This bulletin should be considered as a supplement to United States Department of Agriculture Bulletin 370, which gives the results of the more common physical tests of some 3,650 roadbuilding rock examined by the Office of Public Roads and Rural Engineering to January 1, 1916. The office tested 396 samples of rock in 1916, the results of which tests are given in Table I, the rocks being classified according to their location. It should be noted that in a number of cases, in addition to other tests, the crushing strength of the rock also is given. This test is not made ordinarily when examining rock to determine its suitability for use in various types of broken-stone roads. The test is employed often, however, when considering a rock for use in the manufacture of paving block, and as many requests for records of the crushing strength of various rocks have been received in the past year, it has been thought advisable to give in Table II a complete record of all of the crushingstrength tests made by the office up to January 1, 1917. Following is a brief description of this test, as made by the office. CRUSHING STRENGTH OR COMPRESSION TEST. The compression test is made Upon a cylindrical test specimen 2 inches in diameter and 2 inches high. " Both ends of the specimen, which have been sawed at right angles to the axis of the cylinder, 81335°-Bull. 537-17 1 BULLETIN 537, U. S. DEPARTMENT OF AGRICULTURE. 20 and properly faced, are bedded in plaster of Paris. The cylinder then is crushed in a 200,000-pound universal testing machine. A small 2-inch spherical bearing block is placed between the moving head of the machine and the upper surface of the specimen. The average of at least two determinations is reported as the crushing strength, calculated in pounds per square inch. Crushing strength tests are made upon samples of road-building rock only when especially requested. Of a total of 282 compression tests made up to January 1, 1917, Table II shows that 97 were made on granites, 13 on gneisses, 78 on limestones, 42 on dolomites, 28 on sandstones, and 24 on various other types of material. The percentage variation in the strength of the 110 granites and gneisses and the 120 limestones and dolomites is shown graphically in figure 1.
