# Bonding Performance of Steel Rebar Coated with Ultra-High-Performance Concrete
## 1 Introduction  
Reinforced concrete is a structure with sufficient strength and stiffness for various loads due to its excellent bonding strength between the concrete as the compression member and the rebar as the tensile member [1]. Due to the strong alkalinity of concrete, a passivation film is formed on the surface of the reinforcing bar to prevent fatal corrosion to the structure over a long period of time, while maintaining excellent adhesion to the rebar [2]. In particular, cracks occurring in the tensile part of reinforced concrete lower the stiffness of the member and induce the redistribution of internal stress. [3]. In order to delay the progress of cracks occurring at the interface between the reinforcing bars and the concrete and to maintain the integrity for a long time, it is necessary to form a sufficient bond at the interface. The bond properties at the interface are determined by the crack width, crack spacing, and tension-stiffening phenomenon occurring in the concrete cross-section, as well as the roughness of the attachment surface and the strength of the member [3, 4]. The bond stress, which is the shear stress acting along the reinforcing bar and the concrete surface, is transmitted to the concrete via chemical adhesion at the interface, friction, and the interlocking action between the nodes of the deformed rebar [5, 6, 7, 8, 9]. As a result,the sufficient bonding ability at the interface is one of the very important aspects in the structural behavior of reinforced concrete members. Insufficient bond capacity causes bending and deterioration of the shear performance, leading to premature failure of the structure [10]. With the continuous research and development (R&D) of concrete engineering technology, the number of cases involving the field application of ultra-high-performance concrete (UHPC) has been increasing recently [11; 12; 13; 14; 15; 16; 17]. Ultra-high-performance concrete has excellent strength (compressive strength of 150 MPa or more and tensile strength of 8 MPa or more) among the existing concretes, excellent ductility due to the mixing of steel fibers, low permeability of harmful substances due to minimized internal voids (high durability), and excellent material fracture performance [18; 19; 20; 21; 22; 23; 24; 25; 26]. It has strain-hardening characteristics, inducing multiple micro-cracks by mixing the steel fibers after cracking the concrete under maximum load, meaning it can be applied to various structures [27]. The study to improve the bond performance of ultra-high-performance concrete with steel or FRP bars was conducted by applying them to I-girders and deck joints, confirming their potential as tensile and flexural materials [28; 29; 30; 31]. GFRP rebar does not corrode, has an excellent relative strength-to-weight ratio, and is used as a substitute for existing rebar due to its low-shrinkage stress generation. However, there is a disadvantage in that the redistribution of stress during the drawing process is not smooth due to the relatively low bond strength compared to reinforcing bars [32; 33; 34; 35]. By replacing the concrete matrix with UHPC, a study was also conducted to improve the adhesion strength and ductility, with strong adhesion performance for the GFRP reinforcing bars, confirming the improvement of the adhesion-slip behavior due to the application of UHPC [36; 37; 38]. Although very positive research results have been confirmed for the application of UHPC to improve the performance of reinforced concrete structures, the economic feasibility is also an essential factor for its wide application in construction sites. Due to the material composition of UHPC, which is up to several times more expensive than general concrete, it has limited application in the field. In this study, a strong bond with the rebar was induced by thinly coating UHPC on the attachment surface of the rebar using the spray method.