General properties of clay bricks

Strength. Strength is a property which is so often used to judge the quality of a brick. In fact, however, strength in itself is not always important factor, since the great majority of bricks are used in positions where they are not required to carry anything like their full safe load. It is true that a really strong brick will nearly always be good in many other respects, but the converse does not by any means follow; for there are some excellent types, quite suitable for many purposes, which are of low strength.

To a large extent the strength is dependent upon the type of clay used and method of making, but with most types of brick there is also a considerable variation according to the temperature at which the bricks are burnt.

Where considerable loads have to be carried a strong brick is needed, but it should be remembered that the strength of a wall is derived from a combination of brick and mortar. Building regulations define the type of mortar to be used with bricks of varying strength to carry varying loads.

Resistance to rain penetration. This again is a property that depends upon the brick and mortar combined rather than on the brick itself. In fact it is only with extremely permeable bricks that rain is ever likely to penetrate a 9 in. or even thicker solid wall, but it does so through cracks (often minute) between the bricks and the mortar.

Weathering and appearance. It is well known that brick are usually extremely durable and that although they may change in appearance after considerable exposure, they usually do so in a manner which is pleasant rather than otherwise. In spite of this there are sufficient examples of brickwork which has deteriorated on exposure to make it necessary to consider how such defects occur. The two chief causes of deterioration are frost and the crystallization of soluble salts.

For an estimate of probable resistance to frost damage, an expert view would be based upon a combination of strength, porosity and saturation coefficient. For a quick judgement by the architect probably the best course is to make sure that no underburnt bricks occur in the normal deliveries.

It is important to differentiate between the conditions of exposure to which the bricks will be subjected. Chimneys, parapet walls and retaining walls are very much more severely exposed to both rain and frost than is the general walling of a building, and they are likely to suffer accordingly. Contrary to general belief, brickwork below damp course level is not especially liable to frost damage.

The effect of soluble salts may vary considerably. They may crystallize on the surface, usually in a spell of dry weather after a wet period. When this occurs they form white crystals which may be very disfiguring to the building, but are unlikely to cause any real damage to the bricks unless the quantity present is abnormal. This crystallization can continue for some years and is usually most obvious in the spring. Normally the crystals are gradually washed away and the trouble gets less each year. There is no cure for such efflorescence once the salts are present in the wall, though dry brushing of the efflorescence may help to get rid of the trouble more quickly.

The subject of the appearance of brickwork cannot be dismissed without referring to the importance of the pattern of jointing – which depends upon the brick bond, and also on the thickness of joints, their color and their type. All these things, together with the type of brick itself, give the overall appearance of color and texture to a wall, and they need careful study on actual examples before they can be handled competently as part of the design technique.