"shell" concept in the fabrication of components - only surfaces exposed 

 to the seawater would be fabricated with the organotin resin. 



Antifouling concrete 



Good mixing practice for structural concrete discourages addition of 

 nonaggregate to the uncured concrete mix. Organic additives, in particular, 

 seriously degrade compressive strength, produce poor bonding of cement to 

 aggregate, and are responsible for overall degradation of desirable struc- 

 tural properties. Therefore, few specific areas of organic antifoulant 

 additions to concrete have been attempted. 



Research to Date 



12 

 In one study, Muraoka and Vind impregnated porous expanded shale 



aggregate with various antifouling materials, such as creosote, tributyltin 

 oxide, malachite green, copper naphthenate, and pentachlorophenol . The 

 impregnated shale was substituted for coarse aggregate in concrete mixtures. 

 The resultant concrete than was exposed underwater at representative loca- 

 tions to evaluate the antifouling performance of the cured concrete. Com- 

 pressive strength and adherence to untreated concrete surfaces also were 

 evaluated. TBTO combined with creosote gave the best performance and 

 remained practically fouling-free for 3 yr , Other systems proved less 

 desirable and fouled after shorter periods. Although the organic antifoul- 

 ing additives lowered the compressive strength of cured concretes, those 

 using solvent-washed and dried impregnated aggregates exhibited compressive 

 strengths of about 3500 psi, which is adequate for marine construction. 

 Also, shear strengths for the adherence of antifouling concrete to conven- 

 tional concrete was about 1500 psi. Bonding a thick antifouling concrete 

 shell over the main body of conventional structural concrete to various 

 OTEC structures may provide desirable antifouling properties while retain- 

 ing high structural standards. While bonding fresh mixtures to large areas 

 of fully cured concrete could cause quality control problems, a weaker 

 outer surface may possess desirable renewability properties. Interestingly, 

 the slow deterioration of the outer antifouling shell would constantly 

 expose fresh biologically active sites and extend antifouling protection. 



48 



