Sampling Freshly Mixed Concrete
Why sampling matters
Sampling is the first point at which a test result can be invalidated. The sample must represent the entire batch, not the easiest concrete to reach. Take a composite sample from the middle portion of the discharge, using at least three increments taken across the discharge. Do not take the first or last material from a truck or chute. The minimum sample size is about 1 ft³ (28 L); for strength cylinders plus slump, air, and temperature tests, 1.5 ft³ is safer. Obtain the sample within 15 minutes of discharge and begin tests promptly.
Handling and remixing
Collect the sample in a clean, damp container and protect it from evaporation, rain, and contamination. Before testing, remix the sample with a scoop or shovel so that it is uniform; do not use a trowel to stir only the surface. Complete slump, air, and temperature tests within 15 minutes of sampling. If the concrete has begun to stiffen, do not add water to make it workable.
Traps and rule of thumb
Common traps include sampling only from the top of a wheelbarrow, taking a single scoop, sampling from the very beginning of discharge, and letting the sample sit while paperwork is completed. Another trap is testing a sample that has been remixed with water or that has dried at the edges.
Rule of thumb: one 6 in. by 12 in. cylinder contains about 0.196 ft³ of concrete, so three cylinders require about 0.59 ft³. With slump, air, and temperature, allow at least 1 ft³ and preferably 1.5 ft³. If the sample is short, discard it and resample; do not stretch it.