Workability Evaluation of Foamed Warm-Mix Asphalt

In recent years, a new group of technologies has been introduced in the United States that allow producing asphalt mixtures at temperatures 30 to 100°F (16.7–55.6°C) lower than what is used in traditional hot-mix asphalt (HMA). These technologies are commonly referred to as warm-mix asphalt (WMA). From among these technologies, foamed WMA produced by water injection has gained increased attention from the asphalt paving industry in Ohio since it does not require using costly additives. This type of asphalt mixture is advertised to have better workability than traditional HMA mixtures. However, limited research has been conducted to validate this claim. In this study, a new device was designed and fabricated to evaluate the workability of foamed WMA mixtures and compare it to that of HMA mixtures. Two asphalt binders (PG 70-22 and PG 64-28), two aggregate types (limestone and crushed gravel), and two nominal maximum aggregate sizes (12.5 and 19.0 mm) were used in this study. The test results showed that foamed WMA mixtures are more workable than HMA mixtures, which was attributed to the lower asphalt binder absorption observed for the foamed WMA mixtures. Another factor that might have contributed to the improvement in workability for the foamed WMA mixtures is the presence of vapor pockets entrapped within the foamed asphalt binder that keeps the binder slightly expanded and reduces its viscosity. For both foamed WMA and HMA mixtures, better workability was obtained for PG 64-28 than for PG 70-22, and for the 12.5-mm nominal maximum aggregate size than for the 19.0-mm nominal maximum aggregate size. HMA mixtures prepared using crushed gravel had better workability than those prepared using limestone aggregates. However, foamed WMA mixtures prepared using limestone aggregates had better workability than those prepared using crushed gravel, which indicates that aggregate type affects foamed WMA mixtures differently than HMA mixtures.