3.2 Sampling Locations and Middle-Discharge Control

Matching the Procedure to the Concrete Source

Most ACI candidates picture C172 sampling at a ready-mix truck, and that is the most common field case, but C172 gives a procedure for each delivery type. The unifying concept is source control: whether concrete comes from a stationary mixer, a revolving-drum truck mixer, an open-top or agitating truck, or a paving operation, the technician must obtain concrete that represents the batch and is not contaminated or segregated by the act of sampling.

For stationary mixers and revolving-drum (truck) mixers, the rule is the same: sample the concrete by collecting two or more portions at regularly spaced intervals during discharge of the middle portion of the batch. The first concrete out can be affected by startup flow, chute wetting, or material that has not fully mixed. The last concrete can be affected by tailing, segregation, and cleanup. By concentrating on the middle of the batch and avoiding the very first and very last material, the composite represents the batch average. As a working rule of thumb, do not sample from the first or last 10 percent of the load.

For paving concrete discharged onto the grade or into a hopper, the technician collects the sample from at least five different portions of the pile after the mixer is fully discharged, then combines them into one composite. Five well-spread portions are needed because a paving discharge spreads and segregates more than a chute stream.

Capturing the Stream Without Bias

The container matters as much as the timing. The receptacle should intercept the entire discharge stream, or receive a fully diverted stream, so mortar and coarse aggregate stay in their true proportions. Scooping from one edge of the chute can bias the sample because the stream is not uniform across its width. Letting aggregate bounce out of the pan or paste wash away changes the material before any test begins.

In the verbal performance exam, do not rush past the sampling location. Many weak answers say only that the technician gets concrete from the truck and starts testing. A stronger answer states where in the discharge the portions come from, that two or more portions are taken (or five for paving), and why the first and last material are avoided. That shows the examiner you are describing the ASTM practice, not a habit from one jobsite.

Sampling must also be coordinated with the placing crew. The technician needs room and time to collect portions without interfering with placement. The field habit is to choose the sampling point before concrete is flowing; waiting until the chute is already discharging leads to rushed or unrepresentative sampling.

Performance-exam checklist:

• Identify the concrete source and sampling point before discharge starts.
• For mixers, take two or more portions from the middle of the batch; for paving, at least five portions of the pile.
• Avoid the first and last material of the discharge.
• Capture the full stream into a clean, damp, nonabsorbent container.
• Combine, remix, protect, and move immediately into the timed tests.

When the Truck Will Not Cooperate

Real discharges are not textbook-clean, and the performance examiner may probe how you adapt. If the drum is turning too slowly to discharge, the technician asks the driver to bring the truck to mixing speed for the number of revolutions the delivery ticket or specification requires before sampling, because concrete that has not been remixed in the drum is not representative. If discharge is interrupted, the technician must still keep the first-to-final portion interval within 15 minutes; if the gap will blow that window, the right move is to discard and start a fresh composite rather than report a stale blend.

A common trap is sampling to adjust the mix rather than for acceptance. C172 is for obtaining a representative sample for the standard tests. It is not a tool for fishing out the wettest concrete to make a slump pass. If the technician suspects the load is wrong, the answer is to sample correctly, report the true result, and let the responsible party decide.

Equipment That Keeps the Sample Honest

The sampling equipment itself can introduce error if it is wrong for the job. Containers and tools must be clean, damp, and nonabsorbent. A dry wooden or absorbent container pulls water out of the surface paste and can shift slump and air. A dirty pan adds contamination. The receptacle must be large enough to catch the entire stream for a truck or to hold the combined portions for paving without overflowing.

Thinking about equipment in advance is part of source control. A technician who shows up with a damp, clean, properly sized receptacle has already removed several ways the sample could go wrong before the truck arrives.