790 EXPEEIMENT STATION EECORD. [Vol.35 



Strength and other properties of concretes as affected by materials and 

 methods of preparation, R. J, Wig, G. M. Williams, and E. R. Gates (V. S. 

 Dept. Com., Bur. Standards Technol. Paper 58 (1916), pp. 112, pis. 2, figs. U)- — 

 The results of about 20,000 tests, consisting of compressive and tensile tests 

 upon mortars at different ages including about 240 different sands and stone 

 screenings, and compressive tests on concretes composed of 60 aggregates in- 

 cluding limestones, gravels, granites, cinders, and trap rock, are reported to- 

 gether with tests of the physical properties of the sands, stone screenings, and 

 coarse aggregates. The following conclusions are drawn : 



" No standard of compressive strength can be assumed or guarantied for con- 

 crete of any particular proportions made with any aggregate unless all the 

 factors entering into its fabrication are controlled. 



"A concrete liaving a desired compressive strength is not necessarily guaran- 

 tied by a specification requiring only the use of certain types of materials in 

 stated proportions. . . . The compressive strength of a concrete is just as 

 much dependent upon other factors, such as careful workmanship and the use 

 of the proper quantity of water in mixing the concrete, as it is upon the use 

 of the proper quantity of cement. 



" The compressive strength of concrete may be reduced by the use of an excess 

 of water in mixing to a fractional part of that which it should attain with the 

 same materials. . . . The compressive strength of concrete may be greatly 

 reduced if, after fabrication, it is exposed to tlie sun and wind or in any rela- 

 tively dry atmosphere in which it loses its moisture rapidly, even though suit- 

 able materials were used and proper methods of fabrication employed. The 

 relative compressive strength of concretes to be obtained from any given mate- 

 rials can be determined only by an actual test of those materials combined in 

 a concrete. 



" Contrary to general practice and opinion the relative value of several fine 

 aggregates to be used in concrete can not be determined by testing them in 

 mortar mixtures. They must be tested in the combined state with the coarse 

 aggregate. 



" Contrary to general practice and opinion the relative value of several coarse 

 aggregates to be used in concrete can not be determined by testing them witli a 

 given sand in one arbitrarily selected proportion. They should be tested in 

 such combination with the fine aggregate as will give maximum density, assum- 

 ing the same ratio of cement to total combined aggi'egate in all cases. 



" No type of aggregate such as gi-anite, gravel, or limestone can be said to be 

 generally superior to all other types. There are good and poor aggregates of 

 each type. By proper attention to methods of fabrication and curing, aggre- 

 gates which appear inferior and may be available at the site of the work may 

 give as high compressive strength in concrete as the best selected materials 

 brought from a distance, when the latter are carelessly or improperly used. 



" Density is a good measure of the relative compressive strength of several 

 different mixtures of the same aggregates with the same proportion of cement 

 to total aggregate. The mixture having the highest density need not neces- 

 sarily have the maximum strength, but it will have a relatively high strength. 



" Two concretes having the same density, but composed of different aggre- 

 gates, may have widely different compressive strength. There is no definite 

 relation between the gradation of the aggregates and the compressive strength 

 of the concrete which is applicable to any considerable number of different 

 aggregates. The gradation curve for maximum compressive strength, which 

 is usually the same as for the maximum density, differs for each aggregate. 



