136 PRACTICAL TREE REPAIR 



of one thousand pounds, and that the wood will 

 support the same weight, but that it bends three 

 inches in doing so. If the beams are placed to- 

 gether and a weight of twelve hundred pounds is 

 placed across them, what happens? The wood 

 begins to bend, but it will hardly be supporting a 

 hundred pounds before the concrete beam's limit 

 of elasticity has been reached. The load on it 

 being in excess of its strength, the concrete breaks, 

 and the whole load falling on to the wood, it also 

 gives way. 



The importance of this difference in the char- 

 acter of the strength of the two materials is espe- 

 cially great just at the time the filling is put into 

 the tree. It is well known that concrete, though 

 it hardens with considerable rapidity, does not at- 

 tain to its ultimate strength for a rather long 

 period after it sets. It is this fact which explains 

 the provision in all building codes to the effect that 

 the forms must be left on structural concrete work 

 at least two weeks. During the first day or two 

 that the concrete is in place its tensile strength is 

 very slight, even though the surface of it may 

 seem quite hard. And it is absolutely inelastic. 

 During this first day or two, however, the tree 

 is certain to sway a little, and if there is much 

 wind its trunk will be in motion from top to bot- 

 tom. It is almost impossible to brace a tree so 

 that it cannot bend, and it is quite impossible to 



