13 AN ELECTROCHEMICAL METHOD FOR ACCELERATED TESTING OF CHLORIDE DIFFUSIVITY IN CONCRETE

Transcription

1 13 AN ELECTROCHEMICAL METHOD FOR ACCELERATED TESTING OF CHLORIDE DIFFUSIVITY IN CONCRETE TIEWEI. ZHANG and ODD E. Division of Building Materials, The Norwegian Institute of Technology - NTH N Trondheim - NTH, Norway ABSTRACT In the present paper an electrochemical method for accelerated testing of chloride diffusivity in concrete is presented. The method is based on a theoretical relationship between chloride diffusivity and observed steady-state rate of chloride migration through the concrete. The concentration of the chloride source solution has a significant influence on the rate of chloride migration and, therefore, a correction factor for ionic interaction in the relationship is introduced. It is shown that the relationship can be used for calculation of chloride diffusivity under various testing conditions. Some experimental results are also presented. INTRODUCTION Depending on environmental conditions, chloride ions may penetrate concrete through various mechanisms such as diffusion, permeation and capillary suction. In general, capillary suction may dominate the penetration through a surface layer of the concrete which is very porous and only partly water saturated. However, if the porosity is very low or the concrete is very wet, a diffusion mechanism may dominate the penetration of chloride ions. In order to use chloride diffusivity as a more general parameter for evaluation of concrete quality, it has been an increasing interest to apply various types of accelerated test methods such as migration testing, where an external electrical field is applied for accelerating the chloride penetration. Several types of migration testing have been developed. In the USA, the "Rapid Chloride Permeability Test" (AASHTOT277-83) was introduced in the early 1980's("~). However, this test does not give any specific information about the resistance of the concrete against chloride penetration. In Norway, therefore, a similar test method was introduced in the beginning of the 198O9s, where the "Chloride Permeability" was defined on the basis of the observed rate of chloride migration through the concrete(3). This so-called "Chloride Permeability" is also an empirical quality parameter, which is strongly dependent on the particular testing conditions used. It has been a great need, therefore, both to provide more basic information about the mechanisms which control the chloride penetration and to develop better test methods which can provide more basic information about the resistance of concrete against chloride penetration under various conditions. 105

2 In Sweden, an accelerated, non-steady state migration method for determination of chloride diffusivity was also introduced some time ago(4). After exposure to the electrical field, the diffusivity is calculated on the basis of the observed depth or profile of chloride penetration in the concrete specimen. A more general analysis of the determination of chloride diffusivity in concrete based on migration measurements has recently been carried out by ~ndrade@). A similar study was also carried out by the present authors, from which an accelerated, steady-state migration method for determination of chloride diffusivity was developed. This method is described in the following. THEORETICAL BACKGROUND Although diffusion and migration have different mechanisms, an intrinsic relationship between them exists, which is expressed by the Einstein equation@): where D = diffusion coefficient of the ionic species (cm2d1) U, = absolute mobility of the ions in the same medium (cm-s-'dyne-') k = Boltzman constant (1.38 x10-l6 ergsk1) T = absolute temperature (K) Since both the diffusion coefficient and the mobility reflect the easiness with which the ions move can move through the medium, there is a proportionality between them. When both chemical and electrical driving forces coexist, the flux of the ionic drift can be expressed as: where R and F are the gas constant and Faraday constant, respectively. This equation is normally referred to as the Nernst-Plank equation(617), in which the first and second term represent the contribution of migration and diffusion, respectively. Before a calculation of the diffusion coefficient based on this equation can be obtained, a set of simplified boundary conditions and assumptions have to be made. The migration process PRINCIPLE OF TESTING In principle, the experimental setup is schematically shown in Fig. 1, where Cell (l) contains the chloride source solution and Cell (2) the chloride collecting solution. Normally, Cell (1) contains a NaCl solution, while Cell (2) is filled with a NaOH solution of the same molar 106

3 concentration as that of the chloride source solution. Two mesh electrodes are placed, one on each side of the concrete specimen in such a way that the electrical field primarily is applied across the test specimen. This is the same experimental setup as that used for determination of chloride permeability(39899). Figure 1. Principle of testing. In a well cured concrete on a portland cement basis, the pore solution inside the specimen mainly contains hydroxyl ions in addition to sulphate ions and trace amounts of sodium an potassium ions(lo). When the electrical field is applied, the profile of chloride migration is driven forward by the electrical driving force, while the concentration gradient decreases until a steady-state migration is reached. Then, the chloride flux and the concentration increase in the chloride collecting cell become constant. At this stage, a constant drift velocity of the chloride ions inside the specimen can be assumed and its value determined. With a high enough external electrical field, the influence of the chemical driving force caused by chloride concentration gradient inside the specimen can be neglected. Calculation of diffusivity According to the theoretical relationship between diffusion and migration expressed by the Einstein equation, the diffusion coefficient can be obtained by measuring the actual mobility of the chloride ions in the concrete specimen during the steady-state migration. 107

4 As a first approximation, the chloride source solution is assumed to be infinitely diluted so that the migration behavior of the chlorides through the concrete can be assumed to follow the Einstein equation (Eq. 6). During the steady-state migration process, the following relationship exists: Jin = Jout where J is the flux of the chloride ions (C~-~S-'). Then, by applying the Einstein equation, the following equation can be obtained: 300kT LV D = dc ze,ay cdo dt - - (cm 2-s -l) where D = diffusion coefficient of the diffusing species (cm S-') k = Boltzman constant (1.38 x10'16 ergs K-' T = absolute temperature (K) Zi = valence of ion e0 = charge of proton (or electron) (4.8 X 10-l0 e.s.u.) AY = applied electrical potential (volt) L = distance between the two electrodes which can be put equal to the specimen thickness (cm) A, = cross section of specimen (cm2) CO = source chloride concentration (M) dcldt = steady-state migration rate of chlorides Equation (4) gives the theoretical relationship between the diffusion coefficient and the steady-state migration rate of chlorides (dcldt). From this equation it can be seen that the increasing rate of chloride concentration in the chloride collecting cell (dcldt) is the only variable in the determination of the diffusion coefficient (D). It should be noted, however, that this expression is based on both certain assumptions and testing conditions which need special attention. Assum~tions and testing conditions During testing the following factors may affect the observed test results: (1) Interaction between ions in the migration system. (2) Evolution of hydrogen gas at the cathode and oxygen at the anode due to electrochemical reactions between the electrodes and water. 108

5 (3) Evolution of chloride gas at the anode if the applied voltage is too high and the electrical resistivity of the concrete is too low. (4) Evolution of heat and creation of a temperature increase. Even though all of the above factors will influence the test results to some extent, most of them can be controlled in such a way that the testing can be satisfactory carried out. The chemical driving force will be negligible above a certain voltage. Formation of gas at the electrodes can be controlled by keeping the current low enough, or the voltage low enough in relation to the electrical resistance of the concrete specimen. A moderate voltage will also give a moderate evolution of heat and rise of temperature. In most cases a voltage of 12 V is acceptable, but it may be higher depending on the electrical resistivity of the concrete. Another possible factor which may affect the test results is a reduced chloride concentration (c,) in the chloride source solution during testing. In order to avoid this effect, the volume of Cell (1) in Fig. 1 must be big enough or the time of testing short enough to maintain an approximately constant c,. A chemical binding and a physical adsorption of chlorides inside the concrete specimen may also reduce or block the flow channels through the concrete. Under the influence of a strong electrical field, however, it is assumed that this effect is negligible. From a more thorough analysis of ionic interaction, it is clear that this effect can not be neglected(11112). Eq. (4) is based on the assumption of an infinite dilute chloride source solution. The higher the chloride concentration (c,), the more retarded is the observed drift velocity of the chlorides compared to that from an infinite dilution. Hence, a correction factor for ionic interaction in the calculation of the diffusivity is introduced(12): where: p, = correction factor for ionic interaction v, = drift velocity of chloride ions from an infinite dilute solution without ionic interaction v = drift velocity of chloride ions from a concentrated solution By introducing the correction factor P, in Eq. (4), the following equation for calculation of the diffusivity is obtained: 109

6 The correction factor P,, which depends on both type and concentration of chloride solution as well as temperature, is independent of applied electrical field. Some values of p, for a NaCl solution of various concentrations are shown in Table 1. For an increasing chloride concentration from 0.1 to 0.5 M NaCl, it can be seen from this table that P, increases from 1.22 to Often, a 3% NaCl solution is used, which roughly corresponds to a 0.5 M concentration. If higher chloride concentrations are used, the theoretical basis for making a correction is no longer valid@). EXPERIMENTAL Experimental details As part of a more comprehensive ongoing research program on chloride penetration into concrete, a few preliminary experimental test results on the effect of voltage level as a test parameter are presented. For the present experiments the same experimental setup and roughly the same procedure as that for determination of chloride permeability were used(31819). Based on an ordinary portland cement, one type of concrete and one type of mortar with the same water-cement ratio were tested, the mix proportions of which are shown in Table 2. Test cylinders of Q100 X 200 mm were prepared and cured in water for two months. Each cylinder was first surface dried, and then embedded in epoxy and cut into three 50 mm thick slices from top (T) middle (M) and bottom (B) of the test cylinders, respectively. Before testing, all specimens were subjected to vacuum saturation. Using the same test specimens, three different runs of experiment were carried out with the voltage successively increasing from 6, 9 to 12 V. Before each change of voltage, both the chloride source solution and the solution in the chloride collecting cell were renewed. Further, a NaC1-solution of 0.3 M was used. The increasing chloride concentration in the chloride collecting cell (dcldt) was monitored periodically by use of a spectrophotometric method(13). Test results and discussion All individual test results are shown in Table 3, from which it can be seen that in spite of a constant water-cement ratio, the mortar showed a higher resistance to chloride penetration than the concrete. This is in accordance with previous experience(14915). For the mortar, the diffusion coefficient varied from 11.9 to 19.4 X 10-~, while for the concrete it varied from 28.6 to 36.5 x10-~ cm2s-l. The scatter of test results mostly reflects inhomogeneities in the concrete specimens, but it is partly due also to a certain scatter in the measurements of chloride concentration. 110

7 The test results clearly demonstrate, however, that chloride diffusivity can be used as a general quality parameter for evaluation of the resistance of concrete against chloride intrusion. The experimental results also show that for the level of concrete quality tested, an increasing voltage from 6 to 12 V did not affect the observed chloride diffusivity. In order to reduce the time of testing, the voltage should be as high as possible provided that no evolution of gas at the electrodes develops. Based on the present test results, it appears that a voltage of at least 12 V can be used for the testing of a moderate concrete quality. CONCLUSIONS Based on the theoretical analysis and the experimental results presented in the present paper, the following conclusions appear to be warranted: (1) The electrochemical test method presented can be used for a rapid determination of chloride diffusivity in concrete. (2) The concentration of the chloride source solution has a significant influence on the rate of chloride migration due to ionic interaction. Hence, for a more general determination of chloride diffusivity, the introduction of a correction for ionic interaction is necessary. (3) Eq. (6) can be generally used for calculation of chloride diffusivity with a satisfactory accuracy for practical purposes. The equation is independent of both testing potential and chloride source solution. (4) The chloride diffusivity as determined by the present test method, can be used as a general quality parameter for evaluation of the resistance of concrete against chloride intrusion. Such a quality parameter can be used both for job specification and control of in situ quality, and hence provide a better basis for assuring proper durability. 111

8 REFERENCES Whiting, D., "Rapid Determination of the Chloride Permeability of Concrete", Report No. FHWAIRD , Portland Cement Association, NTIS DB No , Aug AASHTO Designation T , "Standard Method of Test for Resistance of Concrete to Chloride Ion Penetration", American Association of State Highway and Transportation Officials, Washington D.C., Nordtest Method, NT BUILD 355, ISSN , Tang, L. and Nilsson, L.-O., "Rapid Determination of the Chloride Diffusivity in Concrete by Applying an Electrical field", ACI Materials Journal, Vol. 89, No. 1, 1992, pp Andrade, C., "Calculation of Chloride Diffusion Coefficients in Concrete from Ionic Migration Measurements", Cement and Concrete Research, Vol , pp Bockris, J.O.M. and Reddy, A.K.N. "Modern Electrochemistry", Plenum Press, New York, Newman, J.S., "Electrochemical System", Prentice Hall, Englewood Cliffs, New Jersey, Detwiler, R.J., Kjellsen, K.O. and Gjgrv, O.E., "Resistance to Chloride Intrusion of Concrete Cured at Different Temperatures", ACI Materials Journal, Vol. 88, No. l, Zhang, M.-H. and Gjgrv, O.E., "Permeability of High-Strength Lightweight Concrete", ACI Materials Journal, Vol. 88, No. 5, 1991, pp Nixon, P. and Page, C., "Pore Solution Chemistry and Alkali Aggregate Reaction", Concrete Durability, Katharine and Bryant Mather International Conference on Concrete Durability, Proceedings, J. M. Scanlon (Ed) ACI SP-100, Vol. 2, 1987, pp Zhang T. and Gjgrv, O.E.," Diffusion Behavior of Chloride Ions in Concrete", (Submitted to Cement and Concrete Research) Zhang, T. and Gjgrv, O.E., "Effect of Ionic Interaction in Migration Testing of Chloride Diffusivity in Concrete", Cement and Concrete Research (to be published). Vogel's Textbook of Quantitative Inorganic Analysis, 4th. Edition, Longman, 1978, pp Gjgrv, O.E. and Vennesland, g., "Electrical Resistivity of Concrete in the Oceans", 9th. Annual Offshore Technology Conference, Proceedings, Houston, Texas, 1976, Paper No. 17, 12 p. Gjgrv, O.E., and El-Busaidy, A.H.S., "Diffusion of Dissolved Oxygen through Concrete", Materials Performance, Vol. 25, No. 12, 1986, pp

9 Table l. Correction factors for ionic interaction(28) Table 2. Concrete mixtures. Mix Specimen no. w/c Cement Mix proportions (kg/m3) Water Aggregate Air content % Compressive strength (MP4 0-8mm 8-16mm Concrete Mortar A2 D

10 Table 3. Diffusion coefficients (cm2.s-l) x10-~. Specimen no. 6 Voltage (V) 9 12 W n Statistical parameters Xon-l Std. dev.(%) A2-3 T M B Average A2-4 T M B Average D7-4 T M B O Average D7-5 T M B Average