Published in Smart Construction, this study investigates the cyclic bond behavior of fiber reinforced polymer (FRP) bars—an area vital to seismic design yet previously underexplored. By examining carbon (CFRP), glass (GFRP), and basalt (BFRP) fiber reinforced polymer bars under reversed cyclic loading, the research quantifies how bar diameter, embedment length, concrete strength, and rib geometry influence initial bond stiffness, unloading strength, frictional resistance, and energy dissipation. A unified bond stress–slip constitutive model and hysteresis framework are developed to capture interfacial degradation mechanisms under cyclic loads. These contributions offer key insights for improving the seismic performance and reliability of FRP-reinforced concrete structures in earthquake-prone regions.
This study uses physical test methods to systematically study the evolution of bonding performance of different types of fiber reinforced polymer (FRP) bars under cyclic loading. Through positive and negative cyclic pull-out tests on three types of FRP bars, carbon fiber (CFRP), glass fiber (GFRP) and basalt fiber (BFRP), the effects of bar diameter, anchorage length, concrete strength and surface rib shape on bonding stiffness, unloading strength, friction and energy dissipation capacity are comprehensively analyzed.
1. Experimental design
- Material type: Three representative FRP bars, CFRP, GFRP and BFRP, are selected to study the differences in their bonding behavior under cyclic loading.
- Control parameters: Multiple groups of bar diameter, anchorage length, concrete strength grade and rib shape feature combinations are set to systematically examine the effects of various factors on bonding performance.
- Loading method: Apply positive and negative cyclic displacement loading to obtain a complete load-slip hysteresis curve to characterize the degradation characteristics of bonding performance.
2. Key Parameter Analysis
The effects of key influencing parameters—bar diameter, embedment length, concrete compressive strength, and surface rib geometry—were investigated through reversed cyclic pull-out tests to evaluate the bond behavior between FRP bars and concrete. Bond performance indicators such as initial stiffness, unloading strength, frictional resistance, and energy dissipation were analyzed. Main findings include:
- Bar diameter: An increase in diameter generally reduces bond stress due to a lower specific surface area, resulting in decreased frictional resistance and energy dissipation under cyclic loading.
- Embedment length: Greater embedment length enhances anchorage capacity and improves unloading stiffness, but after a threshold, the bond performance gain becomes marginal.
- Concrete compressive strength: Higher concrete strength improves initial stiffness and peak bond strength, while also delaying interface degradation during cyclic loading.
- Rib geometry: Well-defined surface ribs significantly enhance mechanical interlock, improving cyclic bond performance; however, overly aggressive ribs may lead to stress concentration and early interface damage.
3. Constitutive Model and Hysteresis Framework
This study reveals that the bond–slip behavior between FRP bars and concrete under cyclic loading is governed by complex interfacial degradation mechanisms, including frictional loss, stiffness reduction, and progressive slip accumulation. Through systematic reversed cyclic pull-out testing, the evolution of bond performance across different FRP types and influencing parameters was quantified.
To capture these mechanisms, a unified bond stress–slip constitutive model was proposed, incorporating distinct loading, unloading, and reloading branches. The model reflects nonlinearity in initial stiffness, residual strength after unloading, and energy dissipation via slip-dependent degradation rules. A corresponding hysteresis framework was developed to describe the full cyclic response, including pinching effects and strength decay over multiple load cycles.
The proposed model significantly improves prediction accuracy for bond behavior under seismic-like loading and serves as a foundational tool for nonlinear simulation of FRP-reinforced concrete structures. It also lays the groundwork for integrating interfacial damage mechanics into performance-based seismic design. Future work will focus on extending the model to full-scale structural elements and validating it against dynamic loading conditions.
This paper " Bond behavior of FRP bars in concrete under reversed cyclic loading: an experimental study" was published in Smart Construction.
Li B, Li D, Chen F, Jin L, Du X. Bond behavior of FRP bars in concrete under reversed cyclic loading: an experimental study. Smart Constr. 2025(2): 0013, https: //doi. org/ 10. 55092/ sc20250013.
