EVALUATING THE USE OF LOWER VMA REQUIREMENTS FOR SUPERPAVE MIXTURES

The Florida Department of Transportation specifies coarse graded asphalt mixtures for high traffic roadways with the rationale that coarse graded mixtures will offer better rutting performance compared to fine graded mixtures. Contractors struggle to meet minimum voids in the mineral aggregate (VMA) specification requirements, especially when using aggregates native to Florida. Contractors often gap-grade asphalt mixture gradations to obtain enough void space to meet VMA requirements. It is generally believed that gap-grading an asphalt mixture will be detrimental to the mixture's rutting performance. This study examines the effects on laboratory measured rutting, cracking, moisture sensitivity and permeability of asphalt mixtures that have been designed with gap-graded and continuous gradations with the thought that should the continuous gradation provide better performance, then perhaps the VMA specification requirements should be lowered to allow for this type of gradation. The Asphalt Pavement Analyzer and Servopac gyratory compactor were used to determine the mixtures' rutting performance. The Superpave indirect tensile tests (IDT) and calculated parameters (energy ratio, dissipated creep strain energy and fracture energy) were used to determine the mixtures' cracking and moisture sensitivity performance. Additional standard laboratory tests were used to evaluate permeability and moisture sensitivity. Test results indicate that the addition of coarse aggregate on the 12.5 and 9.5 mm sieves of 12.5 mm coarse graded mixtures improved the rutting performance of the mixtures. However, cracking performance was adversely affected by the addition of coarse aggregate. Moisture sensitivity results varied depending on the test method used. Permeability results were unaffected by the gradation change. Since cracking is the predominant form of distress for Florida pavements, it is recommended that no change be made to the Department's specifications at this time. Performance test results indicate that not all mixtures perform at their optimum when designed volumetrically. The Department should continue to conduct research and move towards implementation of one or more performance tests to augment or replace volumetric mix design.