PRESENT STATUS OF THE CONCRETE SHIP. 193 



7. Maximum unit bond stress not over 160 lbs. per sq. in. 



or steel stresses to 16,000 lbs. and proportionally for other steel stresses. 



8. n =— = ID 



Es 



\ STEEL. 



I. Maximum unit tensile stress in all reinforcement except as 



stated below, not to exceed 16,000 lbs. per sq. in. 



(a) In all bulkheads except collision bulkheads 20,000 lbs. per sq. in. 



(b) In shell reinforcement exposed to water 12,000 lbs. per sq. in. 



(c) Maximum unit tensile stress in top steel of keelsons due 



to combined local and hogging or sagging stresses not 



to exceed 20,000 lbs. per sq. in. 



Aside from the design problems, much work has been done in developing mate- 

 rials which would have greater structural strength than ordinary concrete, and less 

 weight. If the concrete ship is to become a permanent institution, it is necessary 

 that the unit weight of the concrete be reduced. Our efforts in this direction have 

 been very satisfying. We have developed an aggregate of bloated clay which floats 

 on water. Concrete made of it weighs less than 1 10 pounds per cubic foot, which 

 is 20 per cent less than that obtained from normal sand and gravel aggregates. In 

 addition to the light weight it has great strength, giving in excess of 4,000 pounds per 

 square inch (in 28 days) in mixtures required in ship construction as compared with 

 2,000 pounds per square inch for concrete used in ordinary building construction. 

 This aggregate also makes a concrete having an impermeability equal to or greater 

 than sand aggregates. The coefficient of efficiency (deadweight divided by total dis- 

 placement) of the concrete ship of present design using the light weight aggregate is 

 about 60 per cent. 



A Portland cement ground to a greater fineness is specified for ship construc- 

 tion. This cement has greater strength and produces a concrete of greater plas- 

 ticity and less volumetric change than normal cement. 



While the concrete does not require any special treatment in order to make it im- 

 pervious to water or ordinary crude oils, it will not hold the lighter oils unless the 

 concrete is of exceptional quality. As a further guarantee of imperviousness, we pro- 

 pose to apply a coating of spar varnish to the concrete which will make it impervious 

 to all mineral oils. 



We have only indirect experience from which to judge the probable life of the 

 concrete ship. We do know that under certain conditions of exposure the reinforc- 

 ing steel of concrete exposed to sea water will corrode. We have no precedent with 

 the rich cement mixtures which we propose to use which in themselves may provide 

 all the protection necessary, but as an added precaution we have developed some 

 paint coatings to be applied to the surface of the concrete. The paints which we 

 are using are as follows : — 



