ASPHALT. 



203 



stating that by the term "pavement" is meant the wearing sur- 

 face of a street or road, when it has been "hardened." 



With this preamble, the essential topics of the paper may 

 now be considered. 



Rock Asphalt, or Bitt t minous Sand or Limestones. 



This material is noteworthy as being the father of the modern 

 asphalt pavement, and in past years has been much more 

 extensively used than it is at present. The reason is not far to 

 seek. Recent investigations have shown that an asphalt pavement 

 depends largely for its life on the grading of the mineral matter, 

 perhaps as much so as on the quality of the bitumen, though the 

 latter, being a component of about only 10 per cent., is a deter- 

 mining factor of long life. However, Nature has no recognition 

 of standard gradings, and she permeates any available pervious 

 stratum with the soft asphalt under pressure ; consequently they 

 vary very considerably in their grading in different deposits, and 

 sometimes even in the same one. However, it must be granted 

 that, whatever the grading, the bitumen must be in the correct 

 amount to fill the voids. This probably governs their success. A 

 fine sand stratum would take up and retain just enough to cover 

 each individual grain with a thin film of bitumen as the stream 

 was forced through and proceeded onward. The amount would 

 be more than in a coarse stratum because of the greater surface 

 area, but because of larger voids the latter would have greater 

 masses, and therefore more be easily displaced when laid as a 

 pavement. For this reason the finer rock asphalts give greater 

 satisfaction than the coarser where the traffic is at all heavy. This 

 non-uniformity of grading is responsible for many of the failures 

 of rock asphalt, so attempts have been made to make it more 

 uniform by breaking up and grinding, especially with the European 

 varieties, and some more successful pavements and sidewalks have 

 been laid in that condition. 



Another reason for the decline in the use of rock asphalts, 

 besides the chance grading, is the expense entailed in shipping and 

 paying for freight on a lot of unnecessary and often inadequate 

 mineral matter which (since sand is available anywhere) can just 

 as well, and in fact better, be introduced where the pavement is 

 to be laid. Another disadvantage is that there is no method of 

 knowing or controlling the penetration of the cementing bitumen, 

 a frequent source of failure. It is generally conceded that rock 

 asphalt pavements are more slippery in wet weather than any other 

 form of pavement except perhaps wood. 



Lake Asphalt may next be considered. 



Lake Asphalts. 



Lake Asphalts may be classed under two varieties, Trinidad 

 as coming from the Island of Trinidad, British West Indies, and 

 Bermudez as coming from the mainland of Venezuela. They both 

 occur as vast deposits which take the form of lakes — hence the 

 name, 

 la 



