Zhuojin Pipe Fitting Co., LtdAFRP-Reinforced Steel Tube Recycled Concrete Column Bearing Capacity Analysis
Literature Overview and Research Background
This 2019 paper published in "Concrete" (混凝土), authored by He Yuanyuan, Dong Jiangfeng, and Wang Qingyuan from Sichuan University, investigates the axial compressive bearing capacity of circular steel tube recycled concrete (RCR) short columns reinforced with externally bonded Aramid Fiber Reinforced Polymer (AFRP) wraps. The study is significant in the context of sustainable construction, where recycled aggregate concrete (RAC) is increasingly used to reduce the environmental impact of cement production and aggregate extraction. The researchers designed 12 test specimens, with 6 as control specimens and 6 as AFRP-reinforced specimens, varying the recycled aggregate replacement rate and basalt fiber content as independent variables.
Core Technical Findings
The experimental program was designed to isolate the effects of three variables on column performance: recycled aggregate replacement rate, basalt fiber dosage, and AFRP confinement. The key results are summarized below:
| Variable | Effect on Bearing Capacity | Effect on Axial Strain | Effect on Hoop Strain |
|---|---|---|---|
| Recycled aggregate replacement rate (increasing) | Most significant negative impact | Reduced ductility | Lower confinement effectiveness |
| Basalt fiber addition | Moderate improvement in toughness | Slightly increased ultimate strain | Improved post-peak behavior |
| AFRP wrapping | Significant increase in bearing capacity | Enhanced ductility | Effective confinement of concrete core |
The most important finding is that the recycled aggregate replacement rate has the greatest influence on specimen behavior. As the replacement rate increases, the interfacial transition zone (ITZ) between the recycled aggregate and the cement matrix becomes weaker due to residual mortar attached to the recycled particles. This weakened ITZ reduces the overall compressive strength of the concrete core, which in turn diminishes the effectiveness of both the steel tube and AFRP confinement.
Theoretical Analysis and Design Implications
The authors developed a theoretical model for calculating the axial compressive bearing capacity of AFRP-reinforced steel tube RCR short columns. The model considers the contributions of three components: the concrete core (with reduced strength due to recycled aggregate), the steel tube (providing passive confinement), and the AFRP wrap (providing additional active confinement). The theoretical predictions showed good agreement with experimental data, validating the approach.
For engineering design purposes, the following considerations are critical:
- Recycled aggregate quality — the source and washing procedure of recycled aggregate significantly affect the ITZ quality.
- AFRP wrap configuration — the number of layers, fiber orientation, and overlap details determine the confinement pressure.
- Basalt fiber dosage — an optimal fiber volume fraction exists beyond which workability decreases and strength gains plateau.
The study demonstrates that AFRP reinforcement can partially compensate for the strength loss caused by recycled aggregate replacement, but it cannot fully restore the performance to that of natural aggregate concrete columns. Engineers must therefore adopt a balanced approach: selecting an appropriate replacement rate based on structural requirements, and supplementing with AFRP reinforcement where additional capacity or ductility is needed.
Engineering Practice Integration
In practical applications, steel tube RCR columns may be used in low-to-medium-rise buildings, temporary structures, or rehabilitation projects where sustainability is a priority. The following engineering considerations should guide implementation:
- AFRP wrapping must be applied to the exterior of the steel tube, which means the steel tube surface must be properly prepared (cleaned, roughened, and primed) to ensure reliable bond.
- The confinement effectiveness of AFRP is reduced when the steel tube already provides significant passive confinement, so the incremental benefit must be evaluated for each specific geometry.
- Long-term durability of AFRP in the presence of recycled concrete (which may have higher permeability) requires attention to moisture ingress and potential degradation of the resin matrix.
Study Insights and Reflections
This research contributes valuable data to the growing body of knowledge on sustainable structural systems. The finding that recycled aggregate replacement rate dominates the performance degradation is consistent with other studies on RAC, and it reinforces the need for quality control in recycled aggregate production. The successful application of AFRP reinforcement to steel tube RCR columns opens new possibilities for strengthening existing structures without adding significant weight or cross-sectional area. However, engineers should note that the study was conducted on short columns, and the behavior of slender columns with higher slenderness ratios may differ, particularly regarding buckling modes and the interaction between AFRP confinement and steel tube buckling. Future research should extend to longer columns and include cyclic loading tests to evaluate seismic performance.