AFRP Strengthening of Steel Tube Recycled Concrete Columns Load-Bearing Capacity Analysis

Literature Overview

This study by He Yuanyuan, Dong Jiangfeng, and Wang Qingyuan from Sichuan University investigates the axial compressive behavior of circular steel tube recycled concrete (RAC) short columns, with a particular focus on the effects of basalt fiber inclusion and external AFRP (Aramid Fiber Reinforced Polymer) wrapping. The research is supported by the National Natural Science Foundation of China (Grant 51408382) and the Ministry of Education Innovation Team Rolling Support Project (IRT_14R37). Published in the journal Concrete in 2019, the paper presents experimental results from 12 short column specimens, comprising 6 control specimens and 6 AFRP-strengthened specimens. The variables examined include the recycled aggregate replacement rate and the dosage of basalt fiber, with the ultimate goal of establishing a theoretical model for calculating the axial compressive load-bearing capacity of AFRP-strengthened steel tube recycled concrete columns.

Core Technical Content and Experimental Design

The experimental matrix was designed to isolate the effects of two primary variables: the recycled aggregate replacement rate and the basalt fiber content within the recycled concrete core. The steel tube outer casing provides lateral confinement to the concrete core, while the AFRP wrapping on the exterior provides additional confinement, creating a multi-layer confinement system. The basalt fiber, being an inorganic fiber with high tensile strength and thermal stability, serves to improve the crack resistance and toughness of the recycled concrete matrix. The recycled aggregate replacement rate directly affects the mechanical properties of the concrete, as recycled aggregates typically possess higher water absorption and lower strength compared to natural aggregates.

The study employed axial compression loading to determine the peak load, load-displacement curves, and axial and circumferential strain distributions. The strain measurements are critical for understanding the confinement mechanism, as the circumferential strain of the AFRP jacket directly correlates to the confining pressure exerted on the concrete core. The control specimens without AFRP wrapping serve as a baseline for quantifying the strengthening effect of the fiber-reinforced polymer jacket.

Key Findings and Technical Interpretation

The most significant finding is that the recycled aggregate replacement rate exerts the greatest influence on the axial compressive load-bearing capacity among all examined variables. This observation is consistent with the well-documented inferior interfacial transition zone (ITZ) characteristics of recycled aggregates, which result from the residual mortar adhering to the recycled aggregate surface. As the replacement rate increases, the ITZ area grows, leading to a progressive degradation of concrete strength and stiffness. The basalt fiber, while beneficial, cannot fully compensate for the strength loss caused by high recycled aggregate replacement rates.

The AFRP wrapping demonstrates a clear strengthening effect, increasing the ultimate load and enhancing the ductility of the columns. The confinement mechanism operates through the hoop strain of the AFRP jacket, which generates a radial confining pressure that counteracts the lateral expansion of the concrete under compression. The theoretical model developed in the study correlates well with experimental data, validating the applicability of the proposed calculation method for engineering design purposes.

Summary of Variable Effects

Engineering Practice Implications

From a practical standpoint, this research addresses a critical issue in sustainable construction: the use of recycled aggregates in structural applications involving steel tube concrete (SRC) columns. The findings suggest that while AFRP strengthening can partially mitigate the strength reduction caused by recycled aggregates, engineers should exercise caution when specifying high replacement rates for load-bearing SRC columns. The theoretical model provides a practical tool for predicting column capacity, which can be integrated into design workflows for retrofitting existing structures or designing new construction with recycled materials. The combination of basalt fiber and AFRP wrapping represents a viable strengthening strategy, though cost considerations and long-term durability of the AFRP system under environmental exposure should be evaluated in actual engineering applications.

Study Insights and Reflections

This study contributes valuable experimental data to the growing body of knowledge on recycled concrete in confined structural members. The emphasis on the recycled aggregate replacement rate as the dominant factor underscores the need for careful material characterization and quality control when specifying recycled aggregates. The theoretical model's good agreement with test data is encouraging, but further validation under more complex loading conditions, such as eccentric loading and cyclic loading, would strengthen the applicability of the proposed method. The integration of basalt fiber as a supplementary reinforcement is a practical approach that warrants further investigation into optimal dosages and fiber length-to-aspect ratio effects. Overall, this work provides a solid foundation for promoting the use of recycled materials in steel tube concrete structures while maintaining structural safety and serviceability.