火灾下自密实钢纤维混凝土损伤机理及力学性能研究

Damage mechanism and mechanical properties of steel fiber reinforced self-compacting concrete under fire

自密实钢纤维混凝土在满足结构可靠度的同时还能有效提高混凝土材料性能,是理想的耐高温建筑材料之一。为研究自密实钢纤维混凝土火灾高温下的损伤机理和力学性能变化规律,配制不同钢纤维掺入比的自密实钢纤维混凝土试件,并在火灾高温至800℃的条件下对混凝土试件分别开展超声波试验、SEM电镜扫描、轴心抗压强度试验,测试相应条件下损伤程度、微观变化特征和相对弹性模量,研究相应条件下试件的高温损伤情况和应力−应变规律。结果表明:自密实钢纤维混凝土高温损伤机理体现为水分子蒸发、水化产物分解和材料高温劣化引起的材料热损伤,以及不同材料差异热膨胀率引起的界面热损伤;高温作用下,自密实钢纤维混凝土,轴心抗压强度、弹性模量随温度升高而快速降低,温度达到800℃时,3种试件轴心抗压强度最大降幅分别达到11.3%、66.68%、69.05%,弹性模量分别降低至2.43、2.39和2.99 GPa;端钩型钢纤维对提高混凝土抗裂能力作用更为显著,端钩型与直线型钢纤维掺入比为1∶1时混凝土力学性能提升更为显著;基于过镇海本构方程,引入温度变量,建立了考虑温度效应的自密实钢纤维混凝土压缩本构方程,该本构可较好预测自密实钢纤维混凝土高温下的应力−应变关系,可为自密实钢纤维混凝土抗火设计提供参考。

Steel fiber reinforced self-compacting concrete(SFRSCC)stands out as an exemplary high-temperature resistant building material,effectively augmenting concrete properties while ensuring structural reliability.This study investigated the damage mechanisms and variations in mechanical properties of SFRSCC under high-temperature fire conditions to explore its behavior.Diverse SFRSCC specimens,featuring distinct steel fiber ratios,were meticulously prepared.Subsequently,ultrasonic testing,scanning electron microscopy(SEM)analysis,and axial compressive strength tests were conducted on these specimens exposed to temperatures reaching 800℃.Damage extent,microscopic change characteristics,and relative elastic modulus were evaluated to scrutinize high-temperature damage and stress-strain behavior.The findings reveal that the high-temperature damage mechanism of SFRSCC involves the evaporation of water molecules,decomposition of hydration products,material degradation,and interfacial thermal damage resulting from differential thermal expansion rates of distinct materials.Under high-temperature exposure,internal damage occurs in SFRSCC specimens,leading to a swift reduction in axial compressive strength,and elastic modulus with increasing temperature.At 800℃,the maximum reduction in axial compressive strength for the three specimen types is 11.3%,66.68%,and 69.05%,accompanied by corresponding reductions in elastic modulus to 2.43 GPa,2.39 GPa,and 2.99 GPa,respectively.The inclusion of hooked-end steel fibers significantly enhances concrete crack resistance,with the 1∶1 ratio of hooked-end to straight steel fibers exhibiting the most notable improvement in mechanical properties.By integrating temperature as a variable in the modified model by Guo Zhenghai,a compressive constitutive model for SFRSCC considering temperature effects was formulated.This model effectively predicts the stress-strain response of SFRSCC under high-temperature conditions.