Mix proportions, water, compaction and curing all decide concrete strength, and none can be seen once the slab is cast. Cubes are the record of what actually went in.
How Cubes Are Made
- Take concrete from the pour as it is being placed — not a special batch.
- Fill 150 mm steel moulds in layers, compacting each by rod or vibration.
- Mark each cube with the date, member and grade.
- Leave in the moulds, covered, for about 24 hours.
- Demould and cure in clean water until testing.
- Test at 7 and 28 days in a compression testing machine; strength = failure load ÷ area.
How Many Samples?
IS 456 sampling frequency:
| Quantity of concrete in the work | Number of samples |
|---|---|
| 1 – 5 m³ | 1 |
| 6 – 15 m³ | 2 |
| 16 – 30 m³ | 3 |
| 31 – 50 m³ | 4 |
| 51 m³ and above | 4 + 1 per additional 50 m³ |
A sample is normally three cubes for each test age. A typical 12 m³ house slab therefore needs two samples — six cubes for 28 days, plus cubes for 7 days.
Reading 7-Day Results
| Cement | 7-day strength as share of 28-day (typical) |
|---|---|
| OPC | About 65 – 70% |
| PPC | Lower; gains more between 7 and 28 days |
So M20 OPC concrete showing roughly 13–14 N/mm² at 7 days is on track. The 28-day result is what counts; IS 456 sets acceptance criteria for groups of results and individual results.
If Results Are Low
- Check the test — cube making, curing and testing can themselves be faulty.
- Non-destructive tests on the structure — rebound hammer, ultrasonic pulse velocity — to map strength.
- Core tests — cylinders cut from the member and crushed.
- Engineer's decision — accept, strengthen, or replace.
The Short Version
Cast cubes from every important pour, at the frequency IS 456 sets, cure them properly and test at 7 and 28 days. They cost little and are the only proof of what is inside your slab.
