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Are you wondering how to measure moisture in concrete? Does it make sense for you to use a concrete moisture meter?

It may surprise you, but a concrete moisture meter is not the preferred tool for accurately and reliably assessing the overall moisture condition of a concrete slab. But that doesn’t mean you shouldn’t use one—especially because meters can provide useful moisture readings when used properly.

concrete moisture meter

Always keep in mind that a concrete moisture meter should not be used for assessing the overall moisture condition of a concrete slab.

It does mean, however, that you should know the limitations of your concrete moisture meter. And if you’re looking for a scientifically proven method for evaluating a concrete slab’s readiness for a floor installation, then you’ll want to turn to a different way of testing moisture.

Ready to know more about what a concrete moisture meter is and what it’s not? Or how and when to use one? Keep reading…

What is a Concrete Moisture Meter?

Let’s start by saying what a concrete meter moisture is NOT.

It is not the preferred tool for getting quantitative measurements of the moisture levels in a concrete slab. Many variables go into the preparation and pouring of concrete, so much so, that concrete cannot be defined by any consistent mixture or concentration of ingredients. This may affect the reliability of quantitative readings taken with a meter.

flooring installer

When preparing to install flooring over concrete, your decision should take into account the moisture gradient of the slab and not rely on surface measurements only.

Another confounding factor is that as a concrete slab dries, a moisture gradient forms within the slab. This means there’s more moisture deeper down than at or near the surface. Since an electromagnetic meter is engineered for surface measurements (measuring moisture only within the top inch or so of the slab), it cannot possibly account for the higher moisture levels deeper down in the slab.

Therefore, a concrete moisture meter is not the most suitable device for measuring concrete slab moisture quantitatively.

On the other hand, a concrete moisture meter is highly suitable as a non-destructive device for obtaining relative readings of the moisture in a slab. This makes it an excellent tool for identifying wet/dry areas.
You can quickly and easily scan the concrete’s surface with a concrete moisture meter to determine areas of the slab that are wetter relative to other drier areas of the slab.

How Do You Use a Concrete Moisture Meter?

Using a concrete moisture meter is quite easy. After turning the meter on, you simply place the meter’s electronic scanning pad onto the surface of the concrete. The meter emits an electromagnetic signal that penetrates the slab, typically down about ¾-inch. Taking measurements at multiple locations across the concrete surface allows one to create a “map” of sorts of the potentially problematic areas within the concrete slab.

In this way, a concrete moisture meter serves as a useful comparative device—allowing you to quickly compare the relative moisture levels of different locations in the slab and identify wet spots for further investigation and testing.

When Should You Use a Concrete Moisture Meter?

ASTM International has developed a standard, known as ASTM F2659, that stipulates that an electronic meter is to be used for “preliminary evaluation of the comparative moisture condition of the concrete, gypsum cement, and other floor slabs and screeds.”

The keywords to remember here are “preliminary evaluation” and “comparative.” A concrete moisture meter provides a preliminary indication of the areas or locations that likely warrant further attention when evaluating the moisture condition of a slab.

This means a moisture meter does not provide definitive answers about concrete moisture levels when making flooring installation decisions. For that purpose, scientific studies at Lund University in Sweden and elsewhere point to the in-situ relative humidity (RH) test.

Rapid RH

The use of in-situ RH probes will give you the most reliable information about when it’s safe to install your flooring.

This test utilizes RH sensors that are placed at a specific depth within the concrete. The depth-specific nature of RH testing is important for getting a reliable, accurate indication of the moisture condition of the slab. Just be sure to follow the specific guidelines spelled out in the ASTM F2170 standard for in-situ RH testing.

This brings us to the answer about when to use a concrete moisture meter. Because a meter provides comparative readings for identifying wet spots, it’s the perfect “targeting” device for quickly and strategically choosing locations in the slab for the placement of in-situ RH sensors.

Your Primary Take-Home Lesson

Always keep in mind a concrete moisture meter’s limitations when measuring concrete moisture. An electronic meter—even if it’s been specially engineered and calibrated for use with concrete—should never be used to make a final determination about when you can safely install a moisture-sensitive finished floor product over concrete.
That decision to install is best made by evaluating the concrete’s moisture condition using in-situ RH sensors.