Characterisation of oxidative ageing in asphalt concrete using a non-collinear ultrasonic wave mixing approach

A study to assess oxidative ageing of asphalt concrete using non-collinear wave mixing of two dilatational waves is presented. Criteria were used to assure that the detected scattered wave originated via wave interaction in the asphalt concrete and not from non-linearities in the testing equipment. These criteria included the frequency and propagating direction of the resultant scattered wave and the time-of-fight separation between the two primary waves and the resulting scattered wave. It was observed that asphalt concrete exhibits non-linear behaviour. It was also observed that the non-linear response decreases with increased ageing until approximately 24 h of ageing, after which the non-linear response exponentially increases. This observation is consistent with previous studies, including the acoustic emission response to thermal loading, and with changes in the dynamic modulus and fracture energy with increasing ageing.