Using the contractor's own personnel to make concrete cylinders to be delivered to a laboratory for testing has advantages: It reduces the testing costs and ensures that someone will always be available to make test cylinders when needed. However, unless proper methods are used in making cylinders, protecting them during the first day, and promptly delivering them to the laboratory, erroneously low strengths may be measured. These can result in construction delays and additional testing costs, and can create unnecessary tension among all involved.
Most concrete cylinders made in the field are consolidated by rodding. The rod should be a standard 5/8-inch diameter with a hemispherical tip. It should not be a piece of rebar. The concrete is rodded in three layers of approximately equal volume. When cardboard molds are used, the thin metal bottom should not be struck forcibly with the rod because this dents the bottom and results in bumps on the concrete surface that could cause capping problems. After each rodding, the sides of the mold should be lightly tapped to close voids left by the rod. This is particularly important for low slump mixes.
After consolidation, the top of the cylinder should be struck off with the tamping rod, a wood float or trowel to provide a reasonably smooth surface; the cylinders should then be placed on a rigid level surface. Failure to perform these post-consolidation procedures can result in bumps on the concrete surface or nonparallel ends, either of which can cause capping difficulties and a potential strength loss if thick caps are used to correct the situation.