HIGHWAY CONSTRUCTION 89 



Its particles must he of such fineness as to pass through a No. X screen; 

 if coarse and containing pebbles, they will not adapt themselves to the 

 irregularities of the bases of the blocks; hence the blocks will be sup- 

 ported at only a few points, and unequal settlement will take place when 

 the pavement is subjected to the action of traffic. The sand must also 

 be perfectly free from moisture, and artificial heat must be used to dry 

 it if necessary. This requirement is an absolute necessity. There 

 should be no moisture below the blocks when laid; nor should water 

 be allowed to penetrate below the blocks; if such happens, the effect of 

 frost will be to upheave the pavement and crack the concrete. 



Where the best is desired without regard to cost, a layer half an 

 inch thick of asphaltic cement may be substituted for the sand, with 

 superior and very satisfactory results. 



Laying Blocks. The blocks should be laid stone to stone, so that 

 the joint may be of the least possible width ; wide joints cause increased 

 wear and noise, and do not increase the foothold. The courses should 

 be commenced on each side and worked toward the middle; and the 

 last stone should fit tightly. 



Ramming. After the blocks have been set, they should be well 

 rammed down; and the stones which sink below the general level 

 should be taken up and replaced with a deeper stone or brought to 

 level by increasing the sand bedding. 



The practice of workmen is invariably to use the rammer so as to 

 secure a fair surface. This is not the result intended to be secured, 

 but to bring each block to an unyielding bearing. The result of such 

 a surfacing process is to produce an unsightly and uneven roadway 

 w r hen the pressure of traffic is brought upon it. The rammer used 

 should weigh not less than 50 pounds and have a diameter of not less 

 than 3 inches. 



Joint Filling. All stone block pavements depend for their water- 

 proof qualities upon the character of the joint filling. Joints filled 

 with sand and gravel are of course pervious. A grout of lime or cement 

 mortar does not make a permanently waterproof joint; it becomes 

 disintegrated under the vibration of traffic. An impervious joint can 

 be made only by employing a filling made from bituminous or asphaltic 

 material; this renders the pavement more impervious to moisture, 

 makes it less noisy, and adds considerably to its strength. 



Bituminous Cement for Joint Filling. The bituminous materials 



