How to Measure the Moisture Content of Concrete


You will almost always have to measure the moisture content of concrete before you can install any kind of flooring or impermeable material onto a slab. There are a number of standard methods to measuring concrete moisture content and each has advantages and disadvantages. Using only one of the tests will not always provide you with all of the information you need when completing a construction job, according to Concrete Construction Magazine. For specific needs you want to choose a specific combination of measurement methods.

  • Use a calcium chloride moisture test. This type of moisture test, "quantifies the volume of water vapor radiating from a concrete slab surface over time," according to Vaprecision. Place the calcium chloride tests so that you have three test spots for every 1,000 square feet of floor. This test will give you a result in pounds, equivalent to the weight of water emitted as vapor over 24 hours across 1,000 square feet.

  • Measure the relative humidity within the concrete slab. You will need to drill a hole into the slab, then place a relative humidity meter into the hole you've drilled. This method can give you the relative humidity as a function of slab depth. Additionally, because you are measuring the moisture content from within the slab, these types of meters are "less sensitive to short-term fluctuations in ambient air humidity and temperature above the slab," according to the Portland Cement Association.

  • Determine the moisture content of the concrete using the plastic sheet method. Tightly tape a plastic sheet onto the concrete slab. Moisture will evaporate from the slab and be trapped under the sheet. Wait 72 hours, then take a humidity reading using a dew point hygrometer.

Tips & Warnings

  • Use more than one measurement method to get an idea of the moisture content both at the surface and in the interior of the slab.
  • Always measure the moisture content of concrete under conditions that are close to those that the slab will be expected to perform under.

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