How to Non-Destructively Test the Moisture Content of Concrete Slabs

Referenced products:

DeFelsko Corporation

Originally published: 11/23/2021

The PosiTest CMM Concrete Moisture Meter being held by a man in a yellow construction vest
The PosiTest CMM Concrete Moisture Meter

There are various methods used for testing the moisture content of concrete. The two most-common methods utilize non-destructive electronic impedance meters and in situ relative humidity (RH) probes. This article will discuss the former—electronic impedance meters; how they operate and how the DeFelsko PosiTest CMM Concrete Moisture Meter is the simple, durable, and accurate way to measure moisture content in concrete slabs in accordance with ASTM F2659.

What are Concrete Moisture Meters and How do They Work?

Non-destructive concrete moisture meters (or electronic impedance meters) 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 moisture content 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 testers provide an instant result, without the need to drill into the concrete (i.e. with in-situ RH probes). However, impedance can also be affected by changes in concrete composition or the presence of rebar within close proximity to the surface. These meters measure the moisture near the surface of the concrete—the top ¾” of the slab. Non-destructive concrete moisture meters are ideal for use as a comparator to locate areas of relatively high moisture within the slab. They pair well with in-situ RH probes, such as the PosiTector CMM IS.

Watch the PosiTest CMM overview video for more information.

How to Measure Concrete Moisture Content with the PosiTest CMM

The PosiTest CMM is the fast, non-destructive impedance meter ideal for inspectors looking to quickly evaluate the moisture condition in concrete slabs. Featuring a simple four-button interface, innovative measurement modes, and 3 measurement scales, taking a moisture content measurement is easy.

1. Power the unit on.

2. Select the preferred measurement scale—Concrete 0-6%, Carbide Measurement Equivalent 0-4%, or Relative 0-100%.

Image shows the 3 measurement scales of the PosiTest CMM

3. Select the preferred measurement mode:

• Continuous Measurement Mode is the default mode and continuously updates the display with the current measurement value.
• Hold Max Mode displays and holds the maximum reading until the memory button is pressed to save it, or the power button is pressed to discard it.
• The innovative Hold Max and Save Mode waits for the reading to stabilize, then automatically saves it into memory and resets the display without needing to press any buttons.*

*The Hold Max and Save Mode is ideal for use with the Telescopic Extender Accessory, which threads into the sturdy tripod mount built into the meter. Quickly measure and record concrete moisture levels over large areas without the need to bend down or kneel.

4. Verify operation: A Check Standard is included to verify the meter’s operation and accuracy. Simply place the check standard on a flat, non-conductive surface and press the meter down firmly until the pins are compressed. If the displayed reading is within the range written on the Check Standard, the instrument is operating correctly.

Photo of the PosiTest CMM and accompanying Check Standard, used to verify operation

5. Ensure the surface being measured, as well as the pins on the PosiTest CMM are free of dirt, dust, and other contaminants. The PosiTest CMM Concrete Moisture Meter’s pins must make direct contact with the concrete surface to take an accurate reading.

6. Place the moisture meter onto the concrete surface and press down to fully compress the sensor pins. The sensor pins are designed to measure accurately on smooth, rough, or uneven concrete surfaces. The moisture measurement reading will display until the gage is lifted from the concrete surface, depending on the Measurement Mode selected.

Image of the PosiTest CMM with and without the telescopic extender accessory taking measurements on concrete

How to Test a Concrete Slab for Moisture Content in Accordance with ASTM F2659

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 describes preparatory tasks to be completed before performing the measurements.

This section will summarize the general procedure on how to comply with 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”. Prior to measurement, the standard must be reviewed to ensure a comprehensive understanding and compliance.

Pre-test Preparation for ASTM F2659

  • The concrete floor slab is required to be at service temperature at least 48 hours prior to testing the moisture content. The airspace above the concrete slab must also be at service temperature and relative humidity should be at a level expected under normal use. If these conditions are not possible, tests should be conducted at 75°F ±10°F (24°C ±5°C) and RH% of 50 ±10%.*
  • No liquids, dirt, coatings, flooring, adhesives, or other contaminants shall be on the concrete surface on selected test sites.

    *See the PosiTest DPM Dew Point Meter for environmental monitoring.

How to Perform the ASTM F2659 Test Procedure with the PosiTest CMM Concrete Moisture Meter

Before beginning, be sure the PosiTest CMM is powered-on and set to the desired Measurement Mode to optimize for the job.

1. Identify Testing Locations — ASTM F2659 stipulates that there are at least 8 tests for the first 1000 ft2 (100m2) and at least 5 additional test locations for each additional 1000 ft2 (100m2).

It is recommend that the inspector test locations across the slab to best understand the moisture condition within. Look for areas suspected of having high moisture that are out of direct sun or other sources of heat. Include areas in the middle of the concrete slab as well as near each exterior wall.

Image depicts several requirements to identify test locations for the ASTM F2659 test method

2. Measure —

A. In accordance with the ASTM F2659 standard, record relative humidity, air temperature, and dew point above the test locations .

Record the concrete surface temperature within 8 in (200 mm) of each test location. PosiTector DPM Dew Point Meters are an accurate and reliable instrument for this requirement.

Photo of the PosiTector DPM taking ambient air readings above an ASTM F2659 test location
Use the PosiTector DPM to take the surface and ambient air readings per ASTM F2659

B. At each test location, use the concrete moisture meter to take 3–5 measurements within a 1 ft2 (929 cm2) area. Report the maximum reading at each location. Use the PosiTest CMM’s Hold Max or Hold Max and Save mode* to make this task quick and simple.

Photo collage of an inspector taking 5 measurements at a test location, per ASTM F2659
Using the Hold Max or Hold Max and Save mode, take the 3-5 required readings per test location

Note: chlorides, carbonates, rebar, and other materials may affect the accuracy of a concrete moisture meter’s readings. Be aware of when these or similar materials are used. Non-destructive devices are best used in conjunction with in-situ RH probes, such as the PosiTector CMM IS, to ensure best accuracy when determining the moisture condition of a concrete slab.

*Use the Hold Max or Hold Max and Save mode with the telescopic extender accessory to make measuring even faster, without the need to bend or kneel.

3. Report — When inspection is complete, use the built-in USB port and included cable to download, view, and print stored readings in professional PDF reports quickly and easily with PosiSoft Desktop.

ASTM F2659 requires the following data to be reported:

• Locations of the test areas on the concrete slab
• The highest moisture content reading at each test location
• The relative humidity, ambient air, and dew point temperatures above each test location
• Additional information pertaining to the location, inspector, as well as additional observations shall be recorded as well. Refer to ASTM F2659 for a complete description.

Image displays a concrete moisture PDF report created in PosiSoft Desktop
Example of a concrete moisture inspection report created in PosiSoft Desktop

If other DeFelsko instruments, such as the PosiTector CMM IS In Situ Concrete RH Probes, PosiTector DPM Dew Point Meter, or the PosiTector 200 Concrete Coating Thickness Gage are used on the same job, measurement data can easily be compiled into one report. Fully customized reports can also be created to match existing paper forms or layouts.


The new PosiTest CMM Concrete Moisture Meter is the Simple, Durable, and Accurate way to measure moisture content in concrete and screed slabs. With an easy-to-use interface, fast measurement speed, memory, and reporting solutions—all backed by DeFelsko’s renowned quality, customer service, and exceptional two-year warranty—the PosiTest CMM is the ideal choice for concrete moisture measurement in accordance with ASTM F2659.

Image thumbnail of the PosiTest CMM cutsheet
Click the image above to view a downloadable PDF
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