2 REINFORCED CONCRETE CONSTRUCTION 



2. Cement. The cement employed in reinforced concrete 

 construction should be of high grade; only portland cement 

 should be used, and the brand selected should conform to the 

 specifications of the American Society for Testing Materials 

 for these specifications are now accepted as the American 

 standard. 



3. Sand. The sand employed should be free from clay, 

 vegetable loam, sticks, and organic matter and should be of hard, 

 dense, tough material. Siliceous quartz sands are the best, 

 although sands from any durable rock will answer. 



Sharp sand was formally a requirement in all important con- 

 struction, but this property is by no means essential. To be 

 sure, by the use of sharp sand there is a slight tendency toward 

 a concrete of greater crushing strength than when sand of 

 rounded grains is employed, but this influence on the result is of 

 less importance than the size of grain, or granulometric composi-. 

 tion. Moreover, the sharper the sand employed the relative 

 sizes of the grains remaining the same the greater the percent- 

 age of voids, and consequently the greater the amount of cement 

 required to produce a given density. (The term density is here 

 used to express the ratio of the volume of the solid particles to 

 the total volume of the concrete.) It is now generally conceded 

 that the requirement of sharpness of sand should be omitted 

 from concrete specifications. 



Tests of mortar and concrete show that strength and water- 

 tightness increase with density, and so the best sand as to size is 

 one which will produce the smallest volume of mortar of stand- 

 ard consistency when mixed with the given cement in the 

 required proportions. To put it somewhat differently, the 

 best sand for strength, for water-tightness, and also for economy 

 (as will be seen later) is one which is so graded from fine to 

 coarse that the percentage of voids in the resulting mortar is 

 reduced to a minimum. Such a sand has a very coarse appear- 

 ance as the amount of fine material required is small. 



It has been found that the densest mixture occurs with par- 

 ticles of different sizes and also that the least density occurs when 

 the grains are all of the same size. Coarse and fine sands are thus 

 inferior to graded sands for concrete, but of the two extremes 

 the coarse sand is preferable. The reason for this is due to the 

 fact that the coarse sand has a less total grain surface in a unit 

 volume, even when the sands considered contain the same pro- 



