ACI method of concrete mix design is based on the estimated weight of the concrete per unit volume. This method takes into consideration the requirements for consistency, workability, strength and durability. This article presents ACI method of concrete mix design.
ACI Method of Concrete Mix Design
Required Data:
Before starting concrete mix design, basic information on raw materials shall be prepared which include:
- Sieve analyses of fine and coarse aggregates.
- Unit weight (dry rodded density) of coarse aggregate.
- Bulk specific gravities and absorptions or moisture content of aggregates.
- Mixing-water requirements of concrete developed from experience with available aggregates.
- Specific gravities of Portland cement and other cementitious materials, if used.
- Relationships between strength and water-cement ratio or ratio of water-to-cement plus other cementitious materials, for available combinations of cements, other cementitious materials if considered, and aggregates.
Procedure for ACI Method of Concrete Mix Design
1. Choice of slump
If slump is not specified, a value appropriate for the work can be selected from Table 1. The values provided in table can be used only when vibration is used to consolidate concrete. To read more about slump, Please click here.
Table 1 Recommended slumps for various types of construction
| Construction type | Slump value, mm | |
| Minimum | Maximum* | |
| Reinforced foundation walls and footings | 25 | 75 |
| Plain footings, caissons, and substructure walls | 25 | 75 |
| Beams and reinforced walls | 25 | 100 |
| Building columns | 25 | 100 |
| Pavements and slabs | 25 | 75 |
| Mass concrete | 25 | 50 |
| *May increased 25mm for methods of consolidation other than vibration | ||
2. Choice of maximum size of aggregate
commonly, maximum aggregate size should be the largest that is economically available and consistent with dimensions of structural element. ACI 211.1-91 specify that, maximum aggregate size shall not surpass:
- One-fifth of the narrowest dimension between sides of forms.
- one-third the depth of slabs
- 3/4-ths of the minimum clear spacing between individual reinforcing bars, bundles of bars, or pre-tensioning strands.
These limitations may be ignored provided that workability and methods of consolidation are such that the concrete can be placed without honeycomb or void.
3. Estimation of mixing water and air content
The quantity of water per unit volume of concrete required to produce a given slump is dependent on:
- nominal maximum size
- particle shape
- grading of the aggregates
- concrete temperature
- amount of entrained air
- use of chemical admixtures.
Table 2 and Table 3 provide estimates of required mixing water for concrete made with various maximum sizes of aggregate, for non-air and air-entrainment concrete, respectively.
Table 2 Approximate mixing water (Kg/m3) and air content for different slumps and nominal maximum sizes of aggregates for non-air content concrete
| Slump, mm | Water, Kg/m3 of concrete for indicated nominal maximum sizes of aggregate | |||||||
| 9.5 mm | 12.5 mm | 19 mm | 25 mm | 37.5 mm | 50 mm | 75 mm | 150 mm | |
| 25-50 | 207 | 199 | 190 | 179 | 166 | 154 | 130 | 113 |
| 75-100 | 228 | 216 | 205 | 193 | 181 | 169 | 145 | 124 |
| 150-175 | 243 | 228 | 216 | 202 | 190 | 178 | 160 | —- |
| Approximate Air content quantity, % | 3 | 2.5 | 2 | 1.5 | 1 | 0.5 | 0.3 | 0.2 |
Table 3 Approximate mixing water (Kg/m3) and air content for different slumps and nominal maximum sizes of aggregates for air content concrete
| Slump, mm | Water, Kg/m3 of concrete for indicated nominal maximum sizes of aggregate | |||||||
| 9.5 mm | 12.5 mm | 19 mm | 25 mm | 37.5 mm | 50 mm | 75 mm | 150 mm | |
| 25-50 | 181 | 175 | 168 | 160 | 150 | 142 | 122 | 107 |
| 75-100 | 202 | 193 | 184 | 175 | 165 | 157 | 133 | 119 |
| 150-175 | 216 | 205 | 197 | 184 | 174 | 166 | 154 | —- |
| Recommended average total air content (%) for different level of exposure | ||||||||
| Mild exposure | 4.5 | 4 | 3.5 | 3 | 2.5 | 2 | 1.5 | 1 |
| Moderate exposure | 6 | 5.5 | 5 | 4.5 | 4.5 | 4 | 3.5 | 3 |
| Severe exposure | 7.5 | 7 | 6 | 6 | 5.5 | 5 | 4.5 | 4 |
4. Selection of water-cement or water-cementitious material ratio
Strength, durability, and determine water to cement ratio:Without strength vs. w/c ratio data for a certain material, a conservative estimate can be made for the accepted 28-day compressive strength from Table 4.
Additionally, if there are severe exposure conditions, such as freezing and thawing, exposure to seawater, or sulfates, the w/c ratio can be obtained from table 5.
Table 4 Relationship between water-cement or water-cementitious materials ratio and compressive strength of concrete
| 28-days compressive strength in MPa (psi) | Water cement ratio by weight | |
| Non-air entrained | Air entrained | |
| 41.4 (6000) | 0.41 | — |
| 34.5 (5000) | 0.48 | 0.40 |
| 27.6 (4000) | 0.57 | 0.48 |
| 20.7 (3000) | 0.68 | 0.59 |
| 13.8 (2000) | 0.82 | 0.74 |
Table 5 maximum permissible water/cement ratios for concrete in severe exposure
| Types of structure | Structure wet continuously of frequently exposed to freezing and thawing | Structure exposed to seawater |
| Thin sections (railings, curbs, sills, ledges, ornamental work) and sections with less than 25mm cover over steel | 0.45 | 0.40 |
| All other structures | 0.50 | 0.45 |
5. Calculation of cement content
The amount of cement is fixed by the determinations made in Steps 3 and 4 above.
6. Estimation of coarse aggregate content
The most economical concrete will have as much as possible space occupied by coarse aggregate since it will require no cement in the space filled by coarse aggregate.
The percent of coarse aggregate to concrete for a given maximum size and fineness modulus is given by Table 6. Coarse aggregate volumes are based on oven-dry rodded weights obtained in accordance with ASTM C 29.
Table 6: Volume of coarse aggregate per unit of volume of concrete
| Maximum aggregate size, mm | fineness moduli of fine aggregate | |||
| 2.40 | 2.60 | 2.80 | 3 | |
| 9.5 | 0.50 | 0.48 | 0.46 | 0.44 |
| 12.5 | 0.59 | 0.57 | 0.55 | 0.53 |
| 19 | 0.66 | 0.64 | 0.62 | 0.60 |
| 25 | 0.71 | 0.69 | 0.67 | 0.65 |
| 37.5 | 0.75 | 0.73 | 0.71 | 0.69 |
| 50 | 0.78 | 0.76 | 0.74 | 0.72 |
7. Estimation of fine aggregate content
At the completion of Step 6, all ingredients of the concrete have been estimated except the fine aggregate.
There are two standard methods to establish the fine aggregate content, the mass method and the volume method. the “volume” method will be used because it is a somewhat more exact procedure.
The volume of fine aggregates is found by subtracting the volume of cement, water, air, and coarse aggregate from the total concrete volume.
Then once the volumes known the weights of each ingredient can be calculated from the specific gravities.
The volume occupied in concrete by any ingredient is equal to its weight divided by the density of that material (the latter being the product of the unit weight of water and the specific gravity of the material).
8. Adjustments for aggregate moisture
Aggregate weights
Aggregate volumes are computed 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.
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.