Its particles must be of such fineness as to pass through a No. 3 screen; if coarse and containing pebbles, they will not adapt themselves to the irregularities of the bases of the blocks; hence the blocks will be sup ported at only a few points, and unequal settlement will take place when the pavement is subjected to the action of traffic. The sand must also be perfectly free from moisture, and artificial heat must be used to dry it if necessary. This requirement is an absolute necessity. There should be no moisture below the blocks when laid; nor should water be allowed to penetrate below the blocks; if such happens, the effect of frost will be to upheave the pavement and crack the concrete.
Where the best is desired without regard to cost, a layer half an inch thick of asphaltic cement may be substituted for the sand, with superior and very satisfactory results.
Laying Blocks. The blocks should be laid stone to stone, so that the joint may be of the least possible width; wide joints cause increased wear and noise, and do not increase the foothold. The courses should be commenced on each side and worked toward the middle; and the last stone should fit tightly.
Ramming. After the blocks have been set, they should be well rammed down; and the stones which sink below the general level should be taken up and replaced with a deeper stone or brought to level by increasing the sand bedding.
The practice of workmen is invariably to use the rammer so as to secure a fair surface. This is not the result intended to be secured, but to bring each block to an unyielding bearing. The result of such a surfacing process is to produce an unsightly and uneven roadway when the pressure of traffic is brought_ upon it. The rammer used should weigh not less than 50 pounds and have a diameter of not less than 3 inches.
Joint Filling. All stone block pavements depend for their water proof qualities upon the character of the joint filling. Joints filled with sand and gravel are of course pervious. A grout of lime or cement mortar does not make a permanently waterproof joint; it becomes disintegrated under the vibration of traffic. An impervious joint can be made only by employing a filling made from bituminous or asphaltic material; this renders the pavement more impervious to moisture, makes it less noisy, and adds considerably to its strength.
Bituminous Cement for Joint Filling. The bituminous materials employed are: (1) The tar produced in the manufacture of gas, which, when redistilled, is called distillate, and is numbered 1, 2, 3, 4, etc., according to its density; thig material under the name of paving pitch is extensively used, both alone and in combination with other bituminous substances; (2) Combinations of gas tar or coal tar with refined asphaltum; (3) Mixtures of refined asphaltum, creosote, and coal tar.
The formula for the bituminous joint filling used in New York City, is: Refined Trinidad asphaltum 20 parts.
No. 4 coal-tar distillate 100 parts.
Residuum of petroleum 3 parts.
In Washington, D. C., coal tar distillate No. is used alone.
In Europe a bituminous cement much used is composed of coal tar, asphaltum, gas tar, and creosote oil, in the proportion of 100 pounds of asphaltum to 4 gallons of tar and 1 gallon of creosote. These proportions are varied somewhat, according to the quality of the asphaltum employed. The mixture is melted, and is boiled from one to tvo hours in a suitable boiler, being then poured into the joints in a boiling state. This mixture is impervious to moisture, and pos
sesses a degree of elasticity sufficient to prevent it from cracking.
The mode of applying the bituminous cement is as follows: After the blocks are rammed, the joints are filled to a depth of about two inches with clean gravel heated to a temperature of about F.; then the hot cement is poured in until it forms a layer of about one inch on top of the gravel; then more gravel is filled in to a depth of about two inches; then cement is poured in until it appears on top of the gravel, more gravel being next added until it reaches to within half an inch of the top of the blocks; this remaining half-inch is filled with cement, and then fine gravel or sand is sprinkled over the joints.
In some cases the joints are first filled with heated gravel; the cement is poured in until the sand beneath and the gravel between the blocks will absorb no more, and the joints are filled flush with the top of the pavement. This method is open to objection; for, if the gravel is not sufficiently hot, the cement will be chilled and will not flow to the bottom of the joint, but, instead, will form a thin layer near the surface, which under the action of frost and the vibration of traffic, will be quickly cracked and broken up; the gravel will settle, and the blocks will be jarred loose, the surface of the pavement becoming a series of ridges and hollows.
The quantity of cement required per square yard of pavement will vary according to the shape of the blocks, the width of the joints, and the depth of the sand bed. With well-shaped blocks, close joints, and a half-inch sand bed, the quantity will vary from 31 to 5 gallons; with ill-shaped blocks, wide joints, and a heavy sand bed, 10 to 12 gallons would not be an excessive amount to use to secure the result obtained by employing well-shaped blocks and close joints.
Stone Pavement on Steep Grades. Stone blocks may be em ployed on all practicable grades; but on grades exceeding 10 per cent, cobblestones afford a better foothold than blocks. The cobblestones should be of uniform length, the length being at least twice the breadth —say stones 6 inches long and 21 to 3 inches in diameter. These should he set on a concrete foundation, laid stone to stone, and the interstices filled with cement grout or bituminous cement; or a bitu minous concrete foundation may be employed and the interstices be tween the stones filled with asphaltic paving cement. Should stone blocks be preferred, they must be laid, when the grade exceeds 5 per cent, with a serrated surface, by either of the methods shown in Figs. 57 and 58. The method shown in Fig. 57 consists in slightly tilting the blocks on their bed so as to form a series of ledges or steps, against which the horses' feet being planted, a secure foothold is obtained. The method shown in Fig. 58 consists in placing between the rows of stones a course of slate, or strips of creosoted wood, rather less than one inch in thickness and about an inch less in depth than the blocks; or the blocks may be spaced about one inch apart, and the joints filled with a grout composed of gravel and cement. The pebbles of the gravel should vary in size between one-quarter and three-quarters of an inch.