Axial Compression Performance of Steel Tube Self-Compacting Recycled Concrete Reinforced RC Short Columns

Study Overview and Motivation

This 2025 publication by Dan Yu, Zhang Chunyu, She Haitao, Chen Hua, and Qin Shitao in Acta Materiae Compositae Sinica (Volume 42, Issue 12, pp. 7099-7112) addresses a timely and practically important problem: the structural retrofitting of existing reinforced concrete (RC) short columns using steel tubes filled with self-compacting recycled concrete (SCRC) and externally wrapped with CFRP sheets. The research is supported by the Guangxi Key R&D Program and the Nanning "Yongjiang Plan" Youth Talent Program, reflecting both the academic and policy-driven interest in green construction and structural strengthening in southern China.

The study is significant from a metallurgical and pipe engineering standpoint because it investigates how the combination of steel tube confinement, recycled aggregate concrete, and CFRP wrapping interacts to enhance the residual load capacity, stiffness, and ductility of deteriorated RC columns. The use of recycled coarse aggregate in the infill concrete introduces additional material variability that must be accounted for in design.

Experimental Programme and Key Parameters

A total of 12 specimens were tested: one unreinforced RC short column as a reference, and eleven reinforced specimens with varying parameters. The test matrix included:

The specimens were subjected to monotonic axial compression loading until failure. Load-displacement curves were recorded throughout, and the full failure process was observed and documented.

Core Technical Results

Load Capacity Enhancement

The ultimate stress enhancement ratio (the ratio of the reinforced column's ultimate stress to the unreinforced column's ultimate stress) ranged from 1.14 to 2.30. This represents a substantial improvement, particularly for columns with lower original concrete strength. The enhancement is attributed to the combined confinement effect of the steel tube and CFRP on the SCRC infill, which in turn provides enhanced lateral support to the original RC core.

Effect of Individual Parameters

• Steel tube wall thickness: Increasing the wall thickness significantly increased the load capacity. This is consistent with the fundamental principle of concrete-filled steel tube columns, where the confining pressure is proportional to the steel tube thickness. The thicker tube provides greater restraint against lateral concrete expansion, thereby increasing the triaxial compressive strength of the concrete core.
• CFRP layer count: Additional CFRP layers contributed incrementally to load capacity enhancement. However, the marginal benefit of each additional layer diminishes, which is consistent with the nonlinear stress-strain behaviour of CFRP under confinement.
• Recycled coarse aggregate replacement ratio: Higher replacement ratios reduced the load capacity. This is expected because recycled aggregates typically have lower strength, higher water absorption, and a more heterogeneous interfacial transition zone (ITZ) compared to natural aggregates. The authors introduced a recycled aggregate safety factor and a material utilisation coefficient to account for this reduction in their analytical model.
• Confining coefficient and cross-section diameter: Interestingly, increasing the confining coefficient or cross-section diameter through geometric modification alone did not yield proportionally significant improvements in load capacity. This suggests that the confinement mechanism reaches a practical limit beyond which additional geometric changes provide diminishing returns.

Ductility and Stiffness

All reinforced specimens exhibited ductile failure modes, in contrast to the brittle failure of the unreinforced reference column. The initial axial stiffness was significantly increased by the retrofitting, primarily due to the high elastic modulus of the steel tube. The ductility improvement is attributed to the steel tube's ability to sustain large inelastic deformations after yielding, combined with the CFRP's contribution to lateral confinement at the ultimate load stage.

Finite Element Analysis and Model Validation

The authors developed ABAQUS finite element models to complement the experimental programme and extend the parametric study beyond the physical specimen limits. The models incorporated:

• Material nonlinearity of concrete (both original and SCRC)
• Elastic-plastic behaviour of the steel tube
• CFRP confinement with appropriate bond-slip modelling
• Contact interfaces between the steel tube, SCRC infill, and original RC core

The finite element results showed good agreement with experimental data, validating the modelling approach. Extended parametric studies revealed that:

• Increasing the steel tube yield strength significantly improves load capacity.
• Higher recycled concrete strength leads to higher ultimate loads.
• Slenderness ratio variations had a negligible effect on load capacity for the short column range studied, confirming that buckling is not a concern for short columns.

Proposed Analytical Model

Based on the limit equilibrium method, the authors proposed an axial compressive load capacity formula for the retrofitted columns. The formula incorporates:

1. The contribution of the original RC column (reduced by a degradation factor).
2. The contribution of the steel tube-SCRC composite annulus, accounting for the confining pressure from both the steel tube and CFRP.
3. The recycled aggregate safety factor to adjust for the reduced strength of recycled concrete.
4. The material utilisation coefficient to account for incomplete stress transfer at interfaces.

The average error between the analytical predictions and experimental results was within 5%, which is excellent for a composite column system with multiple interacting materials.

Welding and Fabrication Quality Considerations

From the perspective of steel pipe fabrication and welding engineering, the following quality control aspects are critical for the successful implementation of this retrofitting system:

Steel Tube Welding Quality

The steel tube used for the retrofitting annulus is typically a welded steel pipe. The quality of the longitudinal weld is paramount because:

• Weld defects: Lack of fusion, porosity, or undercuts in the longitudinal weld can create stress concentration points that initiate local buckling of the tube wall under confinement pressure. Non-destructive testing (NDT) methods such as magnetic particle testing (MT) or ultrasonic testing (UT) should be specified for weld inspection, in accordance with standards such as GB/T 3323 (RT), GB/T 11345 (UT), or JB/T 4730.
• Weld residual stresses: The residual stresses from the pipe welding process (whether ERW, HFW, or LSAW) can influence the local buckling resistance of the tube wall. For thin-walled tubes used in retrofitting applications, the interaction between welding residual stresses and the confining pressure from the concrete can reduce the effective buckling resistance. Preheating and controlled cooling rates during welding can mitigate these residual stresses.
• Heat-affected zone (HAZ) properties: The HAZ of the pipe weld may have different mechanical properties from the base metal. For low-carbon structural steel pipes (e.g., Q235 or Q345 per GB/T 1591), the HAZ is generally acceptable, but for higher-strength grades or in the presence of elevated carbon equivalents, the HAZ may exhibit reduced toughness. Charpy V-notch testing of the HAZ is recommended for critical applications.

CFRP Bonding Quality

The CFRP sheets must be bonded to the outer surface of the steel tube with structural epoxy adhesive. Key quality control points include:

• Surface preparation: Sandblasting to Sa 2.5 grade (per ISO 8501-1) followed by degreasing is essential for achieving the design bond strength.
• Adhesive application: The adhesive must be applied at the manufacturer's specified temperature and humidity conditions. Out-of-autoclave (OAO) bonding requires careful control of cure temperature and pressure.
• Void inspection: Ultrasonic testing (UT) or tap testing can detect delaminations and voids in the CFRP bond line. Any voids exceeding 10% of the bond area should be repaired or the CFRP layer replaced.

Sustainability and Engineering Practice

The use of self-compacting recycled concrete in this retrofitting system represents a meaningful contribution to sustainable construction. Recycled coarse aggregates derived from demolished concrete can be used in structural applications when properly characterised and quality-controlled. The self-compacting property of the concrete ensures adequate filling of the annular space between the steel tube and the original RC column without the need for vibration, which is particularly advantageous in congested construction sites.

However, engineers must be aware that recycled aggregates introduce additional variability in concrete properties. The ITZ between recycled aggregates and the cement paste is weaker and more permeable than the ITZ in conventional concrete. This can affect long-term durability, particularly in environments where chloride ingress or carbonation is a concern. Additional impermeability measures (such as crystalline admixtures or surface treatments) may be warranted.

Reflections on the Analytical Model

The proposed analytical model, with an average error within 5%, represents a significant advance in the predictive capability for this type of composite retrofitting system. The introduction of the recycled aggregate safety factor and material utilisation coefficient is a pragmatic approach that acknowledges the inherent variability in recycled materials. In my experience with composite structural systems, such empirical coefficients are essential for bridging the gap between idealised analytical models and the messy reality of construction materials.

That said, the model is based on monotonic loading and short-term behaviour. For long-term serviceability, the following factors should be considered:

• Creep and shrinkage of the SCRC infill under sustained loads.
• Long-term bond strength degradation between CFRP and steel tube due to environmental exposure (UV, moisture, temperature cycling).
• Progressive corrosion of the steel tube if the bond fails locally.

Conclusion

This study provides a comprehensive experimental and analytical framework for the structural retrofitting of RC short columns using steel tube self-compacting recycled concrete with CFRP wrapping. The dual-confinement mechanism of the steel tube and CFRP, combined with the sustainability benefits of recycled aggregates, makes this system a promising solution for strengthening deteriorated concrete structures. The analytical model, validated with an average error within 5%, offers a practical design tool. Engineers implementing this system must pay careful attention to the welding quality of the steel tube, the bonding quality of the CFRP, and the long-term durability of the recycled concrete infill. The integration of green construction principles with proven structural engineering methods represents a constructive path forward for the built environment.