To estimate 28-day strength from 7-day compressive strength of OPC concrete, the 7-day value should be multiplied by:
Correct Answer: C. 1.5
📚 Detailed Explanation: 28-day Strength = 7-day Strength × 1.5
Why C (1.5) is correct: The 7-day strength is approximately 65% of the 28-day strength (ratio 0.65). Rearranging: f28 = f7 / 0.65 = f7 × 1.538 ≈ 1.5. This multiplication factor of 1.5 is a widely used site approximation in IS codes and field practice.
Derivation:
f7 / f28 = 0.65
f28 = f7 / 0.65
f28 = f7 x 1.538
f28 ≈ f7 x 1.5
f7 / f28 = 0.65
f28 = f7 / 0.65
f28 = f7 x 1.538
f28 ≈ f7 x 1.5
Worked Example (M25 concrete, site test):
7-day cube strength (f7) = 18 N/mm² (average of 3 cubes)
Predicted f28 = 18 x 1.5 = 27 N/mm²
Required f28 for M25 = 25 N/mm² (fck)
Result: OK, as predicted f28 exceeds fck
Standard Site Decision Matrix
| 7-day Cube Result | Predicted 28-day (x 1.5) | Decision for M25 |
|---|---|---|
| 18 N/mm² | 27 N/mm² | Proceed — above 25 MPa |
| 17 N/mm² | 25.5 N/mm² | Borderline — close monitoring required |
| 15 N/mm² | 22.5 N/mm² | Investigate — below required 28-day strength |
| 12 N/mm² | 18 N/mm² | Reject or investigate materials/mix proportions |
Multipliers for Different Cements
| Cement Type | f7/f28 Ratio | Multiplier (f28 = f7 × ?) |
|---|---|---|
| OPC (Ordinary Portland) | 0.65 | × 1.5 |
| PPC (Portland Pozzolana) | 0.55–0.60 | × 1.65–1.80 |
| RHPC (Rapid Hardening) | 0.75–0.80 | × 1.25–1.33 |
- For OPC concrete, multiply 7-day strength by 1.5 to estimate 28-day strength.
- This is a site approximation only — for structural decisions, always test actual 28-day cubes.
- IS 456:2000 acceptance criterion is based on 28-day cube tests (150 mm cubes per IS 516).