Reinforced concrete members without shear reinforcement are still commonly encountered in civil engineering practice, particularly in existing structures and in elements where shear reinforcement is difficult or impractical to arrange. In such members, shear failure occurs in a brittle manner, with limited deformation capacity and little warning. Although many models have been proposed in the literature for the shear behaviour of reinforced concrete members without shear reinforcement, a generally accepted mechanical model for brittle flexural-shear failure has not yet been established.
A major difficulty is that flexural-shear failure is not governed by isolated mechanisms, but by the interaction among several complex mechanisms, including crack propagation, aggregate interlock, dowel action, residual tensile strength and shear stress transfer through the uncracked compression zone. Existing models remain limited in two respects. First, many models do not provide a comprehensive framework in which these mechanisms and their interactions are consistently represented. Instead, they often select one mechanism as the governing mechanism and introduce a calibrated failure criterion based on experimental databases. Second, the available mechanical models for individual shear transfer mechanisms are not always suitable for direct integration into such a comprehensive framework, because they may rely on simplified assumptions or may not be formulated in terms of crack kinematics. As a result, the reliability of these models becomes uncertain when they are applied beyond the range covered by the databases on which they were developed.
The main objectives of this dissertation are twofold. The first objective is to refine the models of selected shear transfer mechanisms to enable their consistent integration into a discrete crack-based mechanical framework. The second objective is to develop a calibration-free mechanical framework that provides a rational basis for assessing shear capacity beyond the current experimental range. In this framework, the critical shear crack is explicitly represented, and the contributions of different shear transfer mechanisms are evaluated along this crack.
The selected shear transfer mechanisms re-examined in this dissertation are dowel action, aggregate interlock and contribution from the uncracked compression zone. A new dowel action model is developed by combining the Beam on Elastic Foundation Theory with concrete fracture mechanics. This model can predict a complete force-displacement relationship for dowel action, whereas most models in the literature are mainly limited to predicting the ultimate dowel capacity. A 3D scanning-based extension of the Two-Phase Model is developed for aggregate interlock by incorporating the real morphology of fractured crack surfaces. This extension provides a systematic framework for quantifying the influence of crack surface roughness on aggregate interlock, whereas currently available models generally rely on empirical factors to account for this influence. The evaluation of the contribution of the uncracked compression zone is further refined by adopting a revised shear stress distribution along the beam depth, motivated by the aggregate interlock stress distribution obtained in the cracked region.
The dissertation further proposes and validates a simplified shear crack model based on comprehensive experimental observations of crack patterns using the author’s database and data reported in the literature. Unlike existing simplified shear crack models, the proposed model considers the propagation of secondary cracks in the compression zone and treats their length as an unknown variable. This crack model, together with the proposed dowel action model, is then integrated into the final mechanical framework for flexural-shear failure. In the proposed framework, moment equilibrium and transverse force equilibrium are first satisfied. The shear capacity is then obtained by minimising the residual in the longitudinal force equilibrium. In this manner, the shear capacity is determined through a conventional limit-state analysis that satisfies three predefined limit-state conditions. Without empirical calibration factors, the calculated shear capacities show good agreement with an extensive shear beam database. Based on these results, a simplified method using the critical shear displacement is further developed to improve the applicability of the proposed model.
Overall, this dissertation contributes to a more rational mechanical understanding of brittle flexural-shear failure in reinforced concrete members without shear reinforcement through the following aspects:
• Development of a mechanical model for dowel action that provides the complete force-displacement relationship and accounts for the development of splitting cracking along the longitudinal reinforcement. (Chapter 3)
• Extension of the Two-Phase Model that incorporates the real morphology of fractured crack surfaces from 3D scanning data, which provides a basis for applying aggregate interlock modelling to concretes with different fracture characteristics. (Chapter 4)
• Development and validation of a simplified shear crack model that captures the main geometric and kinematic features of flexural-shear cracks based on experimental observations. (Chapter 5)
• Development of a calibration-free mechanical model, together with a simplified method based on the critical shear displacement, for predicting the shear capacity of reinforced concrete beams without shear reinforcement. (Chapter 6)
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Reinforced concrete solid slab bridges are often skewed to cross underlying objects, which increases the shear stress concentration at the obtuse corner. Limited experimental evidence on skewed slabs is available, so that both the shear capacity and failure mode in skewed slab bridges are subject to discussion. Therefore, an experimental program at Delft University of Technology investigated the capacity and failure modes in skewed slabs under concentrated loads near the edge. Results from 15 tests on five 1:2-scale slab members result in shear failures and show a decreasing capacity with increasing skew angles. The obtuse corner is found to be critical; the reinforcement layout did not influence the capacity significantly. Comparisons with calculation methods showed reasonable accuracy. A proposed method using a larger integration length around the peak shear stress obtained from linear finite element modeling may be recommended for assessment. ... Read more

The authors regret that the Fig. 6a of the article was incorrect. [...] ... Read more

Bridges are among the most important infrastructure assets, especially reinforced concrete slab bridges. Many were built with a skew angle instead of straight due to the limited space available. The skewness affects the internal shear force distribution of the slabs and causes shear stress concentration at the obtuse corner. Most of the shear design methods are developed based on straight slabs. Whether these methods apply to skewed slabs is unclear. On the other hand, experiments on skewed slabs are quite limited. Therefore, an experimental programme of skewed slabs was conducted at Delft University of Technology. The slabs have a height of 300 mm, representing a half-scale model of representative solid slab bridges in the Netherlands. A concentrated load was applied close to the edge of the slabs to induce a one-way shear failure. Both the top and bottom surfaces of the slabs were measured using stereo Digital Image Correlation (DIC). This paper presents the design and the results of the experiment. By using the stereo DIC from both surfaces, the internal shear crack propagation is captured. All specimens failed in shear and the results demonstrate that the shear capacity of the slabs decreases as the skewness increases. Moreover, the experiment confirmed that the shear stress concentration is more significant in the obtuse corner than in the acute corner. An evaluation method is proposed to calculate the shear capacity of the skewed slab combined with the Linear Finite Element Analysis. The calculated shear capacity given by the proposed method shows a good alignment with the experimental results. ... Read more

Aggregate interlock is considered one of the most important shear transfer mechanisms in concrete members. In the well-established Two-Phase model proposed by Walraven in the 1980s, the shear stress transferred by aggregate interlock is estimated by calculating the projected contact areas of two crack surfaces. As one of the main assumptions in the model, the crack surface is idealized by a plain surface crossing randomly distributed, idealized spherical aggregates. This was a necessary simplification of an actual crack surface in the 1980s because of the lack of measurement equipment as well as computational capacity. With the development of high-accuracy 3D scanning techniques, new possibilities for modelling aggregate interlock have become available. This paper proposes a generalised method to determine the aggregate interlock stresses using the crack surface directly from 3D scanning. The proposed method is cross-verified with the Two-Phase model using the same simplified crack surface. A case study using the scanned crack surfaces of concrete cubes is conducted to investigate the influence of surface roughness. The proposed method provides a new possibility for conducting a refined investigation of the aggregate interlock for new concrete types, especially under the scope of the next-generation Eurocode shear provision. ... Read more

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. ... Read more

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). ... Read more

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. ... Read more

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. ... Read more

For reinforced concrete members without shear reinforcement, the shear failure is characterized by the formation of a critical flexural shear crack. Recently experimental observations making use of Digital Image Correlation (DIC) by many researchers suggested the significance of geometric characteristics and kinematic conditions of critical shear cracks in shear failure. However, limited efforts were reported in literature on the quantification of the geometric characteristics of critical shear cracks. This is mainly due to the lack of understanding of the mechanism of how the flexural shear cracks form. In this paper, the available models in literature for the shear crack trajectory and the underlying theoretical assumptions are reviewed first. Those models include the shear crack model proposed by the authors. Next, the shear crack trajectory models are validated using a collection of shear crack patterns based on the DIC data obtained from the shear failure database from Delft University of Technology. The majority of the crack patterns are from full-scale shear tests of deep beams with an effective depth larger than 1.0 m. The comparison helps us to have a basic understanding of how accurate the available flexural shear crack trajectory models can achieve. ... Read more