Moisture management within concrete is essential before installing floor coverings, but what method should be used, and where? 

Concrete slabs, and the final flooring materials are susceptible to moisture.  

Excess moisture in the concrete could cause latent problems such as condensation or failure of the adhesive under the floor covering. 

Defining moisture 

While relative humidity (RH) and moisture content may sound similar, it’s important to distinguish between the two: 

• RH refers to the moisture level in the air and is expressed as a percentage. 
• By drilling into the concrete and inserting RH probes you can get a good idea of the moisture in the concrete slab by using the RH measurement. 
• Moisture content generally refers to the total amount of water in the concrete itself, often calculated by weight. 

Non-destructive and in-situ RH moisture testing, each have their place in managing moisture in concrete.

Oven Dried Method 

The actual moisture content is obtained using the oven dry method as described in BS and ASTM Methods, where a sample of the material is weighed, dried and weighed again. 

The moisture content is then show as a % of Dry Weight i.e. 

Wet Weight – Dry Weight x 100 = Percentage Moisture Content 

———————– 

Dry Weight 

Most hand held moisture meters try and approximate this oven dried method. Because the actual moisture range of many materials is very tight the WME or wood moisture equivalent reading is often used. Ultimately all materials are trying to get into equilibrium. The WME value is the moisture level that would be achieved by a piece of wood attached to and in moisture equilibrium with the material that is being tested. 

Measuring for moisture 

Tests conducted in the 1990s at the Technical University of Lund in Sweden confirmed that moisture moves from deep within a concrete slab to the surface at a gradient. In other words, moisture in a concrete slab isn’t always the same throughout. 

Elevated moisture levels from older concrete slabs could indicate that the vapour barrier is missing, or damaged, plumbing lines could be broken or leaking, moisture could be entering through an exterior wall, or due to hydrostatic pressure.  

Before installing floor covering, concrete should be tested for moisture immediately before and during the flooring installation process, to avoid any dew point related moisture issues.  

Moisture tests 

In-situ RH testing involves placing a probe into a hole/s drilled into the concrete slab. Image credit: Tramex/ BAMR

Accurate assessment of moisture related issues within a concrete floor slab can require multiple testing methods. The cause of the problem often determines the mitigation process.  

There are international standards for testing concrete, which include one or more of the following test methods: 

• Non-destructive impedance concrete moisture test (ASTM F2659) 
• In-situ Relative Humidity testing of concrete (ASTM F2170) 

Non-destructive testing 

A non-destructive impedance test determines the moisture content of the concrete floor or slab without damaging the surface. 

The ASTM F2659 standard specifies how to perform preliminary evaluation of comparative moisture condition of concrete, gypsum cement and other floor slabs and screeds using a non-destructive electronic moisture meter.  

A low frequency electronic signal is transmitted into the material under test, via the electrodes in the base of the instrument. The strength of this signal varies in proportion to the amount of moisture present in the material.  

The moisture meter, whether a pin or pinless option, determines the strength of the current and converts this to a moisture content value for concrete slabs, and a comparative value for other cementitious floor screeds. 

It is important to note that only a quantitative concrete moisture meter can measure the actual moisture content (%MC) as opposed to a reference meter which only provides a comparative indication. There are moisture meters which include a Hygrometer and optional external pin and RH probe functionalities. 

Non-destructive testing with a moisture meter uses Gravimetric testing as a baseline. Image credit: Tramex/ BAMR

In-situ RH testing 

The in-situ RH test for concrete measures the amount of moisture vapor present in a concrete slab. The RH percentage value provides an indication of the moisture content within the concrete. 

As per ASTM F2170, the in-situ RH test requires a hole/s to be drilled into the concrete slab. Before drilling, it is necessary to locate the best spot on the slab to perform the test. 

This is typically determined by the results of the non-destructive impedance tests, which indicate moisture content values in different areas of the slab. Generally, areas with the highest moisture content value are deemed suitable for conducting the in-situ RH test, also known as the concrete humidity test. 

A low frequency electronic signal is transmitted into the material by the moisture meter’s electrodes. Image credit: BAMR

Like most other materials comprising a building, concrete isn’t immune to moisture issues. Whichever method is employed, an accurate diagnosis of moisture in concrete slabs serves the same purpose: stopping moisture from causing irreparable and expensive damage. 

Issue: Making informed decisions about moisture control in concrete 

Solution: Using the right tools and methods to measure moisture and relative humidity is essential for the longevity and durability of floors. 

Full acknowledgment and thanks go to https://www.bamr.co.za/, https://tramexmeters.com/ and https://www.protimeter.com/ for the information in this article.