Frequently Asked Questions
Cement & Concrete
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 Q:
What could have caused dark spots in a driveway?
Two large areas of our six-week old concrete driveway
are a darker gray color than the rest of the concrete. The discolored
areas are irregular in shape but roughly 3 to 4 ft (1 meter) in
diameter. There’s spiderweb cracking in
the dark areas but not in the rest of the concrete. What could have
caused this?
A: Discoloration of
this type can be caused by a number of factors that include nonuniform
distribution of calcium chloride in the concrete, nonuniform
hard steel troweling, or localized differences
in water-cement ratio.
In this case, the craze cracking provides an additional
clue. When concrete isn’t struck off and floated correctly, low
spots or birdbaths may be present. Puddles of bleedwater
collect in these low spots. Then inexperienced finishers sometimes
dust the bleedwater surface with dry cement
so they can finish the concrete faster. This has two possible effects:
• It may lower the water cement ratio, thus darkening the concrete.
• It may increase the shrinkage of the high-cement-content layer,
thus creating craze cracking.
This may have happened in your driveway. The only way to tell for
sure is to take a core and have it examined by a petrographer.
More causes of discoloration and possible remedies are discussed
in Concrete Slab
Surface Defects: Causes, Prevention, Repair (IS177)
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