Unheated Building with Slab-on-Grade

[JAE]

Just taking a poll of everyone on what to do for a swimming pool bathhouse. This is a stand-alone building with masonry load-bearing walls on conventional footings and an interior slab-on-grade.

The issue is that in the winter, the building is not used and is open to the weather - unheated. It has a roof over everything, but no insulation.

We are concerned that the slab-on-grade will possibly heave and have wrestled with using a flowable fill under the slab for some depth, or using a uniformly graded granular fill for some depth to minimize heave.

Is this a valid concern and what options could be used?

 

[jike]

You can also put 2" extruded insulation (no bead board) below the floor and extend it out beyond the building perimeter 4'-0". Make sure that it slopes away from the building so that water trapped by the insulation will drain away.

I also like using 4'-0" of non-frost susceptible fill. I would also isolate the fill from the surrounding soil with a geotextile to keep it from becoming contaminated. This does not mean it won't freeze, in fact I have read that it freezes quicker than other soils. The thing is that it is designed not to heave.

I assume that there are toilets and this is why any amount of heave is not acceptable because it can easily break pipes or crack fixtures. I also would not want floor mounted urinals/water closets as they would be more susceptible to cracking then wall mounted.

I hope this helps!

 

[jb66]

I think that is better to use granular fill which depth depends on quality of subsoil and the slab reinforce by ribs (my experience). If the internal construction of swimming pool is sensitive to heaving it is necessary to solve very well dilatation between floor slab and the walls.
In extremal cause is possible to use thermal insulation of soil surface (for instance styrofoam for geotechnical application).

jb66

 

[joemarch]

Instead of providing a slab on grade maybe a better option would be to use a structural slab. Just my two cents. Maybe its not an option.

 

[JAE]

Thanks,
jike: I guess I wonder about using insulation - there isn't any head except from the ground below; I've seen it done for heated structures, where you have a close, direct, source of heat.

jb66: When you say "ribs" do you mean thickened slab strips, say about 12" deep and 12" wide or so? Please elaborate.

joemarch: You are right - a structural slab is way too expensive for this application.

 

[jike]

JAE:

I assume this is a small stand alone bathhouse and it does not contain a swimming pool as jb66 mentioned.

 

[JAE]

Correct - the pool is beyond this structure and separate. The bathhouse has a central area for office/customer counter, etc. and each side are the male/female locker/shower/changing areas.

 

[haynewp]

What is the frost depth where it is going to be located? I asked the building department in an area with about a 12" frost depth about how often they see frost protected slabs for an unheated buildings and they didn't know what the hell I was talking about.

Here is some info I sent to someone else that asked me about this before, maybe it will be of some assistance.

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

http://www.pavement.com/PavTech/Tech/Fundamentals/fundfrost.html

 

[JAE]

Midwest USA area with footings usually placed 42" below grade in heated buildings.

 

[jb66]

JAE ... yes,
I thought thickened slab strips, sorry for my english. These strips could be in spacing cca 8´x 8´, 12´´ deep
and 6´´ wide. If the slab would be reinforced with mat near of upper surface and the "ribs" as well (it is enough one strip of the mat vertical in the middle ), so I suppose that this may be solution.

 

[jike]

I dug up a reference to a report called "Design Guide for Frost Protected Shallow Foundations" prepared for the HUD by NAHB Research Center, June 1994. It covers both heated and unheated buildings and uses a combination of non-frost susceptible fill and below grade insulation.

 

[jike]

Please note that insulation requirements vary depending upon the mean annual temperature, the air freeze index and whether the building is heated or unheated.

 

[DeSignO]

This situation is common in Colorado where you have expansive soils and extreme temperature changes during winter months (8 months out of the year) One way to handle this is by using a collapsible spacer between the slab and the expansive soil.

Example of a recommendation for this is as follows:
"Expansive soils require special foundations. The old standard slab-on-grade won't do. Instead, pier and beam foundations are the new standard in Colorado. Basically, concrete pillars are set down onto bedrock. Then concrete beams are placed atop the piers. A thick cardboard spacer can be placed under the beams so that if the soil heaves, the spacer will be crushed but the floor won't move. The walls, especially in finished basements, are separated by 2-4 inches from the concrete slab with small spikes for the same reason."

If money is the main issue (when is it not) and you cannot sell a floor like this to the client, then I would put a disclaimer on your work that covers your butt.
I have not read this Tech Manual, but you may get some useful info that can be applied to your scenario.

http://www.usace.army.mil/inet/usace-docs/armytm/tm5-818-7/basdoc.pdf

I would maybe reach out to a Geotechnical professional in the area, as they know the soils and specify specific slabs and pavement all the time. I would be careful with this one especially because you are using load bearing CMU walls, which will crack badly if you get some differential settlement or heaving. This will be a big visual to the owner that something is amiss in addition to slab issues.

Good Luck ! Simon

 

[CTSeng]

This structure does have a roof over it, right?

And it will have footings below the recommended frost depth, right?

Unless the soil is suspect or there will be a lot of vehicle traffic on the slab, why do anything? 8 feet of air will provide sufficient insulation to prevent frost heave. It is rare for frost to penetrate deep into soil unless there is vehicle traffic on it which is obvious on many northern roads.

 

[JAE]

It has a roof, but the building is not "closed" in that there are gaps throughout where outside air can come into the space. This is located in the midwest (Iowa area) and it gets pretty cold - -10 deg. F sometimes. The fear isn't in the foundations, just the interior slab-on-grade that may have CMU partitions sitting on thickened edges.

Also, no expansive clay issues. An no, we will not set the dang thing on piers - way too expensive for this type of project.

I just get the feeling that a good layer of drainage gravel would do the trick, with minimal conditions where the slab is supporting anything...ie. free to heave a bit...but not too much.

 

[flamby]

If you burry a few hollow pipes beneath the gravel, they may add magic to the trick.

BTW, these are called French Drains. French are always smart people.

Ciao.

 

[FalsePrecision]

Flamby,

Ohh....thats why they say I am half-witted <grin>

 

[haynewp]

Do you think it would be worth putting the partitions on independent footings instead of a thickened slab? If the slab is floating it won't crack around the wall bases if it heaves.

 

[JAE]

Yes - as many as possible will be on true foundation walls, but there are so many (little wing walls on either side of urinals, for example) that I can't do it on all of them. Those little miscellaneous walls will be designed with EJ's to avoid tearing at the main walls.

But with, say, 24" of granular fill under the slab, shouldn't that avoid most frost heave problems.

 

[haynewp]

I doh't know. I would think most.

The whole issue seems like something a geotech should have input on, but I figure there must not be one for this project.