Investigation On The Effects Of Fiber Orientation On Bond-slip Behavior And Shear Performance Of CFRP Bar-reinforced UHPC Beams

Ultra-high-performance fiber-reinforced concrete(UHPC)structures reinforced with carbon fiber-reinforced polymer(CFRP)bars feature exceptional mechanical and durability properties,making them an ideal alternative for ordinary steel bar-reinforced concrete structures.However,the application of CFRP bar-reinforced UHPC structures in practical engineering is limited due to the unclear mechanism of load transfer between these two materials and the incomplete design codes for the shear behavior of CFRP bar-reinforced UHPC structures.This paper systematically investigated the bond-slip behavior and shear performance of UHPC beams reinforced with CFRP bars to provide valuable references for engineering practices.The primary research work and conclusions are outlined below.(1)A total of 36 specimens were subjected to pull-out tests,with the parameters of fiber orientation,fiber content,and diameter of the CFRP bar being taken into account.The failure modes of the specimens and the mechanisms of load transfer between these two materials were primarily focused on.The influence of the aforementioned factors on bond performance was systematically investigated.(2)Two models,a semi-empirical model and a theoretical model based on fiber-matrix discrete,were developed to predict the bond strength.The goodness-of-fit(R~2)of the semi-empirical model was 0.83,while the theoretical model had an average ratio of predicted to tested values of 0.93,with a standard deviation of 18.0%.These predicted results of the two models were in good agreement with the tested results,providing valuable references for evaluating bond strength.(3)Ten CFRP bar-reinforced UHPC beams without stirrups were carried out for shear strength.The experiments focused on the effects of fiber orientation and content on the failure modes,crack propagation,bending and shear crack loads,and shear strength of the beams.The overall performance,ductility,post-cracking carrying capacity,and stiffness degradation of the UHPC beams were investigated,along with an analysis of the fibers’shear transfer mechanism.(4)The UHPC beams exhibited a phenomenon where multiple cracks developed and propagated simultaneously after shear cracking due to the"bridging effect"of fibers.A distinct"creaking"sound,resulting from the fibers being pulled out from the matrix,can be continuously heard from the interior of the beams,and numerous small and dense cracks appear near the critical diagonal crack.Furthermore,the fiber orientation did not alter the failure mode of UHPC beams,but a decrease in fiber content shifted the beams from a semi-ductile to a more brittle diagonal-tension failure.(5)A theoretical model of shear capacity was developed for fiber-oriented UHPC beams based on fiber-matrix discrete.The mean ratio of theoretical values to experimental values for Method I(based on probability theory)and Method II(based on single fiber pull-out load-slip curve)were 1.07 and 1.13,with standard deviations of 4.81%4.86%,when the contribution of UHPC tensile strength was considered.The mean ratio of theoretical values to experimental values for Method I and Method II were 1.02 and 1.09,with standard deviations of 4.57%and 6.22%,when the tensile contribution of UHPC was not considered.These results indicate that the proposed model in this study can accurately predict the shear capacity of fiber-oriented UHPC beams.