Lab Corner – Spring 2008

“You Get Out Of It What You Put In”

We have probably all heard a version of this quote, particularly in reference to workouts. You get more muscle tone and a healthier body when you put in the extra effort in weight-training and cardiovascular activities. While this is true to a certain point, all athletes (and weekend warriors) will reach a point of diminishing returns, where a little more gain can only be achieved by a lot more effort.

Like muscle tone and cardiovascular fitness, you get well-performing asphalt pavements by putting in the effort in using quality materials and proper construction techniques—including achieving an appropriate initial density.

Density affects cracking, rutting and the cohesion of asphalt mixtures. As a result, state agencies have moved to more aggressive compaction specifications to increase pavement construction density from 92 percent of maximum theoretical density to 93 percent in pursuit of better hot mix performance. While the increase in density of one percent seems simple, cost and benefit must be reviewed as with any process. An increase in density without recommending how to properly achieve this increase could result in a negative impact on pavement durability.

The Kentucky Transportation Center at the University of Kentucky has partnered with the Asphalt Institute to conduct a coordinated research program that will address this specific cost-benefit of increasing density. With Phase 1 of the research program nearing completion, some initial results are available that illustrate the effect of density (or percentage of air voids) on performance. For fatigue characterization, a test matrix was developed to represent densities that may be seen if a pavement was under-compacted (88.5 percent of maximum theoretical density) or compacted to the design potential (96 percent), which rarely, if ever, happens. Specimens were prepared at 1.5 percent increments throughout this range. A tolerance of ±0.3 percent was targeted to lessen experimental variability.

Four-point flexural beam fatigue testing was conducted on asphalt mixture beam specimens (2” x 2.5” x 15”) at a temperature of 20°C and various strain levels by repeatedly loading the specimen in the center of the beam. As the specimen fatigues, microcracks are formed and the stiffness of the asphalt mix specimen decreases. As the microcracks increase, the specimen stiffness decreases rapidly, indicating failure. The number of cycles to failure, Nf, is defined as the loading cycle when the mixture stiffness drops to 50 percent of the original stiffness. Research has indicated that the cycles to failure (Nf) in the flexural beam fatigue test could be related to the actual number of loading cycles required to cause fatigue cracking of asphalt pavements. In the laboratory, the fatigue test is often used to compare the expected fatigue performance of different asphalt mixtures.

Figure 1 shows the effect of the percentage of air voids in a specimen on laboratory fatigue life. The data shows two things that are important for this study. First, the maximum fatigue life (or durability) occurs at approximately 5.5 percent air voids (94.5 percent of maximum theoretical density). For this data, compacting past this level gives little benefit for a lot more effort.

The second piece of information that can be gleaned from this graph is that the laboratory fatigue life at 9 percent air voids (91 percent of maximum theoretical density) is 90 percent of the lab fatigue life at 7 percent air voids (93 percent of maximum theoretical density). In other words, achieving 91 percent density instead of 93 percent density means that a pavement expected to last 20 years might only last 18 years.

By obtaining a better understanding of the relationship between density and asphalt pavement performance, more appropriate specification limits and pay adjustments can be developed that directly relate to serviceability and long-term performance. Ultimately, the results of this research may also help producers transition more smoothly to end-result (warranty) specifications. The implementation of these results will not only increase asphalt pavement life, but also contribute to a reduction in user delays and accidents coincident with future work zones.

For more information on the Asphalt Institute’s Research Program, please contact AI Senior Research Engineer Phil Blankenship.