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tract shall not be discharged from liability on their maintenance bond hereunder until the street commissioner shall so determine and certify thereto in writing to the principal under this contract. And it is further expressly agreed, that if at any time during the term for which the contract for the maintenance of the above street is in force, the pavement of said street, or any part thereof, has deteriorated to such an extent as to require, in the opinion of the board of public improvements, reconstruction, the street commissioner shall, with the approval of the board of public improvements and of the mayor, notify the contractor that reconstruction is necessary, and the contractor shall, within three months after receiving such notice, reconstruct the whole or such part of the pavement with the same kind of material as here tofore applied, or with some other material approved by the board of public improvements. And if the contractor fails to reconstruct the street within three months after having been notified, the board of public improvements may, with the approval of the mayor, cancel the contract and relet the work of reconstructing the pavement, and that the cost of such reconstruction shall be paid by the city and the amount collected by suit from the contractor or his sureties, not to exceed fifteen dollars per square of pavement, included in the contract.

And it is further agreed that whenever any repairs of the street are made necessary from the construction of sewers, the laying of pipes or telegraph wires, or from any other disturbance of the pavement by parties acting under permits issued by the city, the contractor shall, on notification from the street commissioner, immediately make all necessary repairs in conformity with the specifications for this class of work. The cost of all such repairs, exclusive of trenching and back filling, which shall be done by the parties who hold the permits, and in the same manner as now required by existing ordinances, shall be paid for at the full contract price for a superficial square of new pavement out of the fund set apart for the payment of contracts for the maintenance of streets, and the amount shall be certified by the street commissioner to the auditor, who shall reimburse, by transfer, the aforesaid fund from the funds of the proper department, if the repairs were made necessary by the construction of any public improvement; and out of the funds to be deposited by persons obtaining permits for opening streets before such permits are granted, if the repairs are made necessary by work done under such permits. And it is agreed that the contractor shall have the right to make all repairs which become necessary by the construction of any public improvement or work done by private parties under permits given by the city.

St. L.

136. Specification for Asphalt Pavement. The following specification for asphalt pavement was prepared in 1892, by Mr. A. P. Boller, of New York City, for such a pavement upon the new Harlem river bridge at 155th st., New York. It probably embodies the latest and most approved methods of making such a pavement, and so far as it is applicable to ordinary street pavements, it might be followed with advantage.

The sub-surface must then be brought to a uniform grade and cross-section not to exceed a crown of three inches in width of roadway by filling all depressions with a fine bituminous concrete or binder, to be composed of clean, broken stone not exceeding one inch in their largest dimensions, thoroughly screened, and coal tar residuum, commonly known as No. 4 paving composition.

If required by the department of public parks, clean, sharp sand may replace a portion of the broken stone.

The stone or stone and sand must be heated by passing through revolving heaters, and thoroughly mixed by machinery with the paving composition in the proportion of one (1) gallon of paving composition to one (1) cubic foot of stone.

This binder must be hauled to the work and spread with hot iron rakes in all holes or inequalities and depressions. below the true grade of the pavement, to such thickness that after being thoroughly compacted by tamping and hand rolling the surface shall have a uniform grade and cross-section, and the thickness of the binder at any point shall be not less than three quarters of an inch.

The upper surface shall be exactly parallel with the surface of the pavement to be laid.

Upon this foundation must be laid the wearing surface or paving proper, the basis of which or paving cement must be pure asphaltum, unmixed with any of the products of coal tar. The wearing surface must be composed of :

1.

Refined asphaltum.

2. Heavy petroleum oil.

3.

Fine sand, containing not more than one per centum of hydro-silicate of alumina.

4. Fine powder of carbonate of lime.

The asphaltum must be specially refined and brought to a uniform standard of purity and gravity of a quality to be approved by the engineer.

The heavy petroleum oil must be freed from all impurities and brought to a specific gravity of from eighteen to twenty

two degrees Beaume, and a fire test of two hundred and fifty degrees Fahrenheit.

From these two hydro-carbons shall be manufactured an asphaltic cement which shall have a fire test of two hundred and fifty degrees Fahrenheit, and at a temperature of sixty degrees Fahrenheit shall have a specific gravity of 1.19, said cement to be composed of one hundred parts of pure asphalt and from fifteen to twenty parts of heavy petroleum oil.

The asphaltic cement being made in the manner above described, the pavement mixture will be formed of the following materials, and in the proportions stated:

[blocks in formation]

The sand and asphaltic cement are to be heated separately to about three hundred degrees Fahrenheit. The pulverized carbonate of lime, while cold, shall be mixed with the hot sand in the required proportions, and then mixed with the asphaltic cement at the required temperature, and in the proper proportion, in a suitable apparatus, which will effect a perfect mixture.

The pavement mixture prepared in the manner thus indicated must be brought to the ground in carts at a temperature of about two hundred and fifty degrees Fahrenheit, and if the temperature of the air is less than fifty degrees, iron carts, with heating apparatus, must be used in order to maintain the proper temperature of the mixture; it shall then be carefully spread by means of hot iron rakes, in such manner as to give a uniform and regular grade, and to such depth that after having received its ultimate compression, it will have a thickness of two inches at crown of roadway, tapering off, if required, to about one inch at gutters. The surface shall then be compressed by hand rollers, after which a small amount of hydraulic cement shall be swept over it, and it shall then be thoroughly compressed by a steam roller weighing not less than two hundred and fifty pounds to the inch run; the rolling to be continued for not less than five hours for every one thousand yards of surface.

The powdered carbonate of lime must be of such degree of fineness that five to fifteen per centum by weight of the entire mixture for the pavement shall be of an impalpable powder of limestone, and the whole of it shall pass a No. 26 screen. The sand must be of such size that none of it shall pass a No. So screen, and the whole of it must pass a No. 10 screen.

In order to make the gutters, which are consolidated but little by traffic, entirely impervious to water, a width of twelve inches next the curb must be coated with hot, pure asphalt and smoothed with hot smoothing irons in order to saturate the pavement to a certain depth with an excess of asphalt.

If rock asphalt be used, it must be natural bituminous limestone rock: (1) from the Sicilian mines at Ragusa, equal in quality and composition to that mined by the United Limmer and Ver Wohle Rock Asphalte Company, Limited; (2) from the Swiss mines at Val de Travers, equal in quality and composition to that mined by the Neuchatel Rock and Asphalte Company, Limited, or (3) from the French mines at Seyssel, equal in quality and composition to that mined by the Compagnie Generale des Asphaltes de France, and it shall be prepared and laid as follows:

(1) The lumps of rock shall be finely crushed and pulverized, the powder shall then be passed through a fine sieve. Nothing whatever shall be added to or taken from the powder obtained by grinding the bituminous rock. The powder shall contain from nine to twelve per cent. natural bitumen, eighty eight to ninety one per cent. pure carbonate of lime, and must be free from quartz, sulphates, iron pyrites, or aluminum. (2) This powder shall be heated in a suitable apparatus to two hundred or two hundred and fifty degrees Fahrenheit, and must be brought to the ground at such temperature in carts made for the purpose, and then carefully spread on the foundation previously prepared, to such depth that, after having received its ultimate compression, it will have a thickness of two inches. (3) It shall be skillfully compressed by heated rammers and rolled until it shall have the required thickness of two inches. (4) The surface to be rendered perfectly even by heated smoothers, and to be rolled with a steam roller weighing not less than two hundred and fifty pounds to the inch run, the rolling to continue for not less than five hours for each one thousand yards of surface. A. P. B.

137. Specification for Granite Pavement. The following specification for granite pavement is that used in the city of Milwaukee so far as the granite paving is concerned. These granite blocks are laid upon a concrete foundation six inches thick, and this latter upon a carefully prepared surface which has been thoroughly rolled with a heavy roller. The concrete is made of natural cement one part, sand two parts, and broken stone five parts. On this is spread a sand cushion two inches thick when compacted, on which the granite blocks are laid.

Granite Block Paving. The blocks must consist of a hard granite uniform in grain and texture, without lamination or stratification and free from excess of mica or feldspar. Neither hard basaltic stone that will take a smooth polish under traffic,

nor soft or weather worn stones nor syenite will be accepted. The blocks must by rectangular in form, of not less than three (3) nor more than four and one half (4%) inches in thickness, nor less than six (6) or more than seven (7) inches in depth, nor less than eight (8) or more than twelve (12) inches in length, and so split and dressed with fair and true surfaces on top, bottom and ends so that when laid close together the end joints will fit close together, and the side joints will not exceed three fourths (34) inch in width. The blocks will be imbedded in the sand bed and laid at right angles to the line of the street, except at street and alley intersections, where the same will be laid at an angle of about 45 degrees with the line of the street. The stone will be laid close together with the top surface smoothly conforming to the crown of the street. Each course is to be of uniform width, with each longitudinal joint broken by a lap of not less than two inches. The blocks are to be immediately covered with sufficient, clean, fine, hot, screened gravel to fill the joints, to not more than 3 1⁄2 inches from the top after which the blocks will be tamped with a heavy paver's ram to a firm, unyielding and uniform smooth surface. The joints will then be filled flush with top of pavement with a hot paving cement obtained by direct distillation of coal tar, immediately after which fine, dry, hot gravel will be run into the joints. Not less than three (3) gallons of paving cement shall be used to each square pard of pavement.*

M.

138. Specification for Granitoid Sidewalks. Sidewalks made after the following specifications are now exclusively used in St. Louis, and have been in use in that city for many years. Where granite can be obtained at a reasonable price, it is thought this composition is more durable and satisfactory for sidewalks than any other material or mixture which has ever been used. The making of these sidewalks has grown to be a very large industry in St. Louis and the price has been gradually reduced because of the great amount of this kind of work done, until in 1895, the total cost of removing old pavement, regrading, laying the foundation and pavement as here described in the most approved manner, and strictly in accordance with this specification is from eighteen to twenty cents per square foot for the "ordinary single flagging."

*This treatment of the joints is especially satisfactory.-Author.

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