12 THE TESTING OF ROAD MATERIALS. 



vegetable matter certain of the rock-making minerals containing mag- 

 iH'sia. potash, soda, lime, iron, and even silica, arc rendered soluble. 

 There is also a small amount of sulphuric acid present in rain water 

 in the vicinity of largo cities. The rocks most affected by those 

 reagents are the limestones and rooks having a matrix of lime and 

 iron, but \ve find the resulting disintegration, in rooks likely to be 

 usod in road making, practically negligible when compared with that 

 due to other agencies to which roads are subjected. Professor Pfaff, 

 of Krlangen, subjected to atmospheric erosion specimens of limestone 

 and granite. The annual loss undergone by the limestone was 

 estimated to be equal to the removal of a uniform layer of the gen- 

 oral surface about <>.ol mm in thickness, while the granite lost 

 0.0076 nun." 



Kven with roads constructed of the most soluble rocks it can be 

 seen that this action from a road-building standpoint is extremely 

 slow. The crushed stone below the surface would be subjected to bul 

 little running water, while the amount of the binder that might be 

 taken into solution would be insignificant when compared with th< 

 l<s through other agencies of wear. The writer compared six thii 

 sections of fresh diabase (trap) with six other sections from the sam< 

 rock ledge which had boon in a roadbed for nine years, and so fai 

 as could be observed with the microscope no change whatever hi 

 taken place in the plagioclase and jiugitc of which the rock is com- 

 p<>-ed. or in the rock as a whole. In specimens of trachyte examin< 

 in the same way. one of which had been in a roadbed for twelve years. 

 no alteration was noticeable. This subject has been fully treated by 

 the writer in a former publication. 8 



PHYSICAL, AGENCIES. 



It is very doubtful whether the effect of freezing is injurious to 

 road materials that absorb water even in considerable amounts, but at 

 jill event- it i- probably very small when compared with the destruc- 

 tion wrought by this same agency on the roadbed as a whole. One 

 hundred volume- of water expand on free/ing to inti volumes of ice. 

 If expansion i< prevented the pressure developed on cooling the 

 water 1 degree below its normal free/ing point is 144 tons per square 

 foot. When we examine the subject closely it becomes apparent that 

 the action of t'rnst on road materials is not so groat as it at first seems. 

 In the following table is given the pen-outage of water absorbed by 

 the more important road material-. 



iff, Sit/, k. k. |',:iyrr. Aka.l., 1888. 

 & Report of the Massachusetts lli-li\\uy ( ,,inini.-i.,n, 1900. 



