Crack Control in Post-Tensioned One-Way Slab Systems

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Title: Crack Control in Post-Tensioned One-Way Slab Systems

Author(s): F. V. Ulloa, S. R. Witthoft, and R. W. Poston

Publication: Special Publication

Volume: 225

Issue:

Appears on pages(s): 213-222

Keywords: crack control; cracking; post-tensioned slab; restrained shrinkage

Date: 3/1/2005

Abstract:
Current structural concrete design standards require that post-tensioned slab systems be provided with mild steel or post-tensioning tendons to control potential transverse cracking due to shrinkage and temperature effects that may occur soon after concrete is cast and over the life of the structure. For the case of prestressed concrete structures, the ACI 318 Building Code requires that the tendons should be designed such that they provide a minimum average compressive stress of 100 psi (0.7 MPa) on a gross concrete area as an effective prestress. This level of prestress is expected to be sufficient to control cracking caused by shrinkage and temperature effects. The Code further requires that the effects of restraint be considered. Field evidence suggests that this minimum value may not be adequate for some post-tensioned one-way slab systems. Relevant case studies are presented and discussed. In these cases, the structural design included the minimum prestress as required, yet significant transverse cracking was observed on some areas of the slab despite the presence of the shrinkage and temperature tendons. This paper presents a design procedure for controlling shrinkage and temperature cracking in post-tensioned one-way slab systems consistent with current code requirements. A method is proposed by which an equivalent shrinkage stress is calculated. This stress is then used to design the shrinkage and temperature tendons. The equivalent shrinkage stress is calculated considering the restraint on the slab provided by elements such as columns and shear walls. It is shown that this restraint, which is a function of the respective stiffness of the restraining elements, is very significant and its effects should be considered if shrinkage and temperature cracking is to be effectively controlled by either tendons or mild steel reinforcement.