Understanding Concrete Durability: Mechanisms and Prevention Strategies
MODULE 2: Concrete Durability
1. Supplementary Cementitious Materials (SCMs)
SCMs are materials used to partially replace portland cement, enhancing both fresh and hardened concrete properties. Common SCMs include fly ash, slag cement, and silica fume, which are byproducts of various industries.
Cementitious/Hydraulic Material: Hydraulic materials like portland cement harden through hydration, a chemical reaction with water. This forms calcium silicate hydrate (C-S-H) for strength and calcium hydroxide (CH), which is susceptible to chemical attack.
Pozzolanic Material: Pozzolans react with CH and water to form additional C-S-H, improving concrete performance.
Benefits of Using SCMs:
- Increased long-term strength (except silica fume, which gains strength quickly)
- Enhanced durability through reduced permeability
- Delayed set time (except silica fume), beneficial in hot weather
- Reduced risk of thermal cracking
- Lower CH content, minimizing oxychloride formation and early joint deterioration
2. Durability Aspects of Concrete
Durability refers to concrete’s resistance to weathering, chemical attack, abrasion, and deterioration. Different concretes require varying degrees of durability based on exposure conditions.
Types of Durability:
- Aging (of polymers)
- Dust resistance
- Fatigue resistance
- Fire resistance
3. Chemical Admixtures in Concrete
Chemical admixtures are added before or during concrete placement to reduce construction costs, modify hardened concrete properties, and ensure quality throughout the process.
Common admixtures include water reducers, superplasticizers, retarders, accelerators, shrinkage preventers, segregation reducers, and heat evolution reducers.
4. Deterioration Mechanisms of Concrete Systems
Corrosion of Reinforcing Steel:
Corrosion is a leading cause of concrete deterioration. Rust expansion creates tensile stresses, leading to cracking, delamination, and spalling.
Mechanical Decay:
Erosion, abrasion, and cavitation can cause mechanical damage to concrete, especially in hydraulic structures.
5. Sources of Sulfate in Concrete
Sulfate attack occurs when sulfate ions react with cement paste constituents. Sources include:
- Oxidation of sulfide minerals in clay
- Bacterial action in sewers
- Seawater or wastewater exposure
- Natural sources like groundwater or soil
6. Mechanism of Sulfate Attack
Sulfate ions react with calcium hydroxide and calcium aluminate hydrate, forming gypsum and ettringite. Ettringite formation causes expansion, cracking, and mass loss.
7. Prevention of Sulfate Attack
Strategies include:
- Low-permeability concrete
- Adequate concrete thickness
- High cement content and low water-to-cement ratio
- Proper compaction and curing
- Sulfate-resisting cements
- Blended cements and pozzolans
- Air entrainment
- High-pressure steam curing
8. Deterioration Due to Acids
Acid attack dissolves and leaches calcium hydroxide from cement paste, increasing porosity, reducing cohesiveness, and causing strength loss.
9. Preventing Acid Attack
Methods include:
- Fixing calcium hydroxide by treating with diluted water and sodium silicate or using surface treatments like coal tar pitch, epoxy resins, etc.
10. Shrinkage in Concrete Structures
Shrinkage is the decrease in concrete volume due to moisture loss or chemical changes. Excess mixing water and drying contribute to shrinkage.
11. Deterioration Due to ASR Reaction
Alkali-silica reaction (ASR) occurs between reactive silica in aggregates and hydroxyl ions in concrete pore fluid. The resulting ASR gel swells, causing microcracking and deterioration.