3.4 Sample Size, Large Aggregate, and Wet Sieving
Key Takeaways
- A sample used for strength specimens must be large enough, commonly at least 1 ft^3 or 28 L, to support representative molding.
- Smaller samples may be acceptable for routine fresh-property tests when no strength specimens are being made.
- Large aggregate can require wet sieving before certain tests, but wet sieving is a controlled procedure rather than a convenience step.
- Removing oversize aggregate changes what is being tested, so records and test selection must make the sample treatment clear.
Planning Sample Size Before Testing Starts
The technician should know the required tests before collecting concrete. If strength specimens will be molded, the sample has to be large enough to make those specimens and support the related fresh-property tests. A common C172 exam number is a minimum composite sample size of 1 ft^3, or 28 L, when strength testing is involved. Smaller samples may be used for routine slump, temperature, or air tests when that is all the sample must support.
Sample size is not just a volume target. The sample must still be representative. A full wheelbarrow of concrete taken from the wrong location is not better than a smaller correctly obtained sample. At the same time, an undersized sample forces the technician to scrape corners, reuse disturbed material, or choose between required tests. Those habits create preventable errors.
Large aggregate creates a second control issue. Some fresh-concrete tests and specimen molds have maximum aggregate-size limits. When the concrete contains aggregate larger than the size allowed for a test, the sample may need to be wet sieved over the appropriate sieve before that test is performed. Wet sieving is not routine cleanup. It is a defined way to remove oversize particles while preserving the mortar and smaller aggregate fraction for the method that requires it.
| Situation | Correct control | Why it matters |
|---|---|---|
| Strength specimens required | Collect at least the required large sample volume | Supports cylinders or beams plus fresh tests |
| Only routine fresh tests required | Smaller sample may be enough | Avoids unnecessary handling |
| Oversize aggregate present | Wet sieve only when required | Keeps test method within scope |
| Aggregate lost during collection | Resample if representation is compromised | Lost stone changes density and air results |
| Sample treatment performed | Record and communicate clearly | Prevents confusion about tested material |
Wet sieving should be handled as part of the planned workflow. If a mix design includes large nominal maximum aggregate, the technician should know which tests are affected before the truck arrives. The sieve, pan, and tools should be clean and damp. The technician must avoid washing paste away, letting aggregate pile up in a way that drains mortar, or delaying timed tests beyond their allowed start windows.
For performance-exam purposes, the candidate does not need to recite every mechanical movement of wet sieving unless asked. The safer JTA-level answer is to recognize when large aggregate can make a test method inapplicable, state that oversize aggregate is removed by wet sieving when the method requires it, and explain that the resulting test is performed on the processed sample.
Use this planning list in the field:
-
Know whether cylinders, beams, air, density, slump, and temperature are required.
-
Collect enough concrete before tests begin.
-
Keep the sample representative while moving it to the testing area.
-
Identify large aggregate issues before the test clock becomes a problem.
-
Document unusual sample treatment and tell the responsible project contact.
What minimum sample size is commonly required when the sample will be used for strength specimens?
When is wet sieving considered in the C172 sampling workflow?
Why is an undersized sample a practical problem?