshly laid mat taken from the paver. Notice the isolated areas of cooler temperature – due most likely to end of load segregation and improper operation of the paver. * A thin layer of mix will cool more quickly in a strong wind than when there is little or no wind. Wind has a greater effect at the surface of the mix than within the mix, and can cause the surface to cool so rapidly that a crust will form and checking of the HMA may occur. The amount of ra