I'll just add one item to consider, if you build your forms yourself, you might consider a trench around the perimeter, so the concrete is thicker around all the edges. This strenght is helpful when driving things on and off that surface.
In my dad's feedlots, we used a true 4" thick w/wire mesh, but with this perimeter trench at 7"-8" and he as never had a break when driving tractors and heavy loaded manure speaders onto the concrete from the "dirt" areas. Note: Because cattle tend to carry dirt away from the edges, many times your tires have to drive UP to get back on the concrete(lots of stress).
We did 13 truck loads in that lot, 30 years ago, and it is still in perfect shape. This may not really apply to your situation, but you probably understand the principle.