Zhuojin Pipe Fitting Co., LtdOrthogonal Experimental Study on Mechanical Properties of Circular Steel Tube Recycled Concrete Composite Columns
Literature Overview
Published in the Journal of Shenyang Jianzhu University in 2019, this research by Wen Yang, Jiang Wenxia, Han Hongpeng, and Fu Liping from Inner Mongolia University of Science and Technology investigates the mechanical behavior of circular steel tube recycled concrete (RAC) composite columns using orthogonal experimental design. Funded by the National Natural Science Foundation of China (Grant 51768056), the study systematically varies steel ratio, recycled aggregate replacement rate, and slenderness ratio to determine their influence on load-bearing capacity and deformation characteristics. The authors derive a practical load-bearing capacity formula suitable for these composite columns.
Core Technical Parameters and Experimental Design
The orthogonal experimental approach is particularly efficient for identifying the most influential parameters among multiple variables. The study varies three key parameters: steel ratio (含钢率), recycled aggregate replacement rate (取代率), and slenderness ratio (长细比). The sensitivity ranking reveals that slenderness ratio is the dominant factor, followed by steel ratio, with replacement rate having the least influence.
| Parameter | Sensitivity Index | Typical Range in Study | Effect on Capacity |
|---|---|---|---|
| Slenderness ratio (λ) | 196 | 20–50 | Higher λ significantly reduces capacity |
| Steel ratio | 147 | 5%–15% | Higher ratio increases capacity |
| Replacement rate | 113 | 0%–50% | Moderate effect on capacity |
The maximum bearing capacity of 1351 kN was achieved at a slenderness ratio of 50, steel ratio of 11%, and replacement rate of 30%. This optimum point is noteworthy because it demonstrates that a moderate replacement rate of recycled aggregate does not compromise structural performance when other parameters are properly selected.
Welding and Fabrication Considerations
From a steel tube fabrication standpoint, the circular steel tubes used in these composite columns must meet stringent requirements for dimensional accuracy and weld quality. The steel tube is typically manufactured by electric resistance welding (ERW) or high-frequency welding (HFW), conforming to GB/T 3091 or ASTM A53 standards. The longitudinal weld must be free of defects such as lack of fusion, undercut, or internal voids, as these would create stress concentration points that could initiate failure under compressive loading.
The steel ratio of 11% implies a significant contribution from the steel tube to the overall structural capacity. This means that the steel tube's material properties, particularly its yield strength and uniform elongation, become critical design parameters. The tube must be fabricated from steel grades such as Q345 or Q390 conforming to GB/T 1591, with certified mill test reports verifying chemical composition and mechanical properties.
Interpretation of Results and Standard Comparison
The study compares the calculated bearing capacity against several design codes and finds that the Chinese CECS 28-2012 standard provides the most suitable basis for these composite columns. The derived formula is expressed as:
N = (-0.0168λ + 1.8093)(f_y·A_s + K_1·f_c·A_c)
where λ is the slenderness ratio, f_y is the yield strength of steel, A_s is the steel cross-sectional area, K_1 is a confinement coefficient, f_c is the concrete compressive strength, and A_c is the concrete cross-sectional area.
| Design Standard | Applicability to RAC Columns | Notes |
|---|---|---|
| CECS 28-2012 (China) | Best fit | Accounts for composite action accurately |
| Eurocode 4 | Conservative | Does not fully capture RAC behavior |
| AISC 360 (USA) | Limited | Primarily designed for conventional concrete |
| GB 50017 | Acceptable | Requires modification for recycled aggregate |
Engineering Practice Integration
In engineering practice, the use of recycled concrete in composite columns presents specific challenges related to concrete quality control. Recycled aggregates typically have higher water absorption and lower density than natural aggregates, which affects the workability and ultimate compressive strength of the concrete. During the concrete filling of the steel tube, proper vibration and compaction are essential to achieve uniform density. The welding integrity of the steel tube becomes even more critical because any defect in the tube wall could allow concrete to escape during pouring or create a weak zone under load.
The finite element simulation results showing maximum stress at the mid-length of the column are consistent with classical column buckling theory. However, the presence of recycled aggregate introduces additional heterogeneity in the concrete material model, which must be accounted for in detailed design calculations. The simulation also confirms that the steel tube effectively confines the concrete, enhancing its ductile behavior and preventing brittle failure.
Key Reflections and Practical Recommendations
The sensitivity analysis reveals a practical design strategy: prioritize slenderness ratio control through appropriate bracing or support conditions, as this has the greatest impact on structural performance. The finding that 30% replacement rate is acceptable for recycled aggregates is encouraging for sustainable construction practices, but engineers must ensure that the recycled aggregate meets minimum quality standards for particle size distribution and contamination levels. The study ultimately demonstrates that composite CFST columns using recycled concrete can achieve reliable structural performance when designed with proper attention to geometric and material parameters, offering a viable path toward more sustainable structural engineering without compromising safety.