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Understanding Vapor Transmission Issues When Coating Concrete
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It would seem that painting a concrete slab or wall should be a simple task with minimal problems.
This is often so, but things change when the concrete is on damp ground, below the water table, or in direct contact
with moisture.
THE PROBLEM
Concrete is not the hard, dense, impermeable mass we tend to imagine it to be. It's a mineral/salt rich, porous
material full of tiny channels and voids. These pathways are just right for the migration of fluids and vapors.
Migration is due to simple physics - forces working to achieve temperature, humidity, concentration, or pressure
(hydrostatic) equilibrium.
But there is more to it than simple equilibrium induced fluid migration. Dissolved metal salts in the migrating
water can form relatively large calcium/potassium silicates (in a typical alkali-silica reaction) upon reaching
the concrete surface. This ‘swelling' can result in pressure build-ups of well over 1000 pounds. That's a lot of
pressure for any coating bond to resist.
Added to this situation is one more wrinkle. Nearly all coatings lose bond strength, or simply will not adhere
at all, when applied to a damp, wet, or saturated surface. How dry is your concrete surface? Chances are it contains
more moisture than the coating manufacturer would recommend for optimum adhesion.
VAPOR BARRIERS
The pre-construction solution is to use a vapor barrier under the concrete. This vapor barrier is nothing more
than a heavy plastic sheet. Unfortunately contractors or owners often discover that no vapor barrier was used or
that the vapor barrier has probably been torn or damaged, possibly when the concrete was poured. Older buildings
and garages often do not have a vapor barrier.
Do You Have A Vapor Barrier? Sometimes it's easy to tell if you're lacking a much needed vapor barrier. There could
be water (smelly water) under the carpet or wood floor sitting directly on the concrete. If the surface is painted,
there might be water filled blisters in the paint. Pop them open and they may literally squirt water several feet
into the air. If the concrete is not covered, it could be covered with tiny crystals, the result of mineral salts
being left behind by the migrating moisture.
One way to test the exposed concrete for moisture migration is to tape a sheet of plastic (several square feet
in area) to the floor and see if water condenses under the plastic. A lack of condensed water doesn't rule out
a possible problem. Moisture migration might occur only after rainstorms, spring thaws, or other occasional events.
TREATING A VAPOR BARRIERLESS FLOOR
How do you fix a floor with a vapor transmission problem? There are lots of partial solutions, but no ‘sure thing'
short of pouring new concrete over a new vapor barrier.
A ‘Breathable' system is one common solution. This lets the moisture pass through. Depending upon your requirements,
especially if you're just looking for an attractive surface, this may be all you need. There are a growing number
of porous rugs, paints, and other surfaces on the market.
Next are special sealers (such as Bio Vee Seal - http://www.epoxyproducts.com/vee4u.html)
that soak into the concrete and form crystals within the voids and channels inside the surface of the concrete.
This reduces permeability and porosity of the concrete, generally restricting the flow of fluids, but not vapors,
through the matrix. If the concrete is exposed, this treatment is easy and quick and just might ‘densify' the concrete
enough to solve, or greatly reduce, the problem. A product like this might also assist in the bonding of regular
paint or epoxy which would normally fail, by lessening the amount of mineral salts, etc. the can migrate to the
concrete - coating interface. When used in this (untested) fashion, the Bio Vee Seal would become a primer of sorts.
Finally, there is at least one line of rollable epoxies and floor epoxies that can bond to wet or damp surfaces
(http://www.epoxyproducts.com) so well that they can resist the pressures (over 1000 pounds) that can build up
on the surface of the concrete at the coating interface. This approach offers a simple, roll on, impermeable barrier
on the inside surface of the concrete. By sealing the concrete surface, moisture, concentration, and pressure equilibriums
would be reached inside the concrete which would stop the flow or movement of moisture, although not its presence.
Bond adhesion, however, is influenced by more than just moisture levels. A good, strong bond requires careful surface
preparation. The surface must be clean, dust free, solid (not chalking or deteriorating), and have a good profile
(microscopic peaks and valleys) to permit maximum surface contact. The presence of pre-existing coatings, sealers
or grease/oil spills could preclude a good bond also.
CONCLUSION
Moisture and salts can and do migrate through concrete slabs as they seek equilibrium with the environment above
the concrete. The resulting pressures and condensing water vapor can ruin painted floors, rugs, wooden flooring
and almost anything placed over the concrete. After-the-fact solutions include using a breathable surface coating,
reducing the porosity and permeability of the concrete and/or using special epoxies that can bond sufficiently
well to moisture rich concrete so as to successfully resist the forces that can develop.
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