12. Mat Problems

12.6.1 Description

Surface (auger) shadows are dark areas that appear in the surface of a mix. In most cases, the shadows cannot be seen until sometime after the pavement has been used by traffic and some of the asphalt binder film has been worn off the exposed aggregate particles by the vehicle tires. Surface shadows are seen most easily when the sun is low on the horizon and the pavement is viewed when looking toward the sun. The shadows are also visible when the pavement surface is damp or when the surface is viewed from the shoulder of the roadway at night and vehicle headlights are shining on the surface.

In severe cases, surface shadows may be visible immediately behind the screed during the laydown operation. Even in this latter case, the shadows will disappear when the mix is being compacted by the rollers, only to be visible again later under the conditions described above. The shadows may be completely across the lane width being placed, or they may be only partially across the width. The extent of the shadows depends on how the paver is operated, particularly the portion of on/off time of the augers on each side of the machine.

12.6.2 Causes

Surface shadows are caused primarily by overloading of the augers on the paver (see Section 7.2.2.4). If the head of material in the auger chamber is large enough to “bury” the augers, the screed will react to the variable forces acting on it. The spacing between the shadows will normally correspond to the starting of the augers when operated in a stop–start manner. Whenever the amount of mix in front of the screed is at or above the top of the augers, the shadows will be formed and seen later in the pavement.

On most pavers it is possible to adjust the distance between the screed and the tractor unit. This is accomplished by unbolting connections on the leveling or tow arms of the paver and moving the tractor forward or backward while the screed remains stationary on the pavement surface. Depending on the make and model of the paver, there is typically 4 inches (100 mm) of adjustment for the screed connection. The severity of surface shadows may increase with the screed in the back position—when more mix is being carried in the auger chamber and the augers are being overloaded. The shadows are thought to be the result of a slight increase in mix density caused by the restarting of the augers and the subsequent forcing of additional mix under the screed. There is no difference in surface texture associated with the location of the surface shadows; they can be seen only from an angle. Their intensity often increases when a tender mix is being laid.

12.6.3 Solutions

The asphalt mixture carried in the auger chamber should be maintained at a level near the center of the auger shaft. This means the flow gates should be set so that the augers operate as close to 100 percent of the time as possible and stopping and starting of the augers is minimized. In no case should the top of the augers be completely covered with mix. Further, the location of the screed should be set as far forward as possible so that the amount of material in the auger chamber is reduced and the head of material in front of the screed is kept to a minimum. The screed should not be set in the back position unless a large-stone mix (one in which the maximum size of the aggregate is more than 1-1/2 inches [37.5 mm]) is being placed.

12.6.4 Effect on Performance

Surface shadows are not necessarily detrimental to the performance of the mix, except for a minor effect on rideability. The difference in the density of the mix in areas with and between shadows is generally not great enough to be determined accurately. The main concern with surface shadows is the visual appearance of the mix to vehicle drivers.