Compressive Strength Laboratory test for Cement

The heart of concrete lies in the cement.  Several tests should be performed to determine the characteristics of cement and its compatibility with other materials in the concrete mix design.  Compressive strength testing of mortar cubes at 3-days, 7-days and 28-days of aging are used to observe the development of the strength gain of the mortar over time.  A chemical analysis of the cement will provide a reasonable estimate of the composition of the cement.  These are just a few of the tests that can be used to determine the quality of the cement. 

Among physical properties of cement, compressive strength is the most important property. When cement is used for important structures, compressive strength test is always carried out to ascertain quality of cement.Strength test is not made on plain cement due to excess shrinkage and cracking of plain cement paste.

Materials and apparatus required :

1. Cement
2. Standard sand
3. Vibration Machine
4. Poking Rod
5. Cube Mould of size 70.6 mm X 70.6 mm X 70.6 mm
6. Gauging Trowel
7. Weigh Balance
8. Graduated glass cylinders – 200 to 250 ml capacity

Preparation of specimen and compressive strength test procedure

1. Mixing of material for each test cube shall be separately prepared. The quantities of cement,standard sand and water shall be as follows:
Cement – 200 gms, Standard sand – 600 gms, water – (p/4+3) % of mass (cement +sand) where P = % of water required to make cement paste of standard consistency.

2.All ingredients shall be dry mixed for one minute. After that, water is gradually added till paste is formed of uniform colour. Mixing time shall be between 3-4 minutes. If uniform colour of paste is not achieved after mixing more than 4 minutes, fresh mortar shall be prepared.

3.Apply mould oil on interior surface of mould and place the mould on vibrating table. Immediately after preparing mortar as describe in point 2, place the mortar

inside cube moulds. Rod 20 times for approximate 8 sec to ensure full compaction of mortar.

4. Further compaction shall be done by using vibrating machine.The period of vibration shall be two minutes at the specified speed of 12 000 ± 400

5.After completion of vibration, remove moulds along with base plate and finish the top surface by trowel.

6.Keep moulds filled with specimen for 24 ± 1 hr in moist environment. Remove samples from mould and immediately submerge in clean water.

7.After curing period is over, remove cubes from water and immediately place in testing machine with side facing upwards. Cubes shall be tested with out packing between steel plattens of the testing machine and cubes surface.Load shall be steadily and uniformly applied, starting from zero at a rate of 35 N/mm2/min. till failure of sample.

8.Calculate compressive strength of specimen by using following formula
compressive strength = P / A

where

P = Maximum load applied

A =  cross-sectional area of test specimen

Compressive strength shall  be calculated as average value of three samples. If value of individual sample is differing more than 10 % of average value, then that value shall not be considered. In that case average of other two samples shall be considered for compressive strength calculation.