Estimation of oxidative ageing in asphalt concrete pavements using non-collinear wave mixing of critically-refracted longitudinal waves

A study using non-collinear ultrasonic wave mixing of subsurface longitudinal waves for the assessment of oxidative ageing in asphalt concrete samples with increasing levels of oven ageing is presented. The use of subsurface waves allows for non-destructive interrogation of bituminous structures when only one side of the test specimen is available for testing, such as asphalt concrete pavements. The following criteria were used to verify that the non-linear wave originated within the test specimen due to the non-collinear wave mixing, and not from non-linearities inherent to the testing instrumentation: (1) frequency; (2) amplitude; (3) propagation direction; and (4) time-of-flight separation. It was observed that the non-linear response of the asphalt concrete mixtures decreases with ageing until about 24 h of oven ageing, after which it increases exponentially with ageing. These results correlated well with the results from previous studies, demonstrating the effectiveness of this technique. Potential applications of the technique for the health monitoring of asphalt pavement surfaces are presented.