Ultrasonic inspection of asphalt pavements to assess longitudinal joints

Longitudinal joints existing in between the lanes of asphalt pavements may initiate deterioration. Traditionally, core density, deflection, and nuclear density tests are used for the quality control. However, such techniques may not suit to the surface at the joints to assess their condition. Alternatively, the ultrasonic surface wave (USW) methods have the potential to both assess the longitudinal joints and estimate the pavement thickness at the same time. In this study, the USW are investigated on two lab-scale asphalt slabs (one laboratory prepared, and the other is cut from an as-built pavement) and on an in-service asphalt pavement to develop an ultrasound-based assessment methodology. Initially, an empirical compaction model is developed to produce the custom-size slab with the desired air-void profile to mimic a pavement with joint. Then, a variety of coupling systems between the pavement and the ultrasonic transducers are trialed to determine the optimum one. The recorded data are processed to determine the dispersion in velocity and the attenuation, which are then interpreted to estimate the pavement thickness and assess the joint quality, respectively. The dispersion curve is found capable of determining the pavement thickness with a precision of 1 cm, while the attenuation curve is observed to be affected by the transducer configuration excessively. Therefore, a normalisation technique, named the Fourier transmission coefficient (FTC), is implemented to reduce the undesired variability caused by the transducer coupling and type. Finally, it is demonstrated on an as-built pavement that the FTC has promising potential to detect, and hence evaluate the quality of longitudinal joints.