Advantages of Design mix concrete over nominal mix
Nominal Mix

Vary widely in strength due to less or more mix in coarse and fine aggregate and another main factor it is not economical design mix:

Designed by the engineers, not much vary in strength and it is economical nominal mix is Prescriptive type,means its ingrediant ratio is fixed,more over the other parameters like maximum size of Aggregate,w/c ratio, type of cement etc.are also fixed,aiming that adherence to such prescriptive specification will result in satisfactory performance. On the other hand, Design mix is Performance based concrete,where the properties of concrete like strength,workability,durability etc. are specified and ingredients ratio,maximum size of aggregate,type of cement etc is user's choice