What’s the typical curing time for concrete poured indoors about three inches thick?
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Replies
There's no absolute curing time. It's relative. Curing to what strength? To a specific psi? To a standard like, "when can I walk on it" or "when can I install flooring"? Theoretically it's still getting stronger for something like 146 years.
Given the standard by which you are measuring, the answer will depend on the mix, the way it's covered and kept misted, the building, and the climate.
Practically speaking, we walked on ours in a day or two. Sooner if you're careful. We'd do regular work on it in a week or so. I'm sure some people go faster, but we'd always schedule other things to do. For installing floor coverings, test it with a meter so that it meets the specs of the floor covering.
See http://www.portcement.org/cb/concretebasics_concretebasics.asp and other links for more info.
you can pretty much walk on it within 24 hr, but it need to cure by being moist and covered with plastic seven days.
Here's the situation. A section of my basement had a dirt floor. I hired a company to pour a lightweight concrete product to seal it. A vapor barrier was laid over the dirt and then the concrete was poured. Within a few months, the floor cracked and began settling. I was told that the product was mixed improperly and also that air pockets created settling between the concrete and vapor barrier. Well, last March they tore out the concrete and this time they poured it without laying a vapor barrier. I was told the improved concrete product would stop moisture. The floor has certainly cured to accomodate foot traffic and storage, but the floor surface is constantly covered with beads of water. I mean it sweats like an ice cold glass of iced tea sitting on a pool deck in the summer Texas heat. I was told that it could take 6-9 months for it to cure including evaporation of all water (used in the mix) to the surface. I expeceted perhaps a clammy surface, not the surface water I'm seeing plus the appearance of some type of surface black mold including little worms. My intuition and logic tell me something is wrong and that I'm not being impatient. What do you think? Thanks.
If it is sweating then that is a different problem caused by a high humidity level in the air hitting the cool floor.
get a piece of 2ft by 2 clear plastic sheet and tape to the floor. Then wait 24 hours and remove it
Is the concrete wet or dry? is the top of the plastic wet or dry?
I'll try that and report back. Thanks.
Bill is right of course that this should be your first test regarding moisture.
Odds are the moisture is permeating through the concrete because you have no VB.
The contractors claim that your problems were due to the air pockets under the VB is BS.
The contractors claim that the special mix would not require a VB is also BS.
Concrete will sweat in the early stages of curing but not for long. Most or all of it would have normally evaporated before forming any beads.
My concern would be more towards the air in your basement and lack of circulation at this point. Check the moisture or RH level in your basement as a precaution.
Gabe
Those are my thoughts exactly. When it rains, I definitely notice more beads of water on the surface, which I'm convinced is groundwater permeating to the concrete surface. I remember reading the FHB magazine article about that CT company that lines dirt crawlspaces with a heavy duty plastic vapor barrier. If my memory serves me correctly, his company didn't bother with a concrete topcoat because the plastic VB solves the moisture problem. Other areas in the basement are fine. Since March, I have been running a dehumidfier in the crawlspace and running an oscillating fan 24x7 with two windows open providing cross ventilation. I'm tired of emptying the dehumidifier each morning.
"I have been running a dehumidfier in the crawlspace...with two windows open providing cross ventilation..."
That may be part of the problem. If you're letting in warm humid air, the humidity will increase dramatically when it cools off in the basement.
Try shutting the windows for a couple of days and see if that helps.
Don't blame you for not wanting to empty the dehumidifier every day. Is there a hose connetion on it? If so, you should be able to run a hose to a floor drain instead of doing the bucket thing.There's only one thing wrong with wife swapping.You get another wife.
For the first month or so after the installation, the windows were closed. The dampness from the floor was so bad that I figured opening the windows would help.
As Boss suggested, close the windows. If you don't have a nearby floor drain for the dehumidifier, as indicated by emptying it ever morning, get a a/c condensate pump. Little Giant makes one with a sump resevoir and auto on/off float switch. Set the dehumidifier on a block and drain it to the condensate pump. The pump will then discharge it through a hose or pvc pipe to a point outside the house or a sink, washer stand pipe,etc. in the house. The last one I installed for a 5 ton a/c unit was a little over $100. I think Granger carries a variety of sizes and prices of similar pumps.
Dave
Thanks. I'll do that. I have a sump pump hole that I can run a hose to.
I was thinking the same line as Boss hog about the humidity coming in the window while you are de-humidifying it. Wasted efforts
IF the moisture is coming from the air and condensing, close the windows and run the de-humidifier.
IF the moisture is percolating up thru the slab, open the windows and fan and turn the de-humidifier off. Then shoot the guy who told you that his slab would waterproof things. He claims it is good for storage? let me imagine you storing a box of books on it.________________
Ok, all done imagining. I hope they weren't archival quality books!
I'm betting the moisture is groundwater coming up thru the slab. If groundwater is present, and no drainage plane of gravel and no barrier such as plastic is down, I would expect water to come up through. I would be suprised if it didn't!
Do you suppose that he wants more practice? Three times is a charm..
Excellence is its own reward!
There's guys here, including Gabe plus some others, who run circles around my experience. Your description takes this outside of "normal pour, everything goes as planned" so I'll sit on the sidelines and anticipate learning from their responses to you.
You didn't mention anything about a few inches of clean gravel and a little vibrating with a compactor before laying the poly and doing the pour. Lightweight needs all the help it can get, I wouldn't use it for a slab and wouldn't pour without gravel and compaction and nothing less than 4".
That's a good point to bring up. Last March, after they tore out the botched concrete, they added maybe an inch of gravel on the perimeter of the area only. No compacting was done. I should also mention that the crawlspace is sloped: It goes from about 6 ft. of head clearance to about 3 ft. Technically, it isn't a "crawlspace" - it's the room under the family room and merely another room in the basement which has a standard poured slab everywhere else. I don't think the concrete product is intended to be a traditional slab. It's sold as a solution to eliminating moisture, mold and rodent problems from dirt basement floor areas. It supports standard foot traffic and may be used for storage.
"Curing" of concrete doesn't mean simply drying out. Curing is the process of controlled hydration of the cement in the concrete. What's at issue with your slab isn't, in my view, a 'curing' process but rather an ambient moisture problem.
First, without a vapor barrier under the slab, if you have any ground water problems in the least, you'll have water coming up through the slab. Concrete is *not* waterproof. That is, unless it's post tensioned and/or other steps are taken to make it so.
If you have a high humidity problem in the 'inside' area, and if the slab isn't insulated, the humidity will condense on the surface of the cold slab. That's elementary physics. Or chemistry. Or some science or the other (grin).
OK, what to do.....
To find out if it's a ground water problem, do the tape-on-plastic sheet test described in another post. What this does is trap the moisture that (might be) coming up through the slab. If the slab is damp, or even darker than the surrounding area, you've got water migrating *up* through the slab. If not, then it's a condensation problem.
If it's an issue of excess humidity in the space, the simplest answer is more ventilation. If heating isn't an issue, install an exhaust fan to push air out and draw new air in from everywhere it can come.
If you'd rather not exhaust heated air to the outside, get a dehumidfier. These things are real eye openers with respect to how much moisture is in the air. We got one at Sears that can be configured to run continuously and drain directly into a sink or floor drain.
There are no 'air pockets' to speak of in concrete. Well, not big ones. Voids in concrete are more likely the end result of too high of a water to cement ratio.
Light Weight concrete isn't worth much with respect to floor slabs - I see no use for using that kind of product as a slab on grade.
Let us know what all happens....
Dennis in Bellevue WA
[email protected]
BTW Post-tensioning does not waterproof concrete.
Gabe
Gabe -
Given the proper mix and PT design, roof slabs can be made close to waterproof. There's still the capillary action through the micro channels left by the evapration of water in the mix but that's where mix design comes into play. No one in their right mind would bet the farm on PT alone, no, but in the underground structure designs I've been involved with, it's a worthy insurance policy none the less.
...........
Dennis in Bellevue WA
[email protected]
I fail to see your logic in thinking that PT has any effect on the permeability of concrete.
Gabe
Post Tensioning aids in the reduction of the inevitable micro-cracking of concrete surfaces. Post-Tensioned bridges have shown a reduction of deck deterioration due to this. For every day items, PT is not very cost effective.
Out in Portland, where concrete is king, the concrete mix quality is incredible. My company designed the Ford Island (Admiral Clarey) pontoon bridge in Hawaii. The pontoons were cast in Portland. The rebar has a purple marine epoxy and there is PT, however the concrete sits in salt water with no waterproofing.
NYSDOT started using a high density concrete mix for bridge decks. The mix uses silica fumes and has very high early strength without using Type III cement. The material exhibited negligible micro-cracking on the surface during testing. It is not often a DOT jumps on a new material like that....that's not a mistake, it's rustic
Post tensioning does nothing to change the permeability of concrete. Certain add-mixtures used in conjunction with post-tension design may decrease the rate of hydraulic conductivity, but it is not due in any way to the post tensioning itself.
Chloride migration is a much greater concern in post tensioning than moisture migration. This is why the tendons are coated.
A number of things can be done to make concrete more water-tight. Mix design and add-mixtures before the pour or micro-crystaline surface application after the pour. It all depends on your design criteria.
Eric
Reread the post. I never said PT changes the permeability of concrete. The reduction in microcracks due to the post tensioning keeps more moisture out of concrete and reduces the chances for deterioration. Chlorides get in with the moisture. Chloride migration is the greatest concern for any reinforcing steel. That is what corrodes and spalls off the concrete. My post mentioned high density concrete mixes. Did you read any part of the post? Micro-crystalline sealers? More like Drylok for the next decade.
...that's not a mistake, it's rustic
Edited 7/12/2003 12:21:41 AM ET by Bungalow Jeff
Gabe -
Look at it this way....
Say you take a sponge, admittedly something less than impermeable. Put in in a vice and compress it to some considerable compression. (the same sort of thing that's being done to concrete when you pull on a non-bonded PT cable anchored at one end and being pulled against the edge of the slab at the other). The more the sponge is compressed, the less permeable it becomes. Until you reach a point where the sponge is compressed sufficiently to eliminate any channels or voids through with water can migrate.
I agree - unless you live in an area where lots of PT work is done, and even then, the cost isn't warranted for ordinary residential work. There are lots of other ways to eliminate water problems associated with on grade concrete slabs. A proper capillary break (6" 3/8" pea gravel), plus rigid insulation between the gravel and slab will do the trick in all but the most ornery ground water situations.
We all install perimeter footing drains, of course! (grin)
...........
Dennis in Bellevue WA
[email protected]
When you can take a concrete slab and compress it to 1% of it's original size with Post tensioning we'll talk.
I'm on a project that has the area of 12 football fields under post tension and trust me, it cost us an additional $300,000 to waterproof the decks before laying the asphalt over the concrete slabs.
Let it go Dennis, you were right on the other 90% of your comments in your post.
Gabe
Congratulations, Gabe. If you got by with a viable membrane over all that acreage for that sum, you got a great deal. By my rough calculations, Kemperall, the only membrane I'd trust under that much an area would have cost in the neighborhood of three times as much.
In any event, I'm not here to seek an arguement, Gabe. Your project was most likely broken down into manageable sections since I can't imagine placing and stressing tendons over that much area without pour breaks - thus cold joints in the slab. What I'm talking about with respect to adding some degree of waterproofing integrity to a concrete slab is more on the order of the footprint of the ordinary residential building. And even then, I wouldn't put all my eggs in the PT basket, either.
What's the project you're working on? Sounds fascinating. Must be an elevated slab?? Asphalt overlay for parking or ....? Just curious.
...........
Dennis in Bellevue WA
[email protected]
It's a $26 M., 1700 car, 4 level parking garage with rental offices.
Not very fascinating, almost finished, can't wait to start the next project.
Membrane was torch on Suprema and only on 3 levels.
Gabe
Thanks, Gabe -
My interest stems from the fact that my neighbor is a licensed Kemper applicator. I've heard many, many sad tales about work he's had to go in and rip out that were done improperly .... or not at all.
I had him install the shower pan for the shower I'm building - held 40 gallons of water for a week so I guess it's safe to continue. And no, it wasn't post tensioned (grin)
...........
Dennis in Bellevue WA
[email protected]
at least we all finally agree concrete isn't waterproof!
JeffBuck Construction Pittsburgh,PA
Fine Carpentery.....While U Waite
Yes, but does it spall when wet?
Dennis, what you mean by "rigid insulation" in the following comment that you made: "A proper capillary break (6" 3/8" pea gravel), plus rigid insulation between the gravel and slab will do the trick in all but the most ornery ground water situations." Thanks.
Capillary action will wick water from the ground up through the concrete. There needs to be a break. The thickness of stone he describes will let water drip down to the ground instead of wicking up.
Imagine trying to suck water up through a broken straw, or to pump water through a hose full of holes. The spaces in the stone under the slab are the breaks.
Then add to that a vapour barrier of plastic, vapobar, or foam sheets, and you have really made it almost impossible for water to come up through, unless your soil is totally saturated to a level well above the slab..
Excellence is its own reward!
Well, I'm 100% sure that groundwater is present. I've seen the damp and in some case, mucky dirt floor. Your explanation of uninsulated slab + humid air makes perfect sense. I was very skeptical when the contractor told me that his proprietary product is waterproof. In addition to the surface water, I have noticed that both the temperature of the room and concrete surface are much cooler relative to the other floor which had a VB. The VB had excellent insulating properties.
Regarding air pockets in concrete, the contractor's technician admitted that the original floor mix was done on a rainy day so the H2O/cement ratio must have been way off. The contractor also told me that an air pocket occurred between the concrete slab and the VB. It led to cracking and when I walked on certain areas, the floor actually cracked and settled. Obviously, these results led to months of fighting until the contractor agreed to fix it. And as you can see, I'm skeptical about the latest fix. Thanks.
28 days works for me.
Edit: I think they even say that for drying up alchy's...lol
andy
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Edited 7/14/2003 6:53:16 AM ET by Andy Clifford(Andybuildz)