Different Grades of Concrete -- Concrete Mix Design

Different grades of Concrete:

          Based on different concrete mix proportions (or) mix ratio’s, concrete mixer is divided into different grades. As per IS 456:2000, concrete mix design has the following mix proportions:

Type of Concrete	Concrete Grade	Mix Ratio (or) Proportion	Characteristic Compressive strength of Concrete @ 28 Days in N/mm2
Ordinary concrete	M5	1:5:10	5 N/mm2
	M7.5	1:4:8	7.5 N/mm2
	M10	1:3:6	10 N/mm2
	M15	1:2:4	15 N/mm2
	M20	1:1.5:3	20 N/mm2
Standard Concrete	M25	1:1:2	25 N/mm2
	M30	Design Mix	30 N/mm2
	M35	Design Mix	35 N/mm2
	M40	Design Mix	40 N/mm2
	M45	Design Mix	45 N/mm2
	M50	Design Mix	50 N/mm2
High Strength Concrete	M55	Design Mix	55 N/mm2
	M60	Design Mix	60 N/mm2
	M65	Design Mix	65 N/mm2
	M70	Design Mix	70 N/mm2

Note:-

In Example: M5

M represents "Mix"

5 represents the compressive strength achieved after 28 days of curing.

GRADES OF CONCRETE

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.

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.