Axial Compression Performance of Recycled Aggregate Concrete-Filled Steel Tube Short Columns

Research Context and Motivation

This paper by Wu Bo, Liu Wei, Liu Qiongxiang, and Xu Zhe from the State Key Laboratory of Subtropical Building Science at South China University of Technology and the Shenzhen Institute of Architectural Design and Research investigates the axial compression behaviour of steel tube columns filled with recycled aggregate concrete (RAC). The research is funded by the Ministry of Education New Century Talent Support Programme (NCET-04-0819) and the State Key Laboratory talent development fund (2008ZB15). The study is motivated by the dual objectives of resource conservation — utilising recycled concrete aggregate — and structural performance verification.

Experimental Programme

The investigation involves 17 test specimens subjected to axial compression loading, comparing recycled aggregate concrete-filled steel tube (RA-CFST) short columns with conventional concrete-filled steel tube (CFST) short columns. The testing programme includes measurements at both 7-day and 28-day curing ages to capture the early-age and long-term behaviour of the recycled concrete under confinement.

Specimen Configuration

Key Experimental Findings

Equivalent Structural Performance

The most significant finding is that despite incorporating 32% to 35% recycled concrete aggregate, the RA-CFST short columns exhibit axial mechanical performance comparable to fully cast-in-place CFST short columns. This result is notable because recycled aggregate concrete typically shows lower compressive strength and higher deformation compared to natural aggregate concrete when used in unconfined conditions. The steel tube confinement effectively compensates for the inherent weaknesses of the recycled concrete.

Stiffness, Strength, and Ductility Comparison

The comparison between RA-CFST and conventional CFST specimens reveals that:

• Stiffness: The initial and secant stiffness values of RA-CFST columns are close to those of conventional CFST columns, indicating that the recycled aggregate does not significantly degrade the column's resistance to deformation under compression.
• Strength: The ultimate compressive load capacity of RA-CFST columns is comparable to conventional CFST columns, demonstrating that the confinement effect of the steel tube is sufficient to develop the full strength potential of the recycled concrete core.
• Ductility: The deformation capacity and energy dissipation characteristics of RA-CFST columns are similar to conventional CFST columns, which is important for seismic and impact-resistant applications.

Code Compliance Assessment

The authors compare their experimental results with predictions from both Chinese and international CFST design codes. The Chinese code JCJ 01-89 (now superseded by JGJ/T 391-2017 but still referenced in this 2010 study) provides a design formula for CFST column capacity that shows good agreement with the test results for RA-CFST columns. This finding is significant because it suggests that existing CFST design codes can be applied to RA-CFST members without modification, at least for the 32-35% recycled aggregate replacement ratio studied.

Technical Analysis

Effect of Recycled Aggregate on Concrete Confinement

The confinement mechanism in CFST columns works through the interaction between the expanding concrete core and the steel tube. When concrete is loaded in compression, it tends to expand laterally, and the steel tube resists this expansion, creating a triaxial stress state that enhances the concrete's compressive strength and ductility. The question is whether recycled aggregate concrete, with its typically higher porosity and lower interfacial transition zone (ITZ) quality, responds differently to this confinement.

The experimental results suggest that the confinement effect is effective regardless of the aggregate type, as long as the recycled aggregate replacement ratio remains within the 32-35% range. This is likely because:

1. The triaxial stress state suppresses the propagation of microcracks in the recycled aggregate ITZ
2. The steel tube prevents lateral expansion that would otherwise cause premature failure
3. The confinement pressure is sufficient to close existing pores and voids in the recycled aggregate

Age-Dependent Behaviour

The inclusion of 7-day and 28-day testing provides valuable information on the age-dependent performance of RA-CFST columns. Early-age testing is particularly relevant for construction sequencing where formwork or adjacent structures may impose loads before the concrete reaches full strength. The comparison between RA-CFST and conventional CFST at early ages helps establish whether recycled aggregate columns require different construction schedules.

Engineering Practice Implications

This research provides direct evidence that recycled aggregate concrete is a viable alternative to natural aggregate concrete in CFST applications, at least for short column members under axial compression. The practical implications include:

1. Sustainability benefit: Incorporating 32-35% recycled concrete aggregate reduces virgin material consumption and construction waste disposal
2. Cost consideration: Recycled aggregate is typically less expensive than natural aggregate, potentially reducing material costs
3. Design simplicity: Existing CFST design codes can be applied without modification, simplifying the design process
4. Quality assurance: Standard CFST inspection and testing procedures remain applicable

However, engineers should note the following limitations of this study:

• The recycled aggregate replacement ratio is limited to 32-35%; higher replacement ratios may exhibit different behaviour
• Only short columns under axial compression were studied; slenderness effects, shear, and combined loading remain unexplored
• The recycled aggregate source and processing method are specific to the study location and may not be universally applicable
• Long-term behaviour including creep and durability under environmental exposure was not investigated

Study Insight

This research contributes meaningfully to the growing body of knowledge on recycled materials in structural engineering. The finding that recycled aggregate CFST columns perform comparably to conventional CFST columns under axial compression is encouraging for sustainable construction practices. The good agreement with existing design codes is particularly valuable because it means that the adoption of recycled aggregate in CFST applications does not require code amendments or special design procedures. For engineers working on green building or sustainable infrastructure projects, this provides a practical pathway for incorporating recycled materials without compromising structural safety.

This collection of five study notes covers a diverse range of topics in the steel pipe and structural engineering domain, spanning seismic design of composite joints, non-destructive testing technology, ultra-high strength composite materials, adaptive inspection equipment, and recycled material applications. Each topic represents a different aspect of the engineering lifecycle — from material development through manufacturing quality control to structural performance verification. Together, they illustrate the breadth of technical challenges and innovations in the steel pipe and structural steel industry, and each contributes specific, actionable knowledge for engineering practice. The common thread across all five studies is the pursuit of improved performance, efficiency, and reliability through targeted technical investigation and practical problem-solving.