IS Code Method of Concrete Mix Design

IS Code Method of Concrete Mix Design
Procedure
1. Target strength for mean strength
The target mean compressive strength at 28 days
K = a statistical value , usually taken as 1.65
S = standard deviation for each grade of concrete( table 8 of I.S. 456 -2000 )
Values of K
Accepted proportion of
low results |
K |
1 in 5, 20% | 0.84 |
1 in 10, 10% | 1.28 |
1 in 15, 6.7% | 1.50 |
1 in 20, 5% | 1.65 |
1in 40, 2.5% | 1.86 |
1 in 100, 1% | 2.33 |
TABLE-8, IS CODE
Grade of concrete | Assumed Standard ,
Deviations (N/mm² ) |
M10 ,M15 | 3.5 |
M 20, M25 | 4.0 |
M30,M35,M40,M45 & M50 | 5.0 |
Selection of water cement ratio
A) The free water cement ratio corresponding to the target strength is to be determined from the graph
Modified graph for Selection of water cement ratio
The water cement ratio, as selected above should be checked against the limiting water cement ratio for the durability requirements as given in table
Durability Criteria as per IS 456- 2000 Adjustments to minimum cement content for aggregates other than 20 mm nominal max. size aggregates as per IS 456: 2000.
Approximate water content (Kg) per cubic metre of concrete (Table 32, SP:23-1982)
Estimation of entrapped air
Depends on nominal max size of aggregate as given in table
Max. size of aggregate (mm) | Entrapped air as % of volume of
concrete |
10 | 3.0 |
20 | 2.0 |
40 | 1.0 |
Selection of water content and fine to total aggregate ratio
The values are given in the tables, based on the following conditions
(a) Crushed (Angular) Coarse aggregate conforming to IS:383
(b) Sand conforming to grading zone II of table 4 of IS 383
(c) Workability corresponds to C.F. of 0.8
Approximate sand and water contents per cu.m of concrete W/C = 0.6, Workability = 0.8 C.F. (For medium strength concrete up to M35)
Maximum size of
aggregate (mm) |
Water content
including surface water per m³ of concrete (kg) |
Sand as % of total
aggregate by absolute volume |
10 | 200 | 40 |
20 | 186 | 35 |
40 | 165 | 30 |
Estimation of Coarse Aggregate Proportion
For W/C ratio of 0.5 use following Table
Correction in Coarse Aggregate values Thetable specified for W/C ratio of 0.5
1. For Every +0.05 change in W/C ratio: -0.01
2. For Every -0.05 change in W/C ratio: +0.01
3. For Pumpable Mix : -10 %
For Other Conditions as per IS:10262,2009
Condition | Correction |
Sub-Angular Aggregates | – 10 Kg |
Gravel + Crushed Particles | – 20 Kg |
Rounded Gravel | – 25 Kg |
For every slump increase
of 25 mm |
+ 3 % |
Use of Water Reducing
Admixture |
– 5 to 10 % |
Use of Superplasticzing
Admixtures |
– 20 % |
Determination of cement content
a) From free W/C ratio and
b) quantity water per unit volume of concrete for workability (as derived in 5.c)
Cement by mass = (Water content) / (w.c.r.)
This cement content should not be less than minimum content from the aspect of durability (given in 2.b)
Calculation of aggregate content (fa and Ca)
where V = absolute volume of concrete
= gross volume (1m 3 ) minus the volume of entrapped air
S c = specific gravity of cement
W = Mass of water per cubic metre of concrete, kg
C = mass of cement per cubic metre of concrete, kg
p = ratio of fine aggregate to total aggregate by absolute volume
f a , C a = total masses of fine and coarse aggregates, per cubic metre of concrete, respectively, kg, and
S fa , S ca = specific gravities of saturated surface dry fine and coarse aggregates, respectively
Determine the concrete mix proportions for the first trial mix.
Prepare the concrete using the calculated
proportions and cast three cubes of 150 mm size and test them wet after 28-days moist curing and check for the strength.
Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.
So the mix proportion works out to be
W : C : fa : ca
= 185 : 411 : 635 : 1150
= 0.45 : 1 : 1.55 : 2.80
This mix will be considered as Trial Mix No.2