Page:Popular Science Monthly Volume 80.djvu/334

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Toughness as here understood is the power possessed by a material to resist fracture by impact. The test piece is a cylindrical rock core similar to that used in determining hardness, and the test is made with an impact machine constructed on the principle of a pile driver. The blow is delivered by a hammer weighing 2 kg. which is raised by a sprocket chain and released automatically by a concentric electro-magnet. The test consists of 1 cm. fall of the hammer for the first blow and an increased fall of 1 cm. for each succeeding blow until failure of the test piece occurs. The number of blows required to cause this failure represents the toughness.

The cementing value, or binding power of a road material, is the property possessed by a rock dust to act as a cement on the coarser fragments comprising crushed stone or gravel roads. This property is a very important one, and is determined approximately as follows:

One kg. of the rock to be tested is broken sufficiently small to pass through a 6 mm. but not a 1 mm. screen. It is then moistened with a sufficient amount of water and placed in an iron ball mill containing two chilled iron balls weighing 25 pounds each and revolved at the rate of 2,000 revolutions per hour for two hours and a half, or until all the material has been reduced to a thick dough, the particles of which are not above 0.5 mm. in diameter. About 25 grams of this dough is then placed in a cylindrical metal die, 25 mm. in diameter, and by means of a specially designed hydraulic press, known as a briquette machine, is subjected to momentary pressure of 100 kg. per square centimeter. Five of the resultant briquettes, measuring exactly 25 mm. in height, are taken out and allowed to dry for 12 hours in air and 12 hours in a hot oven at 100° C. After cooling in a desiccator they are tested by impact in a machine especially constructed for the purpose. This machine is somewhat similar to that used in determining the hardness, and the blow is about the same, excepting that it is given by a 1 kg. hammer and the distance of drop does not exceed 10 cm.

The standard fall of the hammer for a test is 1 cm. and the average number of blows required to destroy the bond of cementation in the five briquettes determines the cementing value.

The specific gravity, is the weight of the material compared with that of an equal volume of water, and is obtained by dividing the weight in air of a rock fragment by the difference of its weight in air and water. Given the specific gravity, the weight per cubic foot of a rock is found by multiplying this value by 62.5 pounds* the weight of a cubic foot of water.

The examination of a rock for structure, its mineral components and the degree to which it has become weathered, is carried out by preparing a thin section of such thickness as to be transparent under the microscope. Its characteristics are then readily determined by the methods employed by the petrographer for this purpose. The appearance of such a section, made from a trap rock of a kind used in the construction of a broken-stone road, is shown in an accompanying illustration, taken from the Bulletin of the Office of Public Roads, which has been referred to.

The application of the methods which have been described to the study of rocks for the purpose of determining their suitability and relative merit for road construction was the first contribution to and application of scientific methods to the subject. Before this road