Chapter 4: Experimental Analysis of Recycled CDW Materials

4.2. COMPRESSIVE STRENGTH TESTING OF CEMENT MORTAR

     The compressive strength test of the cement mortar is done for the purpose of testing the quality of the recycled fine aggregates (less than 1.18mm size) which is used in place of the natural sand. Indirectly our purpose is to test the quality if recycled fine aggregates. As per IS: 4031 (Part 6) and IS: 4031 (Part 7).

APPARATUS: - Compression testing machine, cube mould of size 7.06cmx7.06cm x7.06cm, temping rod, vibrator, others

PREPARTAION OF THE CUBE SPECECIMEN: - The preparation of the cube specimens have been done as per IS code.

SPECIMEN: - 10 cubes of 7.06cm x 7.06cm x 7.06cm in which cube 01-05 made of mix ratio 1:3 and cube 06-10 made of mix ratio 1:6

HAND MIXING 

a) Mix the cement and fine aggregate on a water tight none-absorbent platform until the mixture is thoroughly blended and is of uniform color.

b) The quantity of water shall be (p/4+3) % of combined weight of cement and fine aggregates where, p is the % of water required to produce a paste of standard consistency determined earlier. Add water and mix it until the cement mortar appears to be homogeneous and of the desired consistency.

SAMPLING 

a) Clean the mounds and apply oil 

b) Fill the cement mortar mix in three layers 

c) Compact each layer with not less than 20 strokes per layer using a tamping rod 

d) Level the top surface and smoothen it with a trowel.

CURING 

The test specimens are stored in moist air for 24hours and after this period the specimens are marked and removed from the molds and kept submerged in clear fresh water until taken out prior to test.

PROCEDURE 

a) Remove the specimen from water after specified curing time and wipe out excess water from the surface.

b) The dimension of the cube should be checked.

c) Clean the bearing surface of the testing machine. 

d) Place the specimen in the machine in such a manner that the load shall be applied to the opposite sides of the cube cast. 

e) Align the specimen centrally on the base plate of the machine. 

f) Rotate the movable portion gently by hand so that it touches the top surface of the specimen. 

g) Apply the load gradually without shock and continuously at the rate of 350kg/cm2/minute till the specimen fails. 

h) Record the maximum load for the further calculation of the compressive strength.

CALCULATIONS

Size of the cube =7.06cm x 7.06cm x 7.06cm 

Weight of the Specimen (Kg) = W Kg Area of the specimen = approx. 50cm2 Compressive strength (fck) at 7 days or 28 days = Load/Cross Section Area of cube (N/mm2)

RESULT

Compressive strength at 7 days = 6.0 N/mm2 (average) Compressive strength at 28 days = 9.0 N/mm2 (average)

Note: - For ordinary Portland cement 43 grade, the compressive strength at 3 days, 7 days and 28 days curing shall not be less than 23 MPa, 33 MPa and 43 MPa respectively.

CONCLUSION OF THE TEST

The ordinary Portland cement stands on their strength, but the recycled fine aggregates used in place of the natural sand could not stand on their strength. The binding between the recycled fine aggregates was very weak and hence it could not carry appropriate load during testing in the compression machine. Hence, the result were not satisfactory. The recycled fine aggregates cannot be used in cement mortar and plaster.

Standard Consistency of Cement (Paste)

Experiment:- To determine standard consistency of cement paste.

Purpose- The purpose of this test is to find out the percentage by weight of water to be added to cement to make a paste of standard consistency.

Consistency: -  A certain minimum quantity of water is required to mixed with cement so as to complete chemical reaction between water and cement, less water than this quantity would not complete chemical reaction thus resulting in reduction of strength and more water would increase water cement ratio and so would reduce its strength. So, correct proportion of water to cement is required to be known to achieve proper strength while using cement in structure. This can be found out knowing standard consistency of cement paste.

Standard consistency:  (Normal consistency)

Standard consistency of cement paste is defined as that consistency which permits the Vicat plunger to penetrate to a point 5 to 7 mm from the bottom of the Vicat mould in this test. It is expressed as amount of water as a percentage (by weight) of dry cement.

Necessity and uses of standard consistency

• Generally the water percentage for standard consistency is about 30%. However cement produced by different factories may not have same properties and so it is necessary to standardize the consistency using standard equipment.
• The knowledge of standard consistency is required while performing other important test such as setting time, soundness etc. as these tests are performed on cement paste using a certain percentage of water required for standard consistency.

Gauging time

It is period observed from the time when water is added to cement for making cement paste till commencing the filling of mounld of Vicat Apparatus, in this test.

Apparatus :-

• A Vicat apparatus with plunger 10mm dia
•  Weighing balance
• Trowel weighing 210g ± 10g
•  Measuring glass 200ml
• Clean platform to make cement paste

Materials: -

• Cement 3 to 4 kg
• Water

Procedure: -

• Take a sample off cement 300g. Weigh it correctly.
• Add water 25% by weight i.e., 75g (75ml) to the cement and mix it thoroughly, to make a cement a cement paste.
• Place the Vicat mould on a non-porous plate, fill the mould with cement paste, smooth off the surface of the paste making it level with the top of the mould. To expel air shake the mould slightly. The care that the gauging time is between 3 to 5 minutes. While filling the mould use of operator’s hand and blade of trowel only should be used.
• Place the test block in the mould together with the non-porous plate, under the rod bearing the 10mm dia plunger gently to touch the surface of the test block and quickly release, allowing it to sink into the cement paste. Carry this operation immediately after filling the mould. Note the penetration of the needle from the bottom of mould indicated on the scale.
• Prepare trial pastes with varying percentages of water, increasing water percentage by 1 % each time, until the amount of water necessary for making standard consistency (which permits plunger to penetrate to a point 5 to 7 mm from the bottom of the mould) is determined. 

Observations:

S. No.	Wt. of water, W1 g	Wt of cement, W2 g	(W2/W1) X10	Positon of Vicat needle point after penetration from bottom
1. 2. 3. 4. 5. 6.

Result: - The percentage by weight of water with respect to cement to produce standard consistency is ______%.

Requirement: - As per IS 4031-1968 the standard consistency is obtained when the Vicat plunger penetrates to a point 5 to 7 mm from bottom of Vicat mould.(This is generally 30%)

Conclusion:- For the given sample of cement, the standard consistency is achieved with water percentage equal to _______%.