CALCULATION OF CEMENT, SAND AND WATER REQUIRED FOR PLASTERING

          Plastering is done to protect the wall or ceiling and also to remove surface imperfections caused by brickwork and to make the wall smooth for painting. 

          Plastering is done by using different materials like Cement, Clay, Lime etc., In this post we are discussing about cement plastering work. 

Calculation of Cement and Sand required or plastering:

          We have to remember some general points before going to plastering calculation, they are 

• Cement mortar ratio for wall plastering is 1:6, and for ceiling is 1:4. The ratios mentioned are volumetric ratios of cement and sand. (i.e., Cement:Sand = 1:6, 1 part of cement is mixed with 6 parts of sand for plastering).
• Plastering thickness should not be more than 12-20 mm including two coats.
• In two coat plastering work, the first coat of plastering is done with thickness 12 - 15 mm and this coat is called as "Primary Coating" (or) "Rough Coating".
• Second coating is done with 8 mm thickness, and this is called as "Secondary Coating" (or) "Finish Coat".

Steps involved in calculation of plastering quantities:

1. Calculate the wall area to be plastered in sqm (m²). Take assumption as 100 m².

2. Consider the ratio and thickness of plastering. (i.e., ratio 1:6 & thickness 12 mm).

3. Now we know the area of plastering and thickness, so by using this calculate the Total Volume of Plastering.

4. We know the Total Volume of Plastering and Ratio of plastering so we can calculate the Volume of Cement & Sand required individually.

Step 1 : Assumptions

Area of plastering = 100 m²,

Ratio of plastering = 1:6,

Thickness of plastering = 12 mm.

Step 2 : Volume of plastering

Volume of Plastering = Area of plastering X Thickness

                                        = 100 m² X 0.012 m = 1.2 m³ 

(Take 20% extra quantity, to fill joints & Wastage)

Step 3 : Find individual quantities of cement and sand required

Plastering Ratio = 1:6 

Total parts = 1+6 = 7

Quantity of cement required = 1.2 X 1/7 = 0.17 m³

Density of cement is 1440 kg/m³

Weight of cement = 1440 X 0.17 = 245 kg.

Quantity of Sand required = 1.2 X 6/7 = 1.03 m³

Density of sand is 1920 kg/m³ (It depends on moisture content)

Weight of sand = 1920 X 1.03 = 1978 kg.

Step 4 : Quantity of water required

Quantity of water required is 20% of Total dry material (Cement & sand)

Quantity of water = 20% (weight of cement + weight of sand)

                                 = 20% (245+1978) = 445 kg = 445 ltr

Like that we can calculate the requirement of cement, sand and water for any ratio and thickness of plastering work.

HOW TO CHECK THE QUALITY OF CEMENT ON SITE

          Cement act as a binder in concrete. It binds sand and coarse aggregate together. It is not possible to check all the properties of cement on site, so we can do some basic field tests through which we can get a rough idea about the quality of cement.

          In this article we are suggesting some simple tests which we can perform easily on site without any apparatus to find out the quality of cement on site. 

1. Date Of Packing (Manufacturing Date):

          We know that Strength of cement decreases as time goes on. In general we know that cement which is manufactured 3 months before from that day we use is not suitable for construction. As per IS specification this type of cement is to be tested in lab before using it. 

2. Color of Cement:

          In general cement is present in "Grey color" with light greenish shade. If excess Lime (or) Clay present in cement then its color will change. Check the color of the cement is uniform or not.

3. Lumps in the cement:

          If cement catches moisture automatically lumps will form. Cement undergoes chemical reaction when it reacted with atmospheric moisture. This process is called "Hydration". 

4. Float test of cement:

          Take some quantity of cement and through it in a bucket of water. The cement particle should remain on the surface of water for a while, later it should sink and should not float on the water surface.

5. Smoothness test (or) Rubbing Test:

          To test the cement smoothness take a pinch of cement and rub it between fingers. It feel smooth wile rubbing. If it is rough, it is a sign of cement adulteration with sand.

6. Cement Temperature test:

          Insert your hand into a cement bag. It must give you a cool feeling. The cool feeling implies that no hydration reaction taken place in that bag of cement.

7. Shape test (or) setting test of cement:

          Take a 100 grams of cement and mix it with water to make a stiff paste. Then make a cement cake with sharp edges and place it on a glass plate. Immerse this glass plate in the bucket of water for 24 hrs. Observe that, while settling the shape shouldn't get disturbed. A good cement should be able to set, not crack and attain strength in water. Cement is ready to set under water also and that is why it is also called "Hydraulic Cement".

8. Smell test:

          Take cement into your palm and smell it, if we feel some earthy smell, then we can come conclude that the cement is adulterated with high quantity of silt and clay.

9. Strength test:

          Prepare a block of cement 25 mm breadth, 25 mm height and 200 mm long. Immerse the block in water and cure it for 7 days. Place the immersed block on supports 150 mm apart and then load with a weight of 34 kg (or) 340 N. If the block which is made of good cement should not show any sign of failure.

10. Specific gravity of cement:

          Cement is having specific gravity around 3.1 to 3.6 g/cc, check that the specimen cement is having a value in between it (or) not.

FAQ'S on checking the quality of cement in construction site

How to check quality of cement on site?

What is the cause of lump formation in cement?

How to test cement strength on site?

Why does cement need to be used within 3 months from its manufacturing date?

How to conduct smoothness test on cement?

TECHNIQUES TO CHECK THE QUALITY OF SAND ON SITE

        Sand is also called as "Fine Aggregate". It is the most important constituent in the concrete. To fill the void between coarse aggregate sand (Fine aggregate) is mixed in concrete, and the voids between sand particles is filled with cement. 

          In different regions different varieties of sand is available, like 'Sea sand, River sand, Pit sand' etc., Based on availability we use different varieties. A good grade sand must have almost same size sand particles.

Tests to find the quality of sand:

1. Presence of excessive moisture/ Bulking of sand:

          Excessive moisture content in sand causes increase in sand volume. Moisture content in sand forms a thin layer around the sand particle, due to this each particle exerts pressure on surrounding particle. Thus sand particles move away from each other and causes increase in volume.  

          Before using sand for construction we have to test it for bulking because more than 5% of moisture content in its volume is not suitable for construction.

Bulking of sand depends on two factors, they are

1. Fineness of sand, and

2. Moisture content percentage.

The more fine the sand, more bulking we see in it. i.e., Medium and coarse grade sand possesses less bulking than fine grade sand.

Bulking of sand increases with increase in moisture content up to some limit after that the volume decreases. 

Reference: PCA Major Series 172 and PCA ST20

2. Excessive Silt (or) Clay:

A good quality of sand must process less content of Clay. Clay in sand make sand cohesive. Excessive clay in sand is find in two ways in field, they are

--> Take some dry sand into our palm and throw it, if some sand sticks to our palm then it contains clay.

--> To test in practical way, Take some water in a glass and some sand into it. Shake it thoroughly and  allow it to settle. Check that any layer is formed on the surface water or not. If layer is formed we can come to conclude that clay is present in the sand. A good sand must not have more than 8% clay content.

3. Organic impurities in sand:

          To test the organic compounds presence in sand we need Sodium Hydroxide solution. Take few millilitres of sodium hydroxide solution in a glass and add some sand to it and stir it well, if the color of the solution shows lighter color then there is no organic compounds present in sand. If the solution shows brown color then we can understand that organic compounds present in sand. This type of sand is not suitable for construction.

BRICK WORK CALCULATION IN CONSTRUCTION

          A rectangular unit composed of clay which is used in construction is called a Brick. In different regions of world bricks are available in different sizes and classes. 

In this session we are going to learn 

1. How many bricks required for 1 cubic meter or 1 m³ . (Brickwork calculation).

To calculate how many number of bricks are required for 1 m³  we have to follow below steps:

Step 1: Calculate the volume of a single brick

The size of a brick vary according to the region, the standard size of a brick in India is 19 cm X 9 cm X 9 cm (190 mm X 90 mm X 90 mm). (Specified by Indian Standards institution, New Delhi). 

Then the volume of a single brick = 0.19 m X 0.09 m X 0.09 m 

                                                              = 0.00154 cu. m

Step 2: 

Total volume required = 1 cu. m or m³

Volume of each brick = 0.00154 cu.m or m³

No.of bricks in 1 m³ = 1/0.00154  = 649.39 Bricks = 650 bricks.

hence around 650 bricks are required for 1 m³ .

TO CHECK THE QUALITY OF BRICK ON WORKSITE

           As an engineer we must know how to check the quality of brick on site. In this article we are trying to explain how to check the quality of brick on site.

1. Uniform Color, and Shape:

          A good quality brick should be well burnt and rich red or copper in color. If the brick is under burnt (or) Over burnt the color will be vary. The brick is going to loss its shape if it is under burnt (or) over burnt.

2. Size of Brick:

          A good brick should be uniform in size and sharp at edges and it shouldn't have any bulking on sides. +/- 3 Tolerance is allowed in dimension to a brick.

3. Hardness of Brick:

          If we scratch a brick using finger nail or a sharp tool, it should not show any impression or scratch on its surface then the brick is called a best quality brick. 

4. Homogeneity of Brick:

          To test the homogeneity of a brick, we must break it and examine. It should be homogeneous and having zero lumps.

5. Water absorption:

          To check the water absorption capacity of a brick we must do water absorption test on brick. If we kept a brick in water for 24 hrs, it should absorb less than 20% water then it is called a best quality brick. If the brick is absorbing more than the allowable limit then we should note that the brick during its bonding is going to absorb more water from cement mortar.

6. Efflorescence on Brick:

          The salt deposit seen on brick surface is called Efflorescence. Generally we see some white patches on some bricks,  this is due to presence of salts (Sodium and Potassium) in the brick making material. These type of bricks are not suitable for construction. (Procedure for checking IS: 3495-1992),

7. Soundness of a Brick:

          If we drop a brick from 1.5 mtr height, it shouldn't break. 

8. Frog in Brick:

         The cut impression (or) indent given on brick surface is called Frog. The frog size should be 100 mm X 40 mm X 10 mm. If the frog size is more (or) less then it leads to less (or) more mortor filling while construction. It is uneconomical and effects the strength.