Placed concrete for 2 slabs (22 x 29 and 16 x 14) on thursday and covered with 6 mil plastic immediately following final finishing that evening. Temps have been 50’s during the day and 30’s at night. How long should plastic stay on for optimal curing?
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28 days to dry typically but you can build way sooner than that.
Take the plastic off and just spray the crete a cpl of times a day with H20 to slow down the curring time as long as it doesnt get below freezing.
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Edited 11/14/2004 10:06 am ET by Andy Clifford(Andybuildz)
7 day to cure, 28 days to make strengh
>> Take the plastic off and just spray the crete a cpl of times a day with H20 to slow down the curring time as long as it doesnt get below freezing. <<
Somebody please correct me if I'm wrong, but I'm of the opinion that constant wetting and drying (moisture cycling) can do more harm than good and at best is ineffective.
I say that as long as he has the plastic down, leave it for a week (hopefully) or until he's ready to build on it, or use it as a driveway, or whatever it's intended use is. I think the whole reason for the plastic is to reduce evaporation of moisture.
To the original poster: it's important to understand that while temperature does effect curing time, the hydration of cement (the active ingredient in concrete) is a chemical process that is activated by water. Matt
I thought it had been established that occasionally watering concrete was worse than just letting it cure dry. Either keep it continuously moist or leave it alone.
I have zero experience in this area...
blueWarning! Be cautious when taking any framing advice from me. Although I have a lifetime of framing experience, all of it is considered bottom of the barrel by Gabe. I am not to be counted amongst the worst of the worst. If you want real framing information...don't listen to me..just ask Gabe!
when ever I do a slab , I will buy a cheap soaker hose and leave it under the plastic. and then turn it on once a day for seven days.
Honestly, a correct place slab only need 48 hour cure, the extra five days help the top surface durablity.
Brown bag knows crete better than most of us, but he is in Florida. from the temps you report, you are farther nortth.
Curing ( Hey Andy, what's this about "drying" ?) is a chemical process. How long it takes is dependent on original excess moisture, temperature, and time.
Keeping it warm ( Up to about 90°) decreases curing time and increases strength, but that can result in the surface moisture evaporating before irt enters the reaction.
Thus, in warmer climates or during summer in the north, covering with plastic is a great process. The chemical curing reaction ( not drying - I hate it when carpenters use that term. Paint dries. Concrete cures. An I still respect you in the morning, Andy, LOL) produces heat as a byproduct so the plastic helps hold that heat in to some degree, as well as keeping the water in solution so it can cure thoroughly. Another method sometimes used is to sprinkle the surface occasionally during the fiorst three days to slow the evaporative rate, but the plastic is the better solution - keeping it in.
At you low temps, I would wait three to seven days before doing much on it. if you can scratch it easily with a nail, don't be working on it yet.
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" Paint dries."
Actually paint cures also.
Specially oil based paint and varnishes. They do "dry" with the fashing off of the solvents. But the long term effect is that they curve by oxidizing.
That is why some finishes have a recoat time without sanding. Enough solvent has flashed off that it won't affect the 2nd coat, but the surface has not completely ozidized yet that the 2nd coat can bond with it.
I think that water based finishes cure also. But via a different process.
But the long term effect is that they curve by oxidizing."
Yes and no. Depends on the finish. Non two-part finishes (oils like you mentioned) use the oxygen in the air as a catalyst for their molecular crosslinking, which usually progresses rapidly after the solvents mixed with them have evaporated. After the molecular crosslinking has occurred, no amount of original solvent (not really the right word, as it should properly be called a reducing agent) will reverse the bond (unlike solvent based finishes like lacquer or shellac)
I'm pretty sure it's a similar process with concrete, 'cept my understanding it that it more like a crystalline structure type bond.
BTW Bill, I'm sure you now this, just giving my 2cents to the post.
Jon
I'm surprised no one has mentioned hay.
I've always done plastic and hay or just hay, straw, salt hay in particular. Esp. when temps are down. Hay will keep it moist and warm. Plastic then hay will keep it toasty.
Eric
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After this much time the heat generating capacity of those thin slabs has fallen off. Your best bet is to rent some blankets and cover the 'crete just before the sun sets and uncover them in the morning fo the next 7 days (to be safe). I suggest blankets 'cuz they're easier to handle repeatedly than straw.
As long as there is dew or condensation under the plastice during the day you will not have to add moisture. If the condensation looks like it is slowing down before the seven days are up go ahead and sprinkle the slabs enough to dampen the surace. The water you add should not stay on the surface more than 60 seconds after you move the spray. There is probably enough moisture in the soil under the slabs to do the job.
Ideally, you would have covered the plastic (with hay or blankets) as soon as you had finished the surface, but since you have waited this long, blanketing at night is the best you can do. Leaving them uncovered during sunlight hours will help to raise the internal temp back up where it belongs.
If it looks like you may get a hard freeze, get some heating blankets for the duration.
SamT
SamT
Heating blankets or insulating curing blankets????
I had never heard of heating ones before.
Out in Colorado, the masons had curing blankets that were rag wool encased in plastic. They were a bear to handle.
Laast winter, I had to buy several curing balnkets for a slab, expecting when i ordered them to be the samwe wool rag type. What I got was about 5' x 22' of tripled layered bubble wrap between woven poly, black one side and silvered the other. That was actually good, because they sure got wet. Wet wool would have taken a crane to get out of that cellar hole!
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Piffin,
You can get either. Simple insulating blankets are usually sufficient for any pour over 1' thick and many times good enough for a thin slab as the earth acts like an insulator/heat source too.
In Coz's case, case you di'nt read Coz's case, he has already waited a few days and cold nites. If it gets really cold I would add heat. Since his temps haven't been above 50*, I think he should wait another 7 days even with the added insulation at nite.
But, I like concrete and like givin' it lots o' TLC.
SamT