Experimental Investigation On Flexural And Shear Behaviors Of Ultrahigh Toughness Cementitious Composite

Ultrahigh toughness cementitious composite (UHTCC), as a high performance cementitious composite, shows apparent tensile strain-hardening and fine crack dispersion capacity under uniaxial tension load, with the tensile strain above3%and crack width below0.1mm at ultimate tension state. Based on the outstanding tension property, nonlinear deformation and crack controlling capability, UHTCC can be extensively used in some structure members where the high ductility, durability and seismic resistance are needed, as well as in durability repair of concrete structures. Accordingly, the use of UHTCC in engineering structures has great potential. Based on the study on the basic properties and application, the following investigations on flexural and shear properties of UHTCC were further carried out:(1) The depth effect on flexural properties of UHTCC was studied through the four point bending test on UHTCC specimens with different depths. The experimental results revealed that when the depth of specimen was in the range of100mm, the flexural properties, such as flexural strength, neutral axis depth ratio, and crack width evolution etc., were almost independent of sample depth, and could be considered as intrinsic material performances.(2) Based on ASTM C1609and JCI-SF4, an improved toughness evaluation method was proposed, where a new toughness evaluation index and the equivalent deflection transformation relationship were induced. The theoretical and experimental results both demonstrated that the equivalent deflection transformation relationship enabled the toughness comparison between different depth series, and the proposed toughness index showed no depth effect for UHTCC material.(3) According to the moment-area method, the theoretical solution to the flexural deflection under four point bending load was obtained. Compared with the tested mid-span deflection, the calculated deflection based on the theoretical solution agreed well with the tested value, whereas the computed value based on the simplified equations showed a large difference from the tested value, which further indicated the rationality of theoretical solution.(4) The flexural-shear properties of RUHTCC beam were investigated through the bending experiment under one point loading. Compared with reinforced concrete (RC) counterpart beam, RUHTCC beam failing in typical flexure showed fine multiple-cracking pattern, outstanding deformation and ductility capability. However, the beams that failed in combined flexure and shear underwent a large flexural yielding deformation plateau before the formation of the critical shear crack. In particular, RUHTCC beam with reinforcement ratio of2.28%had a comparable mid-span deflection with RC beam with reinforcement ratio of0.67%. Based on the internal force equilibrium, the theoretical equations for determining the moment of inertia of fully cracked section of RUHTCC beam was proposed, and the calculated maximum deflection at service load according to such proposed equations agreed well with the tested value.(5) The shear behavior of RUHTCC beams without stirrups was investigated through the bending test, and the varied parameters included the shear span ratio and longitudinal reinforcement ratio. The experimental results indicated that the shear strength of RUHTCC slender beams was almost twice larger than that of RC beams, whereas the ultimate shear strength of RUHTCC short beam showed a small enhancement over corresponding RC beam.(6) The combined shear effect of UHTCC and stirrups was studied through the shear experiment of RUHTCC beams with web reinforcement. The tested results showed that the stirrup supplied in shear span of beam constrained the shear deformation of UHTCC, and thus the shear resistance could not be exerted totally.(7) Due to the bridging action of fiber across the diagonal cracks, the stress concentration on reinforcement was eliminated, which ensured the deformation capability between reinforcement and UHTCC. Consequently, the splitting crack along the longitudinal reinforcement was eliminated, resulting in a stable crack propagation mode. Regardless of the shear span ratio and reinforcement ratio, RUHTCC beams presented apparent multiple-cracking pattern in shear, with the maximum crack width of0.1mm at ultimate service state that satisfied the demand of crack width in durability design in crush environment.(8) Based on the experimental results and tested data reported in collected references, the empirical equations for predicting the ultimate shear strength of RUHTCC beams without web reinforcement was proposed through the regression analysis, and the predicted value based on such equations was in good agreement with the tested value. Such suggested equations were applicable to predict the ultimate shear strength of the beams made from strain-softening fiber reinforced concrete. Moreover, taking into account the shear contribution of stirrups and the combined effect between UHTCC and stirrup, the ultimate shear prediction formula was further suggested for RUHTCC beam with stirrups, and the calculated value almost agreed with the tested value.