CAIT project no.: BAY RU9247
Fiscal Year: 2002/2003
Rutgers-CAIT Author(s): Thomas Bennert, Ali Maher, Joseph Smith
An asphalt-rubber hot mix asphalt (AR-HMA) design was created using a Superpave 12.5mm gradation and a #30 (-) mesh crumb rubber at 20% total weight of the asphalt binder. At this point in time, asphalt rubber has only been used with HMA that contains a more open graded nature, such as open graded friction course (OGFC). However, OGFC is limited on it use due to its potential problems (i.e. clogging and winter maintenance). However, a 12.5mm Superpave mix may be used on almost any roadway in New Jersey and is most commonly found as the surface course. It is used for both new construction and rehabilitation projects. Therefore, this mix type has the largest potential for usage, meaning that more tires can eventually be recycled. However, two factors need to be considered prior to acceptance; performance and cost.
Four HMA mixes were constructed and tested in this study. Three baseline mixes using a PG64-22, PG70-22, and a PG76-22 asphalt binder, as well as the AR-HMA mix which used crumb rubber blended with the same PG64-22 asphalt binder. The influence of crumb rubber particle size on the compaction properties during the mixture design procedure was also evaluated. This would provide a method of comparing the final mixture performance of different performance graded binders to the AR-HMA mix. This type of methodology of testing the mixtures is extremely important due to the necessity of ensuring the crumb rubber complying to the aggregate gradation volume limitations, and how the crumb rubber interacts with this volume limitation under performance testing. Therefore, a number of different performance tests were conducted on the four different HMA mixes: 1) Asphalt Pavement Analyzer, 2) Repeated Load Permanent Deformation testing using the Simple Performance test specifications, 3) Dynamic Modulus, 4) Repeated Shear at Constant Height, 5) Frequency Sweep at Constant Height,and 6) Simple Shear at Constant Height.
The results from the mixture design portion concluded that to provide consistent compactibility when using crumb rubber in a 12.5mm Superpave design, the maximum particle size should not be greater than a #30 mesh. The performance tests concluded that the AR-HMA mix performed as well as or better than the PG76-22 for every test conducted. For example, the dynamic modulus test results showed that the AR-HMA had similar stiffnesses to the PG76-22 at the high test temperatures, however, the AR-HMA had a much less stiffness at the low test temperature. This indicates that the ARHMA mix design in this study will provide excellent rut resistance, while also providing excellent low temperature cracking resistance. Essentially, by adding the crumb rubber to the PG64-22, the working temperature range of the asphalt binder increased on both the high and low sides. Also, by performing similarily to the PG76-22, Bayshore Recycling now can establish a cost comparison using the difference in cost between a HMA with a PG64-22 and a PG76-22.