how much does it usually costs to have a truck dump concrete into a 12x20 foundation?
12' X 20' X 4" sounds like a patio to me. You'd need a 6" slab for a garage, extension to the home, etc. One of my neighbors recently had a new patio and walk poured. I asked the guy on the ready mix truck how much it was going for and he

What happens when you have one day to pour a huge concrete foundation and the
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1000 trucks of concrete, 40 hours of consecutive pouring. Apparently, it's the biggest concrete pour in Canadian history.
100 yards of concrete poured for an elevated water storage tank foundation
100 yards of concrete poured for an elevated water storage tank foundation
100 yards of concrete poured for an elevated water storage tank foundation
3" slump! stiff stuff for those of you that know your concrete. the finished surface was not flat.