Materials Performance

OCT 2016

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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43 NACE INTERNATIONAL: VOL. 55, NO. 10 MATERIALS PERFORMANCE OCTOBER 2016 build). Even over 100 years, this would be negligible compared to the embedded CO 2 used to produce the cement portion of con- crete (100 to 300 kg/m 3 ). A full lifecycle analysis would also need to include the life- cycle CO 2 costs associated with items such as the installation and maintenance of the system and manufacture of the anodes. Reinforced Concrete Durability Predictions The long-term corrosion behavior of reinforced concrete structures can be mod- eled as follows. Generally, a chloride ion or CO 2 concentration gradient exists between the outer surface of the concrete and the surface of the embedded steel rebar. Hence, reinforced concrete durability predictive models are mathematically based on the laws of chloride diffusion. With a focus on chloride-induced corrosion damage, chlo- ride-induced corrosion modeling estimates the time it takes for a significant amount of chloride ions to reach the steel surface and initiate corrosion (time of initiation) by promoting the breakdown of the passive film. Once corrosion starts, the corrosion rates and amount of corrosion products generated can be estimated. The accumu- lated corrosion products on the steel sur- face eventually generate stresses greater than the tensile strength of the concrete, which results in cracking and/or spalling. When cracking reaches the concrete sur- face, these cracks become accelerated chlo- ride-intrusion paths that lead to the steel; and corrosion is accelerated. The time of corrosion initiation (t i ) in reinforced concrete structures can be esti- mated from the error-function (erf ) solu- tion of Fick's 2nd law, 10-11 with parameters that include the time and space invariant diffusion coefficient (D), chloride surface concentration (C s ), initial chloride ion con- centration at the surface of the steel bar (C 0 ), chloride corrosion threshold (C T ), and concrete cover thickness (X c ), as shown in Equation (2): (2) This formula is limited since it is one- dimensional and does not consider factors such as chloride binding, the time-depen- dent diffusion coefficient, and the poten- tial-dependent chloride threshold. The estimated impact to t i of adding supplementary cementitious material is re- flected in the value of the chloride diffusion coefficient. Previous work has shown that partially replacing cement in the concrete mix proportions (i.e., 10 to 50%) with mate- rials such as fly ash (FA), silica fume (SF), or metakaolin has resulted in enhanced me- chanical properties with markedly lower values of D with consequent long-term en- hanced concrete performance. 12 An example of how those admixtures and concrete formulation affect the value of D is shown by an empirical equation pro- posed by Sagüés, et al., 13 based on data from several bridges, to determine the effective (time-invariant) chloride diffusion coeffi- cient (D) for 10-year-old marine structures. The result is a function of the blended cement content in the mix (cement factor [CF]), the water-to-cement ratio (w/c), and the range of cement replacement by FA, SF, or blast furnace slag (BFS), as illustrated in Equation (3): (3) where: D is in in 2 /y ; CF is in kg/m 3 (con- verted from the original lb/yd 3 ); CF range = 390446); w/c = 0.32

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