18 METHOD OF CONSTRUCTION OF CONCRETE SHIPS. 



No difficulty was experienced in making watertight construction joints between 

 the several povirs of concrete. The surface where new concrete was to be joined to 

 old was thoroughly roughened by chipping with the pneumatic hammers, and any 

 soft film of cement which might have formed was removed. The surface was then 

 cleaned with compressed air or water and thoroughly saturated with water just be- 

 fore the placing of concrete of the next lift was started. In some cases the surface 

 of the hardened concrete at the joint was grouted also, but this was not necessary 

 with the very rich mixture employed, if care were exercised to insure that the first 

 batches of concrete were thoroughly worked into the surface. 



PATCHING AND POINTING THE CONCRETE. 



Defects of at least a minor charactej- have been found in the concrete of all 

 hulls. This has necessitated some patching and pointing. 



In most cases the patching was done by hand. The cavities were well cleaned, 

 all loose material and dust removed, and the old concrete thoroughly saturated with 

 water. A mortar, usually of one part cement to two parts of sand, was mixed with 

 water to a moist earthy consistency ; that is, it was just wet enough so that when 

 formed into a ball in the hands it could be made to cling together. This mixture 

 was then pounded into the cavity with a hand hammer and hard wood stick or block. 

 Excellent results were obtained by this method, and practically no leaks have oc- 

 curred around patches. 



In some cases, holes were found which extended entirely through the shell, or 

 the defects extended to such a depth that it was necessary to cut away the concrete 

 through the shell. In these cases the repairs were made either by placing forms 

 on both sides, securing them into position by bolting through, and placing the con- 

 crete as originally, or by placing a form on one side only and depositing the con- 

 crete as described for patching. 



Where care has been exercised, the patching has been uniformly satisfactory 

 and watertight walls have been secured. 



The inspection must be very thorough in order to uncover all defects, for in 

 some cases the surface may appear sound and uniform or show only a slight cavity, 

 but by tapping it will be found that the cavity extends for some distance into the 

 mass. 



The cement gun has been used successfully in some cases for patching, but 

 satisfactory results are obtained by the hand methods if carefully done. 



^ LAUNCHING. 



All of the concrete ships described in this paper, with the exception of the two 

 small ships built in private yards, were built for sideways launching.* 



*The reasons which led to the adoption of side lau nching are outlined in a paper by A. L. Bush, "Layout 

 and Equipment of the Government Concrete Shipyards." Pro. Am. Concrete Institute, 1919. 



