The final phase of a 10-year study of deep ocean concrete structures conducted by the U.S. Naval Civil Engineering Laboratory (NCEL) in California has provided answers to some important questions. The results supported the Navy's contention that concrete is an excellent material for deep ocean construction. NCEL started the program when it deployed 18 concrete spheres (66 inches in diameter with 4-inch-thick walls) in water depths ranging from 1800 to 5000 feet. All 18 spheres were designed for a nominal working depth of about 3000 feet at 1300 psi pressure. As expected, the greater pressures crushed specimens placed at 3700 and 5000 feet.
Some spheres were coated with a waterproof phenolic material; others remained uncoated. Two spheres had half of their surface coated and were the only ones to contain steel reinforcement. One of these spheres recovered after 10 years at a depth of 1800 feet had no visible corrosion of the steel, even though in some areas of the model the steel had less than 1 inch of concrete cover. Nor was there any visible deterioration of the concrete material itself in any of the five spheres and blocks retrieved to date.
Studies of changes in strength of concrete in the submerged spheres yielded information applicable to the design of concrete ocean structures. As a rule, the strength of concrete increases with age but the percentage of that strength is lessened each year as aging occurs. By comparison, concrete exposed in the ocean spheres for 5 years showed a compressive strength increase of 15 percent above the 8000-psi design strength. The strength remained the same after 10 years of exposure. The conclusion is that concrete gained considerable strength during the first several years in the 40-degree F seawater and retained that strength under continuing high pressure.