I've seen some videos showing how to make "flagstones" out of concrete. The general process is to create a moulding on the ground (usually the grass is dug out so the dirt is exposed), using some sort of flexible strip of plastic or vinyl. Then, concrete is poured into this mold. In most of the examples I've seen, the stones look to be no more than 2 inches thick, but they can be 2-3 feet across.
I've been thinking of doing this to make a patio (though I'd probably put a layer of gravel below the concrete), but I'm concerned that these stones might be too thin and crack after not much use. Also, we get freezing weather in the winter, and I'm concerned that thin stones might crack in the cold weather.
How thick should these concrete "flagstones" be to ensure they don't crack?
( one example of the technique I'm referring to: https://www.youtube.com/watch?v=MNaVzbwuATI )