A true control test is one which gives results soon enough for the concrete producer to take correction action if results indicate that the concrete is outside specified limits. This restricts control tests to those that are conducted while the concrete is still in the plastic state. As with any testing procedure, sampling is extremely important, and sampling methods described in ASTM C 172, "Standard Method of Sampling Fresh Concrete," should be followed. The tests most commonly conducted on plastic concrete are discussed in this article tests for slump, air content, unit weight and temperature.
FREQUENCY OF TESTING
Tests on plastic concrete should be performed frequently enough that timely mix adjustments can be made, especially for air content, but not so frequently that work is needlessly held up. Slump tests are required each time that strength test cylinders are molded. Specifications may require strength tests for every 100 or 150 cubic yards of concrete placed in any given day. When this is done, visual inspection of concrete consistency between routine tests is all that is necessary to detect major changes that show a need for correction, with the slump test being used to verify the change in consistency.
Air content variations are harder to detect visually, so more frequent testing may be needed for parking ramps, pavements and other exterior concrete exposed to severe weathering conditions. Unit weight tests for normal weight concrete may be needed only once or twice during an entire job to verify that the yield is correct. Temperature readings are so easily obtained that frequent testing does not pose a problem.