This paper describes the use of an ultrasonic imaging technique (Locadiff) for the Non-Destructive Testing & Evaluation of a concrete structure. By combining coda wave interferometry and a sensitivity kernel for diffuse waves, Locadiff can monitor the elastic and structural properties of a heterogeneous material with a high sensitivity, and can map changes of these properties over time when a perturbation occurs in the bulk of the material. The applicability of the technique to life-size concrete structures is demonstrated through the monitoring of a 15-ton reinforced concrete beam subject to a four-point bending test causing cracking. The experimental results show that Locadiff achieved to (1) detect and locate the cracking zones in the core of the concrete beam at an early stage by mapping the changes in the concrete's micro-structure; (2) monitor the internal stress level in both temporal and spatial domains by mapping the variation in velocity caused by the acousto-elastic effect. The mechanical behavior of the concrete structure is also studied using conventional techniques such as acoustic emission, vibrating wire extensometers, and digital image correlation. The performances of the Locadiff technique in the detection of early stage cracking are assessed and discussed.

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International audienc

This paper describes the use of Digital Image Correlation (DIC) techniques for the cracking assessment of reinforced concrete (RC) massive beams and walls. DIC is known to provide accurate and detailed information on displacement and strain fields. Non-contact measurements can be used to evaluate concrete cracking of destructive tests carried out on a wide range of specimen scales. When applied to large RC structures tested outdoors or in difficultly controllable conditions, DIC-based methods may lead to erroneous results. In this study a post-processing procedure is presented to cope with noisy full-field measurements. The proposed cracking assessment approach is validated on a large experimental campaign. Four points bending tests are carried out on RC beams: firstly on full-scale rectangular beams and then on mock-ups scaled down by 1/3. In addition, fours RC walls are tested under in-plane cyclic shear up to failure. Digital images taken throughout the tests are processed by DIC techniques to provide in-plane displacement and strain fields. Full-field measurements are post-processed by the noise-filtering technique and the cracks patterns are identified. Crack widths are measured and compared with measurements obtained from conventional point-based sensors (linear variable differential transformer LVDT and fibre-optic FO transducers). The proposed DIC-based post-processing provides accurate estimation of cracks width for most of the tests. The analyses carried out on the two groups of RC beams show a scale-effect on the cracks width.