Systematic study on the shear behavior of RC members

Shear is a complex problem of structural concrete. Shear failure of slender reinforced concrete members without shear reinforcement usually occurs without warning by formation of a critical shear crack. With sufficiently high amounts of shear reinforcement, the critical shear formation can be avoided that enables another kind of shear cracking and improves the shear resistance of the member. The difference in the shear cracking has been considered as the basis for two totally different approaches to modeling the shear resistance of members without and with shear reinforcement. Although most of structural members only have relatively small amounts of shear reinforcement, possible transition in the shear behavior from members without shear reinforcement to members with low amounts of shear reinforcement has not yet studied by the research community. The goal of this research project is to characterize the shear behavior of members without and with low amounts of shear reinforcement. To meet this goal, the shear crack development and flow of internal forces will be systematically investigated. As a hypothesis, we assume that shear failure of a member with low amounts of shear reinforcement is still characterized by the critical shear crack formation, which is essentially dependent on the support, loading condition and shear slenderness of the member. The proposed experimental program will comprise seventy tests on beams with different support, loading conditions and systematically varied shear reinforcement ratios. The shear crack development and the flow of forces will be analyzed using advanced measuring techniques. Using the observed shear crack geometry and kinematics and appropriate constitutive relationships, different shear transfer mechanisms will be quantified. Based on the analyses and observations obtained from the experimental program, physical models to describe the shear behavior of members without and with low amounts of shear reinforcement will be developed. The shear models should provide background for design concepts that are physically consistent and, thus, generally valid for members with different boundary conditions and amounts of shear reinforcement. The proposed research project will be effective in gaining more insight into this challenging issue, which is of great academic and practical interest. The project fills a gap in basic knowledge of the internal behavior at the transition from shear transfer in members without shear reinforcement to members with sufficiently high mounts of shear reinforcement to establish full truss action.

Shear behaviour is a complex and important problem in structural concrete and has been an object of research for more than 100 years. Despite elaborate investigations a range of open questions still remains. This situation is not satisfactory, especially regarding the assessment of existing structures, as, for instance, bridge superstructures often have deficits in the calculated shear resistance due to insufficient shear reinforcement but do not exhibit any signs of damage. The goal of this research was to characterize the effect of the boundary and loading conditions on the shear behaviour of reinforced concrete (RC) structures without and with small amounts of shear reinforcement. To meet this goal and extensive experimental program consisting of a total of 70 large scale shear with different loading and boundary conditions was conducted. 35 of these tests were conducted at TU-Graz (mainly simply supported beams under point loads and cantilevers under distributed loads) and 35 tests were conducted by the project partner at RWTH Aachen. The project filled a gap in basic knowledge of internal behaviour of shear transfer in RC member without shear reinforcement to members with sufficient shear reinforcement to establish full truss action. The shear in the shear span is transferred via a complex combination of different mechanisms, e.g. direct strut action, contributions by the compression zone, aggregate interlock, crack processing zone, dowel action and the shear reinforcement. To account for the contribution of the individual mechanisms advanced photogrammetric and distributed fibre-optical measurements were utilized. The experimental results have shown that with an amount of shear reinforcement equal to approximately double the required minimum shear reinforcement, the crack pattern in the web tends to change from a single shear crack to an inclined parallel cracking that is commonly assumed in the shear design. The experiments also demonstrated that lower amounts of shear reinforcement than those required for the minimum shear reinforcement in the existing design provisions do not considerably influence the shear resistance of a standard shear test on a simply supported beam subjected to one- or two-point loads. In contrast, lower amounts of shear reinforcement than those required for the minimum shear reinforcement could considerably enhance the shear resistance of cantilevers subjected to uniformly distributed loads. However, the difference in the shear resistance due to different support and loading conditions tended to decrease when to the amount of shear reinforcement was increased. Based on these findings a new approach to model shear resistance of members with low amounts of shear reinforcement was refined and could predict the shear resistance of the investigated members to a satisfactory level.