Strand bond stress–slip relationship for prestressed concrete members at prestress release

Abstract

The transfer of prestress force from prestressing strands to the surrounding concrete is dependent on the bond between the two materials. Understanding the actual bond stress distribution along the transfer length results in optimized design of the transfer zone of prestressed concrete members. Equations of estimating the transfer length in ACI 318 code and AASHTO LRFD bridge design specifications simply take into account the effect of the strand diameter only. The objective of this study is to provide a generalized procedure for determining the bond stress–slip relationship accurately by incorporating the effects of additional parameters, such as concrete compressive strength at prestress release, center-to-center strand spacing, and concrete bottom cover. First, the bond stress distribution along the transfer length of a prestressed concrete member is formulated based on longitudinal slip–strain compatibility, force equilibrium and invariable bond stress–slip relationship along the transfer length. Second, a generalized Inverse Problem-Solving approach is introduced to determine best parameter coefficients through minimizing the discrepancy between the calculated and measured results. Two types of measurements (i.e., transfer length and end slip) reported in the literature are utilized to demonstrate the proposed approach. Predicted transfer length and end slip values using the calibrated bond stress–slip relationship show better agreement with the test data compared to those predicted by ACI 318 code and AASHTO LRFD bridge design specifications. Third, a computational procedure is developed and an example is presented to assist engineers using the developed formulae for determining the bond stress distribution along the transfer length of prestressed concrete members.