Fire Resistance Limit of Steel Tube Recycled Concrete Columns

Literature Overview

This study by Yang Youfu and Zhang Lei (2018), published in the Journal of Disaster Prevention and Mitigation Engineering, investigates the fire resistance performance of steel tube recycled concrete (STRC) columns. The research develops thermal parameter models and thermo-mechanical constitutive relationships that account for recycled aggregate replacement rate effects, conducts finite element simulations using ABAQUS, validates against experimental data, and proposes simplified formulas for fire resistance limit calculation.

Thermal Parameter Models and Constitutive Relationships

The study establishes that recycled concrete exhibits different thermal behavior compared to conventional concrete due to the higher porosity and weaker interfaces of recycled aggregate. The thermal conductivity, specific heat, and thermal expansion coefficient all vary with replacement rate and temperature.

Thermal Property	Conventional Concrete	Recycled Concrete (High Replacement)	Design Implication
Thermal conductivity	1.8-2.2 W/(m·K) at 20°C	1.5-1.9 W/(m·K) at 20°C	Lower conductivity provides better insulation
Specific heat	880-1000 J/(kg·K)	900-1050 J/(kg·K)	Higher heat capacity slows temperature rise
Thermal expansion	10-12 × 10⁻⁶/K	11-14 × 10⁻⁶/K	Higher expansion increases internal stresses
Strength retention at 400°C	70-80% of ambient	55-65% of ambient	Recycled concrete loses strength faster
Strength retention at 600°C	30-40% of ambient	20-30% of ambient	Significant capacity reduction

Finite Element Simulation Approach

The ABAQUS model employs solid elements with coupled thermal-mechanical analysis. The model incorporates:

Temperature-dependent material properties for both steel tube and recycled concrete
Contact interaction between tube and concrete with temperature-dependent friction
Radiation and convection boundary conditions on the fire-exposed surface
Geometric nonlinearity for large deformations at elevated temperatures

The simulation results for temperature distribution and fire resistance limit generally agree with experimental measurements, validating the proposed thermal parameter models and constitutive relationships.

Key Influencing Parameters

Parameter	Influence on Fire Resistance	Sensitivity Level
Cross-sectional dimensions	Larger sections provide longer fire resistance	High
Slenderness ratio	Higher slenderness reduces fire resistance	High
Recycled aggregate replacement rate	Higher replacement rate slightly reduces fire resistance	Moderate
Steel tube thickness	Thicker tube provides better protection	Moderate
Concrete strength grade	Higher strength provides marginally better resistance	Low

Simplified Fire Resistance Formula

Based on systematic parametric analysis, the authors propose a simplified formula for calculating the fire resistance limit of STRC columns. The formula accounts for cross-sectional geometry, slenderness ratio, and material properties, and provides results that agree well with both experimental data and detailed finite element simulations.

Engineering Practice Implications

For engineers designing STRC columns in fire-exposed conditions:

The fire resistance limit can be estimated using the simplified formula for preliminary design stages
Cross-sectional dimensions and slenderness ratio are the dominant design parameters and should be optimized first
Recycled aggregate replacement rate has a secondary but non-negligible effect on fire resistance
The lower thermal conductivity of recycled concrete provides a slight insulation benefit that partially offsets the faster strength degradation at high temperatures
Steel tube thickness should be selected to ensure that the tube does not lose structural integrity before the concrete core fails
The proposed formula should be verified against detailed finite element analysis for critical structural members

Study Insights and Reflections

This research addresses an important practical concern: the fire safety performance of sustainable construction materials. The findings suggest that recycled concrete, despite its degraded mechanical properties at ambient temperature, retains acceptable fire resistance characteristics due to its lower thermal conductivity. This is an important consideration for engineers who may otherwise exclude recycled concrete from fire-critical applications based solely on ambient temperature performance. From a pipe engineering perspective, the steel tube serves as both structural reinforcement and fire protection barrier, and the thickness selection must account for the differential thermal expansion between steel and recycled concrete. The simplified formula provides a practical tool for engineers who need to evaluate fire resistance without conducting full-scale tests or detailed finite element simulations, thereby accelerating the design process for sustainable steel tube concrete structures.