juiyso, I9I7 Infltience of Grading on Value of Fine Aggregate 265 



Toughness is measured by subjecting a cylindrical rock core, i by i 

 inch in size, to the impact produced by the fall of a given weight through 

 successively increasing heights until failure occurs. 



The combined effects of both abrasion and impact are measured to- 

 gether by means of the well-known Deval abrasion test, in which a stand- 

 ard weight of material is rattled in a cast-iron drum in such a way that 

 the pieces composing the sample are subjected to both influences. The 

 weight of charge larger than a certain size after a given number of revo- 

 lutions measures the resistance to wear. 



From a consideration of these tests it is seen at once that, with the 

 exception of rock or slag, they are not adapted to a direct determination 

 of the quality of concrete aggregates. The Deval test in modified form 

 has been used to determine the resistance to wear of gravel and sand, 

 but its use for this purpose, though promising, still is in the experimental 

 Stage. It is manifestly impossible to test the wearing qualities of fine 

 aggregates directly by any of these methods. Either the hardness or 

 toughness test or a wear test may, however, be applied to the fine aggre- 

 gate when combined with cement to form a mortar or v/ith cement and 

 coarse aggregate to form concrete. Wear tests of mortar and concrete 

 have been made with the Deval machine, the Tablet- Jones brick rattler, 

 and other special devices. Of these the adaptation of the brick rattler 

 as described by Abrams ^ appears to be the nearest solution of the prob- 

 lem of obtaining a laboratory wear test. Only preliminary results of 

 tests using this method have been published. Wear tests of concrete in 

 place on the road have been confined largely to suggestions for suitable 

 methods, although tests using the apparatus designed by Goldbeck ^ 

 have been started on an experimental concrete road near Washington, 



D. C. 



EXPERIMENTAL WORK 



In the following tests the effect of varying the grading of sand on the 

 hardness, toughness, tensile strength, and crushing strength of mortars 

 has been studied. The fine aggregate was prepared by artificially grad- 

 ing a quantity of Potomac River concrete sand, the analysis of which is 

 shown in Table I into three sizes as follows : 



(i) A coarse sand (C) composed of equal parts of material passing a 

 X-inch screen but retained on a lo-mesh sieve and that passing a 10- 

 mesh but retained on a 20-mesh sieve. 



(2) A medium sand (M) composed of equal parts of 20 to 30 mesh, 

 30 to 40 mesh, and 40 to 50 mesh material. 



(3) A fine sand (F) composed of the run of the sand below No. 50. 



' Abrams, D. A. a method of making wear tests of concrete. In Ainer. Soc. Testing Materials 

 Proc. 19th Ann. Meeting, 1916, v. 16, pt. 2, p. 194-202. 1916. 



2 Goldbeck, A. T. apparatus for measuring the wear of concrete roads. In Jour. Agr. Re- 

 search, V. s. no. 20, p. 9SI-954. pl. 66. 1916. 



