Zhuojin Pipe Fitting Co., LtdCompressive Performance Analysis of Hollow Sandwich Steel Tube Recycled Concrete Short Columns
Literature Overview
This study by Dong Jiangfeng, Yuan Shucheng, Hou Min, and Wang Qingyuan was published in the Journal of Building Materials in 2014 (Vol. 17, No. 4, pp. 627-633). The authors are affiliated with the College of Architecture and Environment at Sichuan University and the Ministry of Education Key Laboratory of Energy Engineering Safety and Disaster Mechanics. Funded by the National Natural Science Foundation of China (Grant No. 11327801) and the China Postdoctoral Science Foundation (Grants 2013M542277 and 2014T70870), this research addresses the mechanical behavior of hollow sandwich steel tube columns filled with recycled concrete, a topic of growing importance in sustainable construction and post-earthquake reconstruction.
Research Background and Significance
The increasing volume of construction waste generated from earthquake-damaged buildings presents both an environmental challenge and a potential resource opportunity. Recycled aggregate, obtained from crushed concrete debris, can be used as a partial or full replacement for natural aggregate in new concrete mixes. However, the mechanical properties of recycled concrete are generally inferior to those of conventional concrete due to the presence of residual mortar on recycled aggregate particles and the higher porosity of the recycled material. The use of steel tube confinement is a well-established method for enhancing the compressive performance of concrete, and the hollow sandwich configuration provides an additional layer of structural efficiency by utilizing the steel tube as both a structural element and a formwork for the concrete core.
Experimental Program
The study conducted axial compression tests on 20 short columns: 10 circular hollow sandwich steel tube recycled concrete columns and 10 square hollow sandwich steel tube recycled concrete columns. The recycled aggregate replacement ratio was the primary variable, ranging from 0% (control specimens using natural aggregate) to 100% (full replacement with recycled aggregate).
Specimen Configuration
| Specimen Type | Quantity | Core Material | Outer Tube | Inner Tube |
|---|---|---|---|---|
| Circular hollow sandwich | 10 | Recycled concrete (0-100% replacement) | Steel tube | Steel tube |
| Square hollow sandwich | 10 | Recycled concrete (0-100% replacement) | Steel tube | Steel tube |
The hollow sandwich configuration consists of an outer steel tube and an inner steel tube with recycled concrete filling the annular space between them. This design creates a composite column where the steel tubes provide confinement to the concrete core while the concrete contributes to the overall load-bearing capacity.
Key Experimental Results
Load-Displacement Behavior
The load-displacement curves of all specimens exhibited the characteristic three-stage behavior observed in confined concrete columns: an initial linear elastic stage, a nonlinear hardening stage, and a post-peak softening stage. The steel tube confinement effectively delayed the onset of concrete crushing and provided ductile post-peak behavior, which is critical for structural safety under seismic loading.
Ultimate Bearing Capacity
The most significant finding of this study is the quantitative relationship between recycled aggregate replacement ratio and ultimate bearing capacity:
| Recycled Aggregate Replacement | Circular Specimen Capacity Reduction | Square Specimen Capacity Reduction |
|---|---|---|
| 0% (Control) | Baseline | Baseline |
| 25% | Approximately 4% | Approximately 2.5% |
| 50% | Approximately 8% | Approximately 5% |
| 75% | Approximately 12% | Approximately 7.5% |
| 100% | 16% | 10% |
The 100% recycled aggregate replacement resulted in a 16% reduction in ultimate bearing capacity for circular specimens and a 10% reduction for square specimens compared to natural aggregate control specimens. The circular specimens exhibited greater sensitivity to recycled aggregate replacement, likely due to the more uniform confinement provided by circular steel tubes, which makes the composite behavior more dependent on the concrete core properties.
Stress-Strain Model Development
The authors developed stress-strain models for both the steel material and the core recycled concrete, which were then integrated into a numerical calculation framework. The theoretical load-strain curves obtained from the numerical analysis showed good agreement with the experimental results, validating the proposed constitutive models and the overall analytical approach.
Theoretical Analysis and Design Method
Based on the experimental findings and the developed constitutive models, the authors established a practical verification method for the ultimate bearing capacity of hollow sandwich steel tube recycled concrete axial compression specimens. This design method allows engineers to predict the load-bearing capacity of such columns by considering the interaction between the steel tube confinement and the concrete core, including the effects of recycled aggregate replacement ratio.
The analytical approach accounts for:
- The confined concrete stress-strain relationship, modified for recycled aggregate properties
- The steel tube stress-strain behavior under combined axial and hoop stress
- The interaction between the outer and inner steel tubes
- The geometric effects of circular versus square cross-sections
Engineering Implications and Practice Recommendations
This study provides important data for the engineering application of recycled concrete in structural columns. The 16% capacity reduction for circular specimens at 100% replacement ratio is significant but manageable through appropriate design adjustments. Engineers can compensate for the reduced capacity by increasing the column cross-sectional dimensions, using higher-grade steel tubes, or increasing the concrete cover thickness.
For practical applications, the following recommendations emerge:
- Partial replacement is preferred: Using 25-50% recycled aggregate replacement provides a good balance between sustainability goals and structural performance, with capacity reductions of only 4-8% for circular specimens.
- Square sections are more tolerant: Square hollow sandwich columns show less sensitivity to recycled aggregate replacement (10% vs. 16% reduction at 100% replacement), making them a potentially better choice for recycled concrete applications.
- Confinement is critical: The steel tube confinement effectively mitigates the brittleness of recycled concrete, providing ductile failure behavior even at high replacement ratios.
- Design verification is essential: The practical verification method developed in this study should be applied to all recycled concrete column designs to ensure adequate safety margins.
Reflections on Sustainable Construction
This research contributes to the broader goal of sustainable construction by demonstrating that recycled concrete can be effectively used in structural applications with appropriate confinement systems. The hollow sandwich steel tube configuration is particularly attractive because it combines structural efficiency with sustainability, using less material while providing enhanced confinement.
From a materials science perspective, the study highlights the importance of understanding the microstructural differences between natural and recycled aggregate. The residual mortar on recycled aggregate particles creates a weaker interfacial transition zone, which is the primary mechanism for the reduced compressive strength. The steel tube confinement effectively compensates for this weakness by providing lateral support that prevents the concrete from expanding and failing prematurely.
The numerical analysis approach used in this study, combining experimentally calibrated constitutive models with finite element methods, represents a best practice for structural analysis of composite columns. This methodology can be extended to other composite column configurations and materials, providing a versatile analytical framework for engineers.
Summary
The study by Dong Jiangfeng and colleagues provides a systematic investigation of the compressive performance of hollow sandwich steel tube recycled concrete short columns, with recycled aggregate replacement ratio as the primary variable. The experimental results demonstrate that while recycled aggregate reduces the ultimate bearing capacity—by up to 16% for circular specimens and 10% for square specimens at 100% replacement—the steel tube confinement effectively maintains ductile failure behavior and structural reliability. The developed stress-strain models and practical design verification method offer engineers the tools needed to confidently incorporate recycled concrete into structural column designs. This research represents a meaningful contribution to sustainable construction practice, demonstrating that environmental responsibility and structural safety can be achieved simultaneously through intelligent structural design and appropriate material selection.