Question: A lot of the concrete we're now placing would be considered “low slump.” Years ago it would have meant trouble for the crew. But today, we're able to work with most of the low slump concrete we get. Is the slump test outdated?
Answer: The slump test isn't outdated, but concrete mixes have grown much more sophisticated and experienced producers know how to provide concrete that's suited to your particular purpose without relying on slump as an indicator. You could have six different mixes with the same slump and all will behave differently in terms of workability.
Something the slump test still does well, though, is to give you a check on the consistency of the concrete you're receiving. If loads are coming in at a 21/2- to 3-inch slump and you suddenly get one with a 6-inch slump, you can tell that something basic has changed in that load. But the standard test procedure detailed in ASTM C143 says that a variation of 1 to 11/2 inches can be expected in two slump tests performed on the same batch of concrete.
No longer can you make an educated guess at what a batch of concrete's strength would be based on its slump. One reason is the widespread use of admixtures, many of which affect concrete's rheology—the way it stands or flows—so the slump test now tells us just one thing about a batch of concrete: its slump.