J Nondestruct Eval (2013) 32:25–36 DOI 10.1007/s10921-012-0155-7 Evaluation of Concrete Distributed Cracks by Ultrasonic Travel Time Shift Under an External Mechanical Perturbation: Study of Indirect and Semi-direct Transmission Configurations D. Bui · S.A. Kodjo · P. Rivard · B. Fournier Received: 26 October 2011 / Accepted: 8 October 2012 / Published online: 26 October 2012 © Springer Science+Business Media New York 2012 Abstract Techniques based on non-linear acoustics have been proven sensitive to micro-defects in heterogeneous ma- terials, 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 nonlin- ear behaviour of concrete. Moreover, this technique offers the possibility of evaluating linear parameter, such as ultra- sonic 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 lim- ited access to various faces of existing structures, the trans- ducers often have to be set on the same side. Two types of configurations, the indirect transmission (with incident waves at 90 ◦ ) and the semi-direct transmission (with inci- dent waves at 45 ◦ ), were studied with cement base sam- ples 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 D. Bui ( ) · S.A. Kodjo · P. Rivard Civil Engineering Department, Université de Sherbrooke, Sherbrooke, Québec, Canada e-mail: diem.bui@usherbrooke.ca B. Fournier Département de Géologie et Génie Géologique, Université Laval, Québec, Québec, Canada and its versatility (different indicators for different levels of damage) is of interest for rapid testing of structures. Keywords Ultrasonic testing · Non destructive · Nonlinearity · Concrete · Damage · Coda wave · Interferometry 1 Introduction Many non-destructive techniques have been developed to assess concrete damage due to cracking. Ultrasonic pulse velocity (UPV), resonant frequency and attenuation are tra- ditional linear acoustics techniques carried out in the lab- oratories to assess concrete damages. However, it has been observed that non-linear acoustic techniques were more sen- sitive to small defects and low-damage levels [1]. Concrete is a heterogeneous material that contains a system of joints composed of aggregates, cement paste, voids and micro cracks. Such a system confers a nonlinear elasticity to con- crete where, when stress is applied, elastic and viscoelas- tic properties are modified momentarily [2]. Micro damages in concrete accentuate this nonlinear behaviour. Nonlinear acoustic techniques uses high amplitude acoustic waves to apply stress to the concrete in order to open/close the micro cracks and to observe the change in properties [3]. Many techniques have been derived from this phenomenon (non- linear resonance test, quasi static test, etc.). Nonlinear phe- nomena, such as harmonic generation, have shown a very high sensitivity to early damages [1, 4]. Among the nonlin- ear acoustic techniques proposed by several researchers for the characterization of micro defects in concrete materials, the ultrasonic travel time shift technique appears to be the most suitable for field work. This technique takes its roots in the coda wave interfer- ometry technique and combines it with the study of non- linear behaviour in concrete in a methodological way that