Mechanical Properties of Circular Steel Tube Recycled Concrete Columns Based on Orthogonal Experimental Design

Literature Overview

The study by Wen Yang, Jiang Wenlixia, Han Hongpeng, and Fu Liping from Inner Mongolia University of Science and Technology, published in Journal of Shenyang Jianzhu University (2019, Vol. 35, No. 3), investigates the mechanical behavior of circular steel tube recycled concrete (RCRC) composite columns. Using orthogonal experimental design methodology combined with finite element simulation, the researchers systematically evaluated the influence of steel ratio, recycled aggregate replacement ratio, and slenderness ratio on load-bearing capacity and mechanical performance.

Core Technical Analysis

The orthogonal experimental approach allows efficient exploration of multi-parameter interactions with a reduced number of test cases. The study identifies three primary design variables:

The proposed bearing capacity formula derived from the study is:

N = (-0.0168λ + 1.8093)(fy·As + K1·fc·Ac)

where fy is the yield strength of steel, As is the steel cross-sectional area, fc is the concrete compressive strength, Ac is the concrete cross-sectional area, and K1 is a confinement correction factor. The maximum bearing capacity of 1351 kN was achieved at λ = 50, μ = 11%, and ρ = 30%.

Material and Manufacturing Considerations

From a steel pipe manufacturing standpoint, the use of recycled aggregate in concrete infill introduces several quality considerations:

1. Aggregate quality variability: Recycled aggregates from demolished concrete typically contain adherent old mortar, which affects the bond between aggregate and new cement paste. This introduces additional variability in the composite action between the steel tube and concrete core.
2. Concrete workability: Higher recycled aggregate replacement ratios may reduce concrete workability, complicating the infilling process through limited access points in the steel tube. For circular tubes with diameters of 200-500 mm, proper concrete placement and compaction are essential to avoid voids and honeycombing.
3. Steel tube specifications: The study uses circular steel tubes that must meet relevant standards such as GB/T 3091 (welded steel tubes for general cold-rolled) or GB/T 8163 (seamless steel tubes for low-pressure fluid transport). The steel ratio of 11% implies a relatively thick-walled section, which requires careful control of rolling or welding parameters.

Welding Process Implications

The circular steel tube fabrication for these composite columns typically involves either:

• Seamless tube rolling: For smaller diameters (< 400 mm), seamless tubes provide uniform properties and no longitudinal weld seam, which is advantageous for confining concrete under biaxial compression.
• ERW/HFW welded tubes: For larger diameters, electric resistance welding or high-frequency welding provides cost-effective production. The longitudinal weld seam must be positioned to avoid being in the primary compression zone during column loading.

The welding quality directly affects the confinement effectiveness of the steel tube. Any defects in the longitudinal weld (lack of fusion, undercut, or porosity) create weak points where the tube may buckle prematurely under the outward pressure from confined concrete. Ultrasonic testing (UT) of the longitudinal weld is mandatory per GB/T 3323 or equivalent standards.

Finite Element Modeling Insights

The FE simulation results showing maximum stress concentration at the mid-height of the column are consistent with theoretical expectations for columns under axial compression with boundary conditions that allow end rotation. The stress distribution pattern indicates:

• Maximum compressive stress develops at mid-span due to P-Δ effects
• The steel tube experiences higher circumferential stress near the mid-height where lateral bulging is most pronounced
• Recycled aggregate replacement up to 30% does not significantly alter the stress distribution pattern, though it reduces overall stiffness

Engineering Practice Recommendations

For practical implementation of RCRC columns:

• Optimal design parameters: Slenderness ratio should be kept below 40 for economic efficiency; steel ratio of 10-12% provides good confinement without excessive steel consumption; recycled aggregate replacement of 20-30% is recommended as a balance between sustainability and structural performance.
• Quality control: Concrete mix design must ensure adequate workability for tube infilling; slump values of 100-150 mm are typically required for vertical tube placement.
• Weld inspection: All circumferential and longitudinal welds must pass 100% ultrasonic testing with acceptance criteria per GB/T 11345 Level II.
• Standard compliance: The study recommends CECS 28:2012 as the most suitable code for design of such composite columns, which aligns with current Chinese practice for steel-concrete composite structures.

Summary

This research contributes significantly to the understanding of recycled concrete composite column behavior through systematic parametric investigation. The derived bearing capacity formula provides a practical design tool, while the sensitivity analysis clearly identifies slenderness ratio as the dominant design parameter. For steel pipe manufacturers and welding engineers, the study reinforces the importance of tube quality, weld integrity, and proper concrete infill practices in achieving the predicted structural performance of composite columns utilizing recycled aggregates.