28 



BULLETIN" 257, U. S. DEPARTMENT OF AGRICULTURE. 



Table 19. — Average and maximum distance between cracks in concrete experiments at 

 Chevy Chase, Md., March 10, 1915. 



Character of pavement. 



Surface coated with bituminous material: 

 Cement 



Do 



Oil-cement 



Do 



Surface exposed : 



Cement 



Do 



Do - 



Oil-cement 



Do 



Do 



Aggregate. 



Gravel 



Limestone 



Gravel 



Limestone 



Gravel 



Limestone 



Trap 



Gravel 



Limestone 

 Trap 





Average 





distance 



Length. 



between 





cracks. 



Teet. 



Feet. 



675 



37 



575 



64 



370 



25 



226 



75 



425 



27 



625 



78 



300 



23 



260 



26 



200 



50 



290 



36 



Maxi- 

 mum 

 distance 

 between 

 cracks. 



90 

 139 

 55 



77 



63 



148 

 50 

 47 

 84 

 60 



As noted in the previous progress report, the relative distances 

 between cracks continue greater on sections constructed with a 

 limestone aggregate than on those where trap rock or gravel were 



used. 



Experiment No. 6 (Vitrified Brick). 



An inspection of this experiment showed no appreciable difference 

 in wear between the several brands of brick. Three longitudinal 

 cracks were noted, as follows: 



In section D, a crack about one-third the length of the section 

 and about 6 feet from the west curb. In section F, a crack about 

 half the length of the section, beginning at the south end, and about 

 6 feet from the east curb. In seetion G, a crack about 6 feet long 

 and about the same distance from the east curb as that in section F. 

 These cracks are practically straight and so fine that they would 

 hardly be distinguished in a casual observation. Their cause has 

 not thus far been clearly ascertained. 



A traffic census on this experimental road is taken every 13 days 

 for a 24-hour period. A tabulation of the maximum and average 

 of each class of traffic for the period between March 1, 1914, and 

 March 1, 1915, is given in Table 20. The east side of the road carries 

 outgoing traffic and the west side incoming traffic. 



