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What could have caused dark spots in a driveway?

Question:

What could have caused dark spots in a driveway?

Answer:

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)

Question and answer courtesy of Portland Cement Association (PCA). More information is available at: http://www.cement.org/tech/cct_faqs.asp.