"The stone last referred to shall have a percentage of wear not to exceed 5 per cent when tested in the follow ing manner: "The sample to be tested shall be broken into pieces that will pass, in all positions, through a 6 centi meter ring, but not through a 3 centimeter ring. The fragments of stone shall then be cleaned, dried in a hot air bath at zoo° C. and cooled in a dessicator, after which five kilograms shall be weighed out and placed in a cylinder of an abrasive machine and the cover bolted on. This machine (see Art. 37) shall consist of a cast iron cylinder, or cylinders, fastened to a shaft so that the axis of each cylinder makes an angle of 3o degrees with the axis of rotation. Each cylinder shall be 20 centimeters in diameter and 34 centimeters in depth; shall be closed at one end and shall have a tightly fitting cover at the other end. After this, the machine shall be rotated at the rate of 2000 revo lutions per hour for five hours. When the Io,000 revolutions of the machine are completed, the contents of the cylinder shall be placed on a sieve of o.i6 centi meter mesh, and the material which passes through carefully collected and weighed. The ratio between the weight of the fine material and the original five kilograms placed in the cylinder is the percentage of wear.
"After immersion in water for a period of ninety-six hours, a smoothly worn fragment of stone weighing between 20 and 6o pounds shall not absorb more than 3 pounds of water per cubic foot of stone.
"After heating the stone in a rotary mechanical dryer to a temperature of about 250° F. it shall be passed through a rotary screen having six or more sections, with varying sized openings, the maxi mum of which shall not be larger than one and one-half inch, and the minimum one-tenth of an inch in diameter. The several sizes of stone thus separated by the screen sections shall pass into a bin containing six sections or compartments. From this bin the stone shall be drawn into a weight box, resting on a scale having seven beams. The stone from each bin shall be accurately weighed in the proportions deter mined by laboratory tests that will give the greatest density of mineral aggregate and the greatest inherent stability of the mineral aggregate. From the weigh box each batch of mineral aggregate composed of different sizes, accurately weighed, as above described, shall pass into a "twin pug" or other appropriate form of mixer. If the proportions of crushed stone in the mixer do not provide enough fine particles to bring the aggregate to the density desired, there may be added not to exceed 15 per cent of fine sand, gravel, hydraulic cement, and pulverized limestone. To the
stone in the mixer shall then be added a sufficient quantity of Warren's Puritan Brand No. 21 Bituminous Water-proof Cement, to thoroughly coat all the particles of stone and fill all the voids in the mixture. The bituminous cement shall, before mixing with the stone, be heated to between 200 and 250° F. and the amount used in each batch shall be accurately weighed and used in such proportion as have been previously deter mined by laboratory tests to give the best results and fill the voids in the mineral aggregate. The mixing shall be continued until the result is a uniform bituminous concrete. In this condition it shall be hauled to the street and there spread on the prepared foundation to such depth that after thorough com pression with a steam roller it shall have a thickness of two inches. The proportion of the various sizes of stone and of bituminous cement shall be such that the compressed mixture shall have as nearly as possible the density of solid stone.
"After rolling the wearing surface, there shall be spread over it a thin coating of Warren's Quick Drying Bituminous Flush Coat Composition in a plas tic condition, for the purpose of closing any pores or cellular openings, and to thoroughly fill any uneven ness or honeycomb which may appear in the surface. There shall then be applied thereto and combined therewith while plastic, stone chips, with the same qualities required of the stone in the pavement proper, by rolling the same into the surface with a heavy steam roller for the purpose of presenting a gritty surface.
"In order to get the greatest possible density, the pavement shall be rolled continuously from the time the bituminous concrete is brought upon the street until the stone chips have been rolled into the surface and the roller no longer makes a perceptible impression upon the pavement.
" Each layer of the work shall be kept as clean as possible so as to readily unite with the succeeding layer. the bituminous compositions shall in each case be free from water, petroleum oil, water gas, or process tars and shall be especially refined with a view of removing the light oil, naphthaline and other crys talline matter susceptible to atmospheric influences."