(1) Nominal Mix of Concrete
You may have noticed that normally on construction site, the mix ratio of concrete has been already mention in construction drawings. So you have to follow that ratio of concrete. That concrete will give you a concrete strength of ordinary strength normally from M5(1:5:10) to M25(1:1:2). The magnitude mention with the letter M mean the Compressive strength of concrete which in in N/mm2 or Megapascal(Mpa). You can see some Nominal Mix ratios in the table given below.
So this is about the nominal mix when you have to follow the already fix ratios regardless of the material specific properties. This is normally used for residential or other commercial structures where there is no need of high strength concrete.
(2) Design Mix Ratio for Concrete
So the Design mix of concrete is done for high strength of concrete. When the strength requirements beyond M25 (25 Mpa) strength of concrete and when the structures strength is the primary priority then you have to go for Mix Design of concrete.
In Mix Design of Concrete you have to check every material (Aggregates, Water, Admixtures etc) properties that you have to use in your concrete mix and upon some trial you have got the desired strength with some mix ratios. So that Mix proportion of concrete ingredients should be noted with the respective strength. For this you have to bring all the samples of your materials (Aggregates, fine Aggregates, water etc) ingredients to the Mix Ratio Laboratory of your specific area from where you considered to use the concrete ingredients Source. If you want to change the source of the concrete ingredients during construction, then again you to go for mix design. Then that will be your Design Mix ratio for concrete that you have to follow for that specific structures and specific projects. Some Examples of Mix Design ratios or given in the table below but you cannot follow it directly on your site and you to do your own MIX Design for the Concrete ingredients that you have to use in your project. So the values given in table for mix design is M30 to M70.
Thank You if you have still any doubts, you can ask in comment.