Plastic Sheeting Can Permanently Discolor Concrete
Adverse weather conditions such as rain, snow, or freezing temperatures often make it necessary to protect concrete by covering it. One of the most effective materials to guard against rain is plastic sheeting (visqueen). In cold weather, plastic-clad concrete curing blankets are often used to protect the surface from snow and insulate the concrete from freezing. If the concrete is completely covered with plastic, no outside moisture can penetrate to contact the slab. But what about moisture coming from within the concrete itself?
Plastic not only repels water from above, but also from below. This means that water vapor rising up through the slab will not be able to evaporate from the surface, but will instead be trapped there by the impermeable membrane. That’s what makes curing concrete with plastic so effective: the plastic keeps the concrete damp.
Unfortunately, there is a downside to plastic as well. When plastic sheeting or curing blankets are placed atop a concrete slab, much of the blanket is in direct contact with the concrete but there are also wrinkles and air pockets where the plastic remains suspended just above the slab. Condensed water pools wherever the plastic touches the slab, but the concrete under the air pockets remains dry. These pools of water cause the wet concrete to cure differently than the dry spots. When the slab is uncovered, the result is a very blotchy, mottled slab with odd-shaped light and dark shapes throughout. Many people who have observed this phenomenon think that the plastic has somehow “stained” the concrete. In reality, the concrete is not stained but simply cured unevenly.
Unfortunately, the blotches may never fade and the owner is stuck with a permanently discolored slab. The only way to hide this type of discoloration is to cover it with an opaque pigmented stain, sealer, or overlay. Acid stain, translucent stains, or dyes will not hide the discoloration. Instead, they may actually enhance and highlight it.
Although decorative stamped concrete is intentionally mottled with antiquing release anyway, the excessive blotchiness caused by condensation under plastic can ruin the looks of an otherwise beautiful job. Slabs earmarked for acid staining can become problematic as well. Unequal curing can render some spots unstainable while others take the stain normally. *Incidentally, any dense item left atop a new slab such as a bucket, plywood, or board can trap moisture and cause uneven curing and subsequent ghosting of the item’s image when the slab is stained.
To avoid plastic discoloration, the slab should be somehow tented so the plastic is prevented from directly contacting the concrete. This can involve building a framework above the slab and draping the plastic over it. A frame can easily be made with a variety of materials including lumber, 1″ PVC pipe, long strands of rebar, etc. PVC is probably the cheapest, strongest, and easiest material to use.
To get thermal protection while avoiding plastic discoloration, dry straw is often placed atop the fresh concrete and then covered with plastic sheeting. This keeps the plastic from contacting the slab and also keeps precipitation from wetting the straw, causing it to bleed.
Some concrete contractors use fiberglass batt insulation to protect their work. When the slab has set enough that covering it will not mar the surface, the contractor places the insulation atop the concrete and then covers it with plastic to guard against rain. In short, anything that one can do to keep the plastic and framework suspended above the concrete will work. Taking the necessary precautions can prevent unwelcome surprises when the slab is uncovered.