Bearing Capacity Calculation Methods for Steel Tube Self-Compacting Recycled Concrete Short Columns

Literature Overview

This paper by Xiang Xingyun, Zhao Renda, and Liu Yang, published in Railway Construction in 2017 (Vol. 57, No. 2, pp. 34–37), addresses the bearing capacity calculation of steel tube columns filled with self-compacting recycled aggregate concrete (SC-RAC). The research was supported by the National Key Research and Development Program of China (2016YFB1200401). The study recognises that the smaller aggregate size and inferior properties of recycled aggregates compared to natural aggregates create differences in the bearing capacity of steel tube SC-RAC columns relative to conventional steel tube reinforced concrete (SRC) columns.

Motivation and Technical Background

Self-compacting concrete eliminates the need for mechanical vibration during placement, which is advantageous for complex geometries and congested reinforcement. Recycled concrete incorporates crushed waste concrete as aggregate, promoting sustainable construction practices. However, the combination introduces unique challenges:

Existing Calculation Methods and Their Limitations

The paper evaluates the applicability of existing SRC column bearing capacity formulas to SC-RAC columns. The major codes and methods include:

The key limitation of existing formulas is that they assume the mechanical properties of the concrete core follow the conventional relationship between strength grade and constituent properties. In SC-RAC, the reduced aggregate size and recycled aggregate characteristics alter the stress-strain relationship, particularly the post-peak softening behaviour and the effective confinement response.

Proposed Calculation Methods

Axial Compression Bearing Capacity

Based on unified theory and limit equilibrium theory, the authors propose a modified formula that accounts for:

1. The reduced elastic modulus of SC-RAC compared to natural aggregate concrete of equivalent strength.
2. The modified confinement effectiveness due to the different stress-strain characteristics of the recycled concrete core.
3. The interaction between the steel tube confinement and the self-compacting concrete properties.

The formula incorporates a modification factor that relates the SC-RAC properties to the equivalent natural aggregate concrete properties, derived from experimental calibration.

Eccentric Compression Bearing Capacity

An empirical coefficient method is proposed for eccentric compression, introducing adjustment coefficients that account for:

• The non-uniform stress distribution in the SC-RAC core under eccentric loading.
• The modified neutral axis depth due to the different stress-strain curve of recycled concrete.
• The interaction between the steel tube tensile resistance and the SC-RAC compressive zone.

Technical Parameters and Verification

Connection with Steel Pipe and Welding Engineering

The bearing capacity of SRC columns is fundamentally dependent on the quality of the steel tube and the integrity of the steel-concrete interface. For SC-RAC columns specifically:

1. Steel tube fabrication quality – The self-compacting nature of the concrete means it flows into all voids within the tube. Any geometric irregularities in the steel tube, such as ovality, waviness, or weld bead protrusions, directly affect the concrete distribution and confinement effectiveness. The dimensional tolerances per GB/T 17395 must be strictly controlled.
2. Weld seam integrity – Internal weld beads from the pipe manufacturing process (ERW, HFW) create local geometric discontinuities. For SC-RAC columns where the concrete must fully fill the tube, protruding weld beads can create local voids or stress concentrations. The weld bead height should not exceed 0.3 mm for internal seams per GB/T 3091 requirements.
3. End treatment – The ends of the steel tube must be properly prepared to prevent concrete leakage during the self-compacting fill process. Welded end caps or mechanical seals must be leak-tight and structurally sound. The welding of end caps should follow GB/T 3375 and include full-penetration welds with RT inspection.
4. Post-weld heat treatment – For thick-walled steel tubes (t > 25 mm) used in SC-RAC columns, post-weld stress relief is recommended to reduce residual stresses that could influence the long-term durability and fatigue performance of the composite column.

Study Insights and Implications

The research contributes a practical framework for the design of SRC columns using sustainable materials. The proposed formulas provide engineers with reliable tools for calculating the bearing capacity of SC-RAC columns under both axial and eccentric compression. From a steel pipe manufacturing perspective, the study reinforces the importance of maintaining strict dimensional tolerances and weld quality in steel tubes intended for SRC applications. The self-compacting nature of the concrete, while beneficial for placement, also demands higher precision in tube fabrication to ensure uniform concrete distribution and effective confinement. The sustainable aspect of using recycled aggregates aligns with global trends towards green construction, but requires careful attention to the mechanical property variations that affect structural performance predictions. Engineers should validate the proposed formulas against project-specific material properties and consider the long-term durability implications of recycled aggregate exposure to environmental aggressions.