Mechanical properties of prestressing steel in and after fire

Knowledge of the mechanical properties of prestressing steel at elevated temperatures and after cooling is essential to the fire resistance design and post-fire evaluation of the residual load-carrying capacity of prestressed concrete structures. Experiments were carried out using an accurate testing system for the development of empirical formulae to predict the deterioration of prestressing steel at elevated temperatures and after cooling. The helical structure of commonly used seven-wire strands allowed estimation of the mechanical properties of the strand based on those of the central wire. Only the central wire of the strand was thus tested, which enabled better clamping and control and hence more accurate measurements. Grade 1860 strands conforming to GB/T 5224, mostly used in Mainland China, and grade 1860 strands conforming to BS 5896, used in Europe and other countries, were tested. Consistent models for the reduction factors of various properties were developed. Some strands extracted from prestressed concrete specimens after fire testing were also tested for verification.