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Microbial induced calcium carbonate precipitation (MICCP) has applications in improving the quality of cementitious constructions by enhancing its compressive strength, reducing porosity, and increasing durability against environmental degradation. In this study, an attempt has been made to develop an in situ, online, and nondestructive ultrasonic monitoring tool to capture bacterial self-healing in cementitious materials using cheaper source of bacterial nutrition medium. Samples casted and cured in corn steep liquor (CSL) and nutrient broth (NB) demonstrated higher ultrasonic pulse transmitted signals at successive days of curing relative to water cured control (C) specimens. The rise in ultrasonic signal due to bacterial admixing and curing is due to calcite precipitation in bacterial admixed and cured specimens. The compressive strength of CSL specimens was 8.8%, 11.4%, and 11.1% higher, while in NB specimens was 13.5%, 14.6%, and 12.7% higher as compared to water casted and cured specimens at 7, 14, and 28 days, respectively. The water absorption in bacterial treated specimens was significantly reduced due to the filling of pores in the mortar matrix and a reduced sorptivity coefficient of 0.0027 and 0.0022 was observed in CSL and NB specimens respectively as compared to 0.0197 of water casted and cured C specimens. This suggests successful calcite precipitation in bacterial treated mortar specimens and ultrasonic pulse transmission monitoring technique can successfully pick up bacterial intrinsic healing and can serve as in situ and nondestructive monitoring technique in civil engineering applications. Bacterial intrinsic self-healing in cementitious materials is an inherent process and this paper highlights the successful implementation of nondestructive and in situ ultrasonic wave monitoring tool to pick the same. Commonly used nondestructive testing (NDT) techniques like ultrasonic pulse velocity (UPV), acoustic emission monitoring can capture crack initiation in cementitious structures but not its intrinsic healing. In present research, ultrasonic wave monitoring has been used to capture bacterial microstructural development in the initial curing period in MICCP. This innovative NDT technology would go a long way in developing an in situ ultrasonic monitoring technique to pick up efficacy of MICCP in concrete structures for on-site applications. [ABSTRACT FROM AUTHOR]

The main purpose of this study was to understand the failure mechanism of fly ash fiber–reinforced asphalt specimens under uniaxial compression loading and splitting loading, respectively. The variation of acoustic emission (AE) parameters in the acoustic emission tests during this fracture process was analyzed. The results showed that under uniaxial compression, the failure process of the fiber asphalt specimens could be divided into four stages: the formation of internal microcracks, microcrack propagation, rapid crack propagation, and finally, failure. Shear failure is the main failure form of asphalt specimens, and with the gradual increase of fiber content, the failure mode of specimens gradually changes from shear failure to shear-tensile failure. The correlation analysis between peak value and frequency of acoustic emission parameters shows that there are three frequency bands of fly ash fiber asphalt specimen, namely, 0–30, 30–60, and 150–200 kHz, which correspond to the initial dislocation failure of the interface between aggregate and asphalt mortar, the fracture failure of the interface between fly ash fiber and asphalt mortar, and the complete dislocation failure of the interface between aggregate and asphalt mortar. In recent years, acoustic emission technology has been widely used in the field of nondestructive testing of cement concrete and bridge structures. Nevertheless, the damage identification methods commonly used in asphalt pavement, such as the computational simulation method and image processing method, have low working efficiency, large error, and lack of dynamic perception of the whole damage process, which limits their applicability in practical engineering applications. In this paper, acoustic emission technology has been employed to characterize the damage and crack propagation law of fiber asphalt specimens under various stress states. Through the analysis of the evolution characteristics of acoustic emission signal parameters, the fracture damage characteristics of fiber asphalt samples under different stresses are invested. The research results are of great significance for the popularization and application of acoustic emission technology in the field of nondestructive testing of asphalt pavement. [ABSTRACT FROM AUTHOR]

This paper aims to study the strength and damage characteristics of basalt fiber-reinforced concrete (BFRC) under dynamic splitting tensile loads. Brazilian disk splitting tests and acoustic emission (AE) tests were carried out on BFRC herein. The improvement effects of loading rate and fiber content on the dynamic splitting tensile strength of BFRC were studied. Then, AE wavelet analysis methods (wavelet energy spectrum coefficient and maximum wavelet energy value) were employed to analyze the AE signals generated by BFRC. Additionally, a back-propagation (BP) artificial neural network was established to identify the damage stage of BFRC under dynamic splitting tensile loads. The test results showed that the improvement effect of the loading rate on the dynamic splitting tensile strength was enhanced with an increasing loading rate, and the enhancing effect of basalt fiber in the cases of 0.1% or 0.15% fiber dosages was better than that of the others. Most of the AE signals generated by BFRC were low frequency. Furthermore, the loading rate and the addition of fibers had a considerable impact on the wavelet energy spectrum coefficients corresponding to different bands and the distribution of maximum wavelet energy values of BFRC. Finally, the recognition rate of this BP neural network increased with the increment of the number of AE samples with the progress of loading. [ABSTRACT FROM AUTHOR]

The Acoustic Emission (AE) technique is applied to study the failure mode, damage evolution process, and cracking mechanism of reinforced concrete (RC) columns under the coupling effect of exterior compression load and interior reinforcement corrosion. AE signals are collected from different loading stages for corroded and uncorroded RC column specimens. Parameter analyses are conducted to analyze the peak frequency, energy distribution, and phase parameters used to achieve the general changing conditions of AE signals. Fast Fourier Transform is applied to reveal the time-variant rules of signal frequency and energy in different damage stages. Discrete Wavelet Transform is used for a better understanding of the local time-frequency properties of the non-stationary AE signals and optimally interprets the damage evolution process of the RC structure. Results show that the peak frequency has a 3-layered distribution; an obvious "double-peak" trend of the energy distribution indicates two cracking mechanisms: steel/concrete interface damage and aggregate/cement damage. The damage process consists of four stages: micro-crack formation, micro-crack development, crack expansion, and cracking failure. The corrosion behavior remarkably influences the energy distributions of the two cracking mechanisms, the characteristics of the spectra in different stages, and the damage evolution process inside the RC columns. This paper studies the failure mode, damage evolution process, and cracking mechanism of reinforced concrete (RC) columns under the coupling effect of exterior compression load and interior reinforcement corrosion via AE signal analysis. The energy distribution and spectral characteristics show significant differences among different cracking stages. Moreover, those important properties and the damage evolution process are also remarkably affected by the corrosion behavior of the steel reinforcement. The practical meaning of those research outcomes are to provide theoretical supports for the application of AE techniques in the health monitoring of RC structures. The changes in the key properties of AE signals measured and analyzed can be used to capture the state changes and locate the cracking stage, which provides important evidence for the safety evaluation of RC structures. [ABSTRACT FROM AUTHOR]

The progressive damage monitoring and fracture evaluation of plain concrete beam specimens subjected to three-point bending (TPB) are reported in this paper. The objective of the present research is to systematically correlate the fracture behavior of a concrete beam with acoustic emission (AE) data recorded during the present set of experiments. The damage growth and failure mechanisms of the beam specimens are investigated with AE parametric analysis, b-value analysis, and digital image correlation method. Eventually, a reasonable correlation between AE energy and the released fracture energy is established at various stages of loading, which is confirmed with the set of TPB beam experiments and further validated with nonlinear numerical finite-element analysis. The energy released during the cracking processes at different loading stages in the experiment is quantified and systematically analyzed. Acoustic and fracture energies are dependent on active concrete cracking, and their correlation is helpful in determining the extent of damage in the concrete structures. Such a relationship can be used as an operational tool for structural health monitoring of structures as an early warning system supplemented with numerical analysis, a well-calibrated laboratory, and in situ AE experiments. [ABSTRACT FROM AUTHOR]

To study the splitting tensile damage characteristics of concrete, this paper conducted Brazilian disc split tensile tests on concrete and mortar with three water-cement ratios and four loading rates, respectively. Acoustic emission (AE) technology was used for real-time. The test results indicate that in the early loading stage of Brazilian disc specimens, a small number of microcracks are primarily generated at the interface between aggregate and mortar, and microcracks and macrocracks are generated simultaneously at the peak stress. The peak frequency characteristics of AE explain the strength enhancement principle and strain rate effect of concrete damage. The curves of AE cumulative ringing and AE energy counting reflect the internal damage activity of concrete and mortar specimens at different stress stages. Finally, a model that conforms to the macroscopic performance of the concrete dynamic splitting tensile test was established based on the discrete element method (DEM) and Hertz–Mindlin bond model. The simulation was compared with the actual analysis to verify the effectiveness of the parameters. The evolution of concrete damage space was studied through the discrete element simulation results, and the three evolution stages of damage space were summarized, which confirmed that the DEM plays an essential role in the field of micromechanics. [ABSTRACT FROM AUTHOR]

Acoustic emission (AE) has been widely used for damage detection and characterization in reinforced concrete (RC) structures. However, there is little work focusing on correlation of AE signatures and physical phenomena involved within the failure process of RC beams. In this paper, AE monitoring was performed on five small-scale RC beams that were subjected to an incremental cyclic loading regimen. Correlation of AE activity with in situ crack progression was performed by comparing AE signatures with a series of real-time images of crack development. Based on the combination of AE response and development of flexural and shear crack patterns, four phases named microcrack formation, macrocrack initiation, crack opening/closing, and near failure were recognizable. Meanwhile, AE cumulative energy (CE) analysis during loading and unloading demonstrated its potential for damage detection and qualification. The test results show crack initiation and propagation are accompanied with significantly greater AE activity in comparison to existing crack reopening or closing; and during the unloading step when the RC beams are near failure, the calculated CE is greater than that for initial crack appearance. In addition, AE event localization method and waveform analysis were performed to aid in identifying the source mechanisms of meaningful events. Crack propagation events were associated with burst type signals. During unloading step near failure, signals of high amplitude and long rise time are located along the reinforcement indicating a potential loss of bond as the reinforcement interacts with its surrounding concrete. The results showed that the combination of each analysis type aided in determining whether AE generated during loading/unloading of RC can be a meaningful parameter in monitoring damage of RC structures. [ABSTRACT FROM AUTHOR]

To predict the fractured rock failure under deep triaxial stress, true triaxial tests were carried out using thermal infrared monitoring and acoustic emissions (AE). Based on the thermal infrared imagery, which may hold potential information for predicting rock failure, this paper proposes the infrared temperature jumping rate (ITJR) to reflect the jumpiness of the temperature field matrix and establishes an infrared advance prediction method. The results show that the high-temperature area will converge and expand gradually, and cracks propagate along a certain direction. In the sudden-temperature-reduction area, rock stripping is easy to occur. At the boundary between high- and low-temperature areas, it is easy to produce breakage cracks and form rock spalling. In the short quiet period, the rock gradually gathers strain energy, which will be released in the fracture period. By comparing the time of AE sudden increase with the time of ITJR mutation, it shows that the method has a good advance prediction effect for rock fracture. [ABSTRACT FROM AUTHOR]

The accurate identification and quantification of rock damage are important for the reliable safety assessment of rock engineering. In this paper, stress-strain behavior, passive acoustic emission (AE), and active ultrasonic transmission were measured simultaneously under uniaxial compression to characterize the fracture damage of granites. The deformation and acoustic behaviors with respect to crack initiation and crack damage thresholds were analyzed. Meanwhile, irreversible damage was quantified using different methods. The results show that the marked increase in the AE rate and the average hit energy coincide with the crack initiation stress, ranging from 0.41σc–0.45σc (uniaxial compressive strength), which occurs earlier than the decrease of the P -wave velocity and onset of the rock dilation. The moment in which the average hit energy and ratio of low-frequency AEs start to increase rapidly corresponds to the crack damage stress of 0.71σc–0.74σc. In addition, the fracture damage was quantified by the crack volumetric strain, dissipated energy, modulus loss, AE characteristics, b value (defined as the log-linear slope of the frequency-magnitude distribution of AEs), and ultrasonic properties. It is revealed that the majority of rock damage accumulates after the crack damage stress, corresponding to the appearance of most high-magnitude hypocenters. Rock damage quantified by dissipated energy and AE energy with clear physical meaning is more reasonable and less subjective. Moreover, the precipitous reduction of the b value, a sharp increase in AE energy, and the ratio of low-frequency AEs prior to peak failure is a perfect precursor of potentially brittle failure. [ABSTRACT FROM AUTHOR]

The available models for estimating the creep strains of concrete generally assume concrete as a homogeneous material. Since concrete is a composite, such models cannot take into account incompatible strains between cement paste and aggregates during creep loading. The main objective of this paper is to demonstrate that initial defects could increase the creep strain level and justify the application of a coupling between creep and damage at the macroscopic scale. To do so, different contents of air-entraining agent were used to simulate the internal defects of concrete and high-stress continuous load was applied to concrete samples with different initial defects. By analyzing the stress-strain curves of concretes containing different initial defects after compression, it was concluded that the existence of internal defects had a significant effect on the basic mechanical properties of concrete samples. Acoustic emission technology was used to obtain the internal acoustic emission response of concrete samples under high stress. Creep development rate is nonlinear to load holding level based on the analysis of experimental data and internal structure of concrete samples. [ABSTRACT FROM AUTHOR]

This paper investigates damage and healing of cementitious materials based on fly ash (FA) with a special focus on the effect of repeated loading on the self-healing performance. In total, 90 prism specimens and 33 cylinder specimens were prepared and tested. Variables including the FA replacement ratio and preloading level were considered. Different methods were adopted to evaluate the self-healing performance, i.e., mechanical tests, acoustic emission (AE) analysis, water absorption measurement, and scanning electron microscope (SEM) observation. Test results revealed that an appropriate amount of cement replaced by FA was beneficial to the self-healing performance of the specimens subjected to a high level of repeated preloading, where the strength increased by 11% and the water absorption rate decreased by 12%, in comparison with that of the undamaged specimens. This was attributed to the active hydration of FA, which was confirmed by an energy dispersive X-ray spectroscopy (EDS) analysis. This study extends understanding of the self-healing performance when subjected to repeated loading and provides some useful suggestions for the self-healing technique based on mineral additives. [ABSTRACT FROM AUTHOR]

This paper presents piezoelectric-based passive/active methods to reliably detect and quantify defects in mass concrete or grout structures. A dual-mode sensing methodology that makes use of both passive acoustic emission detection and active ultrasonic wave evaluation methods utilizing piezoelectric sensors is investigated. Two types of piezoelectric sensors (PES) transducers are used for acquiring the passive and active signals, respectively. A novel imaging method is developed to map and locate the acoustic emission source caused by the damage growth in the subject structure. Active sensing is further used to evaluate the growth of the damage. A proof-of-concept test using simulated damage, a pencil lead break for acoustic emission, and a drill hole of various diameters for crack growth has been conducted to show the effectiveness of the present passive/active methods. The methods have also been applied to detect and evaluate crack growth caused by stepwise loading. The work presented in this paper lays the foundation for further development of a dual-mode sensing mechanism for in situ crack growth detection and evaluation in massive grout or similar material structures. [ABSTRACT FROM AUTHOR]

Reclaimed asphalt pavement (RAP) and warm mix asphalt (WMA) have become the primary methods for enhancing sustainability in the asphalt industry in recent years. To further enhance sustainability benefits, asphalt producers have begun using RAP and WMA in combination. Research to date has focused on evaluating WMA-RAP mixtures in terms of moisture sensitivity and permanent deformation characteristics as measured in laboratory performance tests. In the present paper, the authors investigate a set of WMA mixtures that encompass a variety of variables, including four WMA additives (Evotherm 3G, Rediset LQ, Sasobit, and Advera) and three RAP contents (0, 15, and 45%). A common belief among practitioners is that the reduced aging in the asphalt binder associated with lower production temperatures in WMA mixtures leaves additional headroom for the incorporation of higher amounts of RAP, which is generally a stiffer, more brittle material. To fully characterize the performance of WMA-RAP mixtures, the authors evaluated the low-temperature, cracking behavior of these mixtures in conjunction with moisture and rutting resistance characterization. They achieved the low temperature testing of WMA-RAP mixtures through the disk-shaped compact tension [DC(T)], indirect tension (IDT) creep compliance, and acoustic emission (AE) tests. Test results showed that chemical additives improved moisture susceptibility according to the AASHTO T 283 standard test and fracture and bulk stress relaxation characteristics using the DC(T) and IDT tests, respectively. The organic Fischer-Tropsch wax-modified WMA mixtures performed the best among the WMA mixtures in terms of rutting resistance. The introduction of RAP led to the increased resistance to permanent deformation and moisture damage. Conversely, RAP reduced thermal cracking resistance according to the low-temperature performance tests in both HMA and WMA. The authors observed the same trend in AE test results as WMA-RAP mixtures exhibited warmer embrittlement temperatures as compared with control mixtures and are therefore expected to be more prone to thermal cracking. On the basis of these findings, thermal cracking resistance remains an issue to be considered in WMA mixtures containing RAP. Additionally, performance testing has shown to be a valuable tool for the evaluation of RAP and WMA mix designs to avoid performance issues in the field. [ABSTRACT FROM AUTHOR]

This paper describes an experimental investigation to assess the development of the localized damage zone in concrete tested in compression. Cylindrical specimens were loaded and unloaded at different maximum strain levels. The mechanical response was measured and compared with acoustic emission activity. As expected, the acoustic emission activity increased with an increase in load. To establish a correlation between mechanical damage and acoustic emission activity, the acoustic emission events were categorized on the basis of their amplitude and duration. Higher amplitude and longer duration events occurred in the specimens at higher damage levels. A linear correlation was observed between the mechanical fracture energy and the measured acoustic emission energy. Triangulation was used to locate the position of the acoustic emission events and to assess the development of the damage zone during testing. The compression damage zone (CDZ) was found to develop at the time of peak stress with a length of approximately 1.2 times the diameter of the specimen. The size of the CDZ was observed to increase to double the diameter of the specimen at 1.6 times the strain at peak. While preliminary, these results indicate how the damage develops to form a zone that influences the stiffness degradation and the postpeak response of the concrete. [ABSTRACT FROM AUTHOR]

Concrete bridges in the United Kingdom represent a major legacy that is starting to show signs of distress. Therefore, the need for monitoring them is an urgent task. The acoustic emission (AE) technique was proposed as a valid method for monitoring these bridges but more study is needed to develop methods of analyzing the data recorded during the monitoring. The writers would like to propose a b-value analysis as a possible way to process AE data obtained during a local monitoring. The b-value is defined as the log-linear slope of the frequency-magnitude distribution of acoustic emissions. This paper presents the results of a b-value analysis carried out on data recorded during a laboratory test on a reinforced concrete beam designed as representative of a bridge beam. During the experiment, the specimen was loaded cyclically and it was continuously monitored with an AE system. The data obtained were processed and a b-value analysis was carried out. The b-value was compared with the applied load, with a damage parameter, and with the cracks appearing on the beam. The damage parameter represents the cumulative damage in terms of total sum of acoustic emissions. The results showed a good agreement with the development of the fracture process of the concrete. From a study of the b-value calculated for a whole loading cycle and for each channel, some quantitative conclusions were also drawn. Further development work is needed to make the b-value technique suitable for practical use on a real bridge. [ABSTRACT FROM AUTHOR]

Due to the differences of physical and mechanical properties of asphalt pavement materials, asphalt pavement (asphalt mixture) presents complex characteristics. To explore the acoustic emission (AE) wave characteristics of asphalt pavement materials, the automatic sensor test (AST) method was used on coarse aggregate, asphalt, and asphalt mixture at different temperatures. The results show that the peak amplitude and frequency of AE waves in coarse aggregate is the largest, followed by asphalt, and asphalt mixture is the smallest. For asphalt mixture, the larger the nominal maximum size of aggregate, the more serious the waveform distortion, the more complex the frequency distribution, and the smaller the amplitude range of AE waves. AE waves in coarse aggregate is not sensitive to temperature, while the peak amplitude of AE waves in asphalt is linearly related to temperature. The peak amplitude and temperature of AE waves in three gradations of asphalt mixtures can be fitted to obtain the corresponding second-order polynomial. AE wave attenuation is mainly reflected in asphalt mixture, while low-temperature sensitivity is mainly reflected in asphalt and asphalt mixture. [ABSTRACT FROM AUTHOR]

This paper experimentally investigates the steel-to-concrete bond phenomenon in two kinds of deformed bars: black steel and hot-dip galvanized steel. The differences in the steel-to-concrete bond behavior are identified in terms of the load-slip curves and the acoustic emission (AE) patterns measured during pull-out tests. Test results show that the bond capacity and the initial stiffness of the bond stress-slip curve of black steel are approximately 20 and 50% greater, respectively, than those of galvanized steel, and the slip corresponding to the maximum bond stress is approximately 20% times larger in the latter. By measuring the AE activity, it is possible to identify the transitional points between the four stages characterizing interaction between the concrete and the bar subjected to pull-out force. Each transition coincides with a sudden drop in the AE activity. [ABSTRACT FROM AUTHOR]

Three-point bending test and acoustic emission (AE) monitoring of notched beams with different volume fractions of steel fiber ultrahigh-performance concrete (UHPC) were carried out. The results show that the initiation toughness, unstable toughness, and fracture energy of UHPC notched beams increase at first and then decrease with the increase of fiber fraction. Based on the stress intensity factor theory, the simplified formulas for calculating initiation toughness and fracture energy were derived. The acoustic emission signal of the UHPC notched beam fracture process was analyzed, and the characteristics of the UHPC material fracture process were characterized by AE energy and cumulative count. Based on the excellent correlation between AE signal and fracture characteristics, it was found that there is an excellent linear relationship between AE energy and fracture energy, and the relationship between cumulative count and crack mouth opening displacement was established. [ABSTRACT FROM AUTHOR]

This study investigates the change in the acoustic emission (AE) parameters emitted in rubberized concrete under abrasion action at a sub-freezing temperature (−20°C). Seven concrete mixtures were developed with two coarse-to-fine aggregate ratios (C/F) (2.0 and 0.7), various crumb rubber (CR) content (0%, 10%, 20%, and 30%), and different rubber particle sizes [4.5 mm CR and 0.4 mm powder rubber (PR)]. Rotating cutter tests were conducted on three 100 mm cubic samples from each mixture at −20°C and 25°C while monitored via an AE system. AE parameters such as amplitude, number of hits, and signal strength were collected and underwent two parameter-based analyses: b -value and intensity analysis approaches, resulting in three additional parameters: b -value, severity (Sr), and the historic index [ H(t) ]. Results showed that testing concrete samples under abrasion at cold temperature, −20°C , resulted in a decrease in the emitted number of hits, cumulative signal strength (CSS), Sr , H(t) , and an increase in b -values compared to testing at 25°C. Furthermore, incorporating rubber particles was found to decrease the AE signals' amplitudes significantly at 25°C and slightly at −20°C , which manifested the higher wave attenuation occurrence at ambient temperature compared to cold temperature. AE analysis also showed a decrease in the abrasion resistance for mixtures with higher C/F, higher CR content, and larger rubber particle size. These decreases were more noticeable at 25°C compared to −20°C. Finally, the study developed two damage classification charts to estimate the ranges of abrasion mass loss percentage and wear depth in terms of the intensity analysis parameters: Sr and H(t). [ABSTRACT FROM AUTHOR]

In this study, inclined shear tests (ISTs) coupled with acoustic emission (AE) monitoring and discrete element method (DEM) numerical simulations were employed to investigate the influence of the water/binder (w/b) ratio (w/b=0.35 and 0.23) and aggregate shape (rounded and angular) on the macro- and microfracture behavior of cement-based materials subjected to shear stress. The experimental results show that macrofracture parameters such as shear strength and stiffness increase as the w/b ratio decreases; however, they are not affected by aggregate shape, in agreement with simulations. The aggregate shape is related to the microcrack behavior. The microcrack distribution in the cement-angular aggregate mixture is confined to the area of the central shear zone, with much lower scatter than that in the cement-rounded aggregate mixture, in which microcracks are scattered around the central shear zone, in line with the macrocrack behavior. This finding is consistent with the numerical simulation results. The DEM simulation results show that the w/b ratio affects macroparameters, namely, Young's modulus (E) and Poisson's ratio (v). The value of E increases as the w/b ratio is reduced and vice versa. The aggregate shape affects only the microparameters (bond properties), which are higher for angular aggregates than rounded aggregates. A method of determination of the DEM input parameters (both micro- and macroparameters) considering different w/b ratios and aggregate shapes was proposed as well. This work provides insights into both the macro and microfracture behavior of cement mortar from the viewpoint of both tests and the relatively new DEM modeling technique. Verified with the IST data, which is rarely found in existing literature, there is generally good agreement between the simulations and tests. Moreover, a unique way of modeling, especially angular aggregate shape has been demonstrated. Further, some reference bond properties and a way to estimate them have been provided for similar simulations. The application of the model presented in this study may be extended to evaluate the strength of rock and assess its suitability as a potentially effective nuclear waste repository site or even evaluate the strength of confined concrete columns (structures) in future studies. These interesting ideas have been presented, for example, because due to the interlocking nature of angular aggregates, the bond properties of cement-based materials made of angular aggregates were found to be higher than those of their rounded aggregate counterparts. [ABSTRACT FROM AUTHOR]

The main cause of degradation for reinforced concrete (RC) structures is the corrosion of the steel reinforcement. Laboratory tests aimed at studying durability are typically accelerated to provide usable results within a reasonable period of time, and well-established electrochemical techniques are used for the assessment of corrosion. However, because these techniques are not suitable for continuous monitoring and are often intrusive, nondestructive testing (NDT) techniques are also suitable for monitoring. Acoustic emission (AE) is one of the NDT techniques used to detect the onset and progression of corrosion. This paper presents an accelerated corrosion test setup and AE monitoring methodology based on the historic index, , for laboratory experiments on RC specimens. Accelerated corrosion is attained by increasing the capillarity suction of RC exposed to chlorides. The frequency spectrum of the AE signals before and after the initiation of corrosion is investigated to isolate the frequency components associated with corrosion. It is shown that the AE signals generated by early corrosion excite a well-defined narrow band of the frequency spectrum. [ABSTRACT FROM AUTHOR]

This paper focuses on the feasibility of using acoustic emission (AE) techniques to evaluate stability of foams produced from paving-grade asphalt binders. The study compared asphalt foamability index [expansion ratio (ER) and half-life (H-L)] measurements with AE measurements for the same foamed asphalt produced using a Wirtgen Model WLB 10 S Laboratory Asphalt Foaming Device. Both ER and H-L are used to characterize the suitability of asphalt binders to produce good-quality foamed asphalt binders for pavement construction. However, current methods for determining foamability are considered as empirical, operator-dependent, and cannot be used for continuous monitoring of the foaming process. Two performance-graded asphalt binders, four foaming water contents, and two foaming temperatures were used. Spectral analyses using a test for white noise suggest AE signals resulting from asphalt foaming are not white noise () and therefore could be useful for evaluating foamed asphalt stability. The evolution with time of AE amplitude during foamed asphalt decay suggests two distinct stages related to foam stability. A clear amplitude dynamics characterized by shorter-duration lower-amplitude foam decay followed by longer-duration higher-amplitude foam collapse was found for all the asphalt binders tested. AE energy, a measure of elastic energy released by collapsing foam bubbles, was found to be sensitive to H-L. The results demonstrate that AE may be useful for characterizing foaming characteristics in a more fundamental manner than existing manual methods. The techniques and results summarized here provide the basis for further investigation into application of AE measurements to monitor and control foamed asphalt stability. [ABSTRACT FROM AUTHOR]

The development of techniques capable of evaluating the deterioration of reinforced concrete structures is instrumental to the advancement of structural health monitoring (SHM) techniques and service life estimate methodologies for constructed facilities. One of the main causes of degradation is the corrosion of steel reinforcement. This process can be modeled phenomenologically, whereas laboratory tests aimed at studying durability responses are typically accelerated to provide usable results within a realistic period of time. Numerous nondestructive methods have been recently studied. Acoustic emission (AE) is emerging as a nondestructive tool to detect the onset and progression of deterioration mechanisms associated with concrete cracking. In this paper, an accelerated corrosion and continuous AE monitoring test setup is presented, providing relevant information on the characteristics of the corrosion circuit, continuous measurement procedure, selection of AE sensors, and AE parameter setting for data acquisition. The effectiveness of AE in detecting and characterizing the initiation of the corrosion process is discussed on the basis of results from small-scale, precracked RC specimens that are representative of areas near the clear cover in typical RC members. The main outcome is a new approach of AE data interpretation based on time-driven parameters. [ABSTRACT FROM AUTHOR]

When cracks occur in concrete, released fracture energy will be proportionally transformed into the energy contained in acoustic emission (AE). According to this physical phenomenon, AE technology provides an effective monitoring method for fracture process of concrete through acquiring generated AE. However, such monitoring is limited in qualitative evaluation of fracture process under most occasions. Quantitative interpretation of fracture process is difficult to accomplish by simply acquiring the amount of AE generated or conducting parameter-based AE analysis. This paper investigates the feasibility of reflecting the fracture energy released during fracture by means of evaluating the energy index of detected AE signals. An effective fracture model based on fracture behavior of concrete cylinder under uniaxial compression is introduced here to provide a mathematical relationship between acquired AE energy index and crack formation energy of concrete. Apart from reasonably estimating the energy consumed for crack formation, cement-based piezoelectric sensors are embedded inside the concrete to carry the task of detecting AE and transforming its energy into the electrical energy of AE signals. On the basis of experimental study, the energy of detected AE signal is satisfactorily correlated to the corresponding fracture energy and quantitatively represents the fracture energy releasing process of concrete during the period of stable cracks development. [ABSTRACT FROM AUTHOR]

This paper deals with two aspects of characterization of the fracture process zone (FPZ) in concrete beams. A comparative study is done between acoustic emission (AE) and scanning electron microscope (SEM) to understand the microlevel aspect of the FPZ in concrete beams. SEM analysis, is mainly based on the preparation and analyzing samples which are considered as being a very important part (poor preparation techniques can lead to erroneous diagnosis of the concrete study). On the other hand, an AE is a localized and quick release of strain energy in a stressed material. The released energy causes stress waves, which propagate through the specimen. These waves can be detected at the specimen surface and analyzed to evaluate the magnitude and the nature of damage. The results obtained from the two experimental studies have shown that there is a small divergence between experimental results (SEM and AE). This may be related to some details during the sample analysis. [ABSTRACT FROM AUTHOR]

The serviceability of concrete structures is a problem in which creep and damage are coupled. In this paper, we present experimental investigations on concrete that involve tertiary creep. We consider residual capacity tests on notched bending beams and investigate the evolution of size effect due to basic creep. In these experiments, beams are first subjected to a constant load, at a given ratio of the maximum capacity during 90 days. They are then loaded up to failure and their residual capacity is obtained. During the creep phase, acoustic emission is analyzed and it shows that damage develops under creep. The comparison between the size effect test results with and without creep exhibits variations of the fracture properties of the material. The fracture energy and the size of the fracture process zone decrease when creep occurs prior to failure. These results, which include size effect fracture tests, may serve for the development and validation of coupled creep-damage models. [ABSTRACT FROM AUTHOR]

Four-point bending tests of self-compacting concrete with different rubber contents were conducted, and the acoustic emission (AE) technique was used to monitor the fracture process. The evolution law of rise angle (RA), average frequency (AF), and AE intensity was analyzed, and the Gaussian mixture model (GMM) was used for clustering analysis of crack mode in different loading stages. The results clarify that the safe regions of self-compacting rubberized concrete (SCRC) were proposed based on AE intensity. Both the slope of the cumulative RA–AF curve and the average value of RA and AF had a quantitative relationship with rubber content. Rubber content had little effect on crack mode. The failure process of SCRC with rubber contents of 0%–30% was dominated by the tensile crack mode, and the maximum proportion of shear cracks appeared in the postpeak loading stage (40%–20%), which can be considered as warning value of concrete bending failure. The results can provide a theoretical reference for the early warning of SCRC failure. [ABSTRACT FROM AUTHOR]

The emergence of strain-hardening cementitious composites (SHCCs) improves brittle cracking of concrete materials. SHCC has the characteristics of strain hardening and multiple cracking under tensile load. In this study, 10 groups of SHCC were developed by incorporating polyvinyl alcohol (PVA) fiber into cement-based materials with high-content fly ash. Testing variables included water/binder ratio, sand/binder ratio, fly ash and cement content, and sand particle. The tensile test on the thin-plate specimen and compression test on cylindrical specimen were carried out. In addition, the double-edge notched plate specimens with relative notch lengths of 0.2, 0.4, and 0.6, respectively, were tested. The results showed that the average ultimate tensile strain of SHCC was 403 times that of ordinary concrete, and the peak compressive strain was about four times higher than ordinary concrete. The ultimate tensile strain of SHCC with high-volume fly ash ratio is generally higher than the low-volume fly ash ratio group. The failure mode of SHCC was ductile with good energy dissipation. Also, SHCC compressive strength is mainly adversely affected by the water/binder ratio and the ratio of cement to fly ash. With the increase of compressive strength, the angle of the failure face was reduced under compression. The acoustic emission (AE) activity captured the initiation and development of cracks at the three stages in the notched specimens under tension. With the increase of the notch length, the fracture energy calculated by the tensile stress–crack mouth opening displacement (CMOD) curve decreases. [ABSTRACT FROM AUTHOR]

The purpose of this study is to clearly understand the self-organized critical phenomenon of concrete in the process of uniaxial compression. During the compression tests, acoustic emission (AE) waveform and its characteristic parameters were collected synchronously. The AE characteristic parameters information entropy was defined by combining information entropy theory with AE characteristic parameters in the time domain and frequency domain. Moreover, the evolution law of the AE characteristic parameters weighted information entropy composed of typical characteristic parameters in the time domain and the AE peak frequency information entropy in the frequency domain are obtained during the loading process. The results reveal that, as the stress grows, the AE characteristic parameters weighted information entropy of concrete shows an evolution characteristic of "descending, stabilizing and rising," while the AE peak frequency information entropy shows the evolution characteristic of "ascending and declining." Meanwhile, as the AE characteristic parameters weighted information entropy reaches the minimum value (e≈0.9 peak stress), the critical state of concrete ends. After that, the information entropy value increases rapidly, and the penetrating cracks initiate. When the specimen is destroyed, the information entropy value reaches the maximum. The evolution process of cracks in concrete, switching between a chaotic state that develops in random and an organized state that develops in order, follows certain laws (disordered–ordered–disordered), which corresponds well to the AE characteristic parameters weighted information entropy. The research results provide a new method for concrete damage monitoring based on AE. [ABSTRACT FROM AUTHOR]

Although the use of nondestructive mechanical characterizations has increased (especially resonant methods), standards do not provide a relation between natural frequencies and mechanical properties for complex geometries such as hollow masonry units. This study uses numerical modeling via finite-element analysis (FEA) and impact acoustic tests to evaluate the dynamic elastic modulus of hollow masonry units based on their vibration modes and frequencies. Static and ultrasonic tests defined the reference values for the elastic modulus and an experimental sample comprised of hollow clay and concrete blocks and half-blocks of different geometries and compressive strengths. The results show the values of elastic modulus obtained from ultrasonic, acoustic, and compressive tests were different due to peculiarities of each test. A numerical simulation enabled the determination of the geometrical constants, and an analytical model proposed related vibration frequency and dynamic elastic modulus values for units of regular geometries and provided a reliable approximation for geometrical constants. [ABSTRACT FROM AUTHOR]

Concrete with coal gangue as aggregate has a self-sensing ability with addition of carbon fiber (CF), which has an outstanding effect in terms of dealing with the large amount of coal gangue discharge and monitoring the concrete structure. However, studies on the development of CF-based gangue concrete (CF-GC) with self-sensing capabilities are limited. Therefore, the uniaxial compression tests of CF-GC were carried out and changes in the electrical resistivity (ER) and acoustic emission (AE) characteristics were monitored throughout testing. The test results suggested that the addition of CF can significantly improve the electrical conductivity properties of gangue concrete. Value of gage factor for CF-GC and gangue concrete is 2.3 and 16.6, which the value of gage factor for the CF-GC increased by 14.3 compared with the GC. The research showed a good correlation between the ER and AE of CF-GC, and an increase in curing time led to a decrease in in the electrical conductivity of CF-GC. This study shows that it is of great significance to be able to predict deformations in gangue concrete structures by monitoring changes in ER and AE. [ABSTRACT FROM AUTHOR]

Long-term corrosion tests were performed on prestressed T-girders measuring 4.98 m (16 ft, 4 in.). The test program was composed of four specimens, including one control. The specimens were subjected to wet/dry cycles using 3% NaCl solution to accelerate corrosion. Two of the specimens were precracked prior to conditioning to examine the effect of crack presence and/or width. The specimens were continuously monitored using acoustic emission (AE). Half-cell potential (HCP) measurements and linear polarization resistance (LPR) were performed daily to serve as a benchmark for corrosion detection. Acoustic emission can detect the onset of corrosion comparable in time to conventional electrochemical methods. Furthermore, AE intensity analysis has the ability to distinguish between different levels of corrosion. The specimens were load-tested to measure the residual capacity, which can give an indication of AE's ability to detect corrosion damage before it affects the strength of prestressed structures. [ABSTRACT FROM AUTHOR]

Research on the usage of industrial by-products to improve mechanical properties of low-quality materials in infrastructure applications is of global interest. Two industrial by-products, namely, fly ash (FA) and asphalt emulsion (AE), were used in this research to develop an FA-AE stabilized marginal crushed rock (CR) as a sustainable pavement base material. A liquid alkaline activator was used, comprised of a mixture of sodium hydroxide (NaOH) and sodium silicate hydrate (Na2SiO3) at Na2SiO3:NaOH=50∶50 and NaOH concentration = 5 M. In this research study, the mechanical properties of FA-AE stabilized CR were investigated via unconfined compressive strength (UCS), indirect tensile strength (ITS), flexural strength (FS), indirect tensile resilient modulus (IT Mr), and indirect tensile fatigue life (ITFL) tests. The UCS of FA geopolymer stabilized CR (without AE) was found to be dependent on the FA content and curing time. For all the curing times studied, the higher FA replacement ratio resulted in higher UCS values. Although the asphalt film enhances particle confinement, it was found to retard the geopolymerization reaction. According to the local road authority, which requires a UCS>1.75 MPa for a stabilized base course, the addition of AE reduced the UCS of FA-AE geopolymer stabilized CR. To meet the minimum 7-day strength requirement for both low and high-traffic roads, a geopolymer mix was suggested with an FA replacement ratio≥20% and an AE content≥1%. Compared with the cement stabilized CR at a similar UCS value, the FA-AE geopolymer stabilized CR had higher FS, ITS, IT Mr , and ITFL but lower carbon footprints. [ABSTRACT FROM AUTHOR]

This study investigates the acoustic emission (AE) and physicomechanical properties of concrete under three sulfate attack conditions: continuous immersion, wetting–drying cycles, and cyclic loading coupled with wetting–drying cycles. The characteristics of AE absolute energy during the concrete compression process were statistically analyzed by the histogram method and maximum likelihood (ML) method. The results from this study showed that the concrete specimens experienced two stages: an enhancement stage and a subsequent deterioration stage. Both wetting–drying cycles and cyclic loading accelerated the sulfate attack process. The probability density distribution of AE absolute energy satisfied a power law, and the distribution exponent was related to the degree of sulfate attack. The filling of expansion products during the enhancement stage led to the ML curves displaying a good plateau with lower exponents. Conversely, the cracking occurring during the deterioration stage created upturned ML curves with higher exponents. The results indicate that the exponent can accurately characterize the degree of concrete degradation under sulfate attack. [ABSTRACT FROM AUTHOR]

This study investigates the performance of two cementitious systems exposed to sodium chloride solution and thermal cycling. The two systems consisting of an ordinary portland cement (OPC) and a calcium aluminate cement (CAC). Several experimental techniques were conducted to evaluate the performance of OPC and CAC pastes exposed to different concentrations of sodium chloride (NaCl) solution and thermal cycling. X-ray powder diffraction was used to study the hydration products. Low-temperature differential scanning calorimetry was used to detect phase changes and study potential chemical interactions between pastes and NaCl solutions. A longitudinal guarded comparative calorimeter equipped with acoustic emission was used to detect phase changes and damage development in pastes saturated with NaCl solutions during thermal cycling. Unlike OPC pastes that can chemically interact with NaCl solutions to form a destructive chemical phase change at temperatures range between −5°C and 8°C, CAC pastes showed no chemical interactions with NaCl solutions, mainly due to possessing different chemistry than the OPC. As a result, no damage associated with the chemical phase change was observed in CAC pastes, indicating that the CAC is more chemically resistant to NaCl damage than OPC. [ABSTRACT FROM AUTHOR]

An acoustic-emission approach to evaluate the low-temperature cracking performance of asphalt binders is presented. The acoustic activity of a thin film of asphalt binder bonded to a granite substrate is monitored while the layer is exposed to decreasing temperatures from around 20°C to approximately -50°C. Results of eight different asphalt binders at three different aging levels, i.e., unaged (TANK), short-term aged (RTFO), and long-term aged (PAV), are presented. The acoustic emission (AE) embrittlement temperatures are found to be sensitive to binder type as well as binder aging level. Results show that for most binders, their AE-based embrittlement temperature is a few degrees lower than their bending beam rheometer (BBR) critical cracking temperatures. [ABSTRACT FROM AUTHOR]

The ingress of chlorides into posttensioned (PT) concrete structures is a leading cause of corrosion of the prestressing strands. This reduces the strength, durability, and service life of the structure and may result in catastrophic failure. A structural health monitoring (SHM) method is needed to improve the maintenance procedures associated with this method of construction. To evaluate the potential of acoustic emission monitoring for this application, long term corrosion monitoring tests were performed on specimens that were representative of internal and external posttensioning methods. Corrosion was induced in the specimens by adding chlorides to the grout and by performing wet/dry cycling with NaCl solution. The corrosion process was monitored by half-cell potential measurements (HCP) and acoustic emission (AE). Results show that AE has the ability to detect corrosion damage in the PT specimens with similar accuracy to HCP measurements. Furthermore, intensity analysis of the AE data shows that the damage in the PT specimens can be categorized according to the level of corrosion present. This investigation demonstrates that AE can be successfully implemented to detect, monitor, and quantify corrosion levels in PT concrete structures, and that AE is a promising nonintrusive method to detect and quantify corrosion in the absence of electrochemical techniques. [ABSTRACT FROM AUTHOR]

Acoustic emission (AE) has been increasingly used for assessment and prediction of fatigue cracks in steel bridge members. Fatigue cracks develop at the transverse weld toe of stiffeners, attachments, and cover plates in steel bridge members. Effectiveness of AE to assess fatigue crack initiation from weld toe is investigated in this study, and an attempt is made to predict the crack growth behavior using AE signal features. Cruciform specimens consisting of a single tension pull plate with transverse fillet-welded plates attached at midspan are tested in this study. The transverse plates represent stiffeners and/or short attachments typical of welded steel bridge details. The specimen provides realistic initial conditions of fatigue crack initiation and growth from high stress concentration regions. Consequently, AE waveform characteristics representative of those expected on bridge structures are produced. Accurate stress intensity factor values are difficult to obtain because of the small, nonuniform crack growth conditions at the weld toe. Finite-element method analysis for welded geometries capturing stress fields at the weld toe of stiffener details is performed, and numerical results are incorporated into an existing analytical stress intensity factor framework. Procedures for assessing the crack size and predicting remaining fatigue life of the specimen using absolute energy feature of AE signals are demonstrated. [ABSTRACT FROM AUTHOR]

Steel degradation due to corrosion in prestressed concrete bridges has been of major concern as it presents a threat to the integrity of structures adjacent to marine environments or where deicing salts are regularly used. To assess the potential for monitoring of the corrosion process, an accelerated corrosion testing program has been conducted. A series of specimens with dimensions () were subjected to constant potential application through the embedded steel strand while being continuously monitored with acoustic emission (AE). Depassivation of the strand was detected by monitoring the fluctuations in applied anodic current. Half-cell potential measurements assessed the probability of corrosion, and all results obtained were compared to acoustic emission data. The mass loss of the corroded strands was correlated to acoustic emission intensity analysis to quantify the degree of damage. Results show that acoustic emission is as sensitive as half-cell potential for detecting the presence of corrosion and may be used to effectively locate corroded areas. The intensity analysis proved useful for categorizing the level of damage, making it a strong candidate for structural prognosis. [ABSTRACT FROM AUTHOR]

In this research, the feasibility of using embedded cement-based piezoelectric sensors to detect the acoustic emission (AE) activities in concrete has been studied. A sensor is presented that is made with the 0-3 cement-based piezoelectric composite as sensing element. The composite is formed by pressing mixed cement-piezoelectric powder into a penny-shaped thin plate. In the terminology of 0-3, 0 represents the dimension of the piezoelectric particle and 3 stands for the dimensions of the matrix. By adjusting the dosage of the piezoelectric ceramic powder, the properties of the composite can be tailored to suit designated applications. Sensors made of the composite (0-3 sensor) are able to be embedded into concrete and have broadband frequency response. It is shown by a four-point bending test on a concrete beam that the sensors have excellent ability in AE activity detection. High sensitivity is also observed for the embedded sensors. [ABSTRACT FROM AUTHOR]

In this experimental study an acoustic emission (AE) technique was used on geometrically similar notched plain concrete three-point bend specimens of different sizes. This study aims to measure the acoustic emission energy released during fracture process of plain concrete beams and to investigate the relation between acoustic emission energy to fracture energy of plain concrete beams of different sizes. The technique of acoustic emission uses one or more sensors to listen to a wide range of events that may take place inside a solid material. A series of concrete specimens of different sizes was tested for fracture energy. Acoustic emission energy release was monitored while testing concrete specimens for fracture energy. A correlation between fracture energy and acoustic emission energy was observed. Fracture energy was calculated from work of fracture. Size effect on the strength of concrete beams was observed. An equation has been proposed to obtain fracture energy by knowing acoustic emission energy. [ABSTRACT FROM AUTHOR]