The ratio of 7-day compressive strength to 28-day compressive strength of OPC concrete is approximately:
Correct Answer: A. 0.65
📚 Detailed Explanation: 7-day/28-day Strength Ratio = 0.65 for OPC
Why A (0.65) is correct: IS 456:2000 and standard concrete practice establish that for Ordinary Portland Cement (OPC), the 7-day compressive strength is approximately 65% of the 28-day reference strength. This means the ratio f7/f28 ≈ 0.65.
Strength ratio: f7 / f28 ≈ 0.65
Example M20 concrete:
f28 = 20 N/mm² (characteristic strength)
Expected f7 = 20 × 0.65 = 13 N/mm²
Estimating 28-day from 7-day:
f28 = f7 / 0.65 = f7 × 1.538 ≈ f7 × 1.5
Cement Hydration and Strength Development
| Compound | Name | Reaction Speed | Strength Contribution |
|---|---|---|---|
| C3S (Alite) | Tricalcium silicate | Rapid (days 1–28) | Early and long-term strength; dominant |
| C2S (Belite) | Dicalcium silicate | Slow (weeks to months) | Long-term strength gain beyond 28 days |
| C3A | Tricalcium aluminate | Very rapid (hours) | Flash set control; very early strength; heat |
| C4AF | Tetracalcium aluminoferrite | Moderate | Minor contribution; gives grey colour |
Strength Development Over Time (OPC, M20)
| Age | Approximate % of 28-day Strength | fck Achieved (M20) |
|---|---|---|
| 3 days | ≈ 40% | 8 N/mm² |
| 7 days | ≈ 65% | 13 N/mm² |
| 14 days | ≈ 80% | 16 N/mm² |
| 28 days | 100% (reference) | 20 N/mm² |
| 90 days | ≈ 120% | 24 N/mm² |
| 1 year | ≈ 135% | 27 N/mm² |
- 7-day / 28-day strength ratio for OPC = 0.65 (65%).
- This ratio is used on site to predict 28-day strength from early-age cube tests → if 7-day strength is known.
- Rapid hardening Portland cement (RHPC): ratio is higher ≈ 0.75–0.80 (more C3S content).