Elsevier

NDT & E International

Volume 68, December 2014, Pages 98-104
NDT & E International

Small crack detection in cementitious materials using nonlinear coda wave modulation

https://doi.org/10.1016/j.ndteint.2014.08.010Get rights and content

Highlights

  • A method aimed at detecting small cracks in concrete and based on Coda Wave Interferometry has been proposed.

  • Very small cracks with widths of around 20 µm have been detected in comparison with an uncracked specimen.

  • Reproducibility and accuracy have been assessed in preparation for tests on larger structures.

Abstract

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, for both 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.

Introduction

Concrete is a widely used construction material by virtue of its cost and mechanical properties. Due to its low tensile strength however, concrete is very sensitive to crack formation. Cracks are taken into consideration in design code recommendations, yet they still endanger the durability of structures since aggressive substances can easily penetrate and deteriorate the material, leading in some cases to structural failure. Cracks are also potentially involved when leakage is detected [1]. Cracks in concrete are therefore responsible for significant inspection, maintenance and repair costs [2], [3]. In order to optimize structural health management, Non-Destructive Testing (NDT) has been extensively studied. Among all NDT techniques, ultrasonic methods are considered advantageous by providing information on mechanical properties in areas not directly accessible from the surface. The damage of a concrete structure, in the form of cracks for example, changes the structure׳s mechanical properties and can therefore be detected and monitored using an ultrasonic NDT method. In general however, linear ultrasonic methods are only able to detect relatively large cracks, either by monitoring the crack influence on the effective acoustic properties of the medium or through the acoustic scattering caused by the crack. In the former case, in-depth knowledge of the medium properties without the crack is required, but such a condition is unrealistic for concrete due, for instance, to the evolution of medium acoustic properties with time or temperature. In the latter case, the identification of an acoustic echo from the crack requires both the use of an acoustic wavelength of the same order of magnitude as the crack size and the absence of strong scattering from the medium matrix. Such only occurs at wavelengths greater than a few centimeters (i.e. frequencies less than ~50 kHz), where concrete acts as an effective propagation medium. To be detected, the cracks need to be opened (in order to exhibit a sufficient acoustic mismatch with the matrix) and have a size of a few centimeters if size quantification is a target of the investigation [4], [5], [6]. Consequently, recent studies have led to developing nonlinear ultrasonic methods to increase the sensitivity to damage [7], [8]. In using diffuse waves, the detection of large cracks/notches and the monitoring of crack evolution have both been achieved [9], [10], [11], [12], [13], which has allowed emphasizing the sensitivity of diffuse ultrasound to a crack opening. Diffuse ultrasound can also be used to monitor the evolution of crack geometry over time [14]. However, the detection of small cracks in cementitious materials remains a great challenge for NDT techniques, despite the special interest in making such detections since these cracks may lead to undesirable premature failure or leakage.

Coda Wave Interferometry (CWI), which relies on an analysis of the last part of the signal formed by multiple scattered waves, offers a sensitive method for detecting time-lapse perturbations on a propagation medium. This method was originally developed in order to measure small changes in wave velocity within the earth׳s crust [15] and was then introduced to the NDT community by Snieder [16]. CWI has been successfully adapted to concrete, which is a highly heterogeneous material, for determining nonlinear elastic properties [17] or detecting and locating a small defect (holes with a diameter of several millimeters) [18], [19]. By controlling the thermal bias [20], CWI can monitor propagation velocities with high precision (10−3% concerning relative velocity variations) in concrete and moreover provides information on the level of microcracking induced by loading [21].

In addition, nonlinear acoustic modulation amplifies the signature of a defect when used with an acoustic load provided by a pump source. The sample is subjected to both a low-amplitude ultrasonic wave (known as a probe wave) and a large-amplitude wave at a lower frequency (known as a pump wave). If the sample contains nonlinearity, caused for example by the presence of cracks or, more generally, contact-type defects, the probe wave will become modulated due to the variation in local and surrounding effective elasticity (elastic modulus or acoustic dissipation) resulting from pump excitation [22], [23], [24], [25], [26], [27]. Many different signal shapes can be input as pump and probe waves; however, the use of higher-order modulation side lobes or amplitude-modulated pump waves has shown greater sensitivity to the presence of cracks than other nonlinear modulation techniques [28].

With the objective of detecting small cracks in cementitious materials, we present herein a novel NDT technique that combines the use of diffuse ultrasound with the nonlinear modulation method. Based on a previous study that had been conducted to detect extended nonlinear damage (cracks) in an initially linear medium (glass) [29], we are proposing a simple and robust method to detect cracks in an initially nonlinear material (concrete or mortar).

Section snippets

CWI analysis by means of stretching

In CWI, multiple scattered waves are introduced to detect temporal changes in a medium considered as an interferometer. The corresponding model [16], [30] allows the detection of temperature fluctuations [31], external loading [18] and damage [19], [20]. CWI has been successfully applied to concrete [32], which contains a large quantity of scatterers of varying composition, size and shape. This paper will focus on the observed variation in elastic wave velocity associated with the presence of a

Specimen preparation

Two series of three mortar samples of equivalent geometry (7 cm×7 cm×28 cm) were mixed with a water-to-cement ratio of 0.35. Mortar mixtures consisted of 1350 g of sand (0/2 sand), 450 g of Portland cement (CEM II), 155 g of water and 8 g of superplasticizer (ChrysoFluid Optima 206). All mortar preparations were carefully vibrated in order to minimize the amount of occluded air. After 1 day of curing under sealed conditions in an air-conditioned room at a temperature of 20 °C, the specimens were

Influence of the presence and size of cracks on CWI measurements

The detailed received signals are comparable in shape and amplitude for all specimens. With the pump wave amplitude increasing from 0 to 60 dB however, the signals recorded from an uncracked (1C) specimen are nearly identical (Fig. 4a and b), while those from a cracked specimen (1B) show a small phase shift and waveform distortion, which is only visible in a late time-window (Fig. 4c and d). This finding therefore indicates that nonlinear modulation has taken place due to the presence of a crack

Conclusion and perspectives

In this study, the implementation of a novel NDT method combining the use of Coda Wave Interferometry (CWI) with a nonlinear acoustic modulation process has been proposed according to the original method developed in [29]. This method displays the capability of global detection for very small defects in mortar using coda signals that potentially originate from multiple scattering or reverberating media. The main idea here is to utilize wide-frequency band signals for both the probing coda

Acknowledgments

The authors gratefully acknowledge the financial support provided for this study by GIS LIRGeC and France׳s Loire Valley Regional Council. Our thanks are extended to Robert Sachs, a native English speaker commissioned to proofread the final version of this paper.

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