Low temperature (non-freezing) during concrete laying and curing:
Correct Answer: A. Increases strength
📚 Detailed Explanation: Low Temperature (Non-Freezing) Increases Final Concrete Strength
SSC JE Note: The official SSC JE answer key for this question gives (a) Increases strength. This refers specifically to non-freezing low temperatures. While low temperature severely delays early strength gain, concrete cured slowly at lower (but above-freezing) temperatures ultimately develops a denser microstructure and marginally higher final compressive strength than concrete cured at higher temperatures.
Why A (Increases strength) is the exam answer: This is a well-established finding in concrete science known as the temperature-maturity effect. At lower curing temperatures (above freezing), cement hydration proceeds more slowly, allowing C-S-H gel to form in a denser, more uniform arrangement with fewer defects and smaller pores. The result is a higher ultimate (long-term) compressive strength, even though early-age strength development is significantly retarded.
Temperature vs. Strength: The Full Picture
| Curing Temperature | Early Strength (7d) | 28-day Strength | Ultimate Long-term Strength |
|---|---|---|---|
| Below 0°C (Freezing) | Very low / zero (hydration stops) | Very low / damaged | Permanently impaired if frozen while fresh |
| 0°C to 5°C (Very cold, no freeze) | Very low | Below standard (40–70% of reference) | Slightly above standard reference — denser microstructure |
| 10°C to 15°C | Slow | Standard or slightly above | Good |
| 20°C to 27°C (Optimal) | Normal | 100% (design reference, IS 456) | Full design strength |
| 30°C to 38°C | Accelerated | Slightly reduced | Below reference |
| >38°C (Hot) | Rapid (coarse C-S-H) | Significantly reduced | Substantially below reference |
Why Does Slow Curing (Low T) Increase Ultimate Strength?
| Mechanism at Low Temperature | Effect on Microstructure |
|---|---|
| Slow C-S-H gel precipitation | Gel forms in fine, uniform crystals; fills pore space more completely; fewer macro-voids |
| More time for inner hydration | C-S-H shell around each cement grain forms slowly; inner core has more time to hydrate fully |
| Reduced thermal stress | Less heat of hydration gradient; less internal thermal microcracking |
| Denser, lower-porosity paste | Higher compressive strength; lower permeability; better durability |
Maturity Method: Explains Low-T Strength Behaviour
M = Σ[Δt × (T − T0)]
M = Σ[Δt × (T − T0)]
At 5°C for 90 days: M = 90 × (5−(−11)) = 90 × 16 = 1440 °C·days
At 20°C for 28 days: M = 28 × (20−(−11)) = 28 × 31 = 868 °C·days
Higher maturity at 5°C (over longer time) → higher ultimate strength
But: 28-day strength at 5°C is much lower than 28-day strength at 20°C
- Low temperature (non-freezing, above 0°C) ultimately increases final concrete strength via slower, denser C-S-H gel formation.
- However, early-age strength is significantly lower at low temperatures; adequate curing duration is essential.
- IS 456: test cubes are cured at 27±2°C as the reference standard; field concrete at lower temperatures needs longer curing to reach equivalent strength.