Compressive strength of cement

AIM
To determine the compressive strength of standard cement mortar cubes compacted by means of standard vibration machine.

APPARATUS
Vibration machine and cube moulds of size 7.06 cms (Conforming to IS: 4031-1988)

STANDARD SAND
The standard sand to be used in the test shall conform to IS: 650-1991 or
sand passing 100 percent through 2 mm sieve and retained 100 percent on 90 micron IS sieve.
sand passing 100 percent through 2 mm sieve and retained 100 percent on 90 micron IS sieve.

2mm to 1mm
1mm to 500 microns
500mm to 90 microns

33.33 percent
33.33 percent
33.33 percent.

THEORY 
The compressive strength of cement mortars is determined in order to verify whether the cement conforms to IS specifications and whether it will be able to develop the required compressive strength of concrete. The average compressive strength of at least three mortar cubes (area of the face 50 cm2 ) composed of one part of cement and three parts of standard stand should satisfy IS code specifications.
verify whether the cement conforms to IS specifications and whether it will be able to develop the required compressive strength of concrete. The average compressive strength of at least three mortar cubes (area of the face 50 cm2 ) composed of one part of cement and three parts of standard stand should satisfy IS code specifications.

PROCEDURE:
Mix proportions and mixing:
1. Clean appliances shall be used for mixing and the temperature of the water and that of the
test room at the time when the above operations are being performed shall be 27 + 2°C.

2. Place in a container a mixture of cement and standard sand in the proportion of 1:3 by weight mix it dry, with a trowel for one minute and then with water until the mixture is of uniform color.

3. The quantity of water to be used shall be as specified below.

4. In any element, it should not take more than 4 minutes to obtain uniform colored mix.

5. If it exceeds 4 minutes the mixture shall be rejected and the operation repeated with a fresh quantity of cement, sand and water.

6. The material for each cube shall be mixed separately and the quantity of cement standard sand and water shall be as follows:

Cement 200 gms
Standard sand 600 grms
Water (P/4 + 3.0) percent of combined weight of cement and
sand, where p is the percentage of water required to produce a paste of standard consistency.

MOULDING SPECIMENS
1. In assembling the moulds ready for use, cover the joints between the halves of the mould with a thin film of petroleum jelly and apply a similar coating of petroleum jelly between the contact surfaces of the bottom of the mould and its base plate in order to ensure that no water escapes during vibration.
2. Treat the interior faces of the mould with a thin coating of mould oil.
3. Place the assembled mould on the table of the vibration machine and firmly hold it is position by means of suitable clamps.
4. Securely attach a hopper of suitable size and shape at the top of the mould to facilitate filling and this hopper shall not be removed until completion of the vibration period.
5. Immediately after mixing the mortar, place the mortar in the cube mould and rod with a rod.
6. The mortar shall be rodded 20 times in about 8 seconds to ensure elimination of entrained air and honey combing.
7. Place the remaining quantity of mortar in the hopper of the cube mould and rod again as specified for the first layer and then compact the mortar by vibrations.
8. The period of vibration shall be two minutes at the specified speed of 12,000 + 400 vibrations per minute.
9. At the end of vibration remove the mould together with the base plate from the machine and finish the top surface of the cube in the mould by smoothing surface with the blade of a trowel.

CURING SPECIMEN
1. Keep the filled moulds at a temperature of 27 + 20 C in an atmosphere of at least 90 % relative humidity for about 24 hours after completion of vibration.
2. At the end of that period remove them from the moulds.
3. Immediately submerge in clean fresh water and keep them under water until testing.

4. The water in which the cubes are submerged shall be renewed every 7 days and shall be maintained at a temperature of 27 °C + 2°C.
5. After they have been taken out and until they are tested the cubes shall not be allowed to
 become dry.

TESTING:
1. Test three cubes for compressive strength at the periods mentioned under the relevant specification for different hydraulic cements, the periods being reckoned from the completion of vibration.
2. The compressive strength shall be the average of the strengths of three cubes for each period of curing.
3. The cubes shall be tested on their sides without any packing between the cube and the steel platens of the testing machine.
4. One of the platens shall be carried base and shall be self adjusting and the load shall be steadily and uniformly applied starting from zero at a rate of 350 Kgs/Cm2/ min.
The cubes are tested at the following periods
Ordinary portland cement 3, 7 and 28 days.
Rapid hardening portland cement 1 and 3 days.
Low heat portland cement 3 and 7 days.

CALCULATION:
Calculate the compressive strength from the crushing load and the average area over which the
load is applied. Express the results in N/mm2 to the nearest 0.05 mm2.
Compressive strength in N/mm2 = P/A =
Where P is the crushing load in N
and A is the area in mm2 (5000 mm2)

PRECAUTIONS
Inside of the cube moulds should be oiled to prevent the mortar from adhering to the sides of the mould.

RESULT
The average compressive strength of the given cement

at 3 days
at 7 days
at 28 days

________N/mm2
________N/mm2
________N/mm2

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