3.4 Sample Size, Large Aggregate, and Wet Sieving

Planning Sample Size Before Testing Starts

The technician must know which tests are required before collecting concrete, because the required volume depends on them. If strength specimens (cylinders under C31/C31M or beams) will be molded, C172 requires a composite sample of at least 1 ft³ (28 L). That volume must support the strength specimens and the fresh-property tests drawn from the same sample. Smaller samples are not prohibited for routine slump, temperature, and air-content testing when those are the only tests being run.

Size is necessary but not sufficient: the sample must still be representative. A full wheelbarrow taken from the wrong location is worse than a smaller, correctly obtained composite. At the same time, an undersized sample forces the technician to scrape corners, reuse disturbed material, or choose between required tests, and each of those creates preventable error. Plan for at least 1 ft³ whenever cylinders or beams are involved.

The 15-Minute Molding Window

C172 also sets a start limit for strength specimens: molding must begin within 15 minutes after fabricating the composite sample. This is separate from the 5-minute window for slump, temperature, and air. The practical consequence is sequencing: the technician typically runs the fresh-property tests first (within their 5-minute window), keeps enough concrete protected and remixed, and then molds cylinders so that filling the first mold begins within 15 minutes of the composite being made.

Large Aggregate and Wet Sieving

Large aggregate creates a second control issue. Certain fresh-concrete tests and specimen molds have maximum aggregate-size limits. When the concrete contains aggregate larger than a method allows, 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 procedure that removes oversize particles while preserving the mortar and the smaller aggregate fraction the method needs.

Wet sieving belongs in the planned workflow. If the mix design has a large nominal maximum aggregate, the technician should know which tests are affected before the truck arrives, and the sieve, pan, and tools should be clean and damp. Avoid washing paste away, letting aggregate pile up so mortar drains off, or letting wet sieving push a timed test past its allowed start window.

For performance-exam purposes, the candidate usually need not recite every mechanical movement of wet sieving. The JTA-level answer is to recognize when large aggregate can make a method inapplicable, state that oversize aggregate is removed by wet sieving when the method requires it, and note that the test is then run on the processed sample.

Field planning list:

• Know whether cylinders, beams, air, density, slump, and temperature are required.
• Collect at least 1 ft³ (28 L) whenever strength specimens will be molded.
• Keep the sample representative while moving it to the test area.
• Identify large-aggregate issues before the clock becomes a problem.
• Document unusual sample treatment such as wet sieving.

Sizing the Sample to the Test Plan

The 1 ft³ (28 L) minimum is a floor, not a target. The technician should size the sample to the whole test plan with a margin for the concrete consumed and discarded along the way. A typical acceptance set might call for slump, temperature, air, density, and a set of cylinders. Each draws from the composite, and some concrete is lost to remixing, rodding, striking off, and cleanup. Collecting only the bare minimum invites a shortfall partway through.

A practical way to estimate is to add up the volume each test needs, then collect comfortably more than that total while still keeping every portion representative. Remember that the minimum applies to samples used for strength specimens; if only routine slump, temperature, and air are required, a smaller sample is acceptable. The judgment call is knowing the test plan before the truck arrives.

Why Removing Aggregate Changes the Concrete

Wet sieving must be understood as changing what is being tested, not just tidying the sample. When oversize particles are removed, the remaining material is mortar plus the smaller aggregate fraction; its density, air content, and behavior differ from the full mix. That can be exactly what a method requires when its equipment cannot accommodate the largest aggregate, but it means the result describes the processed material, not the as-delivered concrete with full-size stone.

For that reason, the technician records when wet sieving was performed so downstream users know the test was run on a wet-sieved sample. The same caution applies any time material is lost: if coarse aggregate bounces out or paste washes away, the sample is no longer representative, and the right action is to resample rather than test biased material.