Do I need to put plastic over a newly poured slab?

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  #1  
Old 10-29-10, 04:00 PM
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Do I need to put plastic over a newly poured slab?

I have a new concrete slab that is 10x14 (floating slab, 3000 PSI concrete in case the info matters). The slab guy (who I no longer trust and won't use again) says there is no need to cover the slab with plastic, everything I read says to cover it to allow it to cure.

So, I do need to cover it with some poly and if so, how long do I allow it to cure before putting the poly over it?

Thanks in advance,

Tim
 
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Old 10-29-10, 04:06 PM
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No Pro...but by covering the slab it will help retain moisture and cure a bit harder. I was always told to take the cover off and mist the slab with a hose (mist..not spray) every day.

I think it was after 5-7 days, that was plenty.

btw...depending on the actual use..the 'crete guy may have been right.
 
  #3  
Old 10-30-10, 05:42 AM
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I just poured a sidewalk. I put plastic over it and left it on for a couple of days. I misted the sidewalk each day.

When I removed the plastic the concrete was discolored wherever the plastic contacted the concrete. I ended up with gray concrete with lighter colored stripes. I thought they would disappear after the concrete fully cured but they didn't. You might want to put stickers down to keep the plastic from contacting the concrete.
 
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Old 10-31-10, 09:42 PM
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placing a sidewalk tomorrow & another driveway the next day,,, no plastic, no mist, & saw driveway jnts the next morning,,, will groove control jnts in s/w as we go,,, same way we did a driveway last wk
 
  #5  
Old 11-01-10, 05:21 AM
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No expert, but my impression was that you cover with plastic to protect the concrete from rain or in hot weather when you need to mist it every day.
 
  #6  
Old 11-01-10, 06:17 AM
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Originally Posted by drooplug View Post
No expert, but my impression was that you cover with plastic to protect the concrete from rain or in hot weather when you need to mist it every day.
Why protect against rain? I thought you want moisture/water (unless it's within a few hours perhaps)?

Also, does anybody know why Wayne's plastic discolored the concrete? That sounds very strange.
 
  #7  
Old 11-01-10, 07:49 AM
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Well, I know you can have too much water in the concrete. I'm not sure if rain could do that, but the drops will put marks in the finish.
 
  #8  
Old 11-01-10, 08:26 AM
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How bout this as a reference? Cement & Concrete Basics: Curing Concrete | Portland Cement Association (PCA)

Apparently there are many reasons..from heat retention to moisture retention.

As to the discoloration...that SHOULD go away with a decent amount of weathering.
 
  #9  
Old 11-01-10, 10:32 AM
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If I get a chance I'll post a pic. It's been a couple of months and it hasn't changed a bit. The discoloration is only where the plastic touched the concrete.

As for the plastic, I thought to was to prevent the concrete from drying before it cured. I read that somewhere. When my garage slab was poured the contractor covered it with a couple of tarps. When the slab for my house addition was poured it was not covered. The contractor said it was protected below grade. Who knows.
 
  #10  
Old 11-01-10, 12:50 PM
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Thumbs up

Typically we use a chemical curing compound that eliminates the need for a "wet" cure and the resultant stains. The compound is applied with a garden sprayer as soon as you can walk on the slab.

Good luck
 
  #11  
Old 11-01-10, 01:20 PM
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A spray on chemical curing compound applied at the proper time is good. Later light sprinkling to increase the moisture on the surface will also delay the concrete from drying as it cures. - Concrete should cure and not "dry" during the first week or two.

Poly can be applied over the surface in ther case of a rain shower that can disturb the surface. It should be removed as soon as possible to maintain even conditions and water praying can provide the moisture.

In India it is common for elevated concrete slabs to be water cured to provide the best concrete since the bottoms can dry out too fast.

Just maintain a moist condition to the concrete in whatever way is necessary to provide moisture, but not physical damage in the early day that can come from rain or walking on it.

In our climate it is normal to use 4000 psi concrete for slabs for durability, but in virginia Beach you have enough natural heat so the only concern would be protection from a quick rain and peridic sprinkling should be enough because of your soil temperature conditions. In a drier climate more surface water would be required and in some hotter area the curing will occur faster (maybe too fast).

Dick
 
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