12. Mat Problems

12.13.1 Description

Dense-graded mixtures should be compacted so that the in-place air voids are at an acceptable level. The NMAS of dense-graded asphalt mixtures has a significant effect on the in-place permeability of the mat (see Section 3.5.3). As the NMAS of the mix decreases, the impermeability increases. If not adequately compacted, the mix will be permeable to air and water and will not have the required durability. If the initial compaction results in air voids of approximately 4 percent or lower, the mix may become unstable under traffic after additional densification; the result will be shoving and rutting of the mixture, as discussed earlier. However, the use of polymer-modified binders in these mixtures, coupled with high-quality angular aggregates, will help mitigate this potential for shoving and rutting. Some specialty mixtures using highly modified binders are actually designed for 2 to 3 percent in-place air voids. Most mixes require a significant level of compaction to reach the desired level of air voids.

12.13.2 Causes

When the mix is too stiff or too tender, compaction is difficult. The primary cause of poor compaction is low design mix density (high design air voids), effectively resulting in a mixture with low design binder content (see Chapter 3). Other causes include inadequate underlying support (see Chapter 5), improper type and weight of rollers, improper tire pressure in rubber-tire rollers, improper rolling procedure (Chapter 8), improper mix design (Chapter 3), mix segregation (Chapter 10), moisture in the mix (Chapter 3), variation in mix temperature, and low mix temperature.

12.13.3 Solutions

Solutions to compaction problems include taking the necessary steps to ensure adequate support, producing an acceptable mixture, and using satisfactory laydown and rolling techniques. When support is inadequate, the compaction requirements may have to be relaxed, or the mix may have to be redesigned to allow for satisfactory compaction.

When the asphalt content is too high, the mix may compact too easily, resulting in low air voids (which may lead to rutting; see earlier discussion). When the asphalt content is too low, the mix may be stiff and difficult to compact to the specified density. A satisfactory mix design will produce a mix with optimum asphalt content that can be compacted with reasonable effort to the required density.

Good laydown and rolling techniques, as discussed earlier throughout this manual, are necessary for good compaction. Density can normally be increased by reducing the speed of the paver or rollers. Density can also be increased by increasing the weight and number of rollers. The compaction process must be adjusted to produce optimum density.

12.13.4 Effects on Performance

When the compaction is inadequate, the mix will be permeable to air and water. Water can flow through the asphalt mixture and reduce the strength of the underlying base course. The high voids also result in excessive oxidation of the asphalt mix, which leads to raveling, cracking, and general deterioration of the asphalt pavement over a period of time. A one percent increase in density has been found to conservatively increase the service life of pavement by 10 percent.

When the air voids are excessively low after compaction (less than 4 percent) for dense-graded mixes, the mix is likely to rut and shove under traffic. Again, the use of polymer-modified binders in these mixtures coupled with high-quality angular aggregates will help to mitigate this potential for shoving and rutting. The low voids are not the result of too much compaction but of an unsatisfactory mixture.