Eccentric Compression Behavior of Square Steel Tube Recycled Concrete Under Acid Rain Environment

Literature Overview

The paper by Huang Hong, Hu Zhi-Hui, Zhu Qi, and Chen Meng-Cheng (East China Jiaotong University and Jiangxi Power Company, 2018) presents experimental and numerical investigation of square steel tube concrete (SSTC) columns incorporating recycled aggregate concrete, subjected to eccentric compression after acid rain corrosion. The study examines the effects of eccentricity ratio, concrete type (recycled vs. normal), and acid rain corrosion rate on the mechanical performance of SSTC members. Nine test specimens were evaluated, providing a comprehensive database for understanding the degradation mechanisms.

Experimental Program and Test Configuration

The test program was designed to isolate the effects of three key variables:

The acid rain corrosion was simulated using Faraday electrochemical corrosion method, which provides controlled and reproducible corrosion conditions. The corrosion rates achieved correspond to approximately 10-30 years of natural acid rain exposure in industrial regions of China.

Key Findings and Technical Analysis

Corrosion Progression and Load Capacity Degradation

The study confirms that the acid rain corrosion follows Faraday's electrochemical corrosion law with good agreement. The degradation of structural capacity with increasing corrosion rate follows a predictable pattern:

• Ultimate load capacity decreases approximately linearly with corrosion rate
• Initial stiffness (pre-peak) decreases more rapidly than ultimate capacity
• Ductility (measured as post-peak deformation capacity) degrades progressively

Effect of Concrete Type

A notable finding is that the concrete type (recycled vs. normal) has limited influence on failure mode, ultimate load capacity, and initial stiffness. However, the recycled aggregate concrete-filled steel tube exhibits slightly lower ductility compared to normal concrete-filled steel tubes under equivalent conditions. This is attributed to the higher porosity and weaker interfacial transition zone of recycled aggregate concrete.

Effect of Eccentricity

As eccentricity increases:

• The relative vertical compressive strain at peak load increases
• Initial stiffness decreases
• The transition from compression-dominated failure to bending-dominated failure becomes more pronounced
• The steel tube on the tension side may experience local buckling before the concrete on the compression side reaches its ultimate strain

Finite Element Modeling Approach

The finite element model employed the following constitutive relationships:

• Steel tube: Von Mises yield criterion with bilinear kinematic hardening
• Concrete: Modified Kent-Park model accounting for confinement effect
• Interface: Cohesive zone model with bonded-slip behavior
• Corrosion degradation: Reduction in steel cross-sectional area and concrete cover thickness based on corrosion rate

The overall agreement between FE predictions and experimental results validates the modeling approach for engineering applications.

Engineering Practice Implications

From a steel pipe manufacturing and welding quality perspective:

• Corrosion protection of steel tubes: The acid rain environment accelerates steel tube corrosion. For applications in industrial or coastal regions, the use of hot-dip galvanized steel tubes (per GB/T 13912) or the application of corrosion-resistant coatings is essential. The galvanized layer thickness should be at least 85 μm per side for moderate corrosive environments and 120 μm per side for severe environments.
• Welding considerations: For square steel tubes, the longitudinal weld is a critical location for corrosion initiation. The heat-affected zone (HAZ) of the longitudinal weld may exhibit reduced corrosion resistance due to microstructural changes. Post-weld heat treatment or the use of corrosion-resistant welding consumables is recommended.
• Recycled concrete quality control: The recycled aggregate used in the concrete should meet the requirements of GB/T 25177 (Recycled coarse aggregate for concrete and mortar). Key quality indicators include: crushing value index, water absorption rate, and sulfate content.

Defect Analysis and Countermeasures

Key Questions and Reflections

The study raises important questions about the long-term behavior of recycled concrete in CFST under combined environmental and mechanical loading. While the Faraday electrochemical corrosion method provides controlled laboratory conditions, the actual acid rain environment involves complex chemical interactions including sulfate attack, chloride ingress, and carbonation that may not be fully replicated.

The relatively limited influence of concrete type on ultimate capacity is encouraging for the use of recycled aggregate concrete in CFST applications, as it demonstrates that the steel tube confinement effectively compensates for the reduced concrete quality. However, the ductility reduction warrants careful consideration in seismic design, where energy dissipation capacity is paramount.

Study Insights and Conclusions

This research provides valuable data for the design of CFST structures in corrosive environments, particularly for applications using recycled aggregate concrete. The finding that concrete type has limited effect on ultimate capacity while slightly affecting ductility suggests that recycled aggregate concrete can be used in CFST applications with appropriate design modifications, such as increased steel tube wall thickness or enhanced detailing at critical sections. The finite element modeling approach validated in this study can be adapted for practical design calculations, incorporating corrosion degradation factors based on the service environment. For steel pipe manufacturers, the key takeaway is that corrosion protection of the steel tube is the most effective strategy for maintaining long-term structural performance, as the tube confinement is the primary mechanism providing ductility and post-peak capacity to the composite member.