How does the compressive strength of concrete change with age?
Correct Answer: A. Increases
📚 Detailed Explanation: Concrete Strength Increases with Age
Why A (Increases) is correct: Concrete gains strength through the ongoing chemical process of cement hydration, in which cement compounds (primarily C3S and C2S) react with water to form calcium silicate hydrate (C-S-H) gel — the primary strength-giving compound. As hydration progresses over time, more C-S-H gel fills the capillary pore space, increasing density, reducing porosity, and continuously increasing strength. This process continues for years if moisture is available.
Strength Development Over Time (OPC Concrete)
| Age | % of 28-day Strength (OPC) | Key Hydration Events |
|---|---|---|
| 3 days | ≈40% | Rapid C3S hydration; initial C-S-H gel network forms |
| 7 days | ≈65% | C3S largely hydrated; C2S begins contributing |
| 14 days | ≈80% | Continued C2S hydration; pore structure refining |
| 28 days (reference) | 100% | Standard design/test age; approximately 70–80% hydration complete |
| 90 days | ≈120% | C2S significantly hydrated; dense microstructure |
| 1 year | ≈135% | Continued slow hydration; approaching asymptote |
| Several years | ≈140–150% | Near-full hydration; strength plateau |
Conditions Required for Continued Strength Gain
| Condition | Effect |
|---|---|
| Adequate moisture | Hydration requires water; if concrete dries out completely, hydration stops and strength gain ceases |
| Favourable temperature (>5°C) | Below 5°C, hydration slows dramatically; below 0°C it stops (fresh) or slows greatly (hardened) |
| No aggressive chemical attack | Acid, sulphate, or chloride attack can reduce strength over time in aggressive environments |
- Concrete strength increases with age as cement hydration continues.
- Rate is rapid early (7-day = 65% of 28-day) then slows asymptotically.
- IS 456: 28-day strength is the design reference; all characteristic strengths (fck) refer to 28-day cube tests.