Zhuojin Pipe Fitting Co., LtdStrength and Lateral Deformation Coefficient of Coal Gangue Concrete Confined by Steel Tubes
Literature Overview
This 2002 study by Li Guochang from Shenyang University and Zhong Shantong from Harbin Institute of Technology investigates the axial compression behavior of steel tube-confined coal gangue concrete (CGC) short columns. Through combined theoretical analysis and experimental investigation, the authors derive formulas for the ultimate compressive strength and lateral deformation coefficient of coal gangue concrete under steel tube confinement. Published in the Journal of Harbin Architecture University, this research addresses an important sustainability concern: utilizing coal gangue (a coal mining waste product) as a concrete aggregate while maintaining structural performance through steel tube confinement.
Core Technical Findings
Confinement Effect on Coal Gangue Concrete
The study demonstrates that steel tube confinement significantly enhances the compressive strength of coal gangue concrete, similar to conventional concrete in CFST members. The confinement mechanism operates through the same fundamental principle: lateral expansion of concrete under axial compression is restrained by the steel tube, generating triaxial compression that increases concrete strength and ductility.
Steel Ratio Influence
The steel ratio (ratio of steel tube cross-sectional area to total cross-sectional area) directly influences the confinement effectiveness:
- Higher steel ratio provides greater confining pressure
- Increased steel ratio leads to higher ultimate compressive strength
- The relationship between steel ratio and strength enhancement is governed by the material properties of both steel and coal gangue concrete
Concrete Strength Grade Effects
A key finding is that lower-strength coal gangue concrete is more suitable for combination with steel tubes to form composite materials. This is because:
- Lower-strength concrete has greater lateral expansion potential
- Greater lateral expansion generates higher confining pressure from the steel tube
- The confinement benefit is more pronounced for lower-strength concrete
- This allows the steel tube to fully utilize its confinement capability
Lateral Deformation Coefficient
The lateral deformation coefficient (ratio of lateral strain to axial strain) is derived as a function of material properties and confinement level. This parameter is critical for:
- Determining confining pressure development
- Predicting steel tube stress state
- Calculating composite member capacity
Technical Parameters and Formulas
Key Derived Relationships
| Parameter | Description | Influence |
|---|---|---|
| Steel ratio ρ | A_steel / A_total | Higher ρ → greater confinement |
| Concrete grade | f_ck of CGC | Lower grade → better confinement benefit |
| Lateral deformation coefficient | ε_lateral / ε_axial | Determines confining pressure |
| Steel tube yield strength | f_y | Higher f_y → greater confinement capacity |
| Wall thickness t | Tube wall thickness | Directly affects confinement |
Design Implications for Coal Gangue Concrete
The finding that lower-strength CGC benefits more from steel tube confinement has important implications:
- Economic optimization: Lower-quality coal gangue aggregates (resulting in lower concrete strength) can be effectively utilized in CFST applications without significant structural penalty.
- Waste utilization: Coal gangue, which is typically a disposal problem for coal mining operations, can be incorporated into structural concrete when combined with steel tube confinement.
- Strength grading: Concrete grades of C20–C30 may be optimal for CFST applications with coal gangue aggregate, as they provide sufficient workability and benefit maximally from confinement.
Process and Manufacturing Considerations
Steel Tube Selection for CGC-CFST Applications
From a steel pipe manufacturing perspective:
- Material compatibility: Steel tubes must be compatible with coal gangue concrete, which may have different pH levels and chemical composition compared to conventional concrete.
- Corrosion protection: Coal gangue may contain sulfides and other reactive components that could accelerate steel corrosion. Protective measures (coating, increased wall thickness allowance) should be considered.
- Dimensional accuracy: Ovality and dimensional tolerances per GB/T 8162 or GB/T 8163 are important for ensuring uniform confinement pressure distribution.
Quality Control for Coal Gangue Concrete
Quality control measures specific to CGC include:
- Aggregate quality verification (coal gangue content, gradation, soundness)
- Concrete mix design optimization (water-cement ratio, admixture selection)
- Strength testing at multiple ages (coal gangue concrete may show different strength development compared to conventional concrete)
- Durability testing (sulfate resistance, freeze-thaw resistance)
Engineering Practice Integration
Sustainability Benefits
The combination of coal gangue concrete with steel tube confinement offers significant sustainability benefits:
- Waste reduction: Utilizes coal gangue waste that would otherwise require landfill disposal
- Resource conservation: Reduces demand for natural aggregates
- Structural viability: Maintains adequate structural performance through confinement
- Carbon footprint reduction: Lower transport requirements for waste material utilization
Application Scenarios
Potential applications for steel tube-confined coal gangue concrete include:
- Non-critical structural members in mining-adjacent construction
- Temporary structures and construction facilities
- Infrastructure in coal-rich regions where gangue disposal is a major concern
- Low-rise building columns and beams where confinement benefits are maximized
Key Questions and Reflections
The finding that lower-strength coal gangue concrete is more suitable for CFST applications raises an important question about the optimal concrete strength for maximizing confinement efficiency. In conventional CFST design, concrete grades of C30–C60 are commonly used. For coal gangue concrete, which typically achieves lower strength grades due to aggregate quality limitations, the confinement benefit may actually be more pronounced, effectively compensating for the lower intrinsic concrete strength.
The lateral deformation coefficient derivation is significant for understanding the confinement mechanism. In coal gangue concrete, the lateral deformation characteristics may differ from conventional concrete due to the different aggregate properties (coal gangue has lower stiffness and higher porosity than natural rock aggregates). This affects the rate of confining pressure development and the ultimate confinement effectiveness.
Another consideration is the long-term behavior. Coal gangue concrete may exhibit different creep and shrinkage characteristics compared to conventional concrete, which could affect the long-term confinement effectiveness. The interaction between steel tube and concrete under sustained loading may differ due to the different elastic properties of coal gangue concrete.
Study Insights and Implications
This study makes a significant contribution to sustainable construction by demonstrating that coal gangue waste can be effectively utilized in structural concrete when combined with steel tube confinement. The finding that lower-strength CGC benefits more from confinement provides a rational basis for using lower-quality aggregates in CFST applications, which has direct economic and environmental benefits. For steel pipe manufacturers, this research opens potential markets in coal-rich regions where gangue utilization is a regulatory priority. The derived formulas for ultimate strength and lateral deformation coefficient provide practical design tools for engineers working with coal gangue concrete CFST members. The study exemplifies how steel tube confinement technology can enable the structural use of unconventional concrete materials, contributing to circular economy objectives in the construction industry.