HIGHWAY CONSTRUCTION 



105 



terior of the cells of the wood so that moisture cannot escape, thus 

 causing fermentation to take place in the interior of the block, which 

 quickly destroys the strength of the fibers and reduces them to punk. 



Expansion of Blocks. Wood blocks expand on exposure to 

 moisture; and, when they are laid end to end across the street, the 

 curbstones are liable to be displaced, or the courses of the blocks will 

 be bent into reserve curves. To avoid this, the joints of the courses 

 near the curb may be left open until expansion has ceased, the space 

 being temporarily filled with sand. The rate of expansion is about 

 1 inch in 8 feet, but Caries for different woods. The time required for 

 the wood to become fully expanded varies from 12 to 18 months. By 

 employing blocks impregnated with the oil of creosote, this trouble will 

 be avoided. Blocks so treated do not contract or expand to any appre- 

 ciable extent. 



The comparative expansion of creosoted and plain wood blocks 

 after immersion in water for forty-eight hours, in percentage on orig- 

 inal dimensions, was: 



Expansion of Wood Paving Blocks 



Manner of Laying. The blocks are set with the fiber vertical, 

 and the long dimension crosswise of the street, the longitudinal joints 

 being broken by a lap of at least one-third the length of the block; the 



Fig. 63. 



blocks should be laid so as to have the least possible width of joint. 

 Wide joints hasten the destruction of the wood by permitting the fibers 

 to wear under traffic, which also causes the surface of the pavement 



