Hotline Question:

How can we make normal-weight concrete floors dry faster in the office space of a commercial building? The flooring installer will put down tile only if the floor moisture content is below 3 %, but after 3 months of drying, he says the concrete moisture content is 12 %.

Hotline Answer:

First, have the installer or the testing firm check the calibration for the testing device used to measure moisture content. Assuming the concrete weighs 4000 pounds per cubic yard, moisture content of 12 % means theres 500 pounds of water per cubic yard still in the concrete (0.125 x 4,000 = 500 pounds). Since typical mixing water content for concrete used in floors is around 300 pounds per cubic yard, the 12 % sounds like an erroneous reading.

Second, find out how many readings are being made, and where theyre being made. Moisture meters are good tools for getting a feel for relative moisture content over a wide area by simply using the dial that reads zero to 100% and spot-checking points all over the floor. This can sometimes help to pinpoint trouble spots in a relatively short time.

Third, see if you can find a reason for, and possible way of correcting for, the wettest spots. For example, if most of the high-moisture-content readings are concentrated near the perimeter of the slab on grade, external water may be entering beneath the slab. This could be caused by a correctable situation such as poor surface drainage of rainwater.

Fourth, use the building HVAC system to either heat or cool the structure to its expected operating temperature under service conditions. Floor fans may help although some contractors have reported that they seem to be ineffective.

If all of these steps fail, the owner may need to take more costly steps. One option is desiccant drying, using a company that specializes in this service. Another option is using one of several proprietary floor-sealing systems that reduce water-vapor emission rates so floor coverings can be placed. We listed some of these in a previous Troubleshooting Newsletter. Use of desiccant drying or proprietary sealing systems can cost a dollar or more per square foot of floor.

The Contractor Hotline and Troubleshooting newsletters are just two of the benefits of membership in the American Society of Concrete Contractors.

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