INFLUENCE OF GRADING ON THE VALUE OF FINE 

 AGGREGATE USED IN PORTLAND CEMENT CON- 

 CRETE ROAD CONSTRUCTION 



By F. H. Jackson, Jr., 



Assistant Testing Engineer, Office of Public Roads and Rural Engineering, United States 



Department of Agriculture 



THE PROBLEM 



Everyone familiar with either the testing of cement concrete or its 

 practical use in various forms of construction realizes the marked effect 

 variations in the grading of the aggregates may have on its strength, 

 density, and other properties. It is known, for instance, that, other 

 things being equal, the use of a coarse sand combined with a uniformly 

 graded coarse aggregate will result in the production of a very much 

 better grade of concrete than will the use of either a fine sand, a poorly 

 graded coarse aggregate, or both. The importance of this matter, 

 with special reference to the use of concrete as a road-surfacing material, 

 has been much emphasized recently before engineering societies and 

 elsewhere. The conclusions so far drawn, however, are apparently 

 based upon the generally accepted principle as applied to the ordinary 

 use of concrete rather than upon definite tests made with the object of 

 determining the effect of variations in either the quality or grading of 

 the aggregate on the resistance of concrete to the peculiar destructive 

 forces encountered on a road. It is, of course, apparent that these forces 

 are not only more severe but are more varied than those which act 

 upon unreinforced concrete as ordinarily used, which is usually subjected 

 only to direct compression. 



Agencies particularly destructive to a concrete road are (i) traffic, 

 (2) weather, and (3) constructional defects, all of which produce stresses 

 in the concrete which should be taken care of as completely as possible 

 through an intelligent selection of materials as well as a proper observance 

 of the details of construction. 



Traffic operates as a destructive force in three ways: (i) Iron tires 

 cause an abrasion or grinding away of the particles composing the 

 surface of the pavement, which varies inversely wth the hardness of 

 the concrete. (2) Suddenly applied loads, horses' hoofs, etc., subject 

 the pavement to impact, tending to loosen and sometimes fracture the 

 individual fragments composing the aggregate, and is resisted by the 

 property of toughness in the material. (3) The actual weight of traffic 

 as transmitted by wheel loads produces also compressive stresses propor- 

 tional to the unit loads, but these are of much less importance in causing 



Journal of Agricultural Research, Vol. X, No. s 



Washington, D. C. July 30, 1917 



jb Key No. D— 12 



(263) 



