Search

Physically, fracture in a material takes place as the release of stored strain energy, which is consumed by nucleating new external surfaces (cracks) and emitting elastic waves. The elastic waves propagate inside the material and are detected by an acoustic emission (AE) sensor. In this concern, important aspects are to select proper sensors and instrumental systems. Depending upon materials and structures, selection of sensors, decision of frequency range, techniques to eliminate noises and conditions for system setting are to be taken carefully into account. Consequently, standardized procedures and requirements for systems are stated. They include the system response based on the linear-system theory, response of PZT element as a contact-type sensor, mounting of sensors and its aperture effect, instrumental bases and data acquisition.

In seismology, linear inversion techniques were proposed to determine the moment tensor component in both time and frequency domains (Stump & Johnson 1977) and (Kanamori & Given 1981). Although all components of the moment tensor must be determined, the moment tensor inversion with constraints has been normally applied to obtaining stable solutions in seismology (Dziewonski & Woodhouse 1981). This is partly because a fault motion of an earthquake is primarily associated with shear motion, corresponding to off-diagonal components in the moment tensor. One application of the moment tensor inversion with constraints is found in rock mechanics (Dai, Labuz et al. 2000).

AE techniques have been extensively studied in concrete engineering since early 1950s. Nowadays, several monitoring tasks are casually performed in large scale, since recently AE tests in concrete are standardized. Others are being developed in laboratory conditions aiming at pushing the limits of reliable characterization. This chapter offers the basic uses, trends and applications of AE in concrete. Issues like damage accumulation, localization, characterization of cracking modes in failure process, corrosion of reinforcement, fatigue, damage and fracture mechanics are discussed in association with ISO standards of AE in concrete.

In this study, to visualize damage and defects, such as cracks and voids in concrete, the SIBIE (stack imaging of spectral amplitudes based on impact echo) procedure is applied and numerically improved to construct a three-dimensional (3D) model of elastic wave propagation behavior. A unit of arrayed accelerometers is installed to detect multi-channel signal waveforms in the frequency domain. The resonant frequencies due to reflections at each node in 3D lattice nodes are computed by using the distances from elastic wave input to multiple output locations. The amplitudes corresponding to the resonant frequencies in the spectrum are summed up as the reflection intensity of elastic wave at each node. The reflection intensity distribution is visualized finally in the targeted area three-dimensionally. Case studies are carried out on the proposal of the improved 3D-SIBIE procedure, applied to a concrete specimen with simulated-damage as well as in-situ highway RC (Reinforced Concrete) slabs in service. As for the signal detection, a non-contact elastic wave detecting system using a laser doppler vibrometer is also introduced to consider and validate the promising remote sensing and inspection technique for damage evaluation in concrete with the 3D SIBIE procedure.

The demand for maintenance and repair of concrete structures in service has been critically increasing worldwide. A variety of techniques and evaluation methods for diagnosis and prognosis of the structures are extensively under development by applying nondestructive evaluation (NDE). As inspection techniques, a major issue is on-site measurement. Thus, acoustic emission (AE) techniques draw great attention to the practices in concrete structures. With respect to AE measurements in concrete, three RILEM recommendations have been published, and are known to be readily available for practical applications. In order to extend the service life of concrete structures, we have proposed the establishment of “infra-dock”, which is named as an inspection system for the prognosis, as referring to the ship dry-dock. Consequently, in relation with establishment of the infra-dock, prospective applications of AE techniques are updated and reviewed on the basis of AE measurements developed. Successful results on prognosis of the fatigue damage in a pre-stressed concrete bridge, the earthquake attack to a concrete aqueduct, nucleation process and mechanisms of corrosion-induced cracks in reinforced concrete are summarized and discussed.

The durability of concrete structures decreases easily due to such environmental effects. For effective maintenance of concrete structures, it is necessary to evaluate not only the values of mechanical properties but also the degree of damage. In this study, damage estimation of concrete is proposed to be performed, applying AE and X-ray CT. Testing concrete core samples were taken from reinforced concrete columns of an existing canal, which is strongly affected by freeze and thawed process. Thus, a relation between AE and X-ray CT parameters is correlated, and the damage of concrete is quantitatively estimated using AE and X-ray CT.

Development of a health monitoring system in situ is so important as to properly maintain concrete structures under severe salt attack. Research Association for Infrastructure Monitoring System (RAIMS) is engaged in research on health monitoring of bridges and thus experiments on concrete bridges under the severe environment are widely conducted. As a preliminary research on an applicability of the health monitoring system, sensing devices for early detection of chloride penetration and rebar corrosion have been applied to a prestressed concrete (PC) bridge in service. Three sensing devices of a corrosion sensor, a reference electrode and an embedded sensor are placed in the bridge. So far, monitoring of the corrosion degree due to chloride attack has been continuously conducted for more than two years. Further, a simple device for easy-installation in a local bridge is proposed.

The elastic wave methods for non-destructive evaluation (NDE) in concrete are theoretically reviewed. These include basics of ultrasonic testing (UT), impact-echo (IE), and acoustic emission (AE). Starting with the generalized theory of AE, it is shown that both elastic waves generated due to impact and AE waves due to cracking are rationally to be analyzed on the basis of elastodynamics. As applications of the techniques, on-site measurements in concrete structures are discussed. This is because the long service-life of infrastructure is to be an evolutional target. Deterioration due to aging and damage due to disasters in concrete structures have updated the urgent demand for inspection and maintenance. In order to extend the service life of the concrete structures, on-site NDE techniques are to be developed for diagnosis and prognosis. Successful applications are updated and summarized.

As a detailed inspection of a concrete structure in service, core samples are usually drilled out and then mechanical properties are measured. In this study, damage estimation of freeze-thawed concrete from concrete-core samples is developed, applying acoustic emission (AE) method. By the authors, the quantitative damage evaluation of concrete has been proposed in RILEM TC −212ACD, by applying AE and damage mechanics in the compression test. In this study, characteristics evaluation of cracking damage effects in concrete mechanical properties by detected AE energy in core test. Prior to the compression test, distribution of micro-cracks in a concrete-core sample is inspected by helical X-ray computer tomography (CT). Then, freeze-thawed damaged samples are tested by the compression test with AE monitoring. Concrete-core samples were taken out of a channel structures in Hokkaido, Japan which is extremely developed cracking damage. Thus, it is demonstrated that the concentration of material damage could be evaluated by comparing geometrical characteristics of cracks with the “energy rate” of AE generation, which is analyzed by AE parameter analysis. A relation between AE energy rate, physical properties and geometrical properties of void is correlated, and thus the damage of concrete is qualitatively estimated by AE parameters.

Making out a draft of a recommendation practice on damage evaluation of concrete by acoustic emission (AE) technique was postponed in RILEM TC 212-ACD (AE and related NDE techniques for crack detection and damage evaluation in concrete). A follow-up RILEM committee TC 239-MCM (On-Site Measurement of Concrete and Masonry Structures by Visualized NDT) was approved in March, 2010. In this committee, therefore, round-robin tests have been conducted and results are under examination for preparing the recommendation. Damage estimation of concrete by the compression test of core samples is developed, by applying AE measurement and the scalar damage parameter in damage mechanics. The procedure is named damage estimation of concrete by acoustic emission technique (DeCAT), which is based on estimating the intact modulus of elasticity in concrete from AE database. In addition to available data already published, results of the round-robin tests conducted by three groups are discussed.

Turkey is located on such a seismically active region that many high-magnitude earthquakes have attacked the areas. Large number of reinforced concrete structures are collapsed or heavily damaged due to these earthquakes. It is crucial to evaluate the condition of the concrete structures in seismically active zones before and after the earthquake to decide whether the structure could continue its service for habitation or not. Consequently, quantitative damage estimation of concrete material is conducted by applying acoustic emission (AE) testing under unconfined uniaxial compression test of core samples taken from the concrete structures. Damaged concrete samples are examined based on fracturing behavior under unconfined compression. AE behavior of concrete under compression is dependent on the degree of damage and could be analyzed by the rate-process analysis. Core samples were collected from two reinforced concrete buildings damaged by Simav Earthquake that hit Kutahya City on May 19, 2011. In compression tests, compressive strengths and Young’s moduli of these samples were determined. After estimating Young’s modulus of the intact state from the correlation between AE rate and a damage parameter in each core, the damage levels of structures are quantitatively estimated.

Steel-making slag concrete (SSC) is an environmental friendly material made mainly of by-products of steel industry without using Portland cement. The fatigue characteristics of SSC under compression in submerged condition are experimentally investigated, for applications to marine and harbor structures. Based on the fatigue tests, calculation formulae are proposed on the averaged fatigue life. The fatigue strength of SSC with hot metal pretreatment slag or converter slag as fine and coarse aggregates is slightly smaller than that of ordinary cement concrete, while using polished hot metal pretreatment slag (PHPS) as aggregates is found to improve the fatigue strength of SSC. The superior characteristics of SSC with PHPS against fatigue failure are studied through the measurements of strain, AE, and hardness of the materials.

The Great East-Japan Earthquake hit Tohoku area in Japan on March 11, 2011. A large number of concrete structures were damaged due to the 9.0 magnitude earthquake of the Richter-scale. Prior to reconstruction and retrofit of these structures, damage evaluation of in situ concrete structures is now in urgent demand. In this concern, quantitative damage estimation of concrete is proposed to be performed, applying acoustic emission (AE) measurement in a uniaxial compression test of core samples. Generating behavior of AE events in the core test is quantitatively analyzed, based on the rate process theory, because notable discrepancy of AE activity is observed between damaged concrete and undamaged concrete. The damage is quantitatively defined by a scalar damage parameter in damage mechanics. Correlating AE rate with the damage parameter, quantitative estimation of damage is proposed in terms of the relative modulus of elasticity (E0/E*). Concrete core samples were taken from reinforced concrete columns of an existing canal in the both periods of pre and post the earthquake. Prior to the compression test, distribution of micro-cracks in a concrete-core sample was inspected by helical X-ray computed tomography (CT), which scans at 1-mm intervals. The results suggest that the damage of concrete could be quantitatively estimated from damage parameter E0/E* without knowing the original state of the concrete at construction. The static moduli E0/E* and dynamic moduli Ed/E* is closely correlated. A relation between AE rate and the damage parameters is correlated, and thus the earthquake damage of concrete is quantitatively estimated using AE.

Investigation on fracture process zones in notched concrete beams under three-point bending is performed by applying acoustic emission (AE). The tests were conducted on beam specimens with different notch depths and the maximum sizes of aggregate in concrete to clarify the generation mechanisms of micro-cracks in the fracture process zone. SiGMA (simplified Green’s functions for moment tensor analysis) procedure was applied to analyze AE signals. The experimental results show the fracture energy increases with increase in the maximum size of aggregate. It is found that dominant motions of micro-cracks vary with the notch depth, regardless of the maximum size of aggregate. The widths of the fracture process zone were estimated from results of AE source location. It is confirmed that the fracture energy correlates with the width of AE cluster, as the energy increase when the width of fracture process zone expands.

The impact-echo method is well-known as a nondestructive test for concrete structures. Methodologically, frequency spectra of elastic waves that are generated by a mechanical impact and propagate in concrete are applied to evaluate the thickness or to locate internal flaws in concrete. Occasionally, spectra analyzed contain so many peak frequencies that resonant frequencies associated with locations of defects are marginally identified in practical applications. A procedure named as stack imaging of spectral amplitudes based on impact-echo (SIBIE) is developed to compensate this drawback and to visually identify locations of defects at a cross-section. In order to evaluate the feasibility of the procedure, three stages of the impact-echo of applying the impact, detecting waves, and identifying peak frequencies are examined. Based on the results, case studies on grout performance in prestressed concrete and surface cracks at delaminated areas in a concrete pier are conducted and the applicability is demonstrated.

Continuous acoustic emission (AE) monitoring is applied to a cyclic wet and dry test of reinforced concrete beams. The onset of corrosion and the nucleation of corrosion-induced cracks in concrete are successfully identified, and then, cross-sections inside the concrete specimen are observed by an electron probe micro analyzer (EPMA). Results of AE parameter analysis are compared with the corrosion mechanisms observed by EPMA. Further, a relation between kinematical information of AE sources and nucleation of micro-cracks inside is identified by the SiGMA analysis and observed by the stereo-microscope. From these results, AE is a promising technique to quantitatively evaluate the corrosion process in concrete due to expansion of corrosion products at an early stage is demonstrated.

Corrosion of reinforcing steel bar is a critical issue in concrete structures. In response, a variety of techniques have been developed for early detection of corrosion phenomena, including nondestructive evaluation (NDE) techniques. Recently, we have reported that continuous acoustic emission (AE) monitoring is applicable to identify the onset of corrosion and the nucleation of corrosion-induced cracks due to the expansion of corrosion products. In order to clarify the kinematics of these AE activities in concrete, cyclic wet and dry tests on reinforced concrete beams were performed. The SiGMA (Simplified Green's functions for Moment tensor Analysis) procedure was applied to AE waveforms to locate micro-cracks and to determine kinematic information about crack types and orientations of corrosion-induced cracks. These results are visually confirmed through comparison with cracking mechanisms observed by a scanning electron micro-graph (SEM).

In the freezing and thawing test of concrete, the frost damage is nondestructively evaluated by elastic-wave methods. In this regard, two alternative methods are recently proposed in RILEM recommendations. One is based on the ultrasonic test, which measures the transmission time of the longitudinal wave (P wave) and the other is associated with the resonant frequencies of vibrations. Re-examining formula to estimate dynamic moduli of elasticity in the standards available, evaluation of the frost damage by the elastic-wave methods is studied. Concerning the resonant frequencies of concrete samples, a three-dimensional analysis is performed by the boundary element method to identify actual vibrating-modes. It is clarified that an assumption of the one-dimensional resonant vibration is not applicable, because the assumption is responsible for a discrepancy between the dynamic and the static moduli. In order to evaluate the frost damage experimentally, the freezing and thawing tests were conducted by employing concrete samples after 1 year curing. The modulus of elasticity obtained from P-wave velocity is recommended to use as the durability factor, although homogeneous damages are only assumed in concrete samples. It is found that this modulus is comparable to the tangential modulus of elasticity in the compression test.

As a detailed inspection of a concrete structure in service, core samples are usually drilled out and then mechanical properties are measured. In this study, damage estimation of concrete structures from core samples is developed, applying acoustic emission (AE) and X-ray computed tomography method. Concrete-core samples were taken from reinforced concrete of an existing canal. These samples are strongly damaged by freezing and thawing process. Crack distributions of concrete-core samples were inspected with helical Computerized Tomography (CT) scans, which were undertaken at 1-mm intervals. After helical CT scan, damage of testing samples was evaluated, based on fracturing behavior under compression and AE activity. AE behavior is dependent on the degree of damage, and could be approximated by applying the AE rate-process analysis and damage mechanics. AE generation behavior is associated with crack volume due to damage accumulation in concrete. The damage is quantitatively defined by a scalar damage parameter in damage mechanics. Applying the scalar damage parameter, quantitative estimation of damage is proposed in terms of relative modulus of elasticity which is named “durability index”. By calculating the durability index, the initial damage of samples is estimated from AE database. The decrease in mechanical properties could be evaluated by comparing the average CT number with the durability index, both of which are affected by internal cracks generated. Thus, the damage of concrete structures is quantitatively evaluated by AE and X-ray computed tomography method.

On-site measurement in concrete piers of a highway structure was conducted by visualized non-destructive testing (NDT). First, the radar technique was applied to identify delaminated areas between pre-stressed concrete (PC) panels covered and original concrete piers. By applying the impact-echo method, Stack Imaging of spectral amplitudes Based on Impact-Echo (SIBIE) procedure is developed as an imaging technique, which can visually display a cross-sectional view based on the impact-echo data. In order to identify voids in delaminated areas and depths of surface cracks, then the impact-echo tests were conducted. From these results, a superior applicability of the SIBIE procedure to on-site measurement is demonstrated in existing concrete structures.

The impact-echo method has been widely applied to identification of defects in concrete. The presence and the location of the defect are estimated from identifying peak frequencies in the frequency spectra, which are responsible for the resonance due to time-of-flight from the defect. In practical applications, however, spectra obtained include so many peak frequencies that it is fairly difficult to identify the defect correctly. In order to circumvent the fact, Stack Imaging of Spectral Amplitudes Based on Impact-Echo (SIBIE) procedure is developed to visually identify the location of the defect. In this study, the procedure is applied to two types of concrete specimens. One type contains grouted and ungrouted sheaths for post-tensioning tendon-ducts. The other specimen with an ungrouted duct is tested to investigate the sensor selection. It is demonstrated that the ungrouted tendon-duct can be identified with reasonable accuracy by the SIBIE procedure. Both an accelerometer and a laser-vibrometer are available for the SIBIE analysis.

The fracture mode of cracking in concrete is normally changing from tensile mode to shear mode at impending failure. As for crack classification in concrete by acoustic emission (AE) techniques, two crack classification methods have been carried out. One of them is parameter-based method (parameter analysis) which has been carried out by employing two parameters of the average frequency and the RA value. The proportion of these two parameters, however, has not been determined yet. The other crack classification is simplified Green’s functions for moment tensor analysis (SiGMA) procedure which is known as signal-based method. The SiGMA analysis is based on the generalized theory of AE, while the parameter analysis results from an empirical relation. Therefore, an optimal proportion of the parameter analysis is in great demand. In this study, these crack classification methods are compared and discussed from results of three types of concrete failure tests. As a result, ratios of the shear crack which are identified by SiGMA analysis are good agreement with those by parameter analysis in the case that the proportion of the RA value and the average frequency is set to 1–200.

For non-destructive evaluation (NDE) of concrete structures, the impact-echo method has been successfully applied to locate defects and voids in concrete. To compensate the difficulty in selecting the resonant frequency and to visually locate a defect, an imaging procedure named stack imaging of spectral amplitude based on impact-echo (SIBIE) has been developed. Combining the SIBIE analysis with the detection by a laser vibrometer, a non-contact procedure is studied to develop an automated system. Since waveforms detected by the laser vibrometer are undistorted, elastic waves generated due to a steel-ball drop were investigated theoretically as Lamb's problem in elastodynamics. It is confirmed that the first motion of detected waves by the impact-echo method is identical to Lamb's solution for a surface pulse, when reflected waves arrive later than the Rayleigh wave. An applicability of a non-contact SIBIE procedure to identify an ungrouted tendon duct of plastic sheath was examined in the impact-echo tests, as well as the duct of metal sheath. In order to study theoretically the SIBIE procedure for identifying ungrouted tendon ducts, the three-dimensional boundary element method (3D-BEM) was applied to synthesize frequency spectra. It is confirmed that frequency spectra detected in the tests are in good agreement with those synthesized. An applicability of the SIBIE procedure is also confirmed by the synthesized spectra. Results of the experiments show that the presence of the ungrouted duct can be visually identified by the non-contact SIBIE procedure in both the cases of plastic sheath and metal sheath.

インパクトエコー（衝撃弾性波）法に基づいてコンクリート内部欠陥の位置を断面部の画像化により検出するSIBIE (Stack Imaging of spectral amplitude Based on Impact Echo) 法を開発中である．この手法をプレストレストコンクリート構造のグラウト未充填部検出に適用した場合の現状の問題点を検討し，改良された手法について実証的に検討した．手法の有効性はモデル試験により考察し，未充填部のシースの大きさや部分未充填部検出の可能性を明らかにした．この結果，SIBIE法の適用により断面画像としてグラウト未充填部を検出する実用的な非破壊検査手法が提案できたと考えられる．

損傷力学に基づいてアコースティック・エミッション（AE）法のレートプロセス解析を適用し，コンクリートの定量的な損傷度評価法を開発中である．一方，衝撃エネルギーの応用技術に関して，高電圧のパルスパワーを利用した再生骨材の製造法を研究中である．そこで，原骨材の特性が不明な再生骨材コンクリートの場合に，その品質が損傷度として評価可能であるかについて検討した． その結果，再生手法の異なる場合に，再生骨材コンクリートの損傷度として粗骨材の品質評価が可能であり，パルスパワーにより製造された品質の異なる再生骨材を用いた強度や空気量の異なるコンクリートの場合にも，再生骨材の原骨材に対する品質低下が損傷度として評価可能なことを明らかにした．

Following the initiation of corrosion in reinforcement, expansion of corrosion products generates corrosion-induced cracks in concrete. At the meso-scale, accumulation and coalescence of micro-cracks create the fracture process zone. Since nucleation of an individual micro-crack can be detected by acoustic emission (AE) techniques, the moment tensor analysis of AE waves is applied to quantitatively identify cracking kinematics of a location, a crack-type and a crack orientation. At the macro-scale, initiation and extension of the corrosion-induced cracks are analyzed by means of classical fracture mechanics, applying the boundary element method (BEM). The generation of corrosion-induced cracks was simulated in expansion tests on concrete specimens. Kinematical mechanisms of micro-cracks were identified by the SiGMA (Simplified Green's functions for Moment tensor Analysis) analysis. By applying the two-domain BEM analysis, extension of the corrosion-induced crack in an arbitrary direction was analyzed. With respect to the orientations of crack extension, results of the BEM analysis were compared with those of the SiGMA analysis, introducing the normalized stress intensity factors. It is demonstrated that extension of the corrosion-induced crack is governed by the mode-I failure both at the meso-scale and at the macro-scale.

The impact echo method is a nondestructive testing method that is based on the use of elastic waves. It has been widely used in the nondestructive evaluation of concrete. With the identification of the peak frequency observed in the frequency spectrum, the depth of a reflector can be estimated when the longitudinal wave velocity is known. However, in most applications, an empirical correction factor (β) is used because the frequency observed in the spectrum is not in good agreement with the frequency calculated by the given equation in the impact echo. This paper presents determination of the empirical shape factor, β, in impact echo for different rectangular shapes. Two-dimensional dynamic boundary element analysis was conducted as a frequency sweep to simulate the impact test. The deformation modes of the first-mode resonance were examined for each shape by applying boundary element method analysis.

鉄筋コンクリート（RC）梁のせん断破壊は，曲げ破壊と異なり破壊の進行が急激で靭性に欠け，構造物に致命的な被害をもたらすことが知られている．本研究では，せん断スパン－有効高さ比（a/d）が1.97のRC梁を作成し，４点曲げ載荷試験を行ってせん断スパン内に発生するアコースティック・エミッション（AE）計測を行った．実験により得られたAE波形にAEパラメータ解析，SiGMA解析を適用し，せん断スパン内の破壊進行状況の考察を行った．その結果，載荷初期から破壊するまでの間に３段階のAE発生挙動が確認され，せん断破壊を，微小クラックの集積過程として機構的に，また，可視的に明らかにできた．

The ultrasonic test (UT) has been conventionally applied to estimate a surface-crack depth in concrete. A test method is based on the time-of-flight of a diffracted wave via a crack tip. It is reported that the depth is mistakenly estimated in the case that water is filled in the crack. In order to visualize a location of an internal defect in concrete, SIBIE procedure has been developed. In the present study, the SIBIE procedure is applied to estimate the depth of surface-cracks, which are artificially made in concrete samples. SIBIE results are compared with those of the conventional UT results. It is found that the depth estimation is marginally available in the UT measurement, while the SIBIE procedure could visually show the depths of a surface-crack, separating the effect of water surface.

Serious shear failures in reinforced concrete (RC) structures were reported in the Hanshin-Awaji Earthquake. In particular, it was demonstrated that a diagonal-shear failure could lead to disastrous damage. However, mechanisms of the diagonal-shear failure in RC beams have not been completely clarified yet. In this study, the diagonal-shear failure in RC beams is investigated, applying acoustic emission (AE) method. To identify source mechanisms of AE signals, SiGMA (Simplified Green's functions for Moment tensor Analysis) procedure was applied. Prior to four-point bending tests of RC beams, theoretical waveforms were calculated to determine the optimal arrangement of AE sensors. Then, cracking mechanisms in experiments were investigated by applying the SiGMA procedure to AE waveforms. From results of the SiGMA analysis, dominant motions of micro-cracks are found to be of shear crack in all the loading stages. As the load increased, the number of tensile cracks increased and eventually the diagonal-shear failure occurred in the shear span. Prior to final failure, AE cluster of micro-cracks was intensely observed in the shear span. To classify AE sources into tensile and shear cracks, AE parameter analysis was also applied. As a result, most of AE hits are classified into tensile cracks. The difference between results obtained by the AE parameter analysis and by the SiGMA analysis is investigated and discussed.

Kinematics of crack nucleation is physically associated with acoustic emission (AE) source mechanisms. Mathematically, kinematical information on cracking is represented by the moment tensor. In quantitative AE waveform analysis, one powerful technique for three-dimensional (3D) visualization of AE source has been developed as SiGMA (Simplified Green’s functions for Moment tensor Analysis), where crack kinematics of locations, types and orientations of AE source is quantitatively analyzed. Starting with theoretical background and principle, the procedure is explained with a sample code.

For detailed inspections of concrete structures, non-destructive tests (NDT) are often conducted and then mechanical properties are compared with detected NDT parameters. In this chapter, damage evaluation of concrete by AE and X-ray computerized tomography (CT) tests are presented. The concrete damage is visualized by X-ray CT. Since nucleation of AE events in damaged concrete is affected by crack distribution, the damage parameter estimated is correlated with the dynamic modulus of elasticity E d which is calculated from P-wave velocity. The results confirm that the damage of concrete could be estimated by applying AE measurement, damage parameter in damage mechanics, CT images and modulus E d. It is also shown that the static modulus E 0 is closely correlated with the dynamic modulus E d. Based on a relation between AE rate and the damage parameter, the damage of concrete is quantitatively estimated.

Corrosion of reinforcing steel-bar (rebar) is modelled phenomenologically as corrosion losses of three phases. Corresponding to these phases, high AE activities are characteristically observed twice during the corrosion process. It is shown that the 1st AE activity corresponds to the onset of the corrosion in rebar and at the 2nd period of high activity AE events result from concrete cracking due to the expansion of rebar caused by corrosion products. The expansion caused by corrosion products generates micro-cracks in concrete as corrosion-induced cracks, of which mechanisms can be investigated experimentally by AE. To this end, SiGMA (Simplified Green’s functions for Moment tensor Analysis) is applied to the corrosion process in reinforced concrete. To compare with cracking mechanisms identified by SiGMA, a numerical analysis by the two-dimensional boundary element method (BEM) is performed for stress analysis and applied boundary nodes were determined from a diffusion analysis of chloride contents by the finite element method (FEM). Thus, fracture mechanisms of corrosion-induced cracks are quantitatively evaluated by AE-SiGMA and BEM. For on-site measurement of the corrosion in reinforced concrete, hybrid nondestructive evaluation (NDE) is going to be developed. Here, acoustic emission (AE) measurement is conducted, along with measuring half-cell potentials and polarization resistances at the surface of concrete. In order to identify the corroded area along rebar, the potential inversion by BEM (PiBEM) analysis is applied. Thus, plausible areas of the corrosion in rebar are identified.

Physical principles of the Acoustic Emission (AE) and the signal processing are presented in this chapter. The mechanism inducing AE waves are explained in comparison with that of an interpolate earthquake. Types and characteristics of AE sources are also explained. Fundamentals of AE propagation in solids are discussed. As for the evaluation several promising AE parameters and such AE source location techniques as 1D, 2D and zonal location are explained.

For nondestructive evaluation (NDE) of concrete structures, the impact-echo method has been successfully applied to locate defects and voids in concrete. To compensate the difficulty in selecting the resonant frequency and to visually locate a defect, an imaging procedure named SIBIE (Stack Imaging of spectral amplitude Based on Impact-Echo) has been developed. This is an imaging technique for visualizing defects of which locations are known or identified. Following the basic concept and procedure, applications to concrete structures are discussed. The presence of the ungrouted duct can be visually identified by the SIBIE procedure in both the cases of plastic sheath and metal sheath. In addition, the depth of the surface crack is visually evaluated.

Corrosion of reinforcement is one of the major causes of deterioration in reinforced concrete structures. Various crack patterns are nucleated around reinforcement in concrete due to expansion of corrosion products. Crack kinematics of locations, types, and orientations are quantitatively determined by the acoustic emission (AE) – simplified Green’s functions for moment tensor analysis (SiGMA) procedure in association with laboratory tests conducted on concrete specimens simulating corrosion-induced damage. These kinematic outcomes are obtained as three-dimensional (3-D) locations and vectors, and are thus visualized in 3-D by using virtual reality modeling language (VRML). Numerical analysis is conducted by the boundary element method (BEM) based on the concept of linear elastic fracture mechanics (LEFM) to clarify the mechanisms of corrosion-induced crack extension. Relationships between dimensionless stress intensity factors and cracking types are studied by BEM. Contributions of mode I and mode II failures are dependent on the cracking types. It is found that the process of crack propagation due to corrosion of reinforcement in concrete is mostly a mode I fracture with mixed-mode and, in a few cases, mode II fracture.Key words: acoustic emission, moment tensor, corrosion cracking, stress intensity factor.

For recycling concrete, the use of recycled aggregate has been extensively studied in Japan. It is widely recognized that compressive strength in concrete of recycled aggregate is lower than normal concrete with the same water-to-cement ratio, because fracture process in concrete of recycled aggregate is not identical to normal concrete. In order to clarify characteristics of fracture process in aggregate-recycled concrete, acoustic emission (AE) technique is applied to detect micro-cracking in concrete under compression. A method to improve the strength of concrete of recycled aggregate was investigated. The test was performed under both monotonous and cyclic loadings. From AE parameters, it is found that cracking behaviors in concrete of recycled aggregate fairly differ from that of normal concrete. Still it is confirmed that the strength could be recovered by vibratory compaction.