Q.: Normal mixing and placing procedures were followed on some recent jobs but the concrete had a tendency to blacken on the surface. Could any variations in city water treatment have caused the problem?
A.: City water treatment is a most unlikely cause. A very common cause is known as "trowel burnishing," which sometimes occurs when flatwork is darkened (usually unevenly) by late, vigorous troweling. It seems to be caused partly by abrading metal from the trowel and partly by a reduction in water-cement ratio at the surface.
When the dark discoloration is uniform, it is perhaps caused by a low water-cement ratio in the mix.