1917] RURAL ENGINEERING. 683 



The specific gravity of nonhomogeneous aggregates, P. Hubbabd and F. H. 

 Jackson, Jb. {Atner. Soc. Testing Materials Proc, 16 {1916), pt. 2, pp. S78-402, 

 figs. 6; abs. in Cement Era, lit (1916), No. 9, pp. 4MS. fiff^- 6).— The object of 

 this investigation was to study methods in common use, or those which gave 

 promise of being most satisfactory, for the determination of apparent and true 

 specific gravity of mineral aggregates, with a view to ascertaining if possible 

 what method is most generally applicable to all classes of materials and also 

 the most accurate. The methods studied were as follows: (1) The ordinary 

 displacement method as conducted by the Office of Public Roads and Rural 

 Engineering of the U. S. Department of Agriculture, (2) the Chapman method 

 for single specimens, (3) the Goldbeck method, (4) the Hubbard- Jackson 

 method, (5) the Chapman wire-basket method, (6) the Le Chatelier method 

 for fine aggregates, and (7) the Jackson method for fine aggregates. Descrip- 

 tions of these tests in detail, together with illustrations, are given. Sixteen 

 samples of crushed rock, three samples of crushed slag, a sample of gravel, and 

 three samples of sand were used in the investigation. The following con- 

 clusions were drawn : 



"(1) In the case of rock and slag an appreciable variation may exist between 

 apparent specific gravity and true specific gravity, depending upon the absorp- 

 tion of the material. (2) It is impracticable by any of the methods studied to 

 determine the apparent specific gravity of samples composed of fragments 

 smaller than 0.5 in. in diameter. (3) Methods employing single test specimens 

 are not safe to use for determining the specific gravity of nonhomogeneous 

 aggregates, even when the average results of three apparently representative 

 samples are taken. (4) In the case of nonhomogeneous aggregates consisting 

 of fragments of not less than 0.5 in. in diameter, all of the methods employing 

 1,000-gm. samples are satisfactory and can ordinarily be depended upon to 

 give check results by different operators working upon the same sample to 

 within one in the third significant figure. (5) When determining the specific 

 gravity of extremely nonhomogeneous aggregates, it is recommended that the 

 average of not less than three tests, made upon different 1,000-gm. samples, be 

 reported. (6) When it is desired to obtain as nearly as possible the apparent 

 specific gravity of aggregates consisting of a mixture of coarse and fine par- 

 ticles, it has appeared advisable to separate a weighed sample of the material 

 by means of a 0.5-in. screen, and to make an apparent-specific-gravity determi- 

 nation upon not less than 1,000-gm. of the coarse fraction and a true-specific- 

 gravity determination upon not less than 50-gm. of the finer fraction. The 

 approximate apparent specific gravity of the whole sample may then be cal- 

 culated from the results obtained." 



A new form of specifications for concrete aggregates, C. M. Chapman 

 (Amer. Soc. Testing MateriaU Proc., 16 {1916), pt. 2, pp. 180-193).— This paper 

 proposes that instead of specifying that concrete aggregates, particularly sand, 

 be of a certain fixed minimum standard of quality, the specifications be made 

 to read in effect as follows: The materials used shall be of such quality and 

 shall be used in such proportions as to produce a concrete which shall show a 

 compressive strength of 2,500 (or 2,000 or 1,500) lbs. per square inch at the 

 age of 28 days under standard test. A brief outline of the manner in which 

 this form of specification is utilized in practice is also given. 



A method of making wear tests of concrete, D. A. Abbams {Amer. Soc. 

 Testing Materials Proc, 16 {1916), pt. 2, pp. 194-208, figs. 6; abs. in Engin. and 

 Contract., 46 {1916), No. 5, pp. 102, 103. figs. 4).— A brief review of tests use^J 

 for determining the wearing resistance of concrete and concrete aggregates, 

 especially for use in the wearing surface of roads, is given, and a method Is 



