I'm not sure that the answer to your question will end up being fully logical, I can only say that this is how I wanted to do it, but I'll give it a try.
Why not just use wood forms and pour the slab on grade?
I could have done that but I was concerned with the pad coming out as perfectly square and straight sided as possible. The side walls will overlap the base by 1" to prevent water from seeping in when it rains. Also, if the base sides were a little bowed the straight square aluminum tubing would not fit as nicely as I would like. When putting in the blocks I dug a little trench for the minimal footing, clamped the blocks between two steel rectangular tubes and suspended them above the trench and then easily slid them into the correct position, I then filled the space between the blocks and the ground with cement. This helped make sure that the sides were perfectly straight and level. Look close at the photo in post #40, you will see the block are suspended.
The rebar used is #3.
Also, it rains like crazy here in the tropics and I wanted the pad surface to be at a height safely above ground in case of flooding.
Wood is very expensive here, cement blocks are less than 2 for $1 plus 11.5% sales tax! I bought 20 blocks for $12, four 2x8's would have cost me over $100! That's a lot of money for termite food.
I hope this helps explain my construction methods, and thanks for asking.
Let me know if I can help with any other questions. I would be very happy to try to explain my doings.
Edited by PirateMike, 01 June 2019 - 07:05 PM.