钢纤维对超高性能混凝土徐变损伤与失效行为的影响

Influence of Steel Fibers on Creep Damage and Failure Behaviors of UHPC

为探明钢纤维对超高性能混凝土(UHPC)在高持久应力作用下的损伤与失效的影响,采用28天龄期的UHPC与普通混凝土试件开展了徐变损伤与失效试验。测试了各个试件加载全过程的轴向与环向应力应变,分析了其破坏模式、残余应变、徐变应变与名义泊松比。结合超声波无损检测与扫描电子显微镜手段,分析了UHPC内部微裂缝扩展与钢纤维与水泥基体的黏结损伤。结果表明:高持久应力的作用会导致UHPC与普通混凝土试件内部微裂缝扩展,引发构件横向膨胀,并最终导致构件破坏。UHPC中钢纤维的桥接约束效应可以很好地控制内部微裂缝扩展,从而限制了构件的横向膨胀。在持荷加载前,UHPC与普通混凝土具有类似的泊松比(0.18~0.19);在持荷破坏时,UHPC的最大泊松比为0.28,而普通混凝土的最大泊松比达到0.6。当持久应力水平超过0.70 f_(c)时,徐变损伤开始出现,具体表现为循环加载的强度与弹性模量下降。随着持久应力水平的提升,钢纤维与水泥基体的黏结出现损伤,钢纤维无法约束试件内部微裂缝的扩展,从而进一步加剧了试件损伤,甚至导致了试件的破坏。

To study the influence of steel fibers on damage and failure behaviors of ultra-high-performance concrete(UHPC)at short-term high sustained stress loading,an experimental program on creep damage and failure was proposed by using UHPC and normal concrete at the age of 28 days.The stress-strain relationship of each specimen was measured during the whole loading processing.The failure modes,irreversible strain,creep strain,and nominal Poisson’s ratio were analyzed according to the tested results.Based on the nondestructive ultrasonic test and scanning electron microscopy test(SEM),the expansion of internal microcracks within UHPC specimens and the bond damage between steel fibers and matrix were characterized.The results show the high sustained stress of UHPC and normal concrete lead to the development of internal microcracks and lateral expansion of specimens,and finally result in the creep failure of specimens.The bridging and confinement effects provided by steel fibers can control the development of internal micro-cracks,and reduce the lateral expansion of specimens.Before sustained loading,UHPC and normal concrete have similar nominal Poisson’s ratios of 0.18~0.19.After sustained loading,the nominal Poisson’s ratio of UHPC becomes 0.28,whereas the normal concrete specimens have a higher nominal Poisson’s ratio of 0.6.When the sustained stress was higher than 0.70 f_(c),creep damage of UHPC specimens occurs,which lead to the reductions in strength and elastic modulus at cycle of reloading.With the increment in sustained stress,the bond between steel fibers and matrix is damaged,and the steel fibers fail to confine the development of internal micro-cracks.As a results,the specimens are continuedly damaged and finally fail.