VITRIFIED BRICK PAVEMENTS FOR COUNTRY ROADS. 7 



midway between the supports by means of a third knife edge. The 

 load is gradually increased until rupture occurs, and the result of 



SPl 

 the test is expressed in terms of the ratio 'WhTnt called the modulus 



of rupture. In the above ratio P represents the breaking load in 

 pounds, while Z, b, and d represent, respectively, the distance between 

 supports, the breadth of the specimen, and the depth of the specimen, 

 all measured in inches. 



The modulus of rupture for good paving brick usually lies between 

 2,000 and 3,000 pounds per square inch, and frequently varies con- 

 siderably even with carefully selected specimens which have been 

 manufactured under identical conditions. 



RATTLER OR ABRASION TEST. 



The rattler or abrasion test is undoubtedly the most important of 

 the tests made on paving brick at present. In making this test the 

 specimen brick are subjected to destructive influences very similar 

 to those encountered in actual service, and the results obtained, there- 

 fore, indicate very closely the effect which traffic may be expected to 

 produce on a pavement constructed of similar brick. The methods 

 of making the test, of which there were formerly a great many, have 

 undergone repeated changes in order that service conditions may be 

 more nearly approached, and also in an effort to bring about uni- 

 formity, so that the results obtained may be of the greatest possible 

 scientific value. The method which is now proposed by the sub- 

 committee on paving brick of the American Society for Testing 

 Materials may be briefly described as follows : 



The apparatus necessary for making the test, ordinarily called 

 the rattler, consists of a 14-sided barrel of regular polygonal cross 

 section supported on a suitable frame and fitted with the necessary 

 driving mechanism. The staves, each of which forms a side of the 

 barrel, are made of 6-inch 15.5-pound structural steel channels 27^ 

 inches long. These staves are double bolted to the cast-iron heads 

 of the barrel, which are provided with slotted flanges for holding 

 the bolts. Cast-iron wear plates are bolted to the inside of the 

 barrel heads. The inside diameter of the barrel is 28f inches. 



In this barrel is placed what is known as the abrasive charge. 

 This charge consists of two sizes of cast-iron spheres having respec- 

 tive diameters of 3| inches and If inches and weighing, respectively, 

 7.5 pounds and 0.95 pound when new. Ten of the larger spheres 

 are used, and the number of the smaller spheres is made such that 

 the weight of the entire charge will approximate 300 pounds. The 

 individual larger spheres are discarded whenever their weight falls 

 to 7 pounds or less and the smaller spheres when they become suffi- 



