264 Journal of Agricultural Research voi x.no.s 



wear than are the stresses produced by abrasion and impact. The com- 

 bined destructive effect of abrasion and impact therefore may be called 

 the effect of wear, so that any concrete which will successfully resist 

 these two forces may be said to possess high resistance to wear. 



The influences of the weather tend to stress the concrete sometimes in 

 tension and sometimes in compression, either through the action of 

 temperature or moisture changes, or both, and usually result in the for- 

 mation of cracks whose edges, unless adequately maintained, subse- 

 quently wear rapidly under the action of traffic. 



Constructional defects usually result in unduly stressing the concrete 

 at some particular point, such as might be caused by settlement due to 

 improper consolidation of the subbase. Traffic undoubtedly is the most 

 important destructive agency in so far as the ultimate life of the road is 

 concerned, because its effects are cumulative and also serve to hasten 

 deterioration started from other causes. With this point in mind, it fol- 

 lows that a determination of the suitability of concrete for use as a road 

 material should be essentially a determination of its resistance to wear; 

 and, since resistance to wear means both resistance to abrasion and 

 impact, it would seem that hardness, and toughness tests, or a single test 

 combining both, should be logical ones to apply. 



It is the purpose of this paper to present some results obtained recently 

 in the laboratory of the Office of Public Roads and Rural Engineering, 

 which show in a general way the possible effect of variations in the 

 grading of fane aggregate on each of these essential properties. Of 

 course, it is realized that grading is only one of a number of properties 

 of the aggregates which may influence the quality of the finished product. 

 The character of the particles themselves, whether they are of a hard 

 siliceous or soft calcareous nature, as well as the amount of impurities, 

 organic or otherwise, present, is of the utmost importance. In the 

 following tests, however, these influences were controlled by the use of 

 a standard aggregate which was artificially graded in the laboratory 

 prior to use. 



In the case of rock used in macadam-road construction the nature of 

 the material is such that its hardness and toughness may be deter- 

 mined readily, either by means of independent tests or by means of a 

 wear test in which both properties are measured. These tests have 

 become well known, but, because the principles involved have been 

 used in this work, they will be discussed very briefly here. The hard- 

 ness of a rock is determined by subjecting a C5^1indrical specimen, pre- 

 pared by means of a diamond drill, to the abrasive action of crushed 

 quartz sand of a definite size. The rock cylinder is held against a hori- 

 zontally revolving steel disk upon which the abrasive is fed. The loss 

 in weight is determined after a given number of revolutions, and this 

 loss is used as an index of the hardness of the material. 



