Investigation On Shear Behavior Of Ultra High Toughness Cementitious Composites Applied In RC Beam

Ultra high toughness cementitious composites(UHTCC)macroscopically presents tensile strain hardening characteristic because of its multiple fine steady cracking and has therefore advantages of excellent crack control capacity,good ductility and high toughness compared to plain concrete.In general,it is believed that UHTCC has good potential for practical applications especially in the repair and strengthening engineerings with high cracking control requirements as well as in the key parts or key components of the structures with high ductility requirements.Therefore,the research work of this thesis is divided into three parts: firstly,the shear behavior of UHTCC/RC composited beams is investigated by using UHTCC replacing partial concrete in tension region,which will provide the references for the application of UHTCC in the repair and strengthening engineerings;secondly,in order to supplement the existing test data,the effect of beam height on shear behavior of reinforced UHTCC beam is mainly investigated,where plain concrete will be fully replaced with UHTCC;thirdly,analytical formulas to calculate shear capacity of the reinforced UHTCC beam are given with main purpose of providing a reference for the future shear design theory.The detailed research work and conclusions are as follows:(1)The effects of the stirrup ratio and the thickness of UHTCC layer on shear behavior of UHTCC/RC composited beams were experimentally examined.Tested results verify that the composite beams containing stirrups have the high maximum shear strength and post-cracking ductility.Also,the mode of the brittle diagonal tension-debonding shear failure occurred in the composited beams containing no stirrups,and however the ductile shear compression failure is obtained in the composited beams with stirrups even if close to the minimum stirrup ratio.In addition,it is found that there is still a need of taking other interface bonding measures to ensure good interface bonding,although the use of stirrups is effective in both delaying the occurrence of interface debonding and mitigating the degree of interface debonding.At last,the results also show that the highest shear strength,reserve shear strength,ductility as well as the well-controlled crack width are obtained in the UHTCC/RC composited beams with the UHTCC layer thickness of 1.5 times the concrete cover thickness.So,it is suggested that 1.5 times the concrete cover thickness can be an optimum to UHTCC layer thickness in UHTCC/RC composited beams.(2)The flexural tests under a center-point load were conducted on RUHTCC beams with and without stirrups and with the depths of 120 mm ~ 240 mm to investigate their shear behavior.The test results show that: 1)nominal ultimate shear strength of RUHTCC beams containing stirrups shows a linear decrease with an increase in the beam depth,and the decreasing degree is greater compared to the RUHTCC beams containing no stirrups;2)for RUHTCC beams without stirrups,normalized shear stress at the same diagonal crack width decreases as the beam depth increases when the diagonal crack width is larger than 0.13 mm,whereas before 0.13 mm such this trend is not significant.For RUHTCC beams with stirrups,if the minimum shear stirrups are provided,normalized shear stress at the same diagonal crack width presents a decreasing change with the increasing of the beam depth.While if the higher shear stirrups are provided,this change is observed only when the diagonal crack width exceeds 0.33 mm.3)a smaller normalized deformation energy is shown for the larger RUHTCC beams without stirrups,and this changing trend is not significant for RUHTCC beams with stirrups;4)RUHTCC beams without stirrups have the lower nominal shear cracking strength as the beam depth increases,and this effect is smaller if the stirrups are provided.(3)Based on truss-arch model and MCFT,the analytical formulas for calculating the shear bearing capacity of RUHTCC beam are respectively proposed with the consideration of the tensile contribution of cracked UHTCC.And,the shear bearing capacity of the tested 67 RUHTCC beams collected from the literatures is estimated,showing a good agreement with the tested values.In addition,the shear contribution to shear capacity,which is provided by the tension property of the cracked UHTCC,is examined based on the truss-arch model and the effects of shear-span ratio and stirrup ratio are subsequently discussed.It is found that a great shear contribution is shared by the cracked UHTCC and the degree increases as shear span ratio increases.For the RUHTCC beams with shear-span ratio of 2,the shear contribution has covered 45 percent of the total shear capacity.Also,the RUHTCC beams with higher stirrups ratio are found to show lower shear contribution ratio.