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The acoustic emission (AE) technique allows monitoring damage in (reinforced) concrete in a non-destructive way by means of piezoelectric sensors attached to the material surface. This approach has disadvantages such as a decrease of the sensor coupling over time, high attenuation of AE waves in concrete, and difficulties in terms of sensor placement. Embedded AE sensors, so-called 'smart aggregates' (SA), can be a valuable addition or alternative to surface-mounted AE sensors. However, the embedment of sensors brings its own challenges. In this paper, the use of SA is investigated to monitor cracking of fiber reinforced concrete during a three-point bending test, and corrosion and related concrete cracking of reinforced concrete during an accelerated corrosion test. The novelty of the paper is the application of SA for passive AE monitoring during concrete degradation processes with a varying cracking behavior and crack orientation. Special emphasis is put on data filtering and localization of AE sources. The results show that, despite a higher level of wide-band noise for the SA sensors, they are able to detect and localize concrete cracking after dedicated filtering. Furthermore, the potential of SA sensors in early-stage detection of corrosion damage is demonstrated, offering enhanced possibilities for predictive maintenance of concrete structures. • Feasibility of smart aggregates for passive acoustic emission monitoring is studied. • Smart aggregates enable to detect and localize concrete cracks after post-filtering. • Potential of smart aggregates for rebar corrosion damage detection is demonstrated. [ABSTRACT FROM AUTHOR]

Rapid urbanizations impose increasing challenges on safety decompositions of those buildings that reached service life, so there is an urgent call for eco-friendly demolition methods. As a green demolition method, the technique of soundless cracking demolition agents (SCDAs) can effectively eliminate dust and noise pollution commonly encountered in conventional methods, and thus attract rising attention in light of increasing concerns for sustainable and low-carbon developments. Herein, this paper provides a state-of-art review of SCDAs and associated applications. SCDA, a powdery material, mainly consists of quicklime (CaO), which tends to expand significantly when mixed with water. The expansion properties of SCDAs are susceptible to internal and external factors, such as compositions, ambient temperature, mixing water temperature, and water/cement (W/C) ratio, restraint conditions, among others. Therefore, substantial efforts have been made to improve the expansion performance of SCDAs by regulating the abovementioned factors. The resulting expansion force from SCDA expansion can effectively crack brittle materials, e.g., concrete and rocks, the cracking process of which can be measured using strain gauges and Acoustic Emission (AE). More recently, digital image correlation and computed tomography have been employed to monitor the SCDA-induced cracking process, and the implementation of those new techniques promotes the observations of crack developments and underlying mechanisms. To gain a cost-effective application of SCDAs, extensive efforts have been undertaken to optimize the design in hole parameters. Accordingly, several empirical equations have been proposed for determinations of optimal hole parameters, but the application of the equations is restricted to particular conditions where they are proposed. Despite the successful usage of the SCDA technique in demolitions of construction structures, mining, unconventional gas and oil recovery, some limitations, namely hard rock fragmentation, precise numerical works and blowout, still need to be addressed for further applications. Overall, this paper gives a brief review of SCDAs, covering scopes of compositions, expansion properties, applications, inadequacies and prospects, deepening our understanding of SCDAs, and providing recommendations for future research priorities. • Factors influencing the expansion characteristics of SCDAs are analyzed. • Some new technologies for observing the process of crack development are presented. • The optimum usage of SCDA related to the hole design parameters is summarized. • Further work in the research and application of SCDA is proposed. [ABSTRACT FROM AUTHOR]

In the process of grouting sealing, the cementation failure of slurry and coal interface exists in the gas extraction borehole, which leads to unstable and gas leakage in the sealing section and affects the efficient gas extraction in the borehole. In order to study the influence of slurry viscosity on the failure characteristics of slurry-coal cemented body, this paper explores the law of influence of slurry viscosity on the acoustic emission response characteristics and deformation characteristics of slurry-coal cemented body under uniaxial compression through laboratory tests, data analysis and quantitative research of image processing. The results show that: (1) Changing the slurry viscosity can achieve the strength control of the cemented body, which can be increased by up to 42.65 %. (2) In the process of uniaxial compression, the cementation structure of slurry-coal cemented body breaks down successively, and its proportion and strength lead to the distribution difference of AE signals and the fluctuation of b-value. Combined with the phase response characteristics of the two, the change of failure mode can be analyzed to provide a reliable evaluation of crack propagation in the cemented body. (3) The b-value of the sample of slurry-coal cemented body with high-viscosity slurry does not fluctuate significantly, and is relatively stable at a low level near 1.0. The results are helpful to promote the research and development of sealing materials and sealing technology. [Display omitted] • The viscosity of slurry changes the compressive strength and brittle-toughness characteristics of slurry-coal cemented body. • The viscosity of slurry affects the formation of uniaxial compression failure crack of slurry-coal cemented body. • The response characteristics of slurry-coal cemented body with different viscosity slurry in the stage of existence. • The change of b value can be used as a warning for the failure of slurry-coal cemented structure. [ABSTRACT FROM AUTHOR]

This paper proposes a precast concrete shear wall based on a hybrid connection using metal corrugated pipe grouting anchors to address the issue that the seismic performance of precast shear wall structures with a single metal corrugated pipe grouting anchor connection is inferior to that of cast-in-place shear walls. This design aims to enable precast shear wall structures to exhibit seismic performance similar to that of cast-in-place shear wall structures under seismic action. The steel bars in the middle wall and edge components are connected using metal corrugated pipe grouting anchors and sleeve grouting, respectively, forming a hybrid connection type of shear wall. A quasi-static loading scheme was adopted to conduct low-cycle repeated load tests on five walls, and two non-destructive testing techniques were used. Observations of the test phenomena and failure characteristics revealed that all specimens exhibited varying degrees of wall corner damage. The hybrid connection shear walls demonstrated higher peak loads and stronger energy dissipation capabilities, and this advantage became more pronounced with the increase in axial compression ratio. During the specimen loading process, two non-destructive testing techniques were used: Digital Image Correlation (DIC) and Acoustic Emission (AE) technology. Among them, the DIC showed that the damage of hybrid connection shear walls was more concentrated. The damage process of the five walls during the test was detected using an AE instrument also, and the internal cracking forms of the concrete were classified using an optimized Gaussian mixture model. Under the same axial compression ratio, the proportion of Cluster1 in the single connection specimen PW-0.1 is 3.31 % higher than that in the cast-in-place specimen CIP-0.1. Conversely, the proportion of Cluster1 in the hybrid connection specimen PGW-0.1 is 3.43 % lower than that in the CIP-0.1. When both RA and AF values were less than 0.5, a large number of signal points were clustered within this range, and the clustering situation of the hybrid connection specimens was more similar to that of the cast-in-place specimens. Additionally, a damage evolution model with 5 walls was established, and the model fits well, indicating its applicability in engineering practice. • The application of grouted corrugated metallic duct hybrid connection in the seismic resistance of reinforced concrete shear walls was studied. • The hysteresis curve, stiffness, displacement ductility, and energy dissipation of shear walls were analyzed. Hybrid-connected shear walls have a better ability to convert damage into energy dissipation and perform similarly to cast-in-place structures. • The shear walls in the proposed static tests were analyzed using non-destructive testing techniques (including the application of digital imaging techniques), the acoustic emission parameters of different shear walls were clustered and damage evolution equations were developed. [ABSTRACT FROM AUTHOR]

In this paper, the fracture performance of nano-metakaolin (NMK) cement mortar during early hydration period was investigated. Different NMK contents (0 %, 3 %, 5 %, 7 %) were considered, and three-point bending tests were conducted. The digital image correlation technology and acoustic emission technology were utilized to analyze P -CMOD curves, initial fracture toughness, unstable fracture toughness, fracture energy, cumulative AE hits, and cumulative AE energy, AE source locations of cement mortar at early hydration stages. Furthermore, SEM experiments were utilized to analyze the mechanism by which NMK affects fracture performance of cement mortar. The results indicate that the incorporation of NMK enhances the fracture properties of cement mortar, with the most suitable NMK content being 5 %. The addition of NMK increases the cracking load, peak load, and the CMOD at the peak load of the cement mortar at 3 d, 7 d, and 14 d. The cracking load and fracture energy of the cement mortar with 5 % NMK increase by 22.5 % and 35.7 % respectively, compared to ordinary cement mortar at 14 d. NMK enhances the initial fracture toughness and unstable fracture toughness of the cement mortar, improving its crack propagation resistance. The unstable fracture toughness of the cement mortar is the largest when the NMK content is 5 % at 14 d, which is 31.6 % higher than that of ordinary cement mortar. Furthermore, NMK increases the cumulative AE energy and fracture energy of the cement mortar, increasing the cumulative ring counts and cumulative AE hits, causing the crack propagation path to deviate and resulting in improved crack resistance. The cumulative ring counts and AE hits for the cement mortar with 5 % NMK are 42.9 % and 66.2 % higher, respectively, than those for ordinary cement mortar at 14 d. NMK significantly modifies the cement matrix, thereby enhancing the fracture resistance of cement mortar. • NMK enhances the fracture resistance of cement mortar at early hydration age. • NMK causes the crack propagation path of cement mortar to deviate. • NMK increases the energy required for the fracture of cement mortar. • The propagation process of cracks in NMK cement mortar is investigated. [ABSTRACT FROM AUTHOR]

• The current similar simulation materials are featured with the contradiction between low strength and high rockburst tendency, thus being unable to simultaneously realize the qualitative rockburst of small size specimen and the impact damage restoration of large size model. Developed a new similar simulation material for coal with both low strength and strong rockburst tendency in this paper. • In this paper, we propose for the first time to develop similar simulation materials using water glass as binder, sodium silicofluoride as curing agent and talcum powder as aggregate. And the optimal doping of the new similar simulation material was obtained by orthogonal experiment and multiple linear regression method. • It was confirmed that the modulus index is a simplified evaluation index that can comprehensively evaluate rockburst. And according to the similarity criterion, the rockburst evaluation criteria of modulus index for similar simulation materials are obtained by deriving theoretical equations. • Micro-macroscopic experiments were utilized to monitor in real time the energy-temperature-strength synergistic change characteristics and similarities between this material and the coal with a strong RT. The feasibility of the material was confirmed in terms of physical parameters, degree of rockburst and homologous energy and temperature characteristics. • This study also revealed the multi-parameter precursor characteristics of rockburst damage in the specimen, which can lay a foundation for the prevention and study of impact ground pressure disaster. The current similar simulation materials (SSM) are featured with the contradiction between low strength and high rockburst tendency (RT), thus being unable to simultaneously realize the qualitative rockburst of small size specimen and the impact damage restoration of large size model. In this case, this study developed a new SSM of low strength and high RT by using water glass (WG) as cement, talcum powder (TP) as aggregate and adding curing agent sodium silicofluoride (SSF). Under the requirements of initial set volume shrinkage rate (IVSR), long-term volume decay rate (LVDR), uniaxial compressive strength (UCS) and modulus index (MI), the optimal ratios of SSF and TP were obtained 20% and 70% by orthogonal experiment and multiple linear regression method, respectively. Micro-macroscopic experiments such as uniaxial compression, scanning electron microscope (SEM), acoustic emission (AE) and Infrared thermal imager were utilized to monitor in real time the energy-temperature-strength synergistic change characteristics and similarities between this material and the coal with a strong RT. The experimental results show that: the UCS value of this material is 6.11 MPa and its MI is 11.57, which meets the requirements for low strength and high RT. Meanwhile it satisfies a certain similar proportionality between its physical parameters and those of coal. In the whole process of uniaxial compression, there are energy and temperature homologous changes in the characteristics of consistency, and both materials conform to the stress–strain phase law. The infrared radiation temperature displays a trend of "slowly decreasing-gradually warming-continuously cooling-suddenly warming-sharply cooling". The AE energy count rate shows a trend of "calm transition and sudden increase before and after the stress peak". All are characterized by strong RT. There is still some difference between the two specimens during energy and temperature changes, due to their different molding characteristics and load-bearing capacity. The feasibility of the material was confirmed in terms of physical parameters, degree of rockburst and homologous energy and temperature characteristics. Meanwhile, this study revealed the multi-parameter precursor characteristics of rockburst damage in the specimen, which can lay a foundation for the prevention and study of impact ground pressure disaster. [ABSTRACT FROM AUTHOR]

• The proposed method compares acoustic emission (AE) events and local crack kinematics. • The comparison is performed along cracks in shear test of full-scale concrete beams. • The number of AE events is related to the complete crack kinematics history. • Two distinctive types of relationships are found in tensile and compressive zones in the beams. • The contact area between two crack faces could essentially explain the relationships. Monitoring or identifying structural cracks is crucial for assessing the health of existing concrete structures. Key information about structural cracking encompasses the location of the crack and its kinematics, which include movements perpendicular and parallel to the crack face. Acoustic emission (AE) is a sensitive technique for detecting the location of internal concrete cracking. However, the state-of-the-art AE monitoring methods offer limited information on crack kinematics, restricting the use of AE in crack assessment. To bridge this gap, this paper uses a recently proposed AE data analysis method that quantifies the spatial distribution of AE events along a crack probabilistically. This method uses a parameter referred to as the probability density of AE events (pdAE). By combining pdAE and crack kinematics measured by digital image correlation in a series of real-scale concrete beam tests, this paper investigates the relationship between AE events and crack kinematics. The analysed cracks are generated by a combination of bending moment and shear forces, as commonly observed in real structural concrete members. We find that the amount of AE events is not only related to crack width (the crack movement perpendicular to the crack face), as most literature suggests, but also to the complete crack kinematics throughout the loading history of the member. We then provide a physical explanation for the observed relationships between concrete crack kinematics and the quantity of AE events. [ABSTRACT FROM AUTHOR]

Underwater non-dispersible polymer (UNDP) is utilized as a grouting material for repairing interlayer gaps in track structures located in water-rich environments. Understanding the compression properties of UNDP is crucial for assessing the longevity of track structures and ensuring the safe operation of trains. In this paper, uniaxial compression tests of UNDP specimens with different curing ages (0.125–28 days) were conducted. The deformation and cracks expansion of the specimens during the loading process were investigated by the acoustic emission (AE) technique and scanning electron microscope (SEM). Furthermore, the compression constitutive model of UNDP was established based on strain-stress curves. Test results indicate that the compressive strength of UNDP reached 22.63 MPa after three hours and stabilized at 30 MPa after 14 days. The stress-strain curves exhibited distinct staged characteristic, which can be divided into the compaction stage, the plastic deformation stage and the damage failure stage. The specimens with different curing ages showed the similar failure modes with a maximum axial deformation of 33%. The internal cracking pattern changed with the increase in curing ages, transitioning from dominant tensile cracks to shear cracks. At a curing age of 14 days, shear cracks were found to be 51.43%. In addition, SEM images revealed that the microstructure of UNDP consisted of a polyurea cross-linked backbone-water glass gel-filled structure. A constitutive model considering the curing ages was also proposed. This study provides a reference for water-rich environment application of this Underwater non-dispersible polymer. • Compression damage evolution and failure modes of UNDP were investigated. • Acoustic emission signal characteristics and microstructure of UNDP were analyzed. • Compression constitutive modes of UNDP considering the curing age was proposed. [ABSTRACT FROM AUTHOR]

This paper investigated the effects of steel fiber admixture (with volume fractions of 0%, 0.5%, 1.5%, and 2.5%) on the flexural performance of concrete by using cryogenic temperature (-165℃) in situ four-point bending tests combined with the acoustic emission (AE) to simulate liquefied natural gas (LNG) storage tanks and the lunar temperature environment. The results indicate that the steel fiber admixture is positively correlated with the flexural strength, and that the strengthening effect is more prominent at the cryogenic temperature. Although the peak flexural strength of HCC with four varying volume fractions increases by 161.9%, 78.3%, 63.4%, and 55.07%, respectively, the relative increase in the peak strength due to the cryogenic temperature decreased gradually; moreover, HCC-SSF 0 -C and HCC-SSF 0.5 -C exhibit obvious cryogenic temperature-induced brittleness effects; however, the flexural toughness indices of HCC-SSF 1.5 -C increase steadily under high deflection, while HCC-SSF 2.5 -C exhibits decrease. The volume fraction of steel fiber admixture is roughly positively correlated with the cumulative ring counts (RC), and the increase in cumulative RC is more significant, especially at cryogenic temperatures; similarly, the presence of steel fibers can increase the energy release rate density (ERR-D) significantly, and the energy release rate magnitude (ERR-M) tends to increase gradually with increasing loading. The frozen water and bonding properties of steel fibers are the main factors affecting the flexural toughness and AE crack signal characteristics of concrete. As the fiber volume fraction increases, shear cracking and AE events increase significantly, primarily due to fiber pullout and slippage, and the steel fiber incorporation increased the percentage of shear cracks. The cryogenic temperatures cause more shear cracks than ambient temperature conditions. The shear cracking dominates at cryogenic temperatures in all four steel fiber content samples, with percentages of 57.01%, 73.52%, 62.03%, and 68.11%, respectively. The issue that there is no uniform or fixed boundary between the two types of cracks was solved by using the machine learning-based Gaussian mixture model (GMM) algorithm and support vector machine (SVM) to classify the percentage of tensile and shear cracks, respectively, and to specify the percentages in different loading stages. A few suggestions were given in this study to guarantee the safe operation of concrete material structures at cryogenic temperatures. • The flexural properties of various steel fiber volume admixtures of concrete at −165℃ was studied. • In situ cryogenic acoustic emission evolution patterns at different loading stages was investigated. • The percentage of tensile and shear cracks was classified by the machine learning. • The mechanism of brittle fracture behavior of steel fiber at cryogenic temperature was analyzed. • Using a high volume fraction of steel fibers in concrete at −165℃ is not recommend. [ABSTRACT FROM AUTHOR]

To tackle the issues of inadequate mechanical properties of recycled aggregate concrete and its limited application in load-bearing structural components, a high-performance hybrid fiber reinforced recycled aggregate concrete (HP-HFRAC) was developed in this study. Sixteen groups of HP-HFRAC specimens fabricated with varying replacement rates of recycled coarse aggregate and different fiber parameters were subjected to uniaxial compression, as well as acoustic emission (AE) tests, to investigate the cyclic behavior and uncover the underlying mechanisms. The findings demonstrated a strong correlation between the AE signals and the macroscopic phenomenon throughout the whole compression process. The AE amplitude distribution was found to be capable of identifying damage mechanisms, including matrix cracking (40–50 dB), fiber sliding (50–80 dB), and fiber pull-out (more than 80 dB). Additionally, the inclusion of hybrid fibers contributed to mitigating the degradation of stiffness and the accumulation of plastic strain, resulting in the ductile failure of the specimens and significantly enhancing the energy dissipation capacity. Specifically, the peak stress of specimens with 1.5% steel fibers increased by 19% compared to that of specimens without steel fibers, compensating for the strength loss due to the presence of recycled coarse aggregate. Finally, constitutive equations governing the skeleton curves and the unloading and reloading curves of HP-HFRAC subjected to cyclic compression were formulated, which were validated using both the test results and the predicted results of the physics-based data-driven model previously published in our companion paper. The findings of this study might contribute to the development of sustainable high performance recycled aggregate concrete for reliable application in load-bearing components. [Display omitted] • The effect of recycled coarse aggregate and hybrid fibers on monotonic and cyclic performance of HP-HFRAC was investigated. • The whole compression process, damage mechanisms and failure modes were revealed using AE technique. • The addition of hybrid fibers helped to slow the stiffness degradation and decrease the cumulative plastic strain. • A constitutive model for HP-HFRAC under uniaxial cyclic compression was developed. [ABSTRACT FROM AUTHOR]

The half-grouted sleeve connection (HGSC) have been widely used as connection tools for prefabricated concrete (PC) buildings. It is crucial to maintain the safety and stability of the nodes and detecting internal defects in the grouting sleeve for the safety of the entire structure. Therefore, understanding the mechanical behavior and damage mechanism of HGSC is essential for the design and construction of PC. Currently, there are few on-site techniques applicable for monitoring the damage progress. The acoustic emission (AE) as a passive method shows great potential to characterize the damage in the HGSC. Therefore, in this paper, we propose to use AE to quantify the damage of the HGSC with defects under the tensile load. The artificial defects were introduced into HGSC with different defective rates. The monotonic tensile load was applied to each HGSC sample with a loading rate of 0.33 mm/s and tested to the failure. In this process, the AE signals were collected by AE transducers. For the classification and prediction of damage signals, we utilized a machine learning framework tailored for HGSC. This framework integrates unsupervised k-means++ cluster analysis and supervised K-Nearest Neighbors (K-NN) for damage classification and prediction. The experimental results showed that four AE signal clusters can be defined during the progress of tensile failure:(i) microcrack expansion;(ii) the friction signals of rebar and high-strength grouts; (iii)the generation of microcracks; (iv) plastic deformation of rebar. The corresponding AE signatures were identified. The K-NN model trained on the training data provides a high accuracy in predicting different class of defects. It was concluded that the proposed machine learning framework is a promising and powerful tool for realizing the on-site damage detection and prediction for HGSC. • AE signatures of rebar pullout and breakage in HGSC were identified. • The precursor AE signals were recommended for the on-site inspection. • Propose a machine learning framework to classify the AE signals and predict the damage. [ABSTRACT FROM AUTHOR]

In the variable water level area, hydraulic concrete structures are susceptible to variable temperature and alternating dry-wet cycles, especially for thin-walled structures. To elucidate alterations in the microstructure and strength of hydraulic concrete subjected to variable temperature dry-wet cycles, this paper firstly conducted a durability test on hydraulic concrete. Subsequently, CT scan images of the specimens are acquired during the variable temperature dry-wet cycle. Secondly, in view of the shortcomings of existing threshold methods for CT image pore segmentation, a new image segmentation method (RASLPSO-TV-Top-hat) is proposed by using Tensor voting and Top-hat technology, combined with a Reverse Adaptive Particle Swarm Optimization algorithm with Self-Learning ability. The evolution process of pore structure (microcracks) of hydraulic concrete under the variable temperature dry-wet cycles is captured. Thirdly, an in-depth analysis is conducted on the compressive strength and acoustic emission characteristics. The analysis shows that with the number of cycles increases, there is a discernible interconnection among the pore structures. This interconnection leads to a noteworthy reduction in the compressive strength of the concrete. Additionally, the cumulative number of Acoustic Emission ringing events exhibits an initial rise followed by a subsequent decline. The study successfully identifies the patterns governing the deterioration of strength and microstructure evolution in hydraulic concrete. The mechanism underlying the decline in performance of hydraulic concrete under variable temperature dry-wet cycles is elucidated. It provides a reliable and valuable basis for the safety assessment of hydraulic concrete structures in the variable water level area. • We conduct a durability test of concrete under variable temperature dry-wet cycles. • We propose a new image segmentation method. • Pore structure, compressive strength and acoustic emission character are analyzed. [ABSTRACT FROM AUTHOR]

Rock formations naturally contain intricate internal fractures due to various environmental factors. Such fractures result in significant weakening of the mechanical properties of the rock mass. As factures have different geometric features and fill material characteristics, it is difficult to replicate their complex behavior in the laboratory. This poses a serious limitation on the experimental investigation of the mechanical properties of fractured rocks. Sand powder 3D printing (3DP) can overcome the limitations of casting methods in preparing samples with complex fractures and thus is widely applied in soft rock mechanics experiments. This paper utilizes Computer Tomography (CT) scanning to obtain the probability distribution patterns of fractures in fractured rock samples. Additionally, it combines sand powder 3DP technology to generate soft rock-like samples with internal networks of filled fractures. Uniaxial compression experiments employing digital image correlation (DIC) and acoustic emission (AE) techniques are used to investigate the mechanical properties, deformation characteristics, and fracture evolution patterns of samples with different fracture densities. By increasing the fracture density, the peak strength of the soft rock-like samples exponentially decreases, and the deformation characteristics linearly decrease. Crack propagation paths mostly follow the prefabricated fracture trajectories and loading direction. An RA-AF analysis suggests that the failure mode of the soft rock-like samples transitions from diagonal shear failure to block-shaped shear failure with increasing fracture density. These research findings represent a novel sand powder 3DP approach for studying the complex mechanics of complex fractured rocks. • A novel method to generate rock analogs with filled internal fractures is proposed. • Internal fracture density has great effect on the mechanical properties of samples. • Effect of fracture density on failure patterns are investigated with DIC and AE. [ABSTRACT FROM AUTHOR]

Semi-rigid base is the main component of highway pavements in China and is prone to cracking damage under vehicle loading and environmental effects. In this paper, three-point bending (TPB) tests on prefabricated cracked specimens of CSM beams were conducted to study the damage evolution and fracture characteristics of cement-stabilized macadam (CSM) under different loading rates. The acoustic emission (AE) technique and scanning electron microscope (SEM) analysis were utilized. The results indicate that the fracture energy and fracture toughness of CSM were enhanced at increasing loading rates, and the amplitude distribution of AE events became wider and more energetic. The damage modes of CSM can be classified into interface transition zone (ITZ) damage, cement mortar damage, and aggregate damage. The fracture surfaces were mainly ITZ damage. The brittle structure of CSM was caused by incomplete hydration of cement particles in ITZ. This study can provide a better understanding of the fracture mechanism of CSM at different loading rates. The findings can also guide the design, construction, operation and maintenance of semi-rigid roadbed pavement structures. • The effect of loading rate on the fracture characteristics of CSM was analyzed. • The evolution of fracture damage in CSM three-point bending tests was investigated using AE technique. • The cracking-damage mechanisms of CSM under different loading rates were revealed. [ABSTRACT FROM AUTHOR]

• An in‐depth literature review and discussion on AE‐based corrosion monitoring in RC is presented. • Consistency of parameter‐ and signal‐based AE methods for corrosion monitoring is analyzed. • AE‐based condition assessment of corroded RC elements during load testing is discussed. • An overview on availability of AE data sets for corrosion tests in lab and in‐situ is presented. • Research challenges for in‐situ monitoring of corrosion damage in RC structures are discussed. Corrosion is a major issue in construction and infrastructure management, giving rise to high repair costs, safety concerns and structural failures. Especially reinforcement corrosion in concrete structures is a complex issue. As degradation starts internally, visual inspection lacks efficiency in early stages of the corrosion process. Acoustic Emission (AE) monitoring provides major advantages as this technique enables to detect internal damage in an early stage. However, the use of AE monitoring also comes with certain challenges. The aim of this paper is to review appropriate protocols for AE-based reinforced concrete (RC) corrosion monitoring based on an overview of reported lab experiments, highlighting challenges and pitfalls, and to provide a solid basis for extension of the research focus towards on-site AE-based corrosion monitoring in RC structures. Therefore, this paper firstly discusses AE monitoring during (accelerated) corrosion tests, including methods for data analysis and best-practice protocols. Secondly, application of the AE technique for condition assessment of corroded RC elements during load testing is discussed. Thirdly, an overview is presented of reported on-site AE monitoring studies on corroding RC structures. The paper concludes with a discussion and future research challenges, especially towards on-site monitoring of existing RC structures. [ABSTRACT FROM AUTHOR]

Backfill mining is an effective technique that can prevent and control the impact of dynamic disasters which can occur in deep underground coal mines. This technique allows to reduce the surface subsidence and achieve a greener mining. The backfill shear mechanical properties under mining disturbances, are of great significance for ensuring the surrounding rock stability in deep mining. Therefore, studying the shear mechanical properties of the cemented gangue backfill body (CGBB) under coupled dynamic and static loads actions in deep coal mining is of great importance. In this paper, shear mechanical tests and the acoustic emission (AE) technology were used to study the shear mechanical response characteristics of CGBB under low-frequency dynamic normal load conditions. The results show that a dynamic normal load with an amplitude of 5 kN improves the shear CGBB strength, while a dynamic load with an amplitude greater than 10 kN will cause a shear strength deterioration, and the degree of deterioration is positively correlated with the load amplitude. Under low-frequency dynamic normal load conditions, the shear CGBB stress-strain curve exhibits classical characteristics. AE indicate that the main CGBB damage changes from a nonlinear deformation to an elastic stage under low-frequency dynamic normal load conditions. During the shear failure process, the cumulative AE counts are positively correlated with the load amplitude. The crack type identification based on RA-AF shows that the low-frequency dynamic normal load affects the CGBB crack type. Under constant normal load (CNL) conditions, tensile or mixed cracks account for 63.95% and are the main fracture mode. Conversely, under low-frequency dynamic normal load conditions, shear cracks account for more than 60% and are the main failure mode. This research provides a reference for the CGBB stability analysis under complex stress conditions for the case of a deep underground coal mining. • The effect of dynamic normal load on the shear mechanical properties of cemented gangue backfill body was studied. • The impact mechanism of dynamic normal load on CGBB was studied through acoustic emission technology. • Based on RA-AF, the effect of dynamic normal load on the type of CGBB cracks was investigated. [ABSTRACT FROM AUTHOR]

Concrete materials are widely used in construction projects. Many concrete structures are in an aqueous environment and subjected to impact loads. Under this condition, the mechanical properties and failure modes of concrete will change significantly. In this paper, the drop hammer impact failure experiment of water-containing concrete was established. The deformation and failure characteristics as well as acoustic emission response law of concrete under dynamic load with different immersion time were studied. The failure mechanism and acoustic emission response precursor characteristics of concrete under drop hammer impact were revealed. The results show that the peak impact force of concrete was exponentially related to the immersion time. Its failure cracks were mainly distributed on the diagonal of the impact surface and the central axis of the observation surface. Due to the dual effect of water on the dynamic mechanical properties of concrete, the cracks and damage degree of concrete increased first and then decreased with the increase of immersion time. The peak acoustic emission (AE) count was exponentially related to the immersion time. The acoustic emission "quiet period" could be used as one of the precursor characteristics of concrete instability and failure, and the "quiet period" gradually became shorter with the increase of immersion time. This research results are of great significance for evaluating the stability of concrete structures and warning of concrete impact damage. • The effect of immersion time on the impact damage of concrete was studied. • Water had a dual effect on the dynamic mechanical properties of concrete. • The peak value of AE count was exponentially related to the immersion time. • As the immersion time increased, the "quiet period" of AE gradually became shorter. • The failure mechanism and AE precursor characteristics of concrete were revealed. [ABSTRACT FROM AUTHOR]

High performance lightweight aggregate concrete has become a popular topic for the development of green concrete due to the incorporation of various industrial wastes to substitute aggregate. However, its brittleness has gradually emerged as an issue with the increasing of compressive strength. To address this concern, the authors in this paper investigated the influence of basalt macro fiber (BMF) on the compressive and splitting tensile strengths of lightweight aggregate concrete at different fiber volume contents and lengths. Moreover, the three-point bending test on notched beams, with the help of acoustic emission and digital image correlation, was also carried out to evaluate the fracture toughness of basalt macro fiber reinforced lightweight aggregate concrete. Finally, the softening constitutive model was determined in the present study. The results showed that the addition of BMF increased the compressive strength of lightweight aggregate concrete and the maximum increase reached 9.03%. As for splitting tensile strength, the maximum increase was 39.88%. In addition, it was observed that the incorporation of basalt macro fiber significantly improved the initial cracking toughness and unstable fracture toughness of plain specimen. The fracture energy of BMF reinforced lightweight aggregate concrete was also increased, and the largest improvement was seen in BMF-0.5–22.5 group, 8.5 times that of plain specimen. • The mechanical properties of basalt macro fiber reinforced HPLAC were investigated. • Acoustic emission and digital image correlation were employed to monitor the three-point bending test on notched beams. • The influences of basalt macro fiber volume content and length on the fracture toughness of HPLAC were discussed. [ABSTRACT FROM AUTHOR]

Aiming to investigate the impact of different shell ash (SA) dosages and particle sizes on the pore structure, mechanical properties, and microstructure of the cemented tailings backfill, NMR tests, the uniaxial compression tests, AE tests, and SEM tests were carried out in this paper. The following conclusions were reached: (1) As the dosage of SA increases, macro pores gradually transform into secondary pores and micro pores, the pore structure distribution is rationalized. The 1250 mesh cemented tailings backfill (SACTB) exhibits the highest degree of hydration reaction, followed by the 3000 mesh SACTB. Thus, adding SA to the filling material significantly increases macroscopic mechanical properties of SACTB. (2) SACTB is primarily dominated by tensile damage, with a small proportion being shear damage or accompanied by shear damage. (3) AE cumulative ringing counts in the time range through the rapidly growing, stabilizing, and continuing rising in three phases. The AE process can be summarized as a rising, stable, and active period. (4) For the improvement of the microstructure and macroscopic mechanical properties of SACTB, it is recommended to use 1250 mesh SA when the dosage is between 3 % and 5 %. Similarly, when the dosage is between 8 % and 10 %, it is advisable to use 3000 mesh SA. • Shell ash (SA) optimizes the pore size structure and promotes specimen densification. • The hydration products occupy a significant ratio inside the shell ash cemented tailings backfill (SACTB). • The AE process can be summarized as a rising, stable, and active period. • When the dosage is 3 %–5 %, it is recommended to use 1250mesh SA. When the dosage is 8 %–10 %, it is recommended to use 3000 mesh SA. [ABSTRACT FROM AUTHOR]

[Display omitted] • The auxiliary cementitious materials can effectively fill the micropores and shorten the interfacial transition zone. • The developed elastoplastic parallel bonding model can record the irreversible damage during freeze–thaw cycles. • The decay constants of UCS and elastic modulus after freeze–thaw cycles can be estimated from the initial parameters. In this paper, pervious concrete (PC) was prepared using fly ash, granulated blast furnace slag, and rice husk ash as supplementary cementitious materials (SCMs), and the mechanical properties, water permeability, and freeze–thaw durability of permeable concrete were mainly investigated. Discrete elements (DEM) were used to mimic the freeze–thaw cycle process of pervious concrete to assess better how freeze–thaw cycles affect the mechanical properties of pervious concrete. To allow water particles of various sizes to fill the model pores, new algorithms were developed. However, since some connected pores release some of the pressure generated by water expansion when undergoing FTCs, two different water particles must be defined. Meanwhile, an elastoplastic parallel bond contact model (EPPBM) was created to account for any residual strain brought on by water freeze–thaw expansion at the aggregate bond. Through experiments, it was determined that adding SCMs increased the compressive strength of the pervious concrete by 1.22 %, 15.32 %, 32.34 %, and 35.20 % at 7, 28, 56, and 90 days compared to the control group. The microstructure of the hydrated calcium silicate (C-S-H) gel could be relatively dense by SEM. The width of the interfacial transition zone (ITZ) was found to have been effectively shortened by further scanning using EDS. As a result, in the tests of mechanical properties after freeze–thaw cycles (25, 50, 75, and 100 cycles), there was a significant improvement in the loss of strength compared to the control group, which was 2.13 %, 6.91 %, 10.04 %, and 17.40 %, respectively. Using numerical simulation to correct the functional correlation between FTCs and the mechanical characteristics of permeable concrete, the trend essentially aligns with the experiment's results, demonstrating the model's viability. Acoustic emission monitoring of model rupture location and strain energy loss after FTCs were utilized and the effect of different porosity and initial uniaxial compressive strength (UCS) on the attenuation constant of the freeze–thaw resistance of pervious concrete was analyzed. The decay constant of UCS increased with increasing porosity and elastic modulus while decreasing with increasing initial UCS. Therefore, the decay constant of UCS and elastic modulus after arbitrary FTCs can be easily estimated using the initial parameters of the pervious concrete. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nanocellulose can empower the anti-disturbance capacity to the concrete. • Anti-disturbance mechanism of nanocellulose concrete was revealed. • The parameter to evaluate the anti-disturbance capacity of concrete was put up. To alleviate concrete performance deterioration caused by disturbance, it is proposed in this paper that nanocellulose can be used to enhance the anti-disturbance capacity of concrete. Macroscopic strength, mesoscopic pore structure, microstructure, and acoustic emission characteristics were considered simultaneously to analyze the anti-disturbance capacity of nanocellulose concrete, and then to reveal the anti-disturbance mechanism. The research shows that the nanocellulose can induce the growth of C-S-H gel, form a bridge structure between hydration products, and then improve hydration products' compactness, optimize concrete pore structure, and enhance the anti-disturbance performance of concrete. The research also shows that acoustic emission parameters of nanocellulose concrete with disturbance do not show significant changes, which is consistent with pore structure characteristics of the concrete, confirming the excellent anti-disturbance capacity. Moreover, the evolution law of the b-value obtained by the acoustic emission test indicates that the first peak reversal of the b-value in time-series can be used to evaluate the concrete sensitivity to disturbance, and the time when the b-value and its first peak appear can characterize the anti-disturbance performance of concrete. [ABSTRACT FROM AUTHOR]

• The mixing ratio of PSVC with high strength and hot-humid resistance is proposed. • The equation of strength considering various microscopic parameters is established. • The failure mode of the PSVC is divided by AE signal and the shape of the cracks. Hot-humid geological environments cause cement substrates to bulge and crack, which seriously affects the strength of lining concrete structures. This paper proposed a composite concrete material suitable for hot-humid environments, using three additives of steel fibers, polypropylene fibers and vitrified beads. The compressive properties of composite concrete were studied, and the relationship between the pore structure and compressive properties was described by a microscopic quantitative method. At the same time, the failure characteristics of the composite concrete in a hot-humid environment were studied in combination with the acoustic emission (AE) events. The results show that the combination of steel fibers, polypropylene fibers and vitrified beads renders the highest compressive strength for the concrete at all temperatures. The strength loss rate is the lowest with increasing temperature, and the toughness is also greatly improved, which indicates that the composite concrete can adequately adapt to the hot-humid environment. The microstructure parameters of concrete have a good nonlinear relationship with the compressive strength. The continuous high number of AE events in an adjacent timing event can be used as precursor information for the failure of the composite concrete. The use of the three additives causes the failure mode to change from tensile to shear. The results can provide a theoretical basis for the research and quantitative evaluation of the tunnel lining structure in hot-humid geological environments. [ABSTRACT FROM AUTHOR]

• Modified Weibull distribution model can describe the whole failure process. • The damage degree of shotcrete is characterized by AE hit ratio. • The damage of shotcrete can be divided into three stages. • The value of Δ H R in initial hit stage cannot be neglected. Many scholars have used the Weibull distribution model to analyze the damage of shotcrete, but it cannot entirely describe the whole process of failure, especially after the peak stress. In this paper, based on the Weibull distribution model modified by the writer with the aim of exploring different mechanical properties of three types of shotcrete, the acoustic emission (AE) technique is used to carry out uniaxial compression tests on different shotcrete, and the failure mechanism of different shotcrete is analyzed according to the AE energy value and cumulative hit counts under loads. The experimental results are consistent with the damage mechanism quantitatively described by the modified Weibull distribution model. This damage model with fewer parameters and simple form, which can quantitatively described the whole process of specimens, including post-peak stress, from the beginning of loading to the complete failure. The research results thus provide a basis for quantitatively describing the damage of shotcrete or rock, and provide a reference for the selection of shotcrete in different engineering environments. [ABSTRACT FROM AUTHOR]

• Early AE is considered as indicator of bonding quality in cementitious media. • RT, RA and AF are very sensitive to the stress transfer at the interfaces. • AE allows evaluation of effectiveness of fibers, bars or patches in concrete. This review focuses on the fiber reinforced cementitious composites for construction, a rapidly advancing material field, from a certain perspective. While fiber reinforcement is known to dominate the cracking and post-cracking response, it seems unambiguous that the fiber influence is also seen at the pre-peak behavior through the restrain they cause to cracking. Acoustic Emission (AE) is often selected to track the transition from solid to damaged state and provide damage assessment on concrete and cementitious composites. However, the sensitivity of AE allows to characterize not only large-scale fracture phenomena but also events in the micro-scale which give no other indication. Herein, information from relevant papers is gathered showing that AE features show strong sensitivity to the shear transfer mechanism between fibers, rebars or patches and matrix, considerably earlier than the visible damage. It is consistently shown that as the quality, quantity or effectiveness of the reinforcement increases, this is reflected on the AE behavior of the early part of the loading, allowing for comparison between different type of reinforcements and prediction on the final mechanical capacity. [ABSTRACT FROM AUTHOR]

• AE fracture mode characterization of masonry constituents. • Successful correlation between full-field strain pattern from DIC with AE indices in masonry components. • Shear fracture mode characterization. • FEM numerical simulation. In this paper, an experimental study conducted on Red Clay brick masonry "triplets" built with Cement and Lime-based mortars is presented. Monitoring of fracture is executed by Acoustic Emission (AE) and Digital Image Correlation (DIC). First, for a profound understanding of individual properties, small scale experiments were conducted for characterization of AE signatures of mortar and brick separately. Representative masonry triplet elements are tested in shear for investigation of the fracture mechanisms in relation to the properties of different mortars. Localization of the cracks as well as source identification is performed by means of AE parameter-based analysis. Their correlation with DIC, accurately showed the location of macro cracking and the developing of strain concentrations even before fracture. This is one of the first investigations where full field strain pattern from DIC are successfully correlated with AE indices in masonry. Numerical simulations were also performed for validation and prediction purposes. [ABSTRACT FROM AUTHOR]

• The effect of small-size healing capsules on concrete damage resistance is assessed. • Fracture toughness of healing concrete increases by 35% compared to reference. • Acoustic emission is applied to assess the capsules effect on fracture processes. • Digital Image Correlation tracks the crack propagation through the capsules. • The capsules benefit perturbating the crack path and forming multiple microcracks. An alternative solution to repair cracks in concrete has been recently established, namely autonomous healing by embedding repair agent into concrete during casting. The agent is included into small-size capsules that break as cracks form. As the agent is released into the crack void, it solidifies fast permitting crack sealing and mechanical restoration. Most of literature studies focus on the healing system design and the repair efficiency assessment. However, limited knowledge exists on the effect of the macro-capsules themselves on concrete mechanical behaviour. This is a crucial subject as the healing mechanism should not compromise the material mechanical properties. The paper investigates the effect of capsules on damage resistance by considering fracture constitutive models. Fracture toughness of samples with capsules increases up to 35% compared to the reference. Acoustic emission is applied to assess the capsules effect on fracture processes. Events localization indicates fracture process zone expansion by 40%. It is proven that the capsules beneficially contribute as local reinforcement perturbating the crack path and forming multiple microcracks (additionally verified by Digital Image Correlation). In summary, an integrated experimental protocol is developed to highlight the concrete toughness enhancement by adding capsules. [ABSTRACT FROM AUTHOR]

• Mechanical properties of pervious concrete reinforced with geogrids are studies. • Geogrids restrain the generation and propagation of the crack of pervious concrete. • Reinforced beams have brilliant post-cracking performance. • Embedding the geogrids at both 1/3 depth and 2/3 depth of the thickness works best. Pervious concrete has been widely used due to the environmental and stormwater management benefits. However, its susceptibility to cracking and low flexural strength limited the widespread use for vehicular traffic pavement. This paper focuses on the flexural behavior of pervious concrete beams reinforced by geogrids with different layer number at various positions. The influences of the embedded geogrids on effective porosity, strength and cracking behaviors are studied. The load-vertical displacement curves are obtained by four-point bending tests to describe the toughness and post-cracking performance of pervious concrete beams. Meanwhile, the acoustic emission (AE) and digital image correlation (DIC) techniques are used to monitor the internal fracture characteristics, strain fields and crack propagation. The experimental results indicate that the compressive strength and flexural strength of pervious concrete can be improved owing to the embedded geogrids. Notwithstanding an increased porosity is induced, the reinforced beams demonstrate significantly superior post-crack performance. Furthermore, the embedded geogrids in pervious concrete restrain the generation and propagation of the crack, resulting in a low strain field level as well as showing a short crack length and a small crack mouth opening displacement (CMOD). Embedding the geogrids at both one-third depth and two-thirds depth of the thickness provides the optimal performance. [ABSTRACT FROM AUTHOR]

• AE analysis was used to assess the crack growth of ECC beam-column connections under reversed cyclic loading tests. • The effect of fibers type on the performance of the specimens was examined. • AE b-value and intensity analyses were completed to analyze the AE data. • Damage classification chart was developed in terms of intensity analysis parameters. This paper presents the results from an experimental program conducted to assess the cracking behavior of reinforced concrete beam-column connections cast with engineered cementitious composites (ECC). Reversed cyclic loading tests in conjunction with acoustic emission (AE) monitoring were completed on all specimens till failure. Four ECC mixtures containing four different types of fibers along with a control conventional concrete (CC) mixture were tested. The AE data obtained from all tests were filtered and analyzed to identify and evaluate the crack growth in all beam-column connections. First, the changes in the AE parameters such as signal amplitudes, number of hits, and cumulative signal strength were considered. Then, additional b-value and intensity analyses on both signal amplitude and strength were performed to develop three further AE parameters including b-value, historic index (H (t)), and severity (S r). The impact of fibers type and sensor location on the aforementioned AE parameters was examined. The AE analysis completed in this study led to the identification of the first cracks prior to their visual observation, regardless of fibers type and/or sensor location. The studied AE parameters were highly correlated to the process of crack propagation in all specimens until failure. The results also showed that varying the types of fibers in ECC yielded non-significant effect on the AE signal characteristics (in terms of signal amplitudes) throughout the tests. The intensity analysis parameters (H (t) and S r) were eventually used to generate a damage classification chart to classify the stages of first crack and maximum load in the tested specimens. [ABSTRACT FROM AUTHOR]

• Development of PCT for AE in 3D space is a novel approach. • Critical mechanisms are distinguished clearer after parameter correction procedure. • As actual amplitude of an activity increases, probability of accurate detection also increases. • Energy is a more deteriorated parameter than amplitude. Deterioration of ultrasonic signals obtained from nonhomogeneous mediums due to dispersion and attenuation is a crucial problem affecting the validities of elastic-wave-based nondestructive testing methods. Concrete, which is the main construction material, is also an excessively heterogeneous material as it consists of cement, water, aggregate and other admixtures. Because Acoustic Emission (AE) is known to be an appropriate method for detecting active damages nondestructively in concrete structures, enhancement of this technique is important for more reliable inspections. This paper describes development of a 3D Parameter Correction Technique (3D-PCT) and first application of AE parameter correction in reinforced concrete (RC). The technique was applied to plain and CFRP-strengthened RC beam specimens. Accordingly, AE data obtained from three-point-bending tests were localized, their signals and also amplitude and energy parameters were corrected in 3D space and parameter analyzes were conducted. By applying 3D-PCT for RC; corrected results were obtained, some AE parameter relations were altered and activity type classification was improved. Moreover, accordance with mechanical observations was enhanced. [ABSTRACT FROM AUTHOR]

Highlights • NaCl solution can interact with concrete, producing a destructive chemical phase that can damage concrete. • The role of supplementary cementitious materials in mitigating the damage was investigated. • Silica fume, class F fly ash, and slag can reduce the damage whereas class C fly ash increases the damage. Abstract It has been reported that concrete can chemically interact with high-concentrated sodium chloride solution to form a new chemical phase change at temperatures between 0 °C and 10 °C. The formation of the chemical phase change has been found to be destructive and can cause damage in concrete. The chemical phase change appears to be the result of reactions between the sodium chloride, the tricalcium aluminate and the calcium monosulfoaluminate hydrate in the concrete matrix. This paper reports results of a study conducted to determine the effects of using supplementary cementitious materials as a partial replacement of the cement on reducing the chemical phase change amount and the associated damage development. Class C fly ash, class F fly ash, slag, and silica fume were used in this study. The amount of the chemical phase change was quantified using low-temperature differential scanning calorimetry. The damage development was monitored and quantified using a low-temperature longitudinal guarded comparative calorimeter equipped with acoustic emission technique. The results indicated that the addition of silica fume, class F fly ash, and slag can reduce the chemical phase change amount and the associated damage development while the addition of class C fly ash showed the opposite effect. X-ray diffraction analysis coupled with thermogravimetric data were also used to estimate the chemical compositions of cementitious pastes in order to determine the relationship between the amounts of the chemical phase change and the chemical constituents present in the cementitious systems. It was found that the formation of the chemical phase change is well correlated with the amounts of tricalcium aluminate and calcium monosulfoaluminate hydrate while a poor relationship existed for other chemical constituents available in cementitious systems. The beneficial aspects of supplementary cementitious materials based on their replacement levels, such as dilution effect and pozzolanic activity were also discussed. The best mitigation strategy to limit the formation of the chemical phase change and the associated damage is to utilize Type V cement or partially replace the Type I cement with silica fume, slag, or class F fly ash. The use of class C fly ash was found to increase the chemical phase change and the associated damage. Therefore, it should not be recommended in concrete exposed high-concentrated NaCl solution. [ABSTRACT FROM AUTHOR]

Highlights • AE-analysis enabled to localise and quantify the amount of corrosion damage. • The developed clustering algorithm allows to distinguish different damage processes. • AE results from accelerated corrosion tests were validated with micro-CT. Abstract Many existing reinforced concrete (RC) structures are suffering from corrosion damage. The development of reliable tools to characterise and localise such damage is essential to assess the structural capacity. The acoustic emission (AE) technique has proven to be promising for this purpose. However, this technique poses challenges to reliably interpret the results and lacks rigourous validation. In this paper, microfocus X-ray computed tomography (micro-CT) is used to validate the use of AE for the localisation and characterisation of chloride-induced corrosion damage. An experimental protocol is designed, where both techniques are applied on small-scale reinforced mortar samples during accelerated corrosion. In addition to the frequently applied localisation of the AE events, a new signal-based clustering algorithm of the AE events is proposed, which allows to distinguish between different damage processes, such as friction due to the emergence of corrosion products and corrosion-induced mortar cracking. The results obtained from the AE localisation and clustering are confirmed by the micro-CT images and are found to be in line with alternative damage characterisation techniques based on AE parameters, that are found in the literature. Finally, it is shown that the combination of clustered AE events and micro-CT for the considered small-scale samples can help to understand the different damage processes induced by corrosion. [ABSTRACT FROM AUTHOR]

Highlights • Kaiser and Felicity effect are demonstrated in masonry under compression. • AE Load-Calm ratios indicate progressive damage in masonry for the first time. • AE-based damage limits are validated with full-field deformation measurements. Abstract Determining the mechanical behaviour of masonry under compression remains challenging, as masonry is anisotropic and heterogeneous in nature. This paper focuses on an Acoustic Emission (AE)-based method to quantify the progressive damage in masonry under cyclic compressive loads. In this study, eight masonry walls were built in combination with four different mortar types. Results indicated that critical limits applied for AE-based damage quantification in concrete were too conservative for masonry under axial loading. However, it is the first time that these AE indices are investigated in masonry. Nevertheless, a good agreement was found between the progressive damage quantification with AE-based methods and through deformation analysis from Digital Image Correlation (DIC) and Linear Variable Differential Transformers (LVDTs). [ABSTRACT FROM AUTHOR]

Highlights • RA replacement ratio of 60%, the bonding strength was the highest. • RAC and ordinary concrete were found to have similar AE signal velocities. • The temporal changes in the AE hits and energy were quite similar. • AE signals' average distance from the steel bar increases with RA replacement ratio. • b-Value in the splitting phase is strongly associated with the effective bonding strength. Abstract This paper reports the experimental studies of the bond between recycled aggregate concrete (RAC) and ribbed steel bars, using the acoustic emission (AE) technique. Four different recycled aggregate replacement ratios (0%, 50%, 60%, and 70%) were considered in pullout tests. The test results indicate that the strength of the bond between RAC and ribbed bars is highly correlated to the RAC strength; the highest RAC bond strength was measured for the recycled aggregate replacement ratio of 60%. The conducted AE signal analysis included AE hits and energy analysis, positioning of AE events, AE signal average frequency analysis, b-value analysis, and RA-AF association analysis. The analysis results demonstrate that the internal damage of RAC is more severe and more dispersed both in space and in time owing to the initial defect of the recycled aggregate. Using the AE monitoring method, the dependence of the RAC bond failure mechanism on the recycled aggregate replacement ratio is explained. [ABSTRACT FROM AUTHOR]

• Influence of wave travel distance on signal parameters is experimentally evaluated. • A new source classification criterion is proposed using peak frequency/partial power. • The new criterion is validated in another structural type, reinforced concrete slab. Acoustic emission (AE) signal parameters can be used to classify the source type in concrete structures. However, signal parameters are influenced by the wave propagation from the source to the receiver, leading to wrong source classification results, especially for monitoring large concrete structures. This paper experimentally evaluates the influence of wave travel distance on signal parameters on a full-scale shear test of a reinforced concrete beam. The evaluated signal parameters include the RA value, average frequency, peak frequency, frequency centroid, and partial power. The evaluation reveals the limitation of using RA value - average frequency trends in large scale structural concrete members. Based on the evaluation, we propose a new source classification criterion using peak frequency or partial power, which can effectively classify the source type. The new criterion is also validated in a reinforced concrete slab test, which is another structural type. Based on the new criterion, we suggest a sensor layout that is suitable for source classification for large concrete structures. The results of this paper can help developing a reliable solution for real-time source classification for large concrete structures in general. [ABSTRACT FROM AUTHOR]

Fibre-reinforced polymers (FRP) are effective for masonry reinforcement, but performance depends on bonding capacity, but performance depends on the bonding capacity. This paper analyses the shear behaviour of the FRP-to-masonry interface and uses the synergistic strengths of different investigation tools to characterise the debonding process. Twelve shear tests were carried out, using FRPs with three different fabrics. Using digital image correlation and Wu's model, the parameters of the bond–slip laws are determined. α is constant. α / β is a function of stiffness at power −0.5. The cumulative acoustic amplitudes increase significantly for a slip inversely proportional to α ( R 2 = 0. 926) and with a slope proportional to the fracture energy ( R 2 = 0. 810). The high frequency partial power increases sharply at a load for which the model predicts debonding at the sensor, or because of a textile tensile failure. The linear location obtained with AE makes it possible to differentiate these two failure modes. • The shear behaviour of masonry-to-FRP interface is investigated. • DIC is used to measure the displacement field of the FRP in the bonded area. • The bond–slip relationship is derived analytically from DIC measurements. • Complementarity of AE, DIC and analytical modelling. • Acoustic emission is effective for monitoring the development of debonding. [ABSTRACT FROM AUTHOR]

• AE and DIC show corresponding fatigue damage evolution in brittle materials. • A novel DIC-based FPZ analysis that uses material and image information is proposed. • AE energy and DIC strains resulted in similar FPZ widths under fatigue loading. Fatigue loading in brittle materials introduces damage at the micro-scale. These micro-fractures can accumulate and cause a significant reduction in material stiffness or even lead to structural failure. Deformation-based monitoring techniques can be inadequate when detecting damage at the micro-level. Hence, here is where advanced non-destructive testing (NDT) methods such as acoustic emission testing (AET) and digital image correlation (DIC) can provide a significant improvement in quantifying fatigue damage. The paper aims to combine AET and DIC for advanced fatigue damage analysis on brittle cementitious mortars. Cylindrical samples with three different sizes are subjected to fatigue loading in a Brazilian splitting test. The damage advancement quantified from cumulative AE event count and energy analysis showed a good correlation with the tensile deformation measurement of the DIC. It was observed that for low cycle loading, the slowly progressing damage evolution may not always occur, instead abrupt failure similar to monotonic loading can occur even after passing several load cycles. The size of the fracture process zone (FPZ) was measured using several AE-based and DIC-based methods. The comparison of the approaches indicated that the AE energy-based method is the most suitable to quantify the FPZ size during fatigue damage progress. From AE-based FPZ analysis of the experimental results, it was concluded that for low-cycle loadings, the lack of a well-developed FPZ can be a cause of the sudden brittle failure. [ABSTRACT FROM AUTHOR]

• Bonded rock-concrete specimens are used to study the shear failure mechanism of interface. • Stress softening and normal dilatancy deformation characteristics gradually strengthen with the roughness. • The shear fracture modes of rock-concrete interface are analyzed. • Acoustic emission is used to evaluate the shear failure process of rock-concrete bonding interface. The shear properties of bonded rock-concrete interfaces play a crucial role in the analysis of engineering structures stability. In this paper, laboratory direct shear tests and acoustic emission (AE) tests are conducted on standard cylinder specimens with varying levels of rock surface roughness. The aim is to gain a better understanding of shear behavior and failure mechanisms of the bonded interface between rock and concrete. The results show that increasing of roughness enhances the brittleness and peak dilatancy characteristics of the specimens. However, the residual shear strength exhibits the opposite trend. In addition, the distribution of residual concrete on the rock surface changes gradually from localized spots to larger areas. The AE test results demonstrate the occurrence of peak AE peak energy and count generation during shear failure. The evolution of the AE-b value indicates that the macro-fracture surface of the bonded surface is the result of the repeated conversion between large and small-scale micro-cracks. Moreover, the fluctuation value of the AE-b value is not influenced by the roughness of the rock interface. Hence, in this experiment, the 90% fluctuation mean value is selected as the primary parameter for rupture warning. Furthermore, The distribution of RA-AF values reveals the generation of numerous tension-shear composite crack types during the fracture process. The number of shear cracks in the failure stage exhibits an increasing trend with increasing roughness. [ABSTRACT FROM AUTHOR]

• Mechanical strength is improved when ceramic waste fine aggregate (CWA) is added into rubber cement mortar as a reinforcing material. • The fracture process of mortar is monitored based on acoustic emission (AE) technology, and the damage process is evaluated by the Ib-value, intensity analysis and the RA-AF value method. • Based on AE localization technology, the damage development along the section is evaluated. • Based on the fast Fourier transform (FFT) and wavelet packet transform (WPT), the frequency of the AE signal is analysed. Adding rubber particles into mortar or concrete has become one of the green ways to deal with waste rubber from the transportation industry. On this basis, this study proposes the addition of ceramic waste aggregate (CWA) to solve the issue of mortar strength reduction after the addition of rubber. This paper mainly analyses the mechanical properties of rubber cement mortar (RCM) mixed with different contents of CWA and the damage analysis of composite mortar in the three-point bending test by using acoustic emission (AE) monitoring technology. The flexural test and compressive test show that the mechanical properties of composite mortar can be greatly improved by adding CWA into the mortar. With increasing CWA content, the strength first increases at low content and then decreases at high content due to the high water absorption of the aggregate. At the same time, combined with AE technology, the cumulative energy, cumulative number of hits, Ib-value analysis, intensity analysis and RA-AF value analysis of different kinds of composite mortar are analysed and compared. Based on the AE source location and damage index, the fracture damage characteristics of three kinds of mortar in the three-point bending test are described. The frequency analysis of the AE signal is carried out by the wavelet packet transform (WPT), and the energy percentage change of the AE signal within a specific frequency interval is analysed. Based on the above AE analysis method, the three kinds of mortar have different AE signals, corresponding to different fracture characteristics. This paper provides the basis of mechanical and AE tests for a new type of green composite mortar. [ABSTRACT FROM AUTHOR]

• The biaxial compression experiment was carried out on CRC, and the strength growth was not as much as that of PC. • Propose the strength failure criterion for CRC under biaxial compression stress states. • Using AE technology to capture the acoustic signal of the concrete in failure process. • Using b-value and RA-AF value analysis methods to describe the characteristics of cracks in CRC and PC. Crumb rubber concrete (CRC) is increasingly becoming one of the environmental protection ways to solve the problem of waste rubber tires. This paper mainly analyzes the mechanical properties and acoustic emission (AE) parameters under biaxial compression stress states of CRC with different rubber contents (0%, 5% and 10%). Two biaxial loading modes, constant lateral pressure loading mode (LPM) and proportional loading mode (PM), were used to determine the failure modes, strength characteristics and peak strain of the concrete under different stress ratio conditions. The experiment results show that the compressive strength and peak strain of CRC are improved under biaxial stress like plain concrete (PC), but due to the weak bond between rubber and cement-based materials, the strength growth rate of CRC is not as much as PC. Therefore, the modified Kupfer failure criterions are proposed to describe its strength growth laws. Meanwhile, under the uniaxial loading mode and LPM, the initiation and growth of concrete cracks were monitored based on AE technology. The analysis used various AE statistical parameters, including the AE ringing counts, cumulative energy, amplitude distribution, b-value and RA-AF value analyses. Based on the above AE parameters can be related to the mechanical characteristics of the concrete, and the similarities and differences between CRC and PC are analyzed in the whole failure stage. The results show that CRC has better integrity in main collapse stage with mainly microcracks, which means the plastic failure characteristics. This paper provides experimental and theoretical foundations for the strength analyses and failure characteristics of CRC structures under complex stress states. [ABSTRACT FROM AUTHOR]

Highlights • Cyclic loading was applied on two concrete beams under flexion and Damage variable was calculated. • An empirical law was found between normalized Acoustic Emission activity and damage variable. • A correlation between Damage variable and Index Damage calculated from Acoustic Emission was proposed. • Temporal-domain and Frequency-domain analysis were performed on signals originated from specific events. Abstract This paper aims to improve knowledge on the degradation mechanisms of concrete beams subjected to flexural loading by using the Acoustic Emission Technique (AET). A strong correlation between damage evolution and acoustic emission activities was highlighted. Damage severity was also assessed by means of the Calm and Load ratio classification and the results obtained were in good agreement with those of mechanical testing. Empirical laws describing the relationship between damage evolution and acoustic emission activities were proposed in order to predict the degradation state of the material from AE activities and it seems that these laws depend only on the type of material tested. Finally, an analysis of the waveforms associated with one event (namely the same source) was carried out. The results show the difficulty of identifying a specific source from the AE signature. [ABSTRACT FROM AUTHOR]

Highlights • High temperature effects were investigated on SC-High Strength and Fiber-Reinforced Concrete. • Steel fibers played a beneficial role in bridging the expansion of heat-induced cracks. • Acoustic Emission (AE) measurements allow scrutinizing thermal damages in cementitious composites. • AE activity was monitored for analyzing bridging fiber mechanisms in SCHSFRC. Abstract This paper investigates the effect of high temperature on two different Self-Compacting (SC) cementitious composites. SC-High Strength (SCHSC) and Fiber-Reinforced Concrete (SCHSFRC) samples were tested in three-point bending after having been exposed to high temperature at 300 and 600 °C. Besides the conventional force-displacement response, Acoustic Emission (AE) activity was monitored during the bending tests with the aim to investigate the possible correlation between the fracture behavior, the rate of AEs and the influence of high temperature exposure. The tests clearly pointed out the effects of heat exposure. More specifically, SCHSC specimens showed a significant decay in mechanical properties as a result of thermal treatments. Then, a lower degradation was observed for the SCHSFRC after heat exposure at 300 °C, whereas the specimens exposed to 600 °C exhibited a tougher response than the corresponding SCHSC. The results showed that the effect of fibers played a beneficial role in bridging the expansion of heat-induced cracks developed in the concrete matrix during the exposure of specimens at high temperature. This bridging effect of fibers was observed also in terms of the AE activity: much less AE events were systematically registered for SCHSFRC specimens with respect to SCHSC ones, for a given value of the imposed Crack-Mouth-Opening-Displacement (CMOD). Therefore, AE measurements confirm their potential in scrutinizing the damage level in cementitious composites. [ABSTRACT FROM AUTHOR]

Electro-chemical techniques that are routinely used in monitoring corrosion of reinforced concrete give an overall estimate of corrosion. Non-destructive techniques that are able to estimate the residual strength of corroding structures would help a great deal in strategizing post-corrosion maintenance. This paper explores two non-destructive techniques based on wave propagation for estimating residual strength of corroding reinforcement in concrete. The specimens undergoing corrosion were simultaneously monitored by acoustic emission (passive) and ultrasonic guided wave (active) techniques. The specimens were instrumented with AE sensors to record acoustic activities inside the specimens during corrosion. Simultaneously, the bars are also monitored by propagating ultrasonic waves through the bars. Active and passive monitoring results are correlated with already established electrochemical techniques to determine the relative efficacies of the NDT technologies. At the end of the exposure, the residual strength of the bars has been determined by tensile testing. [ABSTRACT FROM AUTHOR]

In this study, theoretical modelling and acoustic emission (AE) studies are carried out on damaged reinforced concrete beams strengthened with ultra high performance concrete (UHPC). The contributions from the paper include (i) development of an integrated nonlinear fracture mechanics model to predict the moment carrying capacity of composite beam considering (a) stress intensity factors due to cement paste, steel, UHPC and applied load (b) vectorial contributions of crack displacement from external moment, steel and UHPC force (ii) prediction of load deflection response of strengthened beams by strength based model which applies concepts of force equilibrium and strain compatibility method (iii) analysis of the effect of reinforcement ratio on AE parameters from parametric analysis. The presented results can be effectively utilized for designing UHPC thickness and health monitoring of structures strengthened with UHPC. [ABSTRACT FROM AUTHOR]

For early detection of structural deficiencies due to corrosion, real time damage detection systems are needed to be deployed. With regard to this, present paper explores efficacy of established acoustic emission (AE) based mathematical procedure for corrosion assessment of in-service RC slabs. Two RC slabs (undamaged and damaged due to corrosion) were monitored using AE, half-cell potential and concrete resistivity techniques. The results demonstrate that AE data, captured even for shorter duration, can indicate appropriate probability of corrosion. The corrosion probability predicted by AE technique is well validated with that of electrochemical techniques, establishing AE as a powerful technique for corrosion-induced damage detection of in-service structural elements. [ABSTRACT FROM AUTHOR]

The paper concerns the role of cellulose reinforcements in overall mechanical toughness of fiber concrete panels applied as facade building material. The results of mechanical tests performed on specimens in as delivered state as well as on specimens which underwent the procedure of destruction of internal fiber network structure due to pyrolysis are presented. The results of mechanical tests let the authors to calculate the work of fracture for six different mechanical conditions. Moreover the analysis of registered Acoustic Emission (AE) signal had revealed that the investigated process of destruction begins with the brittle crack generation and growth while major damages of reinforcement system appears in later phase of the process. [ABSTRACT FROM AUTHOR]

• Cyclic load test (CLT) was applied on PC beams to control the level of deterioration. • Assessed PC beam performance estimation techniques with experimental data only. • Static, vibration and acoustic emission results were related via a coefficient of determination. • The effectiveness of the evaluated techniques is related to their capabilities and limitations. Structural health monitoring (SHM) techniques have become a significant subject in structural evaluation due to the development of non-destructive methods used to monitor and assess the integrity of structures. This paper aims to investigate the effect of damage on the flexural performance indicators and dynamic characteristics of post-tensioned concrete (PC) beams, using both static and vibration tests. The beams were tested at several load levels to reproduce different load stages of a PC member: pre-cracking, during and after the formation of initial cracks, and finally, during and after the formation of larger significant cracks. The performance of the elements is evaluated based on static data, energy calculations, and the three criteria of ACI 437 recommendations. Data from the vibration tests were used to identify the changes in modal parameters after different load levels. Acoustic emission data analysis and results were used to classify the level of damage. Experimental bending stiffness and crack length determination via visual inspection was used as control parameters to assess the deterioration levels of the beams. The outcomes of this study show the relationship level among the different experimental responses obtained: flexural performances, the presence of damage in the beams, and natural frequencies. By considering the relationships of the SHM techniques and their individual findings, the evaluation of performance indicators or damage classification in rectangular concrete prestressed beams reveals that the main limitation is the required knowledge about the previous response of the system. [ABSTRACT FROM AUTHOR]

• The damage of water-bearing mudstone during heating has characteristics of stages. • Thermoacoustic emission can reflect the progressive deterioration of mudstone. • The AE signal intensity of mudstone decreases with number of hygrothermal cycles. The periodic effect of an alternating hygrothermal environment makes the microdamage of rock gradually accumulate, thus leading to significant structural damage. The study of the progressive deterioration of mudstone exposed to cyclic hygrothermal conditions is of guiding significance to the stability of bank slopes. In this paper, purple mudstone samples from the Three Gorges Reservoir area were collected for laboratory cyclic hygrothermal experiments. Thermoacoustic emission tests of mudstone samples with different water absorptions under multiple hygrothermal cycles were carried out, and the influence of water content on the macroscopic surface damage and acoustic emission (AE) characteristics of mudstone was analyzed. The results show that the AE counts of mudstone with various water absorptions during heating can be divided into three typical stages of AE activity: active, intense and quiet. The active period of AE activity corresponds to mudstone surface temperatures of 35–50 ℃, which is considered to be the temperature range most likely to induce damage: fracturing, spalling and disintegration. The gradual increase in water absorption of mudstone leads to a decrease in the cumulative AE count during heating as the number of hygrothermal cycles increases, indicating that more clay minerals in the mudstone absorb water and undergo inhomogeneous deformation during each hygrothermal cycle, so that a large number of microcracks or even a network of cracks have been developed in the process of water absorption. This understanding of the deterioration of mudstone provides evidence for the assessment of mudstone strata stability. [ABSTRACT FROM AUTHOR]

• Challenges for acoustic emission monitoring of masonry are elaborated. • AE monitoring is shown to largely enhance experimental creep and fatigue testing. • Monitoring of historical masonry buildings is discussed with cases from literature. • Damage in masonry arch bridges is shown to be efficiently monitored with AE. • Future research challenges for AE monitoring of masonry structures are highlighted. Acoustic emission monitoring is widely used for damage detection in materials research and for site monitoring. Its use for masonry structures is however challenging due to the highly heterogenic nature of masonry and rapid signal attenuation. However, the non-invasive nature and high sensitivity of the technique also provide interesting opportunities, especially for historical masonry structures, to locate damage, identify severity of damage and rate of deterioration. Aim of this paper is to provide an extensive literature review on the application of the acoustic emission technique for masonry structures, addressing specific challenges and recent findings. AE-based methods for damage assessment in masonry are discussed in view of monitoring approaches, wave propagation, source location and crack development under static, fatigue and creep loading. Site applications are discussed for identifying crack location and crack propagation in historical masonry towers, buildings and masonry arch bridges. The paper concludes with future challenges identified in this research field. [ABSTRACT FROM AUTHOR]

• Discussing the frost resistance of gangue concrete under different replacement rates. • Proposing the freeze–thaw damage evolution model of coal gangue concrete with different gangue replacement rate. • Investigating the relationship of freeze–thaw damage value of coal gangue concrete and the change of relative peak strain under different replacement rates. • Establishing the coal gangue concrete freeze–thaw damage constitutive model. This paper aims to solve one of the main problems in the application of coal gangue concrete (CGC), that is, whether its mechanical properties can meet the requirements of norms and actual engineering practice in the freeze–thaw environment. In this work, the CGC with coal gangue coarse aggregate replacement rates of 0%, 20%, 40%, and 60% were subjected to freeze–thaw cycle. The frost resistance of CGC was comprehensively analyzed and evaluated from its appearance, relative mass, compressive strength and relative dynamic elastic modulus. Based on the regular pattern of relative dynamic elastic modulus of CGC, the evolution model of freeze–thaw damage of CGC was established to analyze the development of freeze–thaw damage. The CGC specimens after 0, 25, 50, 75, and 100 freeze–thaw cycles were measured by uniaxial compression constitutive test and acoustic emission test. The mechanical properties of CGC under freeze–thaw environment were analyzed through the whole stress–strain curve and the variation of parameters with freeze–thaw cycles. Finally, based on the acoustic emission characteristics of CGC in compression failure process, a load damage evolution model of CGC was established. Combined with the freeze–thaw damage evolution model, the freeze–thaw damage constitutive relationship of CGC was established, which can consider both freeze–thaw damage and load damage. The results suggest that the replacement rate of gangue for the CGC structure that requires frost resistance should be less than 40%; The freeze–thaw damage evolution model of CGC established in this paper can truly and effectively reflect the development of freeze–thaw damage of CGC under freeze–thaw environment; The CGC freeze–thaw damage constitutive model established in this paper can accurately reflect the overall process damage characteristics of CGC under freeze–thaw and load damage. It can provide a theoretical basis for the durability evaluation and life prediction of CGC structure in cold regions. [ABSTRACT FROM AUTHOR]

Creep of early age concrete is more significant than that of hardened concrete, and it is possible that early age concrete creep presents nonlinearly because of high stresses imposed during construction. In this paper, a viscoelastic–plastic model was proposed to simulate early age concrete creep. Experiments of early age creep of concrete under different stresses, have revealed the nonlinear features of specimens under high stresses. Acoustic emission (AE) technology was applied to collect damage information of early age concrete during creep, and the relationship between the degree of damage and increments of nonlinear creep was deduced according to test data. The parameters involved in the model were regressed. Also, a modified B3 model was proposed to consider nonlinear creep increments of concrete. It can be conclude that the item concerning concrete damage in the proposed model is important for predict the nonlinearity of early-age concrete creep, and it can be used to modify existing models. [ABSTRACT FROM AUTHOR]