Stachiw 



ed, it can be tested for different properties, depending on the 

 requirements of the study. Since so many alternatives were pos- 

 sible the approach was chosen by which only buoyant concrete hulls 

 of maximized pressure resistant shape of the simplest construction 

 were to be considered first. 



The pressure hull shape chosen for the experimental study was 

 a sphere as it represents the optimum pressure resistant hull. 

 The spherical hull is also desirable for its inherent uniform dis- 

 tribution of stresses. Because of this uniformity of stress dis- 

 tribution, the strains measured at any point of the sphere's sur- 

 face can be considered representative of the strains on the sphere. 

 Knowledge of the maximum compressive strain found in simple spher- 

 ical concrete hulls is extremely useful in the evaluation of future 

 concrete hull models where the presence of inserts and penetrations 

 will create stress risers that may lower the critical pressure of 

 the hull. 



The spherical shape is also advantageous for the determination 

 of concrete's permeability under different levels of hydrostatic 

 pressure. Permeability of concrete is probably related to stress 

 level, therefore uniformity of stress in the sphere eliminates 

 those side effects that are associated with the nonuniform distri- 

 bution of stresses. A spherical hull also eliminated any anomalies 

 caused by edge effects of the test sample, as would be found, for 

 example, in a flat specimen mounted in some sort of a flange. 

 Furthermore, since in a spherical hull there is also continuity of 

 curvature, a reasonable assumption can be made that the permeability 

 of water through the walls of the sphere will be uniform through- 

 out, and thus only the level of water in the sphere must be known 

 in order to determine a nominal rate of permeability through the 

 given concrete mix. 



The actual dimensions chosen for the concrete hull models were 

 16-inch outside diameter and 14-inch inside diameter. The outside 

 dimension was controlled by the inside diameter of the largest 

 vessel available at NCEL, while the inside dimension of the con- 

 crete hull was based on the requirement that the resultant concrete 

 hull possess at most a 0.75 weight/displacement ratio. This 

 weight to displacement ratio was considered to be the highest 

 allowable that would permit the fully equipped concrete habitat 

 hull to be either positively, or at worst, neutrally buoyant. 



SCOPE OF INVESTIGATION 



The study of spherical concrete hulls was limited, to-date, 

 to three major phases of experimental investigation. 



PHASE I - Investigation encompassed the testing to destruction 

 of twelve identical spherical concrete hulls of 16-inch, outside 

 and 14-inch inside diameters without penetrations (Figure 3): Six 

 of the models were waterproofed and six were bare. 



PHASE II - Investigation centered around the testing to de- 

 struction under hydrostatic pressure of six 16-inch external and 



Z23 



