6 BULLETIN 23, U. S. DEPARTMENT OF AGRICULTURE. 
whether or not a given quality of brick is suitable for use in construct- 
ing pavements and to furnish a basis for comparing different classes 
of brick. The methods have, therefore, been repeatedly changed, 
not only hi order to make the results obtained indicate more defi- 
nitely the quality of the brick, but also with a view to establishing 
uniformity, so that results obtained hi different laboratories may be 
intelligently compared. A discussion of the most important tests 
follows hi more or less detail. 
FIELD TEST. 
The general appearance of a pavhig 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, however, the knowledge gamed 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 
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 hi 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 
hi 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 
midway between the supports by means of a third knife edge. The 
load is gradually increased until rupture occurs, and the result of 
3 P 1 
the test is expressed in terms of the ratio 9 -, _, 2 , called the modulus of 
rupture. In the above ratio P represents the breaking load in pounds, 
while 1. by and d represent, respectively, the distance between sup- 
ports, the breadth of the specimen, and the depth of the specimen, 
all measured hi niches. 
The modulus of rupture for good paving brick usually lies between 
2,000 and 3,000, and frequently varies considerably even with care- 
fully selected specimens which have been manufactured under iden- 
tical conditions. In making this test a considerable number of speci- 
mens should be used, and the requirements concerning the transverse 
strength should be no less definite as to uniformity hi the results of 
the test than as to the average modulus of rupture. 
