Delayed Ettringite Formation (DEF) is a degradation process in concrete where ettringite, a sulfate-based compound, forms after the concrete has hardened instead of during the early hydration stage. This delayed formation leads to internal expansion as ettringite gradually develops within the hardened matrix, especially in the presence of moisture. The resulting pressure causes microcracks that can grow over time, reducing the durability and structural integrity of the concrete. DEF is particularly associated with precast and mass concrete elements that are exposed to high curing temperatures, which initially prevent normal ettringite formation.
Causes of Delayed Ettringite Formation (DEF):
- High Curing Temperatures: When concrete is cured at temperatures above 65°C, early ettringite formation is suppressed, setting the stage for delayed formation later.
- Moisture Availability: After hardening, exposure to moisture allows delayed ettringite to form and expand within the concrete.
- Sulfate Presence: Sulfates naturally present in cement or introduced from external sources contribute to ettringite formation.
- High-Alkali Cement: Cement with a high alkali content can increase the risk of DEF by encouraging chemical reactions that lead to delayed expansion.
- Large Structural Elements: Mass concrete and precast components often generate significant heat during curing, making them more prone to DEF if temperature control is poor.
Common Areas Where DEF Occurs:
- Bridge piers and abutments
- Dams and spillways
- Retaining walls
- Tunnels and underground structures
- Foundations and footings
- Precast concrete elements (beams, slabs, girders)
- Cooling towers and chimneys
- Thick concrete sections in power plants or industrial units
Signs of Delayed Ettringite Formation (DEF):
- Map cracking on the concrete surface (fine, irregular cracks)
- Expansion or deformation of concrete elements
- Spalling or flaking of the surface
- Discoloration in affected areas
- Progressive internal cracking visible in core samples or through non-destructive testing
Types of Delayed Ettringite Formation (DEF):
1. Internal Delayed Ettringite Formation:
Internal DEF occurs when the sulfates responsible for ettringite formation are already present within the concrete mix, typically from cement or other ingredients like fly ash or slag. During the early curing phase, if the concrete is exposed to high temperatures (usually above 65°C), the formation of ettringite is suppressed. Once the concrete has hardened and is later exposed to moisture, ettringite begins to form and expand within the hardened matrix. This internal expansion leads to microcracking and deterioration over time. Internal DEF is common in precast concrete and mass concrete elements due to the high heat generated during curing.
2. External Delayed Ettringite Formation:
External DEF takes place when sulfates from outside the concrete structure penetrate into the concrete after it has hardened. These external sulfate sources can include sulfate-rich soil, groundwater, seawater, or industrial effluents. Once the external sulfates infiltrate the concrete and react with the aluminates present, ettringite forms and begins to expand within the hardened structure. This expansion leads to cracking and weakening, similar to internal DEF. Structures in marine environments, near wastewater treatment plants, or in sulfate-rich soils are particularly susceptible to external DEF.
3. Combined Delayed Ettringite Formation:
Combined DEF is a situation where both internal and external sources of sulfate contribute to ettringite formation after the concrete has hardened. In such cases, the concrete may already contain some sulfate in the mix, and additional sulfates from the environment enter over time. This dual-source mechanism intensifies the ettringite formation and expansion, often accelerating the deterioration process. Combined DEF is more complex to manage and typically requires both material and environmental assessments for proper analysis and repair.
How to prevent Delayed Ettringite Formation (DEF)?
- Keep the curing temperature below 65°C.
- Use low-heat or sulfate-resisting cement.
- Limit alkali content in the cement.
- Control the water-cement ratio properly.
- Select aggregates and materials with low sulfate content.
- Gradually increase curing temperatures to avoid thermal shock.
- Apply proper moisture protection after curing.
- Use suitable chemical admixtures to reduce heat and improve durability.
- Avoid excessive use of supplementary cementitious materials unless well-controlled.
- Design for good drainage and waterproofing to minimize long-term moisture exposure.
Repair methods for Delayed Ettringite Formation (DEF) in concrete:
- Partial or Full Concrete Removal: For severely damaged areas, remove the affected concrete sections where ettringite has caused significant cracking and expansion. Replace these sections with new, properly mixed concrete to restore structural integrity.
- Crack Injection: For minor to moderate cracks, inject epoxy or polyurethane resins to seal the cracks. This method helps in preventing further moisture ingress and stabilizing the structure, especially in non-structural cracks.
- Sulfate-Resistant Concrete Replacement: When replacing damaged concrete, use sulfate-resistant cement and aggregates to minimize the risk of DEF in the future. This is particularly important for areas exposed to moisture or sulfate-rich environments.
- Strengthening with External Reinforcement: Use external reinforcement techniques like fiber-reinforced polymers (FRP), steel plates, or post-tensioning systems to restore the strength of the concrete structure while preventing further damage.
- Waterproofing and Moisture Control: Install or upgrade waterproofing systems and sealers on the surface to prevent further moisture penetration, which can reactivate DEF. Proper drainage systems are also essential to reduce the chance of water accumulation.
- Surface Coatings: Apply protective coatings to the surface of the concrete, creating a barrier against moisture and chemicals. These coatings help protect the structure from future sulfate attack.
- Regular Monitoring and Inspections:
After repairs, monitor the concrete regularly through inspections, testing, or even using non-destructive testing methods like ultrasonic pulse velocity to detect further signs of expansion or deterioration. - Replacement with Precast Elements: In some cases, it might be more effective to replace the affected sections with precast concrete elements that are more tightly controlled for curing temperatures and material properties.
- Concrete Overlay: Apply a repair mortar or overlay that is designed to bond well with the existing concrete. The overlay can offer an additional layer of protection against moisture and chemicals.
- Reinforcement Bar (Rebar) Replacement or Upgrade: If rebar corrosion or damage has occurred alongside DEF, replace or upgrade the reinforcement bars, ensuring they are adequately protected from further chemical reactions.
Conclusion
Delayed Ettringite Formation (DEF) poses a threat to concrete structures, but with proper prevention and timely repairs, its impact can be minimized. By managing curing conditions, using appropriate materials, and addressing damage promptly, the longevity and safety of concrete can be ensured.
