There are two types of equations for concrete mix designs. One of them is based on volume ratios and the other one is based on the weight ratios of mix parameters. First you should choose which way you want to use and then you can find a specific limit stat equation for each.

I have explained the best way of mixing concrete with water in the following:

The concretes should be produced in a vertical shaft mixer with bottom discharge. Based on the accuracy of the results and proper mix design, before making every concrete mix design in the cup, the mixer machine should be cleaned with some water. The aggregates with sand and 50% of total water should be then placed in the mixer. After 2 minutes of mixing, the cement and microsilica, the additive material, with the rest of the water should be added. The total mixing time is about 6 minutes.

 Thanks for your responses, I need to determine the reliability index of concrete mix but to do that i will first need to establish the failure equation or limit state equation of the concrete mix