ACI METHOD OF CONCRETE MIX DESIGN

In 1991, the American Concrete Institute (ACI) published its guidelines for normal, heavyweight and mass concrete mix design. Use of admixture, silica fume etc. in concrete is beyond scope of this method. Steps for design as per ACI 211.1.91 are enlisted below.

1. The required (target) average compressive strength (f_cr ) at 28 days for mix design is determined byadding up an empirical factor (k) to the design compressive strength ( f_c) as per equation written below

f_{cr =} f_c+k

2. The W/C ratio is selected based on the target strength and the type of concrete (air-entrained or non-air-entrained) (From ACI 211.1-91 table A1.5.3.4)

3. Air content, as percentage of the concrete volume, is estimated depending upon the air-entrained ornon-air-entrained type of concrete, exposure conditions, and NMSA(nominal maximum size aggregates). (From ACI 211.1_91 TABLE A 1.5.3.3)

4. Slump, as measure of workability, is selected depending upon the type of structure and complexity ofthe pouring conditions.

5. Water content is determined based on the NMSA, type of concrete (air-entrained or non-air entrained), and specified slump. Then it is adjusted for the types of aggregates. (From ACI 211.1_91 TABLE A 1.5.3.3)

6. Cement content, is calculated based on the w/c ratio and the water content.

7. Coarse aggregates content, as dry rodded bulk (percentage) of concrete unit volume, is determined.(From table 6.3.6 of ACI 211.1-91)based on the NMSA, and the fineness modulus of sand.

8.Once the water content, cement content, air content, and the coarse aggregate content per unit volume of the concrete is determined, the fine aggregate (F_a) is calculated by subtracting the absolute volume of the known ingredients from unit volume of the fresh concrete (in this case 1 m³)as following:

F_a = 1- X

Where,

X = sum of all other ingredients (air, water, cement and coarse aggregates) in cubic meter calculated for 1 m³ of concrete.

9. Finally, water content is adjusted based on the absorption and the current moisture content of the coarse and fine aggregates, in account of saturated surface dry condition of the aggregates.