Q: I've read about false curl, which results when concrete settles in the middle of a strip pour, making the fresh concrete surface lower than the side forms. While floating and troweling the surface, finishers work material toward the edge, making it higher than the middle of the strip. To prevent false curl when power floating and troweling adjacent to a previously placed floor slab, why can't you allow a small portion of the blades to overlap the hardened concrete surface?
A.: We can think of at least three reasons for avoiding this practice:
If the fresh surface is too low, you could damage the float or trowel blades as they hit the edge of the hardened concrete.
Even if the surface isn't low, paste will be smeared on the hardened surface when the blades pass from the fresh concrete to the hardened concrete. These smears are very hard to remove.
The hardened surface is an elevation control for the newly placed slab. If the power trowel drags paste onto this surface, it also changes the elevation control, making the edge of the newly placed slab higher than intended.