8() THE TESTING OF ROAD MATKRIALS. 



^ach 



the maximum and minimum per cent of water absorbed by eac 



varietv of rock tested and has already been considered in connection 

 with the effect of freezing on road materials. 



SPECIFIC GRAVITY AND WEIGHT PER CUBIC FOOT. 



The specific gravity and weight per cubic foot for all rock samples 

 are calculated from the weights used for determining the absorptive- 

 ne>s of rocks, which are given in Table A of the Appendix. 



TEST FOR HARDNESS. 



The laboratory is equipped with a Dorry machine for testing 

 hardness of materials according to the standard method of the French 

 School of Roads and Bridges. The machine, however, has just been 

 installed and no results have as yet been obtained. 



COMPRESSION AND TENSILE STRENGTH TESTS. 



The laboratory is equipped with a 200,000-pound universal Riehle 

 testing machine and a 30,000-pound Olsen. Although compression 

 and tensile strength tests do not form part of the routine examination 

 of road materials, they are often of the highest value in special cases. 



TESTS FOB PAVING BRICK. 



RATTLER TEST. 



The machine for conducting this test is shown in fig. 9. It con- 

 >i>ts essentially of a rotation drum partially filled with large and small 

 cast-iron blocks or shot. The wearing quality of paving brick is 

 judged by the loss sustained after a sample has been subjected in this 

 drum for one hour to the impact and abrasion of these cast-iron shot. 

 For the lir>t 'on rotations the loss is mainly due to a chipping action 

 which develops the brittleness of the brick; between 1,200 and 1,800 

 rotations the loss is mainly due to an abrasive action which develops 

 the softness of the brick. During the test any defects of structure, 

 ^uch as planc> of >cparat ion. will probably result in rupture. The 

 particular form of rattler shown was designed and built at Purdue 

 I'niver-iiy. Lafayette. Ind. It is an overhanging rattler, and at the 



free end there is convenient access to the drum for the purpose of 

 in-erting and removing the bricks, an operation which in the ordinary 

 form of rattler necessitates the removal of a slat of the drum. 

 tight sheet-iron cover prevents the escape of dust. The worn material 

 is caught in a pan. as >h>wn in the illustration. 



The rattler is ^s inches in diameter and -jo inches long and has 14 

 eaM-stcel slats. It is run at a speed of 30 rotations per minute for 

 1,800 revolutions. The charge to be placed in the rattler consists of 

 9 paving blocks or 12 dry bricks, together with 300 pounds of sh< 





