Recycled Self-Compacting Concrete-Filled Steel Tube Short Column Axial Compression Tests

Research Background and Motivation

This 2011 paper by Zhou Jinghai and colleagues from Shenyang Jianzhu University investigates the feasibility of using recycled aggregate self-compacting concrete in steel tube concrete structures. The study is motivated by the growing environmental concern regarding concrete waste disposal and the potential for incorporating recycled coarse and fine aggregates into structural applications. The authors employed orthogonal experimental design to optimize the mix proportions of recycled concrete and then conducted axial compression tests on short columns filled with the optimized recycled self-compacting concrete. This work is significant from a steel pipe application standpoint because it explores whether recycled concrete can serve as a viable infill material in composite steel tube columns without compromising structural performance.

Orthogonal Test Design and Optimal Mix

The study examined three factors at three levels each: recycled aggregate replacement ratio, water-reducing agent dosage, and fly ash content. The orthogonal test results identified the optimal combination as 50% recycled aggregate replacement, 10% water-reducing agent dosage, and 8% fly ash content. This combination achieved the best balance between workability (essential for self-compacting properties) and compressive strength.

The 50% replacement ratio represents a practical compromise, as higher replacement ratios would further reduce material cost and waste but at the expense of strength and durability. The 10% water-reducing agent dosage is relatively high, which is necessary to maintain the self-compacting properties required for filling steel tubes without mechanical vibration. The 8% fly ash content contributes to workability improvement and long-term strength development through pozzolanic reactions.

Test Results and Performance Comparison

The axial compression tests revealed that the ultimate load of recycled self-compacting concrete-filled steel tube columns was lower than that of conventional concrete-filled steel tube columns. However, the displacement at maximum load was approximately 7 mm for both types, indicating similar ductility characteristics. This is a crucial finding because in structural engineering, ductility often governs the design of composite columns, particularly in seismic zones where energy dissipation through plastic deformation is essential for structural safety.

From a steel pipe metallurgical perspective, the behavior of the steel tube in composite action depends heavily on the confinement pressure exerted by the concrete infill. Recycled concrete typically has a lower elastic modulus than virgin concrete, which means the confinement pressure developed at a given strain level will be somewhat lower. However, the similar displacement at peak load suggests that the overall composite action between the steel tube and the recycled concrete infill remains effective. The steel tube, typically made from ERW carbon steel conforming to GB/T 3091 or similar standards, provides the primary tensile confinement that prevents lateral expansion of the concrete under axial compression.

Feasibility Assessment and Practical Implications

The conclusion that recycled self-compacting concrete can be applied in steel tube concrete structures is supported by the experimental evidence, though with the important caveat that ultimate load capacity is somewhat reduced. For non-seismic or lightly loaded applications, this reduction may be acceptable, particularly when considering the environmental benefits and cost savings. The self-compacting property is particularly advantageous for steel tube concrete applications because it eliminates the need for internal vibration, which is difficult to achieve in narrow-diameter tubes and can potentially damage the steel tube wall if not properly controlled.

In practice, I would recommend that engineers considering this approach verify the long-term durability of recycled concrete-filled steel tubes, particularly regarding carbonation depth, chloride permeability, and potential corrosion of the steel tube due to the recycled aggregate. The higher water demand of recycled concrete can lead to increased porosity and potentially accelerated corrosion of the embedded steel tube. Proper surface treatment of the steel tube interior, such as application of anti-corrosion coatings or use of stainless steel tubes, may be necessary for long-term service life in aggressive environments. The study provides a valuable foundation for further research into sustainable composite steel tube structures.