FRP-confined concrete columns with a stress reduction-recovery behavior: A state-of-the-art review, design recommendations and model assessments

Different from the typical bi-linear stress-strain response, many recent studies have reported a stress reduction -recovery response (i.e., a three-segment behavior of strain hardening-softening-hardening) for fiber reinforced polymers (FRP)-confined concrete in stub columns, which might have detrimental effects on column performance (e.g., excessive deformation or instability). Therefore, the purpose of this paper is to conduct a state-of-the-art review on the stress reduction-recovery behavior of FRP-confined concrete in stub columns, particularly focusing on exploring the underlying causes. Five major root causes for stress reduction have been identified: (1) concrete core brittleness; (2) concrete core shrinkage; (3) non-uniform confinement due to the column shape; (4) insufficient confinement due to the characteristics of FRP materials or fiber orientations; and (5) the arching action. In addition, current strategies for mitigating stress reduction have also been summarized. Although increasing FRP confinement level is a common method, it has little effect for FRP partially confined concrete, for which reducing the area of exposed concrete is more effective. Moreover, increasing steel fiber content in concrete could mitigate stress reductions for columns with brittle concrete cores, while increasing corner radius ratio is more efficient for columns with non-circular shapes (i.e., square and rectangular columns). Furthermore, the hybrid use of small and large rupture strain FRP wraps and the optimum stacking of FRP wraps with different fiber orientations could enhance the FRP confinement and thus alleviate the stress reduction. By establishing a new database containing 317 datasets with stress reduction-recovery response, the performances of six three -segment design-oriented models were assessed. Other than discussing the weakness and strength of these models based on the assessment results, future directions in deriving more accurate models for FRP-confined concrete columns were also outlined.

Automated summary

This paper conducts a review on the stress reduction-recovery behavior of Fiber Reinforced Polymers (FRP)-confined concrete in stub columns, focusing on exploring the underlying causes. Five major causes for stress reduction have been identified: concrete core brittleness, concrete core shrinkage, non-uniform confinement due to the column shape, insufficient confinement due to FRP materials or fiber orientations, and the arching action. Strategies for mitigating stress reduction have also been summarized.