Mechanical Properties of Concrete and Concrete-Filled Steel Tubes Under Constant High Temperature

Literature Overview

This paper by Ding Faxing and Yu Zhiwu (2005), published in the Journal of Railway Science and Engineering, presents a comprehensive re-analysis of existing experimental data on the uniaxial compressive behavior of C20 through C80 grade concrete at elevated temperatures. The authors develop unified formulas for axial compressive strength, elastic modulus, and peak strain, and propose a full stress-strain curve for concrete under constant high temperature. Furthermore, they suggest constitutive models for steel and concrete in confined conditions and establish a concentric cylinder computational model for concrete-filled steel tubes (CFST) under combined compression at high temperatures.

Core Technical Framework

The study addresses a critical gap in fire engineering design: the lack of standardized constitutive relationships for concrete and steel at elevated temperatures. The authors systematically re-analyzed domestic test data covering a wide range of concrete grades (C20 to C80) and temperatures, enabling the derivation of generalized mathematical expressions applicable across the full spectrum of structural concrete grades.

Constitutive Model Development

The unified formulas account for the progressive degradation of concrete mechanical properties with increasing temperature. Key observations from the re-analysis include:

• Compressive strength retention decreases more rapidly for higher-strength concretes at temperatures above 400°C.
• The elastic modulus reduction follows a similar trend but with a more pronounced effect at temperatures between 200°C and 400°C.
• Peak strain increases with temperature, reflecting the transition from brittle to more ductile failure behavior.

Stress-Strain Full Curve at High Temperature

The proposed full stress-strain curve formula incorporates the temperature-dependent parameters as scaling factors, maintaining the general shape of the room-temperature curve while adjusting the key parameters. This approach allows engineers to predict the nonlinear behavior of concrete at any temperature within the validated range without requiring new experimental data for each specific case.

Temperature-Dependent Property Degradation

CFST Concentric Cylinder Model

The computational model treats the CFST column as two concentric cylinders—the inner concrete core and the outer steel tube—each governed by its own constitutive law. The interaction between the two materials is captured through the lateral confining pressure exerted by the steel tube on the concrete, which increases with the axial shortening of the column. At high temperatures, both materials degrade simultaneously, and the confinement effect becomes increasingly important as the concrete loses self-confinement capacity.

The steel tube's multi-axial elasto-plastic constitutive model accounts for the Bauschinger effect and temperature-dependent yield strength reduction. The concrete's axisymmetric triaxial compression model extends the uniaxial behavior to confined conditions using a modified failure criterion that incorporates the confining pressure as a function of temperature.

Engineering Practice Integration

From a fire protection design perspective, this research provides the fundamental material property data needed for performing fire resistance calculations on CFST columns. The proposed formulas can be directly incorporated into finite element analysis software for conducting thermal-structural coupled analyses of steel-concrete composite members under fire exposure. Key engineering implications include:

• CFST columns maintain significantly better residual strength compared to plain concrete columns at elevated temperatures, owing to the confining action of the steel tube.
• The degradation rate of the steel tube's yield strength at temperatures above 500°C becomes the controlling factor for overall column capacity, not the concrete core.
• Fire protection design codes should account for the differential thermal expansion between steel and concrete, which can lead to debonding at the interface at temperatures exceeding 300°C.

Study Insights and Reflections

The value of this paper lies in its systematic approach to harmonizing experimental data across different concrete grades and temperatures into unified analytical expressions. For engineers involved in fire safety design of railway bridges and high-speed rail viaducts, the proposed formulas offer a practical tool for preliminary capacity assessment. However, the study is limited to constant temperature conditions; real fire scenarios involve non-uniform temperature distributions and transient heating rates that may produce additional thermal stresses not captured in the model. Future research should extend these constitutive relationships to incorporate thermal gradients and cooling effects, which are particularly relevant for high-performance concrete (HPC) applications where thermal cracking is a significant concern.

The paper demonstrates that CFST members are inherently more fire-resistant than equivalent steel or concrete members due to the mutual confinement mechanism. This finding supports the use of CFST columns in fire-prone applications such as underground structures, nuclear facilities, and industrial plants where fire protection costs are a significant design consideration.