Axial Compressive Ultimate Bearing Capacity of Rectangular Steel Pipe Micro-Expansive Ceramsite Concrete Short Columns

Literature Overview

This paper by Zhu Hongbing, Zhao Benlu, Li Xiu, Yuan Qiangsong, and Hu Tianyu, published in the Journal of Wuhan University of Science and Technology in 2019 (Volume 42, Issue 1, pages 68-74), investigates the axial compressive ultimate bearing capacity of rectangular steel pipe columns filled with micro-expansive ceramsite concrete. The research was conducted at Wuhan University of Science and Technology School of Urban Construction, Wuhan Transportation Vocational College, and Hubei China Tobacco Cigarette Material Factory, supported by the National Natural Science Foundation of China (Grant 51778630) and Hubei Provincial Department of Education research program (B2017559).

Core Technical Content and Test Configuration

The study designed a comprehensive test matrix with three key design variables: rectangular steel pipe section aspect ratio (1, 1.5, 2), steel ratio (11%, 16%), and expansive agent content in ceramsite concrete (12%, 16%). A total of 12 groups of specimens were fabricated, with each group containing 2 columns, resulting in 24 test specimens in total. All specimens were short columns subjected to monotonic axial compression loading until failure.

The micro-expansive ceramsite concrete is a specialized concrete mix that incorporates an expansive agent to generate internal expansion pressure during hydration. This expansion pressure creates a pre-compressive state in the concrete, which enhances the confinement effect between the concrete core and the steel pipe, thereby improving the overall bearing capacity and ductility of the composite column. Ceramsite aggregate provides lightweight characteristics while maintaining adequate strength, making this composite system suitable for applications where weight reduction is desired without sacrificing structural capacity.

Key Findings and Technical Analysis

The test results revealed that the elastic limit of the specimens can reach approximately 90% of the ultimate load, indicating that the rectangular steel pipe micro-expansive ceramsite concrete columns exhibit excellent elastic behavior with limited plastic deformation before reaching peak capacity. This high ratio of elastic limit to ultimate load is advantageous for structural design, as it provides a large safety margin and predictable load-deflection behavior under service conditions.

The specimens exhibited a pronounced confinement effect, where the steel pipe restrains the lateral expansion of the concrete core, creating a triaxial stress state that enhances the compressive strength of the concrete. The bearing capacity enhancement coefficient was found to decrease with increasing section aspect ratio, which is consistent with the reduced confinement efficiency in rectangular sections compared to circular sections. In rectangular steel pipe sections, the corners provide less effective confinement than the mid-span regions of the flat faces, and as the aspect ratio increases, the proportion of poorly confined concrete increases, reducing the overall confinement effectiveness.

The bearing capacity enhancement coefficient increases with higher steel ratios, as expected, since a greater steel ratio provides more confinement pressure and direct load-bearing contribution from the steel pipe walls. The expansive agent content of 12% produced a larger bearing capacity enhancement coefficient compared to 16%, suggesting that excessive expansion pressure may lead to premature cracking or reduced bond quality between the concrete and steel pipe, ultimately diminishing the confinement benefit.

Based on the experimental data, the researchers proposed a modified formula for calculating the axial compressive bearing capacity of rectangular steel pipe micro-expansive ceramsite concrete short columns. The modified formula was developed by unifying the theoretical basis of several existing concrete-filled steel tube (CFST) bearing capacity formulas and incorporating corrections for the micro-expansive ceramsite concrete characteristics. The calculation results showed good agreement with experimental values, and the formula was deemed suitable for practical engineering applications.

Comparison of Existing Formulas and Modified Approach

The study compared the experimental results with several commonly used CFST bearing capacity formulas, including those proposed by Chinese code GB 50017, the Japanese AIJ code, the Eurocode 4 approach, and the Australian AS 4100 standard. Each formula has different assumptions regarding the stress-strain relationship of confined concrete, the effective confinement pressure, and the interaction between steel and concrete components.

The modified formula accounts for the unique characteristics of micro-expansive ceramsite concrete, including the internal expansion pressure that creates pre-compression in the concrete core, the reduced density of ceramsite aggregate, and the rectangular section geometry effects on confinement efficiency. This approach provides a more accurate prediction of bearing capacity compared to conventional CFST formulas that do not account for these specific material and geometric factors.

Engineering Practice Implications and Reflections

The rectangular steel pipe micro-expansive ceramsite concrete column system offers several advantages for practical engineering applications. The lightweight ceramsite concrete reduces the self-weight of the structure, which is beneficial for seismic design and foundation sizing. The micro-expansive characteristic ensures full filling of the steel pipe section without voids, which is critical for achieving the designed confinement effect. The rectangular section geometry accommodates architectural and layout requirements that are difficult to satisfy with circular columns.

From a steel pipe manufacturing perspective, rectangular steel pipe columns require careful attention to manufacturing quality. The rectangular profile is typically produced by cold forming or hot forming processes, with the latter preferred for thicker wall sections due to reduced springback and improved dimensional accuracy. Key quality control points include: seam weld quality verification through ultrasonic testing per GB/T 3375 or equivalent standards; dimensional accuracy of the rectangular profile including corner radius, side length, and wall thickness uniformity; straightness verification to ensure proper concrete filling during construction; and surface quality inspection to confirm absence of cracks, wrinkles, or deformation that could compromise structural performance.

The finding that 12% expansive agent content outperforms 16% has important implications for mix design optimization. Excessive expansion pressure can lead to internal cracking of the concrete, which reduces the effective confinement area and may cause debonding at the steel-concrete interface. Engineers should carefully calibrate the expansive agent dosage based on the specific concrete mix design, ambient conditions during curing, and the geometry of the steel pipe section. The optimal expansion rate should be in the range of 0.03% to 0.08% of the concrete volume, which corresponds to an internal expansion pressure of approximately 0.3 to 1.0 MPa.

The high elastic limit ratio (approximately 90% of ultimate load) observed in the test results has significant implications for structural design philosophy. This characteristic suggests that the columns can be designed with a higher utilization ratio of the ultimate capacity, leading to more economical designs. However, this should be balanced against serviceability requirements, including deflection limits, crack width control, and vibration performance, which may govern the design in certain applications.

The proposed modified formula provides a practical tool for engineers to design rectangular steel pipe micro-expansive ceramsite concrete columns with confidence. The formula's good agreement with experimental data validates the underlying theoretical approach and provides a foundation for code development or standardization of this composite system. Future research should extend the investigation to include long-term performance under sustained loads, fatigue behavior under cyclic loading, and fire resistance characteristics to provide a comprehensive design basis for this promising structural system.

Summary

The study by Zhu et al. provides comprehensive experimental data and a validated analytical formula for the axial compressive bearing capacity of rectangular steel pipe micro-expansive ceramsite concrete short columns. The key findings include the high elastic limit ratio, the pronounced confinement effect, the optimal expansive agent content of 12%, and the beneficial effect of higher steel ratios on bearing capacity enhancement. The modified formula offers a practical design tool that accounts for the unique material and geometric characteristics of this composite system, making it suitable for engineering applications where lightweight, high-capacity columns are required.