Q.: In a driveway that I placed recently, there is a dime-sized hole about a quarter inch deep that doesn’t want to set up. It has been this way for several days now. I’ve been pouring concrete for 15 years and have never seen anything like this. What is it?
A.: Either at the ready-mix plant or on the job, probably something with sugar content found its way into the concrete. There are stories about finishers who accidentally spilled coffee or pop with sugar in it, fruit juices, or even dropped a piece of a candy bar onto the concrete surface while they were finishing. That’s all that’s needed to retard a small amount of concrete for a long period of time.
There may be other explanations for this, such as an embedded clump of fly ash that temporarily delayed the set. But from the photo and your description, it sounds like sugar.
The dark area around the center hole indicates gross retardation. The larger white area is probably the result of efflorescence caused by the continuous bleed of calcium hydroxide to the surface. You will probably have to remove all the retarded concrete and patch the area.