Analysis of Recycling Agent Effects on the Mechanical Properties of HMA with High Recycled Binder Ratios

The goal of the research presented in this report is to study how the mechanical properties of hot mix asphalt change upon the addition of high contents of reclaimed asphalt pavement (RAP) and the inclusion of any amount of recycled asphalt shingles (RAS), with between 25% and 50% binder replacement and to consider the addition of recycling agents to reduce the increase in stiffness and corresponding decrease in fatigue resistance. To achieve this goal, 16 mixes and the corresponding binders were fabricated and tested in the laboratory. The mix factorial includes a control gradation, two virgin binders (PG 64-16 and PG 58-28, from different sources), two RAPs with different levels of aging (PG high temperatures of 102°C and 109°C), one RAS, and two recycling agents (a petroleum-derived aromatic and a tall oil). The testing of the binders included performance grade (PG), shear stiffness, and Fourier transform infrared spectroscopy. The testing of the mixes included stiffness, four-point flexural fatigue resistance, rutting resistance, and the IDEAL cracking tolerance (IDEAL-CT) test. The main conclusion from this study is that most of the increased stiffness effects of high RAP and/or RAS addition can be offset by using recycling agents and/or reducing the stiffness of the virgin binder by reducing the PG binder grade. Two approaches are proposed to determine an appropriate dosage of recycling agent. The first focuses on restoring the mechanical properties of the mix with high RAP/RAS content back to the properties of a control mix with either no RAP/RAS or a low RAP/RAS content. The second approach focuses on meeting the required performance-related specifications within the balanced mix design framework by using the minimum amount of recycling agent. It was found that restoring the PG high temperature of the binder blend, a commonly followed approach, may result in unnecessarily high recycling agent doses with a consequent increase in cost and greenhouse gas emissions and the over-softening of the mix at intermediate and low temperatures.