M25 cube test gave 16.2 N/mm² at 7 days — is that a problem?

With OPC, concrete usually reaches about 65% of its 28-day strength at 7 days. For M25 that is roughly:

0.65 × 25 = 16.25 N/mm²

So 16.2 N/mm² is on the normal curve — not a reason to panic. A few points:

1. 7-day results are only an early indicator. Acceptance is based on the 28-day results (IS 456 Cl. 16, Table 11).
2. Check curing on site — poor curing in the first week is the most common reason for low early strength.
3. If the 28-day results fail the acceptance criteria, IS 456 Cl. 17.4 allows core tests: the concrete is acceptable if the average equivalent cube strength of the cores is at least 85% of the cube strength required for that grade, and no individual core is below 75%.

Keep the 28-day cubes safe and labelled — they decide it.