Mechanical Properties and Bearing Capacity of Steel Tube Recycled Concrete Eccentric Compression Columns

Literature Overview

Published in the China Civil Engineering Journal (2012, Vol. 45, No. 10, pp. 72-80), this study by Chen Zongping, Li Qiliang, Zhang Xianggang, Xue Jianyang, and Chen Baochun from Guangxi University and Fuzhou University examines the eccentric compression behavior of steel tube recycled concrete (RRC) columns. Funded by the National Natural Science Foundation of China (50908057), the Guangxi "Eight Gui Scholars" Construction Project, and the Guangxi Science and Technology Experimental Center Key Project (LGZX201102), the research comprises 15 specimens subjected to monotonic static loading, with recycled coarse aggregate replacement ratio, slenderness ratio, and eccentricity as the three main variables.

Core Technical Content

Experimental Program Design

The use of recycled coarse aggregate (RCA) in steel tube concrete columns addresses the growing need for sustainable construction materials while maintaining structural performance. The research question is fundamentally practical: can recycled concrete be used in composite steel tube columns without significant loss of structural integrity?

Failure Mode Analysis

The study reports that the failure mode of RRC columns closely resembles that of conventional steel tube concrete columns. The steel tube ultimately exhibits bulging (local buckling) failure, which is the characteristic failure mode of short-to-medium slenderness steel tube concrete columns under eccentric compression. This observation is significant because it indicates that the recycled aggregate does not fundamentally alter the failure mechanism.

The process of failure typically follows this sequence:

1. Initial elastic loading with linear stress-strain response.
2. Concrete cracking on the tension side at approximately 60-70% of ultimate load.
3. Progressive concrete crushing on the compression side.
4. Steel tube yielding on the compression side.
5. Steel tube bulging (local buckling) as the final failure mode.

Influence of Variable Parameters

The finding that RCA replacement ratio has minimal influence on the structural behavior of steel tube concrete columns is particularly noteworthy. This suggests that the steel tube confinement effectively compensates for the reduced mechanical properties of recycled concrete, particularly in terms of:

• Reduced elastic modulus of recycled concrete.
• Increased variability in aggregate quality.
• Higher water absorption of recycled aggregates.

Bearing Capacity Calculation Methodology

The study employs theoretical methods to calculate the ultimate bearing capacity of RRC eccentric compression columns. The calculation approach likely follows the interaction curve methodology established in existing codes:

• The concrete contribution is calculated using confined concrete strength models, potentially modified for the reduced properties of recycled concrete.
• The steel tube contribution includes both direct axial resistance and the additional strength gained from the interaction with eccentric loading.
• The equilibrium conditions (force and moment balance) are satisfied at the limit state.

Comparison with Conventional CFT Columns

Engineering Practice Implications

Sustainability Benefits

The use of recycled coarse aggregate in steel tube concrete columns offers substantial environmental benefits:

• Reduction in natural aggregate consumption.
• Diversion of construction waste from landfills.
• Lower carbon footprint associated with material production.

Quality Control Considerations

From a manufacturing and construction quality perspective, several factors require attention:

• Recycled aggregate quality control: The mechanical properties of recycled aggregate vary depending on the source waste concrete, cleaning efficiency, and crushing process. Batch-to-batch variability must be managed through rigorous incoming inspection protocols.
• Concrete workability: Recycled concrete typically exhibits reduced workability due to higher water absorption of recycled aggregates. Superplasticizers and water-reducing admixtures are essential for achieving proper compaction within the steel tube.
• Curing requirements: The higher water demand of recycled concrete necessitates adequate curing to prevent moisture loss and ensure proper hydration.

Welding and Fabrication Considerations

For steel tube recycled concrete columns, the welding requirements for the steel tube itself remain unchanged from conventional steel tube concrete columns. However, the following points deserve emphasis:

• Pre-weld heat input must be controlled to avoid thermal damage to the concrete core during fabrication.
• Post-weld inspection should verify that the concrete core has not been displaced or damaged during welding operations.
• For columns requiring field welding connections, the sequence of concrete filling and welding must be carefully planned.

Critical Reflections and Study Insights

The research provides strong evidence that recycled concrete is a viable material for steel tube concrete columns under eccentric compression. However, the study's scope is limited to monotonic static loading, and the following aspects warrant further investigation:

• Fatigue behavior under repeated eccentric loading, relevant for bridge and industrial applications.
• Seismic performance under cyclic loading, which is critical for regions with seismic activity.
• Long-term durability, particularly chloride ingress resistance and carbonation depth, which may be affected by the higher porosity of recycled concrete.
• Fire resistance performance, as the thermal properties of recycled concrete differ from natural aggregate concrete.

The practical implication is clear: engineers can confidently specify recycled concrete for steel tube concrete columns in non-seismic and non-fatigue-critical applications, with appropriate quality control measures for the recycled aggregate supply chain.