Why don’t polymers fix my mix?

Raymond “Buzz” Powell, Assistant Director, Senior Research Engineer NCAT | Fan Yin, Assistant Research Professor, NCAT

The benefits of polymers are well documented, with much evidence that shows improvements in both cracking performance and rutting performance. But rutting is no longer a problem for many agencies. The greatest current challenge is cracking performance. It’s not uncommon for agencies to change a mix from neat to polymer modified asphalt as a premium mix treatment for higher traffic volume roadways, with the expectation that the extra cost per ton for polymer modification will more than pay for itself through improved performance.

But, the added cost of premium mix treatments like polymers does not always provide the expected return on investment. A recent study at the National Center for Asphalt Technology (NCAT) at Auburn University was executed to quantify the impact of a premium mix treatment on laboratory cracking performance. The control mix selected for the study was a 9.5 mm nominal maximum aggregate size (NMAS) dense graded virgin aggregate blend. Mix volumetrics were optimized using 60 gyrations to 4.0% air voids with 5.6% optimum binder content and 16.4% voids in the mineral aggregate (VMA). The ductility of the study mix was evaluated with and without a premium mix treatment using IDEAL-CT testing, with the expectation that they would see some improvement in cracking performance. Surprisingly, no statistical difference in cracking performance was observed as a result of the addition of the premium mix treatment.

Figure 1 – Performance optimization of study mix

NCAT researchers hypothesized that they would observe a benefit of the premium mix treatment if the mix contained more asphalt binder. Researchers then tested this theory by subjecting the study mix to further optimization using performance criteria. Both IDEAL-CT and Hamburg wheel tracking tests (HWTT) were run at the volumetrically optimized binder content, volumetric optimum plus 0.5 percent, and volumetric optimum plus 1.0 percent. Using volumetrics as a starting point, the goal of performance optimization was to increase cracking performance without compromising rutting performance. As seen in Figure 1, NCAT researchers found that asphalt binder content could be conservatively increased to 6.0%, which resulted in a 44% increase in average IDEAL-CT index values with HWTT results that were well within the specified performance range.

With the new performance-optimized asphalt binder content set at 6.0%, the original study was repeated with the addition of the premium mix treatment. As seen in Figure 2, a statistically significant increase in IDEAL-CT index values of 24 percent was observed. In addition to proving the effect of the premium mix treatment on laboratory cracking performance, NCAT researchers made two observations that may have great significance to asphalt mix design and construction all over the country. The first observation is that every unique combination of materials (i.e., rock, sand, dust/ filler, asphalt binder/grade, and premium mix treatment) may require a unique performance optimization, even though the volumetric optimization does not change. In other words, when something is added to the mix that will improve rutting performance (e.g., polymer modification), which means the mix can hold more asphalt binder and still not rut, then adding additional asphalt binder can significantly improve cracking performance while not compromising rutting performance.

Figure 2 – Ideal-CT results before and after performance optimization (blue = control, orange and grey = premium mix treatments)

A second, related observation was made by NCAT researchers which may be even more significant for both industry and agencies. The cracking performance of a mix that has been volumetrically optimized will not necessarily be improved by the simple addition of a premium mix treatment (e.g., polymer modification to the asphalt binder). The only way to know the benefit of the treatment is fully realized is to take the additional step of optimizing the mix for performance. In other words, polymers will not necessarily fix a dry mix. If we add premium mix treatments to volumetrically optimized, dry mix that has not been optimized for performance, we can inadvertently increase the price per ton for asphalt pavement without improving cracking performance. In fact, there is evidence beginning to emerge that suggests cracking performance can even go down when polymer modification is added to asphalt binder in a volumetrically lean mix.

For this reason, practitioners should give serious consideration to starting all mix designs with volumetrics, then relaxing volumetric requirements and optimizing mix proportioning for performance. This is the best way to ensure a return on investment when we spend more money on the mix to make roads last longer.