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Solving problems caused by moisture
by Christina Camara
Your customer calls six months after the job is complete and complains about white stuff on the concrete (… and it’s not snowing).
Terry Tomlinson, technical director for White Mountain Products, had a few questions about the above scenario: Has a sealer been used? If so, was it water-based or solvent-based? And most important, why did the “white stuff” appear?
That “white stuff,” or efflorescence, shows up when moisture has moved through the slab, carrying minerals with it. A white residue is left behind when the water evaporates.
One way to gather more information about the problem is to make a $10 investment and purchase a calcium chloride test kit that will determine excess moisture or moisture vapor problems. The dry calcium chloride collects water if moisture is present. The amount of weight the calcium chloride gains indicates how much moisture is coming through, Tomlinson said.
If the job has not been sealed, Tomlinson suggested a simple cleaning with a broom, following up with an efflorescence remover if needed. Applying a solvent-based acrylic sealer will also make the efflorescence disappear.
“What’s happening is the salts dissolve in the sealer, and in the dissolved state they’re invisible,” he said. “When the sealer dries, there’s no change. The salts remain dissolved in the resin.” Keep in mind, though, that the high-gloss finish will give the project a whole new look.
If the job is already sealed with a solvent-based acrylic, Tomlinson suggests correcting the problem by reliquifying the sealer with a solvent, allowing it to repenetrate the slab, and then touching up the sealer with a light coat. If a water-based sealer was used, it must be removed chemically.
Of course, moisture can also be a problem in a newly installed slab. Bart Sacco, owner of Concrete Texturing Tools & Supply, said moisture can get trapped between the slab and the sealer membrane if the sealer is applied before the concrete has cured completely. High-gloss, solvent-based sealers can give a project a high-end look, but if they’re not breathable, moisture problems can result.
If concrete is poured on Tuesday and pressure washed, dried and sealed on Thursday, that’s not nearly enough time for all the moisture to come out of the slab, Sacco said. If the concrete has a white hazy look, it’s likely that moisture is fogging between the solvent-based sealer and the slab.
He suggested first allowing the concrete to naturally aspirate the moisture before taking any corrective measures. “Let it do its thing, and then come back in a week or two and give it a bath with a solvent, a liberal coating,” Sacco said. “I would use xylene. Spray it on and let it emulsify. It lets moisture out when the solvent burns off into the atmosphere. Then put another sealer on top of it.”
 Do a spot test first. If it dries crystal clear, go to work on the rest of the slab, he said.
In the case of chalky-looking efflorescence on a slab, Sacco agreed with Tomlinson, saying that it would be a good idea to strip the sealer, and then clean the surface with an efflorescence remover or TSP. Then allow it to dry thoroughly and reseal.
The most important thing to do is solve the moisture problem, not just cover up the evidence, Tomlinson said. “If you have poorly drained soil, then you have to correct the drainage problem.”
If the concrete was placed over damp soil, then nothing needs to be done except giving it time to dry out and evaporate through the slab. In some cases, he says, that can take as long as a year. “It’s a hard thing to tell people they have to be patient, but the reality is if they’re not, the problems just reoccur,” he said.
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