12. Mat Problems

12.5.1 Description

As the thickness control cranks on the screed are changed, the screed’s angle of attack increases or decreases. As the paver moves forward to place the mix, the screed moves up or down to the new equilibrium point for the newly set mat thickness. When the screed fails to respond to changes in the setting of the thickness control cranks, the operator is unable to alter the depth of the layer being placed. The paver also loses its inherent ability, through the principle of the floating screed, to provide the self-leveling action needed to place a smooth asphalt mat.

12.5.2 Causes

An extremely high paver speed (more than 83 ft [25 m] per min for thin lifts or more than 50  ft [15 m] per min for layers more than 2-1/2 inches [63 mm] thick) may cause a lack of responsiveness of the screed (see Section 7.3.4). The mechanical condition of the screed affects its ability to react. The screed riding on its lift cylinders or loose connections on the thickness control cranks will cause the screed to be unresponsive. If automatic grade controls are used (see Section 7.4.2), an incorrect sensor location will render the screed unable to react to input signals from the grade sensors.

If the maximum aggregate size used in the mix is too great compared with the depth of mix being placed, the screed will ride on or drag the largest aggregate pieces. As a result, the screed will be unable to change its angle and will thus be unresponsive to changes in the thickness control settings. Variations in mix temperature will also cause the screed to be unresponsive to changes in the angle of attack because the mix stiffness variations themselves will cause the screed to continually seek new equilibrium levels for the forces acting on it.

12.5.3 Solutions

The paver and screed must be in good operating condition. The sensor for automatic grade controls must not be located either at the tow points or behind the pivot points of the screed; rather, it should be located in the area between one-third and two-thirds of the length of the leveling arms. If the mix texture is uniform (indicating a proper relationship between course thickness and maximum aggregate size), the screed will be able to respond to changes in the thickness control settings.

12.5.4 Effects on Performance

An unresponsive screed causes a rough asphalt mat. The screed is unable to react to manual changes in the thickness settings. It also loses its ability to self-level on an existing pavement surface because it cannot reduce the thickness of the mix placed over the high points in that surface and increase the thickness placed in the low areas.

Thus, the rideability of the course being placed can be affected significantly if the paver screed is unresponsive.