Q.: On Monday, Tuesday and Wednesday we built 4-inch indoor slabs using 3500-psi concrete of 3-inch slump mixed in a transit mixer. The temperature was 90 degrees F and it was humid, with no breeze. Finishing included use of a 3-bladed power float, followed by a riding double trowel of 4 blades each. On Wednesday, while the riding finishing trowel was being used the top came off from several locations of 15 or 20 square feet each. The material that came off was a dry layer about 1/4 inch thick, and it was powdery underneath. This was not like the trouble known as blistering. What could have caused it?

A.: There are two likely causes and possibly both of them contributed to the trouble. The first is mixing. If the transit mixer blades were worn or if the concrete was not mixed long enough, there could have been some portions of the concrete that remained somewhat dry and never became sufficiently plastic to respond properly to the finishing operations.

It might have been better if a 4-bladed power float had been used. It is said that 3-bladed power floats work faster than 4-bladed, but that they do not make the floor quite as flat. When you followed the 3-bladed power float with a 4-bladed power trowel you may have been tending to cut into the higher spots. There is less likely to be trouble if a 3-bladed machine follows a 4-bladed than the other way around.

Trouble sometimes occurs when the same machine is used for both floating and troweling (not the case on your job) if the operator forgets to set the blades flat for floating. Another possible cause of this kind of trouble would be anything that causes nonuniform movement of water out of the slab, such as a nonuniform subgrade, or gaps in the vapor barrier.