RESULTS AND DISCUSSION 

 Concrete Strength Gain 



The compressive strength of con- 

 crete after curing in the ocean for 1.3 

 and 5.3 years was obtained from 

 uncoated concrete blocks that were 

 attached to the chain of the spheres 

 (Figure 3). The block size was 1^ x 18 x 

 18 inches (356 x k57 x k57 mm), from 

 which four 6 x 12-inch (152 x 305-mm) 

 cores were drilled. The uniaxial com- 

 pressive strength of the cores was com- 

 pared to that of 6 X 12-inch (152 x 305- 

 mm) cast cylinders made from the same 

 batch of concrete and cured under con- 

 tinuous fog room conditions. The 

 strength was also compared to that of 

 on-land cured concrete blocks located 

 about 150 feet (50 m) from the shoreline. 

 These blocks were the same size as those 

 in the ocean, and test specimens were 

 cored from the blocks. The compressive 

 strength results are presented in Table 2. 



A strength differential has been 

 observed to occur between cast and 

 cored specimens of the same concrete 

 due to the effect of drilling. The com- 

 pressive strength of the core specimens 

 in this study was increased by 7% so that 

 it would be equivalent to that of the cast 

 specimens. (Past data supporting the 

 strength adjustment are given in 

 Appendix A). Table 2 presents both the 

 measured strength of the core specimens 

 and the adjusted strength. The following 

 analysis of strength data is based on the 

 adjusted strengths. 



The compressive strength gain of 

 concrete in the different curing environ- 

 ments is presented graphically in Figures 

 6 through 9. Relative strengths are 

 shown where the common denominator is 

 the 28-day fog-cured strength. This 

 strength had a nominal value of 8,000 psi 

 (55 MPa). Reference 1 did not give the 

 coefficients of variation of the 



compressive strengths for spheres at 28 

 days and at the time of emplacement; 

 these data are now given in Appendix B. 

 At 5.6 years of age, the continuously 

 fog-cured concrete showed a relative 

 strength of 1.35 (coefficient of variation 

 was 5.2%). The on-land field-cured 

 concrete showed a relative strength of 

 1.32 (coefficient of variation was 8.0%), 

 and the ocean-cured concrete a relative 

 strength of 1.15 (coefficient of variation 

 was 9.5%). 



The data from the on-land field- 

 cured concrete are interesting but 

 difficult to discuss because the con- 

 crete's actual moisture content varied 

 with time. The compressive strength of 

 dry concrete is greater than that of 

 equivalent wet concrete by an average of 

 20% (Ref 2, 3). The degree of dryness, 

 i.e., the relative humidity of the environ- 

 ment with which concrete is in equilibri- 

 um, also influences the compressive 

 strength. For our case, the equilibrium 

 relative humidity was not known. At 5.6 

 years, spring was starting after a drought 

 year. However, because of the proximity 

 of the concrete blocks to the ocean, the 

 environmental relative humidity was high 

 (100%) most evenings. 



Samples were dried in an oven to 

 obtain an indication of how much 

 absorbed water the on-land concrete had 

 in comparison to the fog- and ocean- 

 cured concrete. These data are given in 

 Appendix C. As expected, the on-land 

 concrete contained less moisture than 

 either the fog-cured or ocean-cured con- 

 crete. It is likely that if the on-land 

 concrete were soaked in water for 

 several days prior to uniaxial testing, the 

 compressive strength would have 

 decreased. The amount of the decreases 

 would have been an estimated 5 to 15%. 



Figure 10 shows a constructed 

 relationship between relative strength 

 and total age for the fog- and ocean- 

 cured concrete. The strength gain 



