Durability . Durability is the long-term resistance of concrete to 

 disintegration of the concrete itself, to corrosion of embedded steel, or 

 both, which may interact .L 20 ' 2 U In general, durable concrete for OTEC 

 applications can be produced with a high degree of assurance by strict 

 adherence to established methods for producing high quality, dense, sul- 

 phate-resistant concrete with low permeability .L ' J The major consid- 

 erations include proper mix design (particularly a low water/cement ratio, 

 < 0.45, and a high cement factor, ^> 7 sacks/yd 3 ), use of proper materials 

 (sulphate-resistant, e.g. ASTM Type II, cement with a C A content between 

 5 and 9%, sound, nonreactive aggregates, careful limitation of 

 chlorides in water, aggregates, and admixtures)', proper placement, com- 

 paction and curing procedures,' and proper structural design and detailing 



r 22 "i 



(adequate cover of embedded steel, control of cracking) X -1 



Durability problems that do exist are frequently the result of not 

 following the known procedures and are often associated with extreme en- 

 vironments such as disintegration of highway bridge decks due to the 

 combined effects of freezing and thawing, abrasion and high salt concen- 

 tration.!? 3 ' 2 J For the OTEC structures, the splash zone is the most severe 

 environmental condition. The concrete is subjected to chemical attack by 

 sulphates, chlorides, carbon dioxide and oxygen, and physical stresses due 

 to alternate wetting and drying, temperature differentials, shock loads 

 from waves, and other conditions. The concrete industry is actively 

 studying these durability problems; for example, federal and state high- 

 way engineers are developing practical field methods for polymer impreg- 

 nation of existing bridge decks .L 2 5 ' 2 6 J Such developments will likely be 

 available for the OTEC structure. 



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