Codes referenced
Sampling, casting, curing, and compressive strength testing of cubes.
Clause 15 (sampling), Clause 16 (acceptance criteria — Table 11).
Sampling at the point of delivery / placement.
Cast-iron mould specifications, dimensional tolerances.
* Earlier editions were IS 516:1959 and IS 1199:1959 — both superseded by the 2018-part-wise revisions while keeping the same standard number. Confirm the latest published revision on the BIS catalogue before quoting on a project.
Sampling frequency
One sample = a set of 3 cubes (tested at 7 & 28 days — actually 6 cubes if testing both ages). Take samples at the point of placement, not at the plant outlet.
| Quantity of concrete (m³) | Minimum samples |
|---|---|
| 1 – 5 | 1 |
| 6 – 15 | 2 |
| 16 – 30 | 3 |
| 31 – 50 | 4 |
| 51 & above | 4 + 1 per additional 50 m³ or part thereof |
Reference: IS 456:2000 Clause 15.2
The procedure, step by step
Sampling
IS 1199 (Part 1):2018
- · Collect concrete at the point of placement using a clean, non-absorbent container.
- · Take the sample from the middle third of the discharge.
- · Re-mix with a shovel or trowel to ensure uniformity before moulding.
- · Slump/temperature record should be noted alongside the sample ID.
Casting
150 mm cubes · 3 layers · 35 strokes each
- · Clean mould, apply release agent, assemble tightly.
- · Fill in 3 equal layers. Compact each layer with 35 strokes of a 16 mm tamping rod.
- · Distribute strokes uniformly across the cross-section.
- · Tap mould sides gently to release trapped air.
- · Strike off flush with a trowel. Label with sample ID, date, grade, location.
Curing
27 ± 2 °C · ≥ 95% RH
- · Cover moulds to prevent moisture loss; keep at 27 ± 2 °C for the first 24 hours.
- · Demould at 24 ± 0.5 hours from time of adding water to mix.
- · Transfer cubes to clean, fresh water tank at 27 ± 2 °C until testing.
- · Test at 7 days or 28 days. Mark the test age on the cube.
Testing
14 MPa/min loading rate
- · Remove cube from water; surface-dry. Weigh and dimension (150 mm assumed).
- · Place centrally in the testing machine between platens.
- · Apply load uniformly at 14 MPa/min (≈ 315 kN/min for a 150 mm cube).
- · Record max load, compute area, calculate strength (load / area).
- · Note the failure pattern (see below).
Assumed σ when you don't have 30 samples
IS 456 Table 8 gives assumed standard deviation values to use when you don't have enough history:
| Grade | Assumed σ (MPa) |
|---|---|
| M10 – M15 | 3.5 |
| M20 – M25 | 4.0 |
| M30 – M55 | 5.0 |
| M60 – M80 | 6.0 |
Worked example — M30 grade
fck = 30 MPa, σ assumed = 5.0 MPa.
- Option A: fck + 0.825σ = 30 + 0.825 × 5 = 34.125 → 34.0 MPa (rounded to nearest 0.5)
- Option B: fck + 3 = 30 + 3 = 33 MPa
- Higher of the two → 34.0 MPa
- fck − 3 = 30 − 3 = 27 MPa
- No single cube (28-day) should drop below this.
Reading the failure pattern
IS 516 specifies a normal conical failure as the qualifying mode. Anything else is a flag for your investigation.
Both ends intact, double cone forms in the middle. Specified pattern — use the result.
Vertical cracks through cube. Platen restraint issue or capping problem. Retest a spare cube.
Diagonal failure along one plane. Possible weak interface or capping weakness. Investigate.
Cube shatters violently. Possibly due to impact loading or very brittle mix. Result may not be valid.
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, and RMC plant operations. NRMCA Certified Trainer (USA) and ISO Lead Auditor.