The time at which the cracks developed is more important. If it happened whwen the concrete was plastic, i.e. within 6 hours, it was plastic shrinkage cracking and was due to the high evaporation. You should have gone for a delayed refinishing and early wetting with a wet hessian cloth or...
Try by reducing sand in total aggregate to 40%. Hence, you may redesign for C.agg - 3/4" = 1950lb and Sand = 1300lb. Normally oversanded mixes entrap more air as well for the same compaction type and shuttering oil.
I need to recommend a granular sub base which is free draining and will act as a good drainage material. Please tell me where I can source a suitable specification.
Very less is available on this subject but personal experience has shown that lignosulphonate based admixtures behave this way when a phospho gypsum is used during cement grinding process. The concrete will regain its full strength at 28 days for sure.
If you have type I cement, why dont you blend it with GGBS (Ground granulated blast furnace slag)at your project site.Blended cements are superior in sulphate resistance as well as resistance to chloride diffusion. Type V is slowly being replaced by blended cements of late.