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 )
Best Answer
Typical pre-cast pavers are about 2" thick, so that's a reasonable experiential datapoint. I've got a few of those in the 16" square size that have been through 11 winters without cracking in a pretty serious winter area.
You can get fibers to mix into your concrete to help reenforce it - 2" is too thin for using steel reenforcement. Ensuring that your mix is not too wet when placed and then kept quite wet for a month after it's set will help ensure you get all the strength your concrete is capable of giving.
You could also pour a 4" steel-reenforced slab on gravel base, and just do patterning/dying on the top surface for the flagstone "effect" - products are sold for this exact purpose (or crews to do so can be hired, preferably after reviewing jobs they have completed.)