n 4 PRACTICAL TREE REPAIR 



is subject to one very serious danger. Many fill- 

 ings made in this way, though they may at first 

 seem hard, gradually " rot." The writer believes 

 that the explanation of this difficulty lies in the 

 dryness of the mixture. There is a saying that 

 a wet mixture tamps itself, but that a dry mixture 

 must be tamped. A dry mixture which is not 

 thoroughly tamped cannot make a dense concrete. 

 Now it is almost impossible to tamp properly con- 

 crete which is being built up in a cavity. In the 

 first place, there is not often much elbow-room 

 and it is difficult to apply sufficient force. Then, 

 too, a hearty ramming would almost surely cause 

 the lower part of the filling to belly out or to fall 

 out bodily. As a result, tree fillings are not often 

 tamped with anywhere near the force considered 

 essential in the manufacture of building blocks by 

 the " dry method," which is practically the only 

 large use of dry concrete. Such fillings are in- 

 evitably porous, and porous concrete is like a 

 sponge. If there is any water about, the concrete 

 will absorb it. 



This brings us to another difficulty which often 

 arises in connection with dry fillings. It often 

 happens that a large part of what water there is 

 in the mixture evaporates the very first day it is 

 in place. Between this evaporation and the orig- 

 inal scantiness of water, not enough is left to crys- 

 tallize the cement in the concrete. When the con- 



