Sampling Freshly Mixed Concrete

Every fresh concrete test assumes the sample represents the load. A sample taken from the first splash out of the chute, or scooped from the edge of a pile, may have more or less paste, water or air than the concrete actually placed. The sampling practice (AASHTO R 60, ASTM C172) sets out where and how to take portions, how to combine them, and how fast to test them (Taylor et al., 2019¹).

The Composite Sample

A sample for acceptance testing is a composite: two or more portions taken from the middle part of the load and mixed together. The portions are taken at regular intervals while the middle of the batch discharges, never from the very first or last of the load, which are the least uniform. All the portions must be taken within 15 minutes of each other. They are carried to the testing site, combined and remixed with a shovel to make them uniform, and protected from sun, wind, rain and contamination until tested.

For strength specimens the composite should be at least 28 L (1 ft³). Smaller samples are allowed for routine temperature, slump and air checks.

Where to Take It

How the portions are collected depends on how the concrete is delivered (Western Alliance for Quality Transportation Construction, 2025²):

• Revolving-drum truck mixers and agitators. Pass a receptacle through the whole width of the discharge stream, or divert the whole stream into the receptacle. Don't let the discharge rate be slowed down to fill the container, because a slower drum changes what comes out. Water and admixtures added on site must be mixed in before sampling.
• Stationary mixers and nonagitating trucks. Take portions from the discharge stream in the same way, at regular intervals during discharge.
• Paving. When concrete is dumped on grade in front of a slipform paver, take portions from at least five different places in the pile, avoiding the edges and anything contaminated by the subgrade.
• Pumps and conveyors. Agencies may sample at the end of the line to account for changes in air and slump during pumping. Where they do, it is usually written into the specification, along with how the results relate to samples at the truck.

Large Aggregate: Wet-Sieving

Strength specimens and some air tests can't hold very large aggregate. When the concrete contains aggregate larger than the test allows, the sample is wet-sieved: passed over the specified sieve while fresh, discarding the coarse particles and the mortar sticking to them, and remixing what passes. The test standard says which sieve. Wet-sieving changes the material: the sieved concrete has more paste and air than the full mix, so the results are compared only with criteria written for wet-sieved concrete (Taylor et al., 2019¹).

Time Limits

Once the composite sample is obtained:

• start the temperature, slump and air tests within 5 minutes; and
• start molding strength specimens within 15 minutes.

Complete each test as quickly as its procedure allows. If the sample sits, it loses workability and its temperature changes, and the results no longer describe the concrete being placed (Taylor et al., 2019¹).

Common Mistakes

• Sampling the first concrete out of the chute because it's convenient. It is often wetter or drier than the rest.
• Taking only one portion. One scoop is not a composite.
• Letting the sample sit in the sun or wind while the paperwork is done.
• Remixing too little, so that the slump and the cylinders come from different-looking concrete.
• Sampling before site-added water is mixed in, which tests concrete that never reaches the forms.

Standard Test Methods

• AASHTO R 60, ASTM C172: Sampling freshly mixed concrete

@misc{pavint-sampling-freshly-mixed-concrete,
  author = {{Pavement Interactive}},
  title = {Sampling Freshly Mixed Concrete},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/testing/cement-tests/sampling-freshly-mixed-concrete/}},
  note = {Accessed [date]}
}