Effects of Curing Time and Curing Temperature on the Standard Test for Strand Bond

Abstract

The purpose of this study was to observe and determine the effects of the curing environment on the STSB method. Through multiple rounds of NASP funded research, the STSB has proven to be a valid test method for assessing performance of prestressing strand bond with concrete and to correlate bond performance to transfer lengths and development lengths in prestressed members. The study focused on altering the curing time and temperature, while maintaining the required mortar strength as specified by the STSB method. Curing conditions lacking in environmental control will affect concrete strength and maturity. In attempting to maintain simplicity of the STSB procedures, the results of this study are potentially valuable to strand producers and prestressed plant operators. The results from the modified STSB test program demonstrated that curing conditions must adhere to the requirements of the STSB procedures to ensure valid pull-out force results. The pull-out forces increase as the w/c ratio of the mixture (decreased curing time) decreases for mixtures of the same mortar strength. In addition, the pull-out values increase as the curing temperature increases for mixtures of the same mortar strength. Interestingly, the STSB method illustrated extreme sensitivity for mixtures with w/c ratios of less than 0.50, and pull-out forces appeared to approach a minimum value, or become less sensitive, as the w/c ratio increased, if the data were extrapolated. The increase in pull-out forces potentially describes a change in behavior in the bond strength between the prestressing strand and mortar at approximately a w/c ratio of 0.50.