QUALITIES FOR A HIGH GRADE PAVING BRICK.
General.—Paving brick should possess the following qualities : 1, Toughness, hardness, and strength. 2. Uniformity of quality through out a given lot of brick. 3. Homogeneity of structure and freedom from laminations. 4. Weather-resisting quality. 5. Regularity in form and size. These qualities are named somewhat in the order of their iniportance, though it should be recognized that several of them are mutually inclusive.
1. Toughness, Hardness, and Strength.—Toughness is that property of a material which indicates its ability to withstand destruction by shock or impact or by a marked distortion of the form of the piece. It is the opposite of brittleness. Of course toughness differs in different ma terials, and it varies in a given material. Mild steel has the property of toughness to a marked degree and will withstand distortion and abuse. One test of the toughness of a specimen of mild steel is to bend the piece cold 180° flat on itself without sign of fracture. Cast iron is a more brit tle material and ordinarily is not used to take shock except in large masses and at low stresses. Different grades of cast iron, however, possess differ ent degrees of toughness, and a good quality of cast iron will bend consid erably before rupturing. With such materials the physical property of toughness which will permit bending and distortion in relatively thin pieces will give ability to withstand blows and the sudden 'application of loads in thicker masses. In the case of paving brick, a lack of tough ness causes the brick to chip and spall under the action of horses' hoofs and not to resist blows and abuse under the action of traffic. This ele ment of toughness is one of the most important qualities in a good paver.
Hardness is that property of a material which indicates its ability to resist abrasion. The necessity for hardness is self-evident. The grinding action of loaded wheels sliding sidewise or even rolling forward wears away the surface of the brick and forms grit or dust. This abrasion is the principal source of wear in a well-constructed pavement made of a good quality of brick. Soft brick will wear rapidly under the action of
traffic. Hardness is therefore a desirable property for paving brick to possess.
Strength is another important element. The loads of wheels are con centrated on a small area, possibly a ton on a fraction of a square inch. With an uneven bedding of a brick or other conditions like its being sup ported on a pebble or by an adjoining brick, considerable flexural action is developed, and even twisting action, and the brick acts as a beam. With uneven surfaces there may be considerable horizontal thrust. It has been argued that lack of strength in the brick does not seriously affect brick pavements and that pavements do not fail from this source, but the writer has seen brick of a mediocre quality spall under the trust of a loaded wheel again and again, and it is not uncommon to see brick broken in two by the passage of loaded wagons. Moreover, when a material is otherwise severely strained the effect of abrasion and impact is greater, and the brick which under heavy stresses remains well below its ultimate strength will be better able to withstand the abrasive action which takes place under such conditions. Besides, high compressive strength is gen erally conducive to hardness, and for granular materials a relatively high tensile strength such as accompanies high values in cross breaking is an indication of toughness and high resilience in the material.
The elements of toughness, hardness, and strength are difficult to differentiate, since one involves the other. On the other hand, a very hard brick may be quite brittle, so much so as to be an inferior article. Some very tough bricks are not hard enough to resist abrasive action sufficiently. Where this is so, there may be some defect in the process or treatment during manufacture. flexural strength goes with toughness and compressive strength with hardness. Not all these qual ities may be expected to exist to the same degree in brick of different makes, and hence the different properties should be considered in discuss ing the merits of a variety of brick.