Fire Resistance Analysis of Hollow Sandwich Steel Tube Recycled Concrete Columns

Research Motivation and Methodology

Zhang Li, Zhao Hui, and Wang Rui from Taiyuan University of Technology investigated the fire resistance performance of hollow sandwich steel tube recycled concrete columns under ISO 834 standard fire conditions. This research addresses two important sustainability and safety concerns simultaneously: the use of recycled concrete (incorporating recycled coarse aggregate) to reduce environmental impact, and the fire safety of innovative hollow sandwich steel tube concrete structural systems. The study employed ABAQUS finite element software to establish parametric models of circular-in-circular hollow sandwich steel tube recycled concrete columns.

The hollow sandwich configuration consists of an outer steel tube, an inner steel tube, and recycled concrete filling the annular space between them. This design offers weight reduction through the hollow center while maintaining structural integrity through the double steel tube confinement. The research was funded by Shanxi Provincial research programs focused on impact and blast performance of high-strength steel tube concrete columns, indicating the broader context of structural safety research.

Key Findings on Fire Resistance Behavior

The parametric analysis examined the influence of recycled coarse aggregate replacement rate, load ratio, hollow rate, steel yield strength, inner steel tube thickness, and concrete strength on the fire resistance limit of the columns. The results revealed several important trends that have direct implications for structural design and material specification.

The non-monotonic relationship between recycled aggregate replacement rate and fire resistance limit is particularly noteworthy. At low replacement rates, the increased thermal conductivity and reduced thermal insulation properties of recycled concrete may degrade fire performance. However, at higher replacement rates, the increased porosity and void content of the recycled aggregate may provide better thermal insulation, leading to improved fire resistance. This finding suggests an optimal replacement rate range for fire-resistant applications.

The finding that increasing the outer steel tube yield strength significantly reduces fire resistance is counterintuitive from a structural strength perspective but can be explained by thermal mechanics. Higher-strength steel typically has lower ductility and may undergo more rapid strength degradation at elevated temperatures. Additionally, higher-strength steels often have different thermal expansion characteristics that may lead to increased thermal stresses at the steel-concrete interface during fire exposure.

Thermal-Mechanical Constitutive Model for Recycled Concrete

The finite element model incorporated thermal-mechanical constitutive relationships for recycled concrete that account for the effects of recycled coarse aggregate replacement rate. The thermal parameters of recycled concrete, including thermal conductivity, specific heat, and thermal expansion coefficient, differ from those of conventional concrete due to the presence of adhered mortar on recycled aggregate surfaces and the increased porosity. These differences affect the temperature distribution within the column cross-section during fire exposure.

The ISO 834 standard fire curve was applied as the boundary condition, representing the standard fire exposure used in fire resistance testing and design. The model captured the progressive degradation of material properties with increasing temperature, including the reduction in concrete compressive strength, the softening of steel, and the development of thermal gradients across the section. The hollow center of the sandwich column creates a unique thermal environment, as the hollow space provides additional thermal insulation while also allowing for temperature measurement and potential fire protection measures such as intumescent coatings.

Engineering Design Implications

For engineers designing hollow sandwich steel tube recycled concrete columns for fire-resistant applications, several practical recommendations emerge from this research. First, the recycled aggregate replacement rate should be carefully selected based on the required fire resistance rating, recognizing that the relationship is non-monotonic and that an optimal range exists. Second, the outer steel tube yield strength should not be unnecessarily high, as this can compromise fire resistance performance without proportional benefit to structural strength at ambient temperature. Third, increasing the hollow rate is beneficial for fire resistance and should be considered in the design optimization process.

The insensitivity of fire resistance to inner steel tube yield strength, inner steel tube thickness, and concrete strength suggests that these parameters can be optimized for other design objectives such as cost, weight, or structural strength without compromising fire safety. This provides design flexibility for engineers balancing multiple performance criteria. From a quality control perspective, the thermal parameters of the recycled concrete used should be verified through laboratory testing, as the variability in recycled aggregate properties can significantly affect fire performance predictions.

Study Insights and Outlook

This research makes a valuable contribution to the sustainable construction field by addressing the fire safety of recycled concrete structural systems. The comprehensive parametric analysis provides clear design guidance for engineers seeking to incorporate recycled materials into steel tube concrete structures without compromising fire safety. The identification of the non-monotonic effect of recycled aggregate replacement rate is a particularly important finding that challenges simplistic assumptions about material substitution. Future research should investigate the effects of fire exposure duration, the influence of ambient temperature and humidity on fire performance, and the development of simplified design formulas for fire resistance rating of hollow sandwich steel tube recycled concrete columns in accordance with relevant building codes and fire safety standards.