Concrete consistency vs. workability: what's the difference?
Consistency and workability are fundamental factors in producing and placing concrete. Although related, they are distinct concepts, and understanding the difference avoids mistakes that compromise the quality and durability of the structure.
What is concrete consistency?
Consistency measures the fluidity of concrete, that is, its ability to flow and take shape. It is a mandatory parameter when specifying concrete, measured by the slump test and standardized by ABNT NBR 16889:2020. That is why the consistency measurement is commonly called slump.
An incorrect slump compromises strength and durability. Adding water on site to soften the concrete up to the desired slump reduces strength, because there is not enough cement to compensate. If the truck arrives with a consistency different from the one requested, by standard it can be returned at no cost to the buyer.
What is concrete workability?
Workability is how easily concrete can be handled, placed and compacted. It depends on the characteristics of the job and on the transport and placement methods. The same slump can have great workability in a slab and be impossible to place in a densely reinforced column. In those cases, the solution is to adjust the mix (smaller aggregates, special admixtures), not to add water.
Conclusion
Consistency is a standardized measure of fluidity; workability is the ease of placement. Both should be assessed regularly during the job to ensure concrete quality, especially in very tall buildings, where placement gets harder floor by floor.
