Pouring garage floor 80x40

[aventino68]

Decades back when I built a garage on a slightly sloped lot we boxed the walls then poured then removed boxing and put in and compacted the fill. Then put up boxing for floor, put down reinforcing and poured again to do the floor.

So now I’m wondering if I dig the foundation’s accurately and only box the outside can I pour the foundations and floor in one pour? Can’t remember if there was a reason why we didn’t before.

 

[The Cobbler]

you're basically talking a monolithic pad pour. they are limited to about 550 sq ft in Ontario I believe

 

[matt_i]

Don't know Ontario but its common to the west in Michigan to earth-form foundations for single story garage-y structures.

Another alternative might be a frost-protected shallow foundation.

 

[aventino68]

you're basically talking a monolithic pad pour. they are limited to about 550 sq ft in Ontario I believe

That’s what I was wondering, I dig my 4ft foundations, only box the outside and then pour the lot in one go.

 

[ConCretin]

It's possible to construct a mono-slab as you describe but there are limits to the practicality of this approach. Basically, the perimeter of your slab would extend down to undisturbed soil with a structural fill to support the interior of the slab. All the usual caveats about removing topsoil, organics, etc still apply.

Achieving proper compaction in the structural fill would require long slopes in the deeper areas, which would take a lot of concrete to fill back in. You could construct a larger fill than you need and then re-excavate to shape steeper slopes but that takes time and money too.

In addition, the pressure applied to formwork by concrete is a factor of height. (Pressure per Sf equals height x 150) In other words, it gets more labor intensive to build a nice, straight perimeter form the deeper the concrete gets. Since there is no way to use ties, it's all got to be braced and there's probably not much out there to brace to.

All and all, I don't think anything over a foot or two in grade change would be very practical. You'd probably be better off with foundation/retaining walls for anything larger.

 

[aventino68]

Lots of great info, i read LLWillysfan links. If I'm doing my foundation walls first do I run them level with the top of the floor and pour the floor inside or do I box and pour the floor on top of the foundation walls?

 

[RAINBURN7]

Here is a detail I recently did for Central NY.

 

[RAINBURN7]

I do heed to LLWillysfan's advice also.

 

[ConCretin]

If I'm doing my foundation walls first do I run them level with the top of the floor and pour the floor inside or do I box and pour the floor on top of the foundation walls?

For a floor that size, I'd place the walls to slab level and pour the slab to the walls. It's better if the slab is free to move rather then be restrained by the perimeter walls.

 

[aventino68]

For a floor that size, I'd place the walls to slab level and pour the slab to the walls. It's better if the slab is free to move rather then be restrained by the perimeter walls.

Sounds good. I then bend my reinforcing rods over from my walls and tie them in with my floor reinforcing mesh? There's a multisport court beside it 120 x 60, would I do that the same? Im not post tensioning it, just using reinforcing mesh.

 

[ConCretin]

Sounds good. I then bend my reinforcing rods over from my walls and tie them in with my floor reinforcing mesh? There's a multisport court beside it 120 x 60, would I do that the same? Im not post tensioning it, just using reinforcing mesh.

I wouldn't tie the slab to the walls at all, in fact I'd install expansion material to isolate them from one another. The wall needs to support building loads and lateral loads from the soils under the slab. The slab is supported by the soil underneath. You want each of these elements to move independently.

You especially want to avoid restraining the slab from contracting inward as it shrinks, which could lead to cracking. Control joints might relieve the tension but if not, you'll have visible random cracks.

Just make sure your wall can do it's job and construct a good solid base under your slab to provide uniform support to your slab.

 

[ODIS]

Use of pea gravel under the slab for the base material? I'm told that material will provide excellent support for a slab.

 

[Beamer]

Decades back when I built a garage on a slightly sloped lot we boxed the walls then poured then removed boxing and put in and compacted the fill. Then put up boxing for floor, put down reinforcing and poured again to do the floor.

So now I’m wondering if I dig the foundation’s accurately and only box the outside can I pour the foundations and floor in one pour? Can’t remember if there was a reason why we didn’t before.

Pretty tough setting wall or haunch forms in the dirt and expecting accuracy.
Another consideration is I'd expect uncontrolled cracking parallel to the haunch/wall length then the concrete goes from thick (wall) to thin (slab). The beauty of a separate wall and slab pour is the nice straight clean joint where they meet.

 

[aventino68]

Pretty tough setting wall or haunch forms in the dirt and expecting accuracy.
Another consideration is I'd expect uncontrolled cracking parallel to the haunch/wall length then the concrete goes from thick (wall) to thin (slab). The beauty of a separate wall and slab pour is the nice straight clean joint where they meet.

I agree. And I can insulate between the floor and wall as well as allowing the floor to move about. Will try to get both sets of walls (Garage/Court) poured together by crew doing the house.