Mentioned below are twelve simple steps for designing any standard concrete mix as per Indian Standards. For mixes of very high strength (say, M60 and above) there may be special requirements over and above these steps or special design procedures itself. These easy-to-remember steps were derived by the author from the on-site mix design procedure and are based on his practical experience in Indian project site. In other words, this set of concise steps are nothing but a summary (or a synopsis) of the usual mix designing procedure adopted in project sites across India.
This, the author reckons, is an easy way to understand and remember the somewhat lengthy and intricate procedure of designing concrete mixes. The procedures adopted in other countries may be somewhat different, yet, the basic concept may be the same.
Step 1: Fix the mix-design parameters such as characteristic strength (fck), max. size & shape of aggregates, degree of quality control (QC) & workability, max. water cement (w/c) ratio, min. cement content based on the severity of environmental exposure. Get the material data such as grade & specific gravity of cement, characteristics of fine & coarse aggregate (FA & CA) such as zone of sand, sp. gravity & water absorption of both, actual size of CA. Often a combination of two sizes of CA (say, 20mm & 10mm) are used for denser and stronger concrete.
Step 2: Find out the target mean strength based on fck & degree of QC.
Step 3: Based on the zone of sand & max size of CA find the water content for the mix and % of sand of total aggregates from code (SP 23).
Step 4: Find the max. permissible w/c ratio from code (IS 456) and adopt a w/c ratio lesser than the suggested one.
Step 5: Subject water content & % sand content to the necessary corrections for the deviations of actual w/c ratio, workability & the zone of sand from the standard conditions. Calculate the new % of sand & CA as well as the w/c ratio after the above corrections.
Step 6: If the mix is to be placed by pumping, reduce the quantity of CA by 10% (ACI 221) and increase sand % accordingly for better pumpability.
Step 7: Decide a % of admixture to be introduced to the mix and reduce the water content accordingly as suggested in code (IS 9103). For example, by using 1% admixture (by weight of cement) water content can be reduced by about 20%.
Step 8: Since, w/c ratio has already been arrived at in Step 5, the cement content will have to be recalculated using the new water content as found in Step 7. Check whether the new cement content is more than the minimum suggested content from environmental exposure point of view. If not, adopt a cement content higher than the min. suggested quantity.
Step 9: Decide the proportion of the CA of the two sizes. Say, the ratio of 20mm & 10mm CA is 60:40. This is a commonly adopted proportion.
Step 10: The volume of entrapped air in a concrete mix depends on the max. size of CA and can be found from relevant IS code. Upon finding that, calculate the quantities of CA & FA using the formulae mentioned in IS code.
Step 11: Thus, arrive at the various quantities of ingredients for the mix. Subject the quantities of CA, sand & water content to batch correction and arrive at the final figures for the first trial mix.
Step 12: Prepare several trial mixes by varying the w/c ratio & cement content suitably from those of the first trial mix. Find out the 7 & 28-day cube strength for each trial mix. Based on the strength results adopt the most suitable trial mix as the final mix for actual concrete work.
For complete details on the mix design procedure read the following post by the same author:
Concrete Mix Design (