Research on deterioration of anchoring performance between anchorages to steel bars for prestressed concrete under fire

In order to research the deterioration of anchoring performance between anchorages to steel bars for prestressed concrete under fire, 65 prestressed anchorage systems were tested under elevated temperatures. The typical tested prestressed anchorage system was specifically designed, which composed by JXM-clip anchorage, 7 steel bar for prestressed concrete (with a characteristic strength f_ptk=970 MPa) with surrounding concrete and a loading frame. A total of 29 unsealed prestressed anchorage systems and 18 prestressed anchorage systems with insulation were conducted as transient-state tests, while 18 prestressed anchorage systems were conducted as steady-state tests. The test variables included stress level (0.2~0.7), temperature (20~800°C), insulation (sealing anchorages by grouting, fire resistive coating and sealing anchorages by grouting and coating) and heating rate (10.0, 26.7°C/min). Transient-state test results from unsealed prestressed anchorage systems show that the failure mode of prestressed anchorage system is through slippage of anchorages. Lower the stress level, temperature and heating rate are, the higher will be the failure time in prestressed anchorage systems. In steady-state tests, failure mode of prestressed anchorage system is rupture of steel bar for prestressed concrete in temperature range of 20~400°C, while failure mode is through slippage of anchorages in temperature range of 400~800°C. Anchorages after elevated temperatures can not be applied in new structures. Test results from prestressed anchorage systems with insulation show that failure mode of anchorage systems is through slippage of anchorages, and insulations do not change the failure mode at elevated temperatures. As compared with unsealed prestressed anchorage systems, the failure time of prestressed anchorage systems with insulations is increased above 100%. It is not safe to apply the recommendation of fire resistive coating thickness for steel structures in GB 14907-2002 'Fire resistive coating for steel structure' to prestressed anchorages in fire safety design.