ACI Method of Concrete Mix Design – Procedure and Calculations

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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.