Mechanical Properties of Concrete and Concrete-Filled Steel Tubes at Elevated Temperatures

Literature Overview

The paper by Ding Faxing and Yu Zhiwu (2005), published in the Journal of Railways Science and Engineering (Vol. 2, No. 6, pp. 9–14), investigates the compressive mechanical properties of concrete and concrete-filled steel tubes (CFST) under sustained elevated temperatures. Funded by the National Natural Science Foundation of China (Grant No. 50578162) and the Hunan Provincial Natural Science Foundation (Grant No. 03JJY308), this study re-analyzes experimental data from domestic tests on C20–C80 grade concrete, proposes unified formulas for axial compressive strength, elastic modulus, and peak strain at elevated temperatures, and develops a full stress-strain curve model. The authors further recommend constitutive models for steel and concrete at elevated temperatures and apply continuum mechanics to establish a concentric cylinder calculation model for CFST short columns under axial compression.

Core Technical Points

Reanalysis of Existing Experimental Data

The authors conducted a systematic reanalysis of compressive test data for C20 through C80 grade concrete at elevated temperatures. This approach is methodologically significant because it consolidates scattered experimental results into a unified framework, enabling the derivation of generalized formulas applicable across a wide range of concrete grades.

Proposed Constitutive Models

The study proposes two key constitutive frameworks:

1. Steel multi-axial elasto-plastic constitutive model at elevated temperatures: This accounts for the progressive reduction in yield strength and elastic modulus of structural steel as temperature increases. The model captures the transition from elastic to plastic behavior under combined stress states experienced in CFST members.
2. Concrete axisymmetric triaxial compression constitutive model at elevated temperatures: This represents the complex behavior of concrete under combined axial and lateral confinement at elevated temperatures, incorporating the effects of thermal degradation on material properties.

Full Stress-Strain Curve Development

The unified formula for the concrete axial compression stress-strain full curve at elevated temperatures is particularly valuable for nonlinear finite element analysis. The curve captures three critical regions:

• Elastic region: Characterized by a reduced elastic modulus at elevated temperatures
• Peak stress region: Where strength degradation is most pronounced
• Post-peak descending branch: Reflecting progressive microcracking and eventual failure

Standards and Code Implications

The findings of this research have direct implications for fire resistance design of steel structures, particularly those governed by standards such as GB 50017 (Design Standard for Steel Structures) and EN 1993-1-2 (Eurocode 3: Design of steel structures – Fire design). The proposed formulas can serve as basis for:

• Determining residual strength of CFST members after fire exposure
• Evaluating fire resistance ratings for composite columns
• Developing performance-based fire engineering methodologies

For pipe and fitting applications, the elevated temperature behavior of steel tubes filled with concrete is relevant to:

• Industrial process piping exposed to thermal cycling
• Fire protection systems using CFST columns
• Nuclear facility containment structures subject to post-accident thermal scenarios

Engineering Practice Integration

In practical engineering, the temperature-dependent mechanical properties of CFST members are critical for several scenarios:

1. Fire protection design: Understanding how the steel tube and concrete core interact at elevated temperatures enables more accurate prediction of fire resistance. The steel tube initially provides confinement that enhances concrete strength, but as steel yield strength drops at high temperatures, confinement effectiveness diminishes.
2. Thermal cycling in process industries: Pipes and columns exposed to repeated thermal cycles (e.g., in power plants, chemical processing facilities) experience progressive degradation of material properties. The formulas developed in this study can inform remaining life assessments.
3. Post-fire structural assessment: After a fire event, the residual load-bearing capacity of CFST members must be evaluated to determine whether repair or replacement is necessary. The proposed constitutive models provide the theoretical basis for such assessments.

Key Questions and Reflections

The study's reliance on reanalysis of existing experimental data, while methodologically efficient, has inherent limitations. The original experimental data may have been obtained under varying conditions (heating rates, specimen preparation methods, testing apparatus), which could introduce systematic errors into the proposed formulas. Ideally, a dedicated experimental program with controlled variables would provide more reliable data for formula derivation.

Additionally, the concentric cylinder calculation model assumes perfect bonding between the steel tube and concrete core, which may not hold under extreme thermal conditions where differential thermal expansion can induce interface separation. The model's validity under such conditions warrants further investigation.

Study Insights and Implications

This research provides a comprehensive theoretical framework for evaluating the compressive behavior of concrete and CFST members at elevated temperatures. The unified formulas spanning C20–C80 grades offer practical utility for engineers designing fire-resistant structures or assessing post-fire structural integrity. The development of coupled constitutive models for steel and concrete at elevated temperatures represents a significant advancement toward more accurate nonlinear analysis of composite structures under thermal loading. For pipe and fitting engineers, the insights into thermal degradation mechanisms inform material selection and protective measures for steel tubes operating in high-temperature environments.