Monday, September 2, 2013

Vicat's Apparatus - Consistency and Setting Time of Cement

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  • Vicat's Apparatus 
is used to find out the consistency, initial setting time and final setting time of the cement. In the normal consistency test we have to find out the amount of water to be added to the cement to form a cement paste of normal consistency.
 Vicat's apparatus consists of an arrangement to hold the plunger of 10 mm diameter and two other needles which are made to freely fall into a mould filled with the cement paste and the amount of penetration of the needles of plunder can can be noted using the vertical graduations from 0 mm to 50 mm.
  •   Consistency Test: 

To find out the consistency test you have to take a sample of dried cement of about 400 g weight which must pass through the 90 micron IS Sieve. Then mix in it about 25% of  water by weight a form a uniform paste withing 2 minutes of time.

 Fill the Vicat's mould with this paste and make the 10 mm plunger fixed to the arrangement to just touch the top surface of the cement paste. Make it freely fall and note the amount of penetration.

When the penetration is of about 42 to 45 mm or when the reading on the vertical graduation is about 5 mm to 7 mm that means cement is of normal consistency. Generally the water required to form a paste of normal consistency is 30%.
Vicat's Apparatus with all the accessories

  • Initial Setting Time: 

Initial time of Cement is the time required by the cement for its early setting. Cement must be applied to the place of its use before its initial setting so it is necessary to find out the initial setting time that is available with us.

Vicat's apparatus is the standard apparatus used to find out this initial setting time. Look in the figure above, there is a needle of diameter 1 mm. This needle is fixed to the movable rod weight.

The cement paste of normal consistency is formed and is filled in the mould. Now the needle is made just touch the top surface of the cement paste and made freely fall in it. Initial setting time is the time from the mixing of the cement and the water to the time when the penetration of the needle is just above 5 mm from the bottom of the base plate or mold.

Generally the initial setting time of the ordinary Portland cement is 30 minutes. For Slow setting cement this time may be increased by adding the admixtures or Gypsum up to 60 minutes.

Similarly, for the final setting time we have to use the third needle which has a enlarged  5 mm hollow cylindrical base. The final setting time is the time from the mixing of the water to the time when this needle just makes the impression on the surface of the cement but do not penetrate into it. Generally the final setting time of cement (OPC) is 10 hrs to 12 hrs.
 
Photograph of the Vicat's Apparatus





















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