Concrete Moisture Measurement

Why Concrete Moisture Measurement is Important

Before applying a protective coating or other flooring materials to a concrete slab, it is critical to ensure that the moisture level inside the concrete has decreased to acceptable levels—typically ≤75% Relative Humidity (RH), 3 lbs/1000 sq. ft./24 hours MVER, or ≤5% Moisture Content (MC).

If not, moisture can accumulate below the coating or flooring causing discoloration, cupping, buckling, blistering, adhesive failure, and/or mold growth. There are various methods used for concrete moisture measurement. The two most-common methods utilize non-destructive electronic impedance meters (ASTM F2659) or in situ relative humidity probes (ASTM F2170).

Proper inspection of the moisture content in concrete before installing a flooring system is a necessary part in quality control and assurance—lowering the chances of a flooring system failure.

For more information about concrete moisture measurement, see below:

Product photo of the PosiTest CMM Concrete Moisture Meter

Non-Destructive Concrete Moisture Meter

PosiTest CMM

Featuring near-instant measurement and no requirement to drill into the concrete, the PosiTest CMM Concrete Moisture Meter is ideal for inspectors looking to quickly evaluate the moisture conditions in concrete slabs.

Conforms to ASTM F2659.

Product photo of the PosiTector CMM IS relative humidity probe

In Situ Concrete Moisture (RH) Meter

PosiTector CMM IS

PosiTector CMM IS (concrete moisture meter in-situ) probes and free mobile app guide users through the ASTM F2170 documentation criteria. View and record readings wirelessly with your Apple or Android smart device, or PosiTector DPM Advanced gage.

Conforms to ASTM F2170.

Principles of Operation of Concrete Moisture Meters

There are many test standards and methods used to determine moisture content and the moisture condition within concrete slabs and screeds—each suited for different tasks with varying levels of accuracy and efficiency. Moisture meters such as the PosiTest CMM Concrete Moisture Meter and in situ probes like the PosiTector CMM IS In-situ Concrete Moisture Meter determine both moisture content and conditions—prerequisites for installing flooring systems or coatings.

Why is Moisture in Concrete Measured?

Concrete is a composite building material comprised of an aggregate that bonds with a binding agent. Usually, the aggregate is a mix of crushed rock, granite, limestone, and sand. The binding agent is most commonly Portland cement. When mixing concrete, water must be added to create a workable mix, and to allow the cement to hydrate and cure. After hardening, much of this water remains as moisture within the porous concrete slab. Over the following weeks and months, this moisture is slowly emitted from the concrete until it eventually equalizes with the surrounding moisture levels.  

If the moisture within the concrete is not allowed to equalize with surrounding conditions before a coating or flooring product is applied, it can accumulate below the flooring or coating, causing discoloration, cupping, buckling, blistering, adhesive failure, and mold growth—leading to potential costly reworks.

Moisture issues are not isolated to new concrete slabs—a missing or inadequate vapor barrier below the slab can allow moisture from wet or saturated soils to travel through the concrete.

Concrete moisture content, %RH (relative humidity), and MVER (moisture vapor emission rate), are all measurements that can help indicate the moisture condition within a concrete slab or screed. Often, the goal is to determine if the slab is ready for a flooring system to be installed.

If readings are not taken—or not taken accurately—before a flooring system is installed, a critical flooring system failure is possible.

Photos of water damaged hardwood and concrete floors
Water-damaged hardwood and coated concrete floors

ASTM F2170 and ASTM F2659, along with other standards, were created to determine the likelihood of issues resulting from excessive moisture levels in concrete.

How is Concrete Moisture Measured?

There are various methods used for concrete moisture measurement. The two most-common methods utilize non-destructive electronic impedance meters and in situ relative humidity (RH) probes.

Non-Destructive Concrete Moisture Meters

Non-destructive concrete moisture meters (or electronic impedance meters)—such as the PosiTest CMM—measure concrete moisture using the principle of electrical impedance of concrete, which is related to concrete moisture. An alternating electric field flows from the device’s transmitting sensor pins through the substrate and is captured by the receiving electrodes. The impedance is measured, with increasing concrete moisture causing an increase in the impedance reading. Using concrete standards with known moisture levels, the meter can be calibrated to read in units of concrete moisture.

Concrete moisture meters provide an instant result, without the need to drill into the concrete. However, impedance can also be affected by changes in concrete composition or the presence of rebar within close proximity to the surface, so the test is less accurate than invasive options. These meters measure the moisture near the surface of the concrete, so high moisture levels near the center or bottom of the slab may go undetected. ASTM F2659 recommends that non-destructive concrete moisture meters are ideal for use as a comparator to locate areas of relatively high moisture within the slab.

Watch the following video or read our article, "How to Non-Destructively Test the Moisture Content of Concrete Slabs", for an overview of measuring concrete moisture content in accordance with ASTM F2659 using the PosiTest CMM.

In Situ Concrete Relative Humidity Probes

In situ concrete relative humidity probes, as described in the ASTM F2170 standard test method, are highly accurate for measuring moisture in concrete. This procedure involves drilling a hole into the concrete slab (typically 40% of the total depth) and then using an electronic relative humidity probe—such as the PosiTector CMM IS—to measure the moisture within the slab.

Since they directly measure moisture/humidity levels near the center of the slab, they provide an accurate quantitative measurement that can be used to determine when a slab is ready for the application of coatings or flooring products. Drilling of a ¾” wide hole within the concrete is required, in addition to waiting 24 hours.

Watch the following video for an overview of measuring concrete moisture content in accordance with ASTM F2170 using the PosiTector CMM IS.

Other methods for Measuring Concrete Moisture

There are other less-popular or older methods for determining the moisture content within a concrete slab, such as the calcium chloride method (ASTM F1869), plastic sheet method (ASTM D4263) or the calcium carbide method (ASTM D4944, MilSpec CRD-C154-77).

Anhydrous Calcium Chloride Moisture Tests

Anhydrous Calcium Chloride Moisture Tests involve measuring the increase in weight of calcium chloride as it collects moisture from an encapsulated area on the concrete surface. A greater increase in weight indicates greater levels of moisture in the concrete.

Calcium Chloride Tests are inexpensive, and relatively simple to use. However, they measure moisture only at the surface of the concrete, and require a waiting period before results are available, typically 24 hours. There is no practical way to calibrate these kits, and they can be heavily influenced by ambient conditions.

Plastic Sheet Method

The plastic sheet method, which involves taping a plastic sheet to the surface of the concrete and observing for a buildup of moisture after a period of time, does not offer a quantitative result and is heavily influenced by ambient conditions. It is better suited for establishing if capillary moisture is currently present, indicating that the slab is not in equilibrium, i.e. not ready for most flooring systems. The plastic sheet method is not recommended to be used as a stand-alone determination.

Calcium Carbide Method

The calcium carbide method is an accurate means to determine moisture content, but it is a complex, destructive test and produces acetylene, which can be prohibitive for some jobsites. It is often performed in a laboratory using samples destructively collected from the jobsite.

What parts of a Concrete Slab are Measured with Different Moisture Meters and Test Methods?

Depending on the test method or instrument used, different depths of the concrete or screed slab are measured. The following graphic illustrates the difference between three of the most popular test methods—anhydrous calcium chloride moisture tests, in situ relative humidity probes, and electrical impedance concrete moisture meters.

Anhydrous Calcium Chloride Moisture Tests, as described by ASTM F1869, measure MVER (Moisture Vapor Emission Rate) from the surface of the concrete slab, usually the top ½–¾ inch of the sample being measured.

In Situ Relative Humidity Probes, such as the PosiTector CMM IS, as described in ASTM F2170, measure at 40% depth of a concrete slab that is being dried on one side (top) or 20% depth of a concrete slab that is drying from two sides (top and bottom). In the illustration below, the slab is representative of being 6 inches thick and being dried on one side. 40% of 6 inches is 2.4 inches.

Non-Destructive Electrical Impedance Concrete Moisture Meters, such as the PosiTest CMM, as described in ASTM F2659, generally measure moisture content down to ¾–1 inch from the top of the concrete slab.

Graphic illustrates the most popular test methods used to measure moisture in concrete and which part of a concrete slab they measure
Illustration depicting the general depth in a concrete slab that anhydrous calcium chloride tests, in situ relative humidity probes, and non-destructive electrical impedance meters measure the moisture condition to

What Standards are Used to Determine the Moisture Conditions and Moisture Content within a Concrete or Screed Slab?

The following standards are used to determine the moisture content and moisture condition within a concrete or screed slab:

ASTM F2659—Standard Guide for Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter

Related product: PosiTest CMM

The outlines and procedures within ASTM F2659 guide the user through conducting relativistic moisture tests with an electrical impedance meter. It defines proper ambient conditions and preparatory tasks to be completed before performing the measurements.*†

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ASTM F2170—Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes

Related product: PosiTest CMM IS

In situ concrete moisture (RH) probes—such as the PosiTector CMM IS—described in ASTM F2170 are designed to determine % RH (relative humidity) within a concrete slab. With the limitation of anhydrous calcium chloride tests and impedance meters only measuring the moisture condition within the top 1 inch of a concrete slab, a full understanding of absolute moisture content has been impossible with those methods.

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ASTM F1869—Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride

The Anhydrous Calcium Chloride method seeks to determine the rate of emissivity of water from concrete slabs—an indication of capillary action or “movement of water”. Once MVER (moisture vapor emission rate) is measured per ASTM F1869, determination to install a flooring system may take place.*†

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ASTM D4263—Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method

ASTM D4263 outlines the use of an 18 x 18 inch (45.7 x 45.7 cm) sheet of polyethylene plastic, stretched taught and taped (sealed) to the surface of a concrete slab. After 16 hours, the inspector removes the plastic and identifies if moisture is present on either the plastic sheet or the underlying concrete.*†

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MilSpec CRD-C154-77—Standard Test Method For Determination Of Moisture In Fine Aggregate For Concrete By Means Of A Calcium-Carbide Gas Pressure Meter

The MilSpec CRD-C154-77 standard guides the user through the calcium-carbide method of determining moisture content of a concrete or screed slab. It requires the use of a pressure chamber and gage to measure the reaction of combining calcium carbide and a sample of the concrete or screed slab.*†

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ASTM F710—Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring

ASTM F710 is not specifically a guide to concrete moisture testing, but a holistic practice used to establish suitability of a concrete slab to receive a resilient flooring system.

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