Bond behavior of steel, CFRP, and GFRP reinforcement in SCC and FRC under varying diameter, embedment length, and confinement

The bond behavior of fiber-reinforced polymer (FRP) and steel rebars in advanced cementitious composites is governed by a complex interplay of rebar type, concrete matrix, confinement, embedment length, and geometric parameters, yet it remains insufficiently understood. This study presents a systematic experimental investigation of 306 RILEM beam specimens to evaluate the bond performance of steel, glass FRP (GFRP), and carbon FRP (CFRP) bars embedded in self-compacting concrete (SCC) and fiber-reinforced concrete (FRC) with steel, polypropylene, and glass fibers. Using a consistent flexural bond testing framework, the integrated effects of rebar diameter, embedment length, bottom concrete cover, transverse reinforcement, fiber type, and fiber content were assessed. Results show that bond strength and failure modes are strongly influenced by rebar diameter and embedment length, with FRP bars displaying distinct responses due to their linear-elastic behavior and surface characteristics. Bottom concrete cover, transverse reinforcement, and fiber-induced confinement critically enhance stress transfer and mitigate splitting failures. Steel and glass fibers significantly improve bond performance via crack bridging and confinement, whereas polypropylene fibers mainly reduce shrinkage-induced cracking. The findings advance understanding of size effects, stress redistribution, and confinement efficiency in SCC and FRC systems and provide a robust experimental basis for developing improved bond models and design guidelines for FRP-reinforced concrete structures.