92 HIGHWAY CONSTRUCTION 



stones a course of slate, or strips of creosoted wood, rather less than 

 one inch in thickness and about an inch less in depth than the blocks; 

 or the blocks may be spaced about one inch apart, and the joints filled 

 with a grout composed of gravel and cement. The pebbles of the 

 gravel should vary in size between one-quarter and three-quarters of 



an inch. 



BRICK PAVEMENTS 



Characteristics of Brick Suitable for Paving. These are: 



(1) Not to be acted upon by acids. 



(2) Not to absorb more than 1-600 of its weight of water in 

 48 hours. 



(3) Not susceptible to polish. 



(4) Rough to the touch, resembling fine sandpaper. 



(5) To give a clear, ringing sound when struck together. 



(6) When broken, to show a compact, uniform, close-grained, 

 structure, free from air-holes and pebbles. 



(7) Not to scale, spall, or chip when quickly struck on the edges. 



(8) Hard but not brittle. 



Tests of Paving Brick. To ascertain the quality of paving brick, 

 they are generally subjected to four tests, namely: (1) Abrasion by 

 impact (commonly called the "Rattler" test); (2) absorption; (3) 

 transverse or cross-breaking; (4) crushing. With a view to securing 

 uniformity in the methods of making the above tests, the National 

 Brick Manufacturers' Association has adopted and recommends the 

 following : 



Rattler Test 



1. Dimensions of the Machine. The standard machine shall 

 be 28 inches in diameter and 20 inches in length, measured inside the 

 chamber. 



Other machines may be used, varying in diameter between 26 and 

 30 inches, and in length between 18 and 24 inches; but if this is done, 

 a record of it must be attached to the official report. Long rattlers 

 may be cut up into sections of suitable length by the insertion of an 

 iron diaphragm at the proper point. 



2 Construction of the Machine. The barrel shall be supported 

 on trunnions at either end; in no case shall a shaft pass through the 

 rattling chamber. The cross-section of the barrel shall be a regular 

 polygon having 14 sides. Tlje heads shall be composed of gray cast- 



