In the symbol MX (e.g., M20), M stands for Mix and the numeric X represents the:
Try it first: pick an option, then read the explanation.
Correct answer: C. 28 days compressive strength
Detailed Explanation: Meaning of MX Grade Designation
Why C (28-day compressive strength) is correct: Per IS 456, the grade designation system for concrete is:M = Mix (indicating it is a concrete grade designation)
X = Characteristic compressive strength in N/mm² at 28 days, measured on 150 mm standard cubes
The 28-day period is the standard curing age for concrete strength assessment because most of the cement hydration (70–80%) is complete by 28 days, giving a stable, representative strength value.
Why 28 Days is the Standard Age
| Age | Approx. Strength (% of 28-day) |
|---|---|
| 1 day | ~20% |
| 3 days | ~40% |
| 7 days | ~65% |
| 28 days | 100% (design reference strength) |
| 90 days | ~115–120% |
- MX designation: M = Mix, X = 28-day characteristic compressive strength in N/mm².
- Measured on 150 mm × 150 mm × 150 mm standard concrete cubes per IS 516.