Construction Technology and Application of C50 Self-Compacting Concrete for Steel Tube Confined Concrete Structures

Literature Overview

This paper by Zhang Haixia and colleagues, published in the journal Concrete in 2016, documents the practical application of C50 self-compacting micro-expansive concrete in the construction of a steel tube confined concrete (CFST) through-tie arch bridge for the South-to-North Water Diversion Project in Pangcun, Henan Province. The project employed the jacking method for concrete placement into steel tubes, representing a challenging construction scenario that demands precise control over concrete rheological properties, pumpability, and bond development.

Core Technical Content and Construction Methodology

The project requirements demanded C50 grade concrete with self-compacting properties and micro-expansive characteristics to ensure full filling of the steel tube interior without vibration and to compensate for shrinkage. The concrete mix design optimization focused on several key aspects:

The jacking method involves pumping concrete upward through the steel tube from the bottom, relying on concrete's own buoyancy and self-compacting properties to fill the tube completely without trapped air or voids. This method is particularly suitable for long, slender steel tubes where conventional pouring is impractical.

Mix Design Optimization Strategy

The concrete mix design incorporated several advanced materials and techniques:

1. Aggregate grading optimization: A carefully designed aggregate grading curve was developed to achieve maximum packing density while maintaining flowability. The use of well-graded fine aggregate with controlled particle size distribution was essential for achieving the required slump flow without excessive paste volume.
2. Micro-expansive admixture: Calcium sulfoaluminate-based expansive agents were selected to provide controlled expansion of 0.02%–0.05%, ensuring that the concrete expands slightly as it hardens, creating compressive pre-stress at the steel-concrete interface and compensating for autogenous shrinkage.
3. High-range water reducers: Polycarboxylate ether (PCE) based superplasticizers enabled low water-binder ratios while maintaining excellent flowability. The dosage was carefully calibrated to balance fluidity with viscosity stability.
4. Supplementary cementitious materials: Fly ash and slag were incorporated to improve workability, reduce heat of hydration, and enhance long-term durability while maintaining the C50 strength requirement.

Construction Process Control

The successful execution of the jacking method required meticulous control at every stage:

• Concrete production: Batch plant calibration and quality control ensured consistent mix proportions and admixture dosages. Each batch was tested for slump flow, T50 time, and viscosity stability before approval for placement.
• Transport and delivery: Temperature-controlled transport vehicles maintained concrete at optimal placement temperature (15–25°C) to prevent premature setting or excessive workability loss during transit.
• Steel tube preparation: Internal surfaces were thoroughly cleaned of rust, mill scale, and debris using wire brushing and high-pressure water jetting. A thin layer of cement mortar was pre-placed at the tube bottom to create a smooth transition and prevent segregation.
• Placement rhythm control: The jacking rate was carefully controlled to maintain a continuous, steady flow without interruptions that could cause cold joints or segregation. Pump pressure was monitored to detect any blockages or flow irregularities.
• Curing: After placement, the steel tube was covered with moist curing cloths for continuous 14-day curing, maintaining adequate moisture and temperature conditions for proper strength development.

Quality Verification Methods

The project employed the hammer testing method (敲击法) for non-destructive evaluation of the concrete-steel tube bond quality. Sound, solid tones indicated proper bonding, while hollow sounds would indicate voids or debonding. This simple but effective method provided rapid verification of placement quality throughout the project.

Additional quality indicators included:

• Visual inspection of concrete surface at tube openings for proper filling
• Core sampling at selected locations for strength verification
• Expansion rate measurement on laboratory specimens

Engineering Practice Implications

From a steel pipe fabrication and welding perspective, this case study highlights several critical considerations:

• Tube internal surface preparation: The quality of the internal steel tube surface directly affects the concrete-steel bond. Mill scale removal, rust prevention, and surface cleanliness are essential. For welded tubes, the internal weld bead should be ground smooth to prevent stress concentrations and ensure uniform concrete flow.
• Tube geometry accuracy: Ovality and out-of-straightness can create regions of high concrete pressure during jacking, potentially causing tube deformation. Dimensional tolerances per applicable standards (e.g., GB/T 8163, SY/T 5037) must be strictly maintained.
• Seam weld quality: Longitudinal welds in ERW or LSAW tubes create potential weak points for concrete penetration during placement. Full penetration and sound welds are essential to prevent concrete leakage and ensure structural integrity.
• Tube thickness uniformity: Wall thickness variations affect the local confinement pressure and concrete-steel interaction. Uniform wall thickness is particularly important for the jacking method where concrete pressure acts on the tube interior.

Key Questions and Reflections

The long-term performance of self-compacting concrete within steel tubes under sustained loading and environmental exposure warrants further investigation. The micro-expansive properties provide initial bond enhancement, but the long-term compatibility between concrete shrinkage/expansion cycles and the steel tube's elastic response remains an open question.

Additionally, the research does not address the behavior of the CFST member under fire conditions. The thermal expansion mismatch between steel and concrete during fire exposure, combined with the potential for concrete spalling in confined conditions, represents a significant design consideration for bridge applications.

Study Insights and Conclusions

This case study demonstrates that C50 self-compacting micro-expansive concrete can be successfully placed in steel tubes using the jacking method, provided that rigorous quality control is maintained throughout the construction process. The combination of optimized mix design, careful surface preparation, controlled placement rhythm, and proper curing proved effective in achieving full, sound concrete filling with adequate bond strength. For steel pipe suppliers and fabricators, the study reinforces the importance of dimensional accuracy, internal surface quality, and weld integrity in CFST applications, particularly where innovative placement methods are employed.