Compressive strength of cement -(Procedure and formula for Test)

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What is compressive strength of cement?

Compressive strength of the cement is the property of cement which specifies how much load it can withstand when cement is made into a hardened mass mixing with standard sand and water.

The process of testing compressive strength of concrete and and cement may be somewhat same but the materials required for preparing cubes are different. Please read carefully without skipping to get full concept.

Why should we do the compressive strength test of cement?

Strength is the most important property of cement. The compressive strength test of cement is done because, with an increase in compressive strength of cement strengths such as flexural strength, resistance to abrasion, and so on also increases. With one test we can determine various aspects of cement and can decide the suitable types of cement for the construction site.

What factors affect the compressive strength of cement?

  • Water-cement ratio
  • Cement-sand ratio
  • Type and grade of sand
  • Manner of mixing
  • Shape and size of the specimen
  • Curing condition
  • Age of specimen
  • Rate of loading

Compressive strength test of cement

Apparatus required

  1. Cube mould of dimension 70.6mm * 70.6mm * 70.6mm.
  2. Weighing machine (with the accuracy of ±1 gm in every 1000gm)
  3. Trowel
  4. Metal tray
  5. Vibrating Machine
  6. Stopwatch
  7. Compression testing machine (With the accuracy of ± 1%)

Materials required

  1. Cement (say OPC)
  2. Sand (standard grade)
  3. Water (for having a standard consistency)

Procedure for Compressive strength test of cement

  • The ratio of cement and sand that should be taken to carry out the test should be 1:3.
  • Let us take 200 gm of cement, then 600 gm of sand should be taken.
  • They should be mixed thoroughly for 1 minute.
  • Then the calculation of water required to make a standard consistency is calculated using the formula:

Where P is the percentage of water required for producing a paste of standard consistency.

Then, we have P = 30 for OPC cement. Putting the value of P in the above equation, we get the amount of water = 84 gm.

  • 84 gm of water is added to the dry mixture and mixed properly for three minutes.
  • The cube mould is assembled and placed on the table of the vibrating machine.
Compressive strength of cement -(Procedure and formula for Test)
Cement and Sand Mixture Cube
  • Then the mould oil is applied in the mould and the mortar is poured in the mould. The mould is then prodded with a rod.
  • Then vibration is applied in the rate of 12000 ± 400 per minute.
  • After 2 minutes, the mould together with the base plate is removed from the machine and the top surface is finished levelling up with the help of a trowel.
  • Now, the above process is repeated for the next 8 cubes.
  • Then the cubes are placed at a place and covered by moist gunny bags for 24 hours.
  • After 24 hours, all the filled cube mould should be demoulded and marked with date and number.
  • After that, the cubes are submerged in the freshwater tank for curing.
Compressive strength of cement -(Procedure and formula for Test)
  • The three cubes each for third, seventh and 28th days are then tested for compressive strength.
  • The weight of cubes is measured and data is recorded.
  • The cubes are placed in the compression testing machine and load is applied in the rate of 35 N/mm²/min.
  • The load at which the cube is crushed is noted.
Compressive strength of cement -(Procedure and formula for Test)
Compression Testing Machine

Then compressive strength for each cube on their respective test day is calculated using the formula:

  • The average compressive strength of three cubes is the final compressive strength of the cement.

Observation Table for Compressive strength test of cement

S.NAge of cube (in days)Weight of cube (gm)Density (gm/cc)Area (mm²)Crushing load of Compression Testing Machine (KN)Compressive strength (N/mm²)Average strength (N/mm²)
13      
23      
33      
47      
57      
67      
78      
88      
98      
  • Now, the procedure from 15- 19 is repeated for day 7 and 28 and data is filled in the above table.

Precaution

  1. The temperature of the room and cube should be maintained 27±2ºC.
  2. The water used for curing should be changed every 7 days.
  3. The cube should not be dried before the completion of the compressive strength test.
  4. If the mixture of cement, sand and water does not obtain uniform colour within 4 minutes of mixing, then the mixture should be discarded and the operation should be started from the start.
  5. Compressive strength depends upon the proportion and properties of raw materials used. So, the raw materials should be selected carefully.

Some important questions

Why should we use the cube mould of the size mentioned above and not the larger one?

This is because if we choose large size cubes, shrinkage and cracks will develop. This will make us unable to perform the compressive strength test.

Why is 33-grade cement called 33 grade?

This is because the maximum amount of compressive strength that can be gained by the cement in the 28 days is 33 N/mm².

Note:

In cement, the relationship between time and strength is non-linear.

Increasement in compressive strength of cement Table

TimeStrength %
1 day16%
3 days40%
7 days65%
14 days90%
28 days99%=say 100%
3 month110%
6 month115%
1 year120%

I hope this post remains helpful for you.

Happy Learning – Civil Concept

Contributed by,

Civil Engineer – Sushmita Niraula

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