Materials Performance

AUG 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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41 MATERIALS PERFORMANCE: VOL. 57, NO. 8 AUGUST 2018 mers (averaging 62 °F [17 °C] and reaching as high as 100 °F [38°C]), and cold winters (averaging –12 °F [–24°C] and dropping as low as –65 °F [–54°C]). It is the application of deicers during these cold winters that drives corrosion of reinforced concrete bridge decks throughout the region. Three such bridges were investigated for corrosion damage along Alaska Route 3/George Parks Highway. A battery of tests was performed to assess the current condi- tion of the decking as well as predict future corrosion damage. These tests included potential mapping of the reinforcing steel (ASTM C876 1 with 6 in [152 mm] spacing), acoustic sounding of the decking (ASTM D4580 2 ), acid-soluble chloride testing of concrete samples (ASTM C1152 3 ), corro- sion rate measurements via linear polariza- tion resistance (LPR), and Wenner 4-pin resistivity measurements of the concrete surface (AASHTO T 358 4 ). The inspection findings were used to identify locations of active corrosion to be included in bridge deck repairs. Corrosion potential and delamination maps were laid over plan drawings of the bridges to allow the contractor to identify predefined repair locations (Figure 4). As can be seen in the figure, potential mapping helps identify areas of corrosion activity that have not yet resulted in delamination of the concrete from the underlying rebar. This, in turn, assists in defining the repair limits. While extremely electronegative potentials are typically an indicator of corrosion activity, potential mapping works by identifying areas of low potential adjacent to areas of much higher potential. A differential in potential is more indicative of corrosion activity than absolute potential values. 1 Corrosion of steel reinforcement in concrete is typically under cathodic con- trol—that is, the corrosion process is lim- ited by oxygen access to the cathode. Even if the chloride content at the anode is suf- ficiently high for corrosion, corrosion will be slow if readily available oxygen is absent. The soffits of the inspected bridges are cov- ered with stay-in-place galvanized steel formwork. The presence of that steel form- work dramatically limits oxygen access to the bottom layer of reinforcement. Following excavation and repair of the contaminated concrete, a 0.75-in (19-mm) polyester concrete overlay with a methyl methacrylate (MMA) primer was used as a restoration method . The polyester con- crete/MMA primer overlay will signifi- cantly reduce future chloride ion penetra- tion into the concrete surface that would otherwise occur from deicer applications. There is already sufficient chloride ion contamination at the rebar level, however, to facilitate corrosion of steel reinforce- ment. In areas with relatively thick con- crete cover over reinforcement that was passive and not corroding, further diffusion of chloride ions to deeper levels in the con- crete will produce corrosion threshold lev- els in the future even with the presence of the overlay. Oxygen entry to the roadway concrete surface of the bridge deck, how- ever, will be dramatically impeded as well by the polyester concrete/MMA overlay. When the overlay is coupled with the stay- in-place formwork on the bottom of the deck, oxygen availability at the steel rein- forcement will be substantially reduced at the level of both rebar mats. Reducing oxy- gen availability will reduce the rate of any future corrosion. To complement reduced oxygen avail- ability at the reinforcing steel, small gal- vanic CP zinc anodes were installed around the repair perimeters. One of the principal criticisms of discrete CP anodes in con- FIGURE 6 Safety Sound Bridge pier elevation. crete is that anions (chloride ions in this case) travel through the electrolyte to the anode. When the anode is eventually con- sumed, this can create a localized environ- ment with high chloride concentrations around each anode, which leads to acceler- ated corrosion . However, since oxygen access is being significantly reduced by the polyester concrete/MMA overlay on top and galvanized stay-in-place formwork on the bottom, the risk of accelerated corro- sion at the anodes once they are consumed is dramatically reduced. After the repairs were completed, but before the polyester concrete/MMA overlay was installed, LPR corrosion current maps were created over the repairs (Figure 5). These maps show the corrosion activity that would be expected in a region of new, c h l o r i d e - f re e c o n c re t e w i t h d i s c re t e anodes. Negligible corrosion currents were observed throughout the patch, except at the anodes where the high corrosion cur- rent of the anode is seen. Case Example 3— Cathodic Protection System Replacement on a Marine Bridge The far north region of Alaska is defined by tundra ; permafrost; short, cool sum- mers; and long, cold winters. There is a Corrosion in Alaskan Infrastructure

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