Paving brick are manufactured by the stiff mud process, which means that the clay is molded into form in a relatively dry condition. To accomplish this, considerable pressure is exerted in forcing the column of clay through the dies, which form the prism from which the brick are cut. If the clay is unsuitable in character or is not properly ground and mixed, the brick will possess planes of weakness between the various layers of clay which have been pressed together, and these planes, called laminations, are a source of weakness if too marked. It is usual to specify that the brick used for road surfaces shall be free from marked laminations.
If the brick is not properly burned it will be only partly vitrified and therefore not of maximum durability. It is customary to specify that the brick shall show a glassy fracture indicating complete vitrification.
Various defects of a minor nature occasionally develop in the brick during the successive steps in the manufacturing process. Check cracks resulting from the burning or from too rapid cooling are often encountered, but unless these are deep, that is 3/16 inch or more, they do not impair the wearing quality of the brick, nor indicate structural weakness. Kiln marks are formed on some of the brick due to the weight of the brick above in the kiln. These depressions are not objectionable unless the brick are so distorted that they will not lie evenly in the pavement.
Spacing lugs or raised letters are formed on one face of the brick to insure sufficient space between the brick for the filler. These lugs or letters are not less than 1/8 inch nor more than ¼ inch high and of such design that they will not obstruct the free flow of filler into the joints between the brick.
Several varieties of paving brick are to be had, the difference being principally in the design or size.
Repressed Brick.—In this type of brick the spacing lugs are formed by pressing the green brick, after it has been cut to size, into a mold on one face of which are recessed letters or other devices into which the clay is pressed, thus forming the spacing lugs.
Vertical Fiber Brick.—These brick are designed to be laid with one wire-cut face up and spacing is provided by two or more beads on the side of the brick. Sometimes the vertical fiber brick has no spacing lug, it being contended that the irregularities of the brick are such as to provide all of the space required. In practice this does not always work out, as the brick are so regular in shape that when laid there is too little space between the brick to permit the introduction of a suitable filler. The use of brick without spacing lugs is just beginning and is not yet a generally accepted practice.
Wire-cut-lug Brick.—This is a type of non-repressed brick which has spacing lugs provided by cutting one face in a special manner which provided lugs for spacing. In this type the wire cut face is the one between the brick as they are laid in the pavement.
Tests for Quality.—The standard test for quality of paving brick is the rattler test. The brick rattler consists of a barrel of 14 sides 24 inches long, mounted so as to rotate at a speed between 29.5 and 30.5 revolutions per minute. The duration of a test is 1800 revolutions. Ten brick constitute a charge and these are placed in the rattler along with 300 lbs. of cast iron spheres. The spheres are of two sizes, the smaller being 1-7/8 inch in diameter when new, and the larger 3¾ inches in diameter when new. Ten of the larger spheres are used and the balance of the charge is made up of the small size.
When tested in the standard manner the loss allowable for the several classes of service are as follows: