THE RELATIONSHIP BETWEEN CHLORIDE DIFFUSION AND MIGRATION COEFFICIENTS IN CONCRETE

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1 THE RELATIONSHIP BETWEEN CHLORIE IFFUSION AN MIGRATION COEFFICIENTS IN CONCRETE Qiang Yuan(1)(2), Geert e Schutter (2), Caijun Shi (3), Katrien Audenaert(2) (1) School of Civil Engineering and Architecture, Central South University, Hunan, China (2) Magnel Laboratory for Concrete Research, epartment of Structural Engineering, Technologiepark-Zwijnaarde 904, B-9052, Ghent (Zwijnaarde), Ghent University, Belgium (3) College of Civil Engineering, Hunan University, Hunan, China Abstract Chloride diffusion coefficient is an indicator of the chloride resistance of concrete and service life prediction of concrete structure subjected to chloride environment. Many methods have been developed to measure the chloride transport in concrete. Based on the driving force, these methods can be classified into diffusion test and migration test. In terms of the change in chloride concentration in concrete, they can be classified into steady-state and non-steady-state testing. However, different methods give different results. This makes the comparison of all these results difficult. In this paper, the relationship between non-steady-state migration test ( nssm ), steady-state migration ( ssm ) test and non-steady-state diffusion ( nssd ) test were investigated. nssm was determined by NT build 492, breakthrough time method and the methods proposed by Castellote, respectively. ssm was determined by upstream method proposed by Truc and downstream method (NT build 355), respectively. nssd was determined according to NT build 443. Four different concretes, containing various mineral admixtures and with water/binder ratio of 0.48, were used for study. A same chloride concentration of 1mol/l, was used for all the tests. The results indicated that all the non-steady-state testing methods gave the same order of the chloride resistance of concretes although the diffusion/migration coefficients were somewhat different. Both NT build 492 and method proposed by Castellote gave a good estimation of diffusion coefficient. Breakthrough time method underestimated chloride diffusion coefficient. Steady-state migration coefficient was one order of magnitude lower than non-steady-state migration coefficients obtained from other methods. Steady-state migration coefficient obtained from upstream method was higher than that obtained from NT build 355. Key words: diffusion, migration, chloride, concrete 553

2 1. INTROUCTION Chloride-induced corrosion is the major durability issue of reinforced concrete structures in marine environment and in cold areas where de-icing salt are used, which causes enormous economic loss for the society. Steel reinforcement in concrete is normally in a passive state due to the high alkalinity of the concrete pore solution. When the chloride content in the concrete reaches a threshold value, the steel is activated and starts to corrode, which can cause a subsequent reduction in the strength, serviceability, and early repair or premature replacement of the structure. Thus, the transport of chloride in concrete is a hot topic in concrete science. In reality, chloride may penetrate into concrete through different mechanisms, such as, capillary suction, advection and diffusion. Chloride penetration into the concrete immersed in the seawater is mainly by diffusion, which is assumed to be water saturated. Since diffusion is a very slow process, an electrical field is often used for laboratory testing in order to accelerate chloride transport in concrete. Many test methods have been proposed or developed to measure the transport of chloride ions in water saturated concretes, based on the driving force, which can be classified into diffusion and migration testing. In view of the change in chloride concentration in concrete, they can be classified into steady-state and non-steady-state testing. In the case of non-steady-state migration coefficient, Andrade and Castellote [1-3], Tang [4], and Halamickova [5] proposed different methods to measure the non-steady-state migration coefficient ( nssm ), respectively. NT Build 355 [6] is a widely used method to measure steady-state migration coefficient ( ssm ). It has two drawbacks: 1) time-consuming; 2) loss of chloride around the anode caused by electrochemical reactions. To overcome these drawbacks, Truc [7] proposed another method to measure the steady-state migration coefficient by monitoring the chloride concentration change in the cathodic cell. NT Build 443 [8] is a non-steady-state diffusion test ( nssd ). Many researchers have attempted to investigate relationships between different test methods [1-3, 9-11], especially the relationship between migration and diffusion testing. ifferent experimental setups, experimental conditions and theoretical bases have been used by different authors. Thus, different test results have been reported. It is well known that chloride diffusion coefficient is concentration dependent [9]. However, even in the three existing standards, i.e. NT build 492 [12], NT build 355, NT build 443, different chloride concentrations are used: 10% sodium chloride is used in NT build 492, 165g/l sodium chloride for NT build 443, 5% sodium chloride for NT build 355. This makes the comparison of results from these three methods difficult. The aim of this paper is to investigate the relationship between non-steady-state migration, steady-state migration and non-steady state diffusion testing by using test setup specified in NT build 492, NT build 355 and NT build 443 respectively. For the purpose of comparison, A sodium chloride solution with a concentration of 1mol/l was used in all the testing methods. The breakthrough time method, methods proposed by Andrade and Castellote, upstream method proposed by Truc were also used at the same time. 554

3 2. EXPERIMENTAL 2.1 Materials and procedures An ordinary Portland cement (CEM I 52.5 N), complying with EN (2000), was used in this study. The chemical composition is shown in Table 1. Fine aggregate with a size range of 1-4mm was used. Gravel with the size of 5-16mm was used as the coarse aggregate. ry ingredients were first added to a 200L capacity flat pan mixer and mixed for 1 min. Water and water reducing admixture (if necessary) were then added into the mixer and mixed for 2 min. After mixing, the concrete was cast in molds, and consolidated with a rod. The specimens were demolded after 24 hours, and then cured in a standard curing chamber for 13 days. Test cores were drilled at the age of 14 days. Specimens with a dimension of mm were cast for compressive strength. Five cores were drilled from prismatic specimens ( mm). The central portions of cylindrical cores (Ф100mm 50mm) were cut for chloride diffusion or migration tests. The surface nearer to the cast surface was labeled, which is the surface exposed to the chloride solution. Before migration and diffusion tests, all the specimens were vacuum-saturated with saturated calcium hydroxide solution. The details of the concrete mixes are shown in Table 2. Table 1: Chemical compositions of cement CaO SiO 2 Fe 2 O 3 MgO Al 2 O 3 SO 3 CO 2 Ignition loss Table 2: etails of concrete mixes Mix proportions (kg/m 3 ) Mix B48 FA48 SF48 SL48 Water Cement Gravel Sand Fly ash Slag Silica fume Water reducer % - Slump(mm) ensity(kg/m 3 ) Air content 1.1% 0.9% 0.7% 1% Strength(MPa) 56 d Porosity accessible to water (%by volume)

4 Three testing setups were used in this study, which are standard testing setups described in NT build 492, NT build 355, and NT build 443, respectively. 1mol/l NaCl was used in the three tests. The tests were carried out on concrete at the age of 56 days. For the steady-state migration test, the upstream solution was changed periodically to maintain the chloride concentration nearly constant (no less than 95% initial concentration). To avoid acidification, the downstream solution was also changed. Both the upstream and downstream chloride concentrations were measured periodically. In the case of Non-steady-state diffusion test, the concrete specimens were immersed in 1mol/l sodium chloride solution for 42 days. The grinding was performed by Profile Grinder The grinding area is 73mm in diameter. Exact depth increments are adjustable, between 0.5 mm and 2.0 mm. The depth increments are accurate within 2% and the variation is less than 1%. The produced powder was collected with a small vacuum cleaner. For every depth increment of 0.5 mm approximately 5 grams of powder is available for analysis. Nitric acid soluble chloride was determined as total chloride content. Since salt may precipitate on the concrete surface, the first layer was omitted. 6-8 points were used for regression analysis. 2.2 Methods of calculation of diffusion/migration coefficients To make the terminologies consistent, all the coefficients obtained from migration test are called migration coefficient. Only the coefficient obtained from diffusion test is called diffusion coefficient in this paper. The non-steady-state migration process is described by the following equation: 2 c c zfu c = nssm 2 t x RTL x (1) Tang [4] obtained the analytical solution of this equation: C 0 ax x+αnssmt x αnssmt C = a erfc + erfc 2 2 nssmt 2 nssmt Under normal laboratory conditions and some simplifications, the non-steady-state migration coefficient is calculated as: (2) (273 + T)L (273 + T)Lx nssm = xd (U 2)t U 2 d (3) Where x d is the chloride ion penetration depth, which is measured by silver nitrate colorimetric method; nssm is the non-steady-state migration coefficient ( m 2 /s); U is the applied voltage (V); T is the temperature ( o C); and t is test duration (hour). Based on a rigorous solution of Nernst-Planck equation in steady-state conditions [1-3], as shown in Eq. 4, nssm can be obtained by Eq. 4 to

5 t 6 v = vcoth 2 2 tdiff v 2 (4) nssm 2 x d = (5) 3t diff zeu v = (6) kt Where e is the elementary charge, k is the Boltzman constant. x d is the penetration depth measured by colorimetric method. In this paper, this method is called ACⅠ. After NT build 492 migration test, the chloride penetration was determined by colorimetric method. This value was used in two methods, i.e. NT build 492 and ACⅠ, for the calculation of the migration coefficient. Castellote and Andrade [3] proposed a method to measure both nssm and ssm from one experiment through monitoring the conductivity of the downstream solution. The theoretical base for obtaining nssm is the same as Eq t diff is obtained by the intersection of the straight line of chloride flux characteristic of steady-state with the x-axis. In this paper, this method was also investigated and was called ACⅡ uring the steady-state migration test, the chloride concentration in the positive cell was periodically measured. The times corresponding to the values of C/C 0 =0.003 and C/C 0 =0.005 were regarded as breakthrough time [5, 13, 14]. C is the downstream chloride concentration (0.003mol/l), and C 0 is the upstream chloride concentration (1mol/l). Thus, the non-steady-state migration coefficient can be calculated by Eq. 7. nssm RT L α L = (7) zfe t Where L is the thickness of specimen; t is the breakthrough time; RTL 1 2 C α= 2 erf 1, when C/C 0 =0.003, ZFU C0 RTL α= ZFU RTL α= ; when C/C 0 =0.005, ZFU According to NT build 355, the steady-state migration coefficient is calculated as: ssm = J down 1 RTL C ΔEF up (8) Where ssm is the steady-state migration coefficient, J down is the chloride flux in downstream cell, C up is the chloride concentration in upstream cell. Truc proposed [7] a method to calculate the steady-state migration coefficient by 557

6 monitoring the chloride concentration in upstream cell. The migration coefficient is calculated by Eq. 9: ssm = J up 1 RTL C ΔEF up (9) Where J up is the chloride flux in upstream cell. According to NT build 443, the non-steady-state diffusion coefficient is calculated by Eq. 10: C(x, t) = C (1 erf (x / 4 t )) (10) s nssd Where C(x,t) is the chloride concentration measured at the depth x at the exposure time t (mass %), C s is the boundary concentration at the exposed surface (mass %), C i is initial chloride concentration measured on the concrete slice (mass %), x is the depth below the exposed surface (m), nssd is the non-steady-state diffusion coefficient (m 2 /s), t is the exposure time, and erf is the error function. 3. RESULTS AN ISCUSSION 3.1 Non-steady-state diffusion/migration coefficients The non-steady-state diffusion/migration coefficients obtained from different methods are shown in Table 3. Obviously, all the methods give the same order of diffusion/migration coefficient. Non-steady-state diffusion coefficient obtained from NT Build 443 is normally considered to be the reference value [10]. The values from migration methods are quite comparable to that of NT Build 443, which are slightly higher in the case of concrete with cement and concrete with fly ash, but a bit lower in the case of concrete with slag and concrete with silica fume. Tang et al. [11] found that the results from NT Build 492 were slightly higher than that of NT Build 443 in all concretes in his study. Tang [9] proposed an equation to account for the relationship between nssm and nssd, however, it seems too complicated to be practical. When comparing NT Build 492 to ACⅠ, the results are quite similar. If we examine the theoretical bases of these two methods, we will find that the theoretical bases of NT Build 492 are sounder than that of ACⅠ. NT build 492 is based on basic electrochemical principles. ACⅠ is based on rigorous solution of Nernst-Planck equation in steady state conditions, and the chloride concentration, the thickness of specimen are not taken into account. 558

7 Table 3: Non-steady-state diffusion and migration coefficients ( m 2 /s) Non-steady state migration test (1mol/l) iffusion Mix Colorimetric method Breakthrough time method ACⅡ (NT build 443) NT build 492 ACⅠ C/C0=0.003 C/C0=0.005 B FA SL SF The results from breakthrough time method and ACⅡ method are much lower than reference values. As for the breakthrough time method, it is not easy to capture the point of C/C 0 =0.003 and Moreover, the chloride front is irregular, as shown in Fig. 1 [14]. Taking the point of C/C 0 =0.003 or as breakthrough point is too arbitrary. The intersection of the straight line of chloride flux with the x-axis is easier to be obtained. In literature [14], the intersection also was defined as breakthrough time, and was calculated by Equation (7). However, the determination of C/C 0 is a problem. ACⅡ is based on the intersection point between the x-axis and the line of chloride flux, which is easier in manipulation than capturing the breakthrough point. The results from ACⅡ are also higher than the reference value. 3.2 steady state migration coefficient The typical evolution of upstream and downstream chloride concentrations with time is shown in Fig. 2. For the upstream method, ordinary concrete only needs 2-3days test duration to give a satisfactory result [7]. Therefore, in the upstream method, only the measurements of four days are used in the linear fit. Truc [7] believed that the flux into concrete was constant from the beginning of the test, and did not depend on chloride binding. However, when chlorides penetrate into concrete, part of them is in pore solution which contributes to the driving force, and part of them is bound to cement hydration products, and the binding isotherm is non-linear. It seems that the assumption that the flux into concrete is independent on chloride binding is invalid. The slope of the upstream method is greater than that of the downstream method. This is because bound chlorides are also counted as free chlorides. The migration coefficients of all mixes obtained from the upstream method are greater than that of the downstream method, as shown in Table 4. It is shown that the results from the upstream method do not correlate with those from the downstream method very well. Since the chloride concentration of the upstream solution is quite high, the solution pipetted from the upstream cell for the determination of the concentration of the solution has to be diluted, which affects the accuracy of this method. Compared to the non-steady-state migration coefficients, the steady-state migration coefficients are one order of magnitude lower. Tang [11] and Tong [14] also observed the same phenomena. Tang proposed an equation to account for this: 559

8 nssm = ssm 1 c + + c a x c c b ε 1+ c m ssm (11) Where 1 c a x =-0.06mol/l, c=0.1mol/l, c W b gel = k, k bm bm = , W gel =weight of c ε gel, is the porosity by volume, ssm Bm = ssm c B m m, m ( v ) B = f 1+β 1. β v = Figure 1: Chloride front in concrete [16] 2500 Chloride loss in upstream Chloride gain in downstream Chloride gain or loss (mg) Upstream four days K=0.159 R 2 =1 ownstream K=0.115 R 2 = Time (min) Figure 2: Evolution of downstream and upstream chloride concentration with time 560

9 Table 4: Steady-state migration coefficients and initial current Mix Steady-state migration ( m 2 /s) Initial Current Upstream ownstream (NT build 355) (ma) B FA SL SF Change solution Current (ma) 30 Change solution Time (min) Figure 3: The change of current with time According to Tang [9], f is the core parameter that reflects the transport property of concrete, which varies in different concretes. It can be assumed that f=11000, W gel =350kg/m 3 and ε =0.13. Inserting these values into Equation (11), nssm = 2.9 ssm is obtained. This does not fit the experimental data. The current was monitored throughout the migration experiments, as shown in Fig. 3. To maintain the chloride concentration nearly constant in upstream (no less than 95% of the original concentration), the solution was changed when needed. As can be seen, the current decreased with time, and decreased very rapidly when the old solution was replaced by new solution. At the end of the experiments, the current almost reached constant. A linear relationship was found to be a good indicator of the chloride resistance of concrete [15]. However, the initial current seems not to have a linear relationship with migration coefficients, as shown in Table CONCLUSIONS The following conclusions can be drawn from the experiment: Under the same chloride concentration (1mol/l), all the non-steady-state testing methods give the same order of the chloride resistance of concretes although the diffusion/migration 561

10 coefficients are somewhat different. Both NT build 492 and ACⅠgave a good estimation of the diffusion coefficient. The breakthrough time method underestimated the chloride diffusion coefficient. The steady-state migration coefficient is one order of magnitude lower than the non-steady state migration coefficients. Equations proposed by Tang, which account for the relationship between nssm and ssm, seem not to fit the experimental data. The steady-state migration coefficients obtained from the upstream method proposed by Truc are higher than those obtained from NT build 355. REFERENCE [1] Andrade C., Castellote M., Alonso C., Gonzάlez C.. Non-steady-state chloride diffusion coefficients obtained from migration and natural diffusion tests. Part 1: comparison between several methods of calculation. Mater. Struct., 2000, 33, January-February: [2] Castellote M., Andrade C., Alonso C., Non-steady-state chloride diffusion coefficients obtained from migration and natural diffusion tests. Part Ⅱ: ifferent experimental conditions joint relations. Mater. Struct., 2001, 34, July: [3] Castellote M., Andrade C., Alonso C., Measurement of the steady and non-steady-state chloride diffusion coefficients in a migration test by means of monitoring the conductivity in the anolyte chamber Comparison with natural diffusion tests, Cem. Concr. Res., 2001, 31: [4] Tang, L., Chloride Transport in Concrete - Measurement and Prediction, Ph.. Thesis. epartment of Building Materials, Chalmers University of Technology, Goteborg, Sweden, [5] Halamickova, P., etwiler, R. J. et al. Water permeability and chloride ion diffusion in portland cement mortars: relationship to sand content and critical pore diameter, Cem. Concr. Res., 1995, 25(4): [6] NORTEST NT BUIL 355, Chloride iffusion Coefficient From Migration Cell Experiments, Finland, [7] Truc O., Olliver J., Nilsson L., LMC test method. In: R. oug Hooton, Michael.A. Thomas, Jacques Marchand, James J. Beaudoin (eds.) Ion and mass transport in cement-based materials. USA, 2001, [8] NORTEST NT BUIL 443, Accelerated Chloride Penetration, FINLAN, [9] Tang L. and Nilsson L., Ionic migration and its relation to diffusion In: R. oug Hooton, Michael.A. Thomas, Jacques Marchand, James J. Beaudoin (eds.), Ion and mass transport in cement-based materials, USA, 2001, [10] Castellote M., Andrade C., Round-Robin test on methods for determining chloride transport parameters in concrete. Mater. Struct., 2006, 39: [11] Tang L. SØrensen H.E.. Precision of the Nodic test methods for measuring the chloride diffuisn/migration coefficients of concrete. Mater. Struct., 2001, 34: [12] NORTEST NT BUIL 492, Chloride Migration Coefficient From Non-Steady-State Migration Experiments, Finland, [13] Yang C.C., Chiang S.C., Wang L.C., Estimation of the chloride diffusion from migration test 562

11 using electrical current, Constru. Build. Mater., 2007, 21: [14] Tong L., Gjørv O.E. Chloride diffusivity based on migration testing, Cem. Concr. Res., 2001, 31: [15] Rolf Feldman, Luiz R. Prudencio Jr.U, Gordon Chan, Rapid chloride permeability test on blended cement and other concretes: correlations between charge, initial current and conductivity, Constru. Build. Mater., 1999, 13: