The approximate ratio of strength of cement concrete at 3 months to that at 28 days of curing is:
📈 Understanding Strength Gain in Concrete
Concrete does not gain its full strength immediately after setting. The process of hydration, where cement reacts with water to form a binding gel, continues for a very long time, provided moisture is available. This means concrete continues to gain strength over months and even years.
- The rate of strength gain is rapid at first and slows down over time.
- The 28-day strength is used as a standard reference point for design purposes (called the characteristic strength), as by this time, concrete has achieved a significant portion of its ultimate strength.
📝 Detailed Analysis of the Options
(a) 1.15
This is the correct answer. Concrete continues to gain strength beyond 28 days. At 3 months (approximately 90 days), it is typically about 10-20% stronger than its 28-day strength. The ratio of 3-month strength to 28-day strength is therefore commonly taken as 1.1 to 1.2. The value of 1.15 falls squarely within this accepted range.
(b) 1.3
This would imply a 30% increase in strength from 28 days to 3 months. While possible under certain conditions or with specific cement types, it is too high for a general approximation for ordinary concrete.
(c) 1
This would mean that concrete stops gaining strength after 28 days, which is incorrect. Hydration and strength development are ongoing processes.
(d) 0.75
This is incorrect as it suggests that concrete loses strength after 28 days, which does not happen under normal curing conditions.
📊 Typical Strength Gain of OPC Concrete Over Time
| Age of Concrete | Approximate Strength (as % of 28-day strength) | Ratio to 28-day Strength |
|---|---|---|
| 3 Days | ~ 40% | 0.40 |
| 7 Days | ~ 65% - 70% | 0.65 - 0.70 |
| 28 Days | 100% (Reference) | 1.00 |
| 3 Months (90 Days) | ~ 110% - 120% | 1.10 - 1.20 |
| 1 Year | ~ 120% - 130% | 1.20 - 1.30 |
💡 Study Tips
- Strength Never Stops (Theoretically): Remember that as long as there is unhydrated cement and moisture, concrete will continue to gain strength, although at a much slower rate.
- Key Ratios: Memorize the approximate strength ratios for key dates: 7 days (~2/3 or 0.67), 28 days (1.0), and 3 months (~1.15).
- 28 Days is the Benchmark: Understand that 28-day strength is not the final strength, but the standard strength used for structural design and quality control.