American Concrete Institute (ACI) Method of Mix Design

This method has been used since 1944 and has undergone several revisions continuously. It has developed an identical procedure for angular or rounded aggregates, regular or light-weight aggregates and air-entrained or non-air-entrained concretes. The ACI mix design is based on certain factors which are established by field experience on large works. The factors are as follows:

1. The fresh concrete of a given slump with a well-graded aggregate (of a given maximum size) will have constant total water content irrespective of variations in water–cement ratio and cement content.
2. It considers the relationship that the optimum dry-rodded volume of coarse aggregate per unit volume of concrete is based on its maximum size and the fineness modulus of the fine aggregate (Table 8.3) irrespective of the shape of the particles.
3. Irrespective of the method of compaction, there is a definite percentage of air that exists which is inversely proportional to the maximum size of the aggregate.

Table 8.3 Dry bulk volume of concrete aggregate per unit volume of concrete

Source: ACI 211.1–91.

Reproduced with permission from the American Concrete Institute, Farmington Hills, MI (www.concrete.org)

The following step-by-step procedure has to be adopted:

1. All the required data, viz., (i) given modulus of the selected fine aggregate, (ii) unit weight of dry-rodded coarse aggregate, (iii) specific gravity of coarse and fine aggregates in SSD condition, (iv) absorption characteristics of both coarse and fine aggregates and (v) specific gravity of cement.
2. The maximum size of the aggregate has to be decided. Generally 20 mm and 10 mm are recommended for RCC and pre-stressed concrete work, respectively.
3. Workability has to be decided based on the slump depending on the work. General guidance may be taken from Table 8.4.Table 8.4 Recommended value of slump for different worksSource: ACI 211.1–91.Reproduced with permission from the American Concrete Institute, Farmington Hills, MI (www.concrete.org)Note: Upper limit of the slump may be increased by 20 mm for compaction by hand.
4. Based on the selected slump and maximum of aggregate, the total water in kg/m³ of concrete can be read from Table 8.5. Also the amount of accidentally entrapped air in non-air-entrained concrete may be obtained from Table 8.5.Table 8.5 Approximate water content requirement for mixing and air content for different workabilities and nominal maximum size of aggregates.Source: ACI 211.1–1994.Reproduced with permission from the American Concrete Institute, Farmington Hills, MI (www.concrete.org)
5. The cement content is calculated by dividing the total water content by the water–cement ratio.
6. The bulk volume of dry-rodded coarse aggregate per unit volume of concrete is selected from Table 8.5 for the particular maximum size of coarse aggregate and the fineness modulus of fine aggregate.
7. On multiplying the bulk volume by bulk density, the weight of the coarse aggregate in one cubic meter of concrete can be calculated.
8. With the knowledge of the specific gravity of coarse aggregate, the solid volume of the coarse aggregate in a cubic meter can be calculated.
9. Similarly, the solid volume of cement, water, and volume of air is calculated in one cubic meter of concrete.
10. The solid volume of sand is calculated by subtracting the volumes of cement, coarse aggregate, water and entrapped air from the total volume.
11. The weight of the fine aggregate is calculated by multiplying the solid volume of the fine aggregate by the specific gravity of the fine aggregate.