Unfortunately the size of building bricks and also of paving blocks varies considerably in different parts of the country. Uni formity of size is very desirable for convenience in making re pairs. Sizes of bricks and blocks range all the way from 2 X 4 X 8 inches to 4 X 5 X 10 inches, but the maximum is seldom more than 3 X 4 X 9 inches. There is no conventional line by which to distinguish bricks from blocks; but as a rule "paving brick" are about 2i" X 4" X 8" and "paving blocks" 3" X 4" X 9".
Rounded corners are very common, the radius of the corner varying from one to three eighths of an inch. The rounding of the corner decreases the loss during the first part of the test of the brick in the rattler (§ 740) and also diminishes the chipping during its earlier use in the pavement; but. the rounded corner increases the initial roughness of the pavement, and makes it at the beginning what it would otherwise have become only after a considerable use, if ever. The rounded corner is a disadvantage, whatever the material with which the joint is filled (§ 773), since it gives a thin edge to the filling which easily crumbles or chips off. The intro duction of the rounded corner was due, in part at least, to the desire of the manufacturer to make a brick that would more readily pass the usual rattler test; and if this test had consisted in determining the loss in a certain number of revolutions after, say, an initial 500 revolutions, this tendency would probably not have occurred. The corner is usually rounded during the re-pressing, which disturbs the initial bond of the clay and weakens the brick, the amount of this weakening varying with the amount of the disturbance and with the character of the clay. The round corner is of doubtful value.
Fig. 122, 123, and 124 shows three forms of the grooves employed to facilitate the introduction of the joint filler and to increase its holding power. The depth of the groove differs with the maker, but is usually about to inch deep. The arrange
ment of the grooves in Fig. 122 is quite objectionable, since the brick spalls off from the edge down to the groove, particularly when the joints are filled with sand. The form shOwn in Fig. 123 is better than that in Fig. 122, since the vertical grooves facilitate the introduction of the joint filler. It would be still better, if the vertical grooves were continuous across the face of the brick. The form shown in Fig. 124 is perhaps a little less objectionable than that in either Fig. 122 or 123. All three of the preceding forms often have the name of the manufacturer in sunken letters in the sides of the brick—partly for advertising purposes and partly to increase the holding power of the material used to fill the joints. The grooves and also the sunken letters are added in re-pressing, a process which breaks the original bond of the clay—EA least to some extent,—and therefore weakens the brick.
One object of the grooves is to facilitate the introduction of the joint filler. Practically the only materials employed to fill the joints are sand, tar, and hydraulic cement; and no difficulty is experienced (when proper methods are employed—see § 773) in filling the joints with any of these materials.
Another object of the grooves and of the sunken letters is to obtain a keying or locking action of the joint filling; but it does not appear that this is either needed or does any good. The resistance to shearing of the portion in the grooves is small in comparison with the adhesion of the joint-filling material to the side of the brick.
The raised letters or buttons are to facilitate the introduc tion of the joint filler; but with the most regular re-pressed paving blocks or bricks the joints when the bricks are laid as close as possi ble are always sufficiently open to permit the introduction of the joint filler. Needlessly wide joints are objectionable, since the ma terial used in filling the joints is not likely to be as durable as the brick itself, and hence the thinner the joints the better. With bricks having a wearing face 3 X 9 inches and joints inch wide, the joints constitute 5 per cent of the surface of the pavement, and finch joints will occupy 10 per cent of the surface.
The value of the grooves and of the recessed letters, as also of the raised letters and buttons, can be better understood after the consideration of the purposes of filling the joints and of the ma terials used—see § 773-80.