Differences in appearance within the mat surface. Extreme differences in mat surface appearance often take on the look of tearing or streaking and should be investigated as such. Typically, mat areas that look rougher and more coarse than other areas are of the greatest concern. HMA mixtures with larger nominal maximum aggregate sizes will naturally appear more coarse on the mat surface than mixtures with smaller nominal maximum aggregate sizes.
In and of itself, a rough surface texture is not necessarily detrimental to pavement performance. Rough surface areas are, however, undesirable in parking lots and other areas of high pedestrian traffic. Furthermore, rough areas can be an indication of low density, aggregate segregation or temperature differentials, which can all substantially reduce pavement life.
There are many potential causes for non-uniform mat texture. Many times, the same mechanisms can cause mat tearing and non-uniform texture. Therefore, an investigation of mat tearing causes is also warranted. The following lists causes that are separate from mat tearing causes:
- Handwork. Handwork behind the paver screed will generally deposit HMA material on top of the mat either in thin layers or individual aggregate particles. Even after compaction, areas of handwork are often noticeable. Although minimal handwork is not detrimental to pavement performance, it can be unsightly and therefore detrimental to pavement appearance.
- Low mix delivery temperature. HMA delivered at low temperatures may not allow adequate time for compaction. If it is cool enough the compaction equipment (including the paver screed) may not be able to achieve a smooth surface texture.
- Soft base course or subbase. If the base course or subbase deforms under the paver screed of compaction equipment, the surface texture may be different from the surrounding mat.
- Segregation. Segregation, especially coarse segregation, is often visibly noticeable as a rough mat texture.
- Temperature differentials. Temperature differentials are often visibly noticeable as a rough mat texture.
Solutions differ depending upon the cause. Typical solutions involve minimizing handwork, delivering the HMA at a high enough temperature to allow for proper compaction, properly preparing base courses and subgrades and taking steps to eliminate segregation and temperature differentials.