Concrete Slab Input needed

[tmars]

I have been reading lots about slab on grade and wanted to see if anyone had some input. Here goes, I am in central Illinois and am planning a 30x40 garage with 9 foot walls. I am planning on 5 or 6 inch 4000 or 4500 psi concrete. I am going to be putting in 5/8 tubing for radiant heat. Not sure if I should use 6x6x6guage mesh or 3/8 rebar with 2 foot squares. Am also not sure about fibers in concrete. I had originally thought about trench foundation and 4 inch slab. I was told it was over kill and 5 or 6 inch slab would be fine. Any thoughts would be much appreciated...Thanks

 

[walrus]

My experience in Maine, never less than #4 rebar 1 foot on center, 4000 psi concrete.

 

[mobetta]

generally you go w/ steel reinforcing or fiber. usually the fibers will sow up thru the finish.

what are you driving on it?

I would say 5" is overkill for autos/ light trucks. 6" is good for heavier stuff.
add more portland = higher psi for a little more money, to an extent.

 

[neverenoughtools]

I poured a small slab a few years ago & I used 1/2" rebar laid out in a 16" grid pattern. The concrete supplier that I used advised me the type , thickness & psi strength of the concrete based on what I was using it for.

 

[tmars]

I have a couple small utility tractors, ford superduty, mustang and a boat. The cost of the concrete to make it thicker is like $400 per inch, so not that big of deal. Thanks for your thoughts

 

[stltikn]

I'm a believer in stem walls. Keeps the framing out of the water, inside and outside of the building. Cold joint between the slab and the wall allows for expansion and contraction due to temperature changes. Also, in case of settling/heaving of either slab or wall not all is affected.

 

[ducati]

On a 30'x40' you will have to do a perimeter wall if you have any building codes. I would strongly suggest it even if you don't. You live in the midwest and your slab will crack all over the place if you don't. A floating slab works great for say a 20'x20' at the very most.

 

[walrus]

I've got a 30 by 34 floating slab, poured 18 months ago, not a crack in it. 6 inch, #4 rebar, 4000psi, 1 row of block and then 12 foot 2 by 6 walls. I have a good and I mean good well drained gravel pad thats ranges from 2 ft thick to 4 ft thick all above grade. Frost is not an issue and neither was any compaction after the fact. The gravel had been there for years. Drainage and proper base is a key to a floating slab. Take it from me if you don't have those 2 things you'll regret doing a floating slab in a cold climate.

 

[nmanitou]

To address your reinforcement steel questions, wire mesh and fiber mesh are essentially used to control temperature cracking - cracking that occurs as concrete cures. Fiber is useful for early cracking, wire mesh can help hold concrete together so that when temperature cracking occurs,the crack doesn't propagate and is held tightly together. Usually you would use one or the other, but not both.

Steel reinforcing bars are intended to give concrete tensile strength-probably a good idea for a "floating" slab, but it's really dictated by the loads placed on the slab and more importantly the underlying support strength of the sand or gravel base.

+1 on the minimum #4 rebar (1/2") as typical. A good combo for your situation might be rebar for tensile strength with fiber mesh for temperature cracking. Finer can be an issue if you require a smooth top finish. It tends to provide a rougher texture.

 

[jayoldschool]

I've got a 30 by 34 floating slab, poured 18 months ago, not a crack in it

Does that meet code in Maine? I would be surprised if it does. Were your plans approved? We can't go that big on floaters here, and our winters are the same as yours. (Ottawa, ON, Canada, btw).

 

[wssix99]

To address your reinforcement steel questions, wire mesh and fiber mesh are essentially used to control temperature cracking - cracking that occurs as concrete cures. Fiber is useful for early cracking, wire mesh can help hold concrete together so that when temperature cracking occurs,the crack doesn't propagate and is held tightly together. Usually you would use one or the other, but not both.

Steel reinforcing bars are intended to give concrete tensile strength-probably a good idea for a "floating" slab, but it's really dictated by the loads placed on the slab and more importantly the underlying support strength of the sand or gravel base.

+1 on the minimum #4 rebar (1/2") as typical. A good combo for your situation might be rebar for tensile strength with fiber mesh for temperature cracking. Finer can be an issue if you require a smooth top finish. It tends to provide a rougher texture.

+1 this is the answer.

Having rebar there for strength is probably overkill and it will work best for you to limit cracking. Another way to have a stronger slab and less cracking is to use a dry mix with a low slump in the concrete. Your finisher and supplier should be able to work with you on this.

 

[walrus]

Does that meet code in Maine? I would be surprised if it does. Were your plans approved? We can't go that big on floaters here, and our winters are the same as yours. (Ottawa, ON, Canada, btw).

What code? We had no State code until a few months ago. They finally passed a statewide code but my town votes on it this coming weekend, bet it doesn't pass. I see no advantage to spending several thousand more dollars for a footer, wall, slab foundation for a shop. Absolutely no advantage in it all if you understand how to do a floating slab that works. Gravel base(in feet not inches), well drained and compacted, rebar and 4000psi concrete. It works for me

 

[tdkkart]

My concrete guy said the only reason for putting mesh in concrete is if you have bunch of it laying around you needed to hide..........

Think about it, what's the sheer strength of the wire?? What's the shear strength of the concrete?? If there's enough force to break the concrete the wire's not gonna help much.

BTW, code here says max. 1250sq/ft for a floating slab, probably with good reason.

 

[aabirdman]

A floating slab is fine for just that, a slab. Footings are there to support the walls and roof weight. People do strange things over time and can overload the perimeter in short order. I would not built any kind of building without a foot And yes I'am a contractor

 

[3x9RT/SE]

A floating slab is fine for just that, a slab. Footings are there to support the walls and roof weight. People do strange things over time and can overload the perimeter in short order. I would not built any kind of building without a foot And yes I'am a contractor

Could'nt agree more!!!!! I pretty much overkill just about anything I build,as a "just to make sure"!!!
My addition I built was a 30'x44',two story building,garage on the bottom w/living space above.Therefore I did the radiant heat in the garage slab to take the load off the upstairs living space during heating season,and knowing this ,and just for that,I made my slab be 6'' thick to retain the heat longer ,ntm,putting in a two post lift.I used 6''x6'' 10ga. 4'x8' sheets on top of 1/4'' bubble wrap insulation.My loops are approx. 250' each and have 5 of them.They are run up and down 12'' on center.I keep my garage at 64 degrees,and even at that,it only calls just about every 2-3 hours,and only needs to run for about 5 minutes to satisfy the t-stat.So glad I did it,it works freaking great!!!!!!
So basically ,my slab never gets to those temps of freezing or even making drastic temp swings.

 

[3x9RT/SE]

And yes,10'' wide foundation frost walls that are 48'' deep,sitting on a 12'' deep by 24'' wide footing.

 

[walrus]

A floating slab is fine for just that, a slab. Footings are there to support the walls and roof weight. People do strange things over time and can overload the perimeter in short order. I would not built any kind of building without a foot And yes I'am a contractor

You should build that way, its your living and reputation. Selling a product to someone and standing beside it is far different than building something for yourself.

A buddy of mine is a Civil Engineer, he gets asked how much foam do I need under a patio in Maine to keep it from heaving. He says am I responsible or are you? He has to design to a 100 year occurrence as he doesn't want any kind of failure on his watch. Me, I can cheat a little and hope I don't see that 100 year incident.

 

[pcmeiners]

Tell the concrete and driver not to "water down" the concrete mix under any circumstance (yet to see a contractor in my neck of the woods not try), look into plasticizer to allow workability, stand guard at truck. Fiber, if you/contractor are not willing to cure the concrete properly (constant wet over the curing period), it may help for the initial cure; overall fiber is an excuse for not doing it right, As to mesh or rebar, I would, as very few site have a properly compacted base/saw cuts/(expansion/contraction measures) . I consider 5" min slab thickness, plus footer and foundation wall.

Think this is a good idea frost protection, saves on footing/foundation, gravel and compaction..

http://www.cs.arizona.edu/people/jcropper/desguide.pdf

Vapor barrier...
http://www.buildingscience.com/docu...floor-problems?topic=resources/flooring-probs

Innovative Ways to Reinforce Slabs-On-Ground....

http://www.wirereinforcementinstitute.org/pages/pubs/pdf/TF 705-R-03.pdf

Expansion, contraction joints...

http://www.cement.org/tech/faq_joints.asp

 

[Falcon67]

Everything depends on where you are and the local codes. I just watched a slab done next door for a 3200ish sq/ft house. This is slab on grade country. The footings under the perimeter and the slab beams were maybe 6" deep below grade and 12" wide. 1/2" rebar run in the beams and perimeter, 3/8 bar on 18" center across the foundation. It stands about 8~10" tops above grade in most places. 7:30am the first truck shows up, 90 yards on the ground in 3 hours, slab all worked and forms gone by 3 PM. Contractor has been building around here for a long time, no issues expected. I didn't ask but I doubt there is any fiber in the mix.

What I learned from watching the whole thing is that unless it's fairly small, you better have a crew of people that know what to do when the trucks show up or you will be screwed trying to DIY. Everything up to that truck showing up is not that complicated.

 

[tmars]

Well thanks for all the input, I have a guy getting a quote to do footings, foundation, and 6 inch slab. I have talked to several people and the general concenus is to use fiber in concrete with no steel. Only steel in footings and foundation. I will have to see what quote looks like.....thanks again

 

[Daniel Dudley]

My general rule on steel is that you cannot have too much, only not enough. I am a big fan of zip ties to secure the radiant to the rebar.

BTW, for those who nay say mesh, you should try jack hammering concrete with mesh in it. it adds considerable strength to concrete if it is placed in the middle of the pour.

 

[Justanoldguy]

BTW, for those who nay say mesh, you should try jack hammering concrete with mesh in it. it adds considerable strength to concrete if it is placed in the middle of the pour.

Actually, having it in the middle does nothing.
Because the slab will usually have a load on it from the top, that places the top of the slab in compression and the bottom of the slab in tension. The centre will be neutral. The steel needs to be nearer the point of tension as this is the weak point of concrete. Put the mesh nearer the bottom of the slab. In saying that, it needs 1 1/2 inches of cover.

 

[Kevin54]

Rebar. x10

 

[walrus]

When people say mesh, they do make mesh other than #10wire, they make stuff thick enough so you aren't bending or cutting it with a pair of pliers. I've used stuff that was 3/8" wire before, not sure how much thicker they make but...

 

[ddawg16]

Lots of good info above.....

I can't say enough about stem walls.....my garage floor has both stem walls and a 2' deep by 16" footing.....it and the slab were poured all at once. I like having the walls about 8" above grade....I don't have to worry about water getting in....and the termites do not like climbing up bare concrete to get to wood.

One last thing.....just remember....if it's not enough, you can't go back and add a little. As you pointed out....the extra cost/inch is not much in the grand scheme of things....

If you look at the link in my sig, some of the pics show the foundation being poured.

 

[wssix99]

Actually, having it in the middle does nothing.
Because the slab will usually have a load on it from the top, that places the top of the slab in compression and the bottom of the slab in tension. The centre will be neutral. The steel needs to be nearer the point of tension as this is the weak point of concrete. Put the mesh nearer the bottom of the slab. In saying that, it needs 1 1/2 inches of cover.

Slabs on grade work differently than suspended floors. For a typical garage floor where cars are driven: (Industrial floors or floors with very heavy industrial loads behave differently and have a different set of guidelines/needs.)

Slabs on grade need to have the reinforcement in the middle. This acts to keep shrinkage and cracking in check. The solid ground under the slab causes the structure to behave differently and tension reinforcement is not needed. (If you think of the floor as a composite beam, the ground underneath the concrete is part of that system and the compression strength of the concrete is the thing that is needed at the top levels.) Good reinforcement in the middle of the floor will keep it looking nice for a very long time. (This is a good technical article on the specifics of all this: http://www.wirereinforcementinstitute.org/pages/pubs/pdf/TF 705-R-03.pdf but going through all these calculations is probably overkill for a garage.)

That being said... putting the reinforcement in the middle is not an easy thing to do. It must be supported from the bottom and suspended in the middle. Shoddy contractor work here is common. You'll see a lot of contractors just laying mesh on the ground or laying it down at half-pour. In these cases, the mesh ends up sinking to the very bottom where it does absolutely nothing.


Its also possible to overdo the steel. If too much reinforcing steel is installed, then the concrete will be weaker in compression. Its important to follow local code regulations and not go beyond. For the strongest system, you want just enough reinforcing to control cracking and nothing more.