The size of the expansion joints between coping and the decking concrete

My flatwork contractor used deck-o-foam between pool shell and flatwork. 1/4” thick foam roll that can be easily removed post cure for the deck-o-seal or left in place if you are running something else on top.
 
A general rule for concrete is it shouldn't exceed 15 feet in any direction without a joint.

I also seem to see most of those joints at 3/4 of an inch but I have never understood why. 1/4 inch seems aesthetically better and still function fine
 
A general rule for concrete is it shouldn't exceed 15 feet in any direction without a joint.

I also seem to see most of those joints at 3/4 of an inch but I have never understood why. 1/4 inch seems aesthetically better and still function fine

3/4” is typical because the 1x4’s they use to form up the sections are 3/4” thick. Between the pool coping and flatwork though, putting a 1x4 there seems odd. At most, I’d put a 1/2” strip of dow foam board that could be easily removed later.
 
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