The most effective method, however, is to exhaust the air from the cylinder after the timber is inserted; then to allow the oil to flow in; and when the cylinder is full, to use a force pump with a pressure of 150 to 200 pounds per square inch, until the wood has absorbed the requisite quantity of oil, as indicated by a gauge, which should be fitted to the reservoir tank.
The oil is usually heated by coils of pipe placed in the reservoir, through which a current of steam is passed.
The quantity of creosote oil recommended to be forced into the wood is from 8 to 12 pounds per cubic foot. Into oak and other hard woods it is difficult to force, even with the greatest pressure, more than 2 or 3 pounds of oil.
The advantages of this process are: The chemical constituents of the oil preserve the fibers of the wood by coagulating the albumen of the sap; the fatty matters act mechanically by filling the pores and thus exclude water; while the carbolic acid contained in the oil is a powerful disinfectant.
The life of the wood is extended by any of the above processes, by preserving it from decay; but such processes have little or no effect on the wear of the blocks under traffic.
The process of dipping the blocks in coal tar or creosote oil is injurious. Besides affording a cover for the use of defective or sappy wood, it hastens decay, especially of green wood; it closes up the ex terior of the cells of the wood so that moisture cannot escape, thus causing fermentation to take place in the interior of the block, which quickly destroys the strength of the fibers and reduces them to punk.
Expansion of Blocks. Wood blocks expand on exposure to moisture; and, when they are laid end to end across the street, the curbstones are liable to be displaced, or the courses of the blocks will be bent into reserve curves. To avoid this, the joints of the courses
near the curb may be left open until expansion has ceased, the space being temporarily filled with sand. The rate of expansion is about 1 inch in 8 feet, but 'cries for different woods. The time required for the wood to become fully expanded varies from 12 to 18 months. By employing blocks impregnated with the oil of creosote, this trouble will be avoided. 'Blocks so treated do not contract or expand to any appre ciable extent.
The comparative expansion of creosoted and plain wood blocks after immersion in water for forty-eight hours, in percentage on orig inal dimensions, was: being broken by a lap of at least one-third the length of the block; the blocks should be laid so as to have the least possible width of joint. Wide joints hasten the destruction of the wood by permitting the fibers to wear under traffic, which also causes the surface of the pavement to wear in small ridges. The most recent practice for laying blocks on 3 per cent grades, has been to remove from the top of one side of each block a strip } inch thick and 1} inches deep, extending the length of the block. When the blocks are laid and driven closely together, there is a quarter-inch opening or joint extending clear across the street in each course. These joints are filled with Portland cement grout. Fig. 63 shows a section of pavement having this form of joint.
Filling for Joints. The best materials for filling the joints are bitumen for the lower two or three inches, and hydraulic cement grout for the remainder of the depth. The cement grout protects the pitch from the action of the sun, and does not wear down very much below the surface of the wood.