juiy3o. 1917 Influence of Grading on Value of Fine Aggregate 269 



PER CENT SAND OVtR •20 SIE.VE.. 



Fig. 2. — Curve of hardness tests of artificially graded sand mortar. 



In figure 2 the results of the i-to-2}4 mortar tests are shown plotted 

 on rectangular coordinates with the percentage of sand over 20 mesh as 

 abscissae and the loss in weight per i ,000 revolutions as ordinates. Each 

 point plotted represents the average of the results on all values obtained 

 originally with the percentage of coarse sand as indicated. For instance, 

 the average of all com- 

 binations of medium 

 and fine sand with 20 

 per cent coarse showed 

 alossof 6 gm., and this 

 value is plotted on the 

 curve. This graph 

 shows the great effect 

 of coarse sand on the 

 hardness of mortar. 

 The corresponding 

 curve (not shown) for 

 the hardness of i-to- 

 I }4 mortars was found 

 to be practically identical, except that the average loss for m^or- 

 tars containing no percentage of sand over 20 mesh was 40 instead 

 of 200. 



Figure 3 shows the results of a number of hardness tests made on a 

 series of mortar specimens, in which the grading of the sand remained 



constant and the per- 

 centage • of cement 

 varied. In these tests 

 the unscreened Poto- 

 mac River sand, simi- 

 lar in quality to the 

 artificially graded 

 sands, was used. It 

 will be seen that the 

 hardness values of the 

 mortar specimens are 

 all greater than neat 

 cement. Further- 

 more, there is very 

 little difference in the loss in weight between the i-to-i, i-to-i}4, i-to-2, 

 and i-to-3 mixes, all of which show about the same hardness as was 

 obtained with the artificially graded coarse sands. The somewhat 

 greater loss sustained by the i-to-4 mortar specimen, no doubt, is due 

 to the very lean mix, which allowed the mortar to disintegrate under the 

 test in much the same way as noted above for very fine sand. 



PER CENT CtMtNT IN MIX. 



Fig. 3. — Curve of hardness tests of natural concrete sand mortars. 



