Detailing Aspects of the Reinforcement in Reinforced Concrete Structures

Abstract

This thesis studies the impact of reinforcement detailing on the behaviour of a reinforced concrete structure. Using a retaining wall as a case-study, the performance of two commonly used alternative reinforcement layouts (of which one is wrong) are studied and compared. Reinforcement Layout 1 had the main reinforcement (from the wall) bent towards the heel in the base slab. For Reinforcement Layout 2, the reinforcement was bent towards the toe. This study focused on the reinforcement details used in the D-region, and on how it impacts the capacity, joint efficiency and failure mode of the structure. First, a literature review is carried out which focused on the behaviour of corner joints from experimental works available in literature. Next, a strut and tie model of the D-region is made. From the strut and tie model, the opening moments acting on the structure subjects the re-entrant corner region to a concentration of tensile stresses, while a compressive stress field acts concurrently with transverse tension within the core of the joint. The internal forces within the D-region are evaluated, and the required steel areas computed. Afterwards, ATENA FEM software is used to model the structure, and to study the impact of the alternative reinforcement layouts on the capacity and behavior of the structure. Some aspects of the structural behavior studied include the stress and strain distribution in the concrete, crack width, crack pattern, steel stress and strain distribution etc. The results obtained from the FEM analysis was sensitive to bond model defined in the material model. When perfect-bond was assumed in the FEM analysis, Reinforcement Layout 1 attained a joint efficiency of 72.4%, while Reinforcement Layout 2 achieved 88% joint efficiency. In his experimental works on similar details, Nilsson (1973) had obtained a joint efficiency of 60% for Reinforcement Layout 1, a range between 82% to 102% for Reinforcement Layout 2. The disparity between FEM result and experimental result for Reinforcement Layout 1 occurred because perfect-bond was assumed in the FEM model. With cracking playing prominent role in this structure, perfect bond assumption is not valid, and some slip is inevitable. To verify, a bond-slip relation is used to model the structure, resulting in 62% joint efficiency for Reinforcement Layout 1, and 82% joint efficiency for Reinforcement Layout 2. These values obtained with bond-slip model are much closer to experimental values than those obtained with perfect bond. The reinforcement layout used also had significant impact on the joint behavior. In Reinforcement Layout 1, the reinforcement (tie) from the wall was not properly anchored in the nodal region in the slab. The compressive stress field (i.e. inclined strut) was observed to flow past the bent part of the reinforcement without much interaction. The force transfer between the inclined strut and the tie was not effective. Also, wide cracks occurred along the inclined strut from the action of transverse tension (caused by the opening moment). These cracks which further weakened the strut. This detail had a diagonal tension cracking failure mode. For Reinforcement Layout 2, a clearly defined nodal region exists. A CTT node formed allowed for effective force transfer (at the node) between the concrete and steel. Furthermore, the bent part of the reinforcement crossed the path of the inclined strut, and helped to control crack width. The reinforcement also provided confinement to the inclined strut which further increased its strength. This detail prevented diagonal tension cracking failure, hence the higher capacity it achieved. Failure was by crushing of concrete along the joint – slab interface, after formation of a wide vertical crack extending from the re-entrant corner downwards into the slab. Adding a diagonal bar, placed 45° around the re-entrant corner, helped to control this re-entrant corner crack, thus ensuring that over 100% joint efficiency is achieved. In conclusion, Reinforcement Layout 1 is a poor detail. Though common in practice, a node is not properly formed in this detail, thus force transfer capacity between concrete and steel is not effective.