HIGHWAY CONSTRUCTION 



by crushing the stone, and by the friction of the fragments on each 

 other while being compacted; its strength varies with the different 

 species of rock, but it exists in some measure with them all, being 

 greatest with limestone and least with gneiss. 



The essential condition of the stone to produce this binding 

 effect is that it bj sound. No decayed stone retains the property of 

 binding, though in some few cases, where the material contains iron 

 oxides, it may, by the cementing property of the oxide, undergo a 

 certain binding. 



A stone for a road surface should be as little absorptive of 

 moisture as possible in order that it may not suffer injury from the 

 action of frost. Many limestones are objectionable on this account. 



The stone used should be uniform in quality, otherwise it will 

 wear unevenly, and depressions w r ill appear where the softer material 

 has been used. 



As the under parts of the road covering are not subject to the 

 wear of traffic, and have only the weight of loads to sustain, it is 

 not necessary that the stone of the lower layer be so hard or so 

 tough as the stone for the surface, hence it is frequently possible by 

 using an inferior stone for that portion of the work, to greatly reduce 

 the cost of construction. 



Size of Stones. The stone should be broken into fragments 

 as nearly cubical as possible. The size of the cubes will depend 

 upon the character of the rock. If it be granite or trap, they should 

 not exceed 1J inches in their greatest dimensions; if limestone, they 

 should not exceed 2 inches. 



The smaller the stones the less the percentage of voids. Small 

 stones compact sooner, require less binding, and make a smoother 

 surface than large ones, but the size of the stone for any particular 

 section of a road must be determined to a certain extent by the 

 amount of traffic which it will have to bear and the character of the 

 rock used. 



It is not necessary nor is it advisable that the stone should be 

 all of the same size; they may be of all sizes under the maximum. 

 In this condition the smaller stones fill the voids between the larger 

 and less binding is required. 



Thickness of the Broken Stone. The offices of the broken 



