We supply concrete for two large flatwork contractors who each place several hundred thousand square feet of concrete floors per year. One experiences curling on some jobs but the other doesn't. Both use 4000-psi concrete and do a good job of curing. There's one difference between the contractors. One uses concrete with 11_4-inch-maximum size rock for all floors 5 inches thick or greater. He doesn't get any curling. The other uses 3_4-inch-rock concrete regardless of floor thickness. His floors sometimes curl. An engineer told me he thinks the large-rock concrete doesn't curl as much because of better aggregate interlock. Is that correct? Or are there other factors involved?
The aggregate interlock theory may explain what you've noticed, but there's another factor that may be more important. Concrete curls when the top of the slab shrinks more than the bottom does. The more a slab shrinks, the more it's likely to curl. So reducing shrinkage reduces curling as well. Concrete made with larger rock doesn't require as much water per cubic yard to produce the same slump as a smaller-rock concrete. The lower water content reduces shrinkage and also should reduce curling.