The effect of high temperature on the strength of concrete is:
Correct Answer: B. Decreases the strength of concrete
📚 Detailed Explanation: High Temperature Decreases Concrete Strength
Why B (Decreases the strength) is correct: While moderate warmth accelerates early strength gain, high temperatures (above ≈30–38°C) ultimately reduce the final compressive strength of concrete. The rapid initial hydration at high temperature produces a coarser, less uniform C-S-H gel microstructure with larger pores, greater shrinkage, and more microcracks — resulting in lower ultimate strength compared to concrete cured at moderate temperatures.
Effect of Temperature on Concrete Strength
| Temperature Range | Early Strength (<7d) | 28-day Strength | Long-Term Strength |
|---|---|---|---|
| <5°C (Cold) | Very low (hydration very slow) | Below standard | May slightly exceed standard if no freezing |
| 10°C to 15°C (Ideal low) | Moderate | Standard or slightly above | Good — dense microstructure |
| 20°C to 27°C (Optimal) | Normal | 100% (design reference) | Full design strength |
| 30°C to 38°C (Warm) | Faster | Slightly lower | Below optimal |
| >38°C (Hot — adverse) | Rapid initial set | Significantly reduced (10–20% loss) | Substantially below design |
Mechanisms by Which High Temperature Reduces Strength
| Mechanism | Effect |
|---|---|
| Rapid initial hydration | C-S-H gel forms quickly but in a coarser, less dense structure; outer shell of C-S-H forms around each cement grain, preventing full inner hydration |
| Accelerated evaporation | Mix water evaporates before it can participate in hydration; less C-S-H gel formed; higher porosity |
| Plastic shrinkage cracking | Surface dries faster than interior; differential shrinkage creates surface cracks before concrete has gained strength |
| Non-uniform microstructure | Coarse C-S-H at grain surfaces; high-porosity zones between grains; micro-cracks at interfaces |
| Increased false set / flash set risk | Rapid stiffening makes placement and compaction difficult |
Hot Weather Concreting Precautions (IS 7861 Part 1)
| Precaution | Purpose |
|---|---|
| Use chilled water / ice in mix | Reduce concrete temperature at mixing |
| Pre-cool aggregates | Large thermal mass; major temperature contributor |
| Shade/insulate transit mixers | Prevent temperature rise during transport |
| Concrete placement temperature ≤32°C (IS) / ≤38°C (ACI 305) | Limit adverse effects on strength and workability |
| Night concreting | Lower ambient temperature; cooler substrate and forms |
| Retarding admixtures | Delay initial set; maintain workability |
| Curing: wet hessian / curing compounds | Retain water; prevent plastic shrinkage |
- High temperature (above ≈38°C) decreases concrete strength — rapid hydration produces coarser microstructure and more shrinkage cracks.
- Ideal concreting temperature: 10°C to 27°C.
- Every 10°C rise in concrete temperature reduces 28-day strength by approximately 5–10%.