Axial Compression Performance and Bearing Capacity of Bimetal Composite Pipe Columns Filled with Seawater Sea Sand Concrete

Literature Overview and Research Background

The literature under review investigates the axial compression behavior and bearing capacity of bimetal composite pipe columns filled with seawater sea sand concrete (SSSC). This research is significant because the global construction industry faces increasing demand for high-performance structural members that combine corrosion resistance with structural efficiency. Seawater sea sand concrete, made from seawater and sea sand without freshwater desalination, offers a sustainable alternative to conventional concrete in marine environments. However, the chloride ions inherent in seawater and sea sand pose severe corrosion risks to embedded reinforcement and steel components. The use of bimetal composite pipes—typically consisting of a carbon steel outer shell bonded to a stainless steel or nickel-based alloy inner layer—addresses this corrosion challenge while maintaining structural integrity. The study examines how the interaction between the bimetal pipe and the SSSC fill material affects load-bearing capacity, ductility, and failure modes under axial compression.

Core Technical Findings

The research demonstrates that bimetal composite pipe columns filled with SSSC exhibit enhanced axial compression capacity compared to conventional concrete-filled steel tube (CFST) columns. The bimetal interface provides a corrosion-resistant barrier that protects the structural steel from chloride attack, while the SSSC fill contributes to confinement effects that improve compressive strength and ductility. Key findings include:

• The ultimate bearing capacity of bimetal SSSC-filled composite columns increases by approximately 15-25% compared to unfilled bimetal tubes, depending on the concrete strength grade and the pipe geometry.
• The failure mode transitions from local buckling in unfilled tubes to a more gradual crushing and outward bulging pattern when filled with SSSC, indicating improved ductility.
• The corrosion resistance of the inner bimetal layer significantly extends the service life of the column in marine environments, with accelerated corrosion tests showing minimal degradation over simulated 50-year exposure periods.

Load-Displacement Behavior and Stress-Strain Response

The axial compression tests reveal a characteristic load-displacement curve with four distinct stages: elastic loading, initial yielding of the bimetal pipe wall, concrete crushing with progressive confinement, and ultimate failure through outward bulging. The confinement pressure exerted by the SSSC on the pipe wall increases the effective yield strength of the steel, following the well-established Mander model but with modified parameters to account for the bimetal interface stiffness. The stress-strain relationship of the confined SSSC shows a higher peak strength and improved post-peak ductility compared to unconfined SSSC specimens.

Bearing Capacity Prediction Models

The literature proposes modified bearing capacity prediction formulas that incorporate the bimetal interface effects and the SSSC confinement mechanism. The proposed models show good agreement with experimental results, with prediction errors generally within ±10%. The key parameters influencing bearing capacity include the outer diameter-to-thickness ratio of the bimetal pipe, the concrete compressive strength, the yield strength of both the structural steel and the corrosion-resistant alloy layer, and the bond strength at the bimetal interface.

Engineering Practice Implications

From an engineering perspective, the findings suggest that bimetal composite pipe columns filled with SSSC are particularly suitable for marine structures such as offshore platforms, coastal bridges, and waterfront buildings where both structural efficiency and corrosion resistance are critical. The use of SSSC eliminates the need for freshwater and desalinated sand, reducing construction costs and environmental impact in coastal regions where freshwater is scarce. However, engineers must carefully consider the chloride diffusion through the concrete matrix and ensure that the bimetal interface remains intact throughout the design life. The bond strength between the SSSC and the inner alloy layer must be verified through pull-off tests during construction quality control.

Key Reflections and Study Insights

The most compelling aspect of this research is the integration of sustainable concrete technology with advanced bimetal construction, addressing both environmental and durability concerns simultaneously. However, several questions remain open for further investigation. First, the long-term durability of the bimetal interface under cyclic loading and sustained chloride exposure warrants additional study. Second, the effect of seawater sea sand concrete on the welding quality of the bimetal pipe during fabrication is not fully addressed, as chloride-rich environments can potentially affect weld metal microstructure and hydrogen-induced cracking susceptibility. Third, the cost-effectiveness analysis comparing bimetal SSSC-filled columns with conventional CFST columns with external corrosion protection systems needs more comprehensive economic data. Overall, this research represents a meaningful contribution to the field of sustainable structural engineering in aggressive marine environments, and the proposed design models provide a practical foundation for engineering applications pending further validation through full-scale testing.