Sheet vinyl lifting off concrete substrates; wood and laminate flooring turning up at the edges, carpet tiles emitting foul odours from reactions with ‘wet’ slabs. All of these failures in floor coverings are regular occurrences around the country, and in almost every case it is due to moisture in the concrete substrate.
Moisture-related problems in flooring are more prevalent today than they used to be, and the most common reasons for this include: fast track construction resulting in reduced slab drying time prior to flooring installation; imperfect construction specifications for on-grade slabs; and an increased use of lightweight concrete on above-grade slabs.
So what has changed? Adhesives have changed from when solvent carriers were used but they are actually better now. Flooring products have changed with the introduction of non-permeable backings which can trap moisture emissions coming from concrete.
Concrete hasn’t changed much but it is finished faster and harder and this limits the moisture’s ability to escape. Most importantly, construction jobs are fast-tracked meaning the job is rushed to completion without enough consideration for the ramifications of this quick action.
Moisture in the concrete, missing or improperly installed vapour barrier/retarders under the slab and cycling HVAC systems to save energy are the three biggest contributors to the problem.
When changes in the internal environment of a building are caused by cycling the HVAC system, and moisture of convenience (water used to hydrate the concrete slab) tries to equalise, moisture will move from high pressure to low. When this happens anything in the way, like flooring, will be affected.
If the flooring material is non-permeable, meaning moisture vapour emissions are trapped and unable to escape, they will eventually push the material off the floor. Sheet vinyl will trap the moisture, wood will absorb the moisture; and moisture and alkalinity can affect the PVC in carpet backing and create an odour.
Failing to place vapour barrier/retarders directly beneath the concrete will allow moisture from beneath to move upward into and through the slab, affecting the flooring material installed on top of it.
Fast-tracking installation often means concrete is not allowed to cure for the appropriate length of time, and installing non-permeable flooring material on top of it is a recipe for disaster.
Installing flooring before the substrate is in the proper condition to receive it allows the laws of physics to work against you. This condition is a regular challenge to flooring installations and is a major source of claims and losses for the industry.
Sources of Moisture
The initial source of moisture in concrete is the mixing water that is used at the time of manufacture. Once the concrete is placed, there are numerous other sources of moisture. These include wet curing, exposure to the weather, wet subgrades (in slab-on-ground construction), condensation (either within the concrete or on the surface), and application of mortar tile bedding and other water-based adhesives.
Wet curing is generally regarded as the most efficient method of curing concrete to ensure the hydration process continues and hence the design strength and other performance requirements of the concrete are achieved.
Wet curing may extend the time required for drying due to saturation of the concrete, rewetting of adjacent elements or creating wet environments (particularly for slab-on-ground work) that provide a source of moisture long after curing has ceased.
Where concrete is required to dry out in the least possible time, curing methods that do not introduce further water should be considered.
Exposure to the weather
If the drying period of concrete is critical then it should be protected from re-wetting. Rainfall on the concrete slab, infiltration into joints and wetting of the subgrade will extend the drying period. For floors, ideally the concrete should not be placed until the building has been enclosed.
The first step to successful concrete moisture testing is to establish good data regarding the conditions of the space. Testing should only be done when a building is fully enclosed with the HVAC units operational. If the building is not fully enclosed, testing can produce results that are inaccurate – results that unfortunately often indicate that it is acceptable to move ahead with the floorcovering installation when it is not.
Testing under the right conditions is a major hurdle for experts, who are often called in too soon and expected to come to a definitive conclusion on whether the floorcovering contractors can move ahead.
Acceptable moisture levels also vary based on what floorcovering is specified. Several of today’s more popular commercial floorcoverings are contributors to concrete moisture problems because of their dense, impermeable constructions.
Fibreglass-backed sheet vinyl is much less permeable than the felt-based vinyl, and modular carpet tile is denser (and therefore less permeable) than broadloom. The denser the construction, the more it is affected by moisture, Because of this, it is especially important that flooring contractors abide by each product’s specifications for installation.
The options for fixing a moist slab are fairly direct. Giving a concrete slab time to dry is the easiest solution to eliminating concrete moisture problems, but it is not always the most practical solution.
Since a slab cannot begin to dry until conditions are right, the process doesn’t begin until doors and windows are installed. Once these elements are in place, the key is to get air moving quickly and constantly, until the slab has naturally dried. Because this is simply not feasible from a time aspect in many situations, other options are utilised.
Some people have been known to use a sand blaster to make the floor porous and breathable, and then apply a topical epoxy seal to the floor. The seal will create a barrier so that water cannot escape through the surface of the slab. This is an effective and long-term solution, but it is also an expensive one.
Penetrant fluids applied to a concrete slab are designed to penetrate the surface and react chemically with the concrete, with the goal of reducing the moisture vapour emission rate. While these are cheaper than epoxies, they are also less effective.
Desiccant drying is another option for solving a moisture problem. In desiccant drying, the air in a space is circulated through absorbent materials, thereby eliminating moisture from the space. However, this process is also expensive and, if used too early, can cause the slab to crack and curl.
Dehumidifiers are sometimes used as well to increase the vapour emission out of concrete, but once again the results are slow.
Acknowledgement and thanks are given to the following for information contained in the compilation of this article: www.floorcoveringinstitute.com; www.floordaily.net.