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

Literature Overview

This paper published in Concrete (2025, No. 6) by Ning Chunzhen and colleagues from Hainan University and Huaqiao University presents experimental and analytical research on the flexural behavior of a novel composite member system designated ACSDST (Aluminum-Concrete-Steel Double-Skin Tube). The work was supported by the National Natural Science Foundation of China (Grant 52268024) and multiple provincial research funds. Four-point bending tests were conducted on ACSDST beam specimens to evaluate failure modes, moment-curvature relationships, flexural stiffness, and bending capacity.

Core Technical Findings

Failure Mode Characterization

The ACSDST composite beams failed due to cracking of the aluminum alloy tube (outer tube) on the tension side. After failure, the aluminum tube on the compression side exhibited varying degrees of outward bulging. This failure sequence is characteristic of the composite action between the aluminum outer tube, concrete core, and steel inner tube. The aluminum tube, having lower yield strength than steel, yields first under tension, while the steel tube and concrete core continue to carry load in the compression zone.

Moment-Curvature Behavior and Flexural Stiffness

The moment-curvature curves exhibit typical three-stage behavior:

Elastic stage — Linear relationship between moment and curvature, governed by the initial flexural stiffness of the composite section
Elastic-plastic transition stage — Gradual yielding of the aluminum outer tube on the tension side, with stiffness degradation
Plastic stage — Significant curvature increase with limited moment gain, until final failure

Code Comparison for Flexural Stiffness

Standard/Method	Application Stage	Prediction Accuracy
EC4 (2004)	Initial flexural stiffness	Best prediction method
AIJ (1997)	Serviceability stage flexural stiffness	Best prediction method
Other methods	Various stages	Generally acceptable but less accurate

The European Code 4 (EC4:2004) for aluminium structures provides the best prediction for initial flexural stiffness, while the Japan Aluminium Institute (AIJ:1997) design method is most appropriate for serviceability-stage flexural stiffness. This distinction is important because the composite action between aluminum and steel tubes through the concrete core evolves with increasing deformation.

Bending Capacity Prediction

By combining the axial compression strength formula for aluminum tube concrete composite sections, the bending capacity formula for hollow sandwich concrete-filled steel tubes can adequately predict the bending capacity of ACSDST sections. This approach leverages the well-established concrete-filled steel tube design methodology while accounting for the additional contribution of the aluminum outer tube.

Technical Analysis from a Steel Pipe and Welding Perspective

Tube Fabrication Requirements

The ACSDST system requires precise fabrication of both the steel inner tube and aluminum outer tube:

Steel inner tube: Typically manufactured as ERW or HFW welded pipe, requiring strict control of weld quality to ensure uniform wall thickness and ovality within ±0.5% of nominal diameter
Aluminum outer tube: Extruded seamless tube, requiring dimensional accuracy for proper concentricity with the steel inner tube
Concentricity tolerance: The gap between inner and outer tubes must be uniform (typically 15-30 mm) to ensure consistent concrete cover and composite action

Welding Considerations for Connection Details

Although this study focuses on beam members, the connection details of ACSDST members in structural applications require careful welding design:

Steel tube connections: Can use conventional steel welding processes (SMAW, GTAW, SAW) with matching filler metals
Aluminum tube connections: Require GTAW or GMAW with appropriate aluminum filler metals (ER4043 or ER5356 depending on base alloy)
Composite joint connections: When connecting ACSDST members to other structural elements, the different thermal expansion coefficients of steel (12 × 10⁻⁶/°C) and aluminum (23 × 10⁻⁶/°C) must be considered in weld design

Quality Control Implications

Quality Parameter	Acceptance Criteria	Testing Method
Steel tube wall thickness	≥95% of nominal	Ultrasonic thickness measurement
Aluminum tube wall thickness	≥95% of nominal	Ultrasonic or caliper measurement
Tube concentricity	≤2 mm deviation	Visual and gauge inspection
Concrete cover uniformity	±3 mm	Radiographic or ultrasonic inspection
Steel tube weld quality	No cracks, porosity ≤1 mm	RT or UT inspection

Study Insights and Engineering Practice Integration

This research contributes to the development of lightweight composite structural systems that combine the corrosion resistance of aluminum with the high strength of steel and the compressive capacity of concrete. From a practical engineering standpoint, the ACSDST system offers potential advantages in marine and coastal environments where steel tube concrete-filled members would require extensive corrosion protection. The key engineering challenge lies in maintaining the concentricity and composite action between the different material tubes throughout the service life. Engineers should pay particular attention to the construction sequence: the steel inner tube must be precisely positioned, the aluminum outer tube concentrically installed, and the concrete carefully placed to avoid voids that would compromise the composite action. The validation of existing design codes (EC4 and AIJ) for stiffness prediction provides confidence for practical design applications, while the bending capacity prediction method based on concrete-filled steel tube theory offers a practical analytical tool for engineers working with this novel system.