An Experimental Study:Various Influence Factors Affecting Interfacial Shear Performance Of UHPC-NSC

Reinforced concrete(RC)has been widely used in building structures at present.With the increase of service years,the RC structures are gradually aging with different degrees of damage(even failure)due to the impact of external loads and environmental variables.The damaged RC structures significantly reduce the safety and durability of the structure.Ultra-high performance concrete(UHPC)is an innovative cement-based material with high tensile and compressive strengths,extremely low permeability,and excellent durability.It is suitable for repair and rehabilitation of damaged normal strength concrete(NSC)structures.However,in order to ensure that the UHPC layer co-work well with the NSC structure,a good shear bond between UHPC and NSC is the main concern.In this paper,the shear properties of UHPC-NSC interface were systematically studied through a series of double-sided direct shear tests,and the shear capacity and corresponding interfacial failure modes were obtained.Different factors affecting the shear capacity of the UHPC-NSC interface were discussed,namely,strength of NSC,ages of UHPC on loading,curing regimes for UHPC,surface treatment of NSC substrate,wettability of NSC substrate,addition of expansive agent for UHPC,and exertion of tensile or compressive stress on the interface except for shear stress.The main conclusions are as follows:(1)The test results show that the interface between the UHPC layer and NSC substrate exhibited an excellent bond behavior.Most specimens failed in a mode of partial interface failure + partial NSC failure or complete NSC failure in shear,and the remaining few specimens developed purely interface failure.Also,shear failure of some test specimens performed a certain degree of brittleness.(2)According to the test results,the NSC strength grade and wettability have significant influence on the shear capacity of UHPC-NSC interface,the higher the strength or wettability of NSC substrate,the greater the shear capacity of the UHPC-NSC interface.(3)The shear capacity of UHPC-NSC interface increased rapidly when the age of UHPC was 3 days to 7 days,and at an age of 7 days,the shear strength of UHPC-NSC interface could reach more than 90% of the final shear strength,but the interface shear strength at the middle and later ages(28 days and 180 days)shows slight change.(4)It was revealed that the shear strength of UHPC-NSC interface with steam curing for UHPC at a temperature of 60℃ is basically the same as that with normal curing,but higher curing temperature up to 90℃ will reduce the shear capacity of the UHPC-NSC interface.(5)The addition of expansive agent to UHPC slowed down the development of shear capacity of UHPC-NSC interface at the early age,and slightly reduced the shear capacity at the later age compared with the specimens without expansion agent in the UHPC.(6)Roughing NSC surfaces(Ro)could significantly improve the interfacial shear capacity.Moreover,the number and area of UHPC tenons in the holes,UHPC grooved joints or post-installed rebars could also obviously affect the shear capacity of the interface.The interfave with grooved joints and post-installed rebars shows ductile failure,the smooth interface has the worst shear capacity.(7)The shear capacity of UHPC-NSC interface under combined tensile and shear stresses decreased significantly,while the shear capacity under combined compressive and shear stresses increased obviously with increment of shear capacity gradually decreased as the increase of normal compressive stress.