210 ASPHALT. 



inch down to the impalpable dust passing a 200 mesh sieve, having 

 40,000 openings to the square inch. The sand is selected to take 

 the gradings from the 10-mesh sieve down to that retained on the 

 200-mesh sieve. The dust carries the grading further from that 

 passing the 200-mesh down to the impalpable dust that will not 

 settle in water during 15 seconds. This decreases the size of the 

 voids to that of the finest material, and the bitumen completes the 

 process by covering each particle and cementing them together. 

 The point of most importance is the one most frequently over- 

 looked, namely, the character of the fine sand. This has a double 

 purpose. First, it increases the surface area and surface energy, 

 and second, it lessens the voids, and distributes the particles of 

 dust throughout the mixture, thereby increasing its stability by 

 making it denser, and also more waterproof. 



The best sand grading has been found by the examination of 

 successful pavements to conform to the following mnemonic 

 figures : — 



Traffic. Heavy. Light. 



Passing 80 mesh sieve 33 per cent. 22 per cent. 



Passing 40 mesh sieve 44 per cent. 44 per cent. 



Passing 10 mesh sieve 22 per cent. 33 per cent. 



It is very seldom that a single sand is found that fulfils the 

 conditions of the above grading, so two or more sands are com- 

 bined in order that the mixture will approximate as closely as 

 possible to the standard. Durban is fortunate in its supply of coarse 

 and fine sand situated as it is between the Umgeni River on the 

 east, and the Umbilo on the west. The former, being a long and 

 swift river, supplies the coarse sand; while the latter, being com- 

 paratively short and sluggish, supplies the fine, or tempering, sand. 

 These effects are due to the well-known transporting power of 

 moving waters. As an example of the grading of the sands from 

 the respective rivers we have — 



Umgeni. Umbilo. Mixed 1 to 4. 



Passing 200 mesh sieve .. — 9 7 



Passing 80 mesh sieve .. 2 38 31 



Passing 40 mesh sieve ... 9 52 43 



Passing 10 mesh sieve ... 89 1 19 



These two sands are supplied to the cold material elevator in 



the proportion of one barrowful of the Umgeni sand to four of the 



Umbilo, which gives a grading as shown in the third column. The 



200 mesh sand is not desirable in a pavement, as it makes a mushy 



mixture, the grains acting rather as ball-bearings than wedges; 



however, it is mostly taken out by the draught fan of the plant. 



The voids in the mixed sands are no smaller than those of the 



finest portions, so as these are >iot fine enough to give a proper 



stability, a stone dust, or Portland cement, is added, of which at 



least 75 per cent, should pass the 200 mesh sieve. Sufficient is 



added to give about 12 per cent, of the 200 mesh material in the 



finished mixture, assuming it is to sustain a heavy traffic. Having 



reduced the voids to the size of those in the finest of the dust, 



the whole, while heated to about 350° F. is mixed with hot 



