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42 OCTOBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 10 CM CORROSION MANAGEMENT the continuing growth and expansion of the country's manmade "built environment." Improving the sustainability of critical social infrastructure relates to the sustain- able development goals (SDGs) established by the United Nations (UN) in December 2015, 4 particularly Goal 9, "Industry, Inno- vation, and Infrastructure" and Goal 12, "Responsible Consumption and Produc- tion." Furthermore, as a result of the COP21 6th annual Sustainable Innovation Forum in 2015, the Cement Sustainability Initia- tive proposed a set of action plans aimed at reducing carbon emissions by one billion tons by 2030. 5 It is therefore timely to con- sider additional ways to improve the sus- t ainabi lity of infra str u cture, n ot only through advances in green concrete and cement production, but by extending the lifetimes of structures so that the overall rate at which resources are extracted, con- sumed, and released to the environment can be reduced. 6 The present work focuses on quantify- ing the sustainability impacts that can be made through corrosion management and the use of cement replacement materials. As an additional consequence, the corro- sion-related costs of repair and rehabilita- tion could be reduced by 15 to 35% accord- ing to a new study on the global cost of corrosion. 7 Corrosion Management and Durability Although several mechanisms contrib- ute to the failure of reinforced concrete structures, corrosion of the reinforcing steel bars (rebar) is one of the greatest concerns; yet it is still widely ignored. Since corrosion is an electrochemical process, one solution for lifetime extension is to control the elec- trochemical potential of the metal. The technique known as cathodic pro- tection (CP) can be achieved by applying a cathodically polarizing current using either sacrificial or impressed current anodes. 8 Metal that has not yet begun to corrode can be maintained in the passive state, and metal that has already begun to experience active corrosion can be brought to a very low corrosion rate or even immunity. When installed at initial construction, the tech- nique is known as cathodic corrosion pre- vention. 9 In contrast, CP is an intervention techniqu e used for lifetim e extension , potentially adding 20 to 40 years of service life to existing structures. For both techniques, a protective cur- rent is applied to the structure from exter- nal anodes, most often sacrificial anodes comprised of Al/Zn/In alloy, or impressed current anodes made of activated titanium mesh installed in the subsurface of the structure. It is essential for the reinforcing steel to be an electrically continuous struc- ture for this method to work successfully. Corrosion monitoring of in-service founda- tions, through sensors and/or inspections, is necessary to determine if and when CP should be applied. Although the impressed current option utilizes electrical power to maintain the electrochemical potential needed to miti- gate corrosion, it contributes much less CO 2 to the environment than would be in- curred should the whole structure require replacement in the interim years. A partial life cycle analysis that considers the CO 2 im- pact from power consumption is presented here under the subheading Impressed Cur- rent and Carbon Emissions. Alternative strategies for achieving long-term durabil- ity through corrosion control are provided in Table 2. Impressed Current and Carbon Emissions Power consumption of impressed cur- rent systems has been estimated to run be- tween 1 to 3 W/1,000 m 2 of concrete for new construction and 3 to 15 W/1,000 m 2 of concrete for extending lifetimes of existing systems. 8 Assuming a thickness of ~0.3 m for systems like highway bridge decks, it is possible to estimate the total energy con- sumption required per year per cubic meter of concrete and compare this to the lifetime emissions. Power consumption for impressed cur- rent for n e w c onstr uction i s 0.3 to 1 W/1,000 m 3 × 24 h/day × 365 days = 3 to 9 W-h/m 3 /y ; and for existing construction is 1 to 5 W/1,000 m 3 × 24 h/day × 365 days = 9 to 44 W-h/m 3 /y. Assuming that the impressed current power is generated from coal, the carbon emissions are ~1 kg CO 2 /kW-h. Then the CO 2 emitted due to impressed current per year per m 3 concrete is 3 to 9 g CO 2 /m 3 /y (new build) and 9 to 44 g CO 2 /m 3 /y (existing TABLE 1. ESTIMATED MATERIALS FLOW (PRODUCTION, WASTE, AND RECYCLING) FOR CONCRETE AND STEEL IN THE UNITED STATES Material Production Rate (Mt/y) Waste Rate (Mt/y) Recycling Rate (Mt/y) Net Demand (Mt/y) Concrete 919 200 100 819 Steel 91 12.5 11 90 Note: Mt = million metric ton. TABLE 2. STRATEGIES FOR ACHIEVING LONG-TERM DURABILITY— CORROSION CONTROL New Structures Existing Structures Enhanced concrete mix designs: achieve low-permeability concretes with low chloride diffusion coefficient CP: cathodically polarize the reinforcement to decrease the corrosion rate Carbon steel (CS) substitutes: replace CS by using other alloys such as corrosion-resistant alloys (CRAs) Coating and sealants: protect the surface of the concrete against aggressive species Cathodic prevention: cathodically polarize the reinforcement from the start to increase the chloride corrosion threshold Chloride ion extraction: apply an electric field with electrodes at the surface of the concrete to extract the Cl – by migration Coatings and sealants: protect the surface of the concrete against aggressive species