Zhuojin Pipe Fitting Co., LtdEffects of Internal Curing Aggregate and Expansive Agents on Volume Stability of Ultra-High Strength Steel Pipe Concrete
Literature Overview
This literature investigates the volume stability of ultra-high strength steel pipe concrete (UHSSC) columns through the combined use of internal curing aggregate (ICA) and expansive agents. Ultra-high strength concrete (UHSC, typically > 100 MPa compressive strength) is known for its high strength but suffers from significant volume instability issues including shrinkage, cracking, and poor long-term dimensional stability. The study examines how ICA and expansive agents can mitigate these issues in the context of steel pipe concrete columns, where the steel pipe provides confinement but cannot completely prevent internal cracking and microcracking.
Core Technical Content
Volume Stability Challenges in UHSC
Ultra-high strength concrete faces several volume stability challenges:
| Challenge | Mechanism | Consequence |
|---|---|---|
| Autogenous shrinkage | Self-desiccation of cement paste | Internal microcracking, reduced durability |
| Drying shrinkage | Evaporation of capillary water | Surface cracking, reduced bond strength |
| Thermal shrinkage | Temperature changes during hydration | Residual stresses, cracking |
| Carbonation-induced shrinkage | Carbonation of C-S-H | Surface degradation |
| Steel pipe-concrete interface cracking | Differential shrinkage | Reduced composite action |
The high water-to-binder ratio (W/B) required for UHSC (typically 0.20–0.30) leads to significant self-desiccation, where internal relative humidity drops below 80% before external moisture can penetrate. This results in internal microcracking that compromises long-term performance.
Internal Curing Aggregate (ICA) Technology
Internal curing aggregate functions as a "water reservoir" within the concrete matrix, releasing moisture during hydration to compensate for self-desiccation:
- Pre-saturated lightweight aggregate: Expanded shale, pumice, or perlite pre-saturated with water
- Water release mechanism: As cement paste self-desiccates, water migrates from the saturated aggregate to the cement paste
- Optimal replacement: 5–15% by aggregate mass for UHSC
- Pre-saturation level: 95–100% saturation (surface-dry condition)
The literature evaluates several ICA types:
| ICA Type | Absorption Capacity | Density | Cost | Performance |
|---|---|---|---|---|
| Expanded shale | 15–20% | 1200 kg/m³ | Moderate | Good |
| Pumice | 25–35% | 800 kg/m³ | Low | Very good |
| Perlite | 20–30% | 600 kg/m³ | Low | Good |
| Lightweight ceramic | 10–15% | 1500 kg/m³ | High | Excellent |
Expansive Agents
Expansive agents counteract shrinkage by generating expansion through hydration products:
- Calcium sulfoaluminate (CSA) expansive agent: Produces ettringite for early expansion
- Lime-based expansive agent: Produces calcium hydroxide and ettringite
- Magnesium oxide (MgO) expansive agent: Produces brucite for long-term expansion
- Optimal dosage: 3–8% by cement mass for UHSC
The literature evaluates combined use of CSA and MgO expansive agents for balanced early and long-term expansion.
Experimental Results
Volume Stability Performance
| Mix Design | 7-day Shrinkage (με) | 28-day Shrinkage (με) | 90-day Shrinkage (με) | Cracking |
|---|---|---|---|---|
| Baseline UHSC | 350 | 520 | 680 | Microcracks observed |
| + ICA (10%) | 220 | 380 | 510 | No visible cracks |
| + Expansive agent (5%) | 280 | 420 | 560 | Minimal microcracks |
| + ICA + Expansive agent | 150 | 260 | 380 | No cracks observed |
The combined use of ICA and expansive agents reduces 90-day shrinkage by approximately 44% compared to baseline UHSC, with no visible cracking observed in the steel pipe concrete columns.
Mechanical Properties
| Mix Design | 28-day Compressive Strength (MPa) | Elastic Modulus (GPa) | Splitting Tensile Strength (MPa) |
|---|---|---|---|
| Baseline UHSC | 125 | 55 | 8.5 |
| + ICA (10%) | 118 | 52 | 8.2 |
| + Expansive agent (5%) | 120 | 53 | 8.8 |
| + ICA + Expansive agent | 115 | 50 | 9.0 |
The mechanical properties remain within acceptable ranges, with the combined mix achieving 92% of baseline compressive strength while significantly improving volume stability.
Steel Pipe-Concrete Interface Performance
The literature evaluates the bond strength and interface integrity:
- Bond strength: Improved by 10–15% with ICA and expansive agent, attributed to reduced interfacial microcracking
- Interface cracking: Eliminated with combined ICA and expansive agent, ensuring full composite action
- Long-term bond stability: Maintained over 180 days of curing, indicating durable interface performance
Engineering Practice Applications
Design Recommendations for UHSSC Columns
Based on the literature findings, the following design recommendations are provided:
- Mix design: Use 10% ICA replacement (by aggregate mass) combined with 5% expansive agent (by cement mass) for optimal volume stability
- W/B ratio: Maintain W/B of 0.22–0.25 for UHSC with ICA
- Steel pipe specification: Q345B or higher grade steel pipe with wall thickness ≥ 8 mm for UHSC confinement
- Curing: Minimum 14 days of moist curing for UHSSC to support expansive agent reactions
- Quality control: Non-destructive testing (ultrasonic, impact echo) to verify internal quality and absence of voids
Construction Considerations
- ICA pre-saturation: Pre-saturate ICA for 24–48 hours before concrete mixing to ensure adequate water content
- Mixing sequence: Add ICA after initial mixing to prevent premature water release
- Placement: Pumped placement with proper vibration to ensure full encasement and avoid segregation
- Curing: Apply moist curing or curing compound immediately after placement to prevent surface drying
- Testing: Conduct compressive strength testing at 7, 28, and 90 days to monitor strength development
Key Technical Insights
The literature reveals several important technical insights:
- Synergistic effect: The combined use of ICA and expansive agents produces a synergistic effect greater than the sum of individual contributions, reducing shrinkage by 44% compared to 30% for ICA alone and 20% for expansive agent alone.
- Optimal dosage: Excessive ICA (> 15%) or expansive agent (> 8%) can lead to reduced strength and potential expansion cracking. The optimal range is 5–15% ICA and 3–8% expansive agent.
- Long-term performance: The volume stability benefits persist over time, with 90-day shrinkage being 44% lower than baseline, indicating durable performance.
- Steel pipe interaction: The steel pipe provides confinement that complements the internal curing and expansive agent effects, creating a robust composite system.
Study Conclusions
The literature demonstrates that the combined use of internal curing aggregate and expansive agents significantly improves the volume stability of ultra-high strength steel pipe concrete columns without substantially compromising mechanical properties. The technology addresses the critical volume instability challenges of UHSC, enabling the use of ultra-high strength concrete in applications where long-term dimensional stability is essential. Engineers should consider this technology for UHSSC applications where cracking prevention and long-term performance are critical. The dual approach of internal curing and expansive agents represents a practical and effective solution for improving UHSC performance in steel pipe concrete applications.