48 THE NAVY OCEAN ENGINEERING PROGRAM 



and 6000 ft. The 2500 ft depth was chosen because it is the level of 

 minimum oxygen concentration at the test site. The 6000 ft depth repre- 

 sents a deep-sea environment on the edge of a major basin beyond the 

 range of present construction operations. 



Results of the STU exposures indicate that corrosion rates for mild 

 steels and high strength, low alloy steels at 6000 ft depths are about one 

 third those at the surface for periods of 400 days or more. 



Additional results to date indicate: (a) the corrosion rates of the copper- 

 base alloys decreased with time; (b) pit depths and corrosion rates of most 

 aluminum alloys increased with time; (c) titanium alloys with one excep- 

 tion were immune to corrosion; (d) depending upon the chemical composi- 

 tion, some nickel-based alloys corroded, while others were uncorroded. 



AppUcation of concrete hulls to undersea installations requires the de- 

 sign, fabrication, and testing of typical spherically shaped models. Experi- 

 ments with spherical concrete hulls of 16 in. outer diameter and one-inch 

 shell thickness have shown that concrete may be suited for underwater 

 applications to depths of about 3500 ft. In the models tested, the con- 

 crete spheres failed at depths of 7000 to 7400 ft, which represents a 

 stress level 46 percent higher (biaxial compression) than identical control 

 cylinders (uniaxial compression). 



The seepage of seawater through unprotected concrete at depths of 

 3500 ft was found to be 6 x 10"^ milliliters per square inch of surface 

 per inch of thickness per hour— very slight. Precoating the spheres with 

 epoxy or self-vulcanizing rubber compounds stopped the seepage com- 

 pletely. Other compounds, such as asphalt or tar, promise much more 

 economical waterproofing for large concrete structures. 



To determine the extent of creep under high compression, several con- 

 crete spherical models were pressurized to 80 percent of their short term 

 critical pressure. When subjected to long term hydrostatic pressurization 

 at a 6750 ft depth, the creep rate immediately after pressurization was 

 high, but declined rapidly and became constant after approximately 50 

 hours. The creep rate at a 13,600 psi stress level was 30, 4, 0.3, and 

 0.02 microinches per inch per minute after five minutes, one hour, 

 30 hours, and 60 hours of pressurization, respectively. 



To provide engineers with data on the safe operational pressure for 

 acryhc windows employed in deep ocean installations, engineering experi- 

 ments are underway on the most important dimensional parameters. The 

 first phase of experiments examined the strength of cone-shaped acrylic 



