ASPHALT. 211 



asphalt cement in sufficient amount to give each particle, large 

 and small, a thin coating. When the asphalt cement in use is 

 made from Trinidad asphalt the voids are still further reduced by 

 the presence of the colloidal clay mentioned in the earlier part of 

 this paper. The reason for the difference between the sand grad- 

 ings for heavy and light traffic is to allow a greater play for con- 

 traction and expansion by reason of the larger voids, and there- 

 fore thicker film of asphalt cement. That this is desirable 1? 

 readily understood, because if a pavement cools while quiescent, 

 the contraction will continue until it reaches the limit of tli4 

 ductility of the asphall cement, and the pavement will crack at 

 the weakest point. If, however, this increasing tension is relieved 

 by the shock of passing traffic, adjustment ensues and rupture is 

 prevented, and the contraction commences again at the zero point. 

 Where traffic is infrequent this shock is not imparted, and the 

 pavement cracks. However, if the film of asphalt is thicker, as in 

 a light traffic mixture, the greater mass of bitumen will stretch 

 farther and accommodate itself to the new condition, so that the 

 pavement will remain intact. This has been frequently observed 

 by the writer where an isolated piece of pavement has cracked 

 while the rest of the pavement, made and laid at the same time 

 with the same mixture, but subjected to traffic, has remained in 

 good condition. Thus it ma}'- be seen that traffic is the life of 

 a pavement. This is also true for the reason that traffic will give 

 a greater compression than that obtained by the use of a roller 1 

 when the pavement is laid and is still hot. It will be readily seen 

 that the weight of a trolley loaded with one ton on two inch tyres 

 will have a greater compressing effect than a five-ton roller sup- 

 ported on two four-foot wheels, and distributed over that distance ; 

 even if the pavement is hot in the latter case and cold in the 

 former. This is generally the reason for the marking that all new 

 pavements should show when first opened to traffic. As traffic 

 continues these marks iron out until the mixture has received it? 

 ultimate compression. A light traffic mixture will naturally mark 

 mere readily than the closer and more compact heavy traffic 

 mixture. 



Dust or Filler.- — Of this ingredient little need be said in 

 this paper except that it should be very fine. As stated before at 

 least 75 per cent, should pass the 200 mesh sieve, and of this 

 portion certainly 50 per cent, should remain suspended in water 

 at 68° F. when agitated and allowed to settle for 15 seconds, as 

 determined by the elutriation test. The dust is added cold in pre- 

 determined amount to the hot sand, and mixed thoroughly with 

 it in the mixer before adding the asphalt cement, and it is here 

 that the value of the fine sand is felt in breaking up the masses 

 of dust and disseminating it throughout the sand; otherwise the 

 masses will be coated with the asphalt cement and cause weak spots 

 in the pavement, and the mixture will be lumpy. 



Asphalt Cement. — A.C. is the technical designation for 

 asphalt cement, and has been mentioned sufficiently under 

 "asphalt." It is modified in penetration (consistency) to suit any 



