LOW-TEMPERATURE FRACTURE TOUGHNESS OF POLYETHYLENE-MODIFIED ASPHALT BINDERS

One of the key factors that determines the susceptibility of a pavement to thermal cracking is the ability of the binder to withstand or relieve thermal stresses before they reach a critical point where cracks form. The mechanisms by which the addition of polyethylene improves the low-temperature properties of asphalt binders are discussed. Also examined is the degree to which the quality of the additive dispersion influences these mechanisms. Notched bending beam tests conducted at -20 deg C yielded fracture toughness values for three different polyethylene-asphalt systems. In all systems studied, the fracture toughness was found to increase linearly with the additive content. The chlorinated polyethylene-modified binders produced the toughest samples, followed by the stabilized polyethylene and the unstabilized polyethylene binders, respectively. High-magnification photographs, made with an environmental scanning electron microscope, yielded additional information concerning the nature of the failure mechanisms in these systems. The combined results show that greater toughening occurs with more finely dispersed polymers and with greater compatibility at the interface between the polymer and the asphalt matrix. These observations are consistent with the mechanisms of crack pinning and shear yielding, both of which are regularly found in multiphase thermoplastic materials.