Axial Compression Behavior of Double Stainless Steel Tube Confined Seawater Sea-Sand Concrete Columns

Literature Overview

This paper by Zheng Kaiqi and colleagues from Nanjing Forestry University presents experimental and theoretical research on double stainless steel tube confined seawater sea-sand concrete (DSTSCC) columns. The study involves 24 test specimens subjected to axial compression loading, investigating the influence of outer tube wall thickness, inner tube wall thickness, and inner tube diameter on failure modes, stress-strain behavior, and load-bearing capacity. The authors propose a new axial compression capacity calculation model based on the test results and theoretical analysis.

Core Findings and Technical Points

The DSTSCC column configuration represents an innovative structural system that combines the corrosion resistance of stainless steel with the sustainability of seawater and sea-sand concrete, making it particularly suitable for marine environments. The double tube configuration provides enhanced confinement to the concrete core compared to single tube systems.

Key experimental findings include:

The failure mode of the DSTSCC specimens is characterized by barrel-type (waist-drum) failure, where the outer tube expands outward in the mid-height region due to the lateral expansion of the confined concrete. The stress-strain curves exhibit no distinct descending branch, indicating good ductility characteristics. This behavior is attributed to the unique constitutive properties of stainless steel, which lacks a distinct yield plateau and can sustain large plastic deformations while continuously providing confinement to the concrete core.

Material and Structural Analysis

The choice of stainless steel for the confinement tubes is motivated by the severe corrosion environment of marine applications. Conventional carbon steel tubes would require extensive corrosion protection measures, such as coating and cathodic protection, which may not be reliable over the long service life of marine structures. Stainless steel, particularly grades such as SUS304 or SUS316 (equivalent to ASTM A270 TP304 or TP316), offers excellent corrosion resistance in chloride-rich environments.

The seawater sea-sand concrete (SSSC) represents a sustainable alternative to conventional Portland cement concrete, utilizing seawater as the mixing water and sea sand as the aggregate. The corrosion resistance of the concrete is achieved through the use of corrosion-inhibiting admixtures and potentially supplementary cementitious materials such as fly ash or silica fume.

The double tube configuration provides a layered confinement mechanism:

1. The inner tube provides primary confinement to the central concrete core.
2. The outer tube provides secondary confinement and structural integrity to the overall column.
3. The interaction between the two tubes creates a more uniform confinement pressure distribution compared to single tube systems.

Engineering Practice Integration

For the practical implementation of DSTSCC columns in marine structures, the following considerations are essential:

1. Welding of stainless steel tubes requires specialized procedures to prevent intergranular corrosion and sensitization. The welding process should include preheating to prevent cracking and post-weld solution heat treatment to restore corrosion resistance.
2. The welding filler metal must be compatible with the base stainless steel grade to ensure metallurgical compatibility and corrosion resistance of the weld.
3. Quality control of stainless steel welds should include intergranular corrosion testing (ASTM A262 Practice E) and potentiodynamic polarization testing to verify the corrosion resistance of the weld and heat-affected zone.
4. The concrete placement within the double tube configuration requires careful attention to ensure complete filling and proper compaction, particularly in the annular space between the two tubes.

The fabrication of double stainless steel tube columns involves precision welding of the inner tube to the outer tube, which requires careful control of welding distortion to maintain the concentricity and dimensional accuracy of the tube assembly.

Key Questions and Reflections

The paper raises the question of whether the double tube configuration is economically justified compared to single tube systems with optimized wall thickness. The enhanced confinement and load capacity of the double tube system may justify the additional material and fabrication costs for critical marine structures, but a life-cycle cost analysis should be conducted to evaluate the economic viability of the DSTSCC system.

Another reflection is that the study focuses on axial compression behavior, while in practice, marine structures are subjected to combined axial, bending, and shear loads. The interaction between the double tube confinement and the structural response under combined loading should be investigated to ensure the applicability of the proposed design model for realistic loading conditions.

Study Insights and Implications

This study provides a comprehensive experimental and theoretical foundation for the design of double stainless steel tube confined seawater sea-sand concrete columns. The key insight is that the unique constitutive behavior of stainless steel, combined with the double tube confinement configuration, creates a highly ductile structural system with excellent load-bearing capacity and corrosion resistance, making it particularly suitable for marine applications. The proposed axial compression capacity calculation model offers a practical design tool for engineers working on marine infrastructure projects. The recommendation is to extend the research to combined loading conditions and to conduct long-term durability testing to validate the corrosion resistance of the DSTSCC system over the design service life of marine structures.