ACI METHOD OF CONCRETE MIX DESIGN

Concrete Mix Design ACI method

The ACI Standard 211.1 is a “Recommended Practice for Selecting Proportions for Concrete ingredients”.

Concrete mix design methodologies are follows: 

Step 1. Choice of slump Step 

Step 2. Choice of maximum size of aggregate 

Step 3. Estimation of mixing water and air content 

Step 4. Selection of water/cement ratio 

Step 5. Calculation of cement content

Step 6. Estimation of coarse aggregate content 

Step 7. Estimation of Fine Aggregate Content 

Step 8. Adjustments for Aggregate Moisture 

Step 9. Trial Batch Adjustments

Step 1 (Choice of Slump):

Concrete slump is a measure of the degree of consistency and extent of workability. In the slump test the plastic concrete specimen is formed into a conical metal mold as described in ASTM Standard C-143. The mold is lifted, leaving the concrete to "slump," that is, to spread or drop in height. This drop in height is the slump measure of the degree of workability of the mix.

Figure 1. Concrete Slump test.

Table 1. Recommended Slumps for Various Types of Construction

Step 2 (Choice of maximum size of aggregate): 

Large maximum sizes of aggregates produce less voids than smaller sizes. Hence, concretes with the larger-sized aggregates require less mortar per unit volume of concrete, and of coarse it is the mortar which contains the most expensive ingredient, cement. Thus the ACI method is based on the principle that the “Maximum size of aggregate should be The largest available so long it is consistent with The dimensions of the structure”.

In practice the dimensions of the forms or the spacing of the rebars controls the maximum CA size.

ACI 211.1 states that the maximum CA size should not exceed:

• One-fifth of the narrowest dimension between sides of forms,
• One-third the depth of slabs,
•  ¾ ^th  of the minimum clear spacing between individual reinforcing bars, bundles of bars, or pre-tensioning strands.


Step 3 (Estimation of mixing water and air content)

The ACI Method uses past experience to give a first estimate for the quantity of water per unit volume of concrete required to produce a given slump. In general the quantity of water per unit volume of concrete required to produce a given slump is dependent on the maximum CA size, the shape and grading of both CA and FA, as well as the amount of entrained air.

One major disadvantage of concrete is its susceptibility to damage by single or multiple freeze-thaw cycles. However, concrete can be made frost-resistant by using air entraining admixtures. Concrete is routinely air-entrained in the Northern U.S. and Canada.

Table 2. Approximate Mixing Water and Air Content Requirements for Different Slumps and Maximum Aggregate Sizes

Step 4 (Selection of water/cement ratio)

The required water/cement ratio is determined by strength, durability and finishability. The appropriate value is chosen from prior testing of a given system of cement and aggregate or a value is chosen from Table 3 and/or Table 4

Table 3.Water-Cement Ratio and Compressive Strength Relationship

Table 4. Maximum permissible water/cement ratios for concrete in severe exposures

Step 5. Calculation of cement content

The amount of cement is fixed by the determinations made in Steps 3 and 4 above. Therefore, calculation of cement content can be estimated from the equation below:

Step 6. Estimation of coarse aggregate content

The most economical concrete will have as much as possible space occupied by CA since it will require no cement in the space filled by CA.

Table 5. Volume of Coarse Aggregate per Unit Volume for Different Fine aggregate Fineness Modulus

Special Note:

* These values can be increased by up to about 10 percent for pavement applications.

* Coarse aggregate volumes are based on oven-dryed weights obtained in accordance with ASTM C- 29.

Step 7. Estimation of Fine Aggregate Content

The volume of fine aggregates is found by subtracting the volume of cement, water, air, and coarse aggregate from the total concrete volume.

Step 8. Adjustments for Aggregate Moisture

Aggregate weights
Aggregate volumes are calculated based on oven dry unit weights, but aggregate is typically batched based on actual weight. Therefore, any moisture in the aggregate will increase its weight and stockpiled aggregates almost always contain some moisture. Without correcting for this, the batched aggregate volumes will be incorrect.

Amount of mixing water.
If the batched aggregate is anything but saturated surface dry it will absorb water (if oven dry or air dry) or give up water (if wet) to the cement paste. This causes a net change in the amount of water available in the mix and must be compensated for by adjusting the amount of mixing water added.

Step 9. Trial Batch Adjustments.

The ACI method is written on the basis that a trial batch of concrete will be prepared in the laboratory, and adjusted to give the desired slump, freedom from segregation, finishability, unit weight, air content and strength.