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A pulse-echo ultrasonic guided wave approach capable of monitoring the viscosity of asphalt binders as a function of temperature is presented. The method consists of sending a torsional wave from one end of a cylindrical steel rod embedded in asphalt binder and receiving the reflected signals. Experiments were performed on several binders of different performance grades, at temperatures ranging from 25ºC to 180ºC. First, the viscosity of the binders was measured using a rotational viscometer in accordance with ASTM standards. The change in signal strength of the end-of-waveguide reflection of the guided wave was also monitored for the same binders over the same range of temperatures. It was observed that the values obtained using the guided wave approach correlate well with the viscosity values obtained using the rotational viscometer. The method also appears capable of monitoring changes in viscosity due to ageing of the binders. The method has the advantage of having no moving parts, which makes it attractive for the development of a system that is capable of monitoring viscosity in asphalt binders in the asphalt industry. Examples of industrial applications are briefly summarised. [ABSTRACT FROM AUTHOR]

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-flight 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. [ABSTRACT FROM AUTHOR]

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, frequency-dependent ultrasonic measurements must also be carried out. Results indicate that complex moduli are sensitive to the level of ageing in asphalt concrete mixtures. [ABSTRACT FROM AUTHOR]