A 3D parameter correction technique for damage assessment of structural reinforced concrete beams by acoustic emission
Introduction
Acoustic emission is an effective nondestructive testing technique providing integrated information for damage detection in concrete. The method bases on propagation of elastic waves within a material due to a fracture, detection of them by appropriate sensors placed on its surface and analysis of the signals obtained by using different algorithms. By this means, even at low load levels, location, orientation, shape, time of origin and propagation of active fractures can be obtained. Obtaining this significant and comprehensive knowledge about the damages is only possible by analyzing the signal parameters of AE waveforms. Accuracy of these parameters directly affects the inspection results. However, due to heterogeneous nature of the concrete, AE signals deteriorate. This deterioration is the result of dispersive and attenuative behavior of this material which consists of cement, water, aggregate and other admixtures. While some studies present that these problems increase by aggregate size, later studies have shown that the real problem is related to the aggregate content [1], [2], [3]. Because, velocity of the elastic wave propagating through the concrete is increased by aggregate content and amount of high-frequency signals received from aggregates, which have higher acoustic impedance than cement paste, decreases [4], [5] revealed effect of aggregate size on attenuation by comparing elastic waves propagating within cement paste and concrete specimens having different water/cement (W/C) ratios. [6] investigated Rayleigh waves due to pencil lead breakages in cement paste specimens including different vinyl particles. They point out that wave velocity is affected by not only damage content, but also their sizes. [7] repaired large scale concrete surfaces with cement grout. According to their results it is shown that because dispersive feature of the material decreases after repairing, wave velocity increases and wave propagation distance and parameters change. [8] created tomographic images of concrete specimens by using attenuation information of ultrasonic waves. Transmitted and received signal forms were different to each other and amplitude of the signals decreased. Moreover, even the waves could not reach to sensor due to long distances and higher attenuations. [9] stated that Rayleigh wave is more sensitive to heterogeneity and is six times more attenuative than the longitudinal wave. [10] revealed that presence of steel fiber increases RA values due to additional propagation paths. Results obtained by [11] indicate that attenuation is higher in concrete due to additional voids aggregate sizes and more sensitive at high frequencies. [12] evaluated AE activities of cement paste, concrete and rock specimens at failure and it was seen that change in specimen size alters the wave propagation path. [13] studied wave propagations in concrete under impact and uniaxial compression affects. They concluded that attenuation does not continuously change with increase of frequency, it increases above certain angular frequency level. [14] investigated failure properties of artificial damaged cement-based materials. Their results show that change of P- and R-wave velocities disorders with respect to frequency. [15] examined wave velocity changes in fresh and hardened concrete. It was seen that while the wave exposes to dispersion in both states of concrete, its velocity increases with higher angular frequencies in fresh state. [16] revealed decrease of AE signal amplitude with longer wave propagation path in reinforced concrete beams. [17] related wave attenuation in steel strand embedded in concrete with concrete properties and wave propagation path. They concluded that amplitude, energy, duration and count decreases exponentially. Moreover, while attenuation ratio increases exponentially with concrete cover; it increases linearly with W/C ratio. [18] examined dispersion and attenuation in fresh cementitious materials under three different perspectives to validate the results. [19] revealed that pre-existing thermal cracks also dampen the wave propagation and attenuate AE signals. All of these situations catch disadvantages and should be solved.
As seen, although a number of studies exist in the literature about revelation of these problems and their effects on damage detection, they were not sufficient to abolish them. [20], [21] suggested Parameter Correction Technique (PCT) for regeneration of these problematic signal parameters. They firstly generated artificial AE sources and investigated their propagation in a carbon fiber reinforced polymer plate. Afterwards, they established relationships between transmitted and received amplitude values and applied these relations to experimental AE data. To prove the effectiveness of this method, they compared the clustering results obtained from raw and corrected parameters. As a conclusion, PCT arrayed amplitude distributions of the AE activities at certain levels and separated clusters more specifically. Thus, damage characteristics could be discriminated clearly. Then they [22] applied the procedure on the same material subjected to tensile-tensile fatigue load. Their results were also validated using C-scanning and computed tomography. Matrix cracking and delamination were identified using PCT.
PCT was firstly utilized for 3D space in previous work [23]. AE parameters due to pencil lead breakages propagating in concrete medium were corrected firstly in concrete which is excessively a heterogeneous material. This paper is focused on damage inspection in reinforced concrete by AE. Differently from the previous studies, in this study 3D-PCT technique has been utilized for AE events related to concrete cracks in structural reinforced concrete beams (plain RC and CFRP-strengthened RC) failed under three-point-bending. In addition, these specimens involved steel reinforcing bars, which increased attenuative and dispersive features.
Section snippets
Parameter correction technique (PCT)
Acoustic Emission (AE) is defined as release of transient elastic waves due to local sources within the stressed materials [24]. AE provides significant information about damage even at low load levels (Fig. 1.a). The method is based on detection of these waves by appropriate sensors located at the surface of the material and analyses of them. To analyze the data, elastic waves are transformed into electrical signals and are processed with different algorithms. The features of these signals are
Experimental work
Within the experimental work of the study, two 2350 × 250 × 150 mm reinforced concrete (RC) beams were produced as their concrete mix design (W/C = 0.66) is presented in Table 1. This composition was chosen to provide appropriate workability, viscosity and gradation. Three aggregate sizes (11.2/22 mm, 4/16 mm, 0/3 mm) were used in concrete mixture. Compressive strength of the standard cylinder concrete was 25 MPa and S420 steel bars were used as reinforcement.
The specimens were designed for
3D-PCT tests results
Firstly, recorded amplitude values of each sensor at all 48 nodes from four different artificial sources were obtained. Then, by stacking amplitude values of all nodes and interpolating them with 300% (X-direction) and 50% (Y- and Z-directions) resolutions; 32 3D volumetric contour maps were created for all sensors and all artificial sources (Fig. 5). By this means, correction relations were obtained for even non-measured 3D-positions. Afterwards, artificial source input vs recorded output
Conclusions
This paper defines development of an AE parameter correction technique for 3D space and its first application on reinforced concrete. Plain and CFRP-strengthened RC beams were produced and tested under three-point-bending. Before the mechanical tests, 3D-PCT tests were conducted on the specimens and attenuation features were revealed. Accordingly, amplitude and energy parameters of AE activities obtained from the flexural tests were corrected by using correction relations and following
Conflicts of Interest
None.
Acknowledgements
Financial support provided by TUBITAK (The Scientific and Technological Research Council of Turkey) to conduct this research under the grant number 118M172 is greatly acknowledged.
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