WOOD PAVEMENTS Wood pavements are formed of either rectangular or cylindrical blocks of wood. The rectangular blocks are generally 3 inches wide, 9 inches long, and 6 inches deep; the round blocks are commonly 6 inches in diameter and 6 inches long.
The-kinds of wood most commonly used are cedar, cypress, juni per, yellow pine, and mesquite; and recently jarrah from Australia, and pyinyado from India, have been used.
The wood is used in its natural condition, or impregnated with creosote or other chemical preservative.
The blocks of wood are laid either .on the natural soil, on a bed of sand and gravel, on a layer of broken stone, on a layer of concrete, or, sometimes, on a double layer of plank. The joints arc filled either with sand, paving-pitch, or Portland-cement grout.
Advantages. The advantages of wood pavement may be stated as follows: (1) It affords good foothold for horses.
(2) It offers less resistance to traction than stone, and slightly more than asphalt.
' (3) It suits all classes of traffic.
(4) It may be used on grades up to five per cent.
(5) It is moderately durable.
(6) It yields no mud when laid upon an impervious foundation.
(7) It yields but little dust.
(S) It is moderate in cost: • (9) It is not disagreeably noisy.
Defects. The principal objections to wood pavement are: (1) It is difficult to cleanse.
(2) Under certain conditions of the atmosphere it becomes greasy and very unsafe for horses.
(3) It is not easy to open for the purpose of gaining access to underground pipes, it being necessary to remove rather a large surface for this purpose, which has to be left a little time after being repaired before traffic is again allowed upon it.
(4) It is absorbent of moisture.
(5) It is claimed by many that wood pavements are unhealthy.
Quality of Wood. The question as to which of the various kinds of wood available is the most durable and economical, has not been satisfactorily determined. Many varieties have been tried. In England, preference is• given to Baltic fir, yellow pine, and Swedish yellow deal. In the United States the variety most used (on account of its abundance and cheapness) is cedar; but yellow pine, tamarack, and mesquite have also been used to a limited extent, and cypress and juniper are being largely used in some of the Southern States.
IIaidwoods, such as oak, etc., do not make the best pavements, as such woods become slippery. The softer, close-grained woods, such as cedar and pine, wear better and give good foothold.
The wood employed should be sound and seasoned, free from sap, shakes, and knots. Defective blocks laid in the pavement will quickly cause holes in the surface, and the adjoining blocks will suffer under wear, the whole surface becoming bumpy.
Chemical Treatment of Wood. The great enemy of all wood pavements is decay, induced by the action of the air and water. Wood is porous, absorbs moisture, and thus hastens its own destruction. Many processes have been invented to overcome this defect. The most popular processes at present are creosoting and modifications of the same, known as the "creo-resinate" and "kreodine" processes. These consist of creosote mixed with various chemicals which are supposed to add to the preserving qualities of the creosote.
Creosoting. This process consists in impregnating the wood with the oil of tar, called creosote; from which the ammonia has been ex pelled, the effect being to coagulate the albumen and thereby prevent its decomposition, also to fill the pores of the wood with a bituminous substance which excludes both air and moisture, and which is noxious to the lower forms of animal and vegetable life. In adopting this pro cess, all moisture should be dried out of the pores of the timber. The softer woods, while warm from the drying-house, may be immersed at once in an open tank containing hot creosote oil, when they will absorb about 8 or 9 pounds per cubic foot. For hardwoods, and woods which are required to absorb more than 8 or 9 pounds of creosote per cubic foot, the timber should be placed in an iron cylinder with closed ends, and the creosote, which should be heated to a temperature of about 120° F., forced in with a pressure of 170 pounds to the square inch. The heat must be kept up until the process is complete, to prevent the creosote from crystallizing in the pores of the wood. By this means the softer woods will easily absorb from 10 to 12 pounds of oil per cubic foot.