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In Japan, the most of waste concrete scraps have been reused as roadbed materials and the recycling ratio of waste concrete scraps has been kept over 95 % from 2000. However, it is expected that the demands of waste concrete scraps as roadbed materials would decrease even though the waste concrete scraps increase with the pulling down buildings in next decade. These facts mean that the recycling of waste concrete scraps would be in the negative situation. Therefore, the development of new recycling technology of waste concrete scraps is paramount importance in Japan. In this work, the pulsed power discharges inside of waste concrete scraps immersed in water were used to reproduce the coarse aggregate. In the experiments, the Marx generator, which storages the energy of 6.4 kJ/Pulse, was used as pulsed power source and the point to hemisphere mesh electrode was immersed in water. The pulsed voltages with 400 kV of peak voltage were applied to the concrete scraps placed on hemisphere mesh and the reproduced coarse aggregates were evaluated after the repetitive discharge treatment. From the experimental results, it is found that the coarse aggregates reproduced by 40∼60 pulsed discharge treatments have the enough qualities of the bone-dry density and the water absorption rate to satisfy JIS (Japan Industrial Standard) regulation (A 5005). In addition, the concretes consisted of the reproduced coarse aggregate also have the enough compressed strength and Young’s module to utilize as the construction material.

Summary form only given. In Japan, the most of waste concrete scraps have been reused as roadbed materials and the recycling ratio of waste concrete scraps has been kept over 95% from 2000. However, it is expected that the demands of waste concrete scraps as roadbed materials would decreases even though the waste concrete scraps increase with the pulling down buildings in next decade. These mean that the recycling of waste concrete scraps would be in the negative situation. Therefore, the development of new recycling technology of waste concrete scraps is paramount importance in Japan. In this work, the pulsed power discharges inside of waste concrete scraps immersed in water were used to reproduce the coarse aggregate. In the experiments, the Marx generator, which storages the energy of 6.4 kJ/Pulse, was used as pulsed power source and the point to hemisphere mesh electrode was immersed in water. The pulsed voltages with 400 kV of peak voltage were applied to the concrete scraps placed on hemisphere mesh and the reproduced coarse aggregates were evaluated after the repetitive discharge treatment. From the experimental results, it is found that the coarse aggregates reproduced by 80 pulsed discharge treatments have the enough qualities of the bone dry density and the water absorption ratio to satisfy JIS (Japan Industrial Standard) regulation (A 5005). In addition, the concretes consisted of the reproduced coarse aggregate also have the enough compressed strength and Young's module to utilize as the construction material.

Cracking in concrete due to corrosion of rebars in reinforced concrete is one of critical problems in concrete structures. To clarify cracking process, acoustic emission (AE) measurement is applied. In an accelerated corrosion test, AE events are detected and monitored continuously. Comparing with permeation of chloride ions, it is found that onset of corrosion and nucleation of cracking can be qualified from AE activity. Applying SiGMA procedure, nucleation mechanisms of cracks due to expansion of corrosive product are identified. During extension of the surface crack, tensile cracks are nucleated dominantly. For the spalling crack, both the tensile and the shear cracks are generated, as the former dominates the latter approaching to a stress‐free surface. In contrast, it is found that the internal crack is nucleated mainly due to shear‐crack motion.

Acoustic emission (AE) techniques have been widely studied in concrete engineering and are presently being applied to monitoring concrete structures in service. In order to assess the damage levels of the structures, a criterion based on the Kaiser effect of AE is proposed in the recommended practice recently published by the Japanese Society for Non-Destructive Inspection. New AE parameters of load and calm ratio are defined for qualification of the damages. Accordingly, the feasibility of the damage qualification is experimentally examined using reinforced concrete (RC) beams damaged under incremental cyclic loading. It was found that the damages qualified by the 2 ratios were in good agreement with actual damages of the beams. This suggests that the damages of such RC structures in service as bridges, docks, and buildings are quantitatively assessed by simply applying cyclic loading and monitoring AE activity.

The moment tensor corresponding to acoustic emission (AE) source contains kinematical information on crack motion in a material. In order to determine the moment tensor components from AE waveforms, the SiGMA (simplified Green's functions fro moment tensor analysis) procedure is developed, where AE sources can be located, classified into tensile cracks and shear cracks, and then the directions of crack motions are determined. In this paper, the SiGMA procedure is further extended in rock-like materials, and a crack micrography is proposed. To this end, a relation between a damage parameter in damage mechanics and the moment tensor is clarified. Thus, damage evolution of the notched beam under bending is estimated from the trace components of the moment tensor. The scalar damage parameter is closely related with the crack density consisting of the crack volume. Therefore, the crack volumes are quantitatively estimated in unaxial compression tests of plate specimens with a slit. In the linear elastic fracture mechanics (LEFM), the direction of crack extension can be determined from the concept of the maximum circumferential stress. Since the direction of crack motion can be derived from the eigenvectors of the moment tensor, the relation is applied to estimate the normalized stress intensity factors.

A quantitative analysis of acoustic emission (AE) waveforms, which was developed as a Simplified Green's Function for Moment Tensor Analysis (SiGMA) code, is revised. By the analysis, cracks of AE sources are located, classified into a tensile crack and a shear crack, and their orientations are determined. For practical application to concrete, both experimental and analytical procedures are reexamined on the basis of the background theory of moment tensor. To apply the SiGMA analysis to AE waveforms recorded, a basic experimental procedure is discussed and established. In the tensile test of an L-shaped reinforced concrete model, nondestructive evaluation (NDE) of cracking mechanisms is investigated. To improve the accuracy of SiGMA solutions, a postanalysis is developed. Unreliable solutions are screened out, based on the discrepancy between SiGMA solutions of the experiment and those of synthesized waveforms. The mechanisms of the fracture process zone are studied in the bending test of a notched mortar beam. These results demonstrate the applicability of the moment tensor analysis to both NDE of concrete structures and to experimental fracture mechanics in concrete.

A nondestructive evaluation method based on ultrasonic spectroscopy was studied to estimate crack depth in concrete members. A surface method, in which both a transmitter and a receiver were attached to the same surface of a specimen, was investigated. Spectral responses were measured by driving sweep-mode sinusoidal waves from a signal generator.

Visualised techniques of non-destructive testing (NDT) are investigated for concrete cracking. Acoustic emission (AE) techniques have been extensively studied and applied to concrete and concrete structures. Currently, source kinematics of crack location, crack type and orientation can be quantitatively identified by the SiGMA procedure of AE. Thus, AE source characteristics in diagonal-shear failure are visually clarified. The impact-echo method is developed for NDT of defects in concrete. Resonance frequencies are applied to estimate the presence and the depth of defects in concrete. Because it is not easy to identify particular peak frequencies only responsible for the presence of defects, SIBIE procedure is developed. The procedure is applied to an ultrasonic test of an artificial surface-crack in concrete. It is shown that the crack-depth is clearly visualised, even in the case that the crack is partially filled with water.

A source inversion technique for acoustic emission (AE) is studied to identify crack locations, crack types, and crack orientations of AE sources. A computer code named SiGMA (simplified Green's function for the moment tensor analysis) is developed to determine moment tensor components from AE waveforms. To classify a crack into a tensile crack and a shear crack, the eigenvalue analysis of the moment tensor is employed and thereby orientations of cracks are determined from directions of eigenvectors. The procedure proposed is applied to a pull-out test and a cylinder-tensile test of concrete. Results confirm the applicability of the AE inversion technique to determine crack kinematics.

The impact-echo method has been applied to nondestructive evaluation for concrete structures. Taking out resonant frequencies responsible for the locations of reflectors in the frequency spectra, identification of reflectors is performed. Stack imaging of spectral amplitudes based on impact-echo (SIBIE) is developed, in order to improve the impact-echo and visually identify the locations of reflectors. In the present paper, two-dimensional dynamic BEM analysis is performed to confirm an applicability of SIBIE. At the peak frequencies in the spectra, distributions of the stress and deformation are clarified.

As a numerical technique, the Finite Element Method (FEM) is versatile for evaluating problems with complex geometries and materials. It is, however, not efficiently applicable to problems of wave scattering in an infinite domain. The Boundary Element Method (BEM) has been used very effectively in elastic wave problems with a homogeneous infinite domain. Therefore, it is reasonable to couple the FEM with the BEM in order to provide a single methodology for analyzing problems including non-homogeneous materials in an infinite domain. In the present paper, a method to couple a BEM domain with a FEM domain is proposed on the basis of the variational principle. To demonstrate the usefulness of the proposed method, P, SV and SH wave fields were investigated. From the results, applicability of the proposed method is discussed.

Surface motions due to a disbonding of a stainless overlay welded on base metal of a pressure vessel have been measured by the use of a commercially available flat‐frequency displacement transducer. The experimental result showed that a buried tensile crack (penny‐shaped crack) can be generated by hydrogen‐induced disbonding of stainless weld overlay from base metal. Acoustic emission (AE) was found to be very sensitive and effective to monitor the disbonding. Some tens of AE events could be located by using two meaningful Δt values. The detected waveforms due to the disbonding exhibited interesting variation as a function of the distance between the source epicenter and the transducer location. On the basis of the theory of elastodynamics and dislocation models, simulation analysis was made for surface motions due to a penny‐shaped crack parallel to the stress‐free surface in a half‐space. Simulated waveforms were obtained as a function of the distance between the epicenter and the observation position. ...

Based on a generalized theory of acoustic emission which we previously proposed, an inverse procedure for the source characterization of AE is discussed. To determine the source kinematics (crack type and crack orientation), a moment tensor representation is introduced. It is clarified that the eigenvalue analysis of the moment tensor can afford essential information on the kinematics of crack movement. Then, an deconvolution analysis provides the kinetics of crack motion as a source-time function. To confirm the applicability of the source inversion procedure, numerical experiments are performed on the surface observation of AE. By employing simulated AE waveforms, source characteristics are inversely solved and determined. Results demonstrate a great promise of the proposed procedure for the source characterization of AE.

The numerical method for analysing elastic wave scattering from surface and near-surface inhomogeneities in a half space is presented. In order to develop an alternative method solving the elastodynamic problems in a half space, we propose the method of applying a fundamental solution in an infinite space and replacing an infinite stress-free boundary in a half space by a truncated finite-lenght boundary. On the basis of the indirect boundary integral equation (the indirect BEM formulation: the single layer potential formulation), the effective lenght of the stress-free boundary is determined. The feasibility of this method is investigated by solving the problems of out-of-plane shear wave (SH wave) fields, and its applications to the seismic responses of canyons, tunnels, and trenches are shown.

An alternative method for calculating the two-dimensional scattering of P, SV and SH waves by near-surface inhomogeneities is presented. As problems are classified into the elastodynamic problems in a half-space, the BEM formulation based on a fundamental solution in a half-space is usually employed. A fundamental solution of the SH wave field in a half-space is obtained with no difficulty, but the problem of the P and SV wave fields in a half-space is of complicated nature.In the present paper, we replace an infinite stress-free surface in a half-space by a trancated boundary of finite length and propose a method to apply the fundamental solution in an infinite space to the problems in a half-space. On the basis of the indirect boundary element method formulation, the feasibility of the method presented is investigated by solving several problems of P, SV and SH wave fields.

Using a commercially available flat-frequency displacement transducer which produces excellent displacement response, we have measured surface motions due to a disbonding of stainless overlay from base metal. The experimental result showed that a buried tensile crack (penny-shaped crack) can be generated by hydrogen-induced disbonding of stainless weld overlay from base metal. Acoustic emission (AE) was found to be very sensitive and effective to monitor the disbonding. Planar source location using a set of three Δt values during this process was impossible because of bounded specimen configuration. However, some tens of AE events could be located by using two meaningful Δt values. The detected waveforms due to the disbonding exhibited interesting variation as a function of the distance between the epicenter of the source and the transducer location. This result suggests potential of a new source location method based on waveform measurement. On the basis of the theory of elastodynamics and dislocation models, simulation analysis was made for surface motions due to a penny-shaped crack parallel and inclined to the stress-free surface in a half-space. Simulated waveforms were obtained as a function of the distance between the epicenter and the observation position. Comparison between the detected and simulated waveforms indicated remarkable agreement in the early time response. This excellent agreement demonstrates the applicability of the transducer and the present simulation analysis to quantitative waveform analysis relating to fundamental AE work.

The development of mathematical theory of acoustic emission (AE) is summarized, and a waveform analysis of the moment tensor solution is proposed. On the basis of the theory, vertical surface motions (AE) in the disbonding process of stainless overlay were simulated, determining source characteristics from the observation of a micrograph. It is confirmed that a simulated displacement agrees quite well with a detected AE waveform up to the arrival of reflection waves, including the scale of amplitude.To check the effect of the inclined angle on results of the deconvolution analysis, simulated waveforms were deconvoluted with error on the inclination angle. According to the results, unless the inclination angle is correctly known, the source time functions obtained may include considerable error except for a rise time. It implies that the determination of crack orientation is of significant importance, before attempting the deconvolution analysis.The applicability of the moment tensor solution was investigated by numerical experiments for inclined tensile and shear cracks. Simulated AE waveforms in a half space were employed, and then components were determined within 8% discrepancy. The results of the eigenvalue analysis show a great promise of the moment tensor solution for determining the crack type and the crack orientation.

Theoretical treatment of acoustic emission (AE) source is investigated in the disbonding process of stainless steel overlay from base metal. Source representations of AE due to disbonding are studied in the simulation analysis based on a generalized theory. Taking into account a crack size, a point dislocation model for a tensile crack is considered as an AE source. The effects of crack shape and crack expansion on waveforms are clarified. Following Eshelby’s process, penny‐shaped tensile cracks are mainly employed as AE sources due to disbonding. An attempt to estimate pressure of hydrogen intrusion suggests that preexisting infinitesimal flaws coalesce as a tensile crack in the disbonding process. The effect of crack orientation is studied by using a moment tensor representation. Since crack orientations can be determined from the eigenvalue analysis of a moment tensor, a technique to determine moment tensor components is applied to simulated waveforms due to inclined cracks. These results confirm that ...

In Japan, the recycling ratio of concrete scraps has been kept over 98 % after the Law for the Recycling of Construction Materials was enforced in 2000. In the present, most of concrete scraps were recycled as the Lower Subbase Course Material. On the other hand, it is predicted to be difficult to keep this higher recycling ratio in the near future because concrete scraps increase rapidly and would reach to over 3 times of present situation in 2010. In addition, the demand of concrete scraps as the Lower Subbase Course Material has been decreased. Therefore, new way to reuse concrete scraps must be developed. Concrete scraps normally consist of 70 % of coarse aggregate, 19 % of water and 11 % of cement. To obtain the higher recycling ratio, the higher recycling ratio of coarse aggregate is desired. In this paper, a new method for recycling coarse aggregate from concrete scraps has been developed and demonstrated. The system includes a Marx generator and a point to hemisphere mesh electrode immersed in water. In the demonstration, the test piece of concrete scrap was located between the electrodes and was treated by the pulsed discharge. After discharge treatment of test piece, the recycling coarse aggregates were evaluated under JIS and TS and had enough quality for utilization as the coarse aggregate.

Subsurface damages in concrete may cause considerable durability loss in the structure since they are not visible from the surface and let water and other chemicals to penetrate into the structure and cause reinforcement corrosion. Ultrasonic methods have been widely applied for defect detection in concrete, however, there are not many applications of the method found in literature for subsurface damage identification. Specifically, when the damage is deeper considering the distance from the top surface. Therefore, in this study concrete blocks including subsurface cracks with different depths as well as a sound concrete block were tested by applying ultrasound. Wave parameters such as velocity and attenuation for each case were evaluated. Rayleigh wave and longitudinal wave velocities do not change for different subsurface crack depths. Even though, wave energy attenuates more in case of a shallower crack, it is difficult to correlate the attenuation rate with crack depth. Still, attenuation rate is an indicative parameter for subsurface damage identification. Numerical results obtained by Boundary Element Method (BEM) analysis are in good agreement with experimental ones. Stack imaging procedure applied to ultrasonic echo data is used as a complementary technique for subsurface crack depth identification. Reflection at the crack depth can be clearly identified.

A source inversion procedure for determining crack types and crack orientations is proposed as the quantitative waveform analysis of acoustic emission (AE), based on the integral representation and the moment tensor. The procedure is applied to AE events monitored during the pull-out tests of hooked anchor-bolts from concrete block.The integral representation is first applied to the stress analysis by the boundary element method (BEM). It confirms that the failure surface is generated as perpendicular to the direction of the maximum tensile stress.AE waveforms detected at five sensor locations during the failure process are analyzed by the source inversion procedure. Crack kinematics determined are in good agreement with experimental findings.

The increase of performance requirements for contemporary cement-based media and structures calls for better control of the material processes. Acoustic emission (AE) is one of the non-invasive techniques that can provide information on the internal condition of the material. This includes the time of crack occurrence, the location as well as the fracture mode. In addition, it can offer valuable insight on the initial, crucial period of curing and hardening that has serious impact on the performance and durability of concrete. The present chapter aims to review the background, offer the reader a basic overview of the application of the technique on fresh cement-based material and at the same time give some new directions for testing of concrete.

The present chapter discusses the parameter-based analysis of AE. This is particularly useful in case full waveform recording is not available, or the acquisition rate is not sufficient to allow waveform streaming. Parameter analysis is based on extraction of descriptors that contain most of the waveform information, without however, the need to store the whole number of points, essentially contributing to data management. The different parameters are introduced and their relation to fracture mode as well as their use for process characterization are discussed in detail. In addition, well established indices based on the total AE activity are also discussed to demonstrate the complete arsenal of AE parametric analysis that has proven very useful to researchers and practitioners, in different material systems.

Concrete structures are referred to as the basis of contemporary technical civilization. They support transportation, storage of goods and shelter humans in large numbers. Despite their long endurance, they have ceased to be considered as maintenance free. They are subject to long term loading due to own weight, dynamic loading scenarios including earthquakes, as well as environmental degradation. Thus, concrete structures should undergo inspection and evaluation of their health condition while proper maintenance actions should be proposed. To prolong their useful life span, robust monitoring methodologies should be applied. The first and basic information is extracted from visual inspection. However, this concerns only the surface distress indications. Defects may start to grow from the interior, and therefore, an assessment based solely on visual observation would not be complete. Among monitoring methodologies that assess the full volume of the structure, acoustic emission (AE) has started to be applied in a wide basis. AE is a passive technique that offers specific advantages like the real-time monitoring of the defects propagation, which enables to characterize the critical moments of the structure in relation to the applied operational load. Three-dimensional localization of the cracking sources is also possible if a number of sensors are used, while the parameters of the recorded waveforms exhibit strong sensitivity to the mode of cracking enabling estimations of the dominant fracture processes. Additionally, even if accessibility is limited, just a few points of the surface would be sufficient to place sensors, through which large structural volumes are inspected, highlighting the global nature of the technique. In this chapter, the basics of AE are mentioned and discussed in terms of instrumentation and correlation of AE parameters to damage of concrete structures. In addition, current studies aiming at improving the characterization reliability are also discussed.

Structural elements commonly obtain plate geometries. This is usual in aeronautics, automotive, naval applications and at the same time becomes more and more common in civil engineering with the expansion in the use of lightweight panels and fibrous media over bulk concrete. Acoustic emission (AE) is typically applied on these geometries yielding excellent results in laboratory and industrial conditions. The plate geometries however, create some specificities compared to bulk geometries. These are mainly related to the conditions of wave propagation since “plate wave dispersion” is exhibited, resulting in strong change of the acoustic signals as they propagate through the material. This influences the received AE waveforms rendering the study of “Lamb waves” of paramount importance in case someone wishes to go into detail in the characterization of plate structures based on their AE behavior. The present chapter offers a firm theoretical basis for guided waves, demonstrates dispersion and tries to examine its influence compared to bulk media from the AE point of view. Several basic applications of AE in plates are given to exhibit the level of the state of the art in laboratory and practice and where it can be further pushed.

The advantages that AE brings in monitoring have enabled its use for following the fracture of biological tissues. Apart from the dedicated tests during mechanical loading of bones and ligaments in vitro, there is large potential for diagnostic studies in patients due to the non-invasive nature of the technique. At the same time, monitoring of very sensitive mobility activities related to moisture and cavitation in plants open new avenues in the field of efficient farming. This chapter intends to summarize the use of AE in biological materials, which despite the inherent complications due to heterogeneity, curvature and very demanding application in general, is very promising and with high social impact.

Acoustic emission (AE) is one of the promising techniques for structural health monitoring (SHM) purposes in concrete structures. Due to its passive and non-invasive nature, it can be applied without disturbing the operation while it provides information that no other monitoring technique can. During the last decades great developments have taken place in the technical equipment and the analysis tools. The present chapter intents to summarize the recent trends in AE parameter analysis mainly related to fracture mode identification. This issue carries high importance since fracture of concrete materials and structures includes shifts between different modes. Therefore, its identification supplies information on the damage status. However, the characterization should remain as simple as possible in order to have the potential to be implemented in situ.

Implementation of Non-destructive Evaluation (NDE) procedures in concrete structures seems to be the only way towards a safe and economic infrastructure management. Since concrete structures have reasonably ceased to be regarded as maintenance-free, periodic inspection is deemed necessary. Although destructive techniques can provide information on the strength of the material, this is only based on sampling and cannot be performed in a wide basis. Therefore, NDE methodologies, especially with the capability of visualizing the interior of the structure, have been developed and applied in structures to answer a question that cannot be addressed destructively. This chapter focuses on the current trends of elastic wave and general techniques that are used and new trends with some examples of how NDE assists in the identification of damage and maintenance of concrete structures are highlighted.