juiy3o, 191 7 Influence of Grading on Value of Fine Aggregate 271 



2° 



- u 

 u n»> 

 II 3 



if 



55» 



20 30 40 sa bO 70 so 30 



PCB CENT 3AN0 PV£« •ZOMEJH. 



Fig. s- — Curve of toughness tests of artificiaUy graded sand mortar. 



in figure 4 in the same way as the results of the hardness tests previously 

 referred to. 



A series of tests made with the view of determining the effect of 

 richness of mix on the toughness of cement mortar are shown, for com- 

 parison, in figure 6. The results are plotted with the percentage of 

 cement as abscissae and toughness as ordinates. Each plotted result 

 is the average of nine 

 tests. These results are 

 of interest in showing 

 the large increase in 

 toughness produced by 

 increasing the cement 

 content. This increase 

 appears to be practi- 

 cally proportional to the 

 percentage of cement up 

 to a I -to- 1 mix, after 

 which the addition of 

 more cement affects the toughness very little. In other words, a neat 

 cement briquette is no tougher than one composed of equal parts of 

 cement and a typical concrete sand, which, in turn, shows twice the 

 resistance to impact of one containing only 20 per cent. Inasmuch as 

 this sand was very well graded and contained only 35 per cent of voids, 

 it is obvious that a considerable excess of cement over that required to 



fill the voids is required 

 to reach the condition 

 of maximum toughness. 

 In other words, these 

 results present an ex- 

 perimental verification 

 of the theory which re- 

 quires the use of a rich 

 mix in the surface of a 

 concrete pavement, es- 

 pecially as has been seen 

 by reference to the hardness test shown in figure 3 that the hardness 

 of a rich i-to-i^ mix is practically the same as a i-to-3 and greater 

 than a i-to-4 mix. 



TENSION AND COMPRESSION TESTS 



The results of the tension and compression tests are shown in figures 

 7 and 8. They were made in the usual way on standard briquettes and 

 2 by 2 inch cylinders. Each value in tension is the average of six tests, 



10 iO JO 40 30 60 70 80 30 

 PpR CCNT CEMENT IN MIX. 



Fig. 6. — Curve of toughness tests of natural concrete sand mortar. 



