CONCRETE 7 



to the barrel is generally taken as the standard. The value 

 3.8 has been selected for convenience since 100 Ib. of cement 

 can thus be considered as 1 cu. ft. 



7. Theory of Proportions. Two well-established laws govern 

 the theory of proper proportioning, namely: 



1. With the same percentage of cement in a unit volume of 

 concrete, the strongest and most impermeable concrete is that 

 which has the greatest density. 



2. If the sand and stone remain the same, the strongest and 

 most impermeable concrete is that containing the greatest 

 percentage of cement in a unit volume. 



The first law is extremely important. Another way of express- 

 ing it is to say that, to obtain the greatest strength and imper- 

 meability, the cement should fill the voids of the sand and the 

 resulting mortar should fill the voids of the stone. The second 

 law means that with the same aggregates the strength and 

 water-tightness increases with the amount of cement used 

 provided, however, that in some cases this amount be not in 

 excess of the voids in the sand, and that the amount of mortar used 

 in each case be the same. If the cement more than fills the voids 

 of the sand, or if the mortar more than fills the voids of the 

 stone, the concrete will be less dense than if the voids were 

 just filled (ordinary concrete has a density between 0.80 and 0.88 

 and hence is denser than either neat cement or cement mortar) ; 

 and thus the strength due to increase of cement may be offset 

 by the decrease in density. 



8. Proportioning by Mechanical Analysis. Certain standard 

 proportions, such as 1:2:4 and 1:21/2:5, are commonly em- 

 ployed in practice; but better results with greater economy can 

 often be secured by the use of mechanical-analysis curves. 

 These curves make it possible to find the best proportions of 

 different aggregates, and they also afford means of finding the 

 best proportions attainable by screening the sand and the stone, 

 and by making artificial combinations of the several portions. 



In proportioning by mechanical analysis, the object to be 

 aimed at is to grade the fine and coarse aggregate so that the 

 densest concrete will result from the use of a given amount of 

 cement. This means that the object to be kept in view while 

 grading should be a minimum percentage of voids; however, the 

 use of much very fine material should be avoided for the reason 

 cited in Art. 3. After this grading is accomplished, an amount of 



