Compressive Strength Test of Cement

The compressive strength of hardened cement is the most important of all properties. Therefore, cement is always tested for its strength at the laboratory before the cement is used in important works. Strength tests are not made on neat cement paste because of difficulties of excessive shrinkage and subsequent cracking of neat cement. The strength of cement is indirectly found on cement sand mortar i specific proportions.
The compressive strength test on hydraulic cement other than masonry cement is performed as per IS 4031 Part-6.
The standard sand to be used shall conform to IS 650

Apparatus

1. Vibration machine: Vibration machine shall conform to IS 10080


2. Poking Rod: Poking rod shall conform to IS 10080


3. Cube Mould: The mould shall be of 70.6 mm size conforming to IS 10080


4. Gauging Trowel: The gauging trowel shall have a steel blade 100 to 150 mm in length with straight edges weighing $210 \pm 10 g$


5. Balance: The permissible variation at a load of 1000 g shall not be more than $\pm 1 g$ for a new balance and $\pm 0.5 g$ for old balance

Preparation of Test Specimens

1. The quantity of cement, standard sand and water to fill a cube is
   Cement: 200 g
   Standard sand: 600 g
   Water: $(\frac{P}{4}+3.0)$ per cent of the combined mass of cement and standard sand. Where $P$ is the water required to produce a paste of standard consistency.
   Note: The material for each cube should be mixed separately.


2. Mix the sand and cement on a non-porous base for a minute and then add water. Mixed it until a uniform colour is obtained. The time of mixing should not be less than 3 minutes nor greater than 4 minutes.

Moulding Specimens

1. 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. Treat the interior faces of the mould with a thin coating of mould oil.


2. Place the assembled mould on the table of the vibration machine and hold it firmly in position by means of a suitable clamp. Attach a hopper of suitable size and shape securely at the top of the mould to facilitate filling and this hopper shall not be removed until the completion of the vibration period.


3. Immediately after mixing the mortar, place it in the cube mould and prod with the poking rod 20 times in 8 seconds. Place the remaining portion of mortar in the hopper and prod again and then compact the mortar by vibration.
   The period of vibration shall be 2 minutes at a specified speed of $12000 \pm 400$ vibrations per minute.


4. 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 the surface with the blade of a trowel.


5. Curing Specimen: Keep the compacted cubes i the mould at a temperature of $27 \pm 2^\circ C$ and at least 90% relative humidity for 24 hours. After 24 hours the cubes are removed from the mould and immersed in clean fresh water until taken out for testing.

Testing of Cubes

Test three cubes for compressive strength for each period of curing mentioned under the relevant specifications for different hydraulic cement, the periods being reckoned from the completion of vibration.

The cubes shall be tested on their sides without any packing between the cube and the steel plattens of the testing machine. One of the plattens shall be carried on a base and shall be self-adjusting, and the load shall be steadily and uniformly applied, starting from zero at a rate of $35 N/mm^2/min$.

Compressive Strength of different types of cement

Type of Cement	Compressive Strength
	1 Day min. MPa	3 Days min. MPa	7 Days min. MPa	28 Days min. MPa
OPC 33	NS	16	22	33
OPC 43	NS	23	33	43
OPC 53	NS	27	37	53
Rapid Hardening	16	27	NS	NS
Low Heat Cement	NS	10	16	35

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