3. Concrete Jackets 



A method sometimes used to protect steel in marine structures 

 from corrosion is to encase or jacket the steel in concrete. Two methods 

 of jacketing steel H piles with concrete as given by Ayers and Stokes 

 (1961) are shown in Figure 13a. Figure 1313 shows methods of protecting 

 steel sheet piling in straight walls and cellular construction with con- 

 crete jackets. To effectively protect steel from corrosion, the concrete 

 must be of good quality, properly placed and cured, and of adequate 

 thickness. Ayers and Stokes (1961) show a minimum thickness of k inches 

 for concrete jackets over steel. 



When concrete is permeated by seawater due to use of a poor quality 

 concrete or inadequate concrete thickness, corrosion of the steel occurs. 

 Since the corrosion product volume is greater than the original steel 

 volume, pressure is exerted on the surrounding concrete. If this pressure 

 is greater than the opposing tensile strength of the concrete, the con- 

 crete will crack and eventually spall, exposing the steel. According to 

 information in Griffin (1965), corroding metal can exert pressures up to 

 about ^,700 pounds per square inch on the concrete. 



Salt- free concrete has a pH value of about 13 (highly basic). Under 

 this condition a tough corrosion film builds up on the surface of steel 

 embedded in the concrete and the steel becomes passive in respect to 

 further corrosion. When chloride ions enter concrete, the pH value is 

 lowered and the passive film of corrosion products is destroyed, allowing 

 further corrosion (Griffin, I965). 



A good quality concrete for jacketing steel in marine structures 

 should have high strength, be relatively impermeable and have good bonding 

 characteristics. Information by Finley in Wood (1963) recommends 7 1/2 

 bags of cement per cubic yard of concrete and 5 gallons of water per bag 

 of cement for corrosion protection in splashing water or alkaline soil. 

 Low water-cement ratios are desirable. 



In areas where freezing and thawing exists, entrained air is recom- 

 mended to prevent concrete deterioration. Mather (l957) reports that 

 tests by the Portland Cement Association showed that proper use of air 

 entrainment improved the performance of concrete with respect to freezing 

 and thawing and to exposure to solutions of sulfate salt. Tests by the 

 Corps of Engineers at Treat Island, Maine, also showed that the proper 

 entrainment of air in concrete was the most important factor in improving 

 the durability of concrete to severe weathering. Tyler (1962) reports 

 that air entrainment in concrete aids by slowing the rate of seawater 

 penetration. Lyse (1961) reports that the percent of air voids in con- 

 crete should be 10 to 12 percent for best resistance to freezing and 

 thawing where exposed to seawater. 



Another factor which may be important when considering concrete pro- 

 tection for steel piles is the rigidity of the structure. It seems quite 

 possible that concrete jackets on seacoast pier piling, for example, 



36 



