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 skin.
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.
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