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 



