Issues pertaining to laboratory evaluation of hot mix asphalt

Since the early 1900's, research in the Hot Mix Asphalt (HMA) industry has focused on the development of a more mechanistic design approach and improving quality control procedures. This paper presents the results of three observational studies conducted to evaluate problems associated with relatively recent developments in HMA design and analysis as follows 1) Aggregate degradation due to laboratory compaction using the Superpave^™ Gyratory Compactor (SGC). 2) Aggregate degradation due to ignition oven testing using the NCAT Asphalt Content Tester. 3) Comparison of the bulk specific gravity, saturated, surface dry (SSD) method with bulk specific gravity, Parafilm^™ method for HMA specimens. These observational studies included HMA samples from over 20 paving projects constructed in the state of Tennessee during the 1998 construction season. The results of these studies suggest that there is no significant degradation of aggregate resulting from the SGC or the NCAT ignition oven for HMA typically produced in Tennessee. The results suggest that the use of the Parafilm^™ method in measuring bulk specific gravity of compacted HMA specimens results in an air-void content approximately 2% higher than using SSD methods. It is not clear which method produces a more correct measure of air-void content. The results also suggest that the SSD method is more suitable for typical HMA specimens containing less than 6% air-voids. It is suspected the internal voids are not interconnected with surface voids at this level of compaction.