12. Mat Problems

12.2.1 Description

There are three general types of mat tearing or pulling of the asphalt mix under the screed of the paver. The three types are defined by the location of the tear marks in the mat: (a) in the center of the lane, (b) on the outside edges, and (c) across the full lane width.

12.2.2 Causes

A gearbox streak can sometimes be seen in the surface of the mat directly behind the center of the main screed. This streak is typically 6 to 8 inches (150 to 200 mm) wide and is normally caused by a lack of asphalt mix being pushed under the auger gearbox located in front of the center of the screed. This lack of mix may be the result of improper flow gate settings—not enough mix being fed back to the screed. It is more likely to be caused, however, by missing, worn, or improperly set reverse augers or paddles on the augers (located adjacent to the gearbox) that are used to force mix underneath the gearbox (see Section 7.2.2.4).

The rough surface texture is the result of a lack of mix at that point in the pavement width—less mix passes under the screed at the auger gearbox than passes under the screed on either side of the gearbox. The rougher texture, or tearing, makes the surface appear more open or segregated. However, this streak can be a form of segregation when gravity allows the mix from the two conveyors to flow under the gearbox. The surface texture of the mat at that location can be more open than that of the adjacent mix and is generally darker in color. Gearbox streaks are more prevalent with harsher mixes—those containing larger-size aggregate, more crushed aggregate, or lesser amounts of asphalt.

A centerline streak can also be caused by improper setting of the crown on the main paver screed. The appearance of streaks behind the screed is caused primarily by an improper relationship between the crowns at the leading (front) and trailing (back) edges of the screed (see Section 7.3.9). A tearing or open texture several ft (m) wide in the center of the mat may be caused by a lack of lead crown in the screed. Conversely, a tearing or open texture along both outside edges of the asphalt mixture is normally caused by an excess of lead crown in the screed. For most mixes, the lead crown of the screed should be set slightly higher (approximately 1/8 inch [3 mm]) than the tail crown. A proper relationship between lead and tail crowns will result in a uniform texture of the mat across its full width. Edge streaks can be caused by improper flow gate settings or incorrect installation of the screed extensions. Partial width tearing can also result from a cold screed plate if the screed has not been uniformly preheated before paving begins (see Section 7.3.8).

Full-width tearing of the mat can be attributed to a number of factors. One such factor is warped or worn screed plates. Another is the forward speed of the paver being too high for a particular mix. The use of a mixture with aggregate that is large compared with the mat thickness being laid can also be responsible for full-width tearing of the mat. A good rule of thumb for the relationship between the maximum aggregate size in the mix and the minimum compacted course thickness is that the depth of the compacted layer should be at least twice the largest coarse aggregate particle size or three times the NMAS. Thus, a mix containing a maximum aggregate size of 19.0 mm (3/4 inch) (NMAS of 12.5 mm [1/2 inch]) should be placed at least 38 mm (1-1/2 inch) thick. Lastly, cold mix temperatures, particularly when combined with a cold paver screed, can significantly affect the amount of tearing that occurs (see Chapter 7).

12.2.3 Solutions

A gearbox streak can usually be eliminated only by changing the amount of mix being forced under the screed at the auger gearbox. This change is made by installing reverse paddles or reverse augers on each side of the gearbox to push more mix under the gearbox. If the paver is already equipped with such devices, they should be checked to see whether they are worn and need to be replaced.

Constant center or outside edge mat tearing can usually be eliminated by adjusting the relationship between the lead and tail crowns on the paver screed. If this change does not solve the problem, the setting of the paver flow gates should be modified. Full-width tearing can be eliminated by increasing the mix temperature, preheating the screed properly before paving starts, replacing warped or worn screed plates, or increasing the lift thickness.

12.2.4 Effects on Performance

Tearing of the mat affects long-term pavement performance by causing changes in density in those areas where the tearing has occurred. Torn areas may appear segregated and are usually deficient in mix quantity. Pavement performance will be reduced in relation to the degree to which the tearing reduces the density and increases the air void content of the mat. In addition, the torn areas will be more susceptible to raveling and to the effects of moisture (stripping).