I just poured an insulated slab for my house addition. The slab is 24’x32′, 5″ thick with a 12″x12″ haunch at the perimeter, #4 rebar @ 12″ o/c, over 2″ insulation, vapor barrier, and 18″ compacted gravel. The site is well drained.
My situation is I may not put up walls and roof until next summer, or maybe the summer after. A local concrete “expert” (the guy has poured a few slabs I guess) says I should cover it in insulation to protect the slab from freezing. This sounds nuts to me…my garage slab is built the same way and has undergone some serious freeze/thaws cycles with no problems after 4 winters.
What do you all think? Will my slab be as good as new after a year or two with just a tarp on it? Do I even need a tarp? The surface will be our “finish” floor for a year or two after moving in until we can install flooring.
Replies
think of all the sidewalks and driveways that are not insulated
A driveway or sidewalk isn't restricted by foundation walls in it's movement by frost; it can go up or down and it may crack, or it may not....
I ran into this on one a few years back. Owner ran out of dough building his house, so after a few years, poured slab inside foundation so kids would have an area to play hoop, skateboard etc. OK it was me.....
Floor was poured on compacted good gravel, 5" thick, w/wire and fibermesh. Conc contractor cut floor at each side of door to back, and cross ways twice.
I got a few cracks; nothing heartbreaking, but on where the slab on one side of the cut at one end is about 1/4" higher than the other, running back to flush. My belief is that the slab getting rained on lets a lot of moisture in around the edges, and when you get a deep freeze, it is more apt to heave there due to higher water content underneath. Slab was there for 2 years before building went up..... Hasn't moved since.
I think a cheap tarp and a few cinder blocks are cheap insurance.
My .02
Bing
Sounds like yours was placed within the confines of foundation walls (as you said) whereas the OP's proposed slab is a monolithic pour - ie no foundation walls. If I understood the OP correctly...
Right you are...missed the monolithic reference...If I'd noticed the word haunch it would have changed my response. Someone started a post here last week something like "Don't you guys read?" Oh wait..............that was me.
corrected:)
Bing
Based on the fact that it is a monolithic pour, I don't see what these other people's concern is about water getting under under the edges of the uncovered slab. True enough it is a concern with any slab, but the fact that the OP isn't gonna be building on it any time soon has nothing to do with - which was what this thread is about "expose slab to elements?".
Although, OTOH, the posters point directly above about putting insulation around the outside perimeter, I thought was always a requirement with these "Alaska slabs".
I'd be worried about spalling which would depend on your weather and your mix. Your garage slab is, I assume covered with a roof so in that case spalling would be unlikely. Sounds like heaving may not be a problem given your insulation and drainage etc. but if it was for a client I'd err on the side of caution.
I have an almost identical monolithic slab, without insulation, on reasonably well drained shale and soil, in Upstate NY. Serves as a basketball half court. Been there for fifteen years, no cracks.
Your gravel base is superior in all respects so, in my opinion, there should be no problems from frost.
Edited 9/17/2008 4:38 am by Hudson Valley Carpenter
>> I just poured an insulated slab for my house addition. The slab is 24'x32', 5" thick with a 12"x12" haunch at the perimeter, #4 rebar @ 12" o/c, over 2" insulation, vapor barrier, and 18" compacted gravel. <<
What's a "Haunch"? A thickened edge similar to a footer that was poured monilithic with the slab I assume?
When you ordered your concrete...did you ask for it to have 6% air entrained?
After the pour, finsh and set, was a sealer applied to the surface?
Check the slab for low areas and drainage patterns and if they exist and you leave the slab open to the weather you may want to consider removing water with a squeegee, or tarping as suggested.
Take a look at the perimeter drainage...loading of the gravel base with rain water followed by deep freeze conditions might put the slab at risk!
If it would frost heave, what would it cost to repair/replace? What would it cost to protect it?
Your call.... your investment!
.....Iron Helix
Edited 9/17/2008 7:06 am by IronHelix
Here, concrete batch plants won't deliver concrete without air entrainment.
I had a neighbor in a place I once lived who built a basement and left it for a few winters. This was on a site that was basically composed of coarse granular fill naturally. I'm sure he was thinking that he was immune to frost heave, but he was wrong. That slab heaved badly(hove?).
Your under slab insulation will help, but if it is a wet fall and then very cold, frost can drive under the slab from the edges. So I would say you are taking some risk.
I'd insulate with some straw to be on the safe side.
Like most things in life .. it depends. I wouldn't call your concrete guys advice "nuts". His advice is simply more conservative than you may think as warranted, but his suggestions are designed to reduce the degree to which winter freezing is able to penetrate the ground in and around your slab. If your slab cracked over a winter/spring and he had not suggested a precautionary measure to protect the slab, you would probably be pissed that you had not been warned of such a possibility.
It's your call. Take whatever chances you are comfortable with. How wet is your fall weather going to be and how severe will winter be? Those are other things that will impact the chances of heaving due to frost.
If you have 18" of free draining material under the slab, and the rest of the surrounding site is very sandy or some other very free draining soil.. then your risk of frost heave is probably not high. You know your site probably better than the concrete guy so if you are comfortable with the soil conditions and are confident that water won't be accumulating in the soils around and under your slab, then perhaps you will be fine as far as frost goes. Do you notice the effects of frost action anywhere else around your house ?
Like someone else pointed out.. I would also be concerned with surface damage to the concrete if exposed for a couple winters/summers. However that is more of a cosmetic concern.
I really wonder why you would pour the slab if you were not planning on finishing the job for a year or two, but that's just me.
My only recomendation would be to add perimeter 2" foam insulation outside of the slab so you don't get any frost sliding under the edge. The exact width is dependent on your location. If the concrete has 28 days to cure before freezing conditions and is sealed I would not bother with any sort of covering. Roger
Kivi already said much of what I was thinking.
I notice that you are in Maine. ( thinks for filling out your profile to save asking)
We do have problems in Maine with cold, clay water - retaining soils, and drainage. so I was surprised to see that were using a monolithic pour for an addition. Is that how the rest of the house foundation is done?
It is common enopugh to do FPSF ( Frost protercted shallow foundation) pours for Maine, but it does need a lot of prep work on the soil and drainage. You mention that this base work is good, but not how good.
The other thing that though that makes a FPSF work is the perimeter insulation and the fact that a structure with heat is built over it. Lacking those, you still have a fairly strong possibility that the slab will heave and possible crack from the subsoil movement.
So your task to ensure that this does not happen is to keep water out of the soil, and add insulation.
Best way to do that would be to cover with plastic, then a foot of straw, then another plastic, and weight it all down well enough so the wind leaves it there all winter. One way to do that without comprsssing the straw too much is to drive stakes every four feet, and loop rope ( oughta be plenty of old float rope around, LOL) or clothesline back and forth to form a retaining web.
Alternatively, I have some old concrete insulation blankets I could sell you.
whether your odds of having problems are 5% or 50% vary a lot by all th evariable ground and water conditions, and what kind of winter we have. If this one gives us as much precipitation as the past year, that four feet of snow on things ought to insulate it just fine, but you can't count on Mother Nature to do your work for you.
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Thanks Piffin and other responders for your advice.
It looks like the safe direction to go here is to put foam board or straw over the slab, and cover all with a good tarp, and keep the water away from the gravel base material. I'll let you all know what this looks like next Spring!
BTW my current houseslab is the same design as the new slab...six years-no problems. The only difference is my house has 2 feet of insul extending out around the slab, my addition slab now has 4 feet of insul around it.
If I could get a structure over it this year, I would, but I am not going to go into debt on this little project so ... one affordable stage at a time.
sounds good.
Reason I asked about current house foundation is that I like same~same so that in extreme conditions, they move - or not- the same way in reaction to same conditions.
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