C60 Concrete Mix Optimization for Concrete-Filled Steel Tubes

Literature Overview

The paper by Jiao Liying, Wang Chao, and Dong Jun, published in Concrete magazine in 2017 (Issue 1, pp. 97–101), presents an experimental study on concrete mix optimization for concrete-filled steel tubes (CFST) using C60 high-strength concrete. The research was supported by the National Natural Science Foundation of China (Grant No. 51378045) and the Beijing Natural Science Foundation (Grant No. 8142012). The authors are affiliated with Beijing Hengkun Concrete Co., Ltd. and Beijing University of Civil Engineering and Architecture. This work addresses a critical practical problem: the high rate of void formation (脱空) between steel tubes and concrete cores in CFST structures, which compromises the composite action between the two materials.

Core Technical Content and Problem Statement

The central problem addressed in this study is the void rate (脱空率) between the steel tube and concrete core in CFST structures. When concrete is pumped or poured into a steel tube, voids can form due to several factors:

• Inadequate concrete workability and flowability
• Insufficient compaction during placement
• Segregation and bleeding of concrete components
• Thermal expansion mismatch during curing
• Shrinkage of concrete during hardening and drying

These voids prevent effective load transfer between the steel tube and concrete core, reducing the composite action that provides the principal advantage of CFST construction. The authors propose that optimizing the concrete mix, particularly using C60 high-strength concrete with carefully selected admixtures and aggregate proportions, can significantly reduce void formation and improve composite performance.

Mix Design Optimization Approach

The study employed a systematic approach to concrete mix optimization, considering the following parameters:

The optimized C60 concrete mix achieved the following performance characteristics:

• 28-day compressive strength: ≥ 60 MPa
• Slump flow: 280–320 mm (sufficient for vertical tube filling)
• Air content: 3–5% (controlled entrainment for workability)
• Drying shrinkage: < 0.4 mm/m (reduced shrinkage for void prevention)
• Fill density: > 98% (minimal void formation)

Void Formation Mechanisms and Countermeasures

The authors identified several mechanisms contributing to void formation in CFST structures and proposed corresponding countermeasures:

1. Pumping voids: During vertical concrete pumping, air can become trapped in the tube, creating voids. Countermeasure: Use high-flow concrete with slump flow ≥ 280 mm, incorporate air-entraining admixtures, and employ vibrator-assisted placement.
2. Segregation voids: Heavy aggregate particles can settle during pumping, creating voids at the top of the tube. Countermeasure: Optimize aggregate gradation for maximum packing density, use viscosity-modifying admixtures, and minimize pumping distance.
3. Shrinkage voids: As concrete cures and dries, shrinkage can create gaps between the concrete surface and the steel tube. Countermeasure: Use shrinkage-reducing admixtures, control curing conditions, and incorporate expansive agents.
4. Thermal voids: Differential thermal expansion between steel and concrete during curing can create micro-voids. Countermeasure: Control concrete temperature during placement, use thermal-matching admixtures, and monitor temperature differentials.

Engineering Practice Integration

The practical significance of this research is demonstrated through field application in an industrial building project. The optimized C60 concrete mix was used in CFST columns and verified through mechanical testing and field inspection. The results showed:

• High strength: 28-day compressive strength consistently exceeded 60 MPa, meeting design requirements.
• Good fill performance: Field inspection confirmed void rates below 2%, compared to typical rates of 5–15% for conventional mixes.
• Low shrinkage: Drying shrinkage measurements confirmed values below 0.4 mm/m, minimizing the risk of shrinkage-induced voids.
• Durable composite action: Long-term load testing demonstrated effective load sharing between steel tube and concrete core, with no evidence of interface degradation.

The PDCA (Plan-Do-Check-Act) cycle was implicitly applied in this research: the Plan phase involved identifying void formation mechanisms and setting optimization targets; the Do phase involved mix design experimentation and field application; the Check phase involved mechanical testing and field inspection; and the Act phase involved refining the mix based on results.

Key Questions and Reflections

The study raises several important questions for engineering practice. First, the long-term durability of C60 high-strength concrete in CFST applications, particularly under cyclic loading and environmental exposure, requires further investigation. High-strength concretes are generally more susceptible to brittle failure and less tolerant of construction defects than conventional concretes. Second, the cost-effectiveness of using C60 concrete with specialized admixtures must be evaluated against the benefits of reduced void formation and improved composite action. Third, the scalability of the optimized mix from laboratory conditions to large-scale field placement requires careful quality control to ensure consistent performance.

From my perspective in materials engineering, the key insight is that concrete mix optimization for CFST applications is not simply about achieving target strength; it is about achieving a specific combination of mechanical, rheological, and volumetric properties that ensure reliable composite action. The void rate is a critical quality indicator that directly affects structural performance, and systematic mix optimization is essential to minimize this risk.

Summary and Outlook

This research provides a practical, field-validated approach to concrete mix optimization for CFST structures using C60 high-strength concrete. The systematic consideration of void formation mechanisms and corresponding countermeasures demonstrates a thorough engineering approach to a critical practical problem. The field application results confirm that the optimized mix achieves the target performance characteristics, including high strength, good fill performance, and low shrinkage. For future work, extending the study to include long-term durability testing, cyclic loading behavior, and cost-benefit analysis would further strengthen the practical applicability of these findings to CFST structural engineering.