cracking grout under coping

Hello all,

The sanded grout between my poured concrete coping and the tile below it is cracking and falling out...this is actually the second time it has happened. My pool builder said this isn't uncommon and sent someone out last time to fix it but all they did was replace the grout with the same thing that was originally put in there.

My questions are, is there any kind of flexible sanded grout that can be used...? If yes, can someone make a recommendation. I've been looking at epoxy & urethane grouts but wasn't sure if either of these are suitable for pool use.

Or, should be using some kind of caulking such as silicone or a polysulfide...? If yes, does anyone know if there are sanded versions of this?

Any guidance would be greatly appreciated.

Thanks,

Adam