火灾作用下锚具对预应力钢棒锚固性能退化规律研究

为研究火灾作用下锚具对预应力（PC）钢棒锚固性能退化规律，完成65组由JXM型锚具、混凝土块、抗拉强度标准值为970 MPa的φ7高强PC钢棒及反力架组成的预应力锚固系统高温性能试验，其中29组为裸露锚具高温瞬态试验，18组为裸露锚具高温稳态试验，18组为带防火保护的锚具高温瞬态试验。研究PC钢棒应力水平（0.2~0.7）、温度（20~800℃）、防火保护措施（灌浆料封锚、涂抹防火涂料、灌浆料封锚后再涂抹防火涂料）和升温速率（10.0、26.7℃/min）对锚固性能的影响。裸露锚具高温瞬态试验结果表明：高温下锚固系统破坏模式为PC钢棒在锚具中发生滑移；PC钢棒的应力水平越高、温度越高、升温速率越大，锚固系统破坏时间越短。高温稳态试验结果表明：温度低于400℃时，锚固系统破坏模式为PC钢棒被拉断，温度高于400℃时，破坏模式为PC钢棒从锚具中滑移；高温后锚具不能再使用。带防火保护的高温瞬态试验结果表明：锚固系统破坏模式为PC钢棒从锚具中滑移，防火保护不改变锚固系统的破坏模式；与无防火保护措施相比，三种保护措施下锚固系统破坏时间均提高100%以上；若将GB 14907-2002《钢结构防火涂料》关于钢结构防火涂料厚度的要求直接应用于预应力锚具防火，将偏于不安全。

Research on deterioration of anchoring performance betweenanchorages 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, φ7 steel bar for prestressed concrete (with a characteristic strength f ptk=970MPa) 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℃), insulation (sealing anchorages by grouting, fire resistive coating and sealing anchorages by grouting and coating) and heating rate (10.0, 26.7℃/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℃, while failure mode is through slippage of anchorages in temperature range of 400~800℃. 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.