Doesn’t Like Vapor Barriers Under Slab??
BilljustBill
| Posted in Construction Techniques on
As I’m getting things together for a 16’X32′ cabin/shed, one contractor said he didn’t like using a plastic vapor barrier between the ground and slab because the water in the concrete can’t get out as fast, thus making it take longer….
He recommended an additive mixed in by the concrete company to prevent moisture from coming up. How good is something like this compared to a heavy plastic vapor barrier?
Thanks for you time and words,
Bill
Replies
Someone who knows more than me will be along any second now, but I can't resist spouting off; I have a big mouth and like to sound smart.
The slower your concrete cures, the harder it will be. The water is part of the chemical reaction. Getting the water out of there "fast" may make it "dry" faster, but it won't make for a better job.
It is true that plastic directly under the concrete can cause problems, specifically "slab curl", which results from water leaving the top faster than the bottom. This is dealt with by keeping the top wet for several days while it cures. A slab without a barrier under it may allow moisture from the soil to come up thru... forever.
david...and bill,
I learned a while back thats why folks "water" a slab and sometimes even place wet burlap on it.
It's a code requirement around here to have a vapor barrier underneath.
Mike
Agree with everybody else, use the vapor barrier.. Personally, I'd not use a contractor that did not want to use it, indicates improper cost cutting. (as if I've ever used a contractor?)
Note: also, as if I dont cut corners on sheds<G>. But anything I heat I use 4 " base of pea gravel well drained thru perf pipe with 4 mil poly on top of that before the concrete.
If you are never going to heat your cabin/shed or drive a vehicle into it, so what. But if any part of it will be heated, the lack of a vapor barrier will cost $$ every year, many $$ if you have a high water table. .
The concrete can be mixed with a lower slump (dryer) so it isn't so soupy and excess water is not as much of a problem.
Run from this contractor. Vapor barriers are required, and proper reinforcing and curing methods mitigate the "problems" created by the vapor barrier. I agree with the previous posts.
Keep the top of the slab wet, and you will end up with a better, stronger slab.
Follow ACI (American Concrete Institute) guidelines, if you care to research the proper methods.
"Good design is good business"
Florence Schust Knoll (1917-)
I'd find another contractor, but I'm interested in what admixture makes concrete impermeable to water. Anyone?
Agree, vapor barrier for sure, new contractor, and could have accelerators added to the mix to "safely" speed "drying" for the sake of being able to get on the slab for finishing.
On the second thought I wouldn't pay for the accelerators, would still use the plastic, and same contractor with a hard bid - no T and M. Than require him to soft cut the joints and wet cure for 14 days with burlap and plastic so he'll be just starting to cover it at about 3 a.m. as punishment for being lazy. Plus his pruned up sore fingers from folding up wet burlap that's soaked up chloride for 14 days should help.
As for the "water proof" additive he could be incorrectly referring to a shake hardner as a topping but it dosn't't replace a vapor barrier in the same way, or a latex additive of liquid styrene butadiene latex that will decrease permeability and better protect reinforcing steel from corrosion, (Concrete International March 07') but still isn't a replacement for a vapor barrier. Not to mention both options are going to be way more expensive and a lot more difficult to finish than just going to work on something else for awhile or run an errand as you wait for mud to sit up.
Maybe the "concrete finisher" doesn't want the plastic because he knows he can't water down the concrete (while still in the truck), use the plastic, and get home in time for dinner :-)
True, it is only a 16'x32'. I'm relating more to the 180-200 yard pours pictured below. One is a slab on grade with vapor barrier, thin enough to push through 300' of hose and 2' thickened edge on one end where it steps down to a mechanical room slab. The finishers didn't make the dinner bell on that one. They wern't able to even get on the first half of the slab until around 2pm.
What is that you're building across the street from those people's houses?
Looks like something I wouldn't want to be next too.
Joe H
It's a 5 story plus basement addition to a hospital. When finished it'll be the new heart surgery wing with 12 new O.R.s, 100-150 new patient bays from ICU, admitting, and recovery bed spaces, New MRI, and radiology labs on 1st floor. New sterilization/decontamination central processing area in basement. Than other stuff that goes along with it like locker rooms for Doctors, few offices, conference rooms, ultra sound labs, etc...
Most of the people that live in the adjacent house actually work in the hospital so they probably don't mind just crossing the street for work.
yer gonna have to type slow for me and maybe switch to a crayon.-buck '07
I never had a problem with character, people've been telling me I was one ever since I was a kid.
You hit it on the nail. But I think he does not want to miss happy hour, or does not want to be seen too much in daylight hours due to his last pour.
Yeah,yeah, they're always pullin' this one. They want to finish, they don't care about moisture in the slab. Get the guy onboard or get a new guy.
Advice on additives is best from the concrete supplier, not the concrete contractor. The supplier will be most familiar with additives, especially that that work best for your local weather and local water.
Fly Ash is a common inexpensive additive to reduce water permeability and related corrosion (it reduces the amount of open air cells in the concrete). Some areas and projects will have 'green concrete' specs that may will require Fly Ash be added (replacing up to 20% or even 25% of the Portland by weight) to make a mix that releases less CO2 (it's a green thing). Fly Ash will push the curing rates out so that the final strength is reached in 56 days instead of 28 days (it still must make 3000 psi by day 28). This also means that a normal 2 or 3 day strength (typical start of construction) may not be reached until day 5 or 6.
Whether you use an additive or not, I recommend you require test cylinders be captured (1 or 2 per truck) for any significant pour. Cheap insurance (~$8 per cylinder + another $20 if you test one) and a clear way to let the contractor and the supplier know that they need to provide your project with the correct mix (they can be looser on the other guy's project).
I'm not the one who started this thread but let me say this: In another thread I asked about pouring my own test cylinders - or do concrete testing companies commonly require that they pour the cylinders?
BTW - I found this web page, but unfornately it is set up so it can't be printed.
we dont pour test clyinders.
>> we dont pour test clyinders. <<Care to elaborate? The customer pours them? Concrete is tested by other means? You got me guessing...
test clyinder are taken in three layer, rodded 25 times per layer with a 5/8 smooth rod. a set of 4 is normal with all 4 clyinder being taken from the same truck. If contractor take the sample, it will only guarenntee the mix if independent lab tech takes the sample then it will guarentee the slab
Thanks BB.
Normally the capture of concrete test cylinders (for at minimum the "threat" of later testing) is made to be part of the foundation contractor's written scope of work. He can buy the cylinders from the same company that supplies rebar chairs, tie wire etc. If you hire a 3rd part pour monitor, the taking of samples can be part of his scope of services. label the cylinder with a sharpie Truck 1, Truck 2, Truck 3, etc. Then use a shovel to scoop 'placed but not worked' concrete into the cylinders. Record the cylinder number on a clip board with the time and approximate location the sample was taken. Some inspectors sketch a 'map' of the slab and then circle and label each sample location. The tests, if done, are inexpensive normally done by the project's geotechnical engineering company lab.
"prevent the moisture from coming up" sounds like he is adding too much water to the truck to make it easier to place and push around. Adding too much water can cut the compressive strenght literally in half. He will finish faster, be long gone, and you will get the call back when it cracks.
Try this approach, tell him you will require that 2 test cylinders be captured for each truck and that you will probably have them tested ($20 per cylinder test around here). Of course you have a written concrete specification that he has read and initialed that requires a minimum 28-day strength of 3000 psi (or he needs to replace it with a jackhammer). That should clearly tell him not to add excess water (or he needs to buy a jackhammer). If he does not know what he is signing on to, do not use him.
I don't know about all the different chemicals, but in general, if it is a problem, for him, he is probably adding too much water in the first place trying to make his work easier, but that increases the odds of shrinkage cracks
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