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.
Step
2
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.
1/5^{th}
of Narrowest dimension between sides of forums,
2.
1/3^{rd}
the depth of slab,
3.
3/4^{th}
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.
Step 3
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.
Step 4
Determination of Water Cement Ratio
The
w/c ratio is determined by following factors
1.
Strength
2.
Durability
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.
Step
5
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
(Table 5)
So this table shows that maximum
amount of w/c ratio for these structures
Step
6
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.
(Table 6)
Step
7
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
(Table 7)
First estimate weight
of Fresh Concrete
Step
8
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
Step
9
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)
0 comments:
Post a Comment