Back-calculation method for determining the maximum rap content in stone matrix asphalt mixtures with good fatigue performance based on asphalt mortar tests

The use of recycled materials in asphalt pavement is commonly accepted both in Europe and in the US. Reclaimed Asphalt Pavement (RAP) is the most highly recycled material in asphalt mixtures since this is associated to significant environmental and economic benefits. Nevertheless, for specific mixture such as Stone Matrix Asphalt (SMA) the use of RAP is not common practice, since the brittle aged binder contained in RAP materials may be detrimental for the overall fatigue performance. In this paper a simple method to determine the amount of RAP that can be added to SMA mixtures without compromising the fatigue resistance is introduced. The procedure is based on a combination of experiments and rheological modeling. First Dynamic Shear Rheometer tests are performed on asphalt binder and mortars composed with selected fine RAP particles (SRAP) and SRAP aggregates (BSRAP). Then, the experimental results are used to calibrate the recently developed modification of the Nielsen model and to back-calculate the complex modulus of the bituminous blend of fresh and SRAP binder in the mortars. The Superpave fatigue parameter G*sinδ is then linked to the SRAP binder content and the maximum SRAP binder percentage which can be allowed in SMA mixtures is estimated. Finally, Linear Amplitude Sweep tests are used to validate the procedure on two mortars. A limiting SRAP binder content of 23% was found for SMA mixtures. The proposed method, based on the modified Nielsen model, represents a simple alternative to more sophisticated solutions which require the preparation of asphalt mixture specimens.