Zhuojin Pipe Fitting Co., LtdC100 Concrete-Filled Steel Pipe Using High Stone Powder Manufactured Sand
Literature Overview
This paper by Ding Qingjun, Xu Yi, and Mu Tingmin, published in the journal Concrete (2017, Issue 11), presents a comprehensive study on developing C100 self-consolidating concrete (SCC) for steel pipe concrete (CFST) applications using manufactured sand with high stone powder content. The research was driven by the growing demand for ultra-high-strength CFST in large-span bridge arch ribs, where traditional natural sand is increasingly scarce and manufactured sand is becoming the dominant aggregate source. The study culminated in the successful application at the Guan-Sheng Ganjiang Extra-Large Bridge in Guang'an, Sichuan Province.
Core Technical Approach
The researchers identified three critical challenges when using high-stone-powder manufactured sand for ultra-high-strength CFST: (1) maintaining workability for pumpability and self-consolidation within confined steel pipes, (2) achieving 28-day compressive strength exceeding 100 MPa, and (3) controlling long-term volume stability to prevent cracking and debonding between concrete and steel pipe walls.
Admixture Selection Strategy
The authors systematically selected admixtures based on a multi-objective optimization approach:
| Admixture Component | Required Characteristic | Rationale |
|---|---|---|
| Polycarboxylate superplasticizer | Low air entrainment, super-dispersing, viscosity-reducing, high plasticity retention | Minimizes voids in dense mix, ensures pumpability, reduces segregation risk |
| Fly ash micro-spheres | High activity, dense filling | Provides pozzolanic reaction and micro-aggregate filling effect |
| Silica fume | Mineral water-reducing, high activity | Enhances interfacial transition zone, reduces permeability |
The combination of fly ash micro-spheres and silica fume was particularly noteworthy. Fly ash micro-spheres act as micro-aggregate fillers in the paste matrix, while silica fume provides ultra-fine particles that densify the pore structure. This dual-particle-size mineral admixture strategy effectively addresses the challenge of maintaining workability at very low water-to-binder ratios required for C100-grade concrete.
Key Performance Parameters
| Parameter | Achieved Value | Significance |
|---|---|---|
| Initial slump | 260 mm | High flowability for SCC |
| Extension degree | 680 mm | Confirms self-consolidating behavior |
| 2-hour slump loss | < 20 mm | Excellent plasticity retention |
| 2-hour extension loss | < 20 mm | Stable workability |
| Air content | 2.0% | Low air entrainment for strength |
| 28-day compressive strength | 122.5 MPa | Exceeds C100 requirement |
| 90-day free expansion rate | 1.06 × 10⁻⁴ | Stable, no excessive expansion |
| 360-day creep coefficient | 1.51 | Acceptable long-term deformation |
Engineering Practice Insights
From a steel pipe manufacturing and welding perspective, this research carries several implications:
- Pipe geometry tolerance requirements: The use of self-consolidating concrete with high flowability (260 mm slump, 680 mm extension) demands that steel pipe inner surfaces be smooth and free from weld splatter, internal protrusions, or significant ovality. Any internal weld bead protrusion exceeding 3 mm could obstruct concrete flow and create voids.
- Welding process considerations: For CFST applications using ultra-high-strength concrete, the steel pipe welds (typically longitudinal or spiral seams) must be ground flush internally to ensure unobstructed concrete placement. The high strength of the concrete (122.5 MPa) means that any stress concentration at weld toes becomes more critical under service loads.
- Concrete-steel bond: The low air content (2.0%) and dense microstructure contribute to a strong interfacial bond between the ultra-high-strength concrete and the steel pipe inner wall. This is essential for composite action and confinement effectiveness.
- Pumpability and construction: The excellent plasticity retention (< 20 mm loss over 2 hours) provides a generous placement window, which is particularly valuable for large-diameter pipe ribs where continuous pumping over several hours may be required.
Critical Reflections
The achieved 360-day creep coefficient of 1.51 is relatively favorable, but engineers should note that long-term creep in ultra-high-strength concrete is influenced by environmental humidity, loading level, and aggregate type. In the context of the Guan-Sheng bridge application, the researchers should have provided environmental monitoring data to validate the creep model predictions. Additionally, the free expansion rate of 1.06 × 10⁻⁴ at 90 days suggests a mild expansive character, which is beneficial for debonding prevention but requires monitoring to ensure it does not exceed the elastic limit of the steel pipe wall.
The use of high stone powder manufactured sand introduces variability concerns. Stone powder content and its mineralogy (quartz vs. calcite vs. feldspar) can significantly affect hydration kinetics and long-term strength development. The study would benefit from additional durability testing including sulfate resistance, chloride diffusion, and freeze-thaw cycling, particularly given the Sichuan province climate conditions.
This research demonstrates that C100-grade CFST using manufactured sand is technically feasible, but the transferability of these mix designs to other geographic regions with different stone powder characteristics requires careful re-optimization.