6 BULLETIN 246, U. S. DEPARTMENT OF AGRICULTURE. 



uniformly over the entire top surface of the test specimen, and may "be 

 much greater if the area over which the load is applied is less than 

 that of the top surface. Since paving brick in use are seldom required 

 to withstand a pressure of more than about 2,000 pounds per square 

 inch, and since inferior brick may possess relatively very high resist- 

 ance to crushing, a knowledge of the crushing strength is clearly of 

 little value in comparing the relative excellence of different makes of 

 brick. It is, therefore, usually considered unnecessary to specify a 

 definite requirement as to the crushing strength of paving brick. 



TESTING THE BRICK. 



Definite methods of testing paving brick have been in general use 

 for only a comparatively few years and have only recently under- 

 gone a pronounced change. The object of all tests is to determine 

 whether or not a given quality of brick is suitable for use in con- 

 structing 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 establish- 

 ing uniformity, so that results obtained in different laboratories may 

 be intelligently 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 will 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 inspect- 

 ing 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 

 evaluation, 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 exami- 

 nation 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 applying a load on the opposite side and 



