The ACI method of concrete mix design is a process that relies on the anticipated weight of the concrete per unit volume. It is designed to meet the necessary standards for consistency, workability, strength, and durability. In this article, we will discuss the ACI method in detail, which takes all of these factors into account.
ACI Method of Concrete Mix Design
Required Data:
To begin designing a concrete mix, certain preliminary information about the raw materials is essential. This includes conducting sieve analyses of both the fine and coarse aggregates, as well as determining the dry rodded density or unit weight of the coarse aggregate. The bulk specific gravities and absorptions, or alternatively the moisture content, of the aggregates must also be determined. It is important to consider the mixing-water requirements of the concrete, which can be determined from previous experience with the available aggregates. Additionally, specific gravities of the Portland cement and any other cementitious materials used must be known. Finally, it is important to understand the relationships between strength and the water-cement ratio or the ratio of water-to-cement plus other cementitious materials for the available combinations of cements, other cementitious materials, and aggregates.
Procedure for ACI Method of Concrete Mix Design
1. Choice of slump
Table 1 provides suitable values for concrete work if no specific slump is indicated. These values are only applicable when utilizing vibration to consolidate the concrete. It is important to note that these values should not be used for other methods of consolidation. If you would like to learn more about slump, please click on the provided link.
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 | ||
Fig. 1: Measuring slump
2. Choice of maximum size of aggregate
The maximum aggregate size used in concrete should ideally be the largest available that is both economically feasible and appropriate for the dimensions of the structural element. To ensure that the concrete is of good quality and free from defects, ACI 211.1-91 sets out certain guidelines that must be followed. Specifically, the maximum aggregate size must not exceed certain limits based on different factors.
For instance, when pouring concrete into forms, the maximum aggregate size should not exceed one-fifth of the narrowest dimension between the sides of the forms. Similarly, when pouring slabs, the maximum aggregate size should not exceed one-third of the slab depth. In addition, the maximum aggregate size should not exceed 3/4-ths of the minimum clear spacing between individual reinforcing bars, bundles of bars, or pre-tensioning strands.
However, in some cases, these limitations may be disregarded, provided that the workability and methods of consolidation are such that the concrete can be placed without leaving behind any honeycomb or voids. Ultimately, the goal is to ensure that the concrete is strong, durable, and able to withstand the loads and stresses to which it will be subjected.
Fig. 2: Coarse aggregate
3. Estimation of mixing water and air content
The amount of water needed per unit volume of concrete to achieve a particular slump is influenced by various factors such as the nominal maximum size of the aggregates, their particle shape and grading, the temperature of the concrete, the quantity of entrained air, and the use of chemical admixtures. To estimate the required mixing water for concrete of different maximum aggregate sizes, Table 2 and Table 3 provide relevant information 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 |
Fig. 3: mixing water
4. Selection of water-cement or water-cementitious material ratio
When dealing with certain materials, it may be necessary to estimate their strength and durability based on conservative assumptions if there is no available data on their strength versus water to cement ratio. In such cases, a reasonable estimate can be made by referring to Table 4 for the accepted 28-day compressive strength.
However, if the material will be exposed to severe environmental conditions such as freezing and thawing, exposure to seawater, or sulfates, it is important to determine the appropriate water to cement ratio. This can be achieved by referring to Table 5, which provides information on the recommended water to cement ratios based on the severity of exposure conditions.
It is important to note that obtaining accurate data on strength and water to cement ratio is crucial for ensuring the durability and long-term performance of the material. By taking into account the specific exposure conditions and using the appropriate water to cement ratio, the material can be designed to withstand harsh environmental factors and maintain its strength and integrity over time.
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 |
Fig. 4:water to cement ratio
5. Calculation of cement content
The quantity of cement needed has been determined through the calculations made in Steps 3 and 4. Therefore, the amount of cement required for the project is fixed and cannot be altered. The computations made during the previous steps have provided the necessary information to determine the exact amount of cement needed to complete the project successfully. Therefore, no further adjustments need to be made to the quantity of cement required.
Fig.5: Cement
6. Estimation of coarse aggregate content
Table 6 provides information on the percentage of coarse aggregate that should be used in concrete in order to achieve maximum economy. This is because the more space that is occupied by coarse aggregate, the less cement will be required to fill the remaining space. This results in a more economical concrete mixture.
The percentage of coarse aggregate recommended for a given maximum size and fineness modulus is based on the oven-dry rodded weights obtained in accordance with ASTM C 29. By using this method to determine the coarse aggregate volume, a more accurate estimation can be made on the amount of coarse aggregate that should be used in the concrete mixture.
Overall, the goal is to maximize the use of coarse aggregate in concrete while still achieving the desired strength and durability. By following the recommendations provided in Table 6 and using ASTM C 29 to determine coarse aggregate volumes, a more economical and efficient concrete mixture can be achieved.
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 |
Fig. 6:Coarse aggregate
7. Estimation of fine aggregate content
To estimate the fine aggregate content of concrete, there are two standard methods available: the mass method and the volume method. In this case, the volume method will be used as it is considered a more accurate procedure. At this point in the process, all of the ingredients for the concrete have been estimated except for the fine aggregate.
To determine the volume of fine aggregates needed, the volume of the other ingredients (including cement, water, air, and coarse aggregate) must first be subtracted from the total concrete volume. Once the volume of the fine aggregate is known, the weights of each ingredient can be calculated based on their specific gravities. The volume occupied by any ingredient in the concrete is equal to its weight divided by the density of the material. The density is calculated as the product of the unit weight of water and the specific gravity of the material.
Fig.7: Fine aggregate
8. Adjustments for aggregate moisture
Aggregate weights
The process of computing aggregate volumes involves using oven dry unit weights. However, when it comes to batching the aggregate, it is typically done based on the actual weight. This presents a challenge as any moisture present in the aggregate will increase its weight. It is a common occurrence for stockpiled aggregates to contain some level of moisture. If the moisture content is not accounted for, the batched aggregate volumes will not be accurate. This can lead to issues in the construction process and may result in costly errors. Therefore, it is important to correct for the moisture content in the aggregate to ensure that the batched volumes are correct.
Amount of mixing water
When dealing with batched aggregate, it is important to consider its moisture content. If the aggregate is not saturated surface dry, then it has the potential to either absorb water or give up water to the cement paste depending on whether it is wet or oven/air dry. This exchange of water can result in a shift in the total amount of water available in the mixture. As a result, it becomes necessary to make adjustments to the amount of mixing water added to the mix in order to compensate for this change. By doing so, the concrete can be produced with the desired properties and characteristics. Therefore, it is essential to carefully manage the moisture content of the aggregate during the mixing process to ensure the quality and consistency of the final product.
Fig.8:aggregate water content
9. Trial Batch Adjustments
The ACI method is based on the preparation of a trial batch of concrete in a laboratory setting, with the aim of achieving specific characteristics such as the desired slump, freedom from segregation, finishability, unit weight, air content, and strength.
To achieve these desired characteristics, the trial batch is adjusted as needed. This adjustment process involves carefully monitoring and tweaking the mixture until the desired properties are achieved.
Overall, the ACI method relies on laboratory testing and experimentation to ensure that the resulting concrete meets the necessary standards and is suitable for its intended use. By carefully controlling the mixture and adjusting it as needed, the ACI method can produce concrete with consistent and reliable characteristics.