14 THE TESTING OF ROAD MATERIALS. 



with a micromotor screw before and after freezing and were found, on 

 the average, to havo increased in volume by about one-half of 1 per 

 cent, the maximum increase being less than 1 per cent. 



The agencies of destruction to roads, which have already been con- 

 sidered, are alike in one respect they cause only abrasion and disin- 

 tegration to the. materials composing the road, while wind and rain 

 carry away all the material loosened b} T these factors. Wind and 

 rain are, therefore, the chief agents that remove material from a 

 road, and as their action is incessant the loss is very great. From 

 elaborate measurements made by the National School of Roads and 

 Bridges of France, it appears that about 7 cubic yards of material 

 per mile per annum are transported from the macadam roads of 

 Franco. Undoubtedly this loss must vary widely in different locali- 

 ties and with different materials, and therefore such general results 

 are not applicable to individual cases. The accumulation of water on 

 the surface and its constant penetration to the foundation, together 

 with the continual washing that takes place, make the action of water 

 the most serious element of destruction that the road builder has to 

 combat, 



SERVICE TESTS. 



The term "service test," as here used, does not refer to the 

 informal tests that every material receives in service, the results of 

 which form the main part of our experience with it, but to those 

 formal, competitive tests that are frequently inaugurated in order to 

 determine the relative merit of various articles or the efficiency of 

 various processes. In the case of paving bricks, for instance, a num- 

 ber of different brands are laid in the same street and subjected to the 

 action of traffic for an extended period, observations of the rate of 

 wear being made from time to time. 



These service te-t> in the case of road materials are of limited value. 

 In the first place, it is difficult to determine the exact significance of 

 the observed iv-ult- because of the variability of the conditions and 

 the indclinitem of our knowledge of them. Further, the result> of 

 the tot can not always be expressed definitely or in units that will be 

 agreed upon, and thus much confusion will arise in the attempt to 

 report the roull-. Again, a given material which proved best under 

 one set of conditions with respect to climate, traffic, and topography, 

 might be entirely unsuitod to different conditions. Another practi- 

 cal objection is that a large expenditure of money and a prohibitory 

 lap-e of time, involving in some cases a change of traffic conditions, 

 are necessiry before any conclusions can be drawn relative to a mat- 

 ter which usually must be settled without great expense or delay. 



The value of materials used for other purposes than road construc- 

 tion is in most ca-e- satisfactorily determined by laboratory tests in 



