Monitoring viscosity in asphalt binders using an ultrasonic guided wave approach

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.