Zhuojin Pipe Fitting Co., LtdMechanical Properties and Design Methodology for Steel Tube Recycled Concrete Members
Literature Overview
This 2006 paper by Yang Youfu from Fuzhou University, published in Industrial Construction (Vol. 36, No. 11, pp. 1-5), presents a comprehensive discussion on the mechanical behavior and design methods for steel tube recycled concrete (STRC) members. The research was funded by the National Science Fund for Distinguished Young Scholars and several provincial research programs, reflecting its significance in the field of sustainable construction materials. The paper reviews preliminary experimental results on STRC axially compressed short columns, pure bending members, and eccentrically loaded members, and then discusses several key issues related to the mechanical performance and design methodology of STRC composite members.
Core Technical Content
Recycled Concrete Background
Recycled concrete (RC) is produced by replacing natural aggregate with recycled aggregate (RA) obtained from the crushing and processing of demolished concrete waste. The key parameter is the replacement ratio, which typically ranges from 0% (conventional concrete) to 100% (fully recycled concrete). In practice, replacement ratios of 30-70% are most common due to the balance between environmental benefit and mechanical performance.
The mechanical properties of recycled concrete differ from conventional concrete in several important ways:
| Property | Conventional Concrete | Recycled Concrete (50% RA) | Difference |
|---|---|---|---|
| Compressive strength | 40-60 MPa | 30-50 MPa | 15-25% reduction |
| Elastic modulus | 30-35 GPa | 25-30 GPa | 10-15% reduction |
| Tensile strength | 3.0-4.5 MPa | 2.2-3.5 MPa | 20-25% reduction |
| Poisson's ratio | 0.15-0.20 | 0.18-0.22 | Slight increase |
| Water absorption | 2-4% | 5-10% | Significant increase |
| Coefficient of thermal expansion | 10-12 × 10⁻⁶/°C | 12-14 × 10⁻⁶/°C | Slight increase |
Confinement Effect in Steel Tube Recycled Concrete
The steel tube provides lateral confinement to the recycled concrete, which enhances the compressive strength and ductility of the composite member. However, the confinement effect for recycled concrete differs from that for conventional concrete due to the lower interfacial transition zone (ITZ) strength and higher porosity of recycled concrete.
The confinement effect coefficient is defined as the ratio of the confined concrete strength to the unconfined concrete strength. For steel tube recycled concrete, the confinement effect coefficient is generally lower than for steel tube conventional concrete at the same confinement pressure level. This is because recycled concrete has a lower Poisson's ratio in the unconfined state, which means less lateral expansion and therefore less effective confinement pressure transfer from the steel tube.
Experimental Results Summary
The paper reports preliminary experimental results on three types of STRC members:
Axially Compressed Short Columns: The load-displacement curves show an initial linear elastic stage, followed by a nonlinear stage where the concrete begins to crack, and finally a post-peak stage where the steel tube progressively yields and provides confinement. The peak load is typically 15-25% higher than the sum of the individual steel tube and concrete contributions, indicating a positive composite effect. The ductility, measured as the displacement at peak load divided by the displacement at first cracking, is approximately 2.5-4.0, which is comparable to steel tube conventional concrete columns.
Pure Bending Members: The flexural behavior of STRC members is characterized by a lower cracking moment compared to steel tube conventional concrete members, due to the lower tensile strength of recycled concrete. However, the ultimate moment capacity is only slightly reduced (5-10%) because the steel tube contributes significantly to the flexural strength. The deflection at ultimate load is larger for STRC members due to the lower elastic modulus of recycled concrete.
Eccentrically Loaded Members: The eccentric loading creates a biaxial stress state in the concrete, and the interaction between the axial and bending components is influenced by the confinement effect. The paper reports that the load eccentricity ratio at which the steel tube begins to yield is higher for STRC members compared to conventional SRC members, because the lower elastic modulus of recycled concrete allows for greater elastic deformation before yielding.
Design Methodology Discussion
Confinement Effect Coefficient Adjustment
The paper proposes an adjustment to the confinement effect coefficient for recycled concrete. The conventional confinement effect coefficient for steel tube concrete is typically calculated using the Mander model or the Usami model. For recycled concrete, the author proposes a reduction factor that accounts for the lower Poisson's ratio and higher porosity:
The adjusted confinement effect coefficient is calculated as:
f_c,confined / f_c,unconfined = 1 + k × (f_l / f_c,unconfined)
where k is the confinement effect factor, f_l is the lateral confinement pressure, and f_c,unconfined is the unconfined compressive strength of the recycled concrete. The value of k for recycled concrete is reduced by a factor of (1 - 0.15 × ρ_RA) compared to conventional concrete, where ρ_RA is the recycled aggregate replacement ratio.
Creep and Shrinkage Considerations
Recycled concrete exhibits higher creep and shrinkage compared to conventional concrete due to the higher porosity and weaker ITZ. This has important implications for the long-term behavior of STRC members:
| Parameter | Conventional Concrete | Recycled Concrete (50% RA) | Design Implication |
|---|---|---|---|
| Creep coefficient (50 years) | 2.0-2.5 | 2.5-3.5 | Larger long-term deflections |
| Free shrinkage strain | 300-500 μɛ | 500-800 μɛ | Higher residual stresses in steel tube |
| Creep compliance | 0.025-0.035 MPa⁻¹ | 0.035-0.050 MPa⁻¹ | Reduced time-dependent stiffness |
The higher shrinkage of recycled concrete induces tensile stresses in the steel tube, which can partially counteract the compressive stresses from the Poisson effect of the confined concrete. This reduces the effective confinement pressure over time and must be accounted for in the design.
Fire Resistance and Post-Fire Performance
The paper also discusses the fire resistance of STRC members. Recycled concrete has a lower thermal conductivity than conventional concrete due to the higher porosity, which provides a thermal insulation benefit. However, the higher water absorption of recycled concrete means that more free water is available for steam generation during fire exposure, which can lead to more severe spalling.
The post-fire residual strength of STRC members depends on the peak fire temperature and the steel tube condition. At temperatures below 400°C, the residual compressive strength of recycled concrete is approximately 80-90% of the original strength. At 600°C, the residual strength drops to 40-50% due to the combined effects of thermal degradation of the ITZ and the steel tube.
Engineering Practice Implications
For steel pipe manufacturers and structural engineers working with STRC members, the following practical considerations are important:
- The steel tube should be designed for the full composite action, including the reduced confinement effect of recycled concrete and the increased long-term shrinkage strains.
- The weld quality of the steel tube is even more critical in STRC members because the reduced confinement effect means that any geometric imperfection in the steel tube has a proportionally larger effect on the member capacity.
- The surface preparation of the steel tube interior should be enhanced to improve the bond with recycled concrete, which has a lower bond strength compared to conventional concrete. Shot-blasting or mechanical roughening of the steel tube interior is recommended.
- Quality control of the recycled aggregate is essential. The recycled aggregate should be tested for crushing value, water absorption, and organic content before use.
Study Insights and Reflections
This paper makes an important contribution to the sustainable construction field by addressing the practical engineering challenges of using recycled concrete in steel tube composite members. The key insight is that the confinement effect, which is the primary mechanism by which the steel tube enhances concrete performance, is reduced for recycled concrete due to its different mechanical properties. This means that simple extrapolation of conventional SRC design methods to STRC members is not appropriate, and specific adjustments are needed.
The paper is somewhat exploratory in nature, presenting preliminary results and design recommendations that require further validation through extensive experimental programs. However, the fundamental concepts presented are sound and provide a useful framework for future research and engineering practice. The increasing availability of recycled aggregate due to building demolition and the growing emphasis on sustainable construction make this research increasingly relevant.