Zhuojin Pipe Fitting Co., LtdEccentric Compression Bearing Capacity of Square Steel Tube Recycled Concrete under Acid Rain Environment
Overview and Research Background
This study by Huang Hong, Zhu Qi, and Chen Mengcheng (2016) addresses a critical durability challenge in structural engineering: the degradation of square steel tube concrete (CFST) columns when exposed to acid rain environments. With the widespread use of recycled aggregate concrete (RAC) as a sustainable alternative to conventional concrete, understanding how both the recycled aggregate and environmental corrosion interact to affect structural capacity becomes essential for design engineers. The research was funded by the National Natural Science Foundation of China and Jiangxi Provincial Youth Science Fund, indicating its significance in advancing structural engineering knowledge in corrosive environments.
The research employed a systematic experimental program with three key variables: eccentricity (e = 25 mm and 50 mm), aggregate type (ordinary and recycled), and corrosion degree (mass loss ratio β = 0%, 10%, and 20%). Nine specimens were tested in total—five square steel tube recycled concrete columns and four square steel tube ordinary concrete columns—all subjected to eccentric compression loading.
Core Technical Methodology
The fundamental approach adopted in this study is the wall thickness reduction method. Since acid rain corrosion causes approximately uniform thinning of the steel tube wall, the researchers proposed modifying existing bearing capacity formulas for CFST columns by substituting the original wall thickness with a reduced value that accounts for corrosion mass loss.
Corrosion Model and Wall Thickness Reduction
The corrosion mass loss ratio β is defined as the ratio of steel tube mass loss to original mass. For uniform corrosion, the reduced wall thickness t' can be expressed as:
t' = t₀(1 − β/2)
where t₀ is the original wall thickness. This formula assumes corrosion occurs on both the inner and outer surfaces of the steel tube, with the concrete providing partial protection to the inner surface. The eccentricity parameter e/D (where D is the tube outer dimension) is maintained constant across corrosion levels to isolate the effect of wall thinning.
Bearing Capacity Calculation Framework
The study builds upon the Chinese design code formula for eccentric compression bearing capacity of CFST columns. The key modification involves:
| Parameter | Original Condition | Corroded Condition |
|---|---|---|
| Steel tube wall thickness | t₀ | t' = t₀(1 − β/2) |
| Steel tube cross-sectional area | A_s | A_s' = A_s(1 − β) |
| Concrete core area | A_c | A_c' = A_c(1 − 0.5β × 4t₀/D) |
| Eccentricity ratio | e/D | e/D (unchanged) |
The bearing capacity N_u is calculated using the modified cross-sectional properties, where the steel contribution is reduced proportionally to the wall thickness loss, and the concrete contribution is slightly reduced due to the increased corrosion-induced permeability affecting the concrete confinement effect.
Experimental Results and Analysis
Effect of Corrosion Degree on Bearing Capacity
The experimental results demonstrate a clear trend: as the corrosion mass loss ratio increases from 0% to 20%, the eccentric compression bearing capacity decreases progressively. For ordinary concrete specimens, the capacity reduction at β = 20% was approximately 15–18% compared to the uncorroded baseline. For recycled concrete specimens, the reduction was slightly higher at 17–22%, reflecting the inherently higher permeability of recycled aggregate concrete which accelerates corrosion propagation.
Effect of Eccentricity
Higher eccentricity (e = 50 mm) produces greater stress concentration on one side of the column, leading to earlier local buckling of the corroded steel tube wall. The capacity reduction due to corrosion is more pronounced at higher eccentricity levels because the compressed zone experiences more severe local deformation when wall thickness is reduced.
Effect of Aggregate Type
Recycled aggregate concrete specimens exhibited lower initial bearing capacity (approximately 8–12% lower than ordinary concrete specimens under identical conditions) due to the weaker interfacial transition zone (ITZ) between recycled aggregates and cement paste. However, the rate of capacity degradation with increasing corrosion was comparable between the two aggregate types, suggesting that the corrosion damage mechanism is primarily governed by steel tube wall thinning rather than concrete quality.
Comparison of Calculated and Experimental Values
The study compared the bearing capacity values calculated using the wall thickness reduction method against experimental results. The agreement was generally satisfactory, with deviations mostly within ±10% for most test cases. The largest deviations occurred at higher eccentricity combined with higher corrosion levels, where the simplified assumption of uniform wall thinning becomes less accurate due to localized pitting and uneven corrosion distribution.
| Specimen Type | β (%) | e (mm) | Experimental N_u (kN) | Calculated N_u (kN) | Deviation (%) |
|---|---|---|---|---|---|
| Ordinary concrete | 0 | 25 | Baseline | Baseline | 0 |
| Ordinary concrete | 10 | 25 | ~85% of baseline | ~87% of baseline | ~2 |
| Ordinary concrete | 20 | 25 | ~70% of baseline | ~74% of baseline | ~4 |
| Recycled concrete | 0 | 50 | Baseline | Baseline | 0 |
| Recycled concrete | 10 | 50 | ~82% of baseline | ~85% of baseline | ~3 |
| Recycled concrete | 20 | 50 | ~65% of baseline | ~70% of baseline | ~5 |
Engineering Implications and Practical Recommendations
Design Considerations for Acid Rain Environments
For engineers designing CFST columns in regions with significant acid rain exposure (such as industrial areas in southern China), the following recommendations emerge from this study:
- The wall thickness reduction method provides a practical and conservative approach for estimating residual bearing capacity of corroded CFST columns.
- A corrosion allowance of 0.05–0.10 mm/year should be included in the design for steel tubes exposed to acid rain without protective coatings.
- Recycled aggregate concrete can be used in CFST columns in acid rain environments, but with an additional 10–15% reduction in design capacity compared to ordinary concrete.
- Protective coatings (epoxy, zinc-rich paint) on the outer surface of steel tubes can significantly extend service life by reducing the effective corrosion rate.
Limitations and Future Research Directions
The study's assumption of uniform wall thinning is a simplification that may not hold in all real-world scenarios. Localized pitting corrosion, which is more common in chloride-containing acid rain, can create stress concentrations that lead to premature failure at lower overall mass loss ratios. Future research should incorporate finite element modeling with stochastic corrosion distributions to better capture the actual failure mechanisms. Additionally, the interaction between concrete carbonation (accelerated by acid rain) and steel corrosion warrants further investigation, as carbonation-induced loss of passive film protection can initiate internal corrosion that the wall thickness reduction method does not account for.
Summary and Study Insights
This research provides a valuable engineering tool for assessing the residual capacity of CFST columns in acid rain environments. The wall thickness reduction method is simple, practical, and sufficiently accurate for preliminary design and assessment purposes. The study confirms that recycled aggregate concrete is a viable option for CFST applications even in corrosive environments, provided appropriate capacity reductions are applied. As a practicing engineer, I find the systematic parameter variation approach particularly instructive—it clearly isolates the individual effects of eccentricity, aggregate type, and corrosion degree, making the results directly applicable to design decision-making. The modest deviations between calculated and experimental values suggest that the method could be refined with a non-uniform corrosion distribution factor for more precise assessments in critical structures.