Expansion Performance of Micro-Expansion Light Aggregate Concrete in Steel Tubes

Literature Overview

The paper by Zhu Hongbing, Yuan Qiangsong, Xiang Jie, and Han Shuai (2017), published in Sichuan Building Science, Vol. 43, No. 3, pp. 135-138, investigates the expansion performance of micro-expansion light aggregate concrete used as the core fill in concrete-filled steel tubes (CFST). The authors, from Wuhan University of Science and Technology's School of Urban Construction, were supported by the Hubei Provincial Natural Science Foundation (Grant No. 2011CDB239). The study examines the deformation behavior of the concrete under both free expansion and restrained expansion conditions, with the objective of optimizing the expansion agent dosage for compensating concrete shrinkage in CFST members.

Core Technical Content

In CFST structures, the differential shrinkage and thermal expansion between the steel tube and the concrete core can lead to gaps at the interface, reducing the effectiveness of the composite action and potentially leading to corrosion of the steel tube. The addition of expansion agents to the concrete mix is a widely adopted strategy to compensate for shrinkage and maintain intimate contact between the steel and concrete. This study focuses on micro-expansion light aggregate concrete, which combines the benefits of lightweight aggregate (reduced self-weight) with the compensating expansion properties of expansion agents.

The experimental program included two types of tests: free expansion tests using prismatic specimens to measure the unconstrained expansion of the concrete, and restrained expansion tests using CFST specimens to measure the actual deformation behavior under steel tube confinement. The results reveal important differences between the two conditions.

Experimental Results and Analysis

Free Expansion Behavior

Under free expansion conditions, increasing the expansion agent dosage progressively reduces the rate of concrete shrinkage at all stages. At higher dosages, the concrete exhibits net expansion rather than shrinkage during the early curing period. This is the expected behavior and confirms the effectiveness of the expansion agent in compensating for autogenous and drying shrinkage.

Restrained Expansion Behavior

Under the restraint of the steel tube, the behavior is more complex. The hoop strain of the steel tube outer surface follows a characteristic pattern: it first increases as the concrete expands and pushes against the tube, then decreases as the expansion is gradually accommodated and the tube reaches equilibrium, and finally stabilizes at a constant value. An important finding is that higher expansion agent dosages result in lower maximum hoop tensile strain in the steel tube during the early curing period.

This counterintuitive result can be explained by the fact that a higher expansion agent dosage produces a more uniform and gradual expansion, rather than a rapid initial expansion that would generate higher transient stresses in the steel tube. The gradual expansion allows the steel tube to accommodate the deformation through elastic deformation without reaching high stress levels.

Optimal Dosage

The study concludes that an expansion agent dosage of 12% provides the best balance between shrinkage compensation and controlled expansion stress. At this dosage, the concrete achieves effective shrinkage compensation without generating excessive hoop stresses in the steel tube that could lead to local buckling or cracking of the concrete.

Engineering Practice Integration

The findings of this study have direct implications for the design and construction of CFST members, particularly in applications where lightweight concrete is required, such as:

• Long-span bridges: Where self-weight is a critical design factor
• High-rise buildings: Where weight reduction improves seismic performance
• Offshore platforms: Where corrosion resistance and weight reduction are both important
• Industrial structures: Where thermal cycling is a concern

Construction Considerations

1. Mix design: The expansion agent dosage must be carefully controlled during batching to ensure uniform distribution throughout the mix. Over-dosage can lead to excessive expansion and cracking.
2. Curing regime: The expansion performance is sensitive to curing conditions. Adequate moisture and temperature control during the early curing period is essential to achieve the designed expansion.
3. Placement method: In CFST members, the concrete must be placed in a manner that ensures full compaction without entrapping air. The expansion agent requires adequate moisture to activate, so the placement method should not compromise the moisture availability.
4. Quality control: The expansion performance should be verified through non-destructive testing (NDT) methods such as ultrasonic testing (UT) to detect voids or incomplete filling at the steel-concrete interface.

Material Compatibility

The compatibility between the expansion agent and the steel tube surface is an important consideration. The expansion agent should not contain chlorides or other corrosive agents that could initiate corrosion of the steel tube. The alkalinity of the concrete should be maintained at a level sufficient to ensure passivation of the steel surface (pH > 12.5).

Key Questions and Reflections

The study provides valuable guidance on expansion agent dosage for CFST applications, but several aspects warrant further investigation. First, the long-term performance of the micro-expansion concrete under sustained loading and environmental exposure (freeze-thaw cycles, carbonation, sulfate attack) has not been examined. Second, the interaction between the expansion agent and the steel tube surface chemistry, particularly the potential for galvanic corrosion in the presence of certain expansion agents, deserves attention. Third, the applicability of the 12% dosage recommendation to different steel tube sizes, wall thicknesses, and concrete grades should be verified, as the confinement effect varies with the geometric parameters of the tube.

From a steel pipe manufacturing perspective, the expansion performance of the concrete fill is directly related to the dimensional accuracy and surface quality of the steel tube. Tubes with tight dimensional tolerances and smooth internal surfaces will provide more uniform confinement, which in turn affects the expansion behavior of the concrete. Therefore, the manufacturing quality of the steel tube is an indirect but important factor in the performance of CFST members with micro-expansion concrete.

Summary

This paper provides practical guidance on the use of micro-expansion light aggregate concrete in CFST members, with the key finding that an expansion agent dosage of 12% optimally balances shrinkage compensation and controlled expansion stress. The study highlights the importance of understanding the difference between free expansion and restrained expansion behavior, and it provides a framework for selecting expansion agent dosage based on the desired performance characteristics. For engineers involved in CFST design and construction, the findings underscore the critical role of material selection and mix design in achieving the intended composite action between the steel tube and concrete core.