Warm-Mix Asphalt Study: Field Test Performance Evaluation

A number of warm-mix asphalt test sections were constructed in California between 2007 and 2010 to assess longterm performance under selected traffic and climate conditions. A range of pavement designs were assessed, but the six projects evaluated in this report focused on open-graded friction courses with polymer-modified (PG 58-34) and rubber-modified (PG 64-16) binders (three projects each). The main purpose of these experiments was to monitor performance under actual conditions and to quantify any benefits associated with using warm-mix asphalt under specific situations, such as with long hauls, in cool and/or damp conditions, under trafficking by large agricultural equipment, etc. Four of the test sections, which were located near Morro Bay, Point Arena, Orland, and Mendocino, had hot-mix controls. Two additional warm-mix asphalt projects, located near Marysville and Auburn, did not include control sections. The warm-mix technologies assessed in these projects included Advera WMA, Evotherm, Gencor Ultrafoam GX, Rediset, and Sasobit. Monitoring included a visual assessment from the shoulder and a photographic record. The six open-graded friction course warm-mix asphalt projects in northern and central California were evaluated for periods of between two and five years. All of the sections performed well. On the projects that included hot-mix control sections, the warm-mix asphalt sections showed equal performance to the controls. On one project (Interstate-5), the warm-mix section showed some early minor rutting in the first six months, which was not observed on the control. However, after 12 months of trafficking rut depths on both sections were the same. This early rutting on the warm-mix section was attributed to less oxidation of the binder due to the lower production and placement temperatures. Once the rate of oxidation had stabilized (after ± 12 months), rutting performance appeared to be the same, and to progress at the same rate, on both sections. This observation was consistent with observations on earlier accelerated loading experiments and is not considered to be a concern given that rut depths were the same on the control and warm mix sections at the end of the testing/evaluation periods. Based on the observations in this study, the use of warm-mix technologies in open-graded friction course mixes with polymer- and rubber-modified binders appears to be beneficial, especially on projects that require long hauls and/or placement in cold temperatures. The use of warm-mix technologies resulted in improved workability of the mix and better compaction, which should improve durability and prevent early raveling.