To date, there is no comprehensive approach available that can explicitly model the complete transient waveforms of acoustic emissions (AE) induced by fracture processes in brittle and quasi-brittle materials like concrete. The complexity of AE modelling arises from the intricate coupling between the local discontinuity of material fracturing and the global continuity of elastic wave propagation in solids. Among others, the lattice type models are promising approaches, as they are known to be a matured modelling approach to simulate the fracturing process in concrete-like materials. Nevertheless, like other discrete element methods (DEM), they are currently limited to describing the number and rate of AE events (broken elements) in the fracture process and cannot explicitly model wave generation and propagation. In this study, we propose a lattice modeling framework to simulate the propagation of complete waveforms of fracture-induced AE signals in concrete. A proportional-integral-derivative (PID) control algorithm is incorporated in an explicit time integration procedure to reduce dynamic noise from spurious oscillations. Additionally, a Rayleigh damping-based calculation method and corresponding calibration procedure are proposed to model the attenuation of AE signals due to material damping. Using the developed approach, we systematically investigate the feasibility of lattice models for elastic wave propagation simulation, the dependence of lattice mesh sizes and the choice of numerical damping parameters. These results lead to a systematic framework which can be employed in simulating wave propagation with attenuation using DEM models in general including lattice models.

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Transportation infrastructure demands reliable, cost-effective, environmentally friendly, and safe solutions. It is, therefore, crucial to leverage both the knowledge gained from current practices and the potential offered by emerging technologies. This paper uses the scoring system approached in the INFRACOMS project to offer a framework for asset managers and technology providers to identify areas of improvement and make informed decisions regarding selecting and implementing remote condition monitoring solutions. We focus on two technologies for bridges, like bridge weigh-in-motion and digital inspection and centre around four areas: data analysis, visualisation and integration and potential for practical decision-making. Technologies are evaluated based on their intended use, acknowledging that some may have multiple applications due to novel sensor installations or data interpretation/visualisation methods. Consequently, a technology may undergo multiple appraisals within this system. We showcase the benefits of the scoring system, alignment with specific use cases, and potential for broad applicability.

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

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To assess the structural capacity of reinforced concrete structures, identifying the internal cracks is important. Acoustic emission (AE) is promising to estimate the location of internal cracks. However, the localization is influenced by many factors like arrival time picking error, presence of crack, etc., resulting in localization error. The error cannot be entirely removed. Considering the inevitable localization error, a probabilistic method was recently developed by the authors. The method estimates the probability of the location of AE events, creating a probability density field of AE events (pdAE field). This method can possibly improve the identification of internal damages. This paper evaluates the performance the pdAE field in identifying internal damages in a reinforced concrete slab. The slab was loaded to failure by a point load. Compared to the conventional localization results, the pdAE field showed a clearer internal crack pattern. Moreover, calculation of the pdAE field was time efficient, thus was suitable for real-time monitoring. With these benefits, the pdAE field indicated the failure of the slab before it occurred. @en

This paper proposes a new damage identification method, namely, the probability density field of acoustic emission (AE) events. This new method provides a different perspective to deal with the uncertainties in the source localization process. We treat the source location as a random variable, and estimate its probability density field based on a probability density function. The function was found from simulations where various uncertainties were included. The probability of AE events falling in a certain space range is the integral of the probability densities over that range. We apply the new method in a failure test of a full-scale reinforced concrete beam. The resultant probability density field clearly reflects the crack patterns of the specimen and a close relationship with the crack width.

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Advanced monitoring methods are required to identify stop criteria in proof-load tests. In this study, the combined methodology of two-dimensional digital image correlation and acoustic emission is investigated for its applicability for future implementation in field tests. The two monitoring systems are deemed to provide valuable insight with external measurements from digital image correlation and internal measurements from acoustic emission. Two overturned T-section reinforced concrete slabs (0.37 × 1.7 × 8.4 m) tested under laboratory conditions are used for the assessment. The first slab test served as a preliminary test to enable sensor placement and creation of a relevant loading protocol. The main scientific results lead to a proposal for a test procedure using the combined methodology based on results, observations, and experiences from an individual stop criteria assessment for the two methods. The results include full-field plots, an investigation of the time of crack detection and monitoring of crack widths with digital image correlation, and a qualitative assessment of activity vs. load followed by a quantitative evaluation of calm ratios using acoustic emission. The individual results show that both digital image correlation and acoustic emission can identify damage occurrence earlier than other secondary methods. At crack detection (415 kN), crack widths were measured at widths between 0.078 mm to 0.125 mm and can be monitored until reaching the stop criterion at 463 kN (Eurocode SLS threshold of wmax = 0.2 mm). The acoustic emission results were limited by the pre-defined loading protocol and thus, only indicated that damage occurred sometime between 300 kN and 500 kN (pre-defined load levels). Therefore, the proposal for test procedure involves a methodology, where the loading protocol may be updated during testing based on monitoring results and thus provide even more valuable data.

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

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Existing concrete bridges require efficient assessment due to increasing traffic load and degrading material. For reinforced concrete structures without shear reinforcement, a typical failure mode is shear, which often means a total loss of structural capacity and does not have an obvious warning beforehand. To indicate the shear failure before it occurs, we need to monitor the structural behaviour like the location and magnitude of the cracks. Acoustic emission (AE) is a favourable technique that can detect the internal cracking sensitively. This dissertation develops an AE system to indicate the shear failure before it occurs. AE methods are improved. A deeper understanding of AE from concrete cracking is provided. Based on the new insights of AE, a ‘traffic light system’ is developed where the green-, yellow- and red-light criteria show three damage levels of the structure. The new system has been validated using experiments on different types of structures. In the end, a general guidance is provided on implementing the AE-based indicators in load testing of a concrete structure.@en

Proof load testing can be an interesting method to assess existing bridges for which analytical methods are unable to provide an accurate assessment. In a proof load test, a load representative of the factored live load is applied to the bridge. If the bridge can carry this load without distress, the proof load test is success-ful, and the bridge proves it fulfils the code requirements. Since large loads are applied, the structure or element that is tested needs to be carefully monitored during the test. This paper reviews the literature on reported load tests and the measurement techniques used during these tests. It also includes the test goals these techniques can address, and the advantages and disadvantages of the contact and non-contact techniques. The result of this re-view is guidance for the selection of appropriate monitoring and measurement techniques during load tests. This practical recommendation can serve engineers during the preparation of a load test, and will be extended in the future with stop criteria validated with experimental results.@en

Acoustic Emission (AE)-based crack tracking is a promising approach to locate cracks in concrete structures. Different from conventional AE, this approach uses AE activities during crack closure. But, AE activities during crack closure may be influenced by the number of load cycles and loading speed. This may challenge the applicability of AE-based crack tracking, especially for existing concrete structures with many load cycles in the service life. This paper investigates the influence of the number of load cycles and loading speed on AE-based crack tracking. A total of 80 load cycles were applied on a pre-cracked concrete beam. The last 5 cycles had increasing loading speeds. AE-based crack tracking was carried out in each load cycle. For calibration, Digital Image Correlation (DIC) was performed to measure the crack opening and closure. We found that AE-based crack tracking can locate the cracks, while, with many load cycles, it cannot indicate the crack width. Loading speed had little influence on AE-based crack tracking. The results of this paper put forward the applicability of AE-based crack tracking for existing concrete structures.@en

For the assessment of existing slab-between-girder bridges, the shear capacity and failure mode are under discussion. Previous research showed that the static and fatigue punching capacity of the slabs is sufficient as a result of compressive membrane action. The girders then become the critical elements. This research studies the shear capacity of prestressed concrete bridge girders. For this purpose, four (half) girders were taken from an existing bridge that was scheduled for demolition and replacement and tested to failure in the laboratory. Two loading positions were studied. The results show that there should be a distinction between the mode of inclined cracking and the actual failure mode. In addition, the results show that for prestressed concrete girders, the influence of the shear span-depth ratio should be considered for shear span-depth ratios larger than 2.5. These insights can be used for the assessment of existing slab-between-girder bridges in the Netherlands.@en

This paper presents a study on the application of the coda-wave interferometry (CWI) technique using embedded transducers for the monitoring of concrete structural members. The coda-wave interferometry (CWI) has been recognized as a promising technique in detecting small changes in the parameters of materials. To reduce the influence of environment and enable the access to the interested region of the target structure we propose to apply CWI with embedded transducers for the monitoring concrete structures. In this paper, the proposed approach is applied on a 0.5-m cylinder under compression and on a 10-m full-scale beam under bending. The results showed a good correspondence between the CWI-deducted wavespeed change and the estimated strain value in both the small-scale and the large-scale test. Besides, the correlation coefficient of the coda, especially the late coda is sensitive to the formation of cracks even at a distance from the sensor pair. Thus, they can be used as crack indicators of the structure.@en

In the Netherlands, many prestressed concrete bridge girders are found to have insufficient shear–tension capacity. We tested four girders taken from a demolished bridge and instrumented these with traditional displacement sensors and acoustic emission (AE) sensors, and used cameras for digital image correlation (DIC). The results show that AE can detect cracking before the traditional displacement sensors, and DIC can identify the cracks with detailed crack kinematics. Both AE and DIC methods provide additional information for the structural analysis, as compared to the conventional measurements: more accurate cracking load, the contribution of aggregate interlock, and the angle of the compression field. These results suggest that both AE and DIC are suitable options that warrant further research on their use in lab tests and field testing of prestressed bridges.@en

Acoustic emission source localization is a promising monitoring technique for concrete structures. However, the accuracy of acoustic emission source localization is influenced by many factors, such as the presence of cracks, which are commonly observed in existing reinforced concrete structures. In this article, the acoustic emission source localization is evaluated using a numerical model with a total number of 11,827,200 independent simulated tests. In this work, the investigated influential factors include the presence of cracks, arrival time picking error, and senor layout. The accuracy of source localization is quantified by the characteristic error defined in this article. Using the proposed wave propagation properties, a relatively stable characteristic error of 150 mm is estimated in the detection zone with the maximum sensor spacing less than 1 m. The evaluation approach and simulated characteristic error are validated experimentally by comparing the 200 manually generated signals using hammer hits on a cracked concrete beam.

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This paper presents a comparative study based on a thorough beam bending test.The intention is to obtain the relationship between the nominal elastic wave velocity obtained from Acoustic Emission tomography analysis and the physical damage of the RC beam in terms of crack opening. The crack width of the specimen was determined by both Digital Image Correlation and the conventional LVDT measurements. The study shows that globally the nominal wave speed derived from AE tomography is related to the crack development of the specimen when the crack width is limited. It is also shown that the scatter of the AE tomography results can be improved by averaging the nominal wave velocity of the cells along the cracked section.@en

Acoustic Emission (AE) monitoring is one of the possibilities to detect the crack distribution inexisting concrete structures. However, the conventional method requires further destruction likeopening of new cracks or propagation of the existing cracks. In this paper, a new strategy ofusing local cumulative AE activities during unloading to track the crack trajectory is proposed.With this strategy, a relatively low load level which does not cause further destruction to thestructure is needed. The possibility of this strategy is experimentally examined using a realscale beam of 10-ton damaged under cyclic loading. For calibration, the crack opening ismeasured by Digital Image Correlation (DIC). The crack patterns detected by the new strategyand DIC show good agreement.@en

Reinforcement corrosion is the most common and most expensive damage mechanism in existing reinforced concrete (RC) structures. Therefore, the qualitative and sustainable renovation of these structures leads to several important technical challenges. Camilo Torres, a student residence of KU Leuven built in 1968, is an example of a RC building dealing with corrosion problems. Corrosion causes the formation of rust and a reduction of the steel bar’s cross section. The former will eventually cause cracking and spalling of the concrete cover and deterioration of the bond at the reinforcement-concrete interface. An important question that can be asked is to which extent the corrosion process affects the structural capacity of the concrete components and how to deal with this in a renovation process. The focus of this paper is the on-site investigation of the corroded RC components of the Camilo Torres residence to provide input for structural assessment and renovation process. Firstly, the history of the building and used materials are investigated. Secondly, on-site analysis is performed to inventorise global and local damage. Results are compared with findings from previous on-site investigations executed in 2000 and 2014 to examine the damage progress. It was found that the main damage is caused by carbonation-induced corrosion leading to cracking and spalling of the concrete cover. An important part of the rebars have a concrete cover less than 25 mm due to construction errors. Also the presence of honeycombs accelerates the corrosion process. The Acoustic Emission technique is able to detect the damage patterns during tomography and passive acoustic emission sensing, however, more effort is needed to optimise the localisation accuracy.@en

Interaction of ultrasonic waves with partially-closed surface-breaking cracks in concrete structures has been studied. Measurements have been conducted on a reinforced concrete beam containing various mechanical-load-induced cracks and compared with the baseline measurements at those locations. Influence of crack width, incident angle of waves with cracks, and distance from the cracks on travel time and amplitude of the waves have been investigated when the beam was unloaded. It has been observed that a measurable part of the waves propagate through the cracks due to the acoustic coupling between the crack faces, although attenuation can be relatively high. The travel time has shown a nearly independent behavior from remaining crack opening in the measured range of 0.05 mm to 3 mm. Measurements in directions orthogonal and parallel to the crack suggest that there is substantial anisotropy in the cracking zone. Furthermore, an effective width of the micro-cracking area around the cracks has been estimated from the measurements.

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Acoustic Emission (AE) technique has promising features for structural health monitoring reinforced concrete structures. However, cracks in existing structures may challenge the commonly-used notion that increased degradation would lead to increased recorded AE activity, and/or adversely affect the accuracy of the localization process. In this article, influence of cracks on the accuracy and reliability of AE results on concrete structures has been quantitatively studied based on measurements on cracks between 0.05 mm and 3 mm. With the demonstrated crack-induced localization errors and substantial attenuation ranges, the presented results may offer new insights into the accuracy and reliability of AE monitoring for existing concrete structures.@en