Eccentric Compression Behavior of Recycled Concrete Columns Confined by Steel Tubes and GFRP Tubes

Literature Overview

The paper by Xiao Jianzhuang, Liu Sheng, and Tresserras Joan, published in Journal of Architecture and Civil Engineering (2015, Vol. 32, No. 2, pp. 21-26), investigates the eccentric compression behavior of recycled aggregate concrete (RAC) columns confined by either steel tubes or glass fiber reinforced polymer (GFRP) tubes. Funded by the National Natural Science Foundation of China and the Shanghai Science and Technology Commission, this research was conducted at Tongji University's Department of Building Engineering. The study addresses the growing need for sustainable structural solutions that incorporate recycled materials while maintaining structural performance.

Experimental Program and Test Parameters

The experimental program included eight column specimens: four confined by steel tubes and four confined by GFRP tubes. Two key parameters were varied: the recycled coarse aggregate replacement ratio (ranging from 0% to 100%) and the expansive agent content. The following table summarizes the test matrix and principal outcomes:

Core Technical Findings

The research reveals several important technical observations regarding the interaction between confinement type, concrete quality, and structural performance under eccentric compression. First, 100% recycled coarse aggregate replacement reduces the ultimate load compared to ordinary concrete columns, which is attributed to the higher interfacial transition zone (ITZ) porosity and lower bond strength of recycled aggregates. However, the deformation capacity of 100% RAC columns is actually greater than that of ordinary concrete columns, suggesting that the increased ITZ porosity provides additional energy dissipation capacity through microcracking.

The comparative performance of steel tube and GFRP tube confinement reveals fundamental differences in structural behavior. Steel tube confinement provides superior ultimate load capacity due to the high elastic modulus and yield strength of steel, which generates a more effective confining pressure. GFRP tube confinement, while offering lower ultimate loads, provides greater deformation capacity and ductility, which is particularly beneficial for seismic applications where energy dissipation is critical.

The expansive agent findings are particularly noteworthy. Expansive agents improve the ultimate load of both steel tube and GFRP tube confined columns, but the improvement is more pronounced for GFRP tube confined specimens. This is because the expansive agent compensates for the lower confinement pressure provided by GFRP tubes by improving the concrete's internal compaction and reducing initial porosity, thereby enhancing the stress transfer efficiency between the concrete core and the GFRP tube.

Standards and Material Considerations

From a materials engineering perspective, this research intersects with several relevant standards and specifications:

The recycled aggregate concrete used in this study should comply with GB/T 50176 or equivalent standards, which specify requirements for the quality of recycled coarse aggregates, including crushing value, water absorption, and content of deleterious substances. The steel tubes used for confinement should meet the mechanical property requirements of GB/T 8163 or ASTM A500, with particular attention to the tensile strength, yield ratio, and elongation properties.

Engineering Practice Implications

The findings have significant implications for sustainable structural engineering practice. In regions with abundant construction and demolition waste, recycled aggregate concrete offers a viable alternative to virgin concrete, and the confinement strategies investigated in this study provide practical solutions for maintaining structural performance. The selection between steel tube and GFRP tube confinement should be guided by the following criteria:

1. For applications where maximum load capacity is the primary design criterion, steel tube confinement is preferred, particularly when combined with expansive agents to optimize the concrete-steel interaction.
2. For seismic applications or structures requiring enhanced ductility and energy dissipation, GFRP tube confinement is advantageous, especially when the concrete contains recycled aggregates.
3. For corrosive environments where steel tube durability is a concern, GFRP tube confinement offers superior corrosion resistance without the need for additional protective coatings.
4. The combination of 100% recycled aggregate replacement with GFRP tube confinement and expansive agent addition represents a promising sustainable solution that balances environmental benefits with acceptable structural performance.

Study Insights and Reflections

The research demonstrates that the mechanical behavior of confined recycled concrete columns is governed by a complex interaction between the confinement type, concrete quality, and loading eccentricity. The fact that 100% RAC columns exhibit greater deformation capacity despite lower ultimate loads suggests that the increased ITZ porosity acts as a damage-tolerant mechanism, which is a valuable property for seismic design. This finding challenges the conventional perception that recycled aggregates are inherently inferior to virgin aggregates, as they may offer complementary advantages in terms of energy dissipation and damage tolerance.

The differential response of steel tube and GFRP tube confined columns to expansive agents highlights an important design principle: the effectiveness of concrete additives depends on the confinement system's ability to transfer and maintain confining pressure. Steel tubes, with their high stiffness, generate confining pressure primarily through elastic deformation, which is relatively insensitive to concrete quality improvements. GFRP tubes, being more flexible, benefit more from concrete quality enhancements because the improved concrete properties directly translate into better stress transfer and higher effective confinement.

This research contributes to the development of sustainable structural engineering by demonstrating that recycled materials can be effectively incorporated into confined concrete systems without catastrophic loss of structural performance. The engineering community should embrace these findings as a pathway toward more sustainable construction practices while maintaining the safety and serviceability of structural systems.