Q: I recently examined a one-month-old black-pigmented slab on grade that was uniformly light gray except for a few small dark gray areas. These dark areas were less than one square yard in size. I scrubbed the surface of the concrete with acid using a clean fiber brush. No change of shade or depth of color could be seen in the scrubbed spot. The pigment was carbon black. What might have caused the very low level of coloration? What suggestions do you have for prevention of future occurrences?