Study Of The Shear Resistance Of The UHPC-NC Interface

Ultra High Performance Concrete(UHPC),as a new type of cementitious composite material,has a good application prospect in the field of engineering repair.At present,there is still no consensus on the shear mechanism of the UHPC-NC interface,and there is a large gap between the calculation formulae in the current code and the test results,and most scholars only focus on the peak bearing capacity calculation method,and there are few calculation models for the normal use limit state.In view of the above deficiencies,this paper carried out experimental research on five groups of Z-type straight shear specimens,carried out simulation analysis of the shear resistance of the UHPC-NC interface using DIANA software,summarized the shear resistance mechanism of the UHPC-NC interface,and established the interface shear resistance calculation model.The main conclusions obtained from the above studies are as follows:(1)Shear reinforcement is a key factor influencing the damage mode of the specimens.Specimens with shear reinforcement show a ductile damage mode,while specimens without shear reinforcement have a brittle damage mode.The first peak load carrying capacity,i.e.the cracking load,is mainly influenced by the interfacial bond strength and is somewhat fluctuating,with no significant relationship to the reinforcement ratio,reinforcement diameter and yield strength of the reinforcement.(2)The reinforcement rate has a significant effect on the second peak load capacity of the specimen,which increases with the reinforcement rate.For the same reinforcement rate,the contribution to the second peak load capacity is greater as the greater number of fine reinforcement bars are more evenly distributed within the interface and are able to share the load better.The use of HRB600 grade reinforcement does not contribute significantly to the second peak load carrying capacity,as it is not possible to achieve ultimate strength with HRB600 grade reinforcement.(3)The analysis results of the simulation model show that there is a stress concentration in the area where the reinforcement is bonded to the interface,and cracks develop from the inside out,eventually appearing on the surface of the specimen.The bond strength has a significant effect on the first peak load bearing capacity,and the magnitude of the second peak load bearing capacity is determined by the action of the reinforcement.With the bond strength at the UHPC-NC interface being higher than the concrete shear strength,the specimen loadbearing capacity increases as the concrete shear strength grows.Face loading causes the bending stresses in the specimen to increase,and the stress distribution along the interface is more uniform in line loaded specimens.(4)The whole process of shear resistance at the UHPC-NC interface can be approximated into four stages,the first stage is the cracking stage,for low reinforcement rates it is the cracking and the first peak bearing capacity stage,the load is basically linearly related to the interface slip and shear reinforcement strain.The shear resistance is mainly borne by the bond at the interface.The second stage for the high reinforcement rate specimens is the first peak bearing capacity stage,where the shear resistance contribution is similar to that of the first stage,but as the crack width becomes larger,the contribution of the reinforcement also increases.For low reinforcement rate specimens the interfacial bond withdraws from the work,the load drops steeply and the shear resistance is provided by the vertical component of the reinforcement as well as the friction between the interfaces,with the reinforcement reaching yield at this stage.The third stage is the second rise of the load,where the slip of the interface has increased significantly,the shear resistance at this stage is mainly provided by the vertical component of the reinforcement after deformation,the ultimate strength of the reinforcement is reached at the second peak load carrying capacity,while the ultimate strength of the high strength reinforcement is not reached.The fourth stage is the damage stage,where the load reaches the second peak load carrying capacity and then instantaneously decreases,the specimen loses its load carrying capacity,the reinforcement is not sheared and the concrete protective layer is destroyed.(5)The first and second peak load carrying capacity equations based on the shear mechanism of the UHPC-NC interface are in good agreement with the results of this test.