184 ANNALS NEW YORK ACADEMY OF SCIENCES 



The effects from less spectacular processes are quite surprising. Where 

 the pressure of the water is such that there is a marked motion of the 

 water within the pores of the concrete, the erosion is internal and far 

 more insidious. In this case, the suspended material is part of the struc- 

 ture itself. Small particles of cement or, in the case of mortar, grains 

 of sand which become detached from the parent mass are whirled around 

 by the water stream and shortly enlarge the cavity in which they are 

 rotating, until it merges with some adjacent cavity. Under favorable 

 conditions this process may continue until the interior of the structure 

 is greatly weakened. 



A factor which to some extent neutralizes the flow of water through 

 concrete is the choking of the pores by sediment, coming from the water 

 itself or furnished by the action of the water upon the concrete. If the 

 flow is oscillatory, as in concrete exposed to the range of the tides, this 

 protective effect will of course not be so marked (54). 



Diatoms and other microscopic marine organisms with siliceous or cal- 

 careous tests undoubtedly play an extensive part in the preliminary- 

 stages of this internal mechanical action, by choking the capillary spaces. 

 At the same time, undoubtedly, the organic debris thus introduced may 

 by its decomposition give rise to substances, carbon dioxide and hydrogen 

 sulphide, for example, which have an accelerating action upon the proc- 

 esses of solution, and the silting effect may thus be neutralized or even 

 overbalanced. 



PART II 

 EXPERIMENTAL INVESTIGATION' 



In Part I, the ways in which water may influence the metamorphism 

 of Portland cement were discussed qualitatively, and their possible effects 

 upon the permanence of the structure of which cement forms the basis 

 were pointed out. This question has now assumed economic and vital 

 importance. 



In the following pages experimental data are offered, in elaboration of 

 the outline laid down in the first portion of the paper. Points in the 

 scheme which have been established beyond doubt by previous investi- 

 gators are here omitted, and only such results are inserted as have been 

 deemed necessary as additional evidence. The last division of the out- 

 line, treating of the action of suspended material in water in effecting 

 the erosion of concrete, has not been experimented upon, not having come 

 within the scope of the writer's activities, and therefore is omitted. 



