燃油火灾下预应力混凝土梁耐火试验

为研究预应力混凝土（PC）桥梁遭遇燃油火灾时的耐火性能，设计制作了3榀大比例PC简支缩尺模型试验薄腹梁，包括1榀箱形截面梁和2榀双T形截面梁，以荷载水平和截面类型为试验参数，开展了燃油火灾升温条件下PC梁局部受火试验。获取了梁截面混凝土温度和预应力钢束温度变化、跨中挠度变化、有效预应力衰变、裂缝开展、爆裂分布与深度以及耐火极限相关试验数据，深入探索了燃油火灾高温下PC梁的损伤演化规律和破坏模式。试验结果表明：梁截面各测点温度在受火期间随着受火时间的增加其整体趋势不断升高，由于水分的蒸发造成温度曲线在100 ℃~120 ℃之间有一明显的缓平段，箱形截面梁箱内温度在达到100 ℃后几乎保持不变。停火后，混凝土内部和预应力钢束温度持续升高，距受火面距离越远，在停火后升温持续时间越长，预应力钢束在停火后最高升温161 ℃。火灾下PC梁挠曲变形分为受火初期显著增长、受火中期缓慢增长和受火后期急速增长3个阶段，最终由于预应力钢束断裂表现出明显的脆性破坏特征。按常温下适筋梁设计的PC模型试验梁在火灾高温下呈现为少筋梁破坏特征；钢束的有效预应力在火灾高温下表现出先增加、后衰减，最后被拉断应力突然降低的三阶段变化特性。箱形闭口截面梁的混凝土温度和预应力钢束温度均低于双T形开口截面梁，其耐火性能明显优于双T形开口截面梁，破坏时预应力钢束临界温度分别为397 ℃和319 ℃。荷载水平由0.35增加至0.55时，火灾下PC梁耐火极限降低21%，破坏时预应力钢束临界温度由416 ℃降低至319 ℃。研究成果可为PC桥梁耐火试验提供方法指导，为其抗火设计和灾后应急提供理论依据。

Experimental Study on Fire Resistance of Prestressed Concrete Girders Under Fuel Fire Exposure

To study the fire resistance behavior of prestressed concrete (PC) bridge girders under fuel fire exposure, three large-scale simply supported PC test girders with thin walls were designed and manufactured, including one box-shape girder and two double T-shape girders. Taking the load level and section type as test parameters, the localized fire test of PC girders under fuel fire exposure was performed. Relevant experimental data, such as the temperature of concrete and prestressing strands, mid-span deflection, decay of effective prestress, crack development, distribution and depth of spalling, and fire resistance, were obtained. The damage evolution law and failure mode of the PC girders under fuel fire exposure were explored in detail. The experimental results show that the temperature of each measuring point increases with fire exposure time during fire exposure. Owing to the evaporation of water, the temperature curves have an obvious flat section between 100℃ and 120℃, and the inner temperature in the box girder remains almost unchanged after reaching 100℃. After the fire is stopped, the temperature of the internal concrete and prestressing strands continues to rise. The duration of the temperature rise increases with the distance from the fire exposure surface, and the maximum temperature rise of the prestressing strands is 161℃ after the fire is stopped. The flexural deformation of PC girders under fire exposure can be divided into three stages:significant increase at initial stage of fire exposure, slow increase at medium stage of fire exposure, and highly rapid increase at final stage of fire exposure. Finally, the PC girders show obvious brittle failure characteristics due to the fracture of prestressing strands. The PC test girders designed according to the balanced-reinforced girder under normal temperature show less-reinforced girder failure characteristics at high temperature. The effective prestress of prestressing strands is characterized by a three-stage variation under high temperature namely; increasing first, then decreasing, and finally suddenly decreasing due to fracture in prestressing strands. The temperatures in the concrete and prestressing strands of the box-shape girder are lower than that of the T-shape girder, and the fire resistance behavior of the box-shape girder is significantly better than that of the T-shape girder. The critical temperatures of the prestressing strands in the box-shape girder and T-shape girder at failure times are 397℃ and 319℃, respectively. When the load level increases from 0.35 to 0.55, the fire resistance of the PC girder decreases by 21%, and the critical temperature of the prestressing strands decreases from 416℃ to 319℃. The results of this study can provide method guidance for fire resistance tests of PC bridge girders and theoretical basis for fire resistance design.