The Perils of Moisture for Concrete Flooring 4

Monday, March 30th, 2015
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Moisture emission issues can cause huge problems for concrete flooring, and in worst case scenarios they can result in extremely costly damage for both new projects as well as renovations.

Despite ostensibly being amongst the most solid and hefty of all building materials, concrete is highly susceptible to moisture vapour problems due to its porous nature.

Concrete slabs are capable of absorbing moisture from the underlying ground, to an extent determined by local geographic conditions such as the depth of the water table and soil gradation.

This moisture is then liable to spread upward through the concrete before either condensing upon its upper surface, or being emitted in the form of a vapour.

Even in the absence of contact with moisture-laden soil, concrete can still suffer from vapour emission problems as a result of water it already contains, which was used during the construction process to either enhance its workability or achieve proper curing.

The emission of vapour or the accumulation of condensation on the surface of concrete floors can in turn cause a range of potentially costly problems, including the buckling and blistering of floor coverings, issues with adhesives, non-breathable coatings and epoxies, the proliferation of mildew and mold, as well as injury to building occupants incurred as a result of accidents on slippery surfaces and related litigation woes.

These problems have worsened over the past few decades for a range of reasons, including the use of absorptive aggregates in lightweight concrete, the absence of vapour retarders, and the removal of asbestos in resilient flooring.

Thankfully for builders, as well as building owners and occupants, there exist a range of methods for mitigating the absorption and emission of moisture by concrete floor slabs.

These methods each possess their own respective advantages and drawbacks, so they can be employed either individually or in tandem to remedy any moisture emission problems.

Image of below slab barrier

Image of below slab barrier

Below-slab barriers placed beneath the floor prior to the pouring of concrete can serve as a highly effective means of preventing the migration of moisture from soggy ground.

While they may not be porous like concrete, they are still susceptible to tears and ruptures as a result of contact with sharp objects or debris such as rocks, rebar or wiring, which can in turn impede their effectiveness.

Similar mitigation methods can be deployed on the opposite side of concrete flooring slabs in the form of a dispersive membrane or surface-applied moisture vapour barrier.

Dispersive membranes are placed on the top of the concrete slab, effectively serving as a barrier between ascending moisture and the upper floor surface.

This method is hampered, however, by its inability to serve as a solid substrate, which makes it a poor choice for floors that are likely to encounter significant amounts of heavy traffic. It also limits options for flooring finishes.

Dispersive membranes can also cause problems because they do not actually prevent moisture from rising through the slab, they only stop it from being emitted as a vapour through the floor.

This means moisture can actually accumulate and condense beneath the membrane, resulting in problems including the proliferation of bacteria, as well as increased pH levels that compromise floor covering adhesives.

Surface-applied moisture vapour barriers can also be applied to the top of concrete floor slabs to prevent moisture emissions from passing through. Compared to dispersive membranes, this solution offers the added advantage of protection from high pH levels that compromise adhesives.

The systems can be installed permanently within a matter of hours, upon both old as well as completely new concrete flooring.

In addition to surface barriers on either the top of bottom of concrete flooring slabs, dehumidification measures can also be employed to prevent the damage caused by moisture emission.

Dehumidification is perhaps the most convenient vapour mitigation system currently available on the market, and simply involves placing a commercial dehumidifying device on top of the concrete flooring slab to deal with existing moisture issues.

While it serves as a quick, easy fix solution at the outset of the construction process, the use of dehumidifiers has its limitations as a long-term or permanent remedy for moisture issues.

This method’s effectiveness can also be uncertain, given the difficulty of determining the amount of time it takes to remove sufficient amounts of moisture from the concrete prior to the installation of floor finishes.

Dehumidifiers are further limited by the inability to deploy them on new concrete slabs, as it can result in damage to the material in the form of cracking or curling due to rapid drying.

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  1. Tom Roberts

    Years ago, there was talk of self-healing concrete whereby the concrete itself produced bacteria which would fill any gaps which occurred because of moisture. It will be interesting to see whether or not advances in technology such as this have a material impact going forward.

  2. Crystal

    Thanks for this information.

  3. Peter Rhodes

    Hi my concrete slab has risen at least 2 inches up above my footpath what could cause this and is there a quick fix I don't want to rip up the slab and Start all over again the slab has been down for 2 years

    • Branko Mladichek

      If your slab has risen 2 inches then you are most likely experiencing slab heave from reactive clay foundation absorbing moisture, swelling and acting like hydraulic jack. Unfortunately unlike a jack it cannot be let down and the damage may be permanent. I have seen slab heave in its various forms over a hundred times in areas of Melbourne and there is over a dozen of possible causes and often multiple causes. You should get very experienced building consultant or a structural engineer to inspect your home for diagnosis.