Thermal creep and relaxation of prestressing steel

Abstract

The thermal creep and relaxation of prestressing steel are crucial to the permanent loss of prestress in post-tensioned concrete structures after fire. Harmathy’s creep model is widely used to account for the irrecoverable thermal creep strain. In view of advances in steel manufacture, it is desirable to determine the relevant parameters of Harmathy’s creep model for common prestressing steel being used. Recently, Gales et al. found that the creep parameters obtained by Harmathy and Stanzak in the 1970s were out of date as the use of these parameters could not give accurate numerical results. They further identified the parameters through testing of prestressing steel to ASTM A417. This study further extended the work of Gales et al. Based on the steady state thermal creep and relaxation tests of prestressing steel to GB/T 5224 (Grade 1860) and BS 5896 (Grade 1860) over wide stress ranges, the parameters of Harmathy’s thermal creep model were identified and calibrated. Using the approach of Maljaars et al., the lower limit of tertiary creep was estimated and the creep model was further fine-tuned to incorporate tertiary creep. Numerical studies were conducted to examine the thermal creep and relaxation of prestressing steel at elevated temperatures using the enhanced creep model. The numerical predictions were found to agree well with the test results in respect of thermal creep and relaxation. In particular, predictions using the enhanced creep model with different sets of thermal creep parameters were compared with results of the thermal relaxation test conducted by MacLean, indicating different thermal creep resistance.