Hope I am in the right channel for this question...
So I have a finshied basement (carpet with underpadding on concrete and drywalled ceiling) that I want to put a new treadmill in. With the height of the raised treadmill and myself running on it, it will be just and too uncomfortable to run. So I have decided to cut a rectangular area in my concrete slab floor to allow me to lower the treadmill into. From reading many posts I have decided to rent a 12inch electric Hilti concrete saw with attached vacuum from my local Home Depot to make clean cuts and then slegdehammer it into manageable pieces to remove. Then I will have to dig 17inches to allow for my 10inch lowered finished trough, 4inches of concrete slab and 2-3inch rock backfill. My big question is how to frame to pour the concrete? Can it be done in one shot or do I have to pour the lowered trough flat bottom first and then once it's dried, make a rectangular frame to create my perimeter walls between the existing concrete slab floor and the new lowered trough walls? I have read advise that I will need to rough up the clean cut lines for the new concrete to adhere well to the existing (cold chisel and hammer or grind channels?) Is drilling holes in the side of the existing slab to insert rebar dowels to connect the old and new concrete a must in this situation, the treadmill weights a couple of hundred pounds plus me on it?
Any advice would be greatly appreciated, especially if I am off track with my thinking.