Study On Interface Bonding Performance Of Precast Keyway UHPC Permanent Template And Normal Concrete

This paper investigates the interfacial failure mode,strengthening mechanism,integrated bearing capacity,and coordinated deformation performance of composite members made of UHPC material as permanent formwork and cast-in-place concrete.The effects of keyway density,the strength of cast-in-place concrete,and curing conditions on interfacial bonding performance are studied through direct shear tests and splitting tensile tests.The influence of factors such as formwork thickness and cast-inplace concrete strength on the bending performance of composite beams is also explored.The research focuses on mechanical properties,load-slip curves,and bearing capacity model calculation.The main conclusions are as follows:(1)In the single-lap direct shear test,the failure modes were classified according to the failure mode of the NC layer.Mode I was the failure mode of the later cast NC layer,while Mode Ⅱ was the interface failure mode.The combination of the contact friction force between the bonding interface,the interaction between the cement matrix and aggregates in the NC layer,the keyway-aggregate interlocking mechanism,and the shear expansion provides the total shear strength of the UHPC-NC keyway bonding surface.As the spacing between the keyways decreases(not less than the maximum particle size of the aggregates),a higher proportion of the aggregate interlocking mechanism is utilized.Steam curing weakens the effects of the cement matrixaggregate interface and keyway-aggregate interlocking mechanism.The strain concentration area in the strain cloud diagram gradually evolved from the area parallel to the loading direction to the tilted area extending into the groove,ultimately forming the main crack along the keyway structure.The formula for the corrected shear strength of the UHPC-later cast NC interface with keyways was established.The calculated value was highly consistent with the experimental value,with most falling within the 90%confidence interval.The points outside the gap were mainly influenced by the age of the curing period.(2)In the splitting tensile test,the initial crack in the specimen extends horizontally from the loading axis to both ends,forming a through-type crack in the shape of the letter "I".The crack propagation path through the coarse aggregate causes it to fracture,leaving the NC matrix embedded inside the UHPC keyway.The strain along the center axis of the interface in the splitting tensile test is more significant than that on both sides,and the interface crack expands from the tensile strain region to the compressive strain region.The keyway causes the internal cracks in the concrete to extend from one end to the other without expanding inward from both ends.The bonding surface’s split tensile strength decreases as the NC strength increases.Steam curing has an adverse effect on the tensile bond strength.There is a significant linear relationship between the composite interface’s splitting tensile strength and the concrete cube’s compressive strength.The fitted calculation parameter is c=1.114(R2=0.976).(3)The laminated beam specimens have better control over crack propagation when subjected to bending loads;after the steel bars bear tensile force,a small amount of short and narrow cracks appeared in the bending-shear zone.No relative slippage of the bonding surface or debonding of the bonding occurs in the laminated beams,but fiber separation and local concrete crushing may arise in the midspan position.The beam has good flexibility,and the laminated beam has a specific residual bearing capacity after unloading.The load-deflection curves of the monolithic and laminated beams have similar changing patterns.Before a displacement of 6 mm,the beam was at its normal service limit state.The ductility coefficient of the laminated beam is mostly lower than that of the monolithic beam of the same size.The laminated beam approximately satisfies the basic assumption of the plane section.The ultimate strain of the laminated beam is lower than that of the monolithic beam,and its bottom strain decreases or even becomes negative after reaching the peak value.Under the same load,the steel bar strain of the UHPC permanent formwork laminated beam lags behind that of the comparison beam,and the effect of the suspended steel bars on the bending capacity of the laminated beam is negligible.(4)Based on the basic assumption of the calculation of the bending capacity of the cross-section,derive the calculation formula for the bending capacity of the permanently laminated beam(with an error of less than 10%).The finite element simulation of the UHPC laminated beam is close to the experimental results regarding the load-displacement curve and crack propagation.