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Concrete Floors – How To Recognize Moisture Issues

Majority of building nowadays are constructed with a concrete substrate poured over prepared soil, also known as slab-on-grade. When done improperly, floor moisture and vapour can cause many issues. These problems that can come about from either the concrete mix itself – where the slab is not completely cured and holding moisture or the soil under the slab-on-grade – where the vapour barrier is not functioning properly.


Floor covering manufacturers of various products like tiles, wood, carpet or other floor coatings publish the maximum allowed moisture content of the concrete where their flooring products can be installed. This is why floors need to be tested before installation begins. There are various ASTM test methods used for this.


Knowing that water is an essential component for any concrete mix, it is important to remember that newly poured concrete needs to be given enough time to cure and release a sufficient amount of moisture before proceeding with the building construction and floor coverings/coating system installation.


How to tell if Moisture is still present.


Concrete is porous by nature and can mislead by giving the false impression that it is dry enough, despite the high content of moisture. What this means is that moisture will evaporate near the surface first, while the moisture from below will slowly make its way through the path of least resistance. This process is called Moisture Vapor Transmission (MVT)

Moisture vapour can be measured in terms of relative humidity (RH) or the rate at which vapour moves through the concrete. Generally, flooring manufacturers suggest tests, methods and limits that should be followed for best results, and in the event of exceeded moisture limits, some manufacturers have effective MVT control solutions for use available with their flooring products.


What Problems Excessive Moisture Vapor Transmission can cause?


  • Cracking or bubbling of the floor covering or coating system
  • Development of uneven floor surfaces that could further bring about slippery or overall hazardous conditions
  • Reduced life span of the flooring products, this, in turn, can invalidate the floor covering warranty
  • Bonding failure, development of mould or other pathogens between the concrete and the floor covering
  • Eventual deterioration of the concrete substrate.


Repairing a concrete floor with moisture problems can be costly and the excess MVT can lead to frequent, unanticipated and expensive repairs.


How to recognize the early signs of concrete moisture issues.


Remember that regular monitoring is critical, as flooring can look fine but still be in the early stages of developing issues.


Look for damp spots on the floor or areas with darker discolouration.

Check for loose pieces, cracking on the flooring installation if you have tiles or other material that required some kind of adhesive for the installation.

Check for black stains on the floor or on the walls.


When should you test the concrete slab for moisture?


Before installing any kind of flooring it is mandatory to check the moisture of the concrete.

For new constructions, the slab should be given enough time to cure. If excess moisture is present it would need to be sorted before installing flooring cover as it can lead to delamination and may require a full re-installation, which in turn will bring more expenses.


For pre-existing floor covering – the floor cover would need to be removed as well as any mortar, grout or adhesive, to expose a clean section of the concrete. Only then you can properly test for moisture.


There are Three Tests available to check the moisture of concrete slabs


1 Concrete moisture test or ASTM D 4263

It is the simplest test you can try to determine moisture accumulation. Duct tape an 18-inch square piece of plastic onto exposed concrete and leave for 16 hours. If there is condensed moisture under the plastic after the 16 hours – that is an indicator there is a problem.


2 Calcium Chloride Test or ASTM F 1869

It is available as a kit and works by comparing the weight of the calcium chloride before and after the test time, which usually takes between 60 to 72 hours. This method will also show the quantity and rate of moisture vapour travelling up through the concrete slab.


3 Relative Humidity Testing or ASTM F2170

This is the most advanced and comprehensive method. It uses special probes embedded in the concrete and measures the presence and quantity of moisture throughout the depth of the slab