Warm-Mix Asphalt Study: First-Level Analysis of Phase 2b Laboratory Testing on Laboratory-Prepared Specimens

This report describes laboratory testing of laboratory-prepared specimens as part of the second phase of a warm mix asphalt study, which compares the performance of a hot mix control produced at 310°F (155°C) against the performance of warm mixes produced at 250°F (120°C) with four different technologies (Advera, Evotherm, Sasobit, and Rediset). Key findings from the study include: The laboratory test results indicate that use of the warm mix technologies assessed in this study, which were produced and compacted at lower temperatures, did not significantly influence the performance of the asphalt concrete when compared to control specimens produced and compacted at conventional hot mix asphalt temperatures. Laboratory performance in all the tests appeared to be mostly dependent on air-void content and less dependent on mix production temperature. Test results were influenced by specimen air-void content, actual stress and strain levels, and actual test temperature. Variation in these parameters needs to be taken into consideration when comparing performance between the different mixes. All mixes were sensitive to moisture content. Rutting performance, fatigue-cracking performance, and tensile strength retained all deteriorated with increasing moisture content in the specimens. All mixes performed significantly better in the Hamburg Wheel-Track Test when subjected to additional curing, indicating that hot and warm mixes are likely to have similar performance on in-service pavements after a short period of aging (e.g., 6 to 12 months). This is consistent with performance on the test track. Test results were consistent with those from earlier testing phases. The laboratory testing completed in this phase has provided no new results to suggest that warm mix technologies should not be used in dense- or open-graded mixes in California, provided that standard specified construction and performance limits for hot mix asphalt are met. It should be noted that lower production temperatures could lead to insufficient drying of aggregates, which in turn could result in moisture-related problems in the road. Moisture content in aggregates should be strictly controlled at asphalt plants and specified mix moisture contents (i.e., less than 1.0 percent by weight of the mix) should be adhered to.