Covering concrete with plastic is an effective way to help prevent cracks from forming as the concrete cures. Plastic acts as a moisture barrier, slowing the rate of water evaporation from the concrete surface. This allows the concrete to cure more evenly, reducing the chances of cracks developing. Here is a step-by-step guide on how to properly cover concrete with plastic.

Choosing the Right Plastic

Not all plastics are created equal when it comes to concrete applications. The plastic you use should:

• Be at least 6 mil thick – Thinner plastics can easily tear.
• Be polyethylene sheeting – This type of plastic allows moisture to pass through while still slowing evaporation.
• Be untinted/translucent – Clear or translucent plastic works best.

Steer clear of:

• Plastic tarps or plastic paint drop cloths – These are too thin and porous.
• Plastic with holes or tears – This allows moisture to escape.
• Thick plastic sheeting – Doesn’t allow adequate moisture flow.

Polyethylene sheeting can be purchased in rolls at your local hardware store or home improvement center.

When to Cover the Concrete

The timing of when you cover the concrete is important:

• Cover as soon as possible after pouring and finishing the concrete.
• If you wait too long, the concrete may begin drying unevenly.
• Don’t cover concrete until bleed water has evaporated – Usually within 1 hour of pouring.

Covering too early traps bleed water on the surface, which can also lead to cracking and other defects.

How to Apply the Plastic

Covering concrete properly takes a little bit of effort but is very straightforward:

• Roll out the plastic over the entire concrete surface. Overlap seams by 6 inches.
• Weigh down the edges of the plastic with boards, stones, or stakes so it doesn’t blow away.
• Seal all the edges with additional stakes, soil, sandbags, or adhesive tape.
• The goal is to eliminate air flow and prevent moisture loss.

Avoid using materials like duct tape directly on the concrete surface, as they may discolor it.

Curing Time

The concrete should be covered with plastic continuously for at least 5-7 days. Longer is often better, especially in hot, dry, or windy conditions.

Inspect the plastic periodically for holes or tears. Patch any openings to maintain the moisture barrier.

Removing the Plastic

• Cut and remove the plastic when ready. Slit it into strips to simplify removal.
• Avoid tearing off the plastic in large sheets. This can damage the concrete surface.
• Dispose of the plastic properly when finished.

Once the plastic is removed, you can proceed with polishing, sealing, or finishing the concrete as desired.

FAQ About Covering Concrete with Plastic

Here are answers to some frequently asked questions about using plastic to cure concrete:

Should I use plastic under or over the concrete?

Plastic should only be used on top of the concrete, not underneath. Underneath plastic can lead to bleed water and alkali buildup issues.

Can I pour concrete directly onto plastic?

No, pouring directly onto plastic leaves a very smooth surface that prevents proper bonding. Plastic should only be placed on top of poured concrete.

How soon can I remove the plastic after pouring?

Plastic sheeting should remain for at least 5-7 full days. Premature removal allows the concrete to cure unevenly.

What kind of adhesive tape can be used?

Avoid duct tape or other tapes that may leave residue. Use an adhesive tape designed for concrete like Gorilla Tape instead.

Can I leave the plastic on for more than 7 days?

Yes, leaving the plastic protection on for longer, especially in hot or arid climates, results in a stronger concrete cure.

What if it rains while the concrete is covered?

The plastic barrier will prevent rainwater from touching the concrete surface. Ensure adequate drainage under the slab.

How do I prevent the plastic from blowing away?

Weigh down the edges thoroughly and seal them to prevent airflow under the plastic. Use adhesive, stakes, or other heavy objects.

Should I re-wet the concrete after removing the plastic?

Yes, continue curing the concrete by re-wetting it periodically for 3-7 days after removing the plastic.

Conclusion

Covering freshly poured concrete with polyethylene plastic sheeting is an easy, affordable way to help ensure proper curing and prevent cracks. For best results:

• Use 6 mil untinted plastic, applied soon after finishing concrete.
• Weigh down edges and seal thoroughly to prevent moisture loss.
• Cover for at least 5-7 days before removing.
• Re-wet the concrete after removing the plastic.

Taking steps to slow the drying time allows the concrete to gain strength evenly and resist cracking as it hardens. Be patient during the curing process and let the plastic do its job protecting the concrete. Your efforts will pay off with a durable, long-lasting slab.