Zhuojin Pipe Fitting Co., LtdSeismic Performance Testing of Steel Tube Recycled Concrete Columns
Literature Overview
This paper by Zhang Xianggang et al. (2014), published in China Civil Engineering Journal (Vol. 47, No. 9, pp. 45-56), presents a comprehensive experimental study on the seismic performance of circular steel tube recycled concrete (STRC) columns. The study is funded by multiple sources including the National Natural Science Foundation of China (No. 51268004) and Guangxi provincial research programs. Ten circular specimens were designed and tested under quasi-static cyclic loading, with four variable parameters: recycled coarse aggregate replacement rate, slenderness ratio, axial compression ratio, and steel ratio.
Core Technical Content
Test Specimen Design
The ten circular STRC column specimens were designed with the following variable parameters:
| Parameter | Variation Range | Purpose |
|---|---|---|
| Recycled coarse aggregate replacement rate | 0%, 30%, 50%, 70%, 100% | Evaluate effect of recycled concrete on seismic performance |
| Slenderness ratio | Multiple values | Assess geometric influence on ductility |
| Axial compression ratio | Multiple values | Evaluate effect of pre-compression on behavior |
| Steel ratio | Multiple values | Assess confinement effectiveness |
Test Procedure
The specimens were subjected to quasi-static cyclic loading, which simulates the repeated loading and unloading experienced during seismic events. The loading protocol involved incremental displacement cycles with increasing amplitude, following standard seismic testing procedures.
Key Experimental Results
- Failure mode: The specimens exhibited failure patterns similar to conventional steel tube concrete (STC) columns, characterized primarily by local buckling and bulging of the steel tube at the base of the column.
- Hysteresis behavior: The hysteresis curves were relatively full and well-formed, evolving from a spindle shape to an arch shape as the loading progressed. This indicates good energy dissipation capacity.
- Effect of recycled aggregate replacement rate: Within the tested replacement rate range, the hysteresis curves were not significantly affected by the recycled coarse aggregate replacement rate. This is a critical finding for the practical application of recycled concrete in seismic-resistant structures.
- Ductility: The displacement ductility coefficient exceeded 3.0 for all specimens, indicating good ductile behavior.
- Energy dissipation: The equivalent viscous damping coefficient at failure ranged from 0.305 to 0.460, demonstrating significant energy dissipation capacity.
Design Calculation Recommendations
The authors compared the calculated flexural compression capacities using several codes and standards, and recommended the use of DL/T 5085-1999 for the design calculation of circular STRC columns under cyclic loading.
Technical Analysis
Recycled Concrete in Structural Applications
The use of recycled coarse aggregate in structural concrete has been promoted as a sustainable construction practice. The primary concern with recycled concrete is the reduced mechanical properties due to the adhered old mortar on the recycled aggregate surface. However, this study demonstrates that the seismic performance of STRC columns is not significantly affected by the recycled aggregate replacement rate within the tested range.
The key factor is the confinement provided by the steel tube, which compensates for the reduced concrete strength. The steel tube confines the recycled concrete core, preventing premature crushing and enabling the column to undergo large inelastic deformations without catastrophic failure.
Steel Tube Confinement Mechanism
The steel tube provides lateral confinement to the concrete core, which enhances the compressive strength and ductility of the concrete. The confinement effectiveness depends on several factors:
- Steel tube diameter-to-thickness ratio (D/t): A lower D/t ratio provides more effective confinement but may be impractical for large columns.
- Steel tube yield strength: Higher yield strength provides greater confining pressure.
- Steel tube buckling resistance: The steel tube must resist local buckling to maintain confinement throughout the loading history.
Welding and Manufacturing Considerations
The steel tubes used in the STRC columns are typically manufactured by HFW or LSAW processes. The weld quality directly affects the confinement effectiveness and the overall seismic performance of the column. Key welding considerations include:
- Weld residual stress: Residual stresses in the longitudinal weld can affect the local buckling behavior of the steel tube.
- Weld defects: Lack of fusion, porosity, or cracks in the weld can serve as initiation sites for local buckling.
- Weld HAZ properties: The HAZ may exhibit different mechanical properties compared to the base metal, affecting the buckling resistance.
The NDT requirements for the steel tube welds should be stringent, with full-length UT or RT inspection recommended for seismic applications.
Comparison with Conventional STC Columns
| Performance Indicator | Conventional STC | STRC (This Study) |
|---|---|---|
| Failure mode | Steel tube buckling at base | Similar to conventional STC |
| Hysteresis curve shape | Full, spindle to arch | Full, spindle to arch |
| Displacement ductility coefficient | > 3.0 | > 3.0 |
| Equivalent viscous damping coefficient | 0.30-0.46 | 0.305-0.460 |
| Effect of recycled aggregate | N/A | Not significant within tested range |
The results demonstrate that STRC columns perform comparably to conventional STC columns in terms of seismic performance, supporting the use of recycled concrete in seismic-resistant structural applications.
Study Insights and Reflections
This study provides strong experimental evidence that recycled concrete can be used in structural columns without compromising seismic performance, provided that the steel tube confinement is properly designed. The finding that the hysteresis curves are not significantly affected by the recycled aggregate replacement rate is particularly encouraging for the promotion of recycled concrete in construction.
From a steel pipe manufacturing perspective, the use of recycled concrete does not impose additional requirements on the steel tube manufacturing process. The same manufacturing and welding quality standards apply to both conventional STC and STRC columns. However, the steel tube must be designed with adequate confinement capacity to compensate for any reduction in concrete strength due to the use of recycled aggregate.
The recommendation to use DL/T 5085-1999 for design calculations is notable, as this standard is specifically tailored for the design of STC structures under seismic loading. Engineers should verify that the standard's provisions are applicable to STRC columns, considering the potential differences in material properties between recycled and conventional concrete.
The study contributes to the broader goal of sustainable construction by demonstrating that recycled materials can be used in critical structural applications without compromising safety. As the construction industry moves toward more sustainable practices, this type of research is essential for building confidence in the use of recycled materials in seismic-resistant structures.