Evaluation Of Accelerated Chloride Ion Diffusion Test And Applicability Of Fick s Second Law

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1 Evaluation Of Accelerated Chloride Ion Diffusion Test And Applicability Of Fick s Second Law AS Poupeleer & D Van Gemert Department of Civil Engineering KU Leuven Belgium Summary: A global test program is set up to find the correlation between the standard diffusion test (ASTM C ) and the accelerated Chloride Penetration Time (CPT) test. The purpose is to find a relation between the steady state and the dynamic diffusion coefficient. In this way only the standard test would be necessary to quantify both diffusion coefficients. However, the CPT test gives extra information about the processes at the reinforcement bar and cylindrical concrete samples and various concrete cover thicknesses can be used. In this paper the results of the first part of the CPT test program are reported. Two concrete mixes with a different porosity are tested. The CPT test is accelerated by a constant potential difference in a 1.5 and 3 % NaCl-solution. Not only the current during the whole test, but also the total amount of charge passed through the samples and the concentration of chloride ions in the concrete at different depths are investigated. Two concrete mixes with different porosity are compared in the CPT-test at 1.5 and 3 % NaCl and at different applied voltages. The applicability of the test method is questioned and suggestions for further investigations are made. Keywords: Accelerated chloride penetration time test, chloride profile, total amount of electrical charge, diffusion coefficient 1 INTRODUCTION One of the main durability problems of concrete is the corrosion of the embedded reinforcement. If the concrete cover on the reinforcements is thick enough, there will be no damage thanks to the alkalinity of the concrete. However, due to carbonation or chloride attack the protection of the reinforcement can be broken down. In this paper only the influence of chlorides will be investigated. Chloride ion ingress is a suction and diffusion phenomenon, which normally takes a long time. This time span must be shortened for control and design purposes, and therefore a number of accelerated chloride ingress test methods are proposed in literature, e.g. Andrade (1968) and Justnes (1990). However, the evaluation of the accelerated test results, their reliability and the transfer of these results to real life behaviour remains uncertain. As a consequence, several accelerated test procedures are used in practice. A correlation is searched between the standardised ASTM C1202 diffusion cell test method and the accelerated chloride penetration time (CPT) test. The purpose is to derive a relation between the diffusion coefficient found in the standard diffusion test and the diffusion coefficient found in the CPT test. This would allow one to relate the accelerated tests to real life behaviour. The first step within this investigation is a detailed study of the transport of chlorides during the CPT test. To get a better view on the mechanism of chloride ion transportation during the accelerated chloride penetration test, two concrete mixes with different porosity are tested. The effect of different voltages (3, 5 and 7 V) is examined by comparing the CPTs and the total amount of electrical charge that has passed through the sample during the tests. The chloride concentration profile is determined at the end of the tests. 2 ACCELERATED CHLORIDE PENETRATION TIME TEST Chloride penetration in the concrete is generated by electrochemical processes in the accelerated chloride penetration time test. The migration test is executed on cylindrical samples. The samples are placed in a 3 % (or 1.5%) NaCl-solution, in such a way that the distance between the surface of the water/salt-solution and the top of the specimen is at least 20 mm (Fig. 1). 9DBMC-2002 Paper 047 Page 1

2 Figure 1: Schematic representation of the electrical circuit of the test The 3% NaCl-solution corresponds approximately to the chloride concentration in seawater. A constant voltage difference of respectively 3, 5 and 7 V is applied between the reinforcement bar in the test sample and a stainless steel cathodic plate in the NaCl-solution. The reinforcement bar in the concrete acts as an anode (positive) in the system and consequently attracts anions (OH - and Cl - ). The stainless steel plate is the cathode (negative) of the system. At this plate the reduction of the oxygen (O 2 ) takes place (4e - + O 2 +2H 2 O 2OH - ). The hydroxyl ions move towards the anode together with the chloride ions present in the water. The electrolytic medium consists of the NaCl-solution and the pore solution in concrete. A process takes place similar to non-accelerated electrochemical corrosion of steel. The Cl - ions break though the passive oxide layer on the reinforcement, and attack the reinforcing bar. This results in pitting corrosion of the reinforcement bar. Figure 2: Schematic plan of the test setup of the accelerated CPT-test The test setup is presented in figuur 2. The electrical current through the system is automatically recorded. Therefor, the voltage drop over a 1 Ohm reference resistance is measured and recorded. According to Ohm s law this voltage drop is converted to a current: V = R.I = 1.I 9DBMC-2002 Paper 047 Page 2

3 Figure 3: Theoretical evolution of the current during a CPT-test. This current is an indication for the corrosion rate. The expected time curve of this current is represented in figure 3. Four stages can be distinguished. 1. In the first part a current decrease is caused by polarisation of the anode. The OH - -ions strengthen the passive oxide layer. These ions are already present in the pore-water long before the chloride ions and they too are negatively charged. The strengthening of this patina makes that less electrons are released and that the current decreases. 2. In a second stage the current increases. This is the moment the chloride ions reach the steel bar in the concrete and start the corrosion. This point of time is called the chloride penetration time or CPT. Shortly after reaching the CPT time cracks will appear in the concrete. Due to the fact that the potential difference between steel and water is now imposed to a much higher level than in natural conditions of corrosion, the concentration of chlorides necessary for corrosion is much lower. Consequently, the corrosion starts at the moment the first chlorides reach the reinforcement bar (CPT). 3. From that moment on the patina becomes weaker and cracks become bigger. Consequently, the chloride ions have better access to the steel and the corrosion process is accelerated. 4. In a final stage the samples totally break. This is visible by a vertical crack all over the specimen. The corrosion products flow out off the concrete and silt up the fissure. The current reaches a maximum value. This is characterised by the time to failure or TTF. Due to the silting process the current remains about constant. The CPT is derived from the current evolution curve at the intersection of two lines tangent to the descending and ascending part of the curve. 3 EXPERIMENTAL DETAILS 3.1 Materials Two different concrete mixes with different porosity are used. Both are made with a Portland cement CEM I, 42.5, according to the standard EN The composition of the mixes is given in table 1. Table 1: Composition of the two concrete mixes. Composition 1 Composition 2 Coarse aggregate 4/ kg/ m kg/ m 3 Fine aggregate 0/5 699 kg/ m kg/ m 3 CEM I, kg/ m kg/ m 3 Water 180 kg/ m kg/ m 3 w/c-ratio porosity vol.% vol.% The 28-day compressive strength of both mixes is 39.4 respectively 45.4 MPa. The original chloride content of the two concrete mixes is negligible. The values are given in table 2. Because no chlorides are added to the mix and the aggregates are free from chloride contamination, the only source of chlorides is the tap water used for mixing. This water has a chloride content of g/l. 9DBMC-2002 Paper 047 Page 3

4 Table 2: Concentration of chloride in the concrete before the experiments Mix 1 Mix 2 w% Cl water soluble (potentiometric) w% Cl acid soluble (NBN B15-250) w% Cl water soluble (potentiometric) w% Cl acid soluble (NBN B15-250) Top Centre Bottom Test specimens The dimensions of the specimens are given in figure 4. It concerns cylindrical concrete samples with a smooth drawn steel bar (ST52) in the centre. The bar has a diameter of 8 mm and is placed 25 mm above the bottom of the sample. The concrete cover of the bar is about 21 mm. After sandblasting, the top and the bottom of the steel bars are covered with epoxy. Only the remaining 100 mm of untreated steel can be attacked by chloride ions. Another important reason for this treatment is to prevent oxidation corrosion at the concrete-steel-air interface. Figure 4: Schematic representation of the cylindrical specimens The test samples are cored out of a concrete plate with a thikness of 175 mm. One day after pouring the concrete, the formwork is removed and the concrete plate is cured for 28 days in water at 20 C. Then the cylinders are cored out of the concrete plate. Subsequently, the cores are saturated for 7 days. In this way capillary suction of water during the first minutes of the test is prevented, and only diffusion of the Cl - is possible to penetrate into the concrete cover. 4 RESULTS AND DISCUSSION 4.1 Results The results of the CPT test are compared according to the composition of the concrete and to the applied voltages. Firstly the focus is on the impact of the porosity on the CPT. Therefor three samples of mix number 1 and two samples of mix number 2 are submitted to a constant voltage of 3 V and a NaCl concentration of 3 %. Secondly the influence of different voltages has been examined, namely 5 and 7 V. In this case the concentration of NaCl is reduced to 1.5 %. Figure 5 gives the current versus time graphs of the samples of mix 1. Three different voltages are applied each time on three samples. Based on these curves the CPT can be determined. As an illustration the determination of the CPT of a sample of mix number 1 tested with 7 Volt and in a NaCl-solution of 1.5 % is given in figure 6. 9DBMC-2002 Paper 047 Page 4

5 Figure 5: Overview current trough samples during the CPT-test Current [ma] CPT = 21.8h 10 7V 1.5% NaCl Ch Time [h] Figure 6: Graphical determination of the CPT The CPTs of the tested cylinders are summarised in table 3. Table 3: Overview of the CPT (Chloride Penetration Time) Samples Individual CPT Mean CPT Standard deviation [hours] [hours] [hours] Comparison between mix 1 and 2 CPT mix 1/ 3V/ 3.0 %-NaCl 38.8, 42.6, CPT mix 2/ 3V/ 3.0 %-NaCl 89.2, Comparison between 5 and 7V CPT mix 1/ 5V/ 1.5 %-NaCl 24.0, 29.2, CPT mix 1/ 7V/ 1.5 %-NaCl 18.1, 27.1, Another indication for the corrosion is the total amount of charge passed through the samples during the test, because each Featom that splits releases not only a Fe 2+ but also 2 electrons. This value can be obtained by integrating (Fig. 6) the current versus time curves. It is a measure of the electrical conductance of the concrete during the period of testing. Although the testing period was not always the same, it is interesting to compare the relative magnitudes of the total electrical charges. 9DBMC-2002 Paper 047 Page 5

6 Table 4: Overview of the charge passed through the samples Samples Time [h] Individual electrical charge [C] Mean electrical charge Spreading [C] [C] Comparison between mix 1 and 2 CPT mix 1/ 3V/ 3.0 %-NaCl , 30487, CPT mix 2/ 3V/ 3.0 %-NaCl , (22975, 24520), 19260, 783, (2705) (21504) Comparison between 5 and 7V CPT mix 1/ 5V/ 1.5 %-NaCl , 28605, CPT mix 1/ 7V/ 1.5 %-NaCl , 46461, At the end of the CPT tests, slices were cut from the samples to determine the chloride concentrations profile. Two slices were cut from the samples of the first mix (5V), at depths of 5 and 17 mm (Fig. 7). In the investigation of the second mix (3V), three thinner slices were analysed, namely at 4, 11 and 18 mm depth, to obtain a more detailed chloride penetration profile (Fig. 7). Figure 7: Position of Cl - -concentration sampling Only the free chloride ions can contribute to the corrosion of the steel bars. In the framework of this investigation the watersoluble and the acid soluble chloride ions are determined (Fig. 8). Neither of the two concentrations really represents the amount of free chloride ions, but they give an estimation of the amount of available Cl - ions in the process. Even the amount of water-soluble chlorides includes dissolved chlorides present in the form of crystalline salts. For the second mix only this amount of the water-soluble chlorides is determined (Fig. 8). Chloride concentration [g Cl./ 100 g concrete] Water Soluble Chloride Acid Soluble Chloride concrete cover (thickness 21 mm) steel bar (diam. 8 mm) Depth [mm] Chloride concentration [g Cl./ 100 g concrete] W ater Soluble Chloride concrete cover (thickness 21 mm) Depth [mm] steel bar (diam. 8 mm) Figure 8: Chloride concentration at different depths in the concrete mixes 1 and 2 after CPT tests 9DBMC-2002 Paper 047 Page 6

7 4.2 Discussion Chloride Penetration Time No general conclusions can yet be made after this first part of the global test program. Comparing the CPTs of mix number 1 and 2, it can be seen that the porosity has a great influence on the penetration time of the chlorides. The more porous the concrete, the lower the CPT value. It takes almost twice as long to penetrate trough the less porous concrete. In a porous water saturated environment the resistance against the transport of chloride ions is low. This confirms the general statement that the concrete cover has to be of a high quality to provide adequate protection of the reinforcement. Not only it has to be of an adequate thickness, also the permeability should be watched. Although the salt concentration in the tank is only 1.5 %, instead of 3 %, in the second comparative investigation, the CPTs here are much lower. This means that lesser chloride ions result in a faster degradation. This could mean that the accelerating effect of the potential difference is very big. On the other hand it might not be justifiable to use excessive potential differences up to 7V. The initial intention was to accelerate the process because normal degradation processes of steel bars in concrete take many years. But, is the influence of the voltages limited to the acceleration? It should be investigated if the high voltages do not ionise the water molecules, because the normal redox potential of water is 1.23V (2 H 2 O O 2 + 4H + + 4e-) (Polytechnic pocketbook 1995). This dissolution would totally change the test conditions. In that case the measured currents are not resulting from chloride diffusion only, but also from the OH - ions (2 H 2 O + 2 e -. H OH - ). Parallel real time tests are necessary to find out the acceleration rate, which would allow the calibration the test results to real life situations. After calibration the diffusion coefficients of the CPT tests can be used for design purposes Total amount of electrical charge passed through the samples The standard ASTM C gives a indication table for the chloride ion penetration based on charge passed during 6 hours. For the CPT test no tables are available yet, but based on the same principle it should be possible to define certain limits. The results in table 4 show that there is a correlation between the diffusion possibility and the total amount of charge. The more porous concrete (mix 1) and therefor also more open for chloride ions gives a higher charge than mix 2. Nevertheless, the results coming from the other applied voltages demonstrate that the eventually established reference table should be adapted for different applied voltages. The higher voltages (5 and 7 V) accelerate the transport process, and thus more ions are transported in a shorter time through the same concrete with the same electrical conductivity Chloride concentration in the concrete The samples of the first mix tested in 1.5 % NaCl solution and with a voltage of 5 V, show a very slightly increasing chloride concentration in function of depth. In a second phase, where more information is gained from the samples of the second mix (with 3 % NaCl and 3 V), the chloride concentration decreases from the concrete surface to the inside and than increases again at the steel bar. Knowing that the samples were saturated, capillary suction is out of the question. Also convection can be eliminated, because the transport occurs inside the concrete. Diffusion seems a possible way of transport. In theory, the laws of Fick can describe the diffusion of chloride ions through concrete. Fick s second law describes the process in a non-steady stage. Consequently, the idea is to apply an adapted version of the second law where the influence of the imposed electric field would be taken into account. In literature many suggestions are made for such adaptation. Andrade et al (1994) proposed the following expression: C x = C 1 erf 2 s x ZF E D RT app t with C x = concentration at depth x [kg/m 3 ] C s = concentration at the surface [kg/m 3 ] D app = diffusion coefficient [m 2 /s] Erf = error function Z = electrical charge of species [equivalent/mol] (z Cl = -1) F = Faraday number = [C/ equivalent] R = gas constant = [J/mol.K] T = absolute temperature [K] 9DBMC-2002 Paper 047 Page 7

8 However, it has to be remarked that this law is mostly used for diffusion of chloride ions through a concrete plate which separates one fluid from another one. The circumstances of the CPT test are different. Because of the treatment with epoxy (Fig. 1, 4) one can consider the problem as circular cylindrical. Consequently, the concentration is only a function of radius r and time t. The diffusion equation becomes: C 1 = t r C rd r r If the diffusion coefficient is considered constant, it can be put in front of the equation. The solution of this expression is determined by both the initial and the boundary conditions of time and place, which determine the specific character of the diffusion process. The initial concentration of chlorides in the concrete of the tested samples is known (table 1) as well as the surface concentration. C = C 0 = 1.5 or 3 % NaCl (9 or 18 g Cl/l), r = 25 mm, t/0, C = f(r) = C1 (table 1), 0 < r < a, t = 0, Because the concentration is initially uniform throughout the cylinder, the solution can be reduced to: C C C 0 1 C 1 2 = 1 a exp( Dα t) J 2 n n= 1 α n J 1( 0 aα ) n ( rα ) n J n = Bessel function of the first kind of order n α n = solutions of J 0 (aα n ) = 0 From the above equation, different values for the diffusion coefficient were found in function of the depth. Also starting from a constant diffusion coefficient it was impossible to find the concentration profile given in figure 8. In the expected theoretical concentration profile corresponding to the law, the concentration of chlorides starts to grow from the surface towards the centre and increases in function of time until the concentration is constant everywhere. In this case the law does not correspond to reality. The main reason for that is most probably the presence of cracks. Physical processes in cracks are totally different from processes in a homogenous medium. Therefor, the CPT test should be redone and the concentration of chlorides should be determined just before cracking. Another reason is the presence of bounded chlorides. The concentration inside the concrete is higher than the enforced concentration at the surface. This confirms the fact that from the moment the first chloride ions diffuse into the concrete they are absorbed to the inner wall of the pores. It is known that the bonding capacity of concrete for chlorides is rather high. Tang and Nilsson (1996) assumed that chloride binding is dominated by adsorption on the CSH-gel. It is also found that the ratio between free and bounded chlorides is not independent of the chloride concentration, nor for chemical binding (e.g. Friedel salt formation) nor for adsorption. Consequently, it is impossible to use a constant factor to determine the free chlorides on the basic of the measured bounded ones. Furthermore, McGrath and Hooton (1996) claim that chemical binding will slow down the break through time and thus reduce the diffusion coefficient. Probably, the chloride ions entered in the surface layers are immediately absorbed due to the presence of positively charged particles on the concrete inner surface of the pores. The chlorides will only start to move if other chlorides or aggressive elements, like sulphates, already occupied the bonding places. Besides, the binding activity was stimulated in our tests because of the young age of the used concrete. It is not appropriate to attempt to calculate the diffusion coefficient by means of an adapted version of the second law of Fick, including the influence of the electric field, the bounding capacity and the time effect of the not yet mature concrete. As it is mentioned in the description of the test procedure, fissures occur. Together with these cracks, the overall structural composition changes. The chloride ions can easily reach the reinforcement bars through these open channels. The main adaptation to the equation should therefore be with respect to these structural changes. 5 CONCLUSIONS The CPT test is an interesting test method to study the penetration of chlorides in saturated concrete samples. Therefore, this test has been evaluated as a first phase of a global investigation. The CPT test consists of different stages and a lot of parameters play a role. The electrical current gives information about the electrical charge that has passed through the sample. The first decreasing part can be explained by the attraction of OH - -ions present in the concrete cover, which cause strengthening of the passive oxide layer of the reinforcement bars. The chlorides penetrate in the concrete, but are immediately adsorbed by the charged particles at the inner surface of the pores. Therefore, the chloride profiles obtained can not be used to determine the diffusion coefficient by means of the second law of Fick, even not after adapting to the electric field used and to the adsorption capacity of concrete. The presence of cracks makes it necessary to review the transport processes. 9DBMC-2002 Paper 047 Page 8

9 In order to use Fick s laws in the future, the concentrations of chlorides should be determined before cracks appear. This moment can be determined by means of the current time curve of the CPT test. Together with the CPT tests the standard diffusion test can be executed on the same mixes to analyse the correlation between the diffusion coefficients. The actual importance of the CPT test lies in its ability to allow quick comparative tests on different compositions and geometrical layouts of reinforced concrete. It can be used as a test method for hydrophobic agents on concrete. Changes in the diffusion coefficient and chloride penetration times can be used as process parameters. One of the most promising applications of this test method is the analysis of corrosion protective products, which penetrate through the concrete cover and fix onto the reinforcement bar itself. For such applications standard diffusion test methods fail. 6 REFERENCES 1. Andrade C.; Monitoring Techniques in Corrosion of steel in concrete Report of the Technical Committee 60 CSC RILEM, edited by P.Schressel, Chapmann and Holl, Andrade C., Sanjuan M.A., Recuero A. and Rio O.; Cement and concrete research 3. Andrade C., Alonso C. and Acha M. ; Chloride diffusion coefficient of concrete containing fly ash calculated from migration tests ; PIC Corrosion and corrosion protection of steel in concrete; Sheffield July, 1994, pp Justnes H., Oye B.A.; Protecting concrete against chloride ion ingress by latex additions, 6th ICPIC, Singapore, China, McGrath, P.F. and Hooton, R.D.; Influences of Voltage on Chloride Diffusion Coefficients from Chloride Migration Tests, Cem. Concr. Res., Vol. 26, No. 8, 1996, pp Polytechnic pocketbook, 1995, edited by Sj. Tysma, F. van Herwijnene, P.H.H. Leijendeckers, 47the edition, pp. A5/30 7. Tang, L. and Nilsson, L.-O.; Service Live Prediction for Concrete Structures under Sea Water by a Numerical Approach, Proceedings of the 7th International Conference of Building Materials and Components, Stockhholm, Sweden, May, 1996, Vol. 1, pp DBMC-2002 Paper 047 Page 9

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