Influence of bond and anchorage conditions of the shear reinforcement on the punching strength of RC slabs

Reinforced concrete flat slabs constitute one of the most common and efficient modern construction methods. The design of such structural system might be governed by a brittle failure in the vicinity of the slab-column connection associated to punching shear. The arrangement of transverse reinforcement in the critical zone increases both the strength and the deformation capacity of the slab-column connection. Recent advances on the understanding of the punching phenomena lead nowadays to a better approach of their differences in efficiency. One of the main parameters identified as governing for the performance of this specific reinforcement is the quality of its anchorage and bond, influencing the development of cracks in the shear-critical region. Such characteristic is generally defined through force-slip relationships and is strongly influenced by the local state of stress and strain. Although the activation of reinforcement often takes place within already cracked concrete for many structural members, the conventional approaches supporting code formulations are still almost exclusively based on tests performed on uncracked specimens. In the coming years, an increased emphasis should be placed on the study of the performance of reinforcing details in such severe conditions, with the aim to improve the knowledge of this rather underrated but critical problematic. Several experimental works were thus conducted in the frame of this thesis on the role of the anchorage of the transverse reinforcement in punching shear phenomenon. A programme of pull-out tests on actual detailing solutions was performed in cracked conditions similar to those developing in slabs at the vicinity of the columns. The results highlighted notable differences amongst the evaluated types of anchorages, confirming therefore the various levels of performance observed in punching tests. The activation of this specific reinforcement is investigated through tests on full-scale slab specimens provided with extended measurements of the force and crack openings. The use of an innovative reinforcing setup allowed to track the concrete and steel contributions in the punching phenomenon, providing the experimental information required to validate the main assumptions of the Critical Shear Crack Theory for the failure mode within the shear-reinforced area. Observations on straight bars with in-plane cracking supported the development of analytical formulations to evaluate the reduction of performance –in terms of strength and stiffness– for various anchorage details by analogous considerations to the aggregate interlock approaches. The model is validated through a refined numerical method and the main test results available from literature. Such developments can be partially used within the frame of the Critical Shear Crack Theory, which calculates the contribution of the shear reinforcement in the punching strength –for the failure mode of interest– with a physical model of activation for the transverse elements. The latter contains a number of general assumptions –perfect bond and anchorage conditions, simplified crack kinematics– which can be improved and refined on the basis of the experimental results of the present research. Proposals are formulated to take into account in the existing model a more realistic activation of the transverse reinforcement in the slab during punching, and thus to improve the understanding and the predictions associated to this failure mode.

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