SCIENCE IN HIGHWAY CONSTRUCTION 329 



water-bound broken-stone road, its physical characteristics and its 

 behavior under certain conditions are important. Before the develop- 

 ment of means of determining these characteristics in the laboratory, 

 the only way in which they could be arrived at was from observation of 

 the behavior of the particular rock in a road when exposed to travel for 

 a considerable period of time. 



The important characteristics of a rock which enables one to judge 

 of its suitability for road' construction are (1) its resistance to wear or 

 abrasion by impact, (2) its hardness or resistance to the displacement 

 of its particles by friction, (3) toughness or resistance to fracture by 

 impact, (4) the cementing properties or value of the rock powder or 

 dust produced by attrition, Vhen moistened, (5) porosity or capacity 

 for absorbing moisture, the latter being closely associated, for the same 

 kind of rock, with the specific gravit}^ and (6) the structure or size of 

 the grain of the rock, the character of the minerals of which it is com- 

 posed, and the extent to which these may have become altered by 

 weathering, upon which all the other characteristics of the rock Avill 

 depend. 



The methods of determining these characteristics have been largely 

 originated and developed in the Office of Public Roads in Washington, 

 and are described in one of its bulletins, 'No. 31, as follows: 



Percentage of wear represents the amount of material under 0.16 cm. in 

 diameter lost by abrasion from a weighed quantity of rock fragments of definite 

 size. It is determined in the following manner: The rock sample is broken into 

 pieces that will pass through a 2.4-inch ring but not through a 1.2-inch ring, 

 and after being thoroughly cleansed, dried and cooled, 5 kg. are weighted and 

 placed in a cast-iron cylinder (34 cm. deep by 20 cm. in diameter) closed at one 

 end and having a tight-fitting iron cover at the other. This cylinder is one of 

 four attached to a shaft so that the axis of each is inclined at an angle of 30° 

 with that of the shaft. These cylinders are revolved for five hours at the rate 

 of 2,000 revolutions per hour, during which the stone fragments are thrown from 

 one end of the cylinder to the other twice in each revolution. At the end of five 

 hours the machine is stopped, the cylinders opened, and their contents j)oured 

 into a basin, in which every stone is carefully washed to remove any adherent 

 detritus. This abraded material is then thoroughly dried, and from the amount 

 lost below 0.16 cm. the per cent, of wear is estimated. 



Hardness is the resistance which a material offers to the displacement of 

 its particles by friction, and varies inversely as the loss in weight by grinding 

 with a standard abrasive agent. The test is made in the following manner: 

 The test piece in the form of a cylinder about 3 inches in length by 1 inch in 

 diameter is prepared by an annular core drill and placed in the grinding machine 

 in such a manner that the base of the cylinder rests on the upper surface of a 

 circular grinding disk of cast iron, which is rotated in a horizontal plane by a 

 crank movement. The specimen is weighted so as to exert a pressure of 250 

 grams per square centimeter against the disk, which is fed from a funnel with 

 sand of about I3 mm. in diameter. After 1,000 revolutions the loss in weight 

 of the sample is determined and the coefficient of wear obtained by deducting 

 one third of this loss from 20. 



VOL. iixxx. — 2:^. 



