Eccentric Compression Capacity Calculation Method for Square Concrete-Filled Steel Tube Recycled Concrete Columns Under Acid Rain Environment

Literature Overview

This 2016 study published in the journal Concrete by Huang Hong and colleagues from East China Jiaotong University and State Grid Jiangxi Electric Power Company investigates the eccentric compression capacity of square concrete-filled steel tube (CFST) columns made with recycled concrete under acid rain corrosion. The research was funded by the National Natural Science Foundation of China (51378206; 51008122) and the Jiangxi Provincial Youth Science Fund (20143ACB21020). The study conducted eccentric compression tests on 5 CFST recycled concrete specimens and 4 CFST ordinary concrete specimens, with eccentricity (e = 25, 50 mm), aggregate type (ordinary, recycled), and corrosion degree (β = 0, 10%, 20%) as the main variables.

Core Findings and Technical Interpretation

The study proposes a practical calculation method for the eccentric compression capacity of CFST recycled concrete columns under acid rain corrosion by reducing the steel tube wall thickness. The key finding is that the corrosion process causes approximately uniform thinning of the steel tube wall, and the existing eccentric compression capacity formulas for CFST ordinary and recycled concrete columns can be applied with a reduced wall thickness to accurately predict the capacity under acid rain conditions.

Test Parameters and Variables

Corrosion Effects on Steel Tube

The study found that acid rain corrosion causes approximately uniform thinning of the steel tube wall. This is consistent with the general behavior of carbon steel in acidic environments, where the corrosion mechanism is primarily uniform attack rather than localized pitting. The uniform thinning simplifies the calculation of the residual capacity, as the cross-sectional properties can be easily adjusted by reducing the wall thickness.

Proposed Calculation Method

The proposed calculation method involves:

1. Determine the corrosion degree (β) from the mass loss of the steel tube.
2. Calculate the reduced wall thickness: t' = t × (1 - β), where t is the original wall thickness.
3. Apply the existing eccentric compression capacity formula for CFST columns using the reduced wall thickness t'.
4. The calculated capacity should be compared with the test results to verify the accuracy of the method.

The study found that the calculated capacities using the reduced wall thickness method are in good agreement with the test results, confirming the validity and applicability of the proposed method.

Welding and Fabrication Considerations

Welding Quality for Recycled Concrete CFST Columns

The fabrication of CFST recycled concrete columns requires careful attention to the welding quality of the steel tube, as the welds are critical for the structural integrity of the column. The following welding considerations are important:

• The steel tube material should be selected based on the expected corrosion environment. For acid rain environments, corrosion-resistant steel grades such as Q345qE or weathering steels may be appropriate.
• The welding procedure should be qualified per GB/T 9445 or ISO 9606-1 for the specific steel grade and wall thickness.
• The weld metal should be compatible with the base metal and provide adequate corrosion resistance.
• Post-weld inspection should include non-destructive testing (NDT) using magnetic particle testing (MT) or ultrasonic testing (UT) to detect surface and subsurface defects.

Effect of Corrosion on Weld Quality

The study does not specifically address the effect of corrosion on the weld quality, but it is important to note that corrosion can affect the weld metal and the heat-affected zone (HAZ) differently from the base metal. The HAZ may be more susceptible to corrosion due to the microstructural changes induced by welding. Engineers should consider the following:

• The corrosion resistance of the weld metal should be verified through coupon testing in the expected corrosion environment.
• The HAZ should be evaluated for corrosion susceptibility, particularly in acidic environments.
• Protective coatings or cathodic protection may be required for the welds to ensure long-term durability.

Engineering Practice Implications

Design Recommendations

Based on the research findings, the following design recommendations can be made:

• For CFST recycled concrete columns in acid rain environments, the eccentric compression capacity should be calculated using the reduced wall thickness method.
• The corrosion degree should be estimated based on the expected service life and the severity of the acid rain environment.
• The design should include adequate corrosion allowance to account for the expected wall thickness reduction over the service life.
• Regular inspection and maintenance should be scheduled to monitor the corrosion progress and ensure structural safety.

Connection to Standards

The design of CFST recycled concrete columns should comply with relevant standards, including:

• GB 50936-2014 for design code of concrete-filled steel tube structures.
• GB/T 50137-2012 for specification for design of concrete structures with recycled aggregates.
• GB/T 28887-2012 for recycled concrete aggregates.

Key Questions and Reflections

Limitations of the Study

The study was based on a limited number of test specimens and a specific range of eccentricity and corrosion degree values. The proposed calculation method should be validated with additional experimental data for different column dimensions, steel grades, and corrosion environments. Additionally, the study did not consider the effect of corrosion on the concrete core, which may also be affected by acid rain.

Independent Thinking

From a materials science perspective, the uniform thinning of the steel tube wall under acid rain corrosion is consistent with the general behavior of carbon steel in acidic environments. However, the actual corrosion rate may vary depending on the specific composition of the acid rain, the steel grade, and the environmental conditions. Engineers should consider the following:

• The corrosion rate may be higher in the presence of chlorides or sulfates, which are common components of acid rain.
• The corrosion rate may be affected by the presence of protective coatings or cathodic protection.
• The corrosion rate may vary over time due to changes in the environmental conditions.

Summary

This study provides a practical and validated calculation method for the eccentric compression capacity of square CFST recycled concrete columns under acid rain corrosion. The method involves reducing the steel tube wall thickness based on the corrosion degree and applying the existing eccentric compression capacity formula. The study confirms that the corrosion process causes approximately uniform thinning of the steel tube wall, simplifying the calculation of the residual capacity. Engineers should use this method for the design of CFST recycled concrete columns in acid rain environments, but careful attention must be paid to the welding quality of the steel tube and the long-term corrosion resistance of the welds and the heat-affected zone.