WHAT ARE THE CHARACTERISTICS OF A GOOD BRICK EARTH ?

The essential requirements for building bricks are sufficient strength in crushing, regularity in size, a proper suction rate, and a pleasing appearance when exposed to view.

SIZE AND SHAPE

The bricks should have uniform size and plane, rectangular surfaces with parallel sides and sharp straight edges.

COLOUR

The brick should have a uniform deep red or cherry colour as indicative of uniformity in chemical composition and thoroughness in the burning of the brick.

TEXTURE AND COMPACTNESS

The surfaces should not be too smooth to cause slipping of mortar. The brick should have precompact and uniform texture. A fractured surface should not show fissures, holes grits or lumps of lime.

HARDNESS AND SOUNDNESS

The brick should be so hard that when scratched by a finger nail no impression is made. When two bricks are struck together, a metallic sound should be produced.

WATER ABSORPTION

Water absorption of good quality brick should not exceed 20 per cent of its dry weight when kept immersed in water for 24 hours.

BRICK EARTH

It should be free from stones, kankars, organic matter, saltpetre, etc.

DIMENSIONAL SPECIFICATION

Bricks can be of modular size or non-modular size. The size of individual brick must conform to the dimensions as given in the following table.

Dimension for Modular Bricks
Length in cm	Width cm	Height in cm
19	9	9
19	9	4
Dimension for Non-Modular Bricks
Length in cm	Width in cm	Height in cm
23	11	7
23	11	3

CRUSHING STRENGTH

A good quality brick should have crushing strength not less than 10 N/mm2.

CLASSES OF COMMON BURNT CLAY BRICKS

On the basis of its average compressive strength, bricks are classified into following classes as shown in the table below. When tested for compressive strength test, brick should fall into any of the following classes.

Class Designation	Average Compressive Strength
	N/mm2	Kgf/cm2, (approx)
35	35	350
30	30	300
25	25	250
20	20	200
17.5	17.5	175
15	15	150
12.5	12.5	125
10	10	100
7.5	7.5	75
5	5	50
3.5	3.5	35

FACTOR AFFECTING THE QUALITY OF BRICK

Strength and density of clay brick are affect by the various factor listed as below :

• Firstly what manufacturing method is used. If the brick is moulded e.g. stock bricks, they tend not to be particularly dense and often are frogged thus leading to a lower strength. 

• However, stocks brick, they still are the most popular brick leading to a 'softer' appearance of the finished masonry. The other main manufacturing process involves feeding the clay through an extruder where it passes through a vacuum. This forces the clay into a dense column which is extruded out of the end often with perforations in them. These perforations further compact the clay as well as reducing overall mass and making it easier to dry and fire. However provided the perforations are not too large they have a minimal affect on strength. 

Other factors which affect strength and density are: 

• The material they are made of certain clay will fire harder than others.

• The temperature they are fired at the higher the temperature, the harder and stronger the brick will be. However to increase the temperature even by 10-20 degree Centigrade is very expensive and you have to be careful the bricks don't melt.