Dear Angie: I'm wondering why my concrete garage slab ended up blotched and discolored. The contractor poured two slabs on a day with a high temperature of 53 degrees and a low of 48. The finisher used 2 percent calcium for the slab that ended up discolored. The other slab, with 1 percent calcium added, looks fine. — Paula B. Richland, Wash.
Answer: A slab can take several months to fully cure, and longer if temperatures are low, so blotchy areas might fade over time. But your spotty slab was poured more than five months ago, which points to calcium as a likely culprit.
Top-rated concrete pros and industry associations we consulted say that calcium chloride, especially if added in amounts that approach 2 percent of the cement weight, can cause blotchiness. They say the discoloration is probably just aesthetic, not a sign of weakness in the slab. That's fortunate, considering a new garage slab can cost $3,500 to $6,500.
Why might a contractor add calcium chloride to concrete? Optimally, concrete should be poured and finished in a temperature of 60 to 75 degrees, with high humidity and no rain. In less-than-ideal situations, concrete experts might add retardants to slow or accelerants to speed the curing process.
Calcium chloride is an accelerant, and using it made sense because your slabs were poured on a cool day. The inconsistency in the amounts used, however, might explain the varying results, or the additive might not have been uniformly mixed.
Experts say that perfectly uniform concrete is not always possible, but excessive discoloration has other possible causes in addition to those I've mentioned:
■ Inconsistent or improper amounts of water added to the concrete mix at the job site.
■ Poor or inconsistent work practices, including insufficient mixing time, improper timing of finishing operations and how the contractor deals with variables, such as a temperature change from morning to afternoon.
■ Covering a slab with polyethylene sheeting. Discoloring can result when not all portions of the plastic are in direct contact with concrete.
■ Putting water on concrete to slow curing, which is more likely to occur in hot weather.
■ Lack of sealing, use of poor-quality sealant or improper application.
■ A mixing problem at the concrete plant.
■ Hard-troweling of the surface.
Experts suggest several methods you can try to even out color in stabilized concrete, although there's no guarantee they'll work. Note that some methods involve ingredients you can get from a local chemical-supply house or by searching online. Use caution or consider hiring help:
■ Alternately flush the slab with hot water, scrub with a brush, then allow to dry overnight.
■ If that doesn't work, wash with a 1 percent concentration of muriatic acid or a 3 percent concentration of acetic or phosphoric acid. Try it on a small section first. Dampen the area before applying the acid wash; flush with clean water 15 minutes after application.
■ If other methods fail, wash with a solution of diammonium citrate (or ammonium citrate, dibasic), two pounds to a gallon of water. Apply to a dried surface for 15 minutes. A whitish gel will form; dilute with water and agitate by brushing. Then, wash away the gel. More than one treatment might be required.
If you can't remove the discoloration and don't want to live with it, consider covering it with a chemical stain or epoxy coating.
When hiring a concrete company, consider only contractors who are appropriately licensed, insured and bonded, and who have positive reviews on a trusted online site. Take time to contact references and consider looking at past work.
Angie Hicks compiles the best advice from the most highly rated service pros on Angieslist.com to answer your questions. Ask Angie your question at firstname.lastname@example.org.
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