Investigating Release-Connection Between Post-tensioned Concrete Slab and Wall

In the construction of post-tensioned concrete slabs, the slab shortening due to concrete shrinkage, creep and post-tensioning compression should be considered in the design of slab-wall connection to minimize the cracking caused by wall restraining effects. Various connection details are used in practice to mitigate the cracking. One Release-Connection involves wrapping the vertical dowel, which connects the slab and wall, with a compressible material. The behavior of this connection has not been investigated in the literature. In this experimental study, the performance of this connection under lateral loading is investigated. Several experiments were performed on full-scale 14-inch thick concrete specimens (typical in post-tensioned transfer slabs) poured over smooth concrete surface separated by a layer of bond breaker. The connecting hooked dowels were wrapped with a one-inch-thick compressible material. Two cases were examined. One with the entire dowels within the slab wrapped, and the other with only the dowel vertical portions wrapped, while the hooks were directly in contact with the concrete. Under seismic actions, the friction between the slab and the wall, which are separated by a bond breaker, is ignored. Furthermore, it is usually assumed that the wrapped dowels do not provide lateral resistance, until the wrapping is fully compressed. For the experiments, in which the surface of base concrete was polished, the friction coefficient was approximately 0.4. This value increased to 0.8 when the surface was hand-troweled. The connection began to resist horizontal force prior to 1ʺ movement. The source of this resistance is concluded to be a combination of the resistance due to densification of the wrapping material, friction between the slab and support, and a complex bending of the dowels. This study investigates and characterizes the lateral load resistance of such connections. The results of this study will help better evaluate the behavior of this type of Release-Connection by providing insight into its response particularly where it is used as a permanent connection. Utilized as a permanent connection, this will considerably reduce the construction time of post-tensioned concrete slabs.