Zhuojin Pipe Fitting Co., LtdResidual Bearing Capacity of Steel Tube Coal Gangue Concrete Axial Compression Short Columns After Fire Exposure
Literature Overview
This research investigates the residual axial compression bearing capacity of short columns composed of steel tubes filled with coal gangue concrete (CGC) after exposure to elevated temperatures simulating fire conditions. Coal gangue, a byproduct of coal mining and washing, is used as a partial or full replacement for natural aggregate in the concrete mix. The study addresses the critical question of whether these economical composite columns can maintain acceptable structural performance after fire exposure, which is essential for their application in industrial and mining structures where fire risk is significant.
Core Technical Findings
Material Properties and Fire Exposure Conditions
Coal gangue concrete exhibits lower compressive strength and reduced thermal stability compared to natural aggregate concrete due to the heterogeneous composition and higher porosity of coal gangue aggregates. The fire exposure conditions follow the ISO 834 standard fire curve, with temperatures ranging from 200°C to 800°C.
| Parameter | Symbol | Typical Value | Notes |
|---|---|---|---|
| Coal gangue replacement ratio | R_cg | 30–100% | By mass of aggregate |
| CGC compressive strength (28d) | f_c,cgc | 25–45 MPa | Lower than natural aggregate |
| Steel tube grade | — | Q235/Q345 | f_y = 235/345 MPa |
| Fire temperature | T | 200–800°C | ISO 834 curve |
| Exposure duration | t | 30–180 min | Time to reach target temperature |
| Residual strength ratio (steel) | f_y(T)/f_y(20) | 0.85–0.20 | At 200–800°C |
| Residual strength ratio (CGC) | f_c(T)/f_c(20) | 0.90–0.30 | At 200–800°C |
Residual Bearing Capacity Results
The residual bearing capacity of the columns decreases with increasing fire temperature, but the rate of degradation is significantly influenced by the coal gangue replacement ratio. Columns with lower coal gangue replacement ratios (30–50%) retain a higher proportion of their original capacity compared to those with full replacement (100%).
| Temperature (°C) | Residual Capacity (R=30%) | Residual Capacity (R=60%) | Residual Capacity (R=100%) |
|---|---|---|---|
| 200 | 95% | 93% | 90% |
| 400 | 88% | 82% | 75% |
| 600 | 72% | 62% | 50% |
| 700 | 58% | 45% | 32% |
| 800 | 40% | 25% | 15% |
Failure Modes After Fire
At temperatures below 400°C, the failure mode remains similar to that of unheated columns, with concrete crushing and steel tube bulging. Above 600°C, the failure mode shifts to premature steel tube failure due to significant strength loss in the steel, with the coal gangue concrete experiencing spalling and explosive popping, particularly when the replacement ratio exceeds 70%.
Thermal Degradation Mechanisms
Coal Gangue Concrete Degradation
Coal gangue contains organic carbon residues, pyrite (FeS₂), and other mineral impurities that undergo thermal decomposition at elevated temperatures. The decomposition of pyrite generates sulfuric acid, which attacks the cement paste and weakens the ITZ. The organic carbon burns out, leaving additional porosity that reduces the concrete's strength and stiffness.
Steel Tube Degradation
The steel tube experiences strength and stiffness degradation according to well-established temperature-dependent models. However, the presence of coal gangue concrete introduces additional thermal effects: the differential thermal expansion between the steel tube and the coal gangue concrete can generate additional hoop stresses, potentially leading to premature tube failure at lower temperatures than expected.
Spalling Risk Assessment
The risk of concrete spalling is significantly higher in coal gangue concrete columns compared to natural aggregate concrete. The heterogeneous composition of coal gangue creates preferential pathways for moisture migration and steam pressure buildup. This is particularly critical at temperatures above 400°C, where rapid heating can cause explosive spalling.
| Temperature (°C) | Spalling Probability (R=30%) | Spalling Probability (R=60%) | Spalling Probability (R=100%) |
|---|---|---|---|
| 300 | 5% | 10% | 20% |
| 500 | 25% | 50% | 75% |
| 700 | 45% | 70% | 90% |
Engineering Practice and Mitigation Measures
Design and Fabrication Recommendations
- Limit coal gangue replacement ratio to 50% for fire-exposed applications to ensure adequate residual capacity.
- Use steel tubes with higher yield strength (Q345 or above) to compensate for the thermal strength loss.
- Apply fire protection coatings to the steel tube exterior to reduce the rate of temperature rise.
- Incorporate polypropylene fibers (0.1–0.3% by volume) in the coal gangue concrete to mitigate spalling risk by creating micro-cracks that relieve steam pressure.
- Ensure adequate concrete cover thickness (tube wall thickness) of at least 8 mm to provide thermal insulation.
Quality Control Procedures
- Conduct thermal conductivity tests on coal gangue concrete to verify the thermal protection effectiveness.
- Perform post-fire visual inspection for spalling, cracking, and steel exposure.
- Apply ultrasonic testing (UT) to assess internal damage in the concrete core.
- Conduct rebound hammer tests on exposed concrete surfaces to estimate residual strength.
- Perform residual compressive strength tests on extracted concrete cores for critical structures.
Study Insights and Implications
This study provides essential data for the safe application of steel tube coal gangue concrete columns in fire-exposed environments. The findings indicate that while coal gangue concrete columns can retain acceptable residual capacity at moderate temperatures (below 400°C), the performance degrades rapidly at higher temperatures, especially with high replacement ratios. The spalling risk associated with coal gangue concrete is a critical concern that must be addressed through material modification and design measures. For engineers involved in mining infrastructure and industrial structures, these results offer a practical framework for evaluating the fire performance of economical composite columns utilizing coal gangue as a sustainable aggregate source.