VITRIFIED BRICK PAVEMENTS FOR COUNTRY ROADS. 5 
PHYSICAL CHARACTERISTICS. 
GENERAL REQUIREMENTS. 
Paving brick should be uniform in size, reasonably perfect in shape, 
free from ragging, due to friction in the die, and from deep kiln 
marks, caused by impressions from overlying brick in burning. They 
should be tough in order to resist crushing, hard in order to resist 
abrasion, and uniformly graded in order that the pavement may wear 
evenly. Each brick should be homogeneous in texture and free from 
objectionable laminations or seams. Fire cracks, caused by too rapid 
firing, should be limited in number and extent, and the entire brick 
should be vitrified and should contain neither unfused nor glassy 
spots. 
COLOR. 
The color is a valuable guide in inspecting brick from the same 
plant, but it is of little importance when the brick to be compared 
are from different factories. For brick manufactured from a particu- 
lar raw material the color indicates, in a measure, the temperature 
to which they have been subjected, provided they have been burned 
under identical conditions. Ordinarily, the darker the color, the 
higher the temperature and, presumably, the better the brick. The 
surface color of brick may be very misleading, however, and the color 
of the interior should be used in making comparisons. 
SPECIFIC GRAVITY. 
The specific gravity of paving brick was formerly considered of 
importance in judging their fitness for use in pavements. But it 
has since been generally conceded that a knowledge of the specific 
gravity is of comparatively little value. The specific gravity of shale 
brick is ordinarily between 2.20 and 2.40, and of fire-clay brick 
between 2.10 and 2.25. 
ABSORPTION. 
The absorptive power of brick, like their color, is a matter of very 
slight importance, except for comparing specimens manufactured 
under identical conditions. It is true that the porosity of the brick 
increases with the power of absorption, but it is very doubtful if any 
paving brick possessing an objectionably high absorptive power could 
pass even a very casual inspection. In other words, a high degree of 
porosity always manifests itself in other ways more clearly than in 
the ability of the brick to absorb water. 
CRUSHING STRENGTH. 
The crushing strength of good paving brick varies from 10,000 
pounds to 20,000 pounds per square inch when the load is applied 
