Zhuojin Pipe Fitting Co., LtdAxial Compressive Bearing Capacity of Steel Tube Coal Gangue Concrete Members
Literature Overview
This study by Zhao Weiping and colleagues from China University of Mining and Technology (Beijing), published in the Journal of Harbin Institute of Technology in 2024 (Vol. 56, No. 10, pp. 24-34), addresses the resource utilization of coal gangue as a coarse aggregate replacement in concrete-filled steel tube (CFST) members. Funded by the National Natural Science Foundation Joint Fund Key Project (U22A20244), this research is particularly relevant to regions with significant coal gangue accumulation, such as northern China's major coal-producing provinces.
Core Technical Findings
The researchers prepared coal gangue concrete with a design strength grade of C40 by substituting natural coarse aggregate with crushed coal gangue at various replacement rates. The study investigated the failure modes, bearing capacity, steel tube confinement effectiveness, and bond stress between the steel tube and the coal gangue concrete core under axial compression.
Loading Configuration and Failure Modes
| Loading Method | Failure Mode | Ultimate Bearing Capacity | Core Concrete Strength Enhancement |
|---|---|---|---|
| Method A (end-loaded) | Shear-type failure | Higher | Moderate |
| Method B (distributed) | Drum-type failure | Lower | More significant |
The study reveals that while Loading Method A produces higher ultimate bearing capacity due to more efficient load transfer through the end plates, Loading Method B generates more uniform confinement stress distribution, resulting in greater enhancement of the core concrete's compressive strength. This distinction is critical for understanding the actual performance of CFST members in structural applications where loading conditions may vary.
Influence of Key Parameters
| Parameter | Range | Effect on Ultimate Bearing Capacity |
|---|---|---|
| Coal gangue replacement rate | Variable (primary factor) | Decreases with increasing replacement rate |
| D/t ratio (diameter-to-thickness) | Variable | Higher D/t reduces confinement effectiveness |
| Loading method | A vs B | Method A yields higher capacity |
Steel Tube Manufacturing and Welding Considerations
From a steel pipe manufacturing perspective, this research has several important implications:
- D/t ratio control: The diameter-to-wall-thickness ratio directly affects the confinement effectiveness of the steel tube. For CFST members designed to confine low-strength concrete cores (such as coal gangue concrete), a lower D/t ratio is preferred to maximize lateral restraint. This typically requires thicker-walled tubes, which increases material cost but provides better structural performance.
- Tube straightness and dimensional accuracy: The bond stress distribution between the steel tube and concrete core is highly sensitive to geometric imperfections. Ovality, waviness, and dimensional deviations can create stress concentrations that initiate premature buckling. Precision manufacturing processes such as HFW (High Frequency Welding) or ERW (Electric Resistance Welding) with strict post-weld calibration are essential.
- Weld integrity: For welded steel tubes used as CFST shells, the longitudinal weld seam represents a potential weak link under the complex stress state of confined concrete. The weld must achieve full fusion with controlled heat-affected zone (HAZ) properties. The residual stresses from welding can interact with the confining pressures from the concrete core, potentially accelerating buckling initiation.
Bond Stress Analysis
The study's investigation of longitudinal strain distribution along the steel tube provides valuable insight into the bond stress mechanism. The bond stress between the steel tube and coal gangue concrete follows a characteristic distribution pattern:
- Maximum bond stress occurs near the loading ends where relative slip is greatest
- Bond stress decreases toward the mid-length where the steel tube and concrete expand more uniformly
- The bond stress magnitude is influenced by the roughness of the tube inner surface, concrete cover thickness, and the mechanical interlock between aggregate and tube wall
Engineering Practice Cases
In practical applications, CFST members utilizing coal gangue concrete can be found in:
- Mining infrastructure support systems where gangue availability is abundant
- Low-rise and mid-rise building columns in mining regions
- Industrial facilities and warehouse structures where ultimate strength requirements are moderate
- Non-critical structural elements where cost reduction is prioritized
The key design consideration is that coal gangue concrete exhibits lower intrinsic compressive strength compared to natural aggregate concrete. The steel tube confinement compensates for this deficiency, but the design must ensure that the combined system meets the required safety margins for all limit states.
Study Insights and Reflections
The most significant finding from an engineering practice standpoint is that the steel tube confinement can effectively compensate for the lower strength of coal gangue concrete. This represents a viable pathway for sustainable construction in mining regions, where the environmental cost of gangue disposal is substantial and the economic benefit of resource recovery is significant.
However, the study also highlights a practical challenge: the bearing capacity reduction with increasing gangue replacement rate means that design optimization must balance environmental benefits against structural performance requirements. For critical structural applications, a partial replacement rate (typically 30-50%) may offer the best compromise.
The distinction between Loading Methods A and B is particularly relevant for engineers designing column connections. The actual loading condition in a building depends on the connection type, panel height, and load transfer mechanism. Understanding how different loading configurations affect failure mode and capacity is essential for accurate structural assessment.
Reference Value and Outlook
This research provides a solid foundation for the development of design guidelines for CFST members with coal gangue concrete cores. Future work should address long-term durability, including the sulfate resistance and carbonation behavior of coal gangue concrete within confined steel tubes. The interaction between steel tube corrosion and coal gangue concrete degradation under environmental exposure remains an important area requiring further investigation. From a steel pipe manufacturing standpoint, the development of surface treatments to enhance bond stress between the tube wall and gangue concrete could further improve structural performance.