Moisture is the single most common cause of epoxy floor failure in Calgary. A slab that looks dry on the surface can have significant moisture vapor moving up from below, pushing coating adhesion off the concrete until delamination begins. Here is where moisture comes from, how to test for it, and what to do if you find it.
Concrete does not dry completely just because you poured it years ago. The slab sits on soil, and that soil contains water. In Calgary, a significant portion of residential foundations are sitting on clay-based soils with poor drainage. Underneath and around those foundations is groundwater. Capillary action pulls water up from the soil into the concrete slab itself. Capillary pull is strongest in concrete over clay. The fine porosity of clay-based soil creates capillary gradients that effectively suck moisture up into the concrete from below. A slab poured in 1995 that has been sitting over clay soil and groundwater is not drying - it is in equilibrium with the moisture content of the soil beneath it. That moisture is moving through the concrete as vapor and as liquid water, steadily, even if there is no visible liquid on the slab surface. Seasonally, the problem gets worse. In Calgary, spring melt and fall rains increase groundwater levels. Slabs that are marginally okay in summer can become significantly wet in spring. Many failures happen in the months following spring thaw, when moisture levels surge beneath the slab. Chinooks add another variable. A warming event in mid-winter can soften the frost, allowing groundwater movement. A few days later when temperature drops again, that water refreezes and ice-lensing can occur in the soil, forcing more water against the slab.
Epoxy and polyaspartic topcoats are hydrophobic - they repel water. They are also essentially impermeable - water cannot move through the coating. But epoxy is not infinitely strong, and moisture vapor from below is relentless. As moisture vapor migrates up from the slab, it accumulates at the interface between the concrete and the epoxy coating. Water molecules are attracted to the concrete surface (they wet it) and repelled by the epoxy coating (which is water-resistant). This creates a pressure differential. Over weeks and months, that vapor pressure and the electrostatic attraction of water to the concrete surface push the epoxy coating away from the concrete. The adhesion bond that was strong the day after application begins to fail. The epoxy lifts slightly, creating a void space underneath. Water vapor and liquid water fill that void space. As the void grows, the epoxy delaminates: the coating separates from the concrete and can be peeled back by hand. Osmotic blistering is a related failure mode. If salts are dissolved in the groundwater moving up through the slab, those salts accumulate at the coating interface. When the topcoat is exposed to sunlight and heat, water is drawn toward the salt concentration (osmotic flow), creating a water-filled blister under the coating. The blister pushes the coating away from the slab, and once the coating is separated, adhesion fails rapidly.
There are two main methods for testing concrete moisture. The calcium chloride test is the more commonly used: it involves placing a test kit on the slab under a sealed dome. The calcium chloride absorbs moisture from the concrete over 24 or 48 hours, and the weight gain indicates the amount of moisture vapor being released from the slab. The result is expressed in pounds of water per 1000 square feet per 24 hours (lbs/1000 sf/24hr). For epoxy and polyaspartic coatings, most manufacturers recommend that moisture reading be below 3 lbs/1000 sf/24hr. Some recommend below 5 lbs/1000 sf/24hr. If the reading is higher than that, the risk of failure is significant. The relative humidity probe method uses a probe inserted into the concrete through a drilled hole. The probe measures the equilibrium relative humidity of the concrete at depth. If the RH is above 75-80%, the slab is too wet to coat reliably. Both tests take time (24 to 48 hours for the calcium chloride test, sometimes hours for the RH probe to equilibrate). Neither can be rushed if the results are to be meaningful. The correct approach is to test before committing to a coating project. If the moisture is high, you have options. You can install a sub-slab depressurization system (radon system) to lower the moisture vapor drive. You can install a moisture vapor barrier on top of the slab before coating. Or you can choose a different use for the slab and accept that coating is not going to work.
If a slab tests high for moisture but you want to coat it anyway, a moisture vapor barrier can be applied over the slab before the epoxy primer goes down. These barriers are typically liquid-applied membranes (polyurethane or acrylic-based) or sheet materials (membrane tape). The goal is to block the vapor drive from below. Barriers are not a cure. They reduce the moisture reaching the epoxy interface, but they do not eliminate it. A barrier system is effective at reducing moisture from 10 lbs/1000 sf/24hr to something in the 3-5 range, which may be acceptable depending on the coating and the acceptable risk level. They are less effective if the moisture is coming from the sides or edges of the slab rather than from below. Alternatively, sub-slab depressurization (also called radon mitigation) uses a vent and fan to actively pull air (and moisture vapor) from below the slab and exhaust it outside. This approach is more aggressive and more reliable for slabs with very high moisture. It also adds cost and requires dedicated space for the ventilation system. The most practical approach for a Calgary residential floor is to test early, understand the moisture situation, and then either proceed with appropriate barriers or decline the project if the moisture is unmanageable.
If your floor is already coated and you start seeing problems, here is what to look for: soft or spongy areas under foot, especially in the center or perimeter of the slab, are a sign of delamination. A topcoat that feels slightly tacky or does not have uniform gloss, or areas where the coating can be lifted with a plastic scraper, indicates loss of adhesion from below. Visible blistering or puckering of the topcoat is osmotic blistering from moisture. If you catch these early, the affected area can sometimes be cut out and recoated. The key is determining whether the moisture problem is ongoing or whether it was a temporary condition (seasonal high water table) that has resolved. If the moisture is ongoing, a simple recoat will fail again as soon as moisture returns. The proper fix for an actively wet slab is moisture remediation first, repair second. That might mean installing sub-slab depressurization, installing a moisture barrier and then recoating, or accepting that the slab is wet and using the space for non-coated purposes (storage, parking). If the floor is failing and the slab is dry (moisture test comes back under 3 lbs/1000 sf/24hr), the failure was from something else - poor prep, incompatible products, physical damage, or improper installation. But if moisture is high, that is the root cause and fixing the symptoms (recoating) without fixing the moisture is waste of time and money.
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