Requirement for Durability of Concrete (Concrete Mix Proportions)

Concrete Mix Proportions 


A) General

The free water-cement ratio is an important factor in governing the durability of concrete and should always be the lowest value. Appropriate values for minimum cement content and the maximum free water-cement ratio are given in Table 5 for different exposure conditions. The minimum cement content and maximum water-cement ratio apply to 20 mm nominal maximum size aggregate. For other sizes of aggregate they should be changed as given in Table 6.


S.No.

Exposure

Plain Concrete

Reinforced Concrete

 

 

Minimum Cement Content

Maximum Free Water-Cement Ratio

Minimum Grade of Concrete

Minimum Cement Content

Maximum Free Water-Cement Ratio

Minimum Grade of Concrete

 

 

Kg/cum

 

 

Kg/cum

 

 

a

b

c

d

e

f

g

h

1

Mild

220

0.60

-

300

0.55

M 20

2

Moderate

240

0.60

M 15

300

0.50

M 25

3

Severe

250

0.50

M 20

320

0.45

M 30

4

Very severe

260

0.45

M 20

340

0.45

M 35

5

Extreme

280

0.40

M 25

360

0.40

M 40

Note -1.

Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions mentioned in IS 456:2000 (5.2). The additions such as fly ash or ground granulated blast furnance slag may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolona and slag specified in IS 1489 ( part 1) and IS 455 respectively.

Note-2

Minimum grade for plain concrete under mild exposure condition is not specified.

 

B). Maximum cement content

Cement content not including fly ash and ground granulated blast furnace slag in excess of 450 kg/cum should not be used unless special consideration has IS 456: 2000 been given in design to the increased risk of cracking due to drying shrinkage in.thin sections, or to early thermal cracking and to the increased risk of damage due to alkali silica reactions.

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Adjustments to Minimum Cement Content for Aggregates Other Than 20 mm Nominal Maximum Size

S.No.

Nominal Maximum Aggregate size

Adjustments to Minimum Cement Contents in Table 5 IS 456:2000

 

Mm

Kg/cum

a

b

c

1

10

+40

2

20

0

3

40

-30

 

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