Use of shear nails connector to improve UHPC-NC interface bonding performance of functional graded concrete components in offshore structure

This paper designed functional graded concrete components to meet the bearing and durability improvement subjected to marine environment, where the normal concrete (NC) core was used to bear loads and was protected by the ultra-high-performance concrete (UHPC) permanent formwork from chloride ion permeability. By inspiring of transfer printing technology and controlled permeability formwork, a new simple shear nails construction treatment on UHPC formwork was proposed for establishing a connection between UHPC formwork and NC core. In order to study shear nails construction measures on the interface bonding performance between UHPC and NC, UHPC-NC bonding specimens were subjected to double-sided shear tests using the density and distribution spacing of shear nails on the surface of UHPC formwork as experimental parameters. The results showed when the shear nail density ρ ≥ 6.4, the specimen failure transformed into axial compression failure of NC core. The shear stress on the bonding surface increases approximately parabolic with the density of shear nails. Under the same density, the shear stress on the bonding surface of specimens with larger nail spacing was enhanced by 19.04%-41.74% compared to specimens with smaller spacing. The results show that the density and the distribution spacing of shear nails have a significant impact on the shear strength of the UHPC-NC surface. The influence of shear nail density on the shear strength of UHPC-NC interface is greater than that of the distribution spacing of shear nails. Based on comparing and analyzing the existing shear strength models of UHPC-NC bonding surfaces, fully considering the failure mode of bonding surfaces, a calculation formula for the shear strength of prefabricated UHPC-NC bonding surfaces was established. The calculation results have a high degree of agreement with experimental values, which can provide reference for the interface design of UHPC-NC composite specimens.