Guidelines for Use of Modified Binders

The Superpave mix design procedure improved mixture quality; however, it still remains a volumetric design devoid of performance tests. A mixture produced for high traffic volumes (above 10 million ESALs), results in low design asphalt content due to the increased gyrations. The lower binder content, although enhancing the rutting performance, might increase the cracking susceptibility of the mix. Modified asphalts have gotten a lot of attention during the past few years due to the promise of enhanced mixture performance. This study consolidated work done in Florida with polymer-modified asphalts - mainly Ground Tire Rubber (GTR) and Styrene Butadiene Styrene (SBS). Issues with the use of modifiers as well as their relative performance have been examined and summarized. Information reviewed included data from the Heavy Vehicle Simulator (HVS), laboratory experiments--Asphalt Pavement Analyzer (APA), Servopac, and Indirect Tension Test (IDT)--as well as field results. Test results clearly show that SBS-modified mixtures outperform control mixtures in rutting experiments, and laboratory information indicates that the same holds for cracking. The basic benefit of GTR is that it can increase binder content without drain-down; the increased binder content subsequently improves cracking resistance. Use of SBS-modified asphalts appears warranted and cost effective. SBS-modified asphalt is highly recommended for intersections (high volume, slow moving traffic) and open-graded friction courses (OGFC); however, environmental benefit of the use of rubber in pavements cannot be overlooked. A project to investigate a hybrid binder--a combination of SBS polymer with rubber--is already under way.

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