Shear and Peel Stresses at the Interface between Hollowcore Slabs and the Topping Concrete
Hollowcore slabs are used in floors and roofs of residential, commercial, industrial and institutional buildings. They are precast/prestressed concrete elements produced using the extrusion process. Their surface finish can be “machine-cast” or “intentionally roughened”. A typical floor consists of a number of hollowcore slabs that are connected together. Prestressing causes hollowcore slabs to camber, which results in an uneven floor surface. A 50 mm topping concrete is commonly cast to level the floor surface. To avoid delamination, engineers require bonding agents to be applied on the hollowcore slab surface before pouring the topping concrete. The concrete topping can be used compositely with the hollowcore slabs to increase the floor’s load carrying capacity. However, North American design standards require intentional roughening of the hollowcore slab surface to consider such composite action. This requirement results in added cost that manufacturers are keen to avoid. This thesis presents a comprehensive experimental program to assess the performance of composite hollowcore slabs with machine-cast and lightly-roughened surface finishes. Three types of tests were performed: pull-off, push-off and full-scale. They provided an overall understanding of the interfacial shear and peel behaviors at the interface between hollowcore slabs and the topping concrete. The tested slabs were found to sustain higher interfacial shear stresses than the limits set by the design standards and to provide adequate composite behavior up to failure. Linear analytical modeling in which closed-form solutions for differential equations governing the interfacial shear and peel stresses during the push-off tests was conducted. Two analytical methods were developed to estimate the shear and peel stresses during the full-scale tests utilizing the interface stiffness determined from the pushoff tests. Linear finite element analysis was performed to validate and compare the proposed methods. To better understand the experimental results and to provide engineers with more accurate tools for estimating the interfacial stresses, nonlinear finite element analysis of the push-off and the full-scale tests were conducted. Interfacial shear and peel stiffness values associated with the tested slabs were also determined to assist design engineers in predicting failure modes of composite hollowcore slab.
hollowcore slabs, composite action, concrete topping, Finite Element Analysis, shear stresses, peel stresses, interface stiffness
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