Effect of bitumen type, temperature and aging on mixed I/II fracture toughness of asphalt binders-experimental and theoretical assessment

The overall characteristics and fracture behavior of asphalt pavements and overlays is significantly controlled by the mechanical behavior of bitumen used in the mixture of asphalt concrete. The bitumen behaves like a dominantly brittle and elastic medium under subzero temperatures. In practice, cracked asphalt pavements experience mixed mode tensile-shear deformations due to moving traffic loads or cyclic thermal gradients. On the other hand, aging phenomenon can affect significantly the overall strength and load bearing capacity of pavements and bituminous asphalt mixtures. Different binders with different penetration grades and performance characteristic are often used for paving the roads. In order to investigate the effect of binder type and aging condition on the mixed mode I/II cracking response of bitumen materials, an experimental and theoretical study is performed in this research. Three binders: namely 30/40 bitumen, 60/70 bitumen and modified bitumen with SBS polymer (i.e. polymeric binder) are used to conduct a full set of mixed mode I/II fracture toughness experiments using an inclined edge notched bend beam specimen. The experimental results showed significant influence of mode mixity, bitumen type and aging on the fracture toughness data. The fracture toughness is reduced for the whole tested binders by moving from pure mode I to pure mode II. While aging has negative effect on the crack growth resistance of 30/40 and 60/70 binders, the mixed mode I/II fracture toughness of polymeric modified bitumen was increased after aging. The morphology of fracture surface and direction of fracture was studied for the tested binders under different environmental and loading conditions. The mixed mode I/II fracture toughness data were also predicted using two generalized tangential stress/strain based fracture criteria.