6 BULiaETIN 373, U. S. DEPARTMENT OF AGEICULTUEE. 



or not a given quality of briclv is suitable for use in constructing 

 pavements and to furnish a basis for comparing different classes of 

 brick. The methods have, therefore, been repeatedly changed, not 

 only in order to make the results obtained indicate more definitely 

 the quality of the brick, but also with a view to establishing uniform- 

 ity, so that results obtained in different laboratories may be intelli- 

 gently compared. A discussion of the most important tests follows 

 in more or less detail. 



FIELD TEST. 



The general appearance of a paving brick is, to an experienced 

 eye, a valuable indication of its quality and Avill frequently suggest 

 the advisability^ of applying routine tests to some particular part of 

 a shipment. Unfortunately the knowledge gained from experience 

 with one kind of brick can not be safely relied upon in inspecting 

 other brick made by a different process or from a different class of 

 raw material. A further limitation to this method of testing lies 

 in the fact that the results obtained do not admit of numerical evalu- 

 ation, and can not, therefore, be very accurately described. This 

 test is nevertheless valuable, and since no apparatus other than a 

 hand hammer is needed, it can always be employed. 



The test consists simply in making a careful inspection of the 

 brick individually and collectively. The size is tested by making 

 measurements, the shape by arranging a number of brick in the order 

 in which they are intended to be placed, and the quality by an exam- 

 ination of both the exterior and interior of a number of samples. 



TRANSVERSE TEST. 



The transverse strength of a brick is determined by supporting it 



upon two knife edges and appljdng a load on the opposite side and 



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



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



3PZ 

 the test is expressed in terms of the ratio ^17^' called the modulus 



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

 pounds, while ?, &, and d represent, respectively, the distance between 

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

 men, 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 



