Concrete arch on fireplace?

[bclark82]

I am designing an outdoor fireplace and trying to keep costs down. My question is if I wanted to make an arch in opening of fireplace out of concrete would that crack or could I add something to concrete to make it heat resistant and still strong. I have read stuff about using Perlite in the mix but have seen other fireplace designs where the face appears to be concrete. Any suggestions?

[dakzaag]

Concrete at the arch opening will crack and most likely fail. Portland cement that goes over 600 degrees F fails, the pictures you see of fireplaces being built of cement are actually refractory casting designed to withstand high temperatures. For the fireplace burn chamber and flue, you will need to line with refractory brick or purchase a kit that is designed for fires. You could use regular brick in the arch and refractory mortar, but if everything else is CMU or whatever then that won't look great either. Perlite will add insulation, but it is not structural so it is not adequate for an arch lintel. I suggest you do some more research on cast refractory or build an arch from firebrick.