Concrete Mix Design is the process of selecting suitable ingredients of concrete and determining their relative proportions to produce a concrete of certain minimum strength and durability as economically as possible.
Concrete Mix designer required a great knowledge so that if there any difficulty comes he handles this problem. He must aware these essential steps which are the basics of concrete mix Design in ACI 211 method.
Essentials steps of Concrete Mix Design ACI 211
1. Determination of Slump
2. Determine the Maximum Size of Aggregate
3. Determination of Mixing water and Air Content
4. Determine Water Cement Ratio
5. Calculate Cement Content
6. Estimation of Coarse Aggregate Content
7. Estimation of Fine Aggregate Content
8. Adjustment of Aggregate Moisture
9. Adjustment of trail Batches
Step 1. Determination of Slump
To measure the consistency of concrete is called Slump
If we are making design first step is to determine the slump that for this project which slump value we will use.
Normally during selection of Slump we determine the how much distance from our batching plant to Our work area i.e. Bridge, Culvert or building. If the Distance is 30 mint so when at last step we will make trail then our slump must come according to the below mentioned table.
The slump ranges shown below apply when Vibrations used to consolidate the Concrete.
So from this table below first we select Slump according to our structure
Recommended Slump For Various Types of Construction
Note. Slump can be increased when chemical admixture used but provided here two cases
1. Admixture treated concrete has lower water cement ration. Because admixture reduce amount of water so in this case we can’t use higher w/c ratio.
2. Use limited amount of admixture so that there is no segregation potential or excessive bleeding. Normally how much dosage we will use is mentioned on admixture.
Determine the Maximum Size of Aggregate
According to ACI 211 large Nominal size of well graded aggregate have less voids than smaller sizes. So concrete with large Aggregate size require less mortar.
The Nominal maximum size of Aggregate should not exceed
1. 1/5th of Narrowest dimension between sides of forums,
2. 1/3rd the depth of slab,
3. 3/4th of spacing between reinforced bars
When high strength concrete is required we must decree Nominal maximum size of Aggregate. So smaller the size greater will be the Strength of Concrete.
Determination of Mixing Water and Air Content
The quantity of Water per unit volume of concrete required to produce a given slump is depends on
1. The nominal maximum Size of Aggregate
2. Particle Shape
3. Grading of Aggregate
4. Concrete Temperature
5. The amount of Air entrained
6. & use of Chemical admixture
Approximation of Mixing Water and Air Content for different Sump and Aggregate Sizes
( Table 2 Non Air entrained)
The table shows three points
1. The Maximum Size of Aggregate is increase the Amount of Water Decreases.
2. As the slump increases Quantity of Water increase.
3. This table shows that as the size of aggregate increases amount of air entrapped decreases
Air Entrained Concrete
You can here clearly observe that in Air entrained concrete we need less amount of water .
Air Content percentage in (Air Entrained Concrete) is different in three condition
So these table shows that greater the size of Aggregate lesser will be air entrained so good will be packing of aggregate.
Determination of Water Cement Ratio
The w/c ratio is determined by following factors
3. So there is relationship is present between w/c ratio and strength.
Note. Values are estimated average strength of concrete contains not more than 2 percent air for non air entrained concrete and 6 percent total air content for air entrained concrete..
This table shows that greater the w/c ration lesser will be the concrete strength.
Calculate Cement Content
Selection of Cement is depends upon above two step
1.Estimated Water content table 2 and table 4
2.Estimated w/c ratio Table 6
So weight of cement will be = Weight of water/
Water to cement ratio
Maximum w/c ratio for concrete in Severe Exposure
So this table shows that maximum amount of w/c ratio for these structures
Estimation of Coarse Aggregate Content
Aggregate of essentially same the same Nominal maximum Size and grading will produce concrete of satisfactory workability when a given volume of coarse aggregate on a oven dry rodded basis.
Below table shows that for equal workability the volume of coarse aggregate in unit volume of concrete is dependent only on its Nominal maximum Size and the fines modulus of fine aggregate.
Estimation of Fine Aggregate Content
The weight of fine aggregate is simply the difference between the Weight of Fresh Concrete and the total weight of other ingredient.
Volume of Fine Aggregate=
1 – Summation of Volume of Cement, Water, Air, Coarse Aggregate.
Weight of Fine Aggregate= ABS Volume of Fine Aggregate* Sp gravity of Fine aggregate
First estimate weight of Fresh Concrete
Adjustment For aggregate moisture
When we take material from stock pile for design Moisture present in Aggregate. Without correcting this batch aggregate will be incorrect.
We correct the weight of Coarse aggregate with this formula
Adjusted Weight of Coarse Agg=Wt of C Agg*(1+%CA Moisture/100)
Adjusted Weight of Fine Agg=Wt of Fine Agg*(1+%FA Moisture/100)
Adjusted Weight of Water
We have to correct the weight of Water which we are choosing for our design. Because Coarse and fine aggregate also has Absorption. For example we select water according to our design is 160 kg. But material also has absorption. So in this case or design need more water.
So to adjust Weight of water we need Value of Absorption and Percentage of Moisture. So we adjust weight of water with this formula.
Adjusted Wt of Water = Wt. C.A. * (%Abs - %MC)/100=(A)
WT. F.A. X (%Abs - %MC)/100=(B)
Adjusted Weight of Water = 160+A+B
Trail Batch Adjustment
The calculated mixture proportions should be checked by means of trail batches prepared and tested accordance with ASTM C 192. Only Sufficient water should be used to produce the required slump
Example of Concrete Mix Design
(Fcr = 0.9Fc+2.3S
Fcr = Design Compressive Strength of Concrete
Fc = Characteristics compressive strength of Concrete
S = Standard Deviation
If the data of Standard deviation is not available then we will use this equation
Fcr = Fc+ 10Mpa)