How to Pour Concrete Over Plastic

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Many construction sites are now following Leadership in Environmental and Energy Design (LEED) building requirements. LEED is a program to encourage contractors to create green buildings. The use of plastic vapor barriers underneath foundation slabs and in crawlspaces is becoming increasing called for as this helps to maintain the structural quality of the concrete and is one of the best ways to prevent energy loss in a building. If you are on your first LEED job, you will need to know how to pour concrete over the plastic of the vapor barrier. There are some steps you need to take before you pour concrete over plastic to ensure it is done properly, but the actual job of pouring the concrete is the same.

Things You'll Need

  • 2-millimeter plastic vapor barrier
  • Box knife
  • Barrier tape
  • Concrete
  • Concrete finishing tools (job specific)
  • Make sure the earth or sand bottom of your pour is completely free of anything that could possibly tear the plastic, such as rocks, brick and trash.

  • Unroll your plastic vapor barrier in your pour area in strips. As you begin each strip, make sure that it overlaps the last one by at least 8 inches. Cover the entire area. Cut a small slit or hole in the plastic with a box knife when going over any protrusions, such as plumbing, so the plastic remains tight to the protrusion.

  • Seal the plastic along the seams where the strips overlap and around any protrusions, using vapor barrier tape. Make sure there are no gaps in the tape where the concrete could get under the plastic during the pour.

  • Begin to pour the concrete. Avoid using too dry a mix as it will not sink into the bottom soil of the pour but will create voids in the bottom of the concrete on the plastic.

  • Instruct your workers, or remind yourself, to be careful when using the various tools involved in the concrete pour. Shovels, pump hoses, vibrators and screed boards can all cause a tear in the plastic.

  • Finish the pour as called for in your building plans. Be extra careful when finishing around any protrusions, so you do not tear the plastic.

Tips & Warnings

  • Nail your plastic vapor barrier up over the sides of your edge form or nail it into the block or existing concrete bordering the new pour above the finished grade. This will help prevent the vapor barrier from being pulled away from the edges by the weight of the concrete.
  • Don't use a thin plastic instead of one designed for vapor barrier, or it will rip during the pour and allow the earth bottom of the slab to mix with the concrete and weaken it.

References

  • Photo Credit fresh concrete image by jimcox40 from Fotolia.com
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