66 BULLETIN 919, U. S. DEPARTMENT OF AGRICULTURE. 



Compression tests. — The specimen to be used in compression tests shall be a cylinder 

 not less than 6 inches in diameter and 12 inches high. When the diameter of the 

 largest particle of aggregate runs over 2 inches an 8 by 16 inch cylinder is recommended. 



The mold should be of metal. A suitable type of mold consists of a 12-inch length 

 of cold-drawn steel tubing 6 inches inside diameter, split along one element, and closed 

 by means of a circumferential band and bolt. Suitable forms can also be made from 

 galvanized iron. Forms should be tight and should rest on level nonabsorbent bases. 



At least three specimens should be made to cover any single point in a series of tests. 

 Only one cylinder of a kind should be fabricated at one time. 



In testing compression specimens the speed of the moving head of the machine 

 shall travel approximately 0.05 inch per minute when the machine is running idle. 

 The bearing plates of the testing machine shall be brought into direct contact with 

 the end of the specimen, and a spherical bearing block shall be used on top of the test 

 piece. The diameter of the bearing block shall be approximately the same as that of 

 the specimen; the radius of the ball in the block should be not over one-half the radius 

 of the test piece. As the testing head of the machine is brought down upon the top 

 of the cylinder, the lower section of the adjustable block should be oscillated to and 

 fro to insure a central bearing and to avoid pulling the cylinder to one side. 



The results of the tests of individual specimens should be reported. 



48. PROPOSED TRANSVERSE TESTS OF CONCRETE. 



Field specimens. — A slab 30 inches long, 8 or 12 inches wide, and of a depth equal 

 to the depth of pavement should be employed. This specimen should be molded 

 at the edge of the pavement with its long dimension parallel to the length of the 

 road. The forms for separating the test piece from the remainder of the road should 

 be made of sheet metal and should be submerged about three-fourths of an inch 

 below the finished surface of the pavement. In order to provide bearing surfaces 

 and a uniform thickness at the center, three strips of wood 3 inches wide should be 

 placed on the subgrade with their axes running transversely with respect to the axis 

 of the test specimen and wide faces parallel to and equidistant from the top of the 

 pavement. The boards should be placed near each end and at the center of the 

 length of the test piece. Specimens should be tested over a 24-inch span and loaded 

 at the center. 



Laboratory specimens. — A specimen 12 inches wide, 8 inches deep, and 30 inches 

 long, to be tested by center load over a 24-inch span, is suggested for laboratory 

 fabrication. The methods of proportioning, mixing, molding, and curing of the 

 transverse test piece should be similar to the method previously outlined. 



The modulus of rupture S r may be found from the following expression: Thus 



36 P 

 J j r = j-j. - in which P is the center load in pounds and b and d the breadth and depth 



in inches of the slab, respectively. 



49. PROPOSED TEST FOR CONSISTENCY OF CONCRETE. 



For the determination of the consistency either in the field or in the laboratory 

 the committee proposes the use of the 4 by 8 by 12 inch conical frustum, as shown in 

 figure 35. 



In making the test, the thoroughly cleaned frustum should be placed on a level 

 nonabsorbent surface and filled with 3-inch layers of concrete. During filling, the 

 mold should be held down by the operator placing his toes on the lip at the bottom 

 of the mo d. As each layer of material is inlroduced it should be puddled by a 

 stirring motion with a one-half-inch rod to uniformly distribute the material. 



After the upper layer has been placed the top shall be struck off and the mold 

 removed by slowly palling it vertically upward. The height of the frustum shall 

 be measured and the slump calculated from the difference of the height of the mold 

 and the frustum. 



