Building Materials, Third Edition

g™

2w2h QPW

for coarse aggregate

V =





   

11

1100

a

cca

C C

W

SpS

Where,
V = absolute volume of fresh concrete; i.e., gross volume, m^3 —volume of entrapped
air
W = mass of water per m^3 of concrete, kg or lit
C = mass of cement per m^3 of concrete, kg
Sc = specific gravity of cement
p = ratio of fine aggregate to total aggregate by absolute volume
fa, Ca = total quantity of fine and coarse aggregate per m^3 of concrete, respectively, kg
Sfa, Sca = specific gravity of saturated surface dry fine and coarse aggregates.
The entrapped air in the concrete depends upon the maximum size of aggregate used in
making the concrete. Table 11.11 gives quantity of entrapped air for different maximum sizes
of aggregate.

„—˜"2II    II e+&

—2e2g

x—!2—

2  & i—2—D2™—2

2——2@--A 

-22™™

IH QH

PH PH

RH IH

i

—$

Example 11.1: At a site, the first 30 test results in N/mm^2 , for M 25 concrete are as follows: 31.4,
32.5, 30.9, 33.2, 32.7, 31.8, 33.0, 32.5, 33.4, 30.0, 34.1, 33.8, 29.7, 26.3, 24.6, 25.4, 27.8, 32.6, 28.3,
31.7, 30.0, 29.8, 24.3, 23.2, 24.9, 27.5, 22.0, 26.1, 25.5, 30.8.
The results of the three test cubes of the 31st sample are 23, 24.5 and 25.
Ascertain the acceptability of concrete.

Solution: As per IS:465–2000, concrete corresponding to 31st sample may be acceptable even if
some of the cubes have strength less than fck.

For 31st sample, 22 23 24.5 2524.2 N/mm^2

3

x , which is less than fck (fck = 25N/mm^2 )

For the given 30 test results, x = 29.32 N/mm^2 and 
 = 3.56

Now applying the provisions of the acceptance criteria:

(i) fck–1.35 
 = 25– 1.35 × 3.56

= 20.19 N/mm^2

(ii) 0.8 fck = 0.8 × 25 = 20 N/mm^2

The greater of (i) and (ii) is 20.19 N/mm^2. The test result is 24.2 N/mm^2 which is greater

than 20.19 N/mm^2 , and hence, the concrete is acceptable.