Study On Interfacial Bonding Mechanical Properties Of NSC-UHPC Wet Joint

With the increase of service life,cracks and water seepage often occur between prefabricated bridge segments and wet joints,which leads to the decline of connection performance,especially the sharp decline of durability.The main reason for the above phenomenon is that the bonding properties of the new and old interfaces can not give full play to the mechanical properties of concrete materials.Ultra high performance concrete(UHPC)with excellent connection performance(cast-in-place)provides a feasible solution to the above problems.However,in recent years,the research on UHPC is only limited to the performance improvement of wet joint itself.the research on the interface properties between UHPC and NSC is relatively less,especially the lack of control parameters to simulate the interface mechanical properties in the finite element calculation.Therefore,in this paper,quasi-static test is carried out for the interface problem of NSC-UHPC,the factors affecting the mechanical properties of the interface are analyzed,and the relevant control parameters are obtained through calculation.(1)The tensile properties of NSC-UHPC interface were studied by splitting tensile test and bending tensile test.The effects of three kinds of roughness and two different interfacial agents were compared and analyzed.The results show that the increase of roughness can significantly affect the tensile strength of the interface: the average bonding strength of type Ⅰ(only removing the surface slurry layer)interface is 1.53 MPa,while the bonding strength of type Ⅱ and type Ⅲ(medium and coarse)interface treated by air hammer is between 1.72 MPa and 2.73 MPa.And the increase of roughness makes the cracking transition from the interface zone to the ordinary concrete gradually;choosing the appropriate interface agent can improve the tensile properties to a certain extent,but compared with the roughness,the effect is smaller.(2)The shear resistance of NSC UHPC interface was studied by direct shear and oblique shear test.In the direct shear test,the interface shear strength of NSC-UHPC with type Ⅲ roughness is the highest.In the oblique shear test,UHPC-UHPC interface shows better shear performance,which is 39% higher than NSC-UHPC interface(type Ⅲ).From the failure mode of interface,it can be inferred that the existence of interface steel fiber increases the interface friction and bond strength.Compared with the increase of interfacial shear strength,the increase of oblique shear test is higher than that of direct shear test.Compared with type Ⅰ NSC-UHPC interface,the shear strength of type Ⅱ and type Ⅲ interface increased by 21% and 76%,respectively,while that of direct shear test increased by 18% and 41.5%.(3)The tensile and shear properties of NSC-NSC interface,NSC UHPC interface and UHPC-UHPC interface were compared and analyzed.It is found that the interfacial bonding effect of UHPC as bonding material is better than that of ordinary concrete in both splitting test and oblique shear test.At the same time,the energy consumption of interface cracking increases with the increase of concrete strength on both sides of the interface.(4)Based on the interface control parameters obtained from the experiment,the finite element modeling analysis of Ⅰ,Ⅱ and Ⅲ interfaces is carried out.The results show that the finite element results are basically consistent with the experimental results.It shows that the interface parameters obtained in the test are effective and reasonable,which can provide the necessary basis for the actual design.