Stachiw 



the compressive stress level as well as time was shown by the differ- 

 ence of time-dependent strain rates measured on the exterior and 

 interior surfaces of the sphere. The interior surface of the sphere, 

 which was under a higher stress, showed a considerably higher time- 

 dependent strain rate than the exterior of the sphere, which was 

 under a lesser stress. 



The long-term Hydrostatic loading was conducted for only 3 days, 

 and thus it is not known how much the time-dependent strain rate 

 decreases after loading duration of several months, or years. The 

 data generated indicates that even at the 6,700 foot depth level to 

 which the waterproofed concrete sphere was subjected, the time- 

 dependent strain rate of dry concrete decreased to 0.01 microinch/ 

 inch/minute after 3 days. This would lead one to believe that at 

 lower stress levels, corresponding to 3,500 foot operational depth, 

 time-dependent strain would not pose any serious engineering prob- 

 lems for concrete spheres with a 0.0625 wall-thickness to diameter 

 ratio. 



Upon depressurization, a time-dependent relaxation strain was 

 observed whose rate decreased to a very small value after only 3 

 days. The difference between the strain level at the beginning of 

 pressurization, and the strain after 3 days of relaxation at zero 

 pressure shows that a nonrecoverable deformation of concrete in the 

 sphere occurred. 



TANGENT MODULUS OF ELASTICITY UNDER SHORT-TERM LOADING - The 

 tangent modulus of elasticity of concrete under short-term uniaxial 

 compression (2,100 psi/minute loading rate) was found to decrease 

 with increasing stress level. The axial strains on the exterior 

 surface of solid dry concrete test cylinders under uniaxial com- 

 pression show that the average tangent modulus of elasticity for 

 concrete mix employed in the casting of spheres is 3.68 x 10° psi 

 in the to 4,500 psi stress range, but decreases rapidly at higher 

 stress levels (Figure 14). What the magnitude of change is in the 

 tangent modulus of elasticity under biaxial or triaxial stresses, 

 as found in the sphere, is not known. The slope of the strain 

 curve for the interior of the spheres shows, however, positively 

 that a decrease in the tangent modulus of elasticity does take place. 

 This makes it necessary to treat Ej- in equation (1) as a variable, 

 and not as a constant. Since curves for E^- of concrete under dif- 

 ferent biaxial and triaxial stress levels do not exist at the pre- 

 sent time, Ej- as determined under uniaxial compression must be used 

 in the meantime. Since Et under uniaxial compression appears to be 

 larger than Et under biaxial and triaxial stress combinations, use 

 of Et under uniaxial loading is a conservative assumption. 



PHASE II 



EFFECT OF PENETRATION INSERT RIGIDITY ON CRITICAL PRESSURE OF 

 SPHERE - It was found that there is no significant difference be- 

 tween critical pressures of concrete hull models with solid pene- 

 tration inserts (Spheres No. 15, 16, and 17) and model without 

 penetration (Spheres No. 18) as long as the rigidity of the insert 

 was equal to, or larger than, the rigidity of the concrete hull 



230 



