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