i io PRACTICAL TREE REPAIR 



density and the strength 'of concrete. Even a 

 slight increase in weight per cubic foot will add 

 very decidedly to the strength. In this connec- 

 tion the importance of the coarser aggregates can- 

 not be too strongly insisted upon. The use of 

 coarse material is essential to density, since coarse 

 material contains the smallest amount of voids. 

 Different kinds of sand, gravel, and stone vary 

 greatly in the amount of their voids; and by judi- 

 ciously mixing coarse and fine materials, the 

 voids may be much reduced, and the weight and 

 density of the concrete increased. 



" If the sand be screened so as to take out the 

 coarse grains, the voids will be increased and the 

 weight reduced, thus injuring the sand for making 

 concrete. Strength may be improved by adding 

 coarse material, even though the proportion of 

 cement is thereby reduced. This has been re- 

 peatedly shown by experiment. A mixture of 

 cement and sand alone will form a rather weak 

 concrete, especially if the sand is fine. By adding 

 gravel say about twice the quantity of gravel 

 that there is of sand, or a little more a con- 

 crete will be obtained containing, of course, a 

 greatly reduced percentage of cement, but of 

 greatly increased strength." 



The strain on concrete fillings in trees is so great 

 that a sacrifice of strength for a fancied esthetic 

 effect is not easily justified. It is, moreover, not 



