Laboratory Studies on Resilient Modulus of Foamed Asphalt Mix and Virgin Aggregate Used in Base Course Layers

The continuous demand for virgin aggregates in construction of new roads and maintenance and rehabilitation of in-service pavements, and the ever-increasing demand to reduce the disposal of road construction waste, is making agencies to look for ways to recycle construction materials. Foamed asphalt base course (FA) mix, which is typically produced by stabilizing reclaimed asphalt pavement (RAP) with foamed (expanded) asphalt, is a material that can help in reducing the use of virgin aggregate in base layers in construction projects. FA mix behaves like unbound granular materials (UGM). A fundamental property used to characterize UGM is the material’s resilient modulus (M_r). Resilient modulus is also one of the primary input parameters for base course materials in mechanistic-empirical (M-E) pavement design. The purpose of this study was to determine the M_r (stiffness) of FA mix and compare that to the M_r of virgin aggregate (VA) used in base course of pavements using the repeated load triaxial resilient modulus test. Using the laboratory test results, the parameters of a resilient modulus model used in M-E pavement design were also estimated. In order to compare the M_r of both materials, they were compacted at three different density levels (98%, 100%, and 102% of maximum dry density). All tests were performed in accordance with the AASHTO T307 procedure, except that the number of conditioning and testing cycles were reduced to prevent potential damage of the FA mix specimens, which appeared to have more brittle behavior after curing in the oven for two days at 40°C. To maintain consistency, the same conditioning and testing protocol was followed to test the virgin aggregate. From the results, it was observed that (1) M_r of both materials increased with increases in density. The coefficient K₁, which is proportional to Young’s modulus, is positive and increased with increase in density for both materials; (2) the coefficient K₂, the exponent for the bulk stress term, was found to be positive for both materials, indicating that an increase in the bulk stress results in an increase in the material stiffness; and (3) the coefficient K₃, the exponent for the octahedral shear stress term, was negative for the foamed asphalt mix but positive for the virgin aggregate, suggesting that an increase in shear stress decreased the stiffness of the foamed asphalt but increased stiffness of virgin aggregate. The resilient modulus of the foamed asphalt mix was found to be between 1.9 and 5.4 times that of the virgin aggregate depending on density level and bulk stress, indicating suitability of foamed asphalt mix as a base course material in lieu of virgin aggregate, at least in the as-compacted state and with the used testing protocol.