Many existing concrete structures require effective assessment of the bearing capacity. A critical failure mode is shear, especially for concrete structures without or with limited shear reinforcement. The shear failure is brittle and often leads to loss of property and lives. Therefore the shear failure should be indicated before it occurs. A potential solution is to use acoustic emission (AE) monitoring, which is sensitive to minor changes in concrete, even micro-cracking, both on the surface and inside the structure. By combining the knowledge of shear failure processes and AE techniques, this paper presents an AE-based shear failure indication system. The system automatically identifies three levels of structural damage levels up to shear failure, which are categorized from minor to severe levels as green-light, yellow-light, and red-light criteria. The 'traffic light system' is validated using six shear tests on full-scale reinforced concrete beams without shear reinforcement. The robustness of the system is also validated across these tests.@en

This paper proposes a new mechanical model to describe the dowel action with the aim of using the model to gain a deeper understanding of the unstable dowel splitting cracking observed in shear experiments of beams without shear reinforcement. The model was developed by combining beam on elastic foundation (BEF) theory and fracture mechanics. The proposed model is able to predict the whole evolution process of dowel action until the propagation of the dowel splitting crack becomes unstable. The model theoretically proves that the development of a dowel splitting crack can become unstable under certain conditions, therefore leading to the unstable shear failure of the whole member. In addition to the derivation of the analytical model, the paper also validates the model using data from the literature. Finally, an analytical solution of the critical shear displacement that triggers the unstable dowel splitting crack is derived. It can be used to improve the failure criterion initially proposed in the Critical Shear Displacement Theory (CSDT).

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Proof load testing for assessment can involve a large risk due to the high loads. Stop criteria can reduce this risk. Stop criteria are necessary for shear, which is a brittle failure mode. This paper describes the development of shear stop criteria for slab strips. The shear stop criteria are developed by combining theoretical concepts related to cracking, as well the relationship between bending- and shear-critical regions, along with insights from the Critical Shear Crack Theory and the Critical Shear Displacement Theory. The shear stop criteria are validated with fourteen beam tests. The result is a set of shear stop criteria in a “traffic light system” with a green light level related to serviceability, and yellow and red light related to the ultimate limit state for shear. These stop criteria serve as the basis for a global approach for proof load testing of reinforced concrete bridges.

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Aging infrastructure globally faces degradation, posing risks and requiring substantial repair investment. Strategic maintenance practices are crucial for evaluating structural conditions and ensuring sustainability. The growing demands on modern materials and structures necessitate enhanced health monitoring approaches. Shifting from reactive to proactive maintenance methodologies is paramount, due to lower investment while keeping the structural performance at acceptable standards. However, quantitative assurance of repair/reinforcement/retrofit programs or self-healing effect in structures is similarly crucial for the operation of the infrastructure. Non-destructive testing (NDT) techniques, such as ultrasound, acoustic emission, and optical methods, play a vital role in assessing structural health. Through real-world case studies, the effectiveness of repair in addition to damage assessment are evaluated, encouraging a more systematic approach to monitoring structural repair efficacy. The paper intends to address the research gap in monitoring the repair effectiveness in civil structures and materials and provides valuable insights to enhance repair strategies in civil engineering.

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As the existing bridge stock is ageing in various parts of the world, the topic of how to assess, maintain and/or improve, and manage existing bridges becomes increasingly important. Existing bridges may be designed according to outdated codes with requirements that may be considered unsafe nowadays, and for loads that are significantly different than those used nowadays. For those bridges, an accurate assessment leads to more efficient management of the bridge stock. This paper outlines various strategies that lead to an improvement of the assessment, and potentially to the extension of the service life of existing concrete bridges. This paper provides a selected examples that engineers who are faced with the assessment of ageing bridge can use. Ultimately, the presented insights can serve to support countries with a younger bridge stock in the development of an assessment strategy.@en

The shear provision for members without shear reinforcement in the second generation of Eurocode has been changed to a new set of formulas based on the critical shear crack theory (CSCT). The formula is based on a shear failure criterion originally developed for reinforced concrete members without shear reinforcement. To allow its application as a design code type for formula, the original CSCT failure criterion undergoes several modifications, such that it can be used to verify the shear resistance of prestressed members as well. Since the new Eurocode shear provision will be applied to design and assess prestressed concrete members in Europe and many other countries in the world, it is important to extensively validate this model. This paper presents a validation study of three different variations of the CSCT strain-based failure criteria, including the one eventually employed in the second generation Eurocode shear provision, using the ACI-DAfStb shear database. The results are also compared with the current Eurocode shear provisions. The second generation Eurocode shear formula appears to be able to determine the shear resistance more accurately than the current one, even for prestressed concrete members without shear reinforcement while it was not actually developed for this. However, Annex I.8 shear formula may lead to an overestimation of the shear resistance for higher values of the effective span to depth ratio (acs/d).

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Dowel action is recognized as one of the major shear resistance mechanisms. Although the dowel action only contributes a relatively small portion of the total shear resistance, the shear failure of reinforced concrete members without shear reinforcement usually occurs accompanied by unstable dowel cracking. This paper presents a new description of the dowel splitting process and a mechanical model based on two theories, namely the Beam on Elastic Foundation (BEF) theory and fracture mechanics. The mechanical model analytically describes the three stages during the dowel splitting of the longitudinal rebar in an RC beam, which are the elastic stage, stable cracking stage and unstable cracking stage. The proposed model can capture the post-peak behaviour of the nature of dowel action. Finally, a simplified equation for engineering practices is proposed. The proposed expression shows promising agreement with the experimental data.@en

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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As the existing bridge stock is aging, assessment of existing bridges becomes increasingly important. In the Netherlands, the shear capacity of reinforced concrete slab bridges is found to be insufficient. In particular, the shear and punching shear capacity of reinforced concrete slab bridges subjected to concentrated loads from the design tandem or truck is subject to discussion, as the shear behavior is situated in between oneway and two-way shear. Currently, an experimental program is being conducted at Delft University of Technology to determine the shear capacity of straight and skewed reinforced concrete slabs under point loads near to the support. This paper presents the results of the 25 tests conducted on six straight slabs of 5m × 2.5 m × 0.3 m subjected to a proof load testing loading protocol. The failure load and modes of the slabs are described in detail. Reinforced concrete slabs under concentrated loads can fail in shear, punching, and flexure, as well as a combination of these failure modes. The results of the experiments are compared to strength predictions obtained by using current design models and current methods for assessment. These experiments demonstrated that the Dutch guidelines, which are based on previous slab experiments, are an improvement as compared to the Eurocode for the assessment of existing reinforced concrete slab bridges. Ultimately, this work provides recommendations for bridge engineers tasked to assess reinforced concrete skewed slab bridges.

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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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Coda wave interferometry (CWI) holds promise as a technique for concrete stress monitoring. This is because the coda, which consists of multiply scattered arrivals, is the result of propagation through the medium over large distances. As such, it is sensitive to both minute structural changes and small velocity changes in that medium. Previous studies focusing on concrete have predominantly utilized the time-domain-based stretching technique to measure travel-time changes. There is, however, a lack of consensus on how to quantify these changes effectively. In this study, we conduct a systematic comparison between two techniques, namely the stretching technique and the wavelet cross-spectrum (WCS) technique, for measuring stress-induced velocity changes in a cylindrical concrete sample. Our comparison focuses on two key aspects: (i) stability against cycle skipping and (ii) consistency in retrieving velocity changes. Experimental results reveal that both the WCS technique and the stretching technique yield consistent velocity changes. In terms of stability, it is challenging to determine which technique performs better, due to differences in the mechanisms triggering cycle skipping. However, when considering waves with frequencies ranging from 50 kHz to 80 kHz, both techniques exhibit comparable performance. Based on our findings, we offer the following recommendations for utilizing these CWI techniques in concrete stress monitoring: For the stretching technique, selecting the time window length based on the wave frequency and the expected magnitude of velocity change. For the WCS technique, operating it in the frequency band where spectral decomposition shows sufficiently high energy in the signal and can accommodate the expected magnitude of velocity change.

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Typically precast girders are designed and utilized as simple supported members. Alternatively, the precast girders can be made continuous at the intermediate support using cast-in-place concrete topping. Once the girders are made continuous, time-dependent restraint moments will occur. The magnitude of the restraint moment is mainly affected by the creep and shrinkage behaviour of the concrete and the age of the girders at continuity. The developing restraint moment may affect the stress conditions near the support region and, in extreme cases, result in the loss of the integrity of the structural member. Currently, full-scale experimental campaign is underway on the shear behaviour precast continuous girders at Delft University of Technology. Inverted T girders are individually cast and later made continuous after a certain period. To investigate the influence of restrained action and quantify the prestress losses, fiber optic sensors (FOS) are embedded in the girders. By utilizing the FOS, the evolution of the concrete strain is monitored. This paper presents the measurement of the time-dependent strains. Furthermore, the concrete strains are analysed to evaluate the prestress loss and time-dependent restraint moment effect.@en

Geopolymer concrete is a new alternative material to conventional concrete with less carbon dioxide emissions. Researchers have reported much research on the material properties of geopolymer concrete. However, research on the behaviour of this newmaterial at the structural level is still limited, especially at a full-scale structural level. Three geopolymer concrete beams with a total height of 700mm were tested till the shear failure. The first two specimens were subjected to the monotonically increasing load until the shear failure. The third specimen was first loaded under sustained load at the level of 80 kN for three weeks to investigate the influence of shrinkage and creep on the cracking behaviour. Then the specimen was then unloaded and reloaded again to failure. Digital Image Correlation (DIC) measurement was used to measure the surface deformation of the whole span of the beam. The crack spacing, crack width and crack development were investigated using the DIC measurement. The experimental results showed that the shear capacity of tested geopolymer concrete beams is lower than the calculated result based on the Eurocode.@en

In this work, dynamic characterization of a simply supported beam is carried out during different steps in a failure load test. The main goal of this work is to evaluate the evolution of the structural dynamic parameters of the beam with different status of damage. Real-scale prestressed concrete beams are tested to investigate its shear behaviour as a part of a large research program at TU Delft. Four dynamic tests are performed at different damage status of the beam: firstly in the initial or undamaged condition; secondly after the first flexural cracks; then, after shear cracking; and finally in the full damaged condition. The dynamic excitation is performed with an impact load at fixed location on the top of the beam and the vibration data is recorded by three different systems. The first one is a cost-effective and open source monitoring equipment, consisting of seven low-cost accelerometers. The second system is based on five trusted high performance accelerometers. The last one is a commercial alternative consisting of four high accuracy piezoelectric accelerometers. Acceleration data is analysed afterwards using Operational Modal Analysis techniques to obtain modal frequencies, modal shapes and damping of the structure in the different states. The obtained dynamic behaviour of the structure and its results are discussed and compared. It is concluded that a change in the frequency of the first flexural mode is only observed when the damage in the beam is very significant, while no changes are observed with the occurrence of flexural and shear cracks.@en

In old Dutch inner cities like Amsterdam a large number of steel-concrete bridge decks built between 1880 and 1960 remain in service nowadays and currently need assessment of their bearing capacity. A significant number of these decks were designed without any mechanical connectors like shear studs in the interface between concrete and steel. Moreover, the concrete decks were designed with only shrinkage reinforcement in both directions on the top layer of concrete. No additional transverse reinforcement was placed that can ensure proper (re)distribution of loads after cracking. In order to study the bearing capacity of this deck typology, two specimens of an existing bridge were taken to the Stevin Lab of TU Delft and tested until failure. In this work, the experimental results of both tests are presented. Then, finite element models including nonlinear behaviour of the materials and the interface are presented and compared with the experimental observations. Experimental results show that the bearing capacity is achieved after yielding of the steel beams. Nevertheless, the ductility and transverse load distribution of the elements is affected by the interface behaviour and the poor detailing. The finite element simulation strategy used shows good agreement with the experiment and can be used for future assessments.
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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 presents a reliability-based calibration method for design code formulas. The method is demonstrated on the shear design formulas in Eurocode 2 and fib Model Code 2010 (MC2010). We found that the partial factor (Formula presented.) in the current Eurocode 2 is about 20% lower than the optimal value and, thus, provides an insufficient safety margin. The obtained optimal partial factor (Formula presented.) in the (modified) Eurocode 2 and MC2010 formulas is 1.53 and 1.36, respectively. The difference stems from higher accuracy and, hence, lower uncertainty of the MC2010 model in predicting experimental results. Hence, on average, the MC2010 formula leads to about 13% larger design resistances compared to Eurocode 2 given that the target reliability for both design formulas is the same. To stimulate and facilitate future structural code development and derivation of partial factors, we make the used computer code freely available.

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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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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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Prestressed hollow-core slabs (PHCS) are generally fabricated through an automated extrusion method by using an extremely dry concrete mixture to maximize productivity in precast factories. In consequence, it is very challenging not only to provide shear connectors along the interface between the PHCS unit and cast-in-place (CIP) topping concrete but also to secure code-compliant intentional roughness due to its extremely hard and flat surface condition. Thus, it is difficult to have sufficient horizontal shear strength required to achieve flexural performances of the prestressed composite members. To this end, extensive pushoff tests were conducted in this study considering various interface roughness conditions, from which some viable options were chosen. Then, flexural tests on full-scale composite PHCS specimens were performed with selected surface conditions from the pushoff tests. On this basis, the applicability of various surface roughness conditions that are inevitably faced in practices is examined based on ACI 318 design criteria and nonlinear analysis. It was confirmed that sufficient horizontal shear strength can be achieved not only by properly roughened surface conditions but also by placing wire meshes between PHCS units even with no intentional roughness.

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