Zhuojin Pipe Fitting Co., LtdEccentric Compression Behavior of Recycled Concrete Columns Confined by Steel Tubes and GFRP Tubes
Overview and Research Context
This paper by Xiao, Liu, and Tresserras (2015) from Tongji University investigates the eccentric compression behavior of recycled aggregate concrete (RAC) columns confined by either steel tubes or glass fiber reinforced polymer (GFRP) tubes. The study addresses the growing need to incorporate recycled materials into structural construction while maintaining adequate load-bearing performance and ductility. Funded by the National Natural Science Foundation (51438007) and the Shanghai Science and Technology Commission (14231201300), the research contributes to sustainable construction practices by evaluating the structural viability of recycled concrete in confined column applications.
The use of recycled coarse aggregate (RCA) in structural concrete is increasingly promoted to reduce construction waste and conserve natural resources. However, RCA typically exhibits lower strength and higher permeability compared to natural aggregate, raising concerns about the structural performance of RAC members, particularly under eccentric loading conditions where both compressive and flexural demands are present.
Experimental Program and Parameters
Eight test specimens were fabricated and tested: four steel tube confined RAC columns and four GFRP tube confined RAC columns. The experimental parameters and their ranges are summarized below:
| Parameter | Levels Tested | Purpose |
|---|---|---|
| Recycled aggregate replacement ratio | 0% (control), 100% | Evaluate effect of RCA on strength and ductility |
| Expansive agent dosage | 0%, 2%, 4% | Assess effect of microcrack control on confinement efficiency |
| Confinement type | Steel tube, GFRP tube | Compare confinement effectiveness |
| Loading condition | Eccentric compression | Simulate realistic loading scenarios |
The specimens were subjected to eccentric compression loading until failure, with measurements recorded for ultimate load, axial deformation, and load-displacement response.
Key Findings
The experimental results reveal several important trends:
- Specimens with 100% recycled coarse aggregate replacement exhibit lower ultimate loads compared to those using natural aggregate, consistent with the reduced strength of RCA.
- At the same concrete strength level, GFRP tube confined RAC specimens achieve lower eccentric compression ultimate loads than steel tube confined specimens, reflecting the lower confinement pressure provided by GFRP tubes due to their lower modulus of elasticity.
- Expansive agents improve the eccentric compression ultimate load for both steel tube and GFRP tube confined specimens, with a more pronounced effect on GFRP tube confined specimens. This is attributed to the fact that expansive agents densify the concrete and improve the bond between the concrete core and the GFRP tube, thereby enhancing confinement efficiency.
- GFRP tube confined specimens exhibit greater deformation capacity (ductility) than steel tube confined specimens.
- 100% RCA replacement specimens show greater deformation capacity than natural aggregate specimens, which is counterintuitive but may be explained by the increased microcracking and energy dissipation in the RAC matrix.
Engineering Practice Implications
The findings have direct relevance to sustainable structural design. The use of 100% recycled aggregate in confined columns is structurally viable, particularly when combined with expansive agents and appropriate confinement design. For engineers specifying recycled concrete in column applications, the following practical recommendations can be drawn:
- Steel tube confinement provides superior ultimate load capacity but may exhibit brittle failure modes.
- GFRP tube confinement offers better ductility and is more sensitive to the use of expansive agents, making it a potentially more efficient solution when combined with chemical admixtures.
- The combination of 100% RCA and expansive agents at 2-4% dosage can partially compensate for the strength reduction caused by recycled aggregate.
Study Insights and Reflections
The observation that GFRP tube confined specimens exhibit greater ductility than steel tube confined specimens is particularly noteworthy. While steel tubes provide higher confinement pressure, the higher stiffness of steel may limit the deformation capacity of the confined concrete, leading to more sudden failure. In contrast, GFRP tubes, with their lower modulus and higher strain capacity, allow for more progressive deformation of the concrete core before failure. This insight is relevant to seismic design, where ductility is a critical performance criterion.
The enhanced effect of expansive agents on GFRP tube confined specimens is also significant. Expansive agents induce controlled microcracking and densification of the concrete matrix, which improves the interfacial bond between the concrete and the GFRP tube. Since GFRP tubes rely more heavily on bond quality for effective confinement (compared to steel tubes where mechanical interlock and friction are more significant), the densification effect of expansive agents is particularly beneficial. This finding suggests that for GFRP tube confined columns, the use of expansive agents should be considered a standard practice rather than an optional enhancement.