FILLING OF CAVITIES 115 



crete dries out only a part of the cement really 

 hardens. The rest, although its cementing value 

 is destroyed, does not harden and is left in the 

 concrete in an unfixed condition. Now when this 

 concrete becomes saturated with water, and the 

 water works downward and out of any drainage 

 openings which may occur, it carries with it some 

 of this loose cement, a condition of things which 

 is betrayed by a light-colored stain on the bark 

 below the opening of the cavity. As this process 

 goes on the porousness of the concrete must ob- 

 viously increase, and before long the filling is ab- 

 solutely saturated, and has become rotten, friable, 

 and weak. This result is the more surprising be- 

 cause a week after the filling is put in it usually 

 seems as hard as flint, due to the fact that trowel- 

 ing and smoothing the surface draws to it a large 

 ampunt of cement and water, which cause the sur- 

 face of the filling to become really hard. 



As a remedy for this state of affairs two solu- 

 tions have been tried. The old method has been 

 rid of some of its faults by making the mixture 

 just as wet as it is possible to handle it, by paying 

 greater attention to the tamping, and, especially, 

 by preventing the access of water to the concrete 

 by means of improved dressings for the inner sur- 

 face of the cavity and the outer surface of the 

 filling. At the same time, efforts have been made 

 to find a practical means of handling a wet mix- 



