Acoustic Emission Characteristics Research Articles

Characterization of Acoustic Emissions from Concrete Based on Energy Activity Coefficient

Single-stage compression loading experiments were carried out on concrete specimens of various strengths to explore the characteristic parameters of the acoustic emission signal and its damage evolution law in the concrete damage process. These specimens were monitored in real time with acoustic emission and DIC instruments during the loading process, and internal pores and slices were scanned with CT scanning instruments after compression. The acoustic emission phenomenon was expressed using the energy activity coefficient, and the law relating to the phenomenon was summarized. The results show that when the peak and mean values in the first adjacent time domain grow rapidly, the specimen produces a large crack and enters the stage of rapid crack development, which can be taken as an indication of the impending damage to the specimen. The energy activity coefficient reflects the damage development intensity as follows: the smaller the energy activity coefficient, the more the cracks developed; the faster the speed, the larger the deformation. With an increase in the load level, the energy activity coefficient gradually tends to stabilize, and the specimen enters the stage of rapid crack development. However, when the energy activity coefficient suddenly increases again, the specimen is destabilized and destroyed. Therefore, the energy activity coefficient responds to the degree of congenital defects in the specimen. As the load increases, the energy activity coefficient is more stable, and the defects are smaller; in contrast, the energy activity coefficient drastically oscillating indicates that the material is very defective.

Study on pore structure and acoustic emission characteristics of Mg(OH)2 modified expired cement

This study investigated the preparation of cementitious mortar specimens (ECM) by blending different dosages of pure magnesium hydroxide (AR), active magnesium oxide, and nano magnesium hydroxide as external additives with expired composite Portland cement. The optimal solution was selected by comparing their “nuclear magnetic resonance” (NMR), mechanical properties, and acoustic emission characteristics. NMR experiments revealed that the addition of 4 wt% nano magnesium hydroxide to expired cement resulted in the largest decrease in harmful pore content in ECM, reaching 21.9%. Mechanical performance testing and acoustic emission monitoring results indicated that the strength of ECM reached a maximum of 21.356 MPa with the addition of 4 wt% nano magnesium hydroxide, representing a 20.2% increase compared to the control group. This suggests that adding 4 wt% nano magnesium hydroxide to expired cement is a preferable option.

A laboratory perspective on accelerating preparatory processes before earthquakes and implications for foreshock detectability

Dynamic failure in the laboratory is commonly preceded by many foreshocks which accompany premonitory aseismic slip. Aseismic slip is also thought to govern earthquake nucleation in nature, yet, foreshocks are rare. Here, we examine how heterogeneity due to different roughness, damage and pore pressures affects premonitory slip and acoustic emission characteristics. High fluid pressures increase stiffness and reduce heterogeneity which promotes more rapid slip acceleration and shorter precursory periods, similar to the effect of low geometric heterogeneity on smooth faults. The associated acoustic emission activity in low-heterogeneity samples becomes increasingly dominated by earthquake-like double-couple focal mechanisms. The similarity of fluid pressure increase and roughness reduction suggests that increased stress and geometric homogeneity may substantially shorten the duration of foreshock activity. Gradual fault activation and extended foreshock activity is more likely observable on immature faults at shallow depth.

Energy Mechanism and Acoustic Emission Characteristics in Rock-Backfill Composite Structure Specimens under Multi-Level Cyclic Loads: Cement-Tailings Ratio Effect

Energy Mechanism and Acoustic Emission Characteristics in Rock-Backfill Composite Structure Specimens under Multi-Level Cyclic Loads: Cement-Tailings Ratio Effect

Influence of Immersion Time on the Frequency Domain Characteristics of Acoustic Emission Signals in Clayey Mineral Rocks.

The frequency domain characteristics of acoustic emission can reflect issues such as rock structure and stress conditions that are difficult to analyze in time domain parameters. Studying the influence of immersion time on the mechanical properties and acoustic emission frequency domain characteristics of muddy mineral rocks is of great significance for comprehensively analyzing rock changes under water-rock coupling conditions. In this study, uniaxial compression tests and acoustic emission tests were conducted on sandstones containing montmorillonite under dry, saturated, and different immersion time conditions, with a focus on analyzing the effect of immersion time on the dominant frequency of rock acoustic emission. The results indicated that immersion time had varying degrees of influence on compressive strength, the distribution characteristics of dominant acoustic emission frequencies, the frequency range of dominant frequencies, and precursor information of instability failure for sandstones. After initial saturation, the strength of the rock sample decreased from 53.52 MPa in the dry state to 49.51 MPa, and it stabilized after 30 days of immersion. Both dry and initially saturated rock samples exhibited three dominant frequency bands. After different immersion days, a dominant frequency band appeared between 95 kHz and 110 kHz. After 5 days of immersion, the dominant frequency band near 0 kHz gradually disappeared. After 60 days of immersion, the dominant frequency band between 35 kHz and 40 kHz gradually disappeared, and with increasing immersion time, the dominant frequency of the acoustic emission signals increased. During the loading process of dry rock samples, the dominant frequency of acoustic emission signals was mainly concentrated between 0 kHz and 310 kHz, while after saturation, the dominant frequencies were all below 180 kHz. The most significant feature before the rupture of dry rock samples was the frequent occurrence of high frequencies and sudden changes in dominant frequencies. Before rupture, the characteristics of precursor events for initially saturated and immersed samples for 5, 10, and 30 days were the appearance and rapid increase in sudden changes in dominant frequencies, as well as an enlargement of the frequency range of dominant frequencies. After 60 days of immersion, the precursor characteristics of rock sample rupture gradually disappeared, and sudden changes in dominant frequencies frequently occurred at various stages of sample loading, making it difficult to accurately predict the rupture of specimens based on these sudden changes.

Study on Mechanical and Acoustic Emission Characteristics of Backfill-Rock Instability under Different Stress Conditions.

Unveiling the mechanical properties and damage mechanism of the complex composite structure, comprising backfill and surrounding rock, is crucial for ensuring the safe development of the downward-approach backfill mining method. This work conducts biaxial compression tests on backfill-rock under various loading conditions. The damage process is analyzed using DIC and acoustic emission (AE) techniques, while the distribution of AE events at different loading stages is explored. Additionally, the dominant failure forms of specimens are studied through multifractal analysis. The damage evolution law of backfill-rock combinations is elucidated. The results indicate that DIC and AE provide consistent descriptions of specimen damage, and the damage evolution of backfill-rock composite specimens varies notably under different loading conditions, offering valuable insights for engineering site safety protection.

Mechanical properties and acoustic emission characteristics of mixed granite after different numbers of freeze‒thaw cycles

The mechanical properties of rocks in cold regions undergo significant changes as a result of decades of freeze‒thaw cycles with seasonal variations, which can lead to a series of geological disasters, such as collapse. This study investigates the evolution of the mechanical characteristics and internal progressive damage characteristics of mixed granite under freeze‒thaw cycling and axial loading. By measuring the mass, wave velocity, and uniaxial compressive strength of rock samples and combining these metrics with acoustic emission (AE) characteristics, the physical and mechanical properties and microfracture development of mixed granite after different numbers of freeze‒thaw cycles were investigated. The results indicate that as the number of freeze‒thaw cycles increases, the longitudinal wave velocity, uniaxial compressive strength, and elastic modulus of the mixed granite decrease nonlinearly, while the peak strain gradually increases. Combined with the stress‒strain curve, the AE characteristics can be divided into four stages. As the number of freeze‒thaw cycles increases, the AE cumulative count decreases, and the AE counts of the four stages are different. The low-frequency-high-amplitude signals first increases and then tends to stabilize, and they only appeared in the third and fourth stages. At the same time, the proportion of the low-frequency ratio gradually increases, and the proportion of the high-frequency ratio decreases. In addition, based on the rise time/amplitude (RA) and average frequency (AF) characteristics and failure modes, it was found that the internal crack types of mixed granite transition from shear cracks to tensile cracks, among which tensile cracks play a crucial role in rock failure.

Anisotropic Mechanical Properties and Fracture Mechanism of Transversely Isotropic Rocks under Uniaxial Cyclic Loading

Transversely isotropic rocks, which are special anisotropic materials, are widely encountered in civil, mining, petroleum, geothermal, and radioactive waste-disposal engineering. Rock is frequently subject to cyclic loads resulting from natural and human-caused events. However, to date, the fracture mechanism of transversely isotropic rocks under cyclic loading remains poorly understood. To address this gap, uniaxial monotonic-loading and cyclic-loading tests were performed on slate specimens by the MTS815 system, during which acoustic emission (AE) signals inside the rock were monitored, and finally the fracture surfaces of the tested rock were scanned by scanning electron microscopy (SEM). Through these tests, the anisotropic mechanical properties, damage evolution, AE characteristics and fracture pattern of slate as a transversely isotropic rock were studied. The results show that the peak strength of specimens varies with the loading–foliation angle under monotonic and cyclic loading, following a U-shaped trend. The deformation modulus during unloading is more capable of characterizing the damage inside the specimen than that during loading. By defining the damage degree based on dissipation energy, it is found that the damage variable is influenced by the loading–foliation angle and the cyclic stress step. The AE characteristics of specimens exhibit significant anisotropy, closely correlated to the loading condition and loading–foliation angle. Regardless of cyclic stress step, the AE counts of specimens with a loading–foliation angle of 0° are mainly distributed near the peak region, whereas those of specimens with other loading–foliation angles occur primarily in the early stage of each cyclic loading. Finally, it is revealed that the fracture mechanism of slate specimens is determined by the loading–foliation angle, loading condition, and cyclic stress step.

A low-power AE b-value sensor for structural health monitoring

An increase in the quantity of aging infrastructure has become a major social issue. It is thus necessary to properly maintain and manage aging infrastructure, which incurs increasingly large maintenance costs. Therefore, there is a demand for maintenance and management using low-cost and wide-application technologies such as Internet of Things (IoT). We propose a unique, ultra-low-power acoustic emission b-value sensor for structural health monitoring. Acoustic emission (AE) is an elastic wave generated by crack generation or growth inside a material. The characteristics of AE, such as waveform, source frequency, source location, and velocity, can indicate damage affecting material strength. When focusing on the source frequency, the b-value, which is the slope of AE amplitude-frequency distribution derived from the source frequency, is important and widely used as an indicator of material integrity. The proposed sensor was developed to detect this b-value trend more conveniently than ever and to achieve continuous structural or machinery health monitoring. We pursued two key points: ultra-low power consumption and long-distance wireless transmission, which enabled the AE sensor to be applied as an IoT sensor. The wake-on-AE algorithm, which wakes the sensor when AE waves reach the sensor, enables AE sensing to consume a very low current of 172μA on two dry batteries. Generally, the waveform of AE is very short in duration, such as microseconds to milliseconds. If the sensor did not wake up and take measurements in such a short time, the AE wave would pass, and the sensor would fail to measure AE. Therefore, we developed a unique analog- digital co-circuit technology that uses a peak-hold circuit to extract AE features with less sampling. In addition, sensor edges reduce data rates through statistical analysis and transmit data using low-power wide-area networks. In this paper, we present validation results obtained with the sensor on bearings. We performed validation experiments with faulty and normal bearings, generating AEs with different b-values. The developed sensor, operated with two dry batteries, and the AE measurement systems for verification simultaneously acquired the AE generated by the bearing rotation. As a result, we successfully confirmed that the developed sensor provides almost the same results as conventional AE systems in b-value measurement with ultra-low power consumption.

Study on acoustic emission and local deformation characteristics of feldspar vein-intrusive metagabbro at different angles of splitting

Feldspar vein-intrusive metagabbro is a special geological structure, and different stress angles have an important influence on the fracture mode and deformation characteristics of metagabbro. A Brazilian splitting test on feldspar vein-intrusive metagabbro was performed using three distinct stress angles (0°, 45°, and 90°), and acoustic emission signals and strain characteristics were monitored synchronously during the test. The results showed that the damage pattern of the feldspar vein-intrusive metagabbro was related to the feldspar mineral perforation damage on the main rupture surface. With the increase in stress angle, the percentage of high peak frequency increased gradually. The phenomenon of strain lagging stress appeared in the rock samples before the peak damage. The feldspar minerals played a controlling role in the expansion of microcracks in the feldspar vein-intrusive metagabbro. Significant differences in the local deformation coordination of rocks under different stress angles were observed. The deformation coordination of rock samples with a stress angle of 0° was much lower than that of other rock samples. This study is of great significance for the understanding of the deformation and damage laws of similar geological structures and also provides an important theoretical basis for the stability of deep chambers.

Experimental Investigation on the Effect of Water Saturation on the Failure Mechanism and Acoustic Emission Characteristics of Sandstone

Experimental Investigation on the Effect of Water Saturation on the Failure Mechanism and Acoustic Emission Characteristics of Sandstone

Revealing evolution law and failure mechanism of interface damage in concrete-encased CFST columns by acoustic emission technology

Concrete-Encased Concrete-Filled Steel Tubular (CFST) has extensive applications in high-rise buildings and bridge engineering as a high-performance composite structure. The severe bond failure between Concrete-Filled Steel Tubular and the outer concrete poses a significant threat to the integrity and safety of the structure. Considering the concealed and random nature of interface damage, this study proposes a real-time monitoring and evaluation method for Concrete-Encased CFST interface damage based on acoustic emission (AE) technology. Four groups of experiments on Concrete-Encased CFST column interface shear was conducted to investigate the influence of the number of shear connectors on the bond failure mechanism between CFST and the outer concrete. The signal characteristics of three kinds of AE sensors with different operating frequency ranges were analyzed to guide the selection of sensors according to the monitoring targets. The structural damage development trend has been recognized by analyzing the change rule of AE feature parameters. RA-AF correlation analysis has been used to identify the evolution law of cracks and the failure mechanism of the structure. The b-value analysis has been used to evaluate the damage severity of the structure. From the above three aspects, the damage evolution law and failure mechanism of the interface can be revealed. The results indicate that the R3α sensor is suitable for early warning of damage, the R15α sensor can be used for monitoring and warning of structural failure, and the R6α sensor can be used to describe and evaluate the whole process of damage. Based on the features of AE hit rate, cumulative hits, and cumulative energy, the damage process of Concrete-Encased CFST column interface can be accurately divided into the elastic stage, local slip stage, shear connector fracture stage, and slip stage. The RA-AF correlation analysis showed that the shear cracks were detected to develop rapidly before the peak load, and a significant number of tensile cracks caused by cracking of the outer concrete appear around the peak load. The changes of b-value can effectively reflect the magnitude of damage and record the occurrence of major damage.

Evolution of the damage precursor based on the felicity effect in shale

Damage precursors during hydraulic fracturing in shale gas reservoirs may be better understood if the deformation, failure, and acoustic emission (AE) characteristics under cyclic loading are known. Therefore, the purpose of this paper is to investigate the quantitative damage based on the Felicity effect under constant stress lower limit uniaxial cyclic loading-unloading rates (0.5, 1.0, 1.5, 2.0, and 2.5 kN/s). Variations in the b-value and the spatiotemporal evolution of cumulative AE were also used to observe how shale fractures formed. The findings reveal that during the unloading stage, there are many cumulative AE events when the stress level is low (≤1.50 kN/s) but that this number drops significantly when the stress level increases above (>2.0 kN/s). The AE amplitude, AE counts, and cumulative AE energy of each cycle in a loading-unloading test show an increasing trend, but the rate increases in the last cycle. During the whole process of loading and unloading, the Kaiser effects were present in the 3rd cycle at stress levels (≤1.5 kN/s). Still, the Felicity effect appeared in the 2nd and 1st cycles during 2.0 and 2.5 kN/s cyclic loading. The Kaiser effect occurs in the linear elastic stage, while the Felicity effect occurs in the crack initiation and crack damage stage. Furthermore, the Felicity ratio (FR) variations during shale deformation and failure can be divided into four phases: (Phase I = 1.01 ≥ FR > 0.89), (Phase II = 0.89 ≥ FR > 0.48), (Phase III = 0.48 ≥ FR > 0.23), and (Phase IV = FR ≤ 0.23). The b-value is relatively higher under the loading rate below (≤1.50 kN/s), indicating an increase in the number of small AE events. In contrast, the fact that the b-value is relatively smaller under the loading rate above (>2.0 kN/s) indicates that, the number of large AE events increases the number of cracks and fractures. These findings provide important design references for damaged precursors during hydraulic fracturing in shale gas reservoirs.

Recognition Algorithm of AE Signal of Rock Fracture Based on Multiscale 1DCNN-BLSTM

Acoustic emission (AE) signals produced by different types of rocks have different characteristics of information. Determining the brittle mineral content of rock according to the acoustic emission characteristics of rock is helpful to understand the mechanical behavior of rock in field monitoring. This article constructs a deep learning algorithm model to identify acoustic emission signals released from rock fractures with different brittle mineral contents. In response to the interference characteristics of acoustic emission signal data, a multiscale one-dimensional convolutional neural network embedded with efficient channel attention (ECA) module was incorporated into the model, and multiscale convolutional kernels were used to extract features of different levels of precision. In the latter half of the model, the BLSTM network was incorporated to extract time series-related features, local spatial uncorrelated features, and weak periodic pattern features from the acoustic emission signal data. To solve the problem that the recognition accuracy of minority samples decreases, this study replaces ReLU activation function with SELU. The results show that the multiscale 1DCNN-BLSTM model embedded in ECA module has a good antinoise performance, and the recognition accuracy can reach over 90%. The discovery of this work provides a new idea for exploring the mechanism of rock mass instability.

Acoustic emission analysis for crack initiation in AA7075-T6 alloy under mixed tensile and bending cyclic loading

In most critical components, fatigue occurs under mixed loadings, such as bending and tensile cyclic loading. Furthermore, in the realm of fatigue loading conditions, scant information has been documented regarding the combination of bending and tensile cyclic loading. Therefore, in this paper, Acoustic Emission (AE) as a non-destructive method was used to investigate crack initiation in AA7075-T6 specimens subjected to fatigue conditions involving bending and tensile cyclic loading. The results showed that generated AE signals had the same trend in all tests, and there was a reasonable correlation between AE and mechanical characteristics. By correlating mechanical data and AE data using the sentry function, the failure process, which includes dislocation movement, plastic deformation, work hardening, micro-crack formation, and crack initiation, respectively, was identified. The S-N curve was plotted by using AE monitoring. This curve was depicted as non-destructive and based on the crack initiation cycle. Obtaining the S-N curve through AE monitoring will greatly assist designers in the design of structures under mixed fatigue loading conditions. This will result in a reduced Safety Factor and, consequently, decreased weight and cost for structures while still ensuring that the structure meets the necessary safety requirements.

Predicting the failure of rock using critical slowing down theory on acoustic emission characteristics

The joint surfaces and size affect the mechanical behavior and failure precursors of shale. Therefore, this paper studies the mechanical behavior, the response characteristics of acoustic emission (AE) and critical slowing down (CSD) characteristics of AE for shale with different joint surfaces and size. The results show that with an increase in the number of joint surfaces and the size of the sample, the crack initiation stress, uniaxial compressive strength (UCS), peak strain, and fractal dimension decrease. During the plastic stage, the occurrence of a sharp rise in AE ringing count, RA, a low-frequency, high-amplitude AE signal, and a rapid drop in AF can be characteristics of impending sample failure. Variance and autocorrelation coefficients can be used to characterize the CSD phenomenon. The optimal window length and lag step are determined to be 400 and 100, respectively. The CSD characteristics of the autocorrelation coefficient and the variance of multi-AE parameters are obtained. The variance-based early warning time provides short-term early warning, while the autocorrelation coefficient-based early warning time provides early warning. The research results can provide theoretical references for the mechanical design and failure prediction of underground rock engineering.

Flexural failure of ultra-high performance concrete subjected to the alternating cryogenic and elevated temperature via acoustic emission characterization

This paper aims to reveal the mechanical evolution and potential mechanism of Ultra-high Performance Concrete (UHPC) exposed to a complex temperature-variation environment typical of Liquefied Natural Gas storage tank. Herein, the effect of steel fiber geometries (straight and hooked-end) on flexural failure of UHPCs in the range of −170 °C ∼ 200 °C was studied, and dynamic fracture evolution during the loading process was tracked using acoustic emission (AE) test. Results indicated that the flexural strength of concrete specimen enhanced at −170 °C but diminished at 200 °C in the first cycle. For multiple cycles, its improved strength subsisted until the third cycle, after which gradually decreased. Moreover, the hooked-end fiber generally had superior strengthening and toughening effects than straight fiber, except for cryogenic temperature and third cycles tests. At cryogenic temperature, the freezing of pore moisture inside specimen enhanced the matrix strength and the interfacial bonding between the matrix and fibers. At elevated temperature, the thawing, diffusion and evaporation of moisture in concrete could induce matrix dehydration and weaken the bonding effect between interfaces. In varying exposure environments, the moisture migration would also undergo secondary hydration reactions with unreacted cement particles. Finally, using AE parameters to effectively record damage progression in UHPCs exposed to extreme temperatures was also highlighted.

Rockburst simulation tests on structural plane of deep high-stress circular tunnels: A true triaxial test study on several different hard rocks

After deep rock excavation, rockbursts can easily occur. The structural plane and rock properties play important roles in controlling rockbursts. To study the role of structural planes and rock properties in controlling rockbursts in the peripheral rocks of tunnels, five different brittle and hard rocks (Red sandstone, Gray sandstone, Granite,Marble, and Limestone) were tested, and cubic specimens with round holes (100 mm*100 mm*100 mm, Φ = 50 mm) and prefabricated fissures were used to simulate the structural plane. A high-speed camera system and an acoustic emission system were used to monitor the damage and microfracture processes of the surrounding rock. The damage process, acoustic emission characteristics, and damage patterns of five brittle rocks with and without specimens with structural planes during loading were comparatively analyzed. The main conclusions were as follows. (1) The presence of structural planes in the rock mass changed the stress and energy propagation paths in the rock mass, resulting in stress concentration; the structural planes in the five brittle rocks could not store energy, hence increasing the propensity for rockbursts. Structural planes in the Marble had the most obvious effect on the promotion of rockbursts. However, structural planes in the Gray sandstone had the weakest effect on the promotion of rockbursts. (2) The presence of structural planes significantly increased the rockburst intensity. In the Marble, regardless of the presence of structural planes, the damage mode of the surrounding rock was mainly shear flexural damage. However, the fine particle contents of specimens with structural planes were significantly higher than those without structural planes. Thus, the rockburst intensities of specimens with structural planes were greater than those without structural planes. For the Red sandstone, Gray sandstone, Granite and Limestone, the surrounding rock specimens with structural planes experienced mainly violent shear ejection damage. (3) The distribution of RA and AF and the number of ruptures in the specimens with and without structural planes were obviously changed, and the numbers of rupture events in the Red sandstone, Gray sandstone, and Granite specimens obviously increased because of the presence of structural planes. The numbers of rupture events for Marble and Limestone specimens significantly decreased due to the presence of structural planes. Gray sandstone and Granite had the most rupture events. (4) Different brittle rock specimens with structural planes had different sizes. The degrees of damage were observed to significantly increase for a period. In addition, the rock body experienced additional internal shear rupture, which intensified due to the structural planes and the tunnels between the rock columns. The ruptures were caused by the destruction of the structural planes during rockburst, and they served as predictions and early warnings, thus providing a certain reference basis. (5) In the Marble during rockbursts, there were structural planes that underwent flexural strain and slip strain. In the Red sandstone, Gray sandstone, and Granite during rockbursts, there were structural planes that underwent compression shear strain and slip strain. The disaster chain was introduced to understand and discuss the mechanism of slip strain for the development of rockburst in structural planes, which should be the final damage form. The disaster chain was a series of rockbursts caused by compression shear and slip of specimens with structural planes. The development of rockbursts in specimens with structural planes occurred sequentially. The final damage pattern was the end result of the development of a series of rockbursts in specimens with structural planes.

Experimental study and numerical simulation verification of the macro- and micromechanical properties of the sandstone–concrete interface under freeze–thaw cycles

In cold-region tunnel engineering, the bonding surface between concrete and surrounding rock is highly susceptible to engineering disasters such as the cracking of support structures under the influence of freeze-thaw cycles, which severely affects the stability of tunnel engineering. This study investigates the macroscopic and microscopic mechanical properties of the sandstone–concrete interface under freeze–thaw cycles and reveals the microdamage and fracture evolutionary laws during the freeze–thaw loading process via the coupled expansion of water–ice particle phase changes via particle flow numerical simulation methods, aiming to provide theoretical support for the design and construction of rock–soil and tunnel engineering in high-altitude frozen soil areas. The results indicate that the interface strength characteristics of the sandstone–concrete specimens exhibit a stable decreasing trend with an increase in the number of freeze-thaw cycles and a decrease in roughness. Taking the most unfavorable condition as an example, the exacerbation of freeze-thaw deterioration resulted in shear strength reductions of 16.53%, 27.01%, and 37.17% for specimens (JRC=3.727), while an increase in roughness led to shear strength increases of 11.00% and 15.14% for specimens (NT=60 cycles). The acoustic emission characteristics during the loading process of the sandstone–concrete interface specimens reflect the microcracking evolutionary activity of the specimens quite well, and setting 1.0 as the recommended value for the precursor determination of the b-value and CV(k) index is most reasonable. Under freeze-thaw cycles, cracks first initiate partial microcracks at the interface and on the outer side of the specimen, primarily dominated by tensile cracks and evolving with a "slow–fast" trend toward both sides of the interface. Under shear stress, particles at the interface first undergo slip dislocation due to their lower bonding strength, generating cracks, subsequently inducing significant displacement of particles on both sides of the interface, resulting in crack propagation toward both sides of the interface, ultimately penetrating and forming shear bands leading to macroscopic failure.

Cyclic compressive behavior of high-performance hybrid fiber reinforced recycled aggregate concrete: Degradation law and acoustic emission features

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.