Compressive strength of concrete cube test provides an idea about all the characteristics of concrete. By this
single test one judge that whether Concreting has been done properly or not.
Compressive
strength of concrete depends on many factors such as water-cement ratio, cement
strength, quality of concrete material, quality control during production of
concrete etc.
Test
for compressive strength is carried out either on cube or cylinder. Various
standard codes recommends concrete cylinder or concrete cube as the standard
specimen for the test. American Society for Testing Materials ASTM
C39/C39M provides Standard Test Method for Compressive
Strength of Cylindrical Concrete Specimens.
Procedure: Compressive Strength Test of Concrete Cubes
For
cube test two types of specimens either cubes of 15cm X 15cm X 15cm or 10cm X
10cm x 10cm depending upon the size of aggregate are used. For most of the
works cubical moulds of size 15cm x 15cm x 15cm are commonly used.
This
concrete is poured in the mould and tempered properly so as not to have any
voids. After 24 hours these moulds are removed and test specimens are put in
water for curing. The top surface of these specimen should be made even and
smooth. This is done by putting cement paste and spreading smoothly on whole
area of specimen.
These
specimens are tested by compression testing machine after 7 days curing or 28
days curing. Load should be applied gradually at the rate of 140 kg/cm2 per
minute till the Specimens fails. Load at the failure divided by area of
specimen gives the compressive strength of concrete.
Apparatus for Concrete Cube Test
- Compression testing machine
- Cube Mould
- weighing balance.
- steel foot rule
Preparation of Concrete Cube Specimen
The proportion and material for making these test specimens are
from the same concrete used in the field.
Specimen
6 cubes of 15 cm size Mix. M15 or above
Mixing of Concrete for Cube Test
Mix the concrete either by hand or in a laboratory batch mixer
Hand Mixing
(i)Mix the cement and fine aggregate on a water tight
none-absorbent platform until the mixture is thoroughly blended and is of
uniform color
(ii)Add the coarse aggregate and mix with cement and fine
aggregate until the coarse aggregate is uniformly distributed throughout the
batch
(iii)Add water and mix
it until the concrete appears to be homogeneous and of the desired consistency
Sampling of Cubes for Test
(i) Clean the mounds and apply oil
(ii) Fill the concrete in the molds in layers approximately 5cm
thick
(iii) Compact each layer with not less than 35strokes per layer
using a tamping rod (steel bar 16mm diameter and 60cm long, bullet pointed at
lower end)
(iv) Level the top surface and smoothen it with a trowel
Curing of Cubes
The test specimens are stored in moist air for 24 hours and
after this period the specimens are marked and removed from the molds and kept
submerged in clear fresh water until taken out prior to test.
Procedure for Cube Test
(I)
Remove the specimen from water after specified curing time and wipe out excess
water from the surface.
(II)
Take the dimension of the specimen to the nearest 0.2m
(III)
Clean the bearing surface of the testing machine
(IV)
Place the specimen in the machine in such a manner that the load shall be
applied to the opposite sides of the cube cast.
(V)
Align the specimen centrally on the base plate of the machine.
(VI)
Rotate the movable portion gently by hand so that it touches the top surface of
the specimen.
(VII) Apply the load gradually without shock and
continuously at the rate of 140 kg/cm2/minute till the
specimen fails
(VIII)
Record the maximum load and note any unusual features in the type of failure.
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 three specimens gives the crushing
strength of concrete. The strength requirements of concrete.
Calculations for Concrete Cube Tests for
Compressive Strength
Size
of the cube =15cm x15cm x15cm
Area of the specimen (calculated from the mean size of
the specimen )=225 cm2
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
Reports of Cube Test
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
Results of Concrete Cube Test
Average compressive strength of the concrete cube =
………….N/ mm2 (at 7 days)
Average compressive strength of the concrete cube =……….
N/mm2 (at 28 days)
Compressive 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
|
excellent information about cube test
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