Steel Tube Self-Compacting Recycled Concrete Strengthening RC Short Columns

Literature Overview

This 2025 paper by Dan Yu, Zhang Chunyu, She Haitao, Chen Hua, and Qin Shitao, published in Acta Materiae Compositae Sinica (Volume 42, Issue 12, pp. 7099-7112), presents a comprehensive experimental and analytical study of steel tube self-compacting recycled concrete (SSCRC) used for strengthening reinforced concrete (RC) short columns. Funded by the Guangxi Key R&D Program (Gui Ke AB24010010), the Nanning "Yongjiang Plan" Youth Talent Program (RC20230103), and Nanning College (2022XJ04), this research addresses both structural performance enhancement and sustainability through recycled aggregate utilization.

Experimental Design and Parameters

The study employed 12 specimens total: 1 un-strengthened control column and 11 strengthened columns with varying parameters. The test matrix was designed to isolate the influence of each variable on strengthening effectiveness.

The self-compacting concrete (SCC) approach eliminates the need for vibrators during placement, which is particularly advantageous for confined spaces around existing columns where traditional concrete placement methods are impractical.

Key Findings and Quantitative Results

Bearing Capacity Enhancement

The ultimate stress enhancement coefficient ranges from 1.14 to 2.30, demonstrating substantial strengthening potential. This wide range reflects the significant influence of parameter interactions:

• Steel tube thickness and CFRP layer count provide the most significant bearing capacity improvement.
• Increasing the confinement coefficient and section diameter yields diminishing returns for capacity enhancement.
• Higher recycled coarse aggregate replacement ratios reduce bearing capacity due to the inferior mechanical properties of recycled aggregate compared to natural aggregate.

Ductility and Stiffness

All strengthened specimens exhibited ductile failure modes, representing a fundamental improvement over the brittle failure typical of un-strengthened RC columns. The load-displacement curves demonstrate:

• Significant improvement in initial axial stiffness due to the steel tube contribution.
• Enhanced post-yield deformation capacity from CFRP confinement.
• Progressive concrete crushing with lateral confinement preventing sudden spalling.

Recycled Aggregate Effect

The introduction of a recycled coarse aggregate safety factor and material utilization coefficient into the analytical model is a notable contribution. These factors account for the increased variability and reduced strength of recycled concrete while maintaining design reliability. The research demonstrates that moderate recycled aggregate replacement ratios (typically 30-50%) are acceptable for strengthening applications where the steel tube provides primary structural capacity.

Finite Element Modeling and Extended Analysis

ABAQUS finite element models were developed to extend the experimental findings beyond the tested parameter range. The model validation against experimental data shows good agreement, confirming the reliability of the numerical approach for parametric studies.

Extended finite element results reveal:

• Steel tube yield strength increase significantly enhances bearing capacity.
• Recycled concrete strength increase provides meaningful capacity improvement.
• Slenderness ratio variation has minimal influence on bearing capacity for short columns.

Bearing Capacity Formula

Based on ultimate equilibrium theory, the proposed axial compressive bearing capacity formula achieves average errors within 5% compared to experimental data. This level of accuracy is considered excellent for structural design formulas and provides a reliable tool for engineering applications.

The formula incorporates:

• Concrete contribution with recycled aggregate modification factors.
• Steel tube confinement pressure contribution.
• CFRP tensile confinement contribution.
• Interaction effects between steel tube and CFRP confinement layers.

Engineering Practice Integration

For steel pipe manufacturing and fabrication, several practical implications emerge:

1. Steel tube specifications: The steel tubes used for column strengthening should meet appropriate standards (e.g., GB/T 9711, API 5L) with attention to wall thickness tolerance, since thicker tubes provide proportionally greater confinement benefit.
2. Surface preparation: The steel tube inner surface must be properly prepared for concrete adhesion. Shot blasting or acid etching may be required to ensure proper bond between steel and concrete interface.
3. Welding considerations: Steel tubes fabricated from rolled plates require careful welding to maintain dimensional accuracy. Weld defects or distortion can reduce the effective confinement geometry.
4. Quality control: Non-destructive testing (MT or UT) of steel tube welds is recommended prior to concrete filling to ensure structural integrity of the confinement element.

Sustainability Perspective

The integration of recycled coarse aggregate represents a meaningful contribution to sustainable construction. By utilizing recycled aggregate in the concrete core while maintaining structural performance through steel tube confinement, this approach reduces the demand for virgin aggregate resources while providing structural strengthening. The recycled aggregate safety factor concept provides a practical framework for incorporating recycled materials into structural applications without compromising safety.

Study Insights and Implications

This research demonstrates that steel tube self-compacting recycled concrete is a viable and effective method for strengthening existing RC columns. The combination of self-compacting concrete for ease of placement, recycled aggregate for sustainability, steel tube for primary confinement, and CFRP for supplementary confinement creates a multi-layered strengthening system that addresses multiple engineering objectives simultaneously. The proposed bearing capacity formula with average error within 5% provides a reliable design tool. For steel pipe fabricators, this research highlights the growing demand for precision steel tubes in structural strengthening applications, where dimensional accuracy and surface quality directly impact structural performance.