ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/697527
42 | ASPIRE Summer 2016 CBP Concrete Bridge Preservation Repairing and Protecting Concrete Piles with Galvanic Jackets by J. Christopher Ball, Vector Corrosion Services From small county roads to major highways, a signifcant number of bridges are built over coastal waters. With exposure to the seawater environment, these structures are at risk of premature corrosion of the reinforcing steel. In particular, conventionally reinforced and prestressed concrete piles in seawater are subjected to high levels of chloride contamination leading to serious deterioration and on-going maintenance repairs. Concrete Pile Corrosion Concrete piles in seawater are exposed to three distinct exposure conditions: • Atmospheric (Dry)—elevated sections of concrete piles are subject to airborne chloride deposition. Once critical concentrations of chloride are present on the steel surface, the passive oxide layer is defeated and corrosion initiates. • Splash/tidal (Wet/Dry)—the splash and tidal zones are intermittently subjected to seawater saturation and are at high risk of corrosion due to wet-dry chloride exposure cycles. • Submerged (Wet)—underwater sections of piles are less frequently affected by corrosion damage. This is due to seawater having low levels of dissolved oxygen, a precondition for corrosion because oxygen is necessary at the cathodic sites to facilitate the reduction reaction (O 2 + 2H 2 O + 4e - → 4OH - ). Pile Jacketing In an effort to repair, protect, and extend the life of corroding concrete piles, damaged piles have been jacketed with • an overbuilt layer of reinforced concrete, • cement-grout-flled stay-in-place forms (fber-reinforced polymer [FRP], corrugated steel, or fabric), or • epoxy-grout-flled FRP jackets. In the 1990s, it was documented that this type of jacketing was ineffective for chloride contaminated concrete piles as it allowed corrosion to continue and the jacketing concealed the pile damage from detection by visual inspection. In 1998, research conducted for the Florida Department of Transportation concluded that only jacketing that included cathodic protection should be permitted and this was implemented as policy. 1 The Virginia Transportation Research Center also reported in 1999 that "Grout jacketing alone is an inadequate protection against corrosion and should be supplemented with cathodic protection (CP)." 2 Cathodic Protection Jacketing Cathodic protection jackets can employ impressed current cathodic protection (ICCP) or galvanic anodes. ICCP is effective but requires a permanently operating power supply and commitment to system monitoring. The importance of ICCP system monitoring cannot be overemphasized, especially when applied to prestressed concrete where overprotection can cause corrosion or hydrogen embrittlement of steel tendons. 3 In the battle against pile corrosion, galvanic jackets are an effective, low-maintenance option for the bridge preservation engineer. Galvanic jackets are installed using zinc anodes that are directly connected to the steel or connected via an CBP CONCRETE BRIDGE PRESERVATION Figure 1. Corrosion of prestressed pile in tidal/splash zone. All Photos and Figures: Vector Corrosion. Figure 2. Fabric form jacket with concrete fll.