How to Measure Moisture in a Concrete Floor

Concrete is the most widely used building material in the world. It is comprised of multiple size aggregates that are bonded together with a binder. Usually, the aggregate is a mix of crushed rock, granite, limestone, and sand. This is dependent on what is locally available. The binder holding all the aggregate and sand together that is commonly used is Portland cement. When mixing concrete, water is added to create a workable mix (fluid), and to promote the cement hydration process. After hardening, much of this water remains as moisture within the porous concrete slab. It is not uncommon this will account for 50% of the total water induced. Over the following weeks and months, this moisture is slowly emitted from the concrete until it eventually equalizes with the surrounding moisture levels.

There are many test standards and methods used to determine moisture content and the moisture condition within concrete slab. Each of the tests are suited for different tasks with varying levels of accuracy and efficiency.

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 repairs.

Why is Moisture in Concrete Measured?

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. Although the additional mix water added while producing the concrete has left the slab, the measured values can show a very wet slab due to the damp sub-base.

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. 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.

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 the moisture is too high it 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 surface meters (ASTM F2659) or in situ relative humidity probes inserted into the slab (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.

How is Concrete Moisture Measured?

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

Non-Destructive Concrete Moisture Meters

Non-destructive concrete moisture meters (electronic resistance meters) measure concrete moisture using the principle of electrical resistance 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. Resistance is measured with increasing concrete moisture causing an increase in the resistance 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, completely nondestructive. However, impedance can also be affected by changes in concrete composition or the presence of rebar within proximity to the surface. So, the test is less accurate than invasive (destructive) 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 tool to locate areas of relatively high moisture within the slab, very quickly and at a low cost.

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 thickness of the slab) and then using an electronic relative humidity probe 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 to have the area in and around the hole come to equilibrium.

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, Mil Spec 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 slab can be measured. The following description describes the difference between three of the most popular test methods, anhydrous calcium chloride moisture tests, in situ relative humidity (RH) probes, and electrical impedance (resistance) concrete moisture meters.

Anhydrous Calcium Chloride Moisture Test - 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 Probe - as described in ASTM F2170, measure at 40% depth of a concrete slab that is being dried on one side (top or slab on grade) or 20% depth of a concrete slab that is drying from two sides (top and bottom). If the slab is 6 inches thick and dried on one side. 40% of 6 inches is 2.4 inches. That is where your RH values will come from.

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

Interpreting Your Results

Non-Destructive Electrical Impedance Concrete Moisture Meter - When using an Electrical Impedance Concrete Moisture Meter on clean new concrete 4.5% MC is considered "dry" in most conditions. In drier conditions, 4% MC is considered "dry". In Desert conditions generally, 3.5% is considered "dry".

In Situ Relative Humidity Probe - Acceptable moisture levels in concrete according to ASTM standard, when testing via ASTM F2170, the RH level of a concrete slab needs to be at or below 75%, unless the flooring system manufacturer provides alternate guidelines. Some flooring system manufacturers have products that can handle 95% RH or more.

Anhydrous Calcium Chloride Moisture Test - The result is expressed as 'pounds' which is the equivalent weight of water, emitted as vapor, over 1,000 square feet in 24 hours. Use this result to decide whether a moisture vapor barrier is required or not below the flooring system. This test required the use of a gram-weight scale with a gradation of 1/10th (0.1) gram or access to a testing lab that can perform the calculations for you.

Dealing with high moisture levels

When installing systems that have minimal to no breathability and that has been determined there are elevated moisture levels, a moisture barrier or control agent needs to be used.

Moisture barriers dramatically slow down the movement of moisture vapor through the topical system. It is a film-forming system that when applied correctly, can be used under many flooring systems.

Moisture control systems, like DiamaPro Moisture Control, control the rate the vapor passes through the concrete surface minimizing the detrimental effect the vapor can have on a non-breathable coating system. It is a non-film-forming system that once it has fully penetrated and dry, will not alter the appearance of the substrate.

Conclusion

In concrete, moisture is observed in two forms, liquid, and vapor. Concrete is like a sponge; it can absorb a tremendous amount of water in a cured state. This water can come from one or multiple sources. When concrete is produced, additional water is used to increase its workability and it takes time before it leaves the slab. Concrete placed on grade (on the ground) can absorb moisture from the ground. This can be a short-term event or continue for many years. Water can also enter the concrete from the top surface.

Many flooring systems have a specific level of moisture that they can tolerate before a failure appears. By understanding which moisture test method the manufacturer approves and the moisture range that is acceptable for the system, you will be on your way to installing a successful project.