COMPRESSIVE STRENGTH OF CEMENT

Compressive Strength of Cement is the capacity of taking compressive loads of cement.

It is determined by compressive strength test on mortar cubes compacted by means of a standard vibration machine. Standard sand (IS:650) is used for the preparation of cement mortar. The specimen is in the form of cubes 70.6mm*70.6mm*70.6mm.

APPRATUS:-

1. Cube Mould of size 70.6mmX70.6mmX70.6mm (As per IS:10080)

2. Vibration Machine (As per IS: 10080)

3. Compression Testing Machine

4. Measuring Cylinder

5. Balance

PROCEDURE:-

1. Environmental condition for cement testing should be of temperature 27± 2⁰ C.

2. Take 200 g of cement and 600 g of standard sand and mix them dry thoroughly.

3. Add (P/4+3)X8 [ P is % of water required for preparing paste of standard consistency] to the dry mix of cement and sand and for a time of 3 to 4  minutes mix them properly si that they form a mix of uniform colour. If the mix is not homogeneous after 4 minutes reject the mix and do the procedure again.

4. Clean the cube moulds of cement thoroughly and place them  the vibrating machine. It should be hold properly in position by clamps provided on the machine..

5. Fill the mould with entire quantity of mortar using a suitable hopper attached to the top of the mould for facility of filling and vibrate it for 2 minutes at a specified speed of 12000±400 per minute to achieve full compaction.

6. Remove the mould from the machine and keep it in a place with temp of 27±2⁰C and relative humidity of 90% for 24 hours.

7. After 24 hrs remove the cube from the mould and immediately submerge in fresh clean water in temperature controlled cube curing tank where its temp. is kept in 27±2⁰C. They should be submerged in waster till testing.

8. Prepare at least 9 cubes in the manner explained .

9. Place the test cube on the platform of a compressive testing machine so that the cubes central line and central line of platform coincides. No packing materials to be used between the cubes and loading plates.

10. At the rate of 35 N/mm²/minute apply the load steadily and uniformly.

11. 3 Cubes to be tested on completion of 3,7 and 28 days of testing and to be reported as shown below.

CALCULATIONS:-

Compressive Strength = P/A

[ P=Maximum load applied to the cube. (N)

A=Cross sectional area (Calculated from the mean dimensions) (mm²) ]

• Compressive strength is reported to the nearest 0.5 N/mm²
• Specimens that are manifestly faulty, or that give strengths differing by more than 10% from the average value of all the test specimen should not be considered.
• Test three cubes for compressive strength for each period of curing.

REPORTING OF RESULTS:-

Test result of Cement Cube Testing is reported as following..

CONSISTENCY OF CEMENT

Consistency is defined as the amount of water that can be added to cement to produce a plastic paste of cement and the ingredients of of concrete.

The standard consistency of a cement paste is defined as that consistency which will permit the Vicat Plunger to penetrate to a point 5 to 7 mm from the bottom of the Vicat mould.

APPARATUS:-

1. Vicat apparatus conforming to IS: 5513 – 1976
2. Balance, whose permissible variation at a load of 1000g should be +1.0g
3. Gauging trowel conforming to IS: 10086 – 1982

PROCEDURE:-

1. Prepare a paste of weighed quantity of Cement with a weighed quantity of potable or distilled water, taking care that the time of gauging is not less than 3 minutes, nor more than 5 min, and the gauging shall be completed before any sign of setting occurs. The gauging time shall be counted from the time of adding water to the dry cement until commencing to fill the mould. Fill the Vicat mould E with this paste, the mould resting upon a non-porous plate. After completely filling the mould,smoothen the surface of the paste, making it level with the top of the mould. The mould may be slightly shaken to expel the air.

2. Place the test block in the mould, together with the non-porous resting plate, under the rod bearing the plunger; lower the plunger gently to touch the surface of the test block, and quickly release, allowing it to sink into the paste.This operation shah be carried out immediately after filling the mould.

3. Prepare trial pastes with varying percentages of water and test as described above until the amount of water necessary for making up the standard consistency as defined above is found.

CALCULATION:-

Prepare trial pastes with varying percentages of water and test as described above until the amount of water necessary for making up the standard consistency i.e. the vicat apparatus shows a reading of 5 to 7mm.

Sample table for calculation and representation of results for standard consistency of cement is shown below.

Soundness Test of Cement

      It is very important that the cement after setting shall not undergo any appreciable change of volume. Certain cements have been found to undergo a large expansion after setting causing disruption of the set and hardened mass.This will cause serious difficulties for the durability of structures when such cement is used.This test is to ensure that cement does not show any appreciable subsequent expansion is of prime importance.

     The unsoundness in cement is due to the presence of excess of lime than that could be combined with acidic oxide at the kiln.This is also due to inadequate burning of insufficiency in fineness of grinding or through mixing of raw materials.It is also likely that too high a proportion of magnesium content or calcium sulphate content may cause unsoundness in cement. For this reason the magnesia content allowed in cement is limited to 6%.

APPARATUS
i) The apparatus for conducting the Le-Chatelier test should conform to IS: 5514 - 1969
ii) Balance, whose permissible variation at a load of 1000g should be +1.0g
iii) Water bath

PROCEDURE
i) The mould is placed on a glass sheet and it is filled with the cement paste formed by gauging cement with 0.78 times the water required to give a paste of standard consistency.
ii) Then the mould is covered with another piece of glass sheet, a small weight is placed on its covering glass sheet and immediately the whole assembly is submerged in water at a temperature of 27 ± 2^oC and kept it there for 24hrs.
iii) The distance separating the indicator points to the nearest 0.5mm is measured (say d _l ).
iv) The mould is submerged again in water at the temperature prescribed above.The water is bought to boiling point in 25 to 30 minutes and it is boiled for 3hrs.
v) Then  the mould is removed from the water, allowed to cool.The distance between the indicator points is measured (say d ₂ ).
vi) (d ₂ – d _l) represents the expansion of cement.

REPORTING OF RESULTS
    The mean of the two values to the nearest 0.5mm to represents the expansion of cement.

Testing of Cement

Testing of Cement can be brought under two categories.
      (a) Field Testing        (b) Laboratory Testing

Field Testing
       It is sufficient to subject the cement to field tests when it is trusted for minor works. The following are the field tests.
(a) Open and the bag and take a good look at the cement. There should not be any visible lumps. The colour of the cement should normally be greening grey.
(b) Thrust your hand into the cement bag. It must give a cool feeling. There should not be any lump inside.
(c) Take a pinch of cement and feel between fingers. It should give a smooth and not a gritty feeling.
(d) Take a handful of cement and through it in to a bucket full of water, the particles should float for some time before they sink.
(e) Take about 100 grams of cement and a small quantity of water and make a stiff paste. From the stiff paste , pat a cake with sharp edges. Put it on a glass plate and slowly take it under water with a bucket.See the shape of the cake is not disturbed while taking it down the bottom of the bucket. After 24 hours the cake should retain its original shape and at the same time it should also set and attain some strength. 
      If a sample of cement satisfies the above field tests it may be concluded that the cement is not bad.The above tests does not really indicates that the cement is really good for important works. For using cement in important and major works it is incumbent on the part of the user to test the cement in the laboratory to confirm the requirements of the Standard Specifications with respect to its physical and chemical properties.The tests which are usually conducted in the Lab to confirm those properties are …
      1.Fineness Test   
      2.Initial and Final Setting Time Test
      3. Consistency of Cement 
      4. Compressive Strength of Cement
      5. Shear Test    
      6. Soundness Test    
      7. Heat of Hydration Test    
      8. Chemical Composition Test