A Proposed Method for Mix Design of Warm Foam Mix Asphalt Produced by Wwo Bitumen Types

Foam bitumen technology has been used in cold mixes in order to reduce bitumen viscosity and to ease mixing of bitumen with cold aggregates. Recently, a new generation of mixes has been developed using foam technology providing the same quality of mixes similar to conventional hot mixes. These are normally produced at lower mixing and compaction temperatures than those of similar HMA. However, there is no proper standard mix design method for these mixes. Regarding to reduce mixing and compaction temperatures, the main challenges of using warm foam mixes in lieu of HMA are their greater moisture susceptibility and rutting potential due to less adhesion properties of foam bitumen to aggregates. In this research, the method of adding two different bitumen types at two stages was adopted. At the first stage, a soft bitumen was added to warm coarse aggregate particles. In the next step, a hard bitumen transformed into foam and was added to the total mix. Warm foam mixes were produced at different mixing and compaction temperatures and their Marshall Parameters were determined and compared with similar HMA. Results indicated that it was possible to reduce mixing temperature of warm foam mixes 30°C below those of similar HMA. The air void content was recognized to be the most effective parameter affecting mixing and compaction temperatures. For investigating the moisture susceptibility of warm foam mixes, indirect tensile test was performed on dry and soaked conditioned samples. The testing results indicated that the moisture susceptibility of warm foam mixes was greater than the specified minimum values of the similar HMA. However, adding anti-stripping agents could compensate this deficiency appreciably. Wheel tracking test results also showed that despite of the larger Marshall Flow values of warm foam mixes compared with similar HMA, by adding hydrated lime powder, it was possible to achieve the same rutting values as for those in HMA. As a result of the above testing results, a mix design method was proposed for designing warm foam mixes.