| Primary value | 29 MPa (mean of 3) (and ≥ 21.25 MPa each cube) |
| Applies to | Routine 28-day cube tests on RCC concrete (M25 grade shown — same logic for all grades) · Acceptance of a sample of 3 cubes cast from the same batch · Site QA / batching plant QC |
| Exceptions | Mean of 3 cubes (Criterion 1) → ≥ fck + 4 = 29 MPa |
| Individual cube (Criterion 2) → ≥ 0.85 × fck = 21.25 MPa | |
| Both criteria must pass → Sample fails if either is short | |
| M30 and above — Criterion 1 → ≥ fck + 5 MPa | |
| Sample size → 3 cubes per 50 m³ or per day, whichever is smaller | |
| Measured as | 150 mm cubes cured in water at 27 ± 2 °C for 28 days, tested in compression on a calibrated CTM. Strength = peak load ÷ 150 × 150 mm² = peak / 22500 N/mm². |
| Source | IS 456 — Clause 16, Table 11 ✓ Verified |
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The two-part criterion (mean AND individual) catches both batching errors (low mean) and consolidation problems (one weak cube while two pass). Many sites only check the mean and pass through borderline batches that have a weak cube — a rejected M25 batch on a slab is far cheaper than a structural review six months later.
Cubes are cast in triplets from each pour and tagged with date / batch / location. Any failed sample triggers a non-destructive testing (rebound hammer + UPV) sweep on the affected pour, and core testing if NDT also flags it.