The restrained thermal expansion of a pretensioned strand causes thermal prestress loss during steam curing until\nsufficient bond strength develops. The amount of thermal prestress loss is directly related to the characteristics of the interfacial\nbond stressââ?¬â??slip relationship at different maturity phases of concrete. For a rational assessment, the bond stressââ?¬â??slip relationship\nneeds to be investigated experimentally at different maturity phases. In this study, a total of 12 pull-out tests were performed using\nseven-wire strand of 12.7 mm diameter, at different concrete equivalent ages of 7.8, 23.5, 53.8 and 85.2 h. Based on the test\nresults, an empirical model of the bond stressââ?¬â??slip relationship was developed. The model comprised four segments: a curvilinear\nascending region, a constant maximum region, a linearly descending region, and a region of constant frictional bond stress. The\ncharacteristic values in the model were expressed as functions of equivalent age. The model was able to predict the test results with\nreasonable accuracy.
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