Insulation required for frost protected shallow foundation 3

[struct6]

Hi Folks,

I am designing a foundation for a unheated storage unit. I am planning to use a slab on grade system with thickened edge. My question is

1. should we provide a continuous insulation layer under the entire slab or should we insulate only at the edge of the wall and extende it out a few feet? The document i am referring to design the insulation calls out for a continuous layer of insulation beneath the slab. However, I have not seen that in regular building practice. Have any of you provided continous insulation beneath the entire slab?

2. Is there any web site which has information on specifying the insulation ?

I have attached the document which I used to design the insulation for your reference.

Thanks all

 

[Splitrings]

ASCE 32-01 specifies ground insulation under the entire slab on grade. I am not familiar with the document you are using.

From personal experience, if you have a soil that is anywhere near being frost susceptible, I would insulate. I watched the center of my new garage floor the first winter heave upwards of 2-3". The outer perimeter of the slab near the frost walls did not heave..needless to say I now have a severely cracked and uneven floor.

 

[paddingtongreen]

The whole floor with only non frost susceptible fill above it. If you don't have that kind of fill, put the insulation under the concrete. If the frost gets to susceptible soil, it will probably heave.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

Split-
I have never personally seen the middle of a floor heave.
Was this a completely detached garage in a very cold area?
Just curious.

 

[Splitrings]

The garage is in Vermont. It has 5' frost walls and is attached. The inside of the wall was backfilled with clean sand. The insitu ML soil was left in the center of the garage (like an island in the center). So the ML soil heaved but the outer edges didn't as they were on sand. Floor now drains from the center towards the walls..

 

[ToadJones]

Ironically enough, I built a new house just a few years ago and back-filled the attached garage with sand. The sand was quite deep around the outsides...5 feet or so. I hogged out the middle about 18" with a skid steer and replaced it with sand and gravel. All fill was tamped.
All this was during the summer, mind you.
The day after my trusted concrete finisher poured the floor, but before he got there to cut it(he poured it very late in the day) the floor had already cracked pretty badly. I was very P%$$ed off and wanted to blame him for not cutting it immediately, but the cracks were more severe than T&S cracks.
The outside show now signs of settlement and I thought confined sand would be a good base.
To this day I cannot think of what would cause the cracks other than settlement.
I have since "routed out" the cracks with an angle grinder and filled them with a flexible Sikadur product. The cracks are now nice and dirty like the floor and aren't nearly as bad looking.
Sorry for the thread hi-jacking.

 

[Splitrings]

I have the same problem with the sand backfill where it is 5' or so deep near the wall. I don't know how much it settled but when I drop things like tires on the floor I can hear it echo or that hollow sound. When the slab gets replaced I will use a coarser material and use extra compactive effort.

Like you, the floor failure was and still is very frustrating. I guess nothing a few thousnad dollars won't fix.

Struct6, insulation is a lot cheaper!

 

[ToadJones]

I am also looking forward to replacing the floor. If i can bust it out myself with a hammer on a skid-steer, then the cost wont be too bad at all. But, my wife would probably think I was insane for breaking out the floor when she has probably never even noticed the cracks.

I really don't think the sand could have settled enough and quickly enough to cause my problem. I also can see the chalk line on the wall the finisher used and the floor hasn't settled. I also tamped the sand/fill in, in lifts.
Old school guys have been telling me for years and years that "confined sand is a great base"
Maybe it was junk concrete?

 

[mmillerpe]

As indicated by Splitrings get a copy of ASCE 32-01, it provides information regaring the amount and type of insulation required. You will need to insulation under the entire area, and fill above the insulation with non-frost susceptible fill. The theory, I believe, is that the soil below the insulation in kept from freezing by geothermal heat. If you only insluated below the edge of the wall, the interior soil could still freeze since the building is unheated, and therefore is not loosing heat to the ground below, which, obviously, is the case with heated structures.

 

[msquared48]

Toad:

Even "good" concrete will crack with a poor base.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BigH]

Where is the groundwater level? What is the nature of the soil beneath the building? For frost heave to happen, you need: 1) a frost susceptible material, 2)availability of water to migrate to the frost zone (doesn't happen in uniform fat clays or in coarse sand to gravel). We had foundations in northern Ontario (frost depth in the order of 3 m) but the groundwater level was down some 5 m (worse case) and the soil was a medium to coarse sand. We put the foundations at 1 m (similar to the structures founded a few decades earlier) - never heard of a problem.

 

[ToadJones]

Mike-
I guess I am fully aware that good concrete (And I am just talking a basic mix done properly) will crack on a bad base but I thought I had a good base.
The reason I suggested the "junk" concrete was the batch plant the finisher liked to use was notorious sending out 10 yd trucks with 8.5 - 9 yards on them and more than once I saw contractors tearing out two day old driveways after using these fools.
I guess what I was getting at is, can a mix be so bad that a 3/16" crack shows up in one day?

 

[Splitrings]

ToadJones-
Did you see the placement? I am wondering if they placed a 10" slump! Was the weather dry and windy?

8.5 yards of concrete plus another 1.5 yards of water and we have 10 yards of concrete for ToadJones' garage! It only cost us 8.5 yards worth and we get to pocket the rest!

You got to love the craftspeople we have today!!!

 

[ToadJones]

something was wrong.
I am actually still P!$$3d about it
I worked for years building houses and this one was of the worst I have seen, ON MY OWN HOUSE.
The funny part is, the guy is a great "finisher"...the floor looks like glass where ever it isnt cracked

Also, the cracks have never gotten any worse.
They are now filled perfectly smooth with a super flexible Sikadur product.

I still think it was a concrete issue.

 

[Splitrings]

Is it possible the edges of the slab bonded to the wall? That would cause restraint in the slab as it cured. That plus excessive mix water maybe enough to cause shrinkage cracks of the size you are talking?

 

[ToadJones]

no bond at wall....thats clear to see
I'm going with the 30" slump you suggested

 

[BAretired]

Toad and Splitrings,

Is it possible the movement you have was caused by swelling of the clay soil? Water can migrate upward by capillary action. The sand around the outside would not be susceptible to swelling.

BA

 

[ToadJones]

My floor cracked in July in Ohio

 

[BAretired]

In July, you say. That sounds like a swelling clay in the middle of the floor causing the floor to heave.

BA

 

[msquared48]

Toad:

The driver probably added water onsite in excess of at added at the batch plant, adding to the shrinkage seen.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask