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of "composite" structures, as in tlie case of filling of spaces around protected piles, 

 such practice is only justified when the concrete is considered as an inert filler. Tremie 

 concrete can be relied upon for structural loads in "simple" structures of mass type. 

 The size of the tremie under such circumstances is a function of the depth of water 

 and the size of pocket to be filled. Sizes from 12 inches to 18 inches are recommended, 

 the latter size for depths of 50 feet and tremie charges of 10 cubic yards. 



In placing tremie concrete in sea water continuous operation is of the greatest 

 importance. Interruption of flow, e\en when the charge is not lost, tends to produce 

 laitance streaks in the concrete. When practicable, storage hoppers should be pro- 

 vided of such capacity that the entire pocket can be charged at one operation. Con- 

 tinuous flow from the accumulated concrete of the storage hopper is preferable to 

 depositing by separate batches, with the possibility of interruption between batches. 

 The principal use of tremie concrete in sea water work should be for sealing the bottom 

 of walls or foundation units preparatory to unwatering. 



The above precautions in manipulation, cited for "simple" concrete structures, 

 are of still greater importance for "composite" structures encasing steel or timber. 

 The reduction by disintegration of the small sections used in these latter structures 

 is not only more serious in weakening the structural stability of the concrete, but it 

 also exposes protected members of steel or wood, upon which the structural stability 

 frequently depends, to rapid destruction by corrosion or marine borer action. Because 

 of smaller sections and more constricted openings, a more flowable consistency must 

 be employed and consequently the proportion of cement is increased, as previously 

 noted. A mushy consistency which will hold the aggregate together without separation 

 during transportation and will flatten into a plastic mass with the tamping specified 

 is satisfactory. Systems of precasting are advantageous because of the better facilities 

 for depositing and tamping the concrete, and of greater accessibility for inspecting 

 and painting exposed surfaces. Systems of casting composite members in place will 

 give good results if special precautions are taken to exclude sea water and to tamp the 

 concrete, but owing to the ditificulty of the work and its cost these precautions are 

 frequenth- neglected under the stress of working conditions. 



Protection of Embedded Steel and Wood 



The principal cause for the disintegration of composite structures composed of 

 concrete and reinforcing or structural steel is the rusting of the embedded steel under 

 the accelerated corrosive action of the sea water. This rusting takes place above 

 mean tide elevation, in that portion of the structure exposed to both sea water moisture 

 and air. The action is increased by the use of porous concrete and by the formation 

 of fine cracks under impact and tension, which assist the penetration of moisture 

 and air; it is retarded and prevented by the use of dense, impervious concrete and by 

 the sealing of cracks, to prevent or retard penetration. The action varies widely 

 with the nature of exposure, being very rapid in "ocean" structures which are re- 

 peatedly bathed with spray, and comparatively slow in "harbor" structures which 

 are infrequently wet directly by spray. The damage consists in the splitting and 

 cracking of the protective coating as the embedded steel expands on rusting. The 

 force of expansion increases with the size of bars and their concentration, and for this 

 reason the spacing and depth of protective coating should be made dependent on the 

 size of bar. Because the cracking destroys adhesion between the steel and concrete, 

 mechanical bond bars are preferable to plain bars. Embedded structural steel may 

 be protected by giving it a heavy coat of paint, so that the salt moisture cannot come 

 in contact with the steel, Init this decreases the bond. It is possible that a system of 



