Principles of mix design; factors not in DOE

Basic principles of mix design

1. Strength-w/c relation (Abrams' law): the w/c ratio governs strength and durability.
2. Workability is governed by the water content, which depends on the aggregate size, shape and grading and on the slump required.
3. Target mean strength = specified strength + a margin for variability ($k s$), based on the degree of quality control.
4. Durability limits (maximum w/c, minimum cement content) for the exposure.
5. Aggregate grading and the proportion of fine to coarse aggregate, to get a dense, cohesive mix.
6. Absolute volume concept: the sum of the volumes of all ingredients = 1 m$^3$ (or the wet density method in DoE).
7. Economy, with trial mixes and field adjustments.

Factors not considered in the DoE method

• Chemical admixtures (plasticisers, superplasticisers) are not covered; the basic procedure is for non-air-entrained OPC concrete, and air entrainment and mineral additions (fly ash/pfa, slag) are handled only as separate modifications.
• The fineness modulus of sand is not used (grading is described by the % passing the 600 µm sieve), and neither is the dry-rodded bulk density of the coarse aggregate.
• The specific gravity of cement is not used, since the aggregate content is found from the wet density of concrete.
• Aggregate shape and texture enter only as "crushed" or "uncrushed"; flakiness and elongation are not considered; the maximum size is limited to 10, 20 and 40 mm.
• Durability is considered only through checks on the maximum w/c and minimum cement content.
• The bulking of sand, absorption and moisture content are applied only afterwards, as site corrections.
• The placing conditions (congestion of reinforcement, method of placing, temperature) are not considered directly, and special concretes (lightweight, pumped, high-strength with superplasticisers) are outside its scope.