142 



TREES IN" WINTER 



Very large openings will need to be bricked np to hold the con- 

 crete in position. The proportions used in making np concrete 

 varies, but most experts use one jDart cement, two parts sand, and 

 four parts crushed stone for the main filling. Just enough water 

 is added to make a mixture that will settle well into all crevices 

 without much tamping. The finishing is done with a stiffer mix- 

 ture made up of one part cement to two parts of fine sand and just 

 enough water to make a good mortar. Sometimes nails are partly 

 driven into the main filling to assist in holding the finishing ma- 

 terial in position until it has hardened. 



The finishing of the surface is one of the most important feat- 

 ures of the work. The filling should be left with a convex surface 

 and the edge of the concrete should be beveled off so that it comes 

 jr«;t beneath the cambium layer or inner bark. If the finishing is 

 properly done the callous will form over the edge of the filling. In 

 long cavities that extend far up the trunk, it is customary to divide 



Fig. 67. Cross-section of a filled cavity, showing the correct position 

 of the concrete in relation to the bark and cambium. B — bark; C — 

 cambium or inner bark; SW — sound wood; MF — main filling of concrete; 

 P — iron pipes for reinforcement; F.C. — finishing concrete. 



