10 BULLETIN" 348, U. S. DEPARTMENT OP AGRICULTURE. 



PHYSICAL PROPERTIES OF ROAD MATERIALS. 



From what has been said it is apparent that many kinds of minerals 

 associated in different ways enter into the composition of rocks for 

 road building. It will now be of interest to discuss briefly the 

 physical properties of the rocks themselves before taking up more 

 specifically the relationship between these properties and the mineral 

 composition and structure of road materials. 



The physical properties selected for this investigation are expressed 

 upon determination by laboratory tests, as follows: (1) Per cent of 

 wear or loss by abrasion, (2) hardness, (3) toughness, (4) cement- 

 ing value, and (5) specific gravity. 



Per cent of wear represents the amount of material under 0.16 

 cm. in diameter lost by abrasion from a quantity of rock fragments as 

 nearly uniform in size as possible and weighing within 10 grams of 5 

 kilograms . This carefully weighed sample which should contain about 

 50 pieces of rock is tested in a Deval type of abrasion machine con- 

 sisting of a cast-iron cylinder, 34 cm. deep by 20 cm. in diameter, 

 closed at one end and having a tight-fitting cover at the other. The 

 cylinder is attached to a horizontal shaft so that the axis is inclined 

 at an angle of 30° with that of the shaft and revolved for 5 hours at 

 the rate of 2,000 revolutions per hour, during which the rock frag- 

 ments are thrown from one end of the cylinder to the other twice in 

 each revolution. This abraded material is then thoroughly dried 

 and screened through a -^-inch mesh sieve and from the amount 

 lost the per cent of wear is determined. This loss may be also 

 expressed by the French coefficient given below : 



n «: • * t onv 20 400 40 



Coefficient ot wear = 20 X -fxr = 



' W W per cent of wear 



where W is the weight in grams of the abraded material under 0.16 

 cm. (-j^g- inch) in diameter per kilogram of rock used. 1 



Hardness is the resistance which a material offers to the displace- 

 ment of its particles by friction and the test is made on a cylindrical 

 rock core, 25 millimeters in diameter. The test piece is held perpen- 

 dicularly under a constant pressure of 1,250 grams, against a revolv- 

 ing cast-steel disk, while standard quartz sand, between 30 and 40 

 mesh, is used as the abrasion agent. From the average loss in weight 

 computed from two runs, the coefficient of hardness is obtained by 

 deducting one-third of this loss, expressed in grams per 1,000 revo- 

 lutions, from an arbitrary constant (20). 



Toughness, as here understood, is the resistance a material offers 

 to fracture by impact. The test piece is a cylindrical rock core 25 

 millimeters high by 25 millimeters in diameter, and the test is made 



i Bulletin 347, U. S. Dept. Agriculture, p. 6. 



