SPONSORED
Compressive strength is the capacity of a material to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g. many concrete structures have compressive strengths in excess of 50 MPa, whereas a material such as soft sandstone may have a compressive strength as low as 5 or 10 MPa
To determine the compressive strength of concrete specimens prepared and to verify the strength requirements as desired in the mix design and stipulated in the IS code.
APPARATUS
APPARATUS
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| a sample type of compression testing machine |
PREPARATION OF CUBE SPECIMENS
Quantity of materials required are calculated as per 6.3
SPECIMEN
6 cubes of 15 cm size Mix. M15 or above
MIXING
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
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
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
The test specimens are stored in moist air for 24hours 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
The water for curing should be tested every 7days and the temperature of water must be at 27+-2oC.
PROCEDURE
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 140kg/cm2/minute till the specimen fails
VIII) Record the maximum load and note any unusual features in the type of failure.
NOTE : A minimum of 3 cubes should be tested.
CALCULATIONS
1. Size of the cube =15cm x15cm x15cm
2. Area of the specimen (calculated from the mean size of the specimen )=225cm2
3. Characteristic compressive strength(f ck)at 7 days =
4. Expected maximum load =fck x area x f.s
5. Range to be selected is …………………..
6. Similar calculation should be done for 28 day compressive strength
7. Maximum load applied =……….tones = ………….N
8. Compressive strength = (Load in N/ Area in mm2)=……………N/mm2
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| Compressive strength of concrete cubes value |
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)
