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Concrete surface condition

by Robert Scantland
(Lac-du-Cerf, Quebec, Canada)

Q. A concrete slab for garage was completed in November and the garage built soon after. There is Thermofoil insulation barrier under the slab laid according the manufacturer instructions. I did not have time to seal the floor. The garage was heated during the winter month at an average temperature of 10C and used mainly as a wood workshop and to store a snowmobile. I intend to seal the floor in the next month when the outside temperature is consistently above 15C of so. I notice that when the floor is wet, the cement does not absorb the water evenly. Specifically, where wet the floor appears like a mosaic before it dries to a smooth finish. Note that the cement was polished when it was poured.

Is this condition a problem and will it affect the type of sealer I use?

A. It sounds like the floor is just fine. When concrete is machine finished it will have areas that are more porous than others which will absorb water much quicker. I say "machined finished" because I assume the floor was finished with a trowel machine. Polishing is a whole other process that's done long after the concrete has hardened and is usually used for commercial floors.

Sealing the floor shouldn't be a problem at all as long as the floor is clean and dry when it's applied. Typically for garage floors, solvent based acrylic sealers are ideal. Just roll on a couple thin coats and you're good to go.