Behavior and analysis of transfer zone in pretensioned prestressed concrete members

Abstract

Transfer length is defined as the distance over which prestressing steel should be bonded to concrete to transfer the effective prestress in the prestressing steel. Estimation of transfer length can greatly affect cracking moment at service limit state as well as shear strength and development length at ultimate limit state due to the lower prestressing force within the transfer zone. Many empirical equations have been proposed for transfer length, however it is well known that there is a significant discrepancy between the predictions from the equations. The first goal of this study is to reassess the influences of the well-known test variables on transfer length and to examine new experimental factors that might affect the estimation of transfer length. In addition, the previous empirical equations assumed a constant bond stress distribution along the transfer zone. This assumption was made based on the observation of a linear distribution of concrete strain. The second goal is to propose a transfer length equation based on the actual distribution of bond stress. The last goal was to evaluate transfer length of high-strength strands that have been recently developed. For the purposes, an extensive experimental program was conducted. Strand strains were measured on the helical wires with electrical resistance strain gauges (ERSGs). Applicability of ERSGs to transfer length test and analytical model for behavior of strand were discussed. Influences of the test variables on transfer length were identified and the empirical equations including the current code provisions were evaluated. Finally, a novel bond model and transfer length equation was proposed based on the actual bond behavior of strand. Test results showed that the effects of initial prestress, concrete compressive strength at transfer, and strand diameter could be accounted for by the equation proposed by Oleśniewicz. It implies a linear distribution of bond stress and a parabolic distribution of strand strain. In the considered range of cover depth, cross section size, and strand spacing, the effects of these factors were negligible if conforming to the current code provisions. The effects of curing condition, debonding, reinforcement spacing, and prestress release method were examined. The current code provisions provided conservative estiamtes for transfer length of high strength strand. The cover depth and strand spacing of the current code are also feasible to high strength strand. Based on the measured strand strain, a novel bond-slip-strain relationship for a strand in the transfer zone of a pretensioned concrete member is presented. Estimates obtained from the proposed model were in good agreement with the test results from other studies as well as those from this work.