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.
Issue link: http://mp.epubxp.com/i/733332
38 OCTOBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 10 CHEMICAL TREATMENT corrosion attack apart from a few small, superficial rust spots—even at the lower end of the wall element, which is the most severely exposed zone, with steel potentials of ~ –250 mV vs. CSE. Chloride Ingress The chloride content data in the con- crete, measured over time at dif ferent depths, were entered into a common model to describe chloride ingress into the con- crete. This approach is based on Fick's sec- ond law of diffusion and can be expressed as follows in Equation (1): 12 (1) where C(x,t) is the chloride content at depth x and time t, C o is the initial chloride content, and C s is the so-called surface chloride content (all in % by mass of con- crete). D a denotes the apparent diffusion coefficient (m 2 /s). To consider the time- dependency of transport processes through concrete, which changes due to proceeding cement hydration, carbonation, eventual formation of surface layers, etc., the appar- ent dif fusion coef ficient i s usually ex- pressed as a function of time, as shown in Equation (2): (2) where D o denotes the diffusion coefficient at a reference age (e.g., 28 d), and α is the aging exponent. The model approach using Equations (1) and (2) is a strong simplification of real- ity. 12 More complex models exist that may, under some conditions, allow more appro- priate descriptions of chloride transport processes in concrete; however, these ex- tended models require more input param- eters, which are not available in the present work. Thus, application of the model ac- cording to Equations (1) and (2) in the present context is considered an appropri- ate approach that yields estimates of the evolution of future chloride content at a certain depth. By assuming a negligible initial chloride content in the concrete (C o = 0) and an aging exponent (α = 0.3 according to litera- ture data 12 ) for each point in time and for each sampled depth, the measured chlo- ride content in the concrete was fitted to the model prediction (Equation [1]). The fitting parameters were D o and C s . The re- sults are summarized in Table 1. The diffu- sion coefficients at reference time (28 d) are in the range of 2 to 3 × 10 –12 m 2 /s. This is in agreement with literature results for concrete with Portland cement. 12-13 Table 1 indicates no dramatic differ- ences in chloride ingress between the two tested concretes. Chloride ingress was only slightly slower in the inhibitor-containing concrete than in the reference concrete, with the apparent chloride diffusion coeffi- cient D o being ~30 to 40% higher in the ref- erence concrete than in the inhibitor - containing concrete. Further indication of a slightly slower mass chloride transport in the presence of the inhibitor was obtained from electrical resistance measurements on embedded probes that suggested a higher concrete resistivity of the inhibitor-contain- ing concrete than of the reference concrete, even after 18 years of exposure. 10 In the lit- erature, similar corrosion-inhibiting admix- tures have occasionally been claimed to af- fect the transport properties, which was explained by the effect of internal hydro- phobic coating or pore blocking. 7,14 Projection of Time-to-Corrosion The model describing chloride ingress permits extrapolating the results from the field test—obtained during 18 years of ex- posure—into the future. This is shown in Figures 3 and 4. Based on re sults of b oth th e non - destructive and destructive inspection techniques, the time of corrosion initia- TABLE 1. PARAMETERS D o (m 2 /s) AND C s (% BY MASS OF CONCRETE) OBTAINED FROM FITTING MODEL AND EXPERIMENTAL DATA (AVERAGE VALUES AND STANDARD DEVIATIONS BASED ON SEVEN SEPARATE FITS PER CONCRETE) E1 E2 Reference Concrete Concrete with Inhibitor Parameter D o C s D o C s Average 2.7 x 10 –12 0.23 2.0 x 10 –12 0.23 Standard deviation 1.0 x 10 –12 0.08 0.3 x 10 –12 0.03 FIGURE 3 Predicted chloride profiles Predicted chloride profiles in the reference concrete based on the measurements taken in the reference Predicted chloride profiles in the reference concrete based on the measurements taken concrete based on the measurements taken over 18 years of exposure. The numbers on the lines indicate the exposure duration in years.