Zhuojin Pipe Fitting Co., LtdPreparation and Engineering Application of High-Strength Self-Compacting Micro-Expanding Concrete-Filled Steel Tubes
Literature Overview
The paper by Zhou Xiao-Jun, Bai Shi-Ming, Li Xin, Jiang Jun-Song, and Ding Qing-Jun (Xihua University and Wuhan University of Technology, 2023) presents the development and engineering application of a high-strength self-compacting micro-expanding concrete (SCMC) for concrete-filled steel tube (CFST) applications. The research was driven by the specific structural requirements of the Jianyang Tuojiang Special Bridge, where the main arch ribs and lacing bars required concrete filling through both pump delivery and manual placement. The paper systematically investigates the effects of binder content, water-binder ratio, sand ratio, aggregate gradation, and expansion agent dosage on the mechanical properties, workability, and volume stability of the SCMC.
Mix Design Technical Route
The mix design followed a systematic approach based on the 5W2H framework:
| Factor | Design Decision | Rationale |
|---|---|---|
| What (material system) | Multi-component binder: OPC + fly ash + slag + silica fume | Synergistic effects on strength and durability |
| What (aggregate) | Optimized fine and coarse aggregate gradation (8:2 ratio) | Minimize void ratio, improve packing density |
| How (workability) | Self-compacting without vibration | Suitable for congested steel tube geometry |
| How (volume stability) | Micro-expanding agent (112×10⁻⁶ at 28 days) | Compensate shrinkage, maintain bond with steel tube |
| Where (application) | Pump delivery and manual placement in arch ribs | Dual placement method requirement |
| When (setting) | 3-hour slump loss < 50 mm | Extended working time for large-volume placement |
Key Mix Design Parameters
The optimal mix design achieved the following performance characteristics:
| Property | Target | Achieved Value | Testing Method |
|---|---|---|---|
| Binder content | 500-600 kg/m³ | 551 kg/m³ | Design calculation |
| Water-binder ratio | 0.28-0.33 | 0.31 | Design calculation |
| Sand ratio | 40-45% | 43% | Design optimization |
| Aggregate gradation ratio (fine:coarse) | 7:3 to 9:1 | 8:2 | Packing density optimization |
| U-box height (flowability) | 300-400 mm | 355 mm | GB/T 50080 |
| V-funnel time | 15-30 s | 23 s | GB/T 50080 |
| 3-hour slump loss | < 50 mm | < 50 mm | Slump retention test |
| 28-day compressive strength | ≥ 100 MPa | 105.1 MPa | GB/T 50081 |
| 28-day expansion rate | 80-150 × 10⁻⁶ | 112 × 10⁻⁶ | GB/T 7649 |
Volume Stability Analysis
The volume stability behavior of the micro-expanding SCMC follows a characteristic pattern:
- Initial expansion phase (0-7 days): The expansion agent (calcium sulfate-aluminum type) reacts with C3A in cement, producing ettringite that causes macroscopic expansion.
- Transition phase (7-14 days): Expansion rate slows as the expansion agent is consumed.
- Stabilization phase (14-28 days and beyond): The concrete reaches a stable micro-expanding state, with the expansion rate converging to a steady value.
The critical design consideration is that the expansion must be sufficient to maintain compressive stress at the concrete-steel tube interface, but not so large as to cause cracking or excessive stress in the steel tube. The achieved expansion rate of 112×10⁻⁶ corresponds to approximately 1.5-2.5 MPa of interface compressive stress, which is adequate for maintaining bond without causing tube deformation.
Effect of Key Variables
Binder Content and Water-Binder Ratio
Higher binder content with appropriate water-binder ratio improves strength through:
- Reduced porosity in the cement paste matrix
- Enhanced pozzolanic reactions with supplementary cementitious materials
- Improved workability without additional water
However, excessive binder content increases shrinkage tendency and cost. The optimal balance was found at 551 kg/m³ with w/b = 0.31.
Sand Ratio and Aggregate Gradation
The sand ratio and aggregate gradation directly affect the packing density of the concrete:
- Higher sand ratio (up to 43%) reduces the void ratio between aggregates
- The 8:2 fine-to-coarse aggregate ratio provides optimal particle packing
- Reduced porosity leads to improved strength, durability, and self-compacting flowability
Expansion Agent Dosage
The expansion agent dosage affects both volume stability and workability:
- Insufficient dosage: Incomplete shrinkage compensation, potential bond loss at steel tube interface
- Excessive dosage: Over-expansion causing cracking, interference with self-compacting flowability
- Optimal dosage: Achieves stable micro-expansion of 100-150×10⁻⁶ at 28 days
Engineering Application at Jianyang Tuojiang Special Bridge
The SCMC was successfully applied to the concrete filling of the main arch ribs and lacing bars of the Jianyang Tuojiang Special Bridge. Key application considerations included:
- Pump delivery: The self-compacting property allowed pumping through long distances and elevated placement points without segregation or blocking.
- Manual placement: For congested sections where pump nozzles could not reach, the high flowability allowed manual placement with minimal effort.
- Placement temperature control: The high binder content generates significant hydration heat, requiring temperature monitoring to prevent thermal cracking. Maximum internal temperature rise was controlled below 65°C.
- Curing regime: Enhanced curing (wet curing for minimum 14 days) was implemented to ensure proper strength development and volume stability.
The engineering results demonstrated high filling density with no voids or honeycombing observed, confirming the effectiveness of the self-compacting property in the complex steel tube geometry.
Quality Control and Inspection
The quality control plan for the SCMC application included:
| Control Point | Inspection Method | Frequency | Acceptance Criteria |
|---|---|---|---|
| Fresh concrete flowability | U-box and V-funnel test | Each batch | U-box: 300-400 mm; V-funnel: 15-30 s |
| Concrete temperature | Embedded thermocouples | Continuous during placement | Internal temp < 65°C; surface-internal diff < 25°C |
| Steel tube cleanliness | Visual inspection | Before each pour | No rust, oil, or debris |
| Filling density | UT or CT scan | Post-hardening | No voids > 50 mm diameter |
| Concrete strength | Cube compressive test | Each batch | ≥ 100 MPa at 28 days |
| Expansion rate | Expansion test on control specimens | Each batch | 80-150 × 10⁻⁶ at 28 days |
Key Questions and Reflections
The research raises important questions about the long-term durability of the SCMC in CFST applications. The high binder content and low water-binder ratio should provide excellent durability, but the micro-expanding component introduces a potential concern: if the expansion agent is not fully consumed within the first 28 days, delayed expansion could occur, potentially causing internal stress buildup in the confined concrete.
Another consideration is the interaction between the micro-expansion and the steel tube. In the confined environment of a steel tube, the expansion is partially restrained, which converts volumetric expansion into compressive stress at the interface. This is beneficial for bond maintenance but must be verified not to exceed the elastic limit of the steel tube, particularly for thin-walled tubes with high D/t ratios.
Study Insights and Conclusions
This research demonstrates a successful integration of advanced concrete technology with practical steel tube engineering requirements. The high-strength self-compacting micro-expanding concrete achieves a remarkable combination of properties: 105 MPa compressive strength, self-compacting flowability suitable for both pumping and manual placement, and stable micro-expansion that compensates for shrinkage while maintaining interface bond. The successful application at the Jianyang Tuojiang Special Bridge validates the technology for large-scale engineering use. For steel pipe manufacturers and structural engineers, the key insights are: (1) the concrete-steel tube interface bond can be actively maintained through micro-expansion technology, eliminating the need for additional bonding agents or mechanical interlock; (2) self-compacting concrete eliminates the risk of vibration-induced segregation and ensures uniform filling in complex geometries; and (3) the optimized aggregate gradation and binder system provides the dual benefit of high strength and excellent workability. Future research should focus on long-term durability monitoring of the bridge application and development of design guidelines for micro-expanding SCMC in CFST structures.