Bearing Capacity of Steel Tube Self-Compacting Concrete Composite Strengthening of Reinforced Concrete Circular Short Columns

Literature Overview and Innovation

This paper published in the Journal of Wuhan University (Engineering Science Edition) (2014, Vol. 47, No. 6, pp. 769-773) by Xue Jifeng, Lu Yiyan, Liang Hongjun, and Gong Tianniu from Wuhan University introduces a novel composite strengthening method for reinforced concrete (RC) circular short columns. The method involves encasing the existing RC column with a steel tube and filling the annular gap with self-compacting concrete (SCC). The research is supported by the National Natural Science Foundation of China (Grant No. 51078294). This approach represents a creative integration of steel tube confinement and high-performance concrete, offering a robust strengthening solution for existing columns that require increased load capacity.

Experimental Program and Test Results

The study conducted axial compression tests on eight specimens: one unreinforced short column and seven composite-strengthened short columns with varying parameters. The parameters investigated included steel tube wall thickness, SCC strength, and interface treatment between the existing RC column and the new steel tube-SCC jacket.

The experimental results demonstrate that composite-strengthened columns exhibit substantially improved bearing capacity and ductility compared to unreinforced columns. The steel tube wall thickness emerges as the dominant parameter governing bearing capacity, followed by SCC strength. Surprisingly, the interface treatment between the existing RC column and the new jacket has minimal effect on the overall bearing capacity.

Theoretical Analysis and Formula Development

Based on the limit equilibrium theory, the authors derive a bearing capacity calculation formula for steel tube SCC strengthened RC short columns. The formula accounts for the contributions of the original RC column, the steel tube jacket, and the SCC annular layer. The calculation results show good agreement with experimental values and are generally conservative, which is desirable from a safety perspective in structural engineering.

The limit equilibrium approach assumes that at the ultimate state, the steel tube yields uniformly, the SCC reaches its confined compressive strength, and the original RC column reaches its residual capacity. The formula provides a practical design tool that can be used for preliminary sizing of the steel tube and SCC parameters required to achieve a target bearing capacity.

Engineering Practice Considerations

From a fabrication and installation standpoint, this composite strengthening method presents several practical challenges. The steel tube jacket must be fabricated to precise dimensions to fit over the existing RC column, which may have surface irregularities. The SCC must be pumped or poured into the annular gap, which may be narrow, requiring careful attention to workability and consolidation. The self-compacting property of the concrete eliminates the need for vibration, which is advantageous in confined spaces, but the SCC mix must be designed to achieve adequate flowability without segregation.

The interface treatment finding is particularly interesting from a materials science perspective. The minimal effect of interface treatment suggests that the composite action between the original RC column and the new jacket is not critically dependent on bond quality. This may be attributed to the significant confinement pressure developed within the SCC annular layer, which effectively locks the components together regardless of the initial interface condition. However, engineers should still ensure adequate surface preparation to maximize the composite action and prevent slippage under service loads.

Welding and Fabrication Implications

The steel tube jacket fabrication requires precise welding of longitudinal and circumferential seams. For circular columns, the steel tube is typically formed by rolling flat steel plates and welding the longitudinal seam, followed by welding circumferential joints to achieve the required height. The welding process must be selected based on the steel grade and wall thickness. For thin-walled tubes (typically 6-12 mm in strengthening applications), gas metal arc welding (GMAW) or flux-cored arc welding (FCAW) are commonly employed.

Quality control of the steel tube jacket welds is essential, as any weld defects could compromise the structural integrity of the strengthening system. Non-destructive testing (NDT) should be performed on all welds, with visual testing (VT) and magnetic particle testing (MT) as minimum requirements, and ultrasonic testing (UT) for critical applications. The welding procedure must be qualified according to applicable standards such as AWS D1.1 or ISO 3834.

Reflections and Design Recommendations

This research contributes a practical and effective method for strengthening existing RC columns. The composite action between the steel tube, SCC, and original RC column creates a synergistic system that outperforms the sum of its individual components. The finding that steel tube wall thickness is the primary design parameter provides clear guidance for engineers: increasing wall thickness should be the first consideration when additional capacity is required.

The conservative nature of the proposed calculation formula is beneficial for design practice, as it provides a safety margin. However, engineers should exercise judgment and not rely solely on the formula for critical applications; detailed finite element analysis should be performed for important structures. The method is particularly suitable for strengthening short columns in existing buildings where additional floor loads are imposed or where seismic retrofitting is required.