Question

Consider the following statements regarding the 'strength of cement':

1. Strength test on cement is made on cubes of the cement-sand mix.

2. Water to be used for the paste is 0.2P, where P is the water needed for normal consistency.

3. The normal consistency is determined on Le Chatelier's apparatus.

4. Cubes are cast in three layers in leak-proof moulds further compacted in each layer by vibration on a machine.

Which of the above statements are correct?
1. 1 and 2
2. 2 and 3
3. 1 and 4
4. 3 and 4

Answer 1

Correct Answer - Option 3 : 1 and 4

Concept:

Compressive strength: this test is carried out to determine the compressive strength of cement. Following procedure is adopted:

The mortar of cement and sand is prepared. The proportion is 1 : 3 which means that x gm of cement is mixed with 3 x gm of sand.

The water is added to the mortar. The water cement ratio is kept as 0.4 which means that x gm of water is added to dry mortar.

Water to be used for the paste is (P/4) +3%, where P is the water needed for normal consistency.

The mortar is placed in moulds in three layers. The test specimens are in the form of cubes with side as 70.6 mm or 76 mm. the moulds are of metal and they are constructed in such way that the specimens can be easily taken out without being damaged for 70.6 mm and 76 mm cubes, the cement required is 185 gm and 235 gm respectively.

The mortar, after being placed in the moulds, is compacted in vibrating machine for 2 minutes.

The moulds are placed in a damp cabin for 24 hours.

The specimens are removed from the moulds and they are submerged in clean water for curing.

The cubes are then tested in compression testing machine at the end of 3 days and 7 days. The testing of cubes is carried out on their three sides without packing. Thus three cubes are tested each time of find out the compressive strength at the end of days and 7 days. The average value is then worked out. During the test the load is to be applied uniformly at the rate of 350 kg/cm² or 35 N/mm².

The compressive strength at the end of 3 days should not be less than 115 kg/cm² or 11.50 N/mm² and that at the end of 7 days should not be less than 175 kg/cm² or 17.50 N/mm^2.

Le Chatelier's apparatus is used to determine the unsoundness due to lime.

Here statement 1&4 are correct.

Other Important Points:

Autoclave test is used to determine unsoundness due to magnesia.

Vicat apparatus is used to determine the consistency of cement.

Le Chatelier's flask is used to determine the specific gravity of cement.

Sieve test and Blainis air permeability apparatus is used to determine the fineness of the cement.