Zhuojin Pipe Fitting Co., LtdAxial Compression Bearing Capacity Calculation Method for Reinforced Square Steel Tube Recycled Concrete Short Columns
Literature Overview
This study addresses a critical gap in the structural design of composite columns utilizing recycled concrete, specifically those with square steel tube confinement and longitudinal reinforcement. Recycled concrete, incorporating crushed waste concrete as coarse aggregate, offers significant environmental benefits by diverting construction and demolition waste from landfills. However, the non-linear mechanical behavior of recycled concrete, combined with the complex interaction between the steel tube, longitudinal bars, and the infill concrete, makes accurate bearing capacity prediction challenging. The authors develop a refined analytical method for axial compression bearing capacity that accounts for the unique properties of recycled concrete and the composite action of all structural components.
Material Properties and Constitutive Models
Recycled concrete exhibits distinct mechanical characteristics compared to natural aggregate concrete, primarily due to the presence of adhered mortar on the recycled aggregate surface, which creates interfacial transition zones (ITZ) with reduced bond strength and increased porosity.
| Property | Natural Aggregate Concrete | Recycled Concrete (50% RA) | Recycled Concrete (100% RA) |
|---|---|---|---|
| Compressive strength (C30 equivalent) | 30 MPa | 26–29 MPa | 22–27 MPa |
| Elastic modulus | 30 GPa | 26–29 GPa | 22–28 GPa |
| Poisson's ratio | 0.20 | 0.19–0.20 | 0.18–0.19 |
| Splitting tensile strength | 3.0 MPa | 2.5–2.8 MPa | 2.0–2.6 MPa |
| ITZ thickness | 5–10 μm | 15–25 μm | 20–35 μm |
The constitutive model for recycled concrete under triaxial compression follows the Mander model framework but incorporates a strength reduction factor α_rc that accounts for the degraded ITZ properties. The confinement pressure provided by the square steel tube is calculated using equilibrium equations modified for the rectangular cross-section geometry, where the stress distribution is non-uniform due to the corner effects and the varying curvature radius along the perimeter.
Bearing Capacity Calculation Method
The proposed analytical method decomposes the total bearing capacity into three components: the contribution from the steel tube (N_st), the contribution from the longitudinal reinforcement (N_sr), and the contribution from the confined recycled concrete (N_rc). The total capacity is expressed as N_u = N_st + N_sr + N_rc.
For the square steel tube component, the yield capacity is modified by a confinement factor that accounts for the local buckling resistance under the lateral pressure from the infill concrete. The effective confined area is calculated using the equation A_e = (b - 2c)^2 for a square section, where b is the outer dimension and c is the concrete cover to the steel tube inner face.
The longitudinal reinforcement contribution is calculated using the yield strength of the steel bars multiplied by the total reinforcement area, with a partial safety factor reflecting the stress redistribution mechanism at failure. The confined recycled concrete contribution employs the modified Mander model with the confinement pressure derived from the steel tube hoop stress equilibrium.
Experimental Validation and Model Accuracy
The analytical method was validated against experimental data from 42 specimens with varying parameters including steel tube dimensions (100×100×4 mm to 200×200×8 mm), recycled concrete replacement ratios (0%, 30%, 50%, 70%, 100%), and longitudinal reinforcement ratios (1.2% to 3.5%). The comparison shows that the proposed method predicts the experimental bearing capacity with an average deviation of less than 8%, significantly outperforming existing design codes that do not specifically account for recycled concrete properties.
Study Insights and Engineering Recommendations
The study highlights that the bearing capacity of reinforced square steel tube recycled concrete columns is not merely a linear superposition of individual component capacities but involves significant interaction effects, particularly the confinement-enhanced strength of the recycled concrete. Engineers adopting this method should note that the accuracy depends on reliable material property data for the specific recycled concrete batch, as the variability in recycled aggregate properties can be substantial. The method provides a practical tool for incorporating recycled concrete into structural design while maintaining safety and economy, contributing to sustainable construction practices. The refinement of this approach for slender columns and eccentrically loaded conditions remains an important area for future research.