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 Ivor, called the modulus 
of rupture. In the above ratio P represents the breaking load in 
pounds, while Z, £>, 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 27i 
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 28| 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- 
