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Flowable Fill and Frost Heave

Flowable Fill and Frost Heave

Flowable Fill and Frost Heave

I've got a swimming pool project where the bathhouse is a concrete masonry structure with wood trusses for the roof.  There is a maze of masonry walls within the bathhouse - partitions between toilets, small closets, etc.  

The architect has told me that the bathhouse may be unheated, and this is located in the midwest with cold winters where the concrete slab-on-grade would potentially heave with freezing of the subgrade.

Has anyone used flowable fill (CLSM) with a low cement content for this purpose?  I'm thinking that we would overexcavate the footprint of the building and then backfill with the flowable fill.  Then the contractor could excavate through the flowable fill and construct the main building footings as flowable fill is diggable if the cement content is kept low.  The interior "maze" of walls could then be placed on the slab.


RE: Flowable Fill and Frost Heave

What will prevent the fill from heaving?

RE: Flowable Fill and Frost Heave

I'd be even more worried about all the plumbing lines freezing during the winter.  How perfect is the draining of the lines and fixtures.  It seems pretty short sighted to me not to heat it, even if you heat the building only to 40 degrees during the winter, it keeps the lines from freezing an the slab from heaving.

That said, I would agree with Haynewp that insulating under the slab and around the perimeter would help, although not guarrantee that the slab would not heave.  Put a clean, course drainage layer on the supgrade, then a minimum of 2" of ridgid styrofoam, then the vapor barrier, then pour the floor slab.  Do not tie it to the exterior foundation walls, but use an expansion joint there to allow some movement.  Don't forget to install control joints in the slab at about 12'-16' centers each way.

RE: Flowable Fill and Frost Heave

Frost heave can only occur when all of the following three conditions are present: 1) the soil is frost susceptible (large silt fraction), 2) sufficient moisture is available (soil is above approximately 80 percent saturation), and 3) sub-freezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of frost damage. Insulation as required in this design guide will prevent underlying soil from freezing (an inch of polystyrene insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on average). The use of insulation is particularly effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat into the foundation soil -- not out through the vertical face of the foundation wall. Second, horizontal insulation projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage. Finally, because of the insulation, the frost line will rise as it approaches the foundation. Since frost heave forces act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards.


RE: Flowable Fill and Frost Heave

Haynewp has hit the nail on the head. I have engineered many an outdoor equipment foundation by using the principle of removing the supply of water by specifying a subbase of granular material. The Canadian Building Digest has an excellant article on this subject.


RE: Flowable Fill and Frost Heave

Thanks so much for the replies....I'll check back in on what we end up doing.

RE: Flowable Fill and Frost Heave

maybe consider a pile supported structural slab cast on bio-degradable void form

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