[27]. Hard limestone may be used where trap is not procurable except at a prohibitive cost; but unless the difference in cost is very great the trap will be the most economical in the end.
[28]. The pressure commonly specified is 5000 lbs per. square inch, but recent investigations make it doubtful if that pressure is ever attained, or is, indeed, practicable with any presses so far constructed.
[29]. Asphalt blocks are now very commonly laid upon their sides, even where they are made as thin as two inches, on streets carrying very considerable travel. The practice is not to be recommended unless the blocks are at least four inches thick. Blocks two inches thick will give good service on private driveways and streets of quite light travel.
[30]. Granite block pavement is, as a rule, used upon, and appropriate for streets of the heaviest class of travel, and should, therefore, be provided with the best and strongest foundation. It is nearly always poor economy to lay granite blocks upon a broken stone, gravel, or sand foundation.
[31]. Like other stones, granite from various localities differs widely in strength, hardness and brittleness. Great hardness, accompanied with comparative brittleness, is not desirable in granite for paving blocks. Such material usually polishes by travel and becomes quite slippery, and it is likely to become “turtle-backed,” that is, the corners are likely to be chipped off or worn off, making the pavement very rough and uneven. The quality of the granite to be used in any one city is generally determined by the available supply, and specifications must be drawn with reference thereto.
[32]. Except on streets of excessively heavy travel there seems to be no good reason for making the blocks more than five inches deep. Blocks of this depth are quite sure to become deformed by irregular wear before the pavement will need to be renewed.
[33]. The widths of joints here specified as allowable are based upon the assumption that they will be filled with Portland cement grout as specified in Sect. 75. If this grout filling is used narrower joints are not necessary, as the grout has sufficient strength to support the corners of the blocks, and sufficient hardness to resist the wear of travel (largely protected as it is by the blocks themselves) and to cause the blocks to wear down quite evenly.
In many European cities the specifications require much closer joints. The granite there available appears to break out naturally to truer lines and better surfaces than that used in the Eastern states, at least, so that the cost of dressing the blocks abroad is not as great as here.
The City of New York has recently adopted specifications for “Special Improved” Granite Block pavement intended to approximate the Liverpool standard. These require that the blocks shall be not less than 6 nor more than ten inches long, not less than 3½ nor more than 4½ inches wide and five inches in depth. “The blocks are to be rectangular with tops and sides uniform in thickness, to lay closely, and with fair and true surface, free from bunches and so cut or dressed that when laid stone to stone the joints shall not exceed ⅜ of one inch. The head of the block shall be so cut that it shall not have more than one-quarter of an inch depression from a straight-edge laid in any direction across the head and held parallel to the general surface of the block.” The joints are filled with bituminous cement.
The above specification is very difficult to meet from the granite available to New York without excessive and expensive cutting, and examination of the pavements laid under these specifications shows that the joints greatly exceed the width specified.