A case for increasing the allowable compressive release stress for prestressed concrete

Two surveys were conducted: one of literature addressing compressive transfer stress and one of professional and producer members of the Precast/Prestressed Concrete Institute (PCI) to ascertain their use of compression transfer stresses that were greater than current code limits. The purpose of these surveys was to determine the current states of research and practice regarding compression transfer stress of prestressed concrete members. After evaluation of the research papers, the authors conclude that compression transfer stresses up to 0.70 f' ci would be acceptable. The PCI survey of professional and producer members provided an indication of the number of plants using compression transfer stresses above the limits set in ACI 318-05, Section 18.4.1.' The PCI study also provided information on the types of products that PCI members use with various compression transfer stresses and on any problems that members have encountered when using stresses greater than the current specified limits. Thirty plants reported using compression transfer stresses greater than 0. 60 f' ci . The most common problems encountered with greater compression transfer stresses, according to the survey, were related to tension cracking and release order of the strands. Camber and prestress loss predictions are slightly more variable with greater transfer stresses. The research reports recommend that care and experience are needed to refine predictive methods. The recommendation of this study is that ACI 318 should raise the permissible compression transfer stress to 0.70 f' ci to comply with the limit currently recommended in the PCI Design Handbook. Refined camber and prestress loss calculations based on the higher trahsfer stress may be needed.