Monolythic pour set on a footer?

[723Monolythic]

Hello all.

I've got a question about a concrete slab. I'm building a 16' x 24' workshop/garage. I'm currently working on the slab. Currently, I have a trench 2 ft wide dug 18 inch deep around the perimeter.

My contractor/neighbor wants to pour a footer approx. 8" thick, THEN later on form the edges and pour the rest of the slab in one pour. Essentially, pouring the stem walls and floor together...in some sort of a hybrid monolythic pour. Has anyone heard of pouring like this, or should I just stop my neighbor now, and form up for a conventional monolythic pour?

The reason my neighbor wanted to pour this way...He said he has done this before, and it allows the slab to "float" on the footers and reduce the chance of cracking.

By the way, I have contacted my local building inspector and they are no help. They keep asking me what my plans say, and I keep telling them my plans only say pour a concrete clab without any details. Then they ask me how my plans ever got approved (by THEM!!!). So, that is frustrating. However, they do encourage me to just do a full monolythic.

Thank you very much.

 

[csp]

Depends on where you are at and what your building codes are.

Where I live there are no monolithic pours of slabs/footers/stemwalls. Each gets its own pour and slabs are isolated from stem walls and/or footers.

 

[ConCretin]

It is a bit unconventional but I don't see any problem with placing the footing and slab separately.

In theory, your neighbor is correct. A haunched slab is 'keyed' in around the perimeter and more likely to crack because the slab can't slide inward. However I really don't think this provides enough of a benefit to make much difference. You're going to need control joints either way.

You mentioned a stem wall. I think it would easier to place the slab and footing monolithically and do the stem wall in a second placement (no hung form)

It's six one, half a dozen the other.

 

[723Monolythic]

by stemwall, I mean the thickend edge of the slab. I plan to have the slab floor level with the wall/edge.
I do plan on control joints, either way.

I just haven't hears od doing it the way he described. All the searches I have looked at on the net describe either a full monolythic pour, or pour footers, pour stem wall (or stack block), and them pour the floor.

His method does have the advantage of not having to deal with all the concrete at once, and being able to float the final form around on a level surface (the slightly wider footers).

I guess if I do it his way, and still have thickened edges, I'll essentially end up with the same thing as a full monolythic, with the exception of a cold joint at the footer junction?

Any other thoughts about this? I don't want to end up with a big mess.

Thanks.

 

[723Monolythic]

By the way, code in my area allows a monolythic pour, or footer, stem wall, then slab...either way.

 

[ConCretin]

Just food for thought. If you are paying any help by the hour it will be more expensive if you go with two forming and placing operations. You might also have more concrete waste and/or incur a small load charge.

It shouldn't be any more difficult to form the perimeter full height than it would be to build off the previous placement.

 

[tmars]

I am in the process of finishing a 30x40 garage and had the same questions as you. I did do the radiant heat in floor. Here is what I ended up doing, I had a trench dug 12inches wide and 2 feet deep all the way around as a foundation. I then put 2 inch high density insulation panels down. Formed up for the floor and poured a 5 inch floor with 3 1/2 inch rat wall. Although it seems odd to pour concrete foundaion and the put insulation down and pour on top of it, I will have a thermal break and so far only very small crack down saw joints....