12. Mat Problems

12.3.1 Description

Nonuniform mat texture (see Figure 165) can be described as differences in the appearance of the mix, both transversely and longitudinally, as it is placed and compacted. Normally, minor differences in surface texture will be apparent because of differences in the alignment of the larger coarse aggregate particles as the mix passes out from beneath the paver screed. In addition, a mix with a higher fine aggregate (sand) content will have a more uniform surface texture than a mix containing a larger percentage of coarse aggregate.

12.3.2 Causes

Many factors related to the operation of the asphalt paver affect the uniformity of the surface texture of the mix (see Chapter 7). A variable amount of mix against the screed, caused by overloading the augers or running the hopper empty between truckloads, can cause variations in the amount of mix tucked under the screed and thus produce a nonuniform texture.

Source: National Asphalt Pavement Association
Figure 165. Nonuniform Mat Texture

Improper screed maintenance, including worn or loose screed plates or screed extensions incorrectly installed, as well as low screed vibratory frequency, may alter the mat texture and cause nonuniformity. In addition, a low mix temperature, caused either by plant problems or by the paver sitting too long between truckloads of mix, can be a factor in uneven mat texture, especially if the paver screed is also cold. The tearing that results when the compacted layer thickness is less than twice the dimension of the largest aggregate particles is still another contributing factor.

A soft or yielding base under the course being constructed may cause the new layer to have a variable surface texture (see Section 5.2). Moreover, segregation of the mix caused by poor mix design (Section 3.4.4) or improper handling of the mix during production (Section 4), loading (Section 6.4), hauling (Section 6.5), unloading (Section 6.6), or placing (Chapter 7) operations can contribute to a nonuniform surface texture. The variability of the texture will be affected as well by any factors that cause nonuniformity in the mix, such as deviations in aggregate gradation, asphalt content, or mix temperature (see Chapter 3).

12.3.3 Solutions

The solutions for nonuniform surface texture are as varied as the causes. Paver operation, particularly regarding the need for a constant head of material in front of the screed, should be monitored closely. The paver and screed should both be well maintained and in good operating condition. The compacted thickness of the mat being placed should be designed so that dense-graded mixes have a lift thickness of at least four times the NMAS when compacted. Lift thicknesses for fine-graded mixtures should be at least three times the NMAS.

Finally, a mix that is tender, variable in aggregate gradation or asphalt content, or easily segregated should be modified to increase its stiffness and improve its properties before it is produced at the plant and delivered to the paver for laydown.

12.3.4 Effects on Performance

Nonuniform surface texture is usually associated with nonuniform density. The same compactive effort will generally achieve lower density in areas in which the coarse aggregate has been dragged by the paver screed or segregation of the mix has occurred, as compared with areas having uniform surface texture. As density decreases and air void content increases, the durability and serviceability of the asphalt mat decrease markedly.