Turn down the exterior edges of the slab so that they extend below the frost line is the best way when you are in an area with a relatively shallow frost depth.
Unfortunately, I'm in an area with like a 5' deep frost depth and only need a small 10'x10' pad for a day tank. May look into replacing some of the soil with some gravel fill (less susceptible to frost heave) and may add some sort of insulation directly under the slab.
In conversations with a lot of geotechs lately, it sounds like using an aggregate base to an elevation below frost to break the capillary action of the groundwater is the most effective way to prevent frost heaving.
The turn down at the perimeter helps, but the entire field of concrete only has a 6" barrier against the cold and will likely still freeze and heave.
To float a slab is to allow it to ride along with frost heave. Depending on your soil profile, you could also suspend the slab on four small helical piles.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
I don't agree with Mike55. Constructing a "frost wall" does nothing for the area under the slab if that zone goes below 32 degrees F. Concrete is a good conductor of heat out of the area under the slab. However, if the contents of the tank are kept above freezing, then you may have no problem, since it keeps the ground below the slab warm. Use flexible connections and forget about the potential frost action.
If water cannot get under the slab area, freezing of any water will not occur, regardless of freezing temperatures. If the water table elevation is not controlled though, all bets are off.
I like the idea of floating the entire slab with flexible connections. However if you ever observed frost heaved ground you will notice that it is usually uneven to some degree or another. To prevent the differential heaving from cracking the slab what sort of reinforcing would be suggested or is this even a factor?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
For a ten foot square slab, I would not be too worried about differential heaving unless there are known anomalies underlying the slab such as previous excavations. If it's a concern, a top and bottom mat of reinforcement could be considered.
