Compressive Strength of Cement

 

Compressive Strength test of Cement and How to find it ?

For any general construction works, Compressive strength is the important basic data to be considered while selecting the cement.

Compressive Strength of Cement: 

Compressive strength is the property of material or structure to resist under compression. 

The Compressive strength of a material is determined by the ability of the material to resist failure in the form of cracks and fissure.

In this test, we will record the impact force applied on both faces of specimen made with Cement.

As well as the maximum compression that cement specimen bears without failure recorded.

In simple Technical way,  compressive strength of cement means :

The ability of cement specimen to withstand or capable of the compressive stress when tested under Compressive Testing Machine [CTM] at 28 days. 

Appratus required for these Test are :

Procedure:

1. Apparatus should be clean & room temperature should be maintained for this test should be 27 ± 2°C.
2. Now, we have to Mix the cement & sand with the trowel for about 1 min on Non–porous plate. Cement should be free from lumps.
3. After that, we have to add water & mix for 3 minutes until the paste is of uniform colour. The quantity of water mixed with the cement, the sand mixture should be
   (P/4 + 3) % where P is the percentage of water required to produce the Standard consistency
4. Clean the mould and apply oil to moulds for easy removal of mortar cube after drying. 
5. Now pour the mortar in steel mould. Tamp the cube with rod for 20 times in 8 sec to eliminate the entrained air.
6. You can also use vibrator instead of a rod. The vibrator is played for the period of 2 mins with the speed 12000±400 vibrations /minute.
7. After completing the vibration, now remove the mould from the vibration machine & place it in room temperature for 24 hrs. 
8. After setting of mortar cube in 24 hours, we have to open the steel mould from mortar cube.
9. Now we have to Keep the test specimens submerged underwater for 7, 14 or 28 days and for every 7 days the water is changed.
10. Test the three cubes, one at 7th day, other at 14th day & another at 28th day.
11. After that these mortar cubes are placed in the space between bearing surfaces of the Compressive strength machine. 
12. During testing, care to be taken that The loading must be applied axially on specimen without any shock & increased at the rate of 35 N/mm2/min. till the specimen collapse.
13. Due to the constant application of load on the face of the cube, the mortar cube starts cracking and fails at a point.
14. We have to note down the reading from Compressive testing machine where the specimen starts failing.

The formula for calculating Compressive strength of cement is

The maximum load carried by the mortar specimen which means the load point on CTM at which the specimen starts breaking is divided with the surface area (contact area).

Example:

Assume the test specimen tested on 28 days  and  max compression load at which the specimen starts breaking is 165KN.

As per above fig, Surface are is 50 Sq.cm = 5000mm2

1 KN = 1000N = 165 x 1000 = 165000

As per formula,

Compressive strength of cement  = 165000/5000 = 33N/mm2 

Hence, the Grade of cement is 33 Grade .

Observations:

Important notes : 

 As per the IS recommendation, at least 3 cubes to be tested on 7 days, 14 days, 28 days.

Ater that we have to take the average of the three cubes are rounded to nearest 0.5N/mm2 . Do not consider the specimen  which differs more than 10% of the average value of compressive strength.

Results: 

The average Compressive strength of cement cube at 7 days _________N/mm2 

The average Compressive strength of cement cube at 14 days ________N/mm2 

The average  Compressive strength of cement cube at 28 days________N/mm2 .

Watch Video: Compressive Strength Test of Cement

Video 1 :

Video 2 :

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