Characterisation of oxidative ageing in asphalt concrete - Part 2: Estimation of complex moduli

The estimation of the dynamic Young's (uniaxial) and shear complex moduli based upon ultrasonic measurements are presented. Phase velocities were used instead of group velocities because of the highly attenuative nature of asphalt concrete. It was observed that the dynamic moduli increase from 0 to 24 h and decrease from 24 to 36 h of oven-ageing. The modulus obtained using ultrasonic measurements is also compared with the modulus obtained using the AASHTO recommended mechanical testing procedure. The differences are due to scattering effects, which are present in ultrasonic testing and absent in mechanical testing. It was also observed that, to avoid the uncertainty associated with assuming a suitable value for the Poisson's ratio, both the dilatational and shear velocities and corresponding attenuation measurements must be carried out. Furthermore, to eliminate the need for traditional mechanical testing during the estimation of complex moduli, frequencydependent ultrasonic measurements must also be carried out. Results indicate that complex moduli are sensitive to the level of ageing in asphalt concrete mixtures.