Theory:
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. The test is therefore carried out on blocks of mortar made of cement, sand and water.
As the quality of sand from different sources varies, it is necessary to use sand of standard quality for this test. The standard sand consists of well graded sand of quartz, light grey or whitish in colour and free of silts and organic impurities. As per IS 650: 1991, standard sand is obtained from Ennore, Tamilnadu which has the following particle size distribution: Smaller than 2 mm but greater than 1 mm
: 33.33%Smaller than 1 mm but greater than 500 μ
: 33.33%Smaller than 500 μ but greater than 90 μ
: 33.33%mm are prepared, compacted and cured properly before testing under direct compression. The cubes are tested in compression testing machine at the end on three days, seven days and twenty eight days. For ordinary portland cement of 43 grade, the average compressive strength after 3, 7 and 28 days should not be less than 23 N/mm2, 33 N/mm2 and 43N/mm2 respectively.
Objective:
Determination of compressive strength of cement.
Reference:
IS 4031-1988 (Part-6).
Apparatus:
Vibration Machine, Poking Rod, Cube Mould of 70.6 mm size conforming to IS: 10080-1982, Balance, Gauging Trowel, Watch, Graduated Glass
Cylinders, etc.
Material:
Ordinary portland cement (43 grade); Water; Grease, Standard sand (IS: 650-
1966).
Procedure:
Preparation of cement mortar cubes:
1. Take 200gms of cement and 600 g of standard sand (i.e. ratio of cement to
sand is 1:3) in a non-porous enamel tray and mix them with a trowel for one
minute.
2. Add water quantity �𝑃𝑃4 + 3.0� % of combined weight of cement and sand and mix the three ingredients thoroughly until the mixture is of uniform colour. (‘P’ is the consistency of cement). The time of mixing should be less than three minutes and not more than four minutes.
3. Immediately after mixing fill the mortar into greased cube moulds of sizes
70.6 mm.
4. Compact the mortar either by hand compaction in a standard specified manner or on the vibrating table.
5. Place the moulds in cabin at a temperature of 27º ± 2º C for 24 hours
6. Remove the specimen from the moulds and submerge them in clean water for
curing.
Testing of cement mortar cubes:
7. Take the cube out of water at the end of three days with dry cloth. Measure the
dimensions of the surface in which the load is to be applied. Let them be ‘L’
and ‘B’ respectively.
8. Place the cube in compressive testing machine and apply the load uniformly at
the rate of 14N/mm2 per minute.
9. Note the load at which the cube fails. Let it be ‘F’.
10. Calculate the compressive strength of the cube by using formula F/𝐴𝐴 . Where
A is the area of loaded surface (i.e. L×B).
11. Repeat the same procedure (steps 7 to 10) for other two cubes.
12. Repeat the whole procedure (Step 7 to 11) to find the compressive strength of
the cube at the end of 7 days and 28 days.
Observations
Results:
The type and grade of cement :
The compressive strength of cement ate the end of
i) 3 days: ___________. N/mm2.
ii) 7 days: ___________. N/mm2.
iii) 28 days: ___________. N/mm2
Conclusions:
Average compressive strength of cement mortar at the end of 3 days, 7 days and 28 days are _______ N/mm2, _______ N/mm2 and _______ N/mm2, respectively. Thus the cement sample satisfies/ not satisfy the codal requirements for the given type and grade of cement.
Precautions:
(Discuss about the precautions to be taken while conducting this experiment)
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Questions:
1. Why strength test is not made on cubes of only cement paste?
2. What do you mean by standard sand? Why we need standard sand for present test?
3. What are the limiting values for minimum compressive strength of cement for following cements: ordinary port land cement (33; 43 and 53 grades) (at 3, 7 and 28 days); rapid hardening cement (1 and 3 days); Portland puzzolana cement (at 3, 7 and 28 days); high alumina cement at (1 and 3 days); low heat cement (at 3, 7 and 28 days); and super sulphated cement (at 3, 7 and 28 days)?
4. What will happen if the rate of application of load is more than the specified value?
