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. Concrete compressive strength for general
construction varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and higher in commercial and industrial
structures.
Compressive strength of concrete depends on many factors
such as water-cement ratio, cement strength,
quality of concrete material, and quality control during production of concrete
etc.
Test for compressive strength is carried out either on cube or cylinder.
Various standard codes recommend 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 moulds 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 this 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.
Procedure for testing Compressive strength of Concrete Cubes following:-
Apparatus for Concrete Cube Test
Compression
testing machine
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 smoothed 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.
Precautions for Tests:
The water for curing should be tested every 7 days and
the temperature of water must be at 27+-2oC.
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.
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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 x 15cm x 15cm
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
|
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