Interface Bonding Performance of Square Steel Tube Sandwich Concrete Retrofitting of CFST Columns

Literature Overview

This study examines the interface bonding performance when retrofitting existing concrete-filled steel tube (CFST) columns by adding an external square steel tube with sandwich concrete layers. The retrofitting approach is relevant for strengthening aging infrastructure, particularly in urban environments where replacement of existing columns is impractical. The research addresses the critical question of how the new sandwich concrete layers interact with the existing CFST surface and how the overall composite section performs under various loading conditions.

Core Technical Points

The retrofitting process involves creating a gap between the existing CFST column and the new external square steel tube, filling this gap with concrete to form a sandwich composite section. The interface between the existing CFST surface and the new sandwich concrete is the weakest link in the system, as it must transfer shear forces and normal stresses between the old and new components. The bonding performance at this interface is influenced by surface preparation, concrete mix design, gap thickness, and the presence of mechanical connectors such as shear studs or dowel bars.

From a materials science perspective, the existing CFST surface may have undergone environmental degradation, including surface oxidation, corrosion product accumulation, or previous coating layers. These surface conditions directly affect the bond strength between the existing steel tube and the new sandwich concrete. Surface rust removal through shot blasting to Sa 2.5 grade per ISO 8501-1 is typically required to achieve adequate bond, though aggressive acid etching or thermal spray texturing may be necessary for heavily corroded surfaces.

The sandwich concrete itself presents unique challenges, as it must be placed in a confined gap with limited access for compaction. The use of self-compacting concrete (SCC) or flowable concrete mixes is recommended to ensure void-free placement around the existing column surface and any embedded mechanical connectors. The concrete mix must also be designed for low shrinkage and creep to minimize long-term bond degradation at the interface.

Process and Standards Analysis

The design should reference GB 50017-2017 for steel structure design, GB 51248-2016 for CFST structures, and relevant retrofitting standards such as GB 50550-2010 for existing building structural assessment and reinforcement. The welding of shear studs to the external square tube must comply with ISO 17643, with full penetration welds verified by ultrasonic testing.

Engineering Practice Integration

The retrofitting process requires a systematic approach to surface preparation, mechanical connection installation, and concrete placement. A typical sequence involves: (1) surveying and marking the existing column surface; (2) surface cleaning and preparation to expose sound steel; (3) fabrication and installation of the external square tube with gap-filling brackets; (4) welding of shear studs or dowel bars to the external tube interior; (5) placement of sandwich concrete using tremie or pump-assisted methods; and (6) curing and quality verification.

The welding of the external square tube to the existing column base or cap is a critical structural connection that must be designed for full moment and shear transfer. Full-penetration butt welds or bolted flange connections are preferred, with weld quality verified by radiographic testing (RT) per ISO 17636 and magnetic particle testing (MT) per ISO 17638 for surface defect detection. Residual stresses from these connection welds can affect the local buckling behavior of the external tube, so post-weld stress relief may be necessary for high-strength steels.

Key Questions and Reflections

A fundamental question in this research is whether the interface bond between the existing CFST and the new sandwich concrete can be relied upon for primary load transfer, or whether mechanical connectors are always necessary. The research suggests that for gap thicknesses below 100 mm and with proper surface preparation, the bond strength alone may suffice for normal load conditions, but mechanical connectors provide essential redundancy for seismic or impact loading.

Another critical consideration is the differential movement between the existing column and the new sandwich section under service loads. Creep and shrinkage of the sandwich concrete, combined with thermal expansion differences between the old and new steel components, can induce interface stresses that may lead to debonding over time. The use of low-shrinkage concrete mixes and flexible gap-filling materials at the top and bottom of the sandwich section can mitigate these concerns.

Study Insights and Implications

The research confirms that the square steel tube sandwich concrete retrofitting approach can effectively enhance the load-bearing capacity and ductility of existing CFST columns, with capacity improvements of 20–50% depending on the retrofitting geometry and connection details. The interface bond performance is the governing factor for the overall effectiveness of the retrofitting, and rigorous quality control at the surface preparation and concrete placement stages is essential.

For welding and fabrication engineers, the key takeaway is that the external tube fabrication and connection welding must meet the same quality standards as new construction, with no compromise on weld inspection and acceptance criteria. The interface between old and new components requires special attention, and non-destructive testing methods such as ultrasonic bond testing or ground-penetrating radar (GPR) should be employed to verify the integrity of the sandwich concrete fill.

Summary

The retrofitting of existing CFST columns with external square steel tube and sandwich concrete is a viable and effective strengthening method, provided that the interface bond is properly designed, executed, and verified. The success of this approach depends on meticulous surface preparation, appropriate mechanical connection design, high-quality concrete placement, and comprehensive quality control throughout the retrofitting process. Engineers should adopt a conservative approach to interface bond design, incorporating mechanical redundancy and regular inspection protocols to ensure long-term structural reliability.