Characterisation of oxidative ageing in asphalt concrete - Part 1: Ultrasonic velocity and attenuation measurements and acoustic emission response under thermal cooling

Asphalt concrete mixtures with different levels of oxidative ageing, prepared by oven-ageing the mixture at 135 degrees C for different amounts of time, were used to study the effects of oxidative ageing upon the ultrasonic velocity and attenuation measurements. The embrittlement temperatures, ie low-temperature cracking, of the mixtures with different amounts of oven-aged times were also evaluated by monitoring the acoustic emission response of the specimens when cooled from room temperature to -50 degrees C. It was observed that the embrittlement temperature increases (ie the onset of embrittlement occurs at a warmer temperature) as the amount of ageing increases. However, the rate-of-change of the embrittlement temperatures with ageing time increases gradually until 24 h of ageing, after which the rate of change significantly increases. It was also observed that both the dilatational and shear velocities increase up to approximately 24 h of ageing, after which they significantly decrease with ageing. Also, both the dilatational and shear attenuation decrease with ageing up to around 24 h of ageing, after which both attenuations strongly increase. The results from both ultrasonic and acoustic emission tests are consistent with results obtained using the mechanical disc-shaped compact tension (DC(T)) fracture tests.