Fundamental concepts of concrete mix design

Concrete mix design is the selection of the proportions of cement, water, fine and coarse aggregate (and admixtures) to produce concrete with the required workability when fresh, the required strength and durability when hardened, and the lowest cost. The following fundamental concepts are commonly adopted:

1. Abrams' water-cement ratio law: strength is governed by the w/c ratio ($f = A/B^{w/c}$); a lower w/c ratio gives higher strength and durability. It is the basis of all methods.
2. Water content for workability: the water required depends on the workability (slump), maximum size, shape and grading of aggregate and air content (tables or charts); cement content = water / (w/c ratio).
3. Target mean strength is higher than the characteristic strength: $f_{target} = f_{ck} + k\,s$ ($k = 1.65$ in IS 10262), allowing for variation (standard deviation $s$) so that not more than 5% of results fall below $f_{ck}$.
4. Durability requirements: limits on maximum w/c ratio and minimum cement content for the exposure condition (IS 456 Table 5), which may override the strength requirement.
5. Aggregate grading and packing: the volume of coarse aggregate and the proportion of fine aggregate are chosen for a well-packed, workable and economical mix (by fineness modulus or grading zone).
6. Absolute volume method: the total volume of all ingredients (each as mass / specific gravity) plus air equals 1 m$^3$.
7. Trial mixes and adjustments: laboratory trial batches are made to check slump and strength; corrections for moisture, absorption and bulking are applied at site.
8. Economy: use the minimum cement that meets the requirements, the maximum practicable aggregate size, and admixtures.