I’m going to be forming up a slab for a 24′ by 28′ garage this weekend. It’s going to have 8/12 pitch roof with attic trusses.
I’ve been told that an 8″ footing/grade beam around the perimieter of the slab (4″) will be enough. I’m leaning towards a 12″ grade beam, as this structure will not be heated and exposed to the ever prevailent frost of northern wisconsin. I’m also going to use rebar as reinforcement, placed 2′ on center throughout the slab and fiber cement. Is this overkill and am I wasting my money on these added items???
Thanks for any input.
upnorth8
Replies
18 inches by 12 inch footer with two # 5 in the footer. trust me increase your thickness to 6 inches. we are not talking that much more, labor be the same.
Are you saying 6 inches thick or 12 inches? YOu said 18 by 12 and then mention 6"??????
the footing 18 x 12 with a 6 inch slab, that way in a later date like ten or twenty years later you could put some weight on the slab is needed.
grade beam or foundation wall?....if grade beam I'm curious as to why? do you mean that you have footing pads with concrete piers supporting your grade beam? or do you have a continuous footing with wall atop? is this thing engineered?
I'm trying to understand what exactly you are doing. you mentioned forming for slab, but are asking about footing/grade beam. I notice that you are forming for a garage ...are you forming for what I 've heard referred to as an alaskan slab, where the slab and footing are poured as one. ( I once did one for a shed like that)
please clarify and I will give you my opinion based on clarification.
It is simply a grade beam. It's also considered a thickened edge slab. My house is built on the same slab, 12" by 12" grade beam around perimiter of slab, and a 4" slab for the rest of the house. It works great for my house, but it's heated all winter, so there's no frost under it.
My garage will have no heat in it, and will be exposed to heaving with frost in the ground.
Hope that makes sense.
upnorth8
I have worked on a number of foundations with grade beams, but are of a completely different nature. it has been where there has been lots of ups and downs over granite rock. we would have a pad(s) down in the "valley" with a pier supporting the grade beam that would span the "crazy" rocky terain. this is easier than having footings or bottom of foundation walls that go up and down at various odd compound angles. although we had those too, the grade beams went into areas with "big" valleys
I understand what you are doing now, I did one like it, just never thought of it as having a grade beam. but I suppose it is essentially a mono pour with grade beam incorperated into it.
how are you building it simply digging out the trenches along the perimiter and forming along the outside? It's easy enough to dig an extra 4" to make 12" if you are concered.
In Minnesota, slab floors for garages are pretty common, that's what I have. It's thickened around the edges, so (if I remember correctly) it's 4 inches in the middle and 12 inches around the perimeter. My garage is 24x40 and is unheated, and it's been holding up fine for nearly ten years now. There are a few minor cracks but nothing's moved around or heaved or anything.
I did a little looking around and found the city code requirements for Hopkins, MN which is a Minneapolis suburb: http://www.hopkinsmn.com/inspections/building/concreteslabs.html Scroll partway down the page.
your post confirms for me I think what he is doing. as mentioned I did a shed like that once. I'm not sure I understand how they work as far as frost heaves go. is the idea that with the thickened edges the whole thing would move as one, if it moves at all?
I've heard these slabs called alaskan slabs..I have no idea why. make more sense to me to call it an arizona slab or someplace where you wouldn't have to worry about a frost line...... but alaska... and you guys in minesota? aren't you like at 4'?
Yeah, the frost line is at least 4', some years it's down to six feet or more. I imagine the whole thing rides up and down on the frost if necessary, although I can't say I've ever noticed any difference from winter to summer.
The idea is that it is a floating slab. using 18" of stone gravel drain base compacted means there is little or no moisture contained in soils under it to freeze so there is little or no expansion stressing it upwards, but the most streess that will occour is at the edges. Th ephotos I just posted above are with a 2x10 form 9" deep edge x 18" wide, then 4" through center. All over 18" inch minus gravel compacted in and a drain line around perimeter to guide ground water away instead of leetting it perc under
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I believe what you may be talking about building is called a monolithic slab. Its basically a slab reinforced at critical points (usually around the edges and under load bearing walls or posts) to prevent excessive flexure. It basically "floats" on the soil as it is made to move upward and downward as the soils below expand and contract. One way to prevent to much movement is to provide plenty of drainage under and around the slab. Also, insulation may be added to prevent frost heave under the slab. I believe this only helps if the structure is going to be heated though.
Oak Ridge National Laboratories has a great free design handbook here:
http://www.ornl.gov/sci/roofs+walls/facts/index.html
You can find other design criteria on the internet if you look up "monlithic slab" or "pour".
I am also building a garage and considered a monlithic slab. However, I didn't plan to heat the structure so the bottom of the beams would have had to extend below the frost line anyway. It turned out to be cheaper to build a conventional "t" foundation with a poured footer, block, and a pour slab above.
Edited 4/18/2006 10:38 pm ET by semipro
Greetings fellow Northerner.
I like the rebar, not overkill in my mind for the slab. Are you also using it in the perimeter grade beam? I would suggest that it is even more important there, than in the slab. I would do both.
I have a question on Fiber Cement.
Is that a concrete mix reinforced with something like polypropylene purported and sometimes sold to strengthen the floor slab?
If so, my understanding is that once the concrete cures, the value of such a fiber is not that great. As I understand it, the fiber is supposed to reduce random shrinkage cracking during curing , and adds little to the ultimate strength of the cured slab. I would be interested in the pricing your concrete supplier applies to fiber reinforcement , if it is not expensive it can't hurt to add, I think.
It may just not do what you might have been told it can.
If shrinkage control is the issue, have you considered any admixtures such as a high range water reducer, sometimes called Super P (as in plasticizer in my neck of the woods)? As I understand it, increasing the proportion of mix water in a concrete mix generally weakens it and increases shrinkage cracking as it cures, Super P works by temporarily improving workability without the addition of any water. You can therefore use a dryer mix (less slump) and still work the stuff.
Less water to evaporate, ie: not needed for cement hydration, means less concrete shrinkage.
Less sand, larger stone (up to a limit), less water , more concrete covering or surrounding your rebar also reduce cracking, I've been advised.
Since the slab is to be unheated, it will likely exposed to freezing temperatures, No? If so, you might want to look into Air Entrainment to increase freeze/thaw durability and also increase workability.
If the intent is to finish the slab yourself, I might suggest that proper finishing techniques can have an important role in ultimate strength and durability. Over-working the concrete, finishing it too early/too late, allowing it to dry before curing, curing in a windy or hot or cold environment can affect results also, I've learned. I've spoken to some foundation guys who won't touch flat work due to the many difficulties in achieving top notch results.
My intent is not to scare you off of DIY, however, just to suggest that you can sometimes get more from a job by being better prepared with knowledge, without spending more money necessarily. You may want to spend more money once you understand what the money gets you in materials or professional help, though.
You can never go wrong by educating yourself, as you are doing now.
LOL
What exactly is Air Entrainment??
As far as the fiber cement goes, I don't fully understand it's characteristics. I do all of the forming of the slab and building of the garage, but I hire help to finish it. They have done it for years and recommend fiber cement in sidewalks, aprons, garage slabs. It only costs $7 extra per yard, so it's worth doing in my eyes.
However, the concrete was $90 a yard!!!!!!! Last summer I paid $70. That was a bit of a shock.
Thanks for the help.
upnorth8
entrainted air is a chemical additive thats added to concrete. Think of it as small air bubbles. That way during the freeze and thaw cycle, the concrete has a place to expand, so cracking and spalling is mimnum. Have you ever seen a old concrete bridge where the concrete seems to be falling off. that is spalling, and is cause by no entrained air.fiber here is $5 a yard, fiber does not and will not ever replace rebar. But it does replace welded wire, which i think is a joke.
Upnorth, Brownbagg's got your question answered re: air entrainment. As he pointed out, alternate freezing & thawing of concrete can be responsible for its complete destruction over time. I can't think of any flatwork exposed to freezing temperatures that I wouldn't spec it.
Ask if your Supplier's premium for Winter concrete is over yet. It may seem strange but they incur added costs to produce concrete during the winter & that may be bumping up your cost per yd3 compared to the last time you purchased. Perhaps a fuel surcharge has been added to correct for increased delivery costs also.
upnorth,
I've got a house, on a monolithic, grade beam and slab. In N.E. Wi.
We did a 12"x18" thickened edge, with a 6" slab, with rebar. Has functioned great, for 14 years now.
One note. I'm in a sandy gravel soil, which is very well draining.
I believe the state has a spec, for a frost protected shallow foundation. It is, from what I recall, 4" of foam insulation, extending out a few feet from the building edge, under the slab, and 2" under the rest. Seems to me that insulation cost, would almost cover the cost of a 4' frost wall.
Brudoggie
I'm with you on this one Brudoggie. Consider a Frost-protected shallow foundation it will perform as well as the monolithic pour with less material. According to NAHB, this system is approved state-wide in Wisconsin. You can find pertanent info. on the NAHB website, or do a search. Here in Michigan our winters are similar, if not the same as Wisc. Our standard here is overkill, 12"x42" concrete trench footings. Most local jurisdictions have enforced this, but I have seen exceptions. If you want the norm for your area talk to a builder. If you want to save money and get decent results your on the right track asking for all info.
Best of luck with your project.
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nice forms. That soil looks more like Carolina clay than Florida sand though.
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thats what it is. georgia red clay road. alabama red. florida red. its looks like clay but its really clasified as a sand base. water goes through it like a sieve.
Thanks for all of the great responses and pictures. They helped me a great deal!
Piffin, i'm going to do the rebar the same way you did in your picture. One question however, how do you get the concrete to the back of the slab (the concrete truck will only be able to access mine from the front). I can't see how a wheel barrow would work in this situation.
upnorth8
You can chute it about twenty feet from the truck with no trouble
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