Predicting Chloride Penetration Profile of Concrete Barrier in Low-Level Radwaste Disposal

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1 Predicting Chloride Penetration Profile of Concrete Barrier in Low-Level Radwaste Disposal Yu-Kuan Cheng*, I-Shuen Chiou, and Wei-Hsing Huang Department of Civil Engineering, National Central University 300 Jungda Road, Jungli Taoyuan, TAIWAN Phone: ext Key words: concrete barrier, chloride attack, low-level radwaste disposal Abstract The final disposal of low-level radioactive wastes requires that the wastes be isolated from human biosphere for a very long period. The use of concrete in the construction of engineered barrier improves the structural integrity and material durability for long-term performance. To fulfill the intended function of barrier material, the degradation mechanism is of utmost importance. The ingress of chloride ions in concrete material is governed by diffusion only transport. This study prepared concrete specimens with various mix proportions and subjected to AASHTO T259 ponding test. The chloride ion penetration profiles in concrete were determined experimentally for chloride ion concentration. The chloride ion penetration profile was measured and compared with the chloride ion profile prediction. Results for the study can be used for the evaluation of service life of concrete barriers in low-level radioactive waste disposal. 1. Introduction Utilizing concrete as the barrier material has the characteristics of durability and the ability to increase the structure completeness for long-term storage of low-level radioactive wastes. It is one of the possible solutions for Taiwan to cope with the final disposal of low-level radioactive wastes. Aiming at the evaluation of barrier functions of concrete, the most important thing is the degradation in the service period. Normally, the loss of durability of concrete is due to the external environmental factors and the internal factors of concrete itself. A good disposal site is difficult to be found in Taiwan due to the high density of population. It is likely that the final disposal site would be located in a remote coastal area. In such circumstances, chloride-induced corrosion of steel bars in reinforced concrete exposed to marine environments may become one of the major causes of deterioration. Chlorides in concrete can be either dissolved in the pore solution, or chemically and physically bound to the cement hydrates and their surfaces. The former is referred to as free chlorides, and the latter bound chlorides. Only the free chlorides dissolved in the pore solution are responsible for initiating the corrosion process. Several mechanisms exist for chloride binding during cement hydration: chloride may be bonded in the C-S-H gel, as a complex calcium oxychloride, Friedel s salt, or its high-iron analogue. Experimental studies have shown that chloride binding causes a slight reduction in the amount of chloride ions being transported by moisture flow. This study emphasizes on how the disposal site influences the environment of nearby coast areas. It focuses on the prediction of chloride ion profiles into the concrete barrier in underground disposal environments. Based on long-term experimental data, it is expected to establish the prediction model for the ingress of chloride ion into the concrete barrier, and the concentration profiles can be assessed. 2. Experimental program In order to verify the developed model, four series of test specimens were prepared. The major variables in the mix proportions was the addition of either fly ash, blast furnace slag, or silica fume. Type I Portland cement was used and the content of fly ash, slag, and silica fume in the mixture was 20%, 40%, and 5% of total cementitious materials, respectively. This was to evaluate the effects of fly ash, slag, or silica fume addition to concrete on the diffusivity of chloride ion in concrete. Four mixes of concrete were prepared in the laboratory, and the mixture proportions are shown in Table 1. Concrete specimens (ψ10 20 cm) were continuously immersed in 3% chloride solutions for 90, 180, 270 days after curing for 49 days at 100% relative humidity. All except the top surface of the concrete specimens were sealed by epoxy resin so

2 Mix Type I cement Table 1 Mixture proportions of the concrete specimens (units: kg/m 3 ) Fly ash Slag Silica fume Coarse aggregate Fine aggregate Water Superplasticizer OPC F S C that the chloride ion penetration can occur only in one-direction. The solution was replenished every 2 weeks to maintain uniform concentration. At the completion of selected immersion periods, the total chloride ion in concrete were measured, in accordance with the titration procedures outlined in AASHTO T Model description Chloride ion ingresses the concrete by ionic diffusion, that process can make concentration gradient between the exposed surface and the bottom. This process is described by Fick s first law of diffusion. (1) where Flux is the flux of chloride ions by diffusion (kg/m 2.s), D c is the effective diffusion coefficient (m 2 /s), C f is the free chloride ion concentration (kg/m 3 ), and x is the distance from the exposed boundary. In saturated concrete, mass conservation of the chlorides gives: (2) where R is the binding factor of chlorides. Substituting Eq. (4) into Eq. (2), the governing equation for total chloride transport becomes the solution of Eq. (5) for semi-infinite concrete is where erfc is the complementary error function, erfc(x)=1-erf (x), and t is the time. 4. Results and Discussion (5) (6) 4.1 Experimental results At the end of designated immersion period, concrete specimens were cut into 5-mm thick discs and determined for their chloride ion content. The diffusion coefficient D for each concrete mixture was determined by curve-fitting of the measured chloride ion profile to Eq. (7). Considering binding isotherms, the bonded chloride can be expressed as (3) where C f is the free chloride ion concentration (kg/m 3 ), C b is the bound chloride ion concentration, and the parameters α and β are empirical constants. In general, β is in the range of 0<β 1. Ifβ=1, the total chloride content C is the sum of free and bound chlorides, thus (4) (7) Figure 1 shows the calculated diffusion coefficient of various concrete mixtures at different ages. It is noted in Figure 1 that the chloride ion diffusion coefficient decreases with an increase in the age of concrete. This indicates that the internal structure of concrete improves continuously as the age of concrete or degree of hydration increases. Figure 1 indicates also that the addition of fly ash, blast furnace slag, or silica fume results in a reduction of diffusion coefficient. This can be attributed to the dense hydration products and refined pore structure produced from the pozzolanic reaction of the pozzolans used. Therefore, partial replacement of cement with pozzolanic materials in concrete proves to decrease the diffusion coefficient and thus

3 improves the resistance of concrete to chloride penetration. Fig. 1 Calculated diffusion coefficient for various concrete mixes as a function of elapsed time Figure 2 presents the predicted and measured chloride concentration profiles for laboratory test specimens of OPC, F20, S40, and C5. It shows that the chloride profiles predicted by the model is in good agreement with measurement, and the shape of chloride concentration profiles is close to that of measured profiles. 4.2 Numerical model analysis The governing partial differential equation given by eq. (2) is widely used for diffusion of chloride ions with constant diffusion coefficient and constant initial and boundary s. To incorporate the effect of ion binding isotherms into the mathematical model, eq. (6), assuming the Freundlich isotherm, can be used to evaluate the effect of chloride binding in concrete. This model can then be used to describe chloride binding in concrete and predict chloride penetration profile of concrete. Chloride may be bonded in the C-S-H gel in the transport of chloride ions in concrete. This causes a reduction in diffusion coefficient and, accordingly, reduces the penetration of free chloride ions into concrete. Chloride binding isotherms describe the relationships between free and bound chlorides in concrete at a given temperature. This means that chloride ions in the saturated environment are transported to the surface of the pores where they either remain dissolved in water or are bound to the pore walls by van der Waals bonds. The binding factor in Eq. (4) gives the proportion of total chlorides to free chlorides and has the effect of retarding the adsorbed chloride relative to the advective velocity of pore water in concrete. The binding factor of different concrete mixtures varies. Thus, a high binding factor tends to have low rate of chloride diffusion into concrete. Fig. 2 Comparison of predicted and measured chloride concentration profile

4 period of 1 year, 10 years, and 100 years for the 0.59 M surface. In these figures, 2 different cases were calculated: one in which binding was neglected (no binding); and another one in which Freundlich binding was accounted for. It can be observed that the level of chloride concentration at a given depth is lower than if no binding at all were assumed. This difference becomes more significant at greater concrete depths as the time of exposure increases. Fig. 3 Predicted chloride profiles for various chloride binding factors The binding factor in Eq. (6) can be considered as a retardation of chloride ingress in concrete. The binding factor is equivalent to the ratio of the velocity of chlorides and the pore water, and range from one to several thousands. A low R factor implies that the retarding effect provided by the pore structure is limited. In other words, more free chlorides can be transported through the concrete. The effects of binding factor on the ingress of chloride into concrete is shown in Figure 3, which was obtained from using the diffusion coefficient of OPC mixtures, i.e cm 2 /s. Figure 3 indicates that the binding factor has a great influence on chloride penetration. It is noted that as the binding factor increases, the amount of free chloride ions penetrated into concrete is reduced. Computations were performed using Eq. (6) to study the effect of binding on the chloride penetration profiles. Assuming that all chlorides in the concrete come from external sources, the initial and boundary s are as follow For t = 0: C f = 0 at x > 0 For t 0: C f = C s at x = 0 C f = 0 at x = L where C s is the chloride concentration of the salt solution in contact with the outside surface of the concrete member with a thickness of L. Two different outside surface s were considered in the computations: 0.59 M and 2.95 M chloride concentration solutions. The former simulates complete submersion in sea water. The 0.59 M concentration was determined from the synthetic sea water as suggested in ASTM-D1141; while the 2.95 M chloride concentration simulates s more characteristic of marine structures in the splash zone. Calculated chloride penetration profiles of OPC and F20 mixes are plotted in Figures 4 and 5, respectively, for Cs = 0.59 M. The chloride concentration profiles are shown for an exposure Fig. 4 OPC chloride concentration profiles at (a) 1 year, (b) 10 years, (c) 100 years for 0.59 M exposure

5 pronounced. This implies that the effect of introducing chloride binding in the diffusion problem becomes more significant as the concrete has been exposed to salt solutions longer. Fig. 5 F20 chloride concentration profiles at (a) 1 year, (b) 10 years, (c) 100 years for 0.59 M exposure Figures 6 and 7 present the chloride penetration profiles of OPC and F20 mixes, respectively, for Cs = 2.95 M. Similar to the Cs = 0.59 M, the chloride profile obtained by taking the binding effect into account are lower in all cases. In addition, it is noted that, as the exposure period increases, the difference between binding and no-binding gets more Fig. 6 OPC chloride concentration profiles at (a) 1 year, (b) 10 years, (c) 100 years for 2.95M exposure

6 (2) With an increase of chloride binding factor, the diffusion of chloride ion into concrete reduces. This effect is reflected and quantitatively calculated in the numerical model. (3) Results of the study indicate it was observed how significantly dependent the calculated chloride profiles are on the chloride binding considered in the analysis. Consequently, the understanding of the binding properties of a given cementitious system and the use of the appropriate binding relationship in numerical model calculations enables the engineer to better estimate the chloride ion penetration depths. (4) The numerical model may be efficiently used to predict the chloride ion penetration of the concrete structures under saline environments, if the chloride diffusion coefficient and the chloride binding factor according to the mixture characteristics are obtained from the material data. Fig. 7 F20 chloride concentration profiles at (a) 1 year, (b) 10 years, (c) 100 years for 2.95M exposure References Andrade, C., Martinez, I., Castellote, M., and Zuloaga, P. (2006), Some principles of service life calculation of reinforcements and in situ corrosion monitoring by sensors in the radioactive wastecontainers of El Cabril disposal (Spain), Journal of Nuclear Materials, Vol. 358, pp Martín-Pérez, B., Zibara, H., Hooton, R. D., Thomas, M.D.A. (2000), A study of effect of chloride binding on service life predictions, Cement and Concrete Research, Vol. 30, pp Han, S. H. (2007), Influence of diffusion coefficient on chloride ion penetration of concrete structure, Construction and Building Materials, Vol. 21, pp Marchand, J., Bentz, D.P., Samson, E., and Maltais, Y. (2001), Influence of Calcium Hydroxide Dissolution on the Transport Properties of Hydrated Cement Systems, Materials Science of Concrete,Vol. 11, Oh, B. H., and Jang, S. Y. (2007), Effect of material and environmental parameters on chloride penetration profiles in concrete structures, Cement and Concrete Research, Vol.37, pp Walton, J. C., Plansky, L.E., and Smith, R.W. (1990), Models for estimation of service life of concrete barriers in low-level radioactive waste disposal, NUREG/CR Conclusion (1) The diffusion coefficient of chloride ion into concrete decreases as the age and exposure period increases. The use of pozzolanic materials in concrete mixes reduces the diffusion coefficient by providing a fine internal pore structure, which, in turn, improves the resistance of concrete to chloride ion ingress.