Brick

A brick is a type of block used to build walls, pavements and other elements in masonry construction. Bricks are obtained by moulding clay in rectangular blocks of uniform size and then by drying and burning these blocks. The term brick denotes a block composed of dried clay, but is now also used informally to denote other chemically cured construction blocks. Bricks can be joined using mortar, adhesives or by interlocking them. In India, standard brick size is 190 x 90 x 90 mm as per the recommendation of BIS. With mortar thickness, the dimension of the brick becomes 200 x 100 x 100 mm which is also known as the nominal size of the modular brick. As bricks are of uniform size, they can be properly arranged, light in weight and hence bricks replace stones. Block is a similar term referring to a rectangular building unit composed of similar materials, but is usually larger than a brick. Lightweight bricks (also called lightweight blocks) are made from expanded clay aggregate.

Fig.1 Bricks

One of the oldest building material brick continues to be a most popular and leading construction material because of being cheap, durable and easy to handle. Clay bricks are used for building-up exterior and interior walls, partitions, piers, footings and other load bearing structures. A brick is rectangular in shape and of size that can be conveniently handled with one hand. Brick may be made of burnt clay or mixture of sand and lime or of Portland cement concrete. Clay bricks are commonly used since these are economical and easily available. The length, width and height of a brick are interrelated as below.

     Length of brick = 2 × Width of brick + Thickness of mortar

     Height of brick = Width of brick

Characteristics of Good Brick

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. The various characteristics of good brick are listed below.

1) Size and Shape

The bricks should have uniform size and plane, rectangular surfaces with parallel sides and sharp straight edges. Bricks should be table moulded, well burnt in kilns, free from cracks and with sharp and square edges.

2) 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.

3) Texture and Compactness

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

4) 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 ringing sound should be produced.

5) Bricks should not absorb water more than 20 percent by weight for first class bricks and 22

percent by weight for second class bricks, when soaked in cold water for a period of 24 hours.

6) Crushing Strength should not be less than 10 N/mm².

7) Brick Earth should be free from stones, kankars, organic matter, saltpetre, etc.

8) Bricks, when soaked in water for 24 hours, should not show deposits of white salts when allowed to dry in shade.

9) Bricks when broken should show a bright homogeneous and compact structure free from voids.

10) Bricks should be low thermal conductivity and they should be sound proof.

11) Bricks should not break when dropped flat on hard ground from a height of about one meter.

Ingredients of Good Brick Earth

For the preparation of bricks, clay or other suitable earth is moulded to the desired shape after subjecting it to several processes. After drying, it should not shrink and no crack should develop. The clay used for brick making consists mainly of silica and alumina mixed in such a proportion that the clay becomes plastic when water is added to it. It also consists of small proportions of lime, iron, manganese, sulphur etc. The proportions of various ingredients are as follows.

1) Alumina

It is the chief constituent of every kind of clay. A good brick earth should contain 20 to 30 percent of alumina. This constituent imparts plasticity to earth so that it can be moulded. If alumina is present in excess, raw bricks shrink and warp during drying and burning.

2) Silica

A good brick earth should contain about 50 to 60 percent of silica. Silica exists in clay either as free or combined form. As free sand, it is mechanically mixed with clay and in combined form; it exists in chemical composition with alumina. Presence of silica prevents crackers shrinking and warping of raw bricks. It thus imparts uniform shape to the bricks. Durability of bricks depends on the proper proportion of silica in brick earth. Excess of silica destroys the cohesion between particles and bricks become brittle.

3) Lime

A small quantity of lime is desirable in finely powdered state to prevents shrinkage of raw bricks. Excess of lime causes the brick to melt and hence, its shape is last due to the splitting of bricks.

4) Oxide of Iron

A small quantity of oxide of Iron to the extent of 5 to 6 percent is desirable in good brick to imparts red colour to bricks. Excess of oxide of iron makes the bricks dark blue or blackish.

5) Magnesia

A small quantity of magnesia in brick earth imparts yellow tint to bricks and decreases shrinkage. But excess of magnesia decreases shrink leads to the decay of bricks.

The ingredients like, lime, iron pyrites, alkalies, pebbles, organic matter should not present in good brick earth.

Harmful Ingredients in Brick

1) Lime

When a desirable amount of lime is present in the clay, it results in good bricks, but if in excess, it changes the colour of the brick from red to yellow. When lime is present in lumps, it absorbs moisture, swells and causes disintegration of the bricks. Therefore, lime should be present in finely divided state and lumps, if any, should be removed in the beginning itself. Experience has shown that when lime particles smaller than 3 mm diameter hydrate and they produce only small pock mark. Particles larger than this might, if present in any quantity, cause unsightly blemishes or even severe cracking.

2) Pebbles, Gravels, Grits

It does not allow the clay to be mixed thoroughly and spoil the appearance of the brick. Bricks with pebbles and gravels may crack while working.

3) Iron Pyrites

It tends to oxidise and decompose the brick during burning. The brick may split into pieces. Pyrites decolourise the bricks.

4) Alkalis (Alkaline Salts)

It forms less than 10 per cent of the raw clay, are of great value as fluxes, especially when combined with silicates of alumina. These are mainly in the form of soda or potash. When present in excess, alkali makes the clay unsuitable for bricks. They melt the clay on burning and make the bricks unsymmetrical. When bricks come in contact with moisture, water is absorbed and crystallise. On drying, the moisture evaporates, leaving behind grey or white powder deposits on the brick which spoil the appearance. This phenomenon is called efflorescence. Efflorescence should always be dry brushed away before rendering or plastering a wall; wetting it will carry the salts back into the wall to reappear later. If bricks become saturated before the work is completed, the probability of subsequent efflorescence is increased and be protected from rain at all times. During laying, the bricks should be moistened only to the extent that is found absolutely essential to obtain adequate bond between bricks and mortar; newly built brickwork should be protected from rain.

5) Organic Matter

On burning green bricks, the organic matter gets charred and leave pores making the bricks porous; the water absorption is increased and the strength is reduced.

6) Carbonaceous Material

It is in the form of bituminous matter or carbon greatly affects the colour of raw clay. Unless proper precaution is taken to effect complete removal of such matter by oxidation, the brick is likely to have a black core.

7) Sulphur

It is usually found in clay as the sulphate of calcium, magnesium, sodium, potassium or iron, or as iron sulphide. Generally, the proportion is small. If, there is carbon in the clay and insufficient time is given during burning for proper oxidation of carbon and sulphur, the latter will cause the formation of a spongy, swollen structure in the brick and the brick will be decoloured by white blotches.

8)Water

A large proportion of free water generally causes clay to shrink considerably during drying, whereas combined water causes shrinkage during burning. The use of water containing small quantities of magnesium or calcium carbonates, together with a sulphurous fuel often causes similar effects as those by sulphur.