It is the amount of water content required for cement to attain the standard consistency or normal consistency of cement.
The standard consistency of cement is that the amount of water required for the cement paste to permit the Vicat plunger to penetrate to a point 5-7mm from the bottom is known as standard or normal consistency of cement.
When we add water to the cement, it starts hydration. In this hydration process the cement and water mixed together and forms a strong bond but if we add more and more amount of water, it results in Increase of water cement ratio and ultimately cement loses its plasticity and also strength. Also let us assume if we add less amount of water to the cement then it may result in less water cement ratio thus results in no gain of strength. So, from this study it is clear that standard amount of water is required to attain proper strength of cement.
For example; The standard consistency or normal consistency of ordinary Portland cement varies between 25-35%.
To prepare a cement paste of standard consistency 25-30%, then the same percentage equivalent amount of water of weight of cement is added. i.e. if we assume the standard consistency of cement is 30% so for 400 g of cement, we add 30% of water i.e. 120g of water content is added to the cement.
Why we calculate Standard or Normal Consistency of Cement?
The standard and normal consistency of the cement test is used to find out the amount of water required to make a cement paste of standard consistency. It is necessary to determine the standard amount of water needed for each case, like initial setting time of cement, final setting time of cement, soundness of cement, and Compressive strength of cement.
Here, P= Normal Consistency of cement
|Name of Test||Amount of Water Required|
|Soundness of cement (Le-Chatelier method)||0.78 P|
|Setting time of cement||0.85 P|
|Compressive strength of cement||(P/4 + 3)% of combined mass of cement and sand.|
Why Normal consistency of cement varies?
There are so many reasons for such variations and we will discuss few important factors affecting the normal consistency of cement:
- Weather is an important factor
- Silica composition in the cement
- The fineness of cement
- Cement produced by different manufacturers doesn’t have the same consistency.
How to calculate standard or Normal consistency of cement?
As already discussed, the standard consistency is varying from region to region and site to site and also composition and fineness of cement. So there needs to be a standard water content that has to be added to cement to gain its desired strength.
To calculate the normal consistency of cement, we have to conduct VICAT APPARATUS test as per IS:4031-PART-1988.
Apparatus required for standard consistency:
- VICAT Apparatus
- Needles (Plunger)
- Movable Rod
- Graduated scale
- VICAT Mould
Procedure for Standard or Normal consistency of cement:
- Take 400g of cement and place it in a tray or bowl.
- Now let us assume the standard consistency of cement is 26% and add the same percentage of water into the cement and mix it.
- Now mix thoroughly the cement paste within 3-5 minutes and the time taken for obtaining cement paste from which water is added is known as gauge time.
- Take care that the gauge time is not less than 3 minutes and not more than 5 minutes, and it is completed before setting occurs.
- Now fill the cement paste in Vicat mold and any excessive paste is removed by the trowel.
- Then, place the Vicat mold on the glass plate and ensure that the plunger is gently placed on the surface of the mold.
- Release the plunger and allow it to sink into the Vicat mold.
- Note down the reading from the bottom of the mold indicated on the scale.
- Repeat the same experiment by adding different percentages of water, let’s say 28%, 30%, 32%, etc… until the reading is in between 5-7mm on the Vicat apparatus scale.
- Type of cement___________
- Name of the cement________
- Room temperature_________
Table: Following values are noted while conducting experiment.
Formulae for Standard or Normal consistency of cement;
Standard or Normal consistency of given cement sample is _______%
- Gauging time should be observed properly.
- Clean the apparatus before and after testing.
- Care should be taken that the area should be safe from vibrations and disturbances.
- The room temperature and humidity are maintained specifically.