10 



THK TKSTING OF ROAD MATERIALS. 



results of this study, which have already been published as a con- 

 tribution from the Bureau of Chemistry, have been very satisfactory." 



It appears that the cementing value depends upon a certain hydrated 

 colloid condition of the particles, or some proportion of the particles. 

 All rock powders that cement well are hydrated, i. e., contain water 

 of combination) although it does not follow that all hydrated rock 

 powders will cement. It seems that only a certain kind of water of 

 combination is concerned with and measures the cementing value. 

 This property is undoubtedly related to that of plasticity in clays, and, 

 in a few words, is due to amorphous, inorganic particles which by 

 reason of their characteristic porous structure are able to absorb and 

 hold water, thereupon assuming a plastic and coherent condition. 

 Heating above a certain temperature destroys this structure, and the 

 powder no longer possesses the slightest cementing value. It is prob- 

 able that this theoretical investigation will lead to im^ ortant practical 

 developments. 



Under the head of physical tests will be found a description of the 

 methods, as used in this laboratory, for testing the cementing power 

 of road materials, in 'Fable III. page 33, are given the maximum and 

 minimum results obtained on various kinds of rock, and in Table A 

 of the Appendix the individual results on each sample. 



THE CAUSES OF THE DETERIORATION OF ROADS. 



The agencies to be discussed as tending to the deterioration of roads 

 include traffic (pounding of horses' feet, action of wheel.-, etl'ect of load). 



weathering or chemical decomposition, frost, wind, and water, and may 

 be classified as mechanical, chemical, and physical agencies. 



MECHANICAL AGENCIES. 



The severe pounding to which a road is subjected by the action of 

 horse>" feet lends to destroy the binding surface and loosen the under- 

 lying >tonex. The line material when loosened is more readily carried 

 oil by wind or rain and water can then more readily penetrate to the 

 foundation. The properties necessary to resist this action are. with 

 stone blocks or brick pavements, high resistance to impact, and with 

 crushed >tone and gravel, binding or cementing power in the tine mate- 

 rial of the Mirfaee and toughness in the larger fragments. 



The etl'eet produced on roads by the wheels of vehicles varies much 

 with the material of which the road i> composed and with the charac- 

 ter of the tratlic passing over it. If the surface of a road is perfectly 

 smonth. and the load per running inch of tire is sufficiently within the 

 limit of reliance of the surface, the amount of wear is probably 



On the ( 'ausr of the (Vmcntini: Yalur of Rock Powders and the Plasticity of 

 Clays. Allfrtoii s. ( 'uslmian. Jour. Am. Chcin. Soc.. May, 1903. 



