Processing conditions—fracture toughness relationships of asphalt concrete mixtures

Abstract

The effect of processing conditions (dynamic compaction) on the fatigue crack propagation behaviour of AC-20 asphalt concrete mixture was studied. Beams were prepared from AC-20 asphalt binder containing 8% asphalt by weight with and without dynamic compaction. The gradation used was Ohio Department of Transportation item 403, and was kept the same. Flexural static tests were conducted to determine the effect of dynamic compaction on both the ultimate strength and flexural modulus. Flexural fatigue tests were conducted on three identical notched specimens prepared using each of the two compaction techniques. Parameters controlling the crack propagation process were evaluated; namely, the energy release rate and the change in work, W _i expended on damage formation and history dependent viscous dissipation processes within the active zone (region ahead of the crack tip). The modified crack layer (MCL) model was employed to extract the specific energy of damage, γ′, a material parameter characteristic of the asphalt concrete mixture's resistance to crack propagation, and the dissipative coefficient, β′. It has been found that the dynamically and statically compacted AC-20 mixture displayed superior fracture resistance, as reflected in γ′ and β′. Also, the ultimate strength and modulus increased by about two-fold. Scanning electron microscopic examination revealed an obvious change in the morphology of the fracture surface. This is manifested in the appearance of a finer more dense texture in the case of the dynamically and statically compacted mixture. In addition, smaller more frequent dimples in binder rich areas are indicative of better adhesion between the binder and the aggregate. This in turn contributes to the increased fracture resistance of the dynamically and statically compacted AC-20 asphalt concrete mixture.