- Q.What causes a concrete slab
to form a thin top coat that can flake off or, if something is
dropped on it, chip off in big pieces? Some customers want a
glasslike finish on their garage floors, but after screeding
and power troweling, the surface often becomes a thin, flaky
material that doesn’t adhere to the rest of the mix. Is
this caused by too much troweling?
A.Carl Hagstrom responds: The situation you
described is most likely caused by troweling the surface too
soon, not by troweling it too long. After the slab is
screeded and bull-floated, "bleed water" will find its way to
the surface. This bleed water must be allowed to evaporate
before troweling begins. Troweling the surface before the bleed
water evaporates causes the bleed water to be troweled back
into the concrete, significantly weakening the surface
When bleed water is present, you will be able to see
reflections on the surface of the slab. Begin troweling when
you can no longer see any standing water or reflections.
In cool, damp weather, bleed water can stubbornly refuse to
evaporate, and there have been instances where I "pulled off"
the standing water by dragging a hose across the surface (the
alternative was troweling the slab at midnight under the glare
of my truck’s headlights). This method doesn’t get
a five-star rating, but it is preferable to troweling bleed
water back into the slab.
Carl Hagstrom is an associate editor at the Journal
of Light Construction.