Numerical Simulation of Axial Compression Performance of Square Steel Tube Recycled Concrete Long Columns

Literature Overview

Xu Yang, Zhang Zhaoqiang, and Yao Yong from the School of Civil Engineering and Architecture, Southwest University of Science and Technology, published their research in Green Building (Vol. 8, No. 5, 2016, pp. 74-79). Based on eight axial compression tests of square steel tube recycled concrete (SRC) long columns, the authors conducted numerical simulations using ANSYS finite element software. Funded by multiple sources including the National Natural Science Foundation (51308479) and Sichuan Provincial Education Department (13ZB0179), this work investigates the mechanical behavior of SRC columns incorporating recycled aggregate concrete, addressing sustainability concerns in construction materials.

Experimental and Numerical Framework

The research combines experimental investigation with numerical modeling:

Experimental program: Eight square steel tube recycled concrete long columns with varying slenderness ratios and diameter-to-thickness ratios were tested under axial compression.
Finite element modeling: ANSYS was used to create nonlinear models incorporating material nonlinearity (steel and recycled concrete constitutive models), geometric nonlinearity, and contact nonlinearity between steel tube and concrete core.
Validation: Numerical results were compared with experimental data for failure modes, load-displacement curves, and ultimate load capacity.

Parameter	Symbol/Range	Effect on Ultimate Load	Effect on Stiffness
Slenderness ratio	λ = 5, 8, 10, 12	Decreases with increasing λ	Increases then decreases
Diameter-to-thickness ratio	η = 30, 40, 50, 60	Increases with increasing η	Moderate effect
Concrete compressive strength	f'c = 20-40 MPa	Increases with f'c	Increases with f'c
Steel tube yield strength	fy = 235-355 MPa	Increases with fy	Increases with fy
Recycled aggregate replacement rate	0-100%	Decreases (10-25% reduction)	Decreases (15-30% reduction)

Key Findings and Failure Mode Analysis

The study revealed several important mechanical behaviors:

Overall buckling failure: All long column specimens failed by global buckling rather than local wall buckling or concrete crushing, confirming that slenderness governs the failure mode for long columns.
Slenderness effect: As λ increases from 5 to 12, the ultimate load capacity decreases progressively. The reduction follows the classical Euler buckling pattern modified by material nonlinearity.
Diameter-to-thickness ratio effect: Higher η values increase ultimate load capacity because thicker walls provide greater confinement and resistance to local deformation, though the benefit diminishes at very high η values.
Recycled concrete impact: The incorporation of recycled aggregate reduces both strength and stiffness compared to natural aggregate concrete, with the magnitude depending on replacement rate and recycled aggregate quality.

Engineering Practice Implications

This research has direct relevance to sustainable construction practices:

Material sustainability: Recycled aggregate concrete reduces construction waste and virgin material consumption. The quantified strength reduction (10-25%) provides design engineers with the data needed to specify appropriate safety factors.
Section design optimization: For long SRC columns, increasing the diameter-to-thickness ratio improves load capacity more effectively than increasing concrete strength, which has implications for steel pipe selection in fabrication.
Quality control considerations: The variability in recycled aggregate properties requires enhanced quality control measures. Key parameters include recycled aggregate crushing value, water absorption, and impurity content.

From a steel pipe manufacturing perspective:

Pipe specification selection: The optimal diameter-to-thickness ratio for SRC columns may differ from conventional STC columns due to the lower confinement pressure provided by recycled concrete.
Welding and fabrication: Square steel tubes require four longitudinal welds (for welded tubes) or four-corner forming (for hot-finished tubes). The welding quality at corners is critical for buckling resistance.
Surface treatment: Recycled concrete may contain chlorides from seawater sources, necessitating corrosion protection for the steel tube interior (internal coating or increased wall thickness).

Study Insights and Outlook

The numerical simulation approach validated against experimental data provides confidence in using FEA for design optimization of SRC columns, reducing the need for extensive physical testing. The findings support the feasibility of using recycled aggregate concrete in structural applications, though with appropriate design modifications. Future research should address:

Cyclic loading behavior of SRC columns for seismic applications
Long-term durability and creep behavior of recycled concrete confined by steel tubes
Combined loading conditions (axial force plus bending) typical of real structural members
Life-cycle assessment comparing conventional STC columns with SRC columns

The work contributes meaningfully to the growing field of sustainable structural engineering and provides practical guidance for engineers seeking to incorporate recycled materials without compromising structural safety. The numerical methodology developed can be readily extended to other recycled material combinations and structural configurations.