Zhuojin Pipe Fitting Co., LtdEffect of Internal Curing Aggregates and Expansive Agents on Volume Stability of Ultra-High Strength Concrete-Filled Steel Pipes
Literature Overview
This study note examines the volume stability behavior of ultra-high strength concrete (UHSC) filled into steel pipe columns, with particular focus on the synergistic effects of internal curing aggregates (ICA) and expansive agents (EA). Volume instability in UHSC—manifesting as drying shrinkage, autogenous shrinkage, and thermal cracking—is a critical concern for the long-term durability and structural integrity of concrete-filled steel pipe (CFST) members. The literature provides systematic experimental data on how ICA and EA can mitigate these volume changes, offering practical guidance for the design of high-performance CFST structures.
Core Technical Content
Ultra-high strength concrete, defined as concrete with a 28-day compressive strength exceeding 100 MPa, is increasingly employed in CFST applications for high-rise buildings, long-span bridges, and energy-absorbing structural systems. However, the high cement content and low water-to-binder ratio (typically 0.20–0.25) inherent to UHSC mixtures lead to severe autogenous shrinkage, which can reach 800–1200 microstrain within the first 28 days. This internal restraint-induced shrinkage, particularly when the concrete is confined within a steel pipe, generates significant tensile stresses that may exceed the tensile strength of the concrete, leading to microcracking.
Internal Curing Aggregates
Internal curing aggregates function as distributed water reservoirs within the concrete matrix. Pre-saturated lightweight aggregates (typically expanded perlite, pumice, or ceramsite) are incorporated into the UHSC mix, releasing absorbed water during the early hydration period to sustain continued cement hydration and reduce autogenous shrinkage. The literature evaluates ICA at replacement levels of 3%, 5%, and 7% by volume of total aggregate:
| Parameter | 0% ICA (Reference) | 3% ICA | 5% ICA | 7% ICA |
|---|---|---|---|---|
| Autogenous shrinkage at 28d (μɛ) | 950 | 780 | 620 | 480 |
| Compressive strength at 28d (MPa) | 128 | 122 | 115 | 108 |
| Porosity (%) | 8.2 | 9.5 | 10.8 | 12.1 |
| Relative humidity at 28d (%) | 72 | 82 | 88 | 92 |
The data clearly demonstrate the trade-off between shrinkage reduction and strength development. At 5% ICA replacement, a favorable balance is achieved with a 35% reduction in autogenous shrinkage and only a 10% reduction in compressive strength.
Expansive Agents
Expansive agents, typically calcium sulfoaluminate (CSA)-based compounds or calcium hydroxide-based products, are incorporated to generate controlled expansion that counteracts shrinkage. The literature evaluates expansive agents at dosages of 2%, 4%, and 6% by mass of cement:
| Expansive Agent Dosage | Expansion at 7d (mm/m) | Expansion at 28d (mm/m) | Compressive Strength Change |
|---|---|---|---|
| 0% | 0 | 0 | Reference |
| 2% | +1.2 | +2.5 | -3% |
| 4% | +2.8 | +5.1 | -8% |
| 6% | +4.5 | +8.2 | -15% |
Excessive expansive agent dosage leads to over-expansion, which can cause surface cracking and delamination at the steel-concrete interface. The optimal dosage of 2–3% provides sufficient compensatory expansion without adverse effects on strength or surface integrity.
Synergistic Effect of ICA and Expansive Agents
The combined use of ICA and EA produces a synergistic effect that outperforms either measure alone. The literature reports that the combination of 5% ICA and 3% EA achieves:
- Autogenous shrinkage reduction of 55% compared to the reference mixture.
- Compressive strength retention of 92% of the reference.
- Internal relative humidity maintained above 85% throughout the curing period.
- No observable surface cracking or interface delamination at 28 days.
The mechanism of synergy involves the ICA providing sustained water supply for hydration and the EA generating controlled expansion to counteract residual shrinkage. Together, they create a self-moistening and self-compensating concrete system that maintains volume stability within the confined steel pipe environment.
Volume Stability in Confined Steel Pipe Environment
The confinement effect of the steel pipe on UHSC volume stability is a critical consideration. In a CFST member, the steel pipe exerts radial confining pressure on the concrete core, which restricts lateral expansion but also provides beneficial confinement during loading. The literature highlights that:
- The steel pipe confinement reduces visible cracking but does not eliminate internal microcracking caused by autogenous shrinkage.
- Residual stresses at the steel-concrete interface due to differential shrinkage can affect the bond strength and load transfer efficiency.
- The combined ICA-EA approach minimizes interface residual stresses to below 2 MPa, which is well within the elastic range of both materials.
Engineering Practice Recommendations
Based on the literature findings, the following practical recommendations are offered for UHSC-filled steel pipe columns:
- Select ICA with appropriate water absorption capacity (15–25% by mass) and particle size distribution compatible with the UHSC aggregate grading.
- Use CSA-based expansive agents at 2–3% dosage, ensuring compatibility with the cement type and admixture system.
- Implement adequate external curing (wet curing or curing compound) for at least 7 days to complement the internal curing effect.
- Monitor internal relative humidity using embedded sensors during the first 28 days to verify the effectiveness of the internal curing system.
- Design the steel pipe-to-concrete interface with appropriate surface treatment (e.g., shot blasting) to enhance bond strength and accommodate differential deformations.
Study Insights and Conclusion
This literature provides a systematic and well-documented investigation into the volume stability enhancement of UHSC-filled steel pipes through the combined use of internal curing aggregates and expansive agents. The quantitative data on shrinkage reduction, strength retention, and internal humidity maintenance offer engineers a clear basis for mix design optimization. The identification of the synergistic mechanism and the recommended optimal dosages (5% ICA and 3% EA) represent practically actionable knowledge that can be directly applied to CFST structural design. The emphasis on the confined environment effects and interface behavior addresses a gap in previous research that focused primarily on unconfined concrete specimens. For the growing field of ultra-high performance concrete in structural applications, this work contributes essential understanding of how to achieve volume stability without compromising the mechanical performance that makes UHSC attractive for advanced structural systems.