Jack I'd say that two import issues are the base and no sealing. The base is the most important without a good base even the best of concrete could crack. The sealing is to keep the hydration process going so the slab will gain strenth with time. in your area the concrete design should have been 4000lb min, air entrained, fibers, poured at a 4 slump. The time of the year is important also, here in NH we don't pour exterior slab work much after the 1st of Oct so that the concrete can attain strength enough to withstand the freeze thaw cycle. Now did the concrete just crack or did it also scale? that would be a sign of sealing (sealing it in as with a steel trowel not with the sealer that helps the cure) water in and it freezing spaling the slab.