ArtMetal

I haven't made a concrete pitch bowl, (I use cast iron ones), but I have made a number of similar objects from concrete. Based upon my experiences I'll make the following suggestions:

1. Don't use the ready-mixed stuff in the bags, such as Sakrete. The aggregate in that is too large for what you're doing, unless you're planning a pitch bowl the size of a bath tub. Instead, mix 1 part Portland Cement with 2 parts sharp sand and 3 parts pea-sized aggregate. Mix this together thoroughly in the dry state before adding any water.

2. Add to the above mix some shredded polyester fiber reinforcing. You can get this at most places that sell supplies for concrete and plastering work. It is made from recycled soda bottles and adds tremendous tensile strength to thin sections of concrete. Without it, concrete has little tensile strength, that is, resistance to breaking from stress.

3. Use just enough water to make the mix plastic, but not at all "loose." The more water you use, the longer it takes to set and the weaker it ultimately is. Additional working to get it shaped and compacted will be better than more water.

4. Your concrete bowl will need to set for 30 days to achieve maximum strength. After two weeks it will have about 75% of full strength, but don't rush it. When you use the pitch bowl, the forces pushing it out from the inside can be considerable and that is the direction it is weakest. Be patient.

5. After the bowl has set completely, you can sand and/or wax the outside to achieve a surface finish that will be comfortable to move in a support ring. I use old solid rubber tires from scrapped toy wagon wheels and such for support donuts.

I hope this is some help to you. Let us know how it comes out.

Rich