PACIXI, METAMORPHI8M OF PORTLAND CEMENT 



20' 



TABLE 17 

 Rate of Flow in Gallons per Square Foot per Hour under 20-ineli Head 



In denser concretes, this effect was not found so marked. It will be 

 noted that after storage following the first exposure to the effect of per- 

 colating water, these specimens appear to offer less resistance to the flow 

 of water. This may be due to the fact that in lean concretes the propor- 

 tion of capillary and subcapillary voids is smaller and that of super- 

 capillary voids greater, and that cementing and clogging actions, which 

 have their greatest effect in capillary and subcapillary passages, are not 

 so effective. 



The greater flow along the bedding planes has been observed in the 

 case of rock, and is in all respects a phenomenon of the same nature. In 

 the case of a stratified sandstone cited by King (51), the reason is ad- 

 vanced that no more water can pass the more open layers, when advancing 

 -across the bedding planes, than was able to pass those of the closest tex- 

 ture ; whereas when the flow is along the bedding planes, each particular 

 .stratum carries water in proportion to the coarseness of its texture, 

 uninfluenced by any other. 



In the case of water percolating into a concrete tunnel this would 

 tend to emphasize lateral percolation, and in the case of disintegration 

 would exercise, in general, a localizing influence. It is not to be as- 

 sumed that this is a rigid rule, inasmuch as a large number of factors, 

 evidently, may neutralize this influence. 



From these considerations, it will be seen that the solvent effect of 

 water upon set cement is of high importance in considering the perma- 

 nence of concrete structures, and that this solvent effect tends to diminish 

 .as the set cement ages. This is not the onlv wav, of course, that water 



