Efflorescence part 3: Example of repairing a concrete floor

Consider the following scenario: A new urban condo has acid-stained concrete floors finished with an acrylic sealer. The unit downstairs is unoccupied and unheated. The owner notices efflorescence starting soon after move-in and progressively worsening over several months.

We use the example of a floor because concrete floors are generally more susceptible to efflorescence than concrete countertops, and because this actually happened in my condo building in downtown Raleigh, NC.

Efflorescence is occurring because the moisture in the slab from construction and from acid staining has been locked under the acrylic. Water vapor leaving the slab is drawing soluble salts to the warmer side of the concrete.

Fixing this floor starts with stripping off the sealer and then physically removing the efflorescence. Common means are scrubbing or washing with a dilute acid solution. However, you can’t just mop dilute acid (with a lot of water) all over the place. Doing so would pump more water into the concrete. The best solution is to use an automatic scrubber that washes, scrubs and vacuums in one step. This minimizes the amount of water that penetrates into the concrete.

Then the concrete must be allowed to dry thoroughly before lithium silicate densifiers are applied. Commercial dehumidifiers can speed drying.

The final step to finishing the floor depends upon the floor’s water vapor transmission rate, the aesthetics and the desired level of durability. The simplest solution is either to leave as is or to apply a renewable “wax” to the floor. Unlike acrylics, wax is unlikely to trap efflorescence. If it does, wax can easily be stripped and replaced. This process may not halt 100% of the efflorescence, but it will allow everyday cleaning to remove the slight residue that occurs. In areas such as under a bed that don’t get cleaned regularly, efflorescence may still occur.

For floors that require more protection (like restaurants), first conduct tests to determine the rate of vapor transfer. If the moisture levels are low enough, choose a vapor-pressure-resistant sealer based upon the manufacturer’s recommendations. Impermeable sealers that are able to resist the existing vapor pressure will not develop efflorescence because the water vapor cannot pass through the sealer.

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