Q.: I seem to remember reading that testing for slump was very outdated and that there is really no difference between a 5-inch slump and 6 1/2-inch slump. Is this true?
A.: While there has been a lot of valid criticism of the slump test in recent years, the bottom line is that the slump test is good for comparing two samples of what you believe to be the same concrete.
For example, if you get two batches of what is supposed to be the same mix and one has a significantly different slump, then you know something is different. It could be the mix proportions or the temperature or the time since it was mixed. But even with two identical mixes, the tolerance on slump is typically plus or minus 1 1/2 inches (for slumps greater than 4 inches), so a 5-inch slump is not considered to be significantly different from a 6 1/2-inch slump.
The slump test is a good indicator of fresh concrete's movement under its own weight. When testing different mixes, though, slump is often not adequate. You could have six different mixes with exactly the same slump, and all will behave very differently in terms of workability and the ability to place. The change in slump over time is also likely to vary significantly. This is compounded by the addition of admixtures, which can have a big effect on slump.
So what's better? The answer, according to Richard Szechy in a January 2001 article in The Concrete Producer, is rheology. "Rheology is the study and science of matter flow," he states. Measurement based on rheology may be an improvement over the slump test because the results are based on fundamental physical properties, have numerically similar values (indicating similar properties), and are reproducible. Unfortunately there is no simple way to measure rheology in the field it tends to be a subjective evaluation of the concrete's workability based on its yield strength and plastic viscosity. Research is in progress, though, so rheology may be a word to know in the near future.