42 THK TESTING OF ROAD MATERIALS. 







have found time to examine the question only superficially have mis- 

 understood the bearing and value of road-material testing, hi all 

 ca>es the results obtained in testing materials of construction are of 

 relative rather than absolute value. Even quite a large variation in. 

 the results yielded by different test pieces of the same sample should 

 not condemn the practical value of the figures if they are properly 

 applied and interpreted. The necessary qualities of road materials 

 have to be considered from the double standpoint of furnishing a 

 strong, unyielding, well-drained road foundation and a hard, coherent 

 binder surface. In road building the attempt should be made to get 

 a perfectly smooth surface, neither too hard, too slippery, nor too 

 noisy, and as free as possible from mud and dust, these resujts to be 

 attained and maintained as cheaply as possible. Such results, how- 

 ever, can be achieved only by selecting the material and methods of 

 construction best suited to the conditions. 



Given a number of materials for laboratoiy examination, it is not 

 pretended that an actual practical grade of excellence can be estab- 

 lished. On the other hand, if more than one material is available, it 

 is quite possible for the laboratoiy to point out which one would yield 

 the best results both as to immediate excellence and length of life 

 under known conditions of climate and traffic. Undoubtedly in many 

 ca-es large sums of money have been wasted in building roads from 

 unsuitable material, which might have been saved by referring the 

 materials to the laboratory. If, for instance, it is desired to know 

 whether an available rock will be useful as a top dressing to form the 

 binder surface, no better method of obtaining preliminary information 

 on the subject is known than to test the cementing value. Undoubt- 

 edly >ome rocks will yield powder from successive test pieces which 

 shows very wide variations in this value under the conditions of the 

 te-t. but there is at present no difficulty in distinguishing between 

 good and bad material. It is true that before the discovery that cer- 

 tain road materials increase in binding power after being in use fora 

 time some errors were made. A notable, although exceptional, example 

 of this kind was a southern chert from Alabama. The method formerly 

 u-e<l ( f mixing the rock dust with water and immediately molding 

 the briquettes, as previously described in this bulletin, yielded a low 

 cementing value, and the rock was not recommended as a surfacing 

 material if a better one was available. It appears, however, that 

 these southern cherts develop a high binding power under traflic and 

 make excellent roads. Under the present method of conduct ing the 

 cementation tests i. e. . allowing the dough to stand lie fore molding it. 

 the briquettes of this same, chert gave a comparatively high cementing 

 value. It has been impossible, under the pressure of routine work 

 during the current year, to prepare ihe figures and results of these 

 and many other investigations for publication in this bulletin. These 



