Evaluation of Concrete Distributed Cracks by Ultrasonic Travel Time Shift Under an External Mechanical Perturbation: Study of Indirect and Semi-direct Transmission Configurations

Techniques based on non-linear acoustics have been proven sensitive to micro-defects in heterogeneous materials, such as concrete, but their implementation on-site is very restrictive. Ultrasonic travel time shift, a technique where a high frequency ultrasonic wave probes the medium while a low frequency elastic wave disturbs it to create a "time delay", is a new promising technique that may be used efficiently on-site. This technique is based on nonlinear behaviour of concrete. Moreover, this technique offers the possibility of evaluating linear parameter, such as ultrasonic pulse velocity of direct waves. The scope of this paper is to study the applicability of the technique at different level of concrete damage and define its advantages, limitations in order to optimize its use. Because of the large dimensions, the geometry and limited access to various faces of existing structures, the transducers often have to be set on the same side. Two types of configurations, the indirect transmission (with incident waves at 90(a similar to)) and the semi-direct transmission (with incident waves at 45(a similar to)), were studied with cement base samples at different levels of damage (generated by freeze-thaw cycles). Up to now, test results have shown that time-shift is more sensitive when used in an indirect configuration of transmission rather than in a semi-direct configuration. Overall, the non-linear time-shift technique is much more sensitive to the initiation of cracking than linear indicators and its versatility (different indicators for different levels of damage) is of interest for rapid testing of structures.