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.

This meta-analysis of prospective cohort studies demonstrates that obesity negatively influences the overall complication rate, dislocation rate, functional outcome and operative time of primary total hip arthroplasty.

In this paper, we describe an experimental study of concrete behavior under a uniaxial tensile load by use of the thermally-compensated Coda Wave Interferometry (CWI) analysis. Under laboratory conditions, uniaxial tensile load cycles are imposed on a cylindrical concrete specimen, with continuous ultrasonic measurements being recorded within the scope of bias control protocols. A thermally-compensated CWI analysis of multiple scattering waves is performed in order to evaluate the stress-induced velocity variation. Concrete behavior under a tensile load can then be studied, along with CWI results from both its elastic performance (acoustoelasticity) and plastic performance (microcracking corresponding to the Kaiser effect). This work program includes a creep test with a sustained, high tensile load; the acoustoelastic coefficients are estimated before and after conducting the creep test and then used to demonstrate the effect of creep load.

This paper presents an ultrasonic method, based on the nonlinear acoustic mixing of coda waves with lower-frequency swept pump waves, for providing an efficient global detection of small cracks in cementitious materials. By simultaneously comparing, forboth uncracked and cracked mortars, the ultrasonic velocity variations and decorrelation coefficients between the unperturbed and perturbed signals with pump amplitude, this method makes it possible to accurately detect cracks with widths of around 20 μm in correlation with velocity variations of approximately 0.01%. The potential influence of certain material parameters such as microscopic damage is also discussed

An ultrasonic method providing for an efficient global detection of defects in complex media (multiple scattering or reverberating media) is reported herein; this method is based on the nonlinear acoustic mixing of coda waves (stemming from multiple scattering) with lower frequency-swept pump waves. Such a nonlinear mixing step is made possible by the presence of nonlinear scatterers, such as cracks and delamination, yet remains absent when the waves are scattered only by linear scatterers, as is the case in a complex but defect-free medium. A global inspection is achieved thanks to the use of wide-band coda and pump signals, which ensure the excitation of many resonances along with a homogeneous acoustic energy distribution in the medium. We introduce the existing sensitivity tools developed for Coda Wave Interferometry in extracting the pump amplitude-dependent parameters of the coda waves associated with effective nonlinear parameters of the medium. By comparing results at two damage levels, these effective nonlinear parameters are shown to be correlated with crack presence in glass samples. The mechanisms potentially responsible for the observed amplitude dependence on the tested elastic parameters and waveform modification are discussed.

The Coda Wave Interferometry (CWI) analysis serves to monitor the variation of propagation velocity in a heterogeneous medium with high precision (10(-3)% in relative terms). In combination with acoustoelastic theory, this type of analysis offers an NDT method for stress evaluation and/or damage detection. Since the CWI method is intended to evaluate extreme levels of accuracy, the presence of bias under certain circumstances can undermine evaluation results and/or test repeatability. In this paper, we offer a bias control technique involving the use of a second (reference) specimen for CWI analysis that is designed to compensate: (1) the thermally-induced velocity variation due to environmental temperature fluctuations; and (2) bias originating from experimental procedures. The presentation of this technique contains both a theoretical analysis and experimental protocol for the purpose of implementation. Furthermore, comparisons of experimental results have been included in order to demonstrate the utility of this bias control technique under laboratory conditions.

Locadiff, an innovative imaging technique based on diffuse waves, has recently been developed in order to image mechanical changes in heterogeneous geological or man-made materials. This manuscript reports the on-site application of Locadiff to locate several pre-existing cracks on an aeronautical wind tunnel made of pre-stressed concrete. Using 32 transducers working at ultrasonic frequencies (80-220 kHz) where multiple scattering occurs, we monitor during 15 minutes an area of 2.5 m×2.5 m of a 35 cm thick wall. With the wind tunnel in its routine operation, structural changes around the cracks are detected thanks to their closing or opening due to slight pressure changes. By mapping the density of such microstructure changes in the bulk of the material, the locating of three pre-existing cracks is properly performed in 3D.

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