# Discoloration of concrete from plastic sheet



## pdrewcock (Aug 13, 2013)

I'm curious as to how to prevent the discoloration of a concrete pad when using a plastic sheet or membrane to cure. Would it be reasonable to move the plastic every day or two so that the condensation is not concentrated in one area for the full cure time? It's possible that this question has already been asked, but I didn't know how to frame the search.
Cheers! Pete.


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## oh'mike (Sep 18, 2009)

I'm not a concrete guy---but around here we just spray the slab with water--some use a sprinkler---plastic is only used in the winter---


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## pdrewcock (Aug 13, 2013)

oh'mike said:


> I'm not a concrete guy---but around here we just spray the slab with water--some use a sprinkler---plastic is only used in the winter---


Thanks oh'mike,

Since concrete should be kept from drying for at least 2 weeks (if possible 4), a sprinkler would have to run continuously or at least every 10 or so minutes. This is a lot of water and a chance of it not working. Also I understand that continuous watering could wash out the entrainment chemicals in the top layer leading to spalling. I don't know if this is fact.
Cheers! Pete.


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## Tscarborough (Mar 31, 2006)

Jute or burlap under the poly is the only way to protect it. If water is cheap, you can make a sand dam around the perimeter and keep a sprinkler going.

The reality is that you do not need to hydrate a slab for a week, or 2 weeks, or 28 days. The curing of concrete is not linear it is exponential, so you really need to wet cure it for 2 or 3 days under the worst conditions.

If it is exterior in full sun and wind and will not be acid stained or dyed, buy a white pigmented cure and seal and be done with it.


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## pdrewcock (Aug 13, 2013)

Tscarborough said:


> Jute or burlap under the poly is the only way to protect it. If water is cheap, you can make a sand dam around the perimeter and keep a sprinkler going.
> 
> The reality is that you do not need to hydrate a slab for a week, or 2 weeks, or 28 days. The curing of concrete is not linear it is exponential, so you really need to wet cure it for 2 or 3 days under the worst conditions.
> 
> If it is exterior in full sun and wind and will not be acid stained or dyed, buy a white pigmented cure and seal and be done with it.


Thanks Tscarborough,
There are so many different opinions out there. The burlap sounds like a good idea but fairly expensive since I don't know anyone to borrow it from. Flooding may be a workable alternative.
You say that concrete cures exponentially. What percent in the first day/second/third etc? Are ther charts at different temperatures that one can use? I ask because I have designed my slabs for minimum thickness and one is a carport that will be driven on.
Cheers! Pete.


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## jomama45 (Nov 13, 2008)

More than you'll ever want to know about curing concrete, and it's a reliable source as well:

http://www.cement.org/tech/cct_curing.asp

We've had this discussion here as recent as this Spring, and YES, plastic sheeting (Visqueen) will discolor concrete when left in direct contact for more than a very short time. Painter's dropclothes, bed sheets, etc... also work well between the concrete & plastic. My typical curing choice is spryed membrne though, as Tscar mentioned.........


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## pdrewcock (Aug 13, 2013)

jomama45 said:


> More than you'll ever want to know about curing concrete, and it's a reliable source as well:
> 
> http://www.cement.org/tech/cct_curing.asp
> 
> We've had this discussion here as recent as this Spring, and YES, plastic sheeting (Visqueen) will discolor concrete when left in direct contact for more than a very short time. Painter's dropclothes, bed sheets, etc... also work well between the concrete & plastic. My typical curing choice is spryed membrne though, as Tscar mentioned.........


Thanks jo,
Another interesting read. The chart is helpful and reconfirms the 28 day optimum for curing. I'll probably manage 14 days, but not being able to access my front door for 28 days might be pushing it. I won't be using a sprayed membrane since I may want to paint or stain the carport floor at some point.
Cheers! Pete.


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## stadry (Jun 20, 2009)

w/o studying the ' chart ' & respectfully to 'mama, you should easily be able to walk on the conc the next day - even the afternoon if its placed in the am,,, IF you're curing w/wtr spray, take off your shoes prior to entering the front door :yes: we paved 1/2mile of conc per day & had to saw/slice it into slabs,,, couldn't do that w/o walking on it :no:

28d is only a time period for breaking to confirm strength,,, the fastest strength gain is the 1st 12hrs, then the 1st 24hrs ( steep curve ),,, typically strengths are ck'd in 24hrs, 7d, & 28d,,, after 7d we had const trks driving on it :laughing:

IF the weather's extremely hot & windy, you might need to wtr 3d but i doubt it


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## pdrewcock (Aug 13, 2013)

I wasn't concerned about walking on the concrete. My front door is four feet above the new slab and I will have to build a new entrance on the slab. I suppose I will be able to build on it after a few days. Just that trying to keep it wet and building on it becomes a. Bit of a trick. That's what I was referring to about not being able to access my entrance.
Anyway, an update: I have poured the slabs and covered with plastic. After the slabs were well set up, I was able to flood the top by misting under the plastic. The result was that the thin plastic sheet sucked down onto the slab with almost no wrinkles. It looks like a shiny new slab! I will continue to mist under the plastic if required. I will follow up with the results. Thanks to all for their input. :thumbup:
Cheers! Pete.


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## pdrewcock (Aug 13, 2013)

An update:
It's been over a month now and the plastic is off. The slab looks great with no discoloration. I flooded under the plastic every day for two weeks and then left the plastic on without flooding for another two weeks. A little anal but the results are terrific. 
Thanks to all for their input.


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