Paving bricks do not strictly come within the province of the architect, but as he may have occasion to use such bricks for paving driveways, etc., it is well to know something about them.
Paving bricks are most commonly made by the stiff clay process, and the bricks, after being cut from the bar, are generally, although not always, repressed to give them a better shape. The clay used for making these bricks is generally shale, almost as hard as rock, although it is sometimes found in a semi-plastic condition. With the shale a certain proportion - often 30 per cent. - of fire clay is generally added.
The principal difference in the manufacture of paving brick from common building brick is in the burning. Paving brick, to stand the frost and wear, must be burnt to vitrification, or until the particles of the body have been united in chemical combination by means of heat. Besides being vitrified paving brick are also annealed, or toughened, by controlling the heat and permitting the bricks to cool under certain conditions.
Paving bricks, to enable them to endure the various sources of wear and disintegration to which they must be exposed in a street or driveway, or even on a roof, must be homogeneous and compact in texture, and must possess the qualities of vitrification and toughness. They should be free from loose lumps or uncrushed clay, or from extensive laminations, or fine cracks or checks of more than superficial character or extent, and should not be so distorted as to lay unevenly in the pavement. They should be free from lime or magnesia in the form of pebbles, and should show no signs of cracking or spalling after remaining in water ninety-six hours. They should have a crushing strength of not less than 8,000 pounds per square inch.*
The best test of vitrification is that of porosity. A common hard-burnt brick may be very dense and strong and still absorb 10 or 15 per cent, of water. The same brick when vitrified will hold very little water, and should absorb none, in the chemical sense of the word.
Engineers, when specifying brick for pavements, generally limit the absorption to 4 per cent., and sometimes to 2 per cent., the brick bo be first dried to 2120 F. Paving bricks are made that do not absorb more than 1 per cent. It is claimed, however, that a brick may be vitrified and still absorb as high as 6 or 8 per cent., owing to its containing considerable air spaces. The density or specific gravity also gives a valuable idea of the degree of vitrification of paving brick. A great density or high specific gravity usually indicates durability.
For testing the toughness and resistance to wear under the horses' feet a machine called a "rattler" is used. The rattler resembles a barrel, and into it several bricks are put together with pieces of scrap iron and the rattler is then revolved rapidly for a given length of time. The amount that the bricks lose in weight is taken as the test of their durability.
It is claimed by good authorities that the rattler test when properly conducted is the most important test for durability, and that any brick which will successfully withstand this test will be found satisfactory.