CONCRETE 3 



portion of solid matter and voids. Less total grain surface means 

 less cement and water to coat the grains, and less labor required 

 in mixing. The additional amount of cement and water required 

 in the case of the fine sand reduces the density of the resulting 

 mortar and likewise its strength. (The density of neat cement 

 ranges between 0.49 and 0.59, while the density of a sand mortar 

 ranges from 0.60 for a fine sand to 0.75 for a coarse sand or a 

 well-graded sand.) 



A fine sand is one containing more than 30 per cent of par- 

 ticles that will pass a No. 40 sieve (diameter of hole = 0.015 in.). 



0025 0.050 



0.075 0.100 0.125 0.150 0.175 



Diameter of Particle in Inches. 



0.20O 0225 



FIG. 1. Typical mechanical analyses of fine, medium and coarse sands. 



The finer the sand, the more nearly uniform the size of the grains, 

 and consequently the greater the proportion of voids. Fine 

 sand is seldom satisfactory and should not be used unless a 

 coarse sand is not available. Even in such cases, tests of strength 

 should be made with the idea of determining what extra cost 

 may be justified in securing a coarser material. 



The most accurate method of determining the value of a sand 

 with reference to its size is by means of a sieve analysis. This 

 consists of sifting the sand through several different sieves, and 

 then plotting upon a diagram the percentage by weight which is 

 passed (or retained) by each sieve abscissae representing size 

 and ordinates representing percentage. Fig. 1 represents the 

 analyses of three natural sands a fine, a medium, and a coarse 

 well-graded sand. Uniform grading is indicated by an approach 

 to a straight line. A standard size of sieve is 8 in. in diameter 

 and 21/4 in. high. Woven brass wire sieves are employed for 



