Moisture issues in concrete floors are one of the main reasons why epoxy flooring fails. Moisture in the concrete occurs in a newly poured concrete or when there are high levels of moisture underneath the concrete slab due to poor drainage, naturally damp land, or a pipe leak.
Areas such as Victoria, Tasmania, and Queensland where there are higher levels of rainfall tend to get more moisture issues than drier areas such as Western Australia and the Northern Territories.
As concrete is porous it is important to know how much moisture your slab transmits, before you decide to apply epoxy or paint. As epoxy does not breathe, any moisture trapped between the concrete and the epoxy can potentially delaminate or peel the epoxy when trying to evaporate.
Your concrete might already show signs of moisture issues, without you realizing it. If you have areas on your garage floor where there is fine white powdery residue, which comes back after cleaning, chances are there is efflorescence. Efflorescence happens when moisture travels through the concrete, condenses, and then evaporates, leaving a residue of white powdery calcium.
Another sign of moisture issues is if your floor and garage always feel damp. This can be caused by poor water drainage underneath the slab, leaky pipe, or naturally damp soil due to climate. Concrete might seem hard and dense, but in reality is porous, acting like a sponge when there is water around.
If you have dark spots on your concrete, which feel damp to touch, chances are your concrete is absorbing water from underneath.
The simple thing to do when you are doubting about moisture issues is to perform a moisture test.
This consists of an A4 plastic sheet and some tape. Place the plastic sheet on the concrete floor and tape all four sides to the floor. Let the sheet stay on the floor for 24 hours. Then peel it off and look for signs of condensation underneath the plastic sheet or dark spots on the floor. If there is condensation or dark spots then it's highly likely you have moisture issues.
If you do and you are curious how much moisture your concrete emits, you can use a moisture measuring device or do a simple calcium chloride test to find the exact amount of moisture.
If you have moisture levels below on average 1.3kg per 90 m2, then you are good to apply most coatings on top of your garage. If the moisture levels are above these you'll need to consider applying a moisture blocking sealer or moisture blocking epoxy primer.
One important thing to consider is that moisture tests are only indicative at the time of performing them. If you do the test during the dry period of the year, you might not have any moisture issues, compared to if you do the test during a rainy period.
The same applies if your concrete slab is above grade or below grade. Slabs that are below ground level experience more moisture issues than slabs that are above ground level.
The important thing to remember is that if you are doubting your garage is experience moisture, is best to perform a moisture test, before applying any kind of coating. This will save you a lot of time and money in the long run.