72 HIGHWAY CONSTEUCTION 



If the use of motor vehicles equipped with pueumatic tires be- 

 comes general, it is possible that some other description of binding 

 material will be necessary. The pumping action of suction created 

 by pneumatic tires, especially when propelled at a high speed, causes 

 a considerable movement of the fine particles of the binding material, 

 which on being displaced will convert the covering into a mass of 

 stones. This objection can probably be overcome by watering. 



Compacting the Broken Stone. Three methods of compacting 

 the broken stone are practiced : (1 ) by the traffic passing over the road ; 

 (2) by rollers drawn by horses; (3) by rollers propelled by steam. 



The first method is both defective and objectionable. (1) It is 

 destructive to the horses and vehicles using the road. (2) It is waste- 

 ful of material ; about one-third of the stone is worn away in the oper- 

 ation. (3) Dung and dust are ground up with the stone, and the 

 road is more readily affected by wet and frost. 



Steam=rollers were first successfully introduced in France in 1860, 

 since which time they have been almost universally adopted on account 

 of the superiority and economy of the work done. Their use shortens 

 the time required for construction or repair, and effects an indirect 

 saving by the reduced wear and tear of horses and vehicles. They are 

 made in different weights ranging from 3 to 30 tons. For the compact- 

 ing of broken stone roads the weights in favor are from ten to fifteen 

 tons; the heavier weights are considered unwieldy and their use is 

 liable to cause damage to the underground structures that may be in 

 the roadway. 



The advantage of steam rolling may be summed up as follows: 



(1) They shorten the time of construction. 



(2) A saving of road material, (a) because there are no loose 

 stones to be kicked about and worn ; (b) because there is no abrasion 

 of the stone, only one surface of the stone being exposed to wear ; (c) 

 because a thinner coating of stone can be employed ; (d) because no 

 ruts can be formed in which water can lie to rot the stone. 



(3) Steam-rolled roads are easier to travel on account of their 

 even surface and superior hardness and they have a better appearance. 



(4) The roads can be repaired at any season of the year. 



(5) Saving both in materials and manual labor. 



