Bending Performance of Hollow Sandwich Aluminum Tube-Concrete-Steel Tube Composite Members

Literature Overview

This study published in Concrete (2025, No. 6, pp. 5-10) by Ning Chunzhen, Zeng Xiang, Zeng Weizhe, and Xu Bin investigates the flexural mechanical properties of hollow sandwich aluminum tube-concrete-steel tube (ACSDST) composite members. Funded by the National Natural Science Foundation of China (52268024), Hainan Provincial Natural Science Foundation (521RC504), and other sources, the research was conducted at Hainan University and Huaqiao University. The paper presents four-point bending test results on ACSDST beam specimens and compares the calculated flexural stiffness and bending capacity with existing international code provisions.

Core Technical Findings

Specimen Configuration and Test Methodology

The ACSDST composite member consists of an outer hollow aluminum tube, an inner steel tube, and concrete filling the annular space between them. This triple-layer composite cross-section is designed to combine the lightweight characteristics of aluminum with the high strength of steel and the compressive capacity of concrete. The four-point bending test configuration ensures a constant moment region in the central span, allowing for accurate measurement of flexural stiffness and ultimate capacity.

Failure Mode Analysis

The failure of ACSDST specimens occurred due to cracking of the tension-side aluminum tube (outer tube). After failure, the compression-side aluminum tube exhibited varying degrees of outward bulging. This failure mode indicates that:

• The aluminum tube governs the ultimate flexural capacity of the composite section
• The steel tube and concrete primarily contribute to stiffness and compressive resistance
• The composite action between the three materials is effective up to the point of aluminum tube cracking

Flexural Stiffness Comparison with Code Provisions

The study compared the experimental flexural stiffness with predictions from various international codes:

The bending capacity calculation formula derived by combining the aluminum tube concrete composite axial compressive strength formula with the hollow sandwich steel tube concrete bending capacity formula provides good predictions for the ACSDST section bending capacity.

Engineering Practice Insights

Material Compatibility and Interface Behavior

From a metallurgical and materials science perspective, the ACSDST composite member presents interesting challenges:

• The coefficient of thermal expansion mismatch between aluminum (approximately 23 x 10^-6 /K) and steel (approximately 12 x 10^-6 /K) must be considered in design, particularly for structures exposed to significant temperature variations.
• The galvanic corrosion potential difference between aluminum and steel in the presence of concrete pore solution requires careful attention. The concrete layer serves as a physical barrier, but any cracks in the concrete could expose the steel tube to corrosion in the presence of aluminum.
• The bond interface between aluminum and concrete, and between steel and concrete, governs the composite action. Shear connectors or mechanical interlocking features may be necessary to ensure full composite behavior.

Welding and Fabrication Considerations

The fabrication of ACSDST members involves several critical welding and forming operations:

Design Recommendations

Based on the test results, the following design recommendations can be made:

1. The initial flexural stiffness should be calculated using the EC4 (2004) methodology, which accounts for the elastic properties of all constituent materials without considering cracking or slip effects.
2. For serviceability limit state checks, the AIJ (1997) approach provides the most accurate prediction of the actual flexural stiffness under sustained loads.
3. The ultimate bending capacity should be calculated using the combined formula that integrates aluminum tube concrete composite axial strength with the hollow sandwich steel tube concrete bending formula.
4. The failure mode being governed by aluminum tube cracking suggests that the aluminum tube thickness should be the primary design variable for ultimate capacity, while the steel tube and concrete primarily control stiffness and compressive resistance.

Study Insights and Reflections

This research represents an innovative approach to composite structural members that leverages the advantages of multiple materials in a single cross-section. The finding that EC4 and AIJ codes provide the best predictions for different design stages is practically valuable, as it gives engineers specific code references for different limit state checks. However, I note that the study does not address the long-term durability of the aluminum-steel-concrete interface, which is critical for real-world applications. The thermal expansion mismatch and galvanic corrosion potential difference between aluminum and steel are significant concerns that require further investigation, particularly for structures in aggressive environments. From a steel pipe manufacturing perspective, the inner steel tube in this composite member must maintain precise dimensional accuracy and surface quality to ensure proper concrete filling and composite action. The welding of the steel tube seam should be performed with careful heat input control to minimize distortion, as dimensional accuracy is paramount for the assembly process. This study opens up interesting possibilities for lightweight composite structural members, but practical implementation will require resolution of the material compatibility and durability issues identified here.