Monolithic slabs

[Arky217]

A question on monolithic slabs:

When pouring the footing and slab all together, I understand the bottom of the footing
needs to be below the frost level. But what keeps the frost upheaval from pushing up
on the slab area in the case of an unheated structure such as an unheated garage ?

On an unheated slab, it would seem like the optimum way would be to pour the footing first and
then pour the slab so the slab could 'float' with the frost upheaval
and not affect the footing.

Yet, many unheated garages are poured slab and footing together.
What am I missing here ?

Arky

[NathanS]

http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-26.html

That is a really good article published in 1962 on frost heave.

The three basic requirements for frost heaving are: 1) a freezing plane in the soil; 2) a fine grain soil through which moisture can move; and 3) a supply of water. If any one of these factors can be controlled, frost heaving can be prevented. Since it is seldom economically possible to control soil temperature, frost heaving is usually prevented by replacing the fine grain soil with a coarse granular material. Soil moisture can also be controlled by careful attention to drainage, so that the extent of frost heaving is greatly reduced.

Where a detached building is located on a concrete slab on grade, protection will be provided by placing the slab on a mat of coarse granular material, which will act as a buffer against any movement of the soil under the mat. A mat 12 to 18 in. in thickness is usually adequate.

The packed gravel prevents #2 from happening, which is water movement through capillarity. Add perimeter drainage and you should be good to go.

[UK4X4]

Mines not a slab, but its a monolithic design with rim beams and cross beams

we used crushed rock under ours to move water away from the foundation, we looked at gravel as its cheaper but being roundish it does not pack together as well...our issue was spring runoff along with frost lift.

we are 3 years in with no movement noted....yet