Application of Expansive Concrete in Concrete-Filled Steel Tube Structures

Literature Overview

This 2006 technical paper by Liu Xiao and Chen Bing from Shanghai Jiaotong University addresses a fundamental issue in CFST design: the absence of initial hoop confinement force between the steel tube and concrete core during the early loading stage. The authors explore the use of expansive concrete as a solution, providing a comprehensive review of the working principles, influencing factors, advantages, and engineering applications of steel tube expansive concrete (STEC) systems in China.

Core Technical Content

The Problem of Initial Confinement

In conventional CFST members, the steel tube and concrete core act independently under initial loading. The hoop confinement force between the steel tube and concrete core develops only after the concrete has undergone sufficient lateral expansion under axial compression. This means that during the early loading stage:

• The steel tube does not provide confinement to the concrete.
• The concrete does not benefit from the enhanced compressive strength that confinement provides.
• The composite action between steel and concrete is not fully utilized.

This limitation reduces the structural efficiency of CFST members, particularly under low axial loads or in members subjected to combined loading where early-stage behavior is critical.

Expansive Concrete Solution

Expansive concrete incorporates expansive agents (such as calcium sulfoaluminate or calcium hydroxide-based expanders) that cause the concrete to expand during hydration. When placed inside a steel tube, this expansion generates an initial radial pressure on the steel tube wall, creating a pre-compression state that provides immediate confinement.

The working principle can be summarized as:

1. Expansive concrete is poured into the steel tube.
2. During hydration, the expansive agents react and cause volumetric expansion.
3. The steel tube constrains this expansion, generating radial pressure.
4. This radial pressure creates a hoop tension in the steel tube and a confining pressure on the concrete core.
5. The pre-established confinement enhances the compressive strength and ductility of the concrete from the initial loading stage.

Key Influencing Factors

The paper systematically examines four primary factors affecting STEC performance:

Expansive Rate

The expansive rate (typically expressed as a percentage of volume change) directly determines the magnitude of initial confinement pressure. Higher expansive rates generate greater initial confinement but may lead to:

• Excessive internal stresses that could cause cracking.
• Long-term volume instability if the expansion is not properly controlled.
• Construction difficulties due to rapid expansion during placement.

Concrete Strength

The strength of the expansive concrete affects both the initial confinement pressure and the ultimate structural capacity:

• Higher strength concrete can sustain greater confinement pressures without cracking.
• The interaction between expansive stress and compressive stress must be considered in the design.
• High-strength expansive concrete may require specialized mix designs to achieve both high strength and controlled expansion.

Steel Ratio

The steel ratio (steel tube area to total section area, or steel tube thickness to diameter ratio) influences:

• The capacity of the steel tube to resist expansive pressure.
• The confinement effectiveness (higher steel ratio provides greater confinement).
• The overall structural capacity and ductility.

Creep

Long-term creep of the expansive concrete affects the sustained confinement pressure:

• Creep may cause gradual loss of expansive stress over time.
• The long-term confinement effectiveness depends on the creep characteristics of the concrete.
• Proper mix design can minimize creep and maintain long-term confinement.

Technical Parameter Summary

Engineering Applications in China

The paper highlights the growing application of STEC in Chinese engineering projects:

Application Areas

1. Bridge piers: CFST bridge piers using expansive concrete provide enhanced load capacity and improved long-term performance.
2. High-rise building columns: STEC columns in tall buildings benefit from the enhanced confinement for improved seismic performance.
3. Industrial structures: Heavy industrial buildings with CFST columns use expansive concrete to maximize structural efficiency.
4. Offshore platforms: Marine structures with CFST members benefit from the enhanced confinement for improved fatigue and corrosion resistance.

Construction Considerations

The use of expansive concrete in steel tubes requires careful construction management:

• Pouring sequence: The expansive concrete must be placed in a manner that allows uniform expansion and avoids differential stresses.
• Curing conditions: Proper curing is essential to achieve the desired expansive reaction and avoid excessive early expansion.
• Temperature control: Temperature affects the expansive reaction rate and magnitude; construction temperature should be controlled within specified limits.
• Quality testing: Non-destructive testing methods should be employed to verify the internal confinement pressure and concrete quality.

Engineering Practice Integration

Design Methodology

For designing STEC members, the following approach is recommended:

1. Determine the required confinement pressure based on the structural loading conditions and performance requirements.
2. Select the expansive concrete mix to achieve the target expansive rate and concrete strength.
3. Calculate the steel tube dimensions to resist the expansive pressure and provide adequate confinement.
4. Verify long-term performance considering creep, shrinkage, and environmental effects.
5. Conduct construction quality control to ensure the design intent is achieved in the as-built structure.

Quality Assurance

Study Insights and Reflections

The concept of using expansive concrete to create initial confinement in CFST members is elegant in its simplicity. By leveraging the natural expansive properties of certain concrete mixes, the system achieves a pre-stressed state that enhances structural performance from the initial loading stage.

However, several practical challenges must be addressed for widespread adoption:

1. Long-term reliability: The sustained confinement pressure over the service life of the structure must be verified. Creep and environmental effects may reduce the effective confinement over time.
2. Construction variability: The expansive reaction is sensitive to temperature, humidity, and mixing conditions. Ensuring consistent performance across different construction environments requires robust quality control.
3. Cost-benefit analysis: The use of expansive concrete may increase material costs. The structural performance benefits must be weighed against the additional costs for economic justification.
4. Standardization: The development of design codes and standards for STEC members is essential for widespread engineering acceptance. Current design guidelines may not fully address the unique behavior of expansive concrete in steel tubes.

The paper's comprehensive review of the technology and its applications in China provides a valuable foundation for further research and development. The identification of key influencing factors and their effects on performance offers a clear framework for design optimization. As CFST structures continue to gain popularity in China and worldwide, the STEC technology represents a promising avenue for enhancing structural performance and efficiency.