My grandpa used to be a concrete inspector (for the state of Minnesota - if you ever drive i394 there he was one of the inspectors for that). Different plants within a normal commute of his house often had very different sand and so needed a different mix.

Oh cool, did he work for MNDOT? I sell and run a lot of work at their facilities, they have a materials lab over in Maplewood off Hwy 36 and English St, and also the weird test surface area on 94 west of St Cloud.