These findings raise a question as to 

 the appiicability of "concrete strength 

 increases with age" as generally 

 accepted (Ref 8). Massive offshore 

 structures are typically fabricated in a 

 seawater environment. If saturation is 

 considered to occur, then the following 

 interim guide can be used for strength 

 gain with age. The initial 28-day fog- 

 cured strength should be reduced by 10% 

 to account for saturation effects. Subse- 

 quent in-situ strength increases with 

 time may depend on the depth at which 

 the concrete is located. Depth is impor- 

 tant because it can influence the degree 

 of saturation. At present, data are 

 available at depths of a few thousand 

 feet. For this case, the strength increase 

 relative to the 28-day fog-cured strength 

 appears to be nil at 1 year, 5% at 2 

 years, and 15% at 5 years. 



For cases where the concrete is at a 

 depth of a few hundred feet, it is hard to 

 estimate the strength gain behavior. 

 First, it is unknown how much of the wall 

 thickness will become saturated. It could 

 take months for several feet of thickness 

 to become saturated. If the interior of 

 the structure were to be at a relative 

 humidity of less than 100%, the concrete 

 would never become saturated. How- 

 ever, some of the concrete would be 

 saturated near the outside wall, and that 

 portion would exhibit a strength dif- 

 ferent from that not saturated. As a 

 guide, the compressive strength should 

 be reduced by 10% to account for satura- 

 tion effects; then it is probably reasona- 

 ble to permit a strength increase of 10% 

 at 6 months and 15% at 12 months. These 

 values are conservative from the on-land 

 increase factors of 20% at 6 months, and 

 2if% at 12 months. 



Numerous cylinders, both cast and 

 cored, were instrumented with strain 

 gages to determine the elastic modulus 

 and Poisson's ratio of the concrete. 

 Table 3 summarizes the data, and 

 Figures 11 through l^ show the stress- 



strain curves. The behavior of cored 

 cylinders is presented as raw data, i.e., it 

 has pot been adjusted for drilling effect. 

 The concretes were all linear up to about 

 0.5 f^. The elastic modulus of ocean- 

 cured concrete was 30% less than that of 

 the continuously fog-cured concrete. 

 Although the elastic modulus was not 

 determined for fog-cured concrete at the 

 age of 28 days, it is reasonable to 

 assume, based on data from similar 

 concrete (Ref 5), that the elastic 

 modulus of the ocean-cured concrete did 

 not change significantly from that of the 

 28-day fog-cured specimens. 



Short-Term Loading of Spheres 



Implosion . The three spheres 

 retrieved from the ocean were returned 

 to the Laboratory for short-term loading 

 tests. These tests were conducted in a 

 pressure vessel where the external 

 hydrostatic pressure was increased 

 steadily until implosion. 



While in the ocean, these spheres 

 were subjected to a sustained load of 

 about 50% their short-term ultimate 

 strength for a period of 5.3 years. In 

 previous work (Ref 5) identical speci- 

 mens were tested under short-term 

 loading where the specimens were not 

 exposed to the long-term preloading. 

 Table ^ shows the results from both 

 types of short-term tests, with and with- 

 out preload. Table 5 summarizes the 

 results. 



For the uncoated spheres, the pre- 

 loaded sphere showed a decrease in 

 implosion strength of about 8% compared 

 to that of the non-preloaded spheres. 

 The compressive strength of the 

 saturated concrete was fairly well- 

 defined for these tests. For the coated 

 spheres, the preloaded spheres showed an 

 increase in implosion strength of about 

 5% compared to that of the non- 

 preloaded spheres. The actual increase 



15 



