3.2 Sampling Locations and Middle-Discharge Control
Key Takeaways
- Sampling from a truck mixer normally targets two or more regularly spaced portions from the middle part of discharge.
- The first and last parts of a discharge are avoided because they can be less representative of the batch average.
- A sample container should intercept the full discharge stream or receive diverted flow without losing mortar or coarse aggregate.
- Sampling from piles, dirty ground, or segregated edges can invalidate the connection between the sample and the batch.
Choosing the Right Sampling Location
Most ACI Field Testing Grade I candidates picture C172 sampling at a ready-mix truck. That is a common field situation, but the broader concept is source control. Whether concrete comes from a stationary mixer, truck mixer, paving operation, or other delivery setup, the technician must obtain concrete that represents the batch and is not contaminated or segregated by the sampling method.
For a revolving-drum truck mixer, the key phrase is the middle portion of the batch. The first concrete out of the chute can be affected by startup flow, chute wetting, or material that is not fully representative. The very last concrete can also be less representative because of discharge tailing, segregation, or cleanup effects. The technician should collect two or more portions at regularly spaced intervals during the middle part of discharge and combine them.
The container matters as much as the timing. The sample should intercept the full stream or receive a diverted full stream so the mortar and coarse aggregate stay in their real proportions. Taking a scoop from only one edge of the chute stream can bias the sample. Letting aggregate bounce out of the pan or allowing paste to wash away also changes the material before testing begins.
| Sampling decision | Good field practice | Problem it avoids |
|---|---|---|
| Discharge interval | Use middle portion of the batch | Biased first or last material |
| Number of portions | Use two or more portions when required | One-point sample bias |
| Stream capture | Intercept the whole stream | Mortar or aggregate loss |
| Container condition | Use clean, damp, nonabsorbent equipment | Contamination and moisture change |
| Ground contact | Keep sample off soil and debris | Foreign material in tests |
In the verbal performance exam, do not rush past the sampling location. Many weak descriptions say that the technician gets concrete from the truck and then starts testing. A stronger answer states where in the discharge the portions come from and why the first and last material are avoided. This shows the examiner that you are describing the ASTM practice, not a habit from one jobsite.
Sampling should also be coordinated with the placing crew. The technician may need enough room for a wheelbarrow, pan, or receptacle and enough time to collect portions without interfering with placement. The field habit is to plan the sampling point before concrete is flowing. Waiting until the chute is already discharging often leads to rushed, unsafe, or unrepresentative sampling.
Performance-exam prompts can be answered with this checklist:
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Identify the concrete source and sampling point before discharge.
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Avoid the first and last portions of the batch when sampling truck discharge.
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Obtain the required portions at regular intervals from the middle discharge.
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Capture the full stream into a clean container and prevent loss of any fraction.
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Combine, remix, protect, and move immediately into the timed tests.
When sampling a revolving-drum truck mixer, which part of the discharge is normally targeted?
What is a major problem with dipping a small scoop from one edge of the chute stream?
Why should the technician avoid sampling from dirty ground or a segregated pile edge?