NOTE

Minimum three specimens should be tested at each selected age. If strength of any specimen varies by more than 15 per cent of average strength, results of such specimen should be rejected. Average of there specimens gives the crushing strength of concrete. The strength requirements of concrete.

CALCULATIONS
Size of the cube =15cm x15cm x15cm
Area of the specimen (calculated from the mean size of the specimen )=225cm2
Characteristic compressive strength(f ck)at 7 days =
Expected maximum load =fck x area x f.s
Range to be selected is …………………..
Similar calculation should be done for 28 day compressive strength
Maximum load applied =……….tones = ………….N
Compressive strength = (Load in N/ Area in mm2)=……………N/mm2
=……………………….N/mm2
REPORT
a) Identification mark
b) Date of test
c) Age of specimen
d) Curing conditions, including date of manufacture of specimen
f) Appearance of fractured faces of concrete and the type of fracture if they are unusual
RESULT
Average compressive strength of the concrete cube = ………….N/ mm2 (at 7 days)
Average compressive strength of the concrete cube =………. N/mm2 (at 28 days)

Percentage strength of concrete at various ages:

The strength of concrete increases with age. Table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.
Age
Strength per cent
1 day
16%
3 days
40%
7 days
65%
14 days
90%
28 days
99%

Compressive strength of different grades of concrete at 7 and 28 days

Grade of Concrete
Minimum compressive strength N/mm2 at 7 days
Specified characteristic compressive strength (N/mm2) at 28 days
M15
10
15
M20
13.5
20
M25
17
25
M30
20
30
M35
23.5
35
M40
27
40
M45
30
45