What are recommended mix proportions for good concrete?

According to me I feel Water-cementitious materials ratio is the best mix proportion. Many researchers have also concluded that the most important variable in achieving high-strength concrete is the water-cement ratio. However, most high-strength concretes contain binding materials other than cement. Consequently, the water-cementitious materials ratio must be considered instead of the water-cement ratio where the cementitious materials include cement, fly ash, silica fume, and GGBFS as appropriate. In general, as the water-cementitious materials ratio decreases, the concrete compressive strength increases.

All the ideas are just generic and they cant be anything else. Do you really think that there is some proportion which will fit all the requirements? If yes then pleas go back to texy books. Concrete proportioning depends upon:

Characteristic Strength requirements (grade of concrete)

Early strength requirement (removal of formwork, shifting-in precast work, stressing-in PSC etc)

Durability requirements (quality of air, water etc and structure's exposure o them)

Method of concreting (manual, pump, crane & bucket, screed, PCC etc)

Pouring conditions (Pavement, underwater, thin elments, complicated geometries-self compacting)

Now tell me which proportion will suit them all?

Proportion of ingredients to get good quality of concrete mix is essential hence,

Concrete ingredients should be selected to make the most economical and having the required strength, workability, and durability. 

To know whether these properties are attained to the required level or  the maximum water-cement ratio or minimum cement content to produce the required strength will determined by laboratory tests therefore, you should need to established basic laboratory tests, by taking cubic samples of different proportions until you get the required strength by actual trial and adjustment in the field,

 Mix proportion should be selected to produce concrete with:-

                       -The stiffest consistency

                       -The maximum size of aggregate economically available & work type.

                       -Adequate durability.

                       -Sufficient strength

                 (As per the requirement of the project work since there are different grades of concretes)                         

Note that: For different environmental condition the concrete character well not matching at all and the ratio of concrete ingredients worked say for India may not work for Ethiopia because so many environmental influencing factors like temperature etc.

Making of good concrete is an art, for which science can be used,

there is alwaya a variance in basic constituents(materials) and soft conditions(daily temprature/ moisture conditions/ concrete timings/ chemistry of ingradients).

With these dynamic variance, you can not have a general formulae of ingadients, but if the concrete manufacturer is aware of role of each parameter, he can definetly provide good conrete mix.

The question "Recommended mix propotion is vague". First it should be clear for what purpose the concrete is to be used. Is it used in beam, slab, column or foundation?, ie what is the recommended ultimate/working strength? In other word its characteristic strength.

First it is necessary to identify the purpose of the concrete to be used, ie its characteristic strength as explained above. Then according to which code or specifiction or by what method of mix design is to be adopted eg. IS, BS, ASTO etc.

At last the qualities of ingradients ie, cement type, sand, aggregate, moisture in sand and aggregate and  quality of water will  determine mix proportions for a good concrete. 

Water-reducers or high-range water-reducers are essential in high-strength concrete to ensure adequate workability while achieving a low water-cementitious materials ratio. Retarding admixtures may also be used. The optimum dosage of an admixture or combination of admixtures should be determined by trial mixtures using varying amounts of each additive. It is also important to be sure that admixtures are compatible when used in combination.

Mix designs doesnt hav any fixed rcomended mix proportions. coz place 2 place materials varies n dats why its properties also varies.  Properties of the material plays a very vital role in d mix design. Dats why mix proportion of same grade of concrete varies place 2 place.

Mix design only hav some recomended guide lines like max. water cement ratio, min. cement content (in some cases max. also) n properties of aggregate n other materials which u can find in IS:456 & MOSRTH. 4 properties of d aggregate u can check IS: 383 & MOSRTH also. 4 mix design procedures u can check IS:10262 & SP:23 whatever u prefer allthough volumetric method is nt metioned dere.

Now back 2 admixture part, admixture is mainly used as a water reducing agent 2 increase d workability of the concrete. Depending upon d purpose sometimes returder, high early strength admixture, etc are used.