Zhuojin Pipe Fitting Co., LtdFire Resistance Analysis of Hollow Sandwich Steel Tube Recycled Concrete Columns
Literature Overview
This paper by Zhang Li, Zhao Hui, and Wang Rui, published in Fire Science and Technology (2019, Vol. 38, No. 10, pp. 1362-1367), presents a finite element analysis of the fire resistance performance of hollow sandwich steel tube recycled concrete columns. The research was supported by the Shanxi Provincial Applied Basic Research Project Excellent Youth Fund (201701D211006) and the Shanxi Provincial Higher Education Science and Technology Innovation Project (2017130). The authors, from the School of Civil Engineering at Taiyuan University of Technology, developed a thermomechanical constitutive model for recycled concrete considering the influence of recycled coarse aggregate replacement rate, and used ABAQUS software to simulate the behavior of the columns under ISO 834 standard fire conditions.
Structural Configuration and Material Properties
The hollow sandwich steel tube recycled concrete column consists of an outer steel tube, an inner steel tube, and recycled concrete filling the annular space between the two tubes. The hollow region in the center of the inner tube creates a unique structural configuration that differs from conventional steel tube concrete columns. The use of recycled concrete, which contains recycled coarse aggregates from demolished concrete structures, introduces additional complexity due to the modified thermal and mechanical properties of the concrete.
Recycled Concrete Thermal Properties
The thermal properties of recycled concrete differ from those of virgin concrete due to the presence of adhered old mortar on the recycled aggregates and the increased porosity of the recycled aggregate surface. These differences affect the heat transfer characteristics within the column cross-section, influencing the temperature distribution and the rate of temperature rise in the steel tubes and concrete core during fire exposure.
The authors selected thermal parameters and thermomechanical constitutive relationships for recycled concrete that account for the recycled coarse aggregate replacement rate. This is a significant contribution because most existing fire resistance models for steel tube concrete columns assume virgin concrete properties, which may not accurately predict the behavior of columns incorporating recycled materials.
Core Technical Content
Finite Element Model Development
The finite element model was developed using ABAQUS software and subjected to ISO 834 standard fire conditions, which prescribe a temperature-time curve representing the temperature of the fire environment as a function of time. The model incorporates coupled thermal-mechanical analysis, where the temperature field is solved first and then used as input for the structural response analysis. This approach captures the temperature-dependent degradation of material properties, including the reduction in steel yield strength and concrete compressive strength at elevated temperatures.
Parametric Study Results
The parametric study examined the influence of several key parameters on the fire resistance limit of the hollow sandwich steel tube recycled concrete column:
| Parameter | Influence on Fire Resistance Limit | Mechanism |
|---|---|---|
| Recycled aggregate replacement rate | Non-monotonic: first decreases then increases | Changes in thermal conductivity and mechanical properties |
| Hollow ratio | Increases with increasing hollow ratio | Reduced heat absorption, delayed temperature rise |
| Load ratio | Decreases with increasing load ratio | Higher loads accelerate failure under thermal degradation |
| Outer steel tube yield strength | Significant decrease with higher strength | Higher strength steels lose strength more rapidly at elevated temperatures |
| Inner steel tube yield strength | No significant influence | Inner tube is shielded from direct fire exposure |
| Inner steel tube thickness | No significant influence | Limited contribution to overall structural response |
| Concrete strength | No significant influence | Concrete strength degradation dominates over initial strength |
Temperature Field Distribution
The analysis of the cross-sectional temperature field distribution under different recycled aggregate replacement rates revealed that increasing the replacement rate generally leads to lower temperatures throughout the cross-section. This is attributed to the increased porosity and thermal insulation properties of recycled concrete, which slow the heat transfer rate from the fire environment to the inner structural components. However, the effect on the fire resistance limit is non-monotonic, with an initial decrease followed by an increase as the replacement rate increases. This complex behavior is explained by the competing effects of improved thermal insulation (beneficial) and reduced mechanical strength of the recycled concrete (detrimental).
Hollow Ratio Effect
The finding that increasing the hollow ratio increases the fire resistance limit is counterintuitive at first glance, as a larger hollow region reduces the overall structural capacity. However, the mechanism is explained by the thermal behavior: the hollow region acts as a thermal insulator, reducing the heat transfer to the inner steel tube and the concrete in the annular region. This delayed temperature rise in the critical structural components allows the column to maintain its load-bearing capacity for a longer duration under fire conditions.
Steel Strength Effect
The significant decrease in fire resistance limit with increasing outer steel tube yield strength is a critical finding for design purposes. Higher strength steels, while offering greater initial load-bearing capacity, experience more rapid strength degradation at elevated temperatures compared to lower strength steels. This is because the strength of high-strength steels is more dependent on microstructural features such as dislocation density and precipitate strengthening, which are more susceptible to thermal degradation. In contrast, lower strength steels rely more on the basic metallic bond strength, which is less affected by temperature.
Engineering Practice Integration
Design Considerations for Recycled Concrete Columns
| Design Factor | Recommendation | Justification |
|---|---|---|
| Recycled aggregate replacement rate | Optimize for specific application; consider non-monotonic fire resistance behavior | Balance thermal insulation benefits against mechanical strength reduction |
| Hollow ratio | Can be increased to improve fire resistance without compromising structural capacity | Thermal insulation effect of hollow region |
| Steel grade selection | Prefer lower yield strength steels for fire-exposed components | Slower strength degradation at elevated temperatures |
| Concrete strength | Standard strength grades are adequate; higher strength does not improve fire resistance | Concrete strength degradation dominates |
| Load ratio | Keep service load ratio below 0.5 for adequate fire resistance margin | Higher loads accelerate failure under thermal degradation |
Quality Control and Inspection
For the fabrication and installation of hollow sandwich steel tube recycled concrete columns, the following quality control measures should be implemented:
- Material Verification: Verify the recycled concrete mix design, including the recycled aggregate replacement rate, through laboratory testing of thermal conductivity, compressive strength at room temperature, and compressive strength at elevated temperatures.
- Steel Tube Inspection: Inspect steel tubes for dimensional accuracy, wall thickness uniformity, and surface quality using ultrasonic testing and visual examination.
- Concrete Placement: Ensure proper compaction of recycled concrete in the annular region between the steel tubes, using appropriate vibration methods and monitoring placement density.
- Weld Quality: If welding is involved in the assembly of the column components, inspect welds using appropriate non-destructive testing methods (RT, UT, MT) per relevant standards.
- Fire Protection Verification: After construction, verify the fire protection system effectiveness through thermal imaging or other appropriate methods.
Key Reflections and Insights
The non-monotonic relationship between recycled aggregate replacement rate and fire resistance limit is a particularly important finding that challenges the common assumption that higher replacement rates always have negative effects on structural performance. The initial decrease in fire resistance limit at lower replacement rates is attributed to the reduction in concrete mechanical strength, while the subsequent increase at higher replacement rates is attributed to the improved thermal insulation properties. This suggests an optimal replacement rate range that balances thermal and mechanical performance, which should be determined for specific structural applications through detailed analysis.
The finding that the hollow ratio can be increased to improve fire resistance without compromising structural capacity opens up new design possibilities for recycled concrete columns. By optimizing the hollow ratio, engineers can achieve better fire performance while reducing material usage and weight, contributing to sustainability goals. However, the hollow ratio should not be increased excessively, as this may compromise the structural integrity under lateral loads or seismic events, which are not addressed in this study.
The significant influence of outer steel tube yield strength on fire resistance limit has important implications for the selection of steel grades in fire-exposed structural components. Engineers should avoid using high-strength steels for the outer tubes of columns that may be exposed to fire, as the rapid strength degradation of these steels at elevated temperatures can lead to premature failure. Instead, lower strength steels with better high-temperature strength retention should be preferred, even if they result in slightly thicker tube walls or reduced initial load capacity.
The use of recycled concrete in structural columns is an important direction for sustainable construction, and this research provides valuable guidance for the fire design of such columns. The development of thermomechanical constitutive models that account for the recycled aggregate replacement rate is a significant contribution to the field, as it enables more accurate prediction of fire behavior for columns incorporating recycled materials. Future research should extend this work to include experimental validation of the finite element predictions, investigation of long-term fire exposure effects, and consideration of combined fire and seismic loading scenarios.
The integration of recycled materials in structural engineering is a critical aspect of sustainable development, and the fire resistance performance of structures incorporating recycled materials must be thoroughly understood and properly designed. This research provides a foundation for the safe and efficient use of recycled concrete in hollow sandwich steel tube columns, and the findings should be incorporated into design codes and guidelines for recycled concrete structures. Engineers working on projects involving recycled materials should use the insights from this research to ensure that the fire safety performance of their structures meets or exceeds the requirements of relevant building codes and standards.