Zhuojin Pipe Fitting Co., LtdAxial Compression Test of Composite Stainless Steel Tube High-Strength Recycled Block Concrete Short Columns
Literature Overview and Context
The paper by Wang Weihua, Chen Zijian, Cai Minwei, and Dong Yuli, published in Advances in Engineering Science in 2022 (Vol. 54, No. 3, pp. 180-191), investigates the axial compression behavior of composite stainless steel tube concrete-filled columns incorporating high-strength recycled concrete blocks. This research was supported by the National Natural Science Foundation of China (51208217, 51678256) and the Quanzhou Science and Technology Support Program (2017T002).
The composite stainless steel tube concrete-filled column (CFST) represents an advanced structural system that combines the corrosion resistance and fire resistance of stainless steel with the high strength and sustainability of recycled concrete. The innovative aspect of this research is the incorporation of recycled concrete blocks within the stainless steel tube, which addresses the challenge of utilizing recycled aggregate in structural applications while maintaining high load-bearing capacity. The addition of an internal carbon steel tube within the square stainless steel tube further enhances the confinement effect and structural performance.
From a metallurgical perspective, stainless steel tubes are typically fabricated from austenitic stainless steel grades such as SUS304 or SUS316 (per GB/T 14976 or ASTM A213), which exhibit different mechanical behavior compared to carbon steel, particularly regarding strain hardening, work hardening exponent, and stress-strain curve shape. The welding of stainless steel tubes requires careful control of heat input and filler metal selection to avoid sensitization, chromium carbide precipitation, and intergranular corrosion.
Experimental Program and Parametric Study
Sixteen composite stainless steel tube high-strength recycled block concrete short column specimens were tested, investigating the effects of:
| Parameter | Variation Range | Number of Specimens | Purpose |
|---|---|---|---|
| Recycled block replacement rate | 0% to higher rates | Multiple | Evaluate effect on bearing capacity |
| Confinement effect coefficient | Varying tube dimensions | Multiple | Evaluate confinement effectiveness |
| Old and new concrete strength | Different strength combinations | Multiple | Evaluate interface effects |
The key design parameters studied are the recycled block replacement rate (the proportion of recycled concrete blocks relative to the total concrete volume), the confinement effect coefficient (related to the geometry of the steel tubes), and the strength relationship between the old (recycled) and new (matrix) concrete.
Core Findings and Technical Analysis
The research yielded several important findings:
The axial compression bearing capacity decreases with increasing recycled block replacement rate, with a maximum reduction of 6.8% at a 20% replacement rate. This is a relatively modest reduction, indicating that the composite stainless steel tube system effectively compensates for the inferior properties of recycled concrete through its confinement mechanism.
The failure process and failure mode of the composite stainless steel tube recycled block concrete columns are similar to those of conventional composite stainless steel tube concrete columns. This similarity in failure behavior is important for design purposes, as it means that existing failure mode classifications and design provisions can be applied with confidence.
The internal circular steel tube concrete exhibits two failure types: barrel-shaped (bulging) failure and shear failure. The barrel-shaped failure is characteristic of columns with adequate confinement, where the concrete expands laterally under compression and the steel tube bulges outward. Shear failure occurs when the confinement is insufficient or when the concrete strength is too high relative to the steel tube strength.
A particularly significant finding is that all specimens maintained a residual bearing capacity exceeding 63% of the peak load. After the peak load, the load-displacement curve experiences a brief decline before stabilizing at a horizontal or slightly ascending level. This excellent post-peak behavior indicates superior ductility and energy dissipation capacity, which are critical for seismic design.
Stainless Steel Welding and Metallurgical Considerations
The fabrication of composite stainless steel tube columns involves several critical welding operations:
- Stainless steel tube fabrication: The square stainless steel tube is typically fabricated from stainless steel plates using GTAW (Tungsten Inert Gas Welding) or GMAW (Gas Metal Arc Welding) with appropriate filler metals (e.g., ER308L for 304 stainless steel). The heat input must be controlled to prevent sensitization (chromium carbide precipitation at grain boundaries) and to maintain the corrosion resistance of the weld zone.
- Internal carbon steel tube fabrication: The internal carbon steel tube may be fabricated from carbon steel plates using SAW (Submerged Arc Welding) or GMAW. The weld quality must meet the requirements of relevant standards such as NB/T 47013 for pressure equipment or ISO 3834 for general welded structures.
- Tube-to-tube connections: If the columns require field fabrication or splicing, the connections between stainless steel tube sections and between carbon steel tube sections must be designed and welded with appropriate procedures. Dissimilar metal welding (stainless steel to carbon steel) requires special consideration for thermal expansion mismatch, galvanic corrosion, and weld metal compatibility.
The following table summarizes the key welding and material considerations:
| Component | Material Grade | Welding Process | Key Quality Concern |
|---|---|---|---|
| Outer square tube | SUS304 or SUS316 | GTAW/GMAW | Sensitization, intergranular corrosion |
| Inner circular tube | Q235 or Q345 | SAW/GMAW | HAZ toughness, residual stress |
| Tube connections | Same as parent material | GTAW/SMAW | Penetration, defect-free welds |
| Dissimilar joints (if any) | SUS304 + Q345 | GTAW with transition filler | Thermal expansion mismatch, galvanic corrosion |
Confinement Effect and Design Methodology
The authors proposed a simplified method for calculating the confinement effect of the steel tube cross-section on the core concrete in the composite stainless steel tube cross-section. Four bearing capacity calculation methods were compared, and the assumption of multiplying the circular section portion's bearing capacity by a reduction factor was found to provide good agreement with experimental results.
The confinement effect is the primary mechanism by which the steel tubes enhance the performance of the recycled concrete core. The confinement pressure increases the triaxial compressive strength of the concrete, which is particularly beneficial for recycled concrete that may have lower uniaxial compressive strength due to the presence of residual mortar on the recycled aggregates. The stainless steel's superior strain hardening capacity (compared to carbon steel) provides sustained confinement even at large deformations, which explains the excellent residual bearing capacity observed in the tests.
Study Insights and Engineering Implications
This research represents a significant advancement in sustainable structural engineering by demonstrating that recycled concrete blocks can be successfully incorporated into composite stainless steel tube columns with minimal impact on structural performance. The maximum 6.8% reduction in bearing capacity at 20% replacement rate is an encouraging result that supports the practical viability of this approach.
The excellent post-peak behavior (residual capacity exceeding 63% of peak load) is particularly significant for seismic applications, as it indicates that the columns can sustain large deformations without catastrophic failure. This is consistent with the known superior ductility of stainless steel, which has a higher work hardening exponent and more gradual strain hardening compared to carbon steel.
From a quality assurance standpoint, the fabrication of these composite columns requires careful attention to the welding quality of both the stainless steel and carbon steel tube components, the dimensional accuracy of the tube geometry (which affects the uniformity of concrete confinement), and the quality of the recycled concrete blocks (aggregate replacement rate control, mixing, and curing). The research provides a solid experimental foundation for the design and application of composite stainless steel tube recycled concrete columns in sustainable and high-performance structural systems.