Effect of Low-Temperature Modifiers on HMA Mixture Aging and Cracking Resistance

The Superpave Performance Grade of an asphalt binder can be improved by adding different combinations of polymers and low-temperature modifiers. Although using various modifiers is not new, their effect, in particular softening oils, on long-term aging is not yet well understood. The objective of this study is to evaluate the effect of two widely-used oils to adjust binder grades on volumetrics, workability and fatigue cracking resistance of mixtures subjected to short- and long-term aging. Eight combinations of softening oils and polymers were used to prepare PG 58-34 binders. The mixtures prepared with these combinations were oven-aged for six and 14 h and used to compare their Flexibility Index (FI) results following Semi-Circular Bending Illinois FI test (SCB-IFIT) procedure. In addition, the effects of binder composition and aging on the compaction effort were assessed using the Superpave Gyratory Compactor (SGC) data. Furthermore, extracted and recovered binders of these mixtures were tested following the Linear Amplitude Sweep (LAS) test at intermediate temperature. Overall results indicate that oils used and aging susceptibility of the eight mixtures affected FI results differently. There is a good relationship (R2 = 83%) between the FI values and the absolute value of the LAS-B parameter of the extracted and recovered binders. This finding shows that FI sensitivity to aging is at least initially driven by the binder properties. The main finding from the analysis of the LAS-B parameter is that ΔTc measured for the 20-hr Pressure Aging Vessel (PAV) aging cannot be assumed to represent long-term oven aging of mixtures exceeding six-hours of oven aging at 135 °C.