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Concrete Dec 2018

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35 COATINGSPRO CONCRETE COVERED DECEMBER 2018 exposed to fresh or brackish water, bare bulk anodes may not be effective. Tidal Zone Because the tidal zone is subjected to repeated, intermittent seawater exposure, the tidal zone portions of marine piles are at increased risk of corrosion. A lthough the risk of corrosion is increased, the corrosion rate in this zone is often limited by the rate oxygen can diffuse to the reinforcing steel, since the concrete in this zone generally remains saturated. e tidal section of marine piles may be protected using zinc mesh or zinc strip anodes installed with cement grout or concrete inside a stay-in-place form. e stay-in-place form may be FRP or poly vinyl chloride (PVC). Tidal zone jackets are installed such that the bottom of the jacket is in contact with water. Exposing the zinc anode directly to salt water provides a corrosive environment that allows the zinc to corrode relative to the reinforcing steel. As a result, galvanic CP jackets provide good protection to the tidal zone. Protection diminishes quickly above the high tide line as the splash zone is only wetted periodically, and salt water may not reach this level inside the stay-in-place form. Splash Zone e splash zone is subjected to occasional seawater wetting and is generally at greater risk of corrosion since this zone is exposed to both chlorides and oxygen. e splash zone is not consistently or regularly exposed to salt water, which limits the ability of submerged or tidal systems to provide effective corrosion protection. Two approaches have been developed to provide CP in this zone: 1. Jackets with "wicking" anodes include anodes with fabric designed to wick salt water up along the surface of the anode. This directly wets the anode/concrete interface with salt water and keeps the anode active at least 6 feet (2 m) above the high tide line. 2. Alkali-activated galvanic jackets include alkali-activated anodes inside the stay-in-place form. Alkali keeps the anodes active without the need for salt water exposure. As such, these jackets can provide cathodic protection at any elevation relative to the salt water. Atmospheric Zone T he upper sections of concrete marine piles are often above the splash zone and may appear to be less v ulnerable to corrosion; however, these sections are subject to airborne chloride deposition. In this zone, it may take more time for chlorides to reach critical concentrations at the reinforcing steel surface, but when this happens, corrosion can be severe as ox ygen availability is not limited. Galvanic CP of these areas requires the use of alkali-activated galvanic jackets. Relative humidity of the concrete alone is sufficient for this system to perform. W hen suitably designed, galvanic CP can be provided for 20 to 40 years. Galvanic Jacket Installation Once the jacket is in place, the annular space between the jacket and exterior face of the pile is filled with a cement grout or concrete. In the case of tidal or splash zone jackets with wicking anodes, the base of the jacket is typically placed at or below the low tide level to ensure that the bottom of the jacket remains wet. Together with an open-bottom design, seawater is allowed to enter and wick upward inside the jacket. is keeps the concrete wet and exposes the zinc anodes to sufficient chlorides to keep the zinc electrochemically active in this area. A bulk zinc anode is often installed below the jacketed area to provide cathodic protection of the steel in the submerged zone. In the case of atmospheric zone jackets, the base of the jacket does not need to extend to the low-tide level. Case History: Florida DOT Bridge Columns (Above Splash Zone) Florida Department of Transportation (FDOT) marine bridges located in South Florida and the Florida Keys were suffering from corrosion above the tidal and splash zones due to long-term exposure to chlorides (see Figure 2). FDOT Figure 2: Deteriorated columns of the Niles Channel Bridge in the Florida Keys (top) and column condition after removal of damaged concrete (SR802 - South Florida) (bottom). Corrosion Protection

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