Determining the allowable content of RAP in HMA with regard to fatigue resistance

The determination of the allowable percentage of Reclaimed Asphalt Pavement (RAP) that could be used in Hot Mix Asphalt without compromising fatigue cracking is a crucial task. The principal issue is related to the evaluation of the fatigue resistance of the aged binder contained in the RAP and how does it influence the fatigue resistance of the fresh and aged binder blend. In order to solve this issue a new approach is proposed to estimate the allowable RAP binder content in the asphalt mixture, with regard to fatigue resistance, by considering the Superpave parameter G*sin δ. This parameter is determined by using a recently proposed methodology, which avoids the extraction and recovery of the RAP binder to calculate the complex modulus and phase angle master curves of the blend of fresh and RAP binders. This procedure allows evaluating the rheological properties of the bituminous blend by the results of Dynamic Shear Rheometer tests carried out on mortars composed of a selected fine fraction of the RAP, totally passing the 150 µm sieve, mixed with fresh binder at different volumetric fractions of the selected RAP. These properties are estimated by using the Nielsen model specifically modified to take into account the influence of lower frequencies on the stiffening effects of fine particles in bitumen mortars. The allowable content of RAP, determined by this way, is then validated performing time sweep tests on mortars composed of RTFOT+PAV aged and fresh binders, confirming the reliability of the proposed procedure.