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Concrete Mix Design

Concrete Mix Design

Concrete Mix Design

Concrete Mix Design

It means the selection of suitable ingredients & their relative quantities to obtain an economical concrete with the values of certain properties not less than the desired minimum for the Concrete Mix Design, such as

These are the main requirements of mix design

Mix Design Procedure for Concrete Mix Design

There are three methods to be used for mix design generally

  • ASI Methods
  • BS Methods
  • Road notes

But we will discuss only ACI Method

Step: 1 Determine the Target Strength for Concrete Mix Design

fc´ = min strength obtained while testing several specimens
fcr´ = avg. 28 day strength of a test
So we may have two situations

i.      If previous statistical data is not available

fcr´ = (fc´ + 8.3) MPa

ii.      If previous stastical data is available
Then fcr´ is greater of the follow

a)    fcr´= fc´ + 1.34S
b)     fcr´     = fc´ + 2.33S- 3.5

Where S is the standard deviation of the least 30 different tests of existence data (or data already available by performing tests previously) & every test consists of two cylinder results.
Previous data for at least 30 differ tests (28 day strength) to be done same lab under same conditions & new one must be different from them.

Step: 2 Type of Cement for Concrete Mix Design

Selection of type of cement depends upon

i.      Required rate of gain of strength
ii.     Likelihood of chemical attack

Step: 3 Durability for Concrete Mix Design

Summarize 1st the requirements of durability for your project. There are some factors affecting durability, which can be controlled in the start,

i.      w/c ratio
Lesser w/c ratio, less voids & hence greater durability

ii.      Cement Content (kg/m3)
Larger cement, more durability

iii.      Minimum Cylinder Strength
It is mainly gives compressive strength of concrete but indirect indication of all other properties

BS´-8110 gives a table for the requirements of durability

Environment exposure conditions

Max. w/c ratio

Min. cementations material content in kg/m3 for following agg. Size

Min. fc´

(MPa)

   

40mm

20mm

14mm

10mm

 

1) Mild

0.80

150

180

200

220

15

2) Moderate

0.65

245

275

295

315

24

3) Severe

0.6

270

300

320

340

27

4) Very Sever

0.55

295

325

345

365

30

5) Extreme

0.5

320

350

370

390

36

 Exposure Conditions

I.      Mild Environment:
I.e. protected concrete might be inside of a building and not exposed to environment i.e. surroundings
II.     Moderate Environment:
Concrete surface sheltered from sever rain but occasional rain can be there.
III.    Sever Environment:
Frequent rains, open atmosphere, alternate freezing & thawing. (Continues dipping of steel in water give less rusting then       open one)
IV.      Very Sever Environment:
Sea water sides i.e. coastal are building having tides of water which come & go back &giving many cycles of drying & wetting. Factories may also come under this.
V.      Extreme:
Having flow of acidic water i.e. drains of factories. Also when machines having steel tire is dragged on concrete surface, scratch the surface. Fatigue does not come under this even though it is also an environmental phenomenon. So fatigue is kept separate.

ACI Requirement against Sulfate Attack:

Exposure condition

Water resoluble sulfate in soil w.r.t. %age of soil max.

SO4 present in water

(PPm)

Type of cement

Max. w/c ratio

Min. fc´

1) Negligible

0-0.1

0-150

-

2) Moderate

0.1-0.2%

150 – 1500

Modified or Type-2 having OPC & pozzolan or blast furnace sludge

0.5

28

3) Sever

0.2 – 2 %

1500 – 10,000

Sulfate resulting 0.45 cement or Type – 5

0.45

31

4) Very severe

>2%

> 10,000

Sulfate resisting or Type -5 cement

+pozzolan or blast furnace sludge

0.45

31

Tables are also available for many other chemical like chloride attack etc)

Step: 4 W/C Ratio which Satisfy the Required Strength, fcr´ for Concrete Mix Design

In mix design we always use w/c ratio for strength.
Graph can be drawn for different w/c ratio & strength of specimens, though it is the most  accurate method but problem is that we don’t have previous data available
That’s why we have to go for table provided by ACI code.

Avg. compressive strength (MPa)

w/c ratio

45

0.38

40

0.43

35

0.48

30

0.55

25

0.62

20

0.70

15

0.80

Now take the smaller value of w/c ratio out of those required for durability & strength, will satisfy both.

Step: 5 Workability / Total Water Content for Concrete Mix Design.

When ever we go for w/c ratio or water content, we exactly mean the saturated surface dry condition ofaggregates
So all these conditions are based on saturated surface dry condition of agg.
Most important factor workability is water content in kg/m3 i.e. how many kgs of water is required per m3 of concrete
Table for workability

Slump (mm) Amount of water in kg/m3 for following sizes of concrete aggregates
  10 12.5 20 25 40
30 – 50 205 200 185 180 160
80 – 100 225 215 200 195 175
150 – 180 240 230 210 205 185

 

Normal type of concrete won’t have value greater than this
Linear interpolation is to be done for any other value of slump in between

Step: 6 Estimate the Amount of Air % Present in Concrete %age by Volume for Concrete Mix Design

  Amount of water in kg/m3 for following sizes of coarse aggregates
  10 12.5 20 25 40
Air content % 3.0 % 2.5 % 2.0 % 1.5 % 1.0 %

Step: 7 Calculate the Cement Content Required

W/c = known already i.e. a fixed value which is decided already

                                         Water content (kg/m3)

Cement content, C =       w/c ratio (known value)

This cement content is for strength, now check it for durability.
Hence take the larger value for durability & strength

Step: 8 Selection of Fine & Coarse Aggregates Quality for Concrete Mix Design

Decide their sizes & check the grading

Step: 9 Calculate the Amount of Coarse Aggregates Required for Concrete Mix Design

Based upon max size $ fineness modulus value, we decide this. If coarse aggregates have large size, we will need more fine & vice verse

Max. aggregates size

Dry bulk volume of rodded coarse agg. In m3 per 1m3 of concrete for the givenfineness modulus of fine agg.

 

2.0

2.2

2.4

2.6

10

0.54

0.52

0.50

0.48

12.5

02.63

0.61

0.59

0.57

20

0.70

0.68

0.66

0.64

40

0.75

0.73

0.71

0.69

Step: 10 Calculate the Amount of Fine Aggregate for Concrete Mix Design

This is calculated indirectly after calculating the whole mix volume.
Af   =   ðf [1000-(w + C/ð+ Ac/ðc = 10 A)]
(Kg/m3)
Where
Af = fine agg. Content
ðf = bulk specific gravity of fine aggregate, rodded
W = water content (hg/m3)
C = cement content (kg/m3)
ð= specific gravity of cement (3.15 for OPC)
Ac = volume of rodded aggregate × bulk density
ðc = bulk specific gravity of coarse aggregate rodded
A = % age air voids.

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My name is Mustafa Awais, I am basically from Pakistan & Currently living in Saudia Arabia, I am Civil Engineer as well as Web Master.

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