Text 9. Bricks and Brickwork

The traditional brick of the building trade consists of blocks of clayey earth which have been baked or burnt. Other more modern types include concrete and sand-lime bricks. The quality and properties of a clay brick depend on three factors, namely: a) the chemical and mineralogical composition of the earth used; b) the processes through which it passes prior to burning or baking; c) the temperature of burning and the care with which the burning is carried out.

Material suitable for the making of clay bricks consists essentially of clay and sand, i. e. of silica and alumina. Other constituents include oxides of iron, iron pyrites, chalk or limestone, salt, and small proportions of various minerals which yield manganese, sodium,, potassium and traces of other metals, together with a certain amount of organic matter.

The colour of a brick is largely governed by the chemical composition of the brick earth, but the temperature of burning is also important. Shades of orange and red are found in bricks made from clay containing oxides of iron, the exact shade depending on the amount of the iron and the temperature of burning while blue bricks are made from clay with a high iron content and a very high temperature of, burning. Clays which are free from iron yield white bricks; yellow bricks are obtained by the addition of magnesia to an iron-free clay, but in clamp burnt bricks a yellow colour may be due to sulphur released from the breeze during burning. The colour of facing bricks is often varied by sprinkling selected sand, sometimes with specially added pigments such as manganese dioxide, either on the raw brick or in the brick mould prior to firing. The surface of the brick then takes on the required colour during burning.

The actual making of a brick consists of two main processes, namely: 1) the preparation of the unburnt brick and 2) the burning or baking of the brick. The preparation of the brick may further be considered in three distinct stages, i. e. 1) the preparation of the earth, 2) the shaping of the brick, and 3) the drying of the brick prior to burning.

Preparation of the brick earth. Brick earths are quarried or dug from open pits. Stone-picking, i. e. the removal of large stones, is carried out by hand and then the clay is spread out to expose it to the action of the weather. The weathering is facilitated by placing alternate layers of materials such as sand, chalk or breeze with the brick earth, piled in banks up to a height of some 6 feet. Tempering follows weathering and in big brickfields it is carried out by means of pug-mills

If the clay as dug is not suitable for brickmaking purposes, but requires conversion to malm, it is placed in a wash- mill, immediately it is dug. This machine resembles a pug-mill, but the cylinder is made of brick instead of iron. The other materials to be added, such as sand and chalk, are ground in water and added to the clay in the wash-mill. The mixture is reduced to a liquid of a creamy consistency, known as slurry, and then passed through iron gratings to retain any large particles, and then into settling pits, or backs. The water is allowed to evaporate until the clay is almost solid.

The shaping of the brick may be done by hand or by machine. The hand-made brick is shaped by means of a wooden mould. Thus this mould is larger than the finished burnt brick, to allow for shrinkage.

In moulding the brick, the inside of the wooden mould is sprinkled with either water (slop-moulded) or sand (sand- moulded) to prevent the clay from sticking to the wood. In the best-quality bricks, the sand is carefully chosen so as to give the required colour and finish to the burnt brick.

The moulder throws a clot of clay into the prepared mould, with sufficient force to fill the mould, and removes excess clay from the top with a wooden strike, leaving a level top surface.

Slop-moulded bricks are wetter than sand-moulded ones, and they are left in the mould on covered drying-floors or drying-rooms for some 48 hours before being taken to the hacks for the final airdrying. Sand-moulded bricks are taken directly to the hacks, which are simply long rows of bricks so placed that they are dried by winds, but are protected from rain by pent roofs.

The machine-made bricks may be moulded, pressed or wire-cut. Moulded machine-made bricks go through the same processes as the hand-made bricks, i. e. weathering, grinding and pugging, but the pugged material is mixed with enough water to make it of a workable consistency and this mix is run by machinery into moulds. The moulded brick is dried and fired in the normal way.

A pressed brick made by the stiff plastic, process needs little or no drying when it comes from the press. A suitable clay is ground when dry, and then passes from mixer, into a rough shaper and finally to a die box. Here the brick is pressed, sometimes twice, then fired.

The semi-dry process for pressed bricks consists in running the ground, screened clay mechanically into a container which passes under the press head.

A wire-cut brick is made by extruding the clay from a machine through a shaped die. This gives a continuous block of clay which is cut by wire into brick lengths. This may be done by hand or in more modern types of machinery the cutting may be automatic. Some wire-cut bricks are partially dried and then pressed.

The drying of bricksoften takes place naturally in the hacks. Artificial drying is, however, used where waste heat is available from the kilns. Hot air flues or steam pipes are led from closed kilns to the shed in which the bricks are stacked, and this gives quicker drying, in from 8 to 10 days, than if wind drying alone is utilised. In general it is not possible to hasten the process of drying without damaging the quality of the brick, since too rapid drying produces flaws and cracks.

The burning of the bricks may be carried out in either a clamp or a kiln, the latter being either of the intermittent, the continuous or the tunnel type.

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Text 8. Properties And Manufacture of Concrete	\|	Text 10. Prestressed Concrete

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