214 ASPHALT. 



upon to bear. In this connection the investigations of the 

 Massachusetts Highway Commission on the bearing power of soil* 

 and the distribution of wheel loads will be of interest: — 



'•The Commission has estimated that non-porous soils drained 

 of ground water, at their worst will support a load of about 41b. 

 per inch, and having in mind these figures the thickness of broken 

 stone has been adjusted to the traffic. 



"On a road built of fragments of broken stone the downward 

 pressure takes a line at an angle of 45 degrees from the horizontal, 

 and is distributed over an area equal to the square of twice the 

 depth of the broken stone. If a division of a load in pounds at 

 any one point (i.e., of contact) by the square of twice the depth 

 of the stone in inches gives a quotient of 4 or less, then the road 

 foundation will be safe at all seasons of the year. On sand or 

 gravel the pressure can be safely put at 201bs. per square inch. 



"Acting on this theory the thickness of the stone varies from 

 4 inches to 16 inches, the lesser thickness being placed over good 

 gravel or sand, the greater over heavy clay, and varying thick- 

 nesses or other solids. In cases where the surfacing of broken 

 stone exceeds 6 inches in thickness the excess in the base may be 

 broken stone, stony gravel, or ledge stone, the material used for 

 the excess depending entirely upon the cost, either being equally 

 effective." 



If now the broken stone mentioned in the above extract were 

 cemented together by asphalt the angle of distribution would be 

 lessened by about 25 per cent., and the area of support more than 

 doubled, so that the thickness could be decreased by nearly one- 

 half. 



Conclusions 



It is verv evident that asphalts may differ considerably in tlu 

 quality of the bitumen they contain. Since in a sheet asphalt 

 pavement the bitumen only amounts to about 10 per cent., )t 

 necessarily follows that the asphalt should be well chosen in order 

 not to endanger the permanence of the other 90 per cent. It is 

 poor economy to save pennies on the asphalt and pay pounds on 

 the upkeep. No asphalt pavement is more lasting than the 

 bitumen it contains, for the dry mineral aggregate has no inherent 

 stability. 



No satisfactory laboratory test, or series of tests, will ensure 

 a long life to a pavement; the only reliable test is that of service, 

 but it must be borne in mind that even an asphalt with a good 

 record can be ruined by careless workmen. A street pavement has 

 the same relation to the mixture in the plant as a finished photo- 

 graph has to the negative in the camera ; there are many points 

 to consider in each case all bearing on the final product. For 

 instance a perfect wearing surface may be influenced by the fol- 

 lowing defects : — 



(a) Too much bitumen; (h) too little bitumen; (c) improper 

 type of bitumen; (d) too much flux; (e) too little flux; 

 (/) character of flux ; (g) too much coarse sand (10-40) ; (h) too 

 little coarse sand; (?) too much fine sand (80-100); (j) too little 

 fine sand; (/,•) improper type of sand; (/) too much filler (200); 

 (m) too little filler; (n) improper type of filler; (o) too hot a sand; 

 (p) insufficient dry or wet mixing. 



