A comparison of transport properties for concrete using the ponding test and the accelerated chloride migration test

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1 Available online at Materials and Structures 38 (April 2005) A comparison of transport properties for concrete using the ponding test and the accelerated chloride migration test C. C. Yang Institute of Materials Engineering National Taiwan Ocean University Taiwan R.O.C. Received: 13 October 2003; accepted: 19 July 2004 ABSTRACT In order to develop a better understanding of the relationship between 90-day salt ponding test and accelerated chloride migration test (ACMT; the electrochemical technique is applied to accelerate chloride ion migration) the transport properties for concrete obtained from ACMT are compared to the diffusion coefficient from ponding test. The plain cement concrete fly ash concrete and slag concrete with different w/b ratios ( and 0.65) were used. In this study the total chloride content and penetration depth of concretes were measured after the ponding test and the Fick s second law of diffusion was fitted to the data from experiment to determine the diffusion coefficient. The non-steady-state diffusion coefficient the migration coefficient and the current corresponding to the coulomb charge passed obtained from ACMT in the previous works were compared with the diffusion coefficients obtained from ponding test. Parallel tests show that the diffusion coefficients obtained from ponding test correspond well with the non-steady-state diffusion coefficient the migration coefficient and the current corresponding to the coulomb charge passed obtained from ACMT although the diffusion coefficient measured by ponding test is different from that measured by the ACMT in non-steady state and steady state RILEM. All rights reserved. RÉSUMÉ Pour mieux comprendre la relation entre l essai de trempage dans l eau salée pendant 90 jours et l essai accéléré de migration de chlorure (ACMT; la technique électro-chimique est employée pour accélérer la migration des ions chlorure) les propriétés de transport du béton obtenues par l ACMT sont comparées au coefficient de diffusion de l essai de trempage. Du ciment simple des cendres volantes ainsi que des scories avec des rapports eau/liant différents ( et 0.65) ont été utilisés. Dans cette étude la teneur totale en chlorure et la profondeur de pénétration des ciments ont été mesurées après l essai de trempage et la seconde loi de diffusion de Fick correspond aux données tirées d expériences visant à déterminer le coefficient de diffusion. Le coefficient de diffusion de l état non stationnaire le coefficient de migration et le courant de charge passé correspondant à coulombs obtenu à partir de l ACMT d expériences précédentes ont été comparés avec les coefficients obtenus à partir de l essai de trempage. Des essais parallèles montrent que les coefficients obtenus à partir de l essai de trempage correspondent bien au coefficient de diffusion de l état non stationnaire le coefficient de migration et le courant de charge passé correspondant à coulombs après l ACMT bien que le coefficient de diffusion mesuré par l essai de trempage soit différent dans les états non stationnaire et stationnaire de celui mesuré par l ACMT. 1. INTRODUCTION Chloride ions are one of the major causes affecting the corrosion of rebar embedded in concrete. Therefore the resistance to chloride ion penetration into concrete is a crucial parameter affecting the durability of reinforced concrete structures. In order to improve concrete performance it is essential to have a reliable method for determining the transport properties in concrete. For the ponding test and the conventional diffusion method chloride ions require considerable time to penetrate the concrete specimen. Several accelerated chloride ions diffusion test methods by the application of an electrical field were developed to accelerate the movement of chloride ions. The mechanism of chloride penetration in the ponding test (a longterm test) and that in an accelerated migration test are different. Andrade et al. [1] applied an electrical potential in the ponding test and the diffusion coefficient was calculated from the chloride concentration in the penetration depth. Tang and Nilsson [2 3] introduced an accelerated non-steady-state migration test method and the penetration depth of chlorides was used to determine the chloride diffusivity. In electrical migration tests [4-8] the evolution of chloride concentration in the downstream cell is monitored and the change of chloride concentration with time allows the calculation of diffusion coefficients. In the ASTM C (Rapid Chloride Pene RILEM. All rights reserved. doi: /14140

2 314 C.C. Yang / Materials and Structures 38 (2005) tration Test; RCPT) [9] developed by Whiting [10] the current passing through the specimen in 6 hours is measured and the charge passed is used to rate the concrete penetrability. The 90-day salt ponding test is a long-term test for measuring the penetration of chloride into concrete. Patrick et al. [11] found that there is a poor correlation between the results of RCPT and ponding test when the standard tests were conducted and the ponnding data was analysed in the conventional manner. Truc et al. [12] presented a method for measuring the chloride diffusion coefficient from a steadystate test by measuring the drop in chloride concentration in the cathodic solution. Tong and Gjørv [13] found a correlation between the initial electrical conductivity and the chloride diffusivity from non-steady state. In order to avoid the sampling and analyzing chlorides during the migration test Castellote et al. [14] have established a linear relationship between the chloride concentration and conductivity in the anodic compartment from the migration test. Samson et al. [15] presented a multiionic model for measuring the chloride diffusion coefficient from the migration test based on current measurements. In this study the total chloride content and penetration depth of concretes were measured after the 90-day salt ponding test and the Fick s second law of diffusion was fitted to the data to determine the diffusion coefficient. Using the same mixtures as the present study the non-steady-state diffusion coefficient the migration coefficient and the current corresponding to the coulomb charge passed in the previous studies by using the accelerated chloride migration test (ACMT) method were compared and correlated with the diffusion coefficients obtained from ponding test. 2. EXPERIMENTAL PROGRAM 2.1 Materials and specimen preparation Concrete is a cement-based composite in which fine aggregate and coarse aggregate are embedded in a matrix of cementitious paste. In this study ASTM Type I Portland cement (specific gravity: 3.15) fly ash (specific gravity: 2.21) and slag (specific gravity: 2.83) were used as matrix. River sand was used as fine aggregate and crushed limestone with a maximum size of 10 mm was used as coarse aggregate. The concrete proportions are summarized in Table 1 three different binders and each binder with four w/b ratios ( and 0.65) were used. Twenty five percent of cement was replaced by fly ash in mix F series and by slag in mix S series respectively. For each mix a number of cylindrical specimens ( 100 x 200 mm) were cast and cured. After demolding the specimens were cured in water (23 C) for 1 year. A disc of 50 mm thick for the ponding test was cut from central portion of the cylinder. 2.2 Test procedures Two types of arrangements were used to find the transport properties for concrete. Salt ponding test is for the tests in non-steady-state conditions and the accelerated chloride Table 1 - Mix design Unit content: kg/m 3 Mix w/b Fine Coarse Water Cement Slag Fly ash agg. agg. C C C C S S S S F F F F migration test (ACMT) is for the tests in non-steady-state and steady-state conditions Salt ponding test In this study the ponding test similar to the test described in AASHTO T259 [16] was used. Before the test specimens were air-dried then the lateral surface of specimens was coated with epoxy that was allowed to harden for up to 24 h. The specimens were sealed around the outside edge using the acrylic ring to create the dam for chloride solution (Fig. 1a). The specimens with dams were subjected to continuous ponding with 3.0% NaCl solutions to a depth of 15 mm for 90 days. The top of acrylic ring was sealed with plastic wrap to minimize evaporation and additional solution was added if necessary to maintain the 15 mm depth. After 90 days of exposure the solution was removed and the specimens were allowed to dry. And then the surface of specimen was wire brushed until all salt crystal buildup was completely removed. After brushing three cylindrical specimens ( 20 x 50 mm) were obtained by coring from the ponding specimen ( 100 x 50 mm). Each cylindrical specimen was starting dry cut from the exposed to chloride surface into ten 5-mm thick slices. The concrete slices obtained were then dried at 105 C to constant mass and ground to pass a 300-m sieve. The powder samples were analyzed for total chloride content in accordance with AASHTO T [17]. Fig. 1 (a b) - Experimental arrangement used (a) in ponding test and (b) in ACMT.

3 C.C. Yang / Materials and Structures 38 (2005) After sieving a 3-g sample of the powder is weighted and digested using concentrated HNO 3 solution. Heat the acid solution to boiling on a hot plate and boil for about 1 min and then left to cool. The cooled samples were filtered through double filter paper (Whatman No. 41 over No. 40 filter paper) the residue being washed with boiling deionesed water. Filtrate and washings were made up to 125 ml with deionised water. And the sample of filtrate was analyzed using a Metrohm 792 Basic ion chromatograph Accelerated Chloride Migration Test (ACMT) In a previous study the accelerated chloride migration test (ACMT) was used and described in [18]. The concrete specimen (30 mm in thickness) obtained by sawing the midportion of cylindrical specimen and water-saturated under vacuum following the specification in ASTM C1202. A schematic presentation of the ACMT setup is illustrated in Fig. 1b. The specimen was placed between two acrylic cells. One of the cells was filled with 0.30 N NaOH solution and the other cell with 3.0% NaCl solution. Each solution volume in the cells is 4500 ml. The cells were connected to a 24 V DC power source in which the NaOH electrode becomes the anode and the NaCl electrode becomes the cathode. The current was measured and recorded at 5 minute intervals by a data logger and the chloride concentration in anode cell was measured periodically using a Metrohm 792 basic ion chromatograph. 3. RESULTS AND DISCUSSION Diffusion is the process by which matter is transported from one part of a system to another due to concentration gradient. The diffusion coefficient obtained from 90-day ponding test in this study and the non-steady-state diffusion coefficient migration coefficient and the current for a given charge passed in steady state were obtained from ACMT in the previous works. Comparison of the results of this work for the diffusion coefficient obtained from ponding test with the transport characteristic from ACMT is described follows. For ponding test Equation (2) and the following conditions are used to obtain the solution of Equation (1) where: initial condition: C = 0 x > 0 t = 0; boundary condition: C = 0 x = t 0; infinite-point condition: C = x = 0 t = 0. The instant plane source solution for an instantaneous limited supply of ions for Equation (1) is [19]: C 2 m exp x. D t 4 D pt p A typical profile after the salt ponding test and the curve of experimental results fitted by Equation (3) are shown in Fig. 2. The chloride content is expressed as a percentage of the dry mass of sample and the depth is the mid-point of each slice from the exposed to chloride surface. Fick s second law of diffusion was fitted to the data using Equation (3) to determine the diffusion coefficient. The total amount of diffusing substance (m) and diffusion coefficient (D p ) were both allowed to varying. The surface chloride content C s was obtained from Equation (3) as x = 0. The diffusion coefficients and chloride surface contents obtained from ponding test and Equation (3) for all specimens are listed in Table 2. The depth of penetration at 0.1% chloride content and the chloride content at 20 mm depth for the 90-day salt ponding data obtained from fitting by Equation (3) are also listed in Table 2. Amount of chloride near the surface (at about 5 mm) were detected in all specimens indicating that concrete is not able to keep the transport of chloride into the near-surface cover under the condition of the 90-day ponding test. It appears that the concrete diffusion coefficient from ponding test increases with (3) 3.1 Diffusion coefficient from ponding test In this study chloride ions are considered to penetrate the concrete sample solely by diffusion in the 90-day ponding test. Fick s second law of diffusion was fitted to the data using Equation (1) to determine the diffusion coefficient from ponding test as: 2 dc d C D (1) p 2 dt dx where C is concentration of ions as a function of distance x at any time t and D p is diffusion coefficient in this work. According to Climent et al. [19] assuming a semi-infinite medium and the total amount of diffusing substance m is initially deposited on the semi-infinite medium surface. The total amount of diffusing substance is constant and can be calculated as: m Cdx t 0. (2) 0 Fig. 2 - Typical chloride content profile (cires) after 90-day ponding test.

4 316 C.C. Yang / Materials and Structures 38 (2005) Mix Table 2 - Diffusion coefficient and results from ponding test Ponding diffusion coefficient (cm 2 /s; 10-8 ) Surface chloride content (%) Depth at chloride content 0.1% (cm) Chloride content at 20 mm (%) Specimen No. Specimen No. Specimen No. Specimen No C C C C S S S S F F F F The diffusion characteristic from ACMT In a previous study [20] chloride ions are transported in concrete under an applied voltage by using ACMT. The measured chloride concentration in anode cell is normalized by the original chloride concentration in anode cell and the typical result of normalized chloride concentration is plotted in Fig. 3 as a function of time. Fig. 3 shows that two stages exist non-steady state and steady state with respect to the change of the chloride concentration. The chloride ions are in the process of migrating through saturated pores in concrete and have not yet reached the anode cell in the non-steady state. In the steady state the flux of chloride ions passing through the concrete specimen becomes constant and the migration coefficient is calculated from the steady state portion of the curve Non-steady-state diffusion coefficient The non-steady-state diffusion coefficient (D n ) of the same mixture as the present study was measured using the ACMT method in the non-steady state and that can be calculated from the modified Fick s second law [3]: dc dt 2 d C D n 2 dx z FE RT dc dx where: D n is non-steady-state diffusion coefficient; z is the electrical charge of chloride F is the Faraday constant E is the strength of the electric field between the anode and cathode; R is the universal gas constant; T is absolute temperature. For ACMT the following conditions are used to obtain the solution of Equation (4) where: initial condition: C = 0 x > 0 t = 0; boundary condition: C = C o x = 0 t > 0; infinite-point condition: C = 0 x = infinite t = large number. The analytical solution for Equation (4) is: (4) Fig. 3 - The typical results of normalized chloride concentration in anode cell as a function of time in ACMT. increasing w/b ratio for all series because more pores and diffusing paths may form as the w/b ratio increase. The diffusion coefficient obtained from ponding test for C series are higher than those of other two series (S and F) with the same w/b ratios. Concrete containing mineral admixtures (fly ash and slag) has a significant influence on both chloride content and penetration depth. At 90-day exposure time the concretes with slag and fly ash (S and F series) result generally in lower chloride contents and reduced penetration depths. The mineral admixture can improve the distribution of pore size and pore shape of concrete since more C-S-H gel is formed when fly ash and slag concrete hydrate. C C o x ad t x ad t (5) Dnt Dnt ax n n x t e erfc erfc z FE where a C o is chloride concentration in cathode RT cell and erfc is complementary error function. From Equation (5) when the electrical field is large enough and the penetration depth is sufficient the nonsteady-state diffusion coefficient can be calculated as [3]: 1 x x D (6) n a t 1 where 1 2C 2 erf 1 1 and erf is the inverse of a C o error function. In ACMT since the chloride concentration was

5 C.C. Yang / Materials and Structures 38 (2005) not continually monitored a value of C/C o = was used to obtain the time-span t (in Fig. 3) for chloride ion penetration through specimen [21]. Using the same mixtures as the present study the non-steady-state diffusion coefficient (D n ) obtained from ACMT in a previous study [20] is calculated from Equation (6) and listed in Table 3. In Fig. 4 the values of diffusion coefficient (D p ) obtained from ponding test are graphically correlated with the nonsteady-state diffusion coefficient (D n ) from ACMT for all mixes. The cire points represent the average of three test data for all mixes and the linear regression is carried out. By linear regression the empirical relationship between D p and D n is statistically derived as 8 D 0.46D (7) p n where D p and D n are in cm 2 /s. By linear regression the regression coefficient R 2 is and it appears that D p correlates linearly with D n regardless of concrete mixes Migration coefficient from steady state As shown in Fig. 3 linear regression is carried out for the steady state portion of the ACMT to obtain the chloride migration rate K which is the slope of the chloride concentration obtained in anode cell vs. time curve. In order to calculate the migration coefficient of chloride ions for concrete under ACMT test the flux (J ) is calculated from the chloride migration rate as J V C V (8) A t A K where V is the volume of solution in anode cell A is the crosssectional area of specimen. Since the concrete is saturated the velocity of solute can be neglected and under the influence of an electrical field across the sample the contribution of diffusion in concrete is small and can be neglected [22]. The migration coefficient of chloride ions for concrete M is calculated on the basis of the Nernst-Planck equation [22] where the convection and diffusion terms are neglected. RT M J (9) z C FE 0 where C o is chloride concentration in the upstream cell at the cathode. Using the same mixtures as the present study Table 3 - Non-steady state diffusion coefficient migration coefficient and the current corresponding to the coulomb charge passed from ACMT Mix * Non-steady state diffusion coefficient (cm 2 /s; 10-8 ) * Migration coefficient (cm 2 /s; 10-8 ) * Current at coulomb (ma) C C C C S S S S F F F F Fig. 4 - The relationship between diffusion coefficient from ponding test and non-steady diffusion coefficient from ACMT. Fig. 5 - The relationship between diffusion coefficient from ponding test and migration coefficient from ACMT.

6 318 C.C. Yang / Materials and Structures 38 (2005) the migration coefficient of concrete (M ) in a previous study [23] is calculated and listed in Table 3. Fig. 5 shows the relationship between diffusion coefficient (D p ) obtained from ponding test and migration coefficient (M ) from ACMT for all mixes and the corresponding regression results are also shown in this figure. By linear regression the empirical relationship between M and D p is statistically derived as 8 D 2.63M (10) p where M and D p are in cm 2 /s. It can be seen in Fig. 5 a linear relationship exists between D p and M with a regression coefficient R 2 of Current for a given charge passed The transference number of an ion in a given electrolyte solution is the fraction of the total electrical current carried in the solution by that ion [15 22]. Every specie in the electrolyte has a transference number t i which must always be positive and between zero and one. According to the definition the chloride transference number t can be calculated by the following equation as dq I dt t (11) I I where I is the electrical current carried in the solution by chloride ions and Q is the charge passed carried by chloride ions. According to Faraday s results the charge passed carried by chloride ions (Q ) is determined by the mole number of chloride that pass through the circuit and from Equation (11) the chloride transference number is rewritten: dn z F dt t (12) I where n is the mole number of chloride in the anode cell in mole. For a given charge passed (Q s ) the slope of Q-time curve at Q s is the current (I s ) corresponding to the given charge passed (Q s ). By definition I s dq (13) dt t t Q where t Q is the time corresponding to the given charge passed (Q s ). From Equations (12) and (13) when chloride flux reached steady state at a given charge passed the chloride migration rate K is written: dc dn t I s (14) dt Vdt z FV and t K I s. (15) z FV Equation (15) shows that at a given charge passed in steady state there is a relationship between the chloride Fig. 6 - The relationship between diffusion coefficient from ponding test and current corresponding to charge passed coulomb from ACMT. migration rate and the current corresponding to the given charge passed. In a previous study [23] the current corresponding to the coulomb charge passed (I 30kQ ) of the same mixture as the present study was measured using the ACMT under steady-state condition. The results for all mixtures show that the chloride migration rate (K ) and the I 30kQ in steady state are linearly correlated. Fig. 6 illustrates the relationship between the diffusion coefficient (D p ) obtained from ponding test and the current corresponding to when the charge passed is coulombs. By linear regression the empirical relationship between the diffusion coefficient (D p ) obtained from ponding test and the current corresponding to the coulombs charge passed is statistically derived as 8 D 0.099I (16) p 30kQ where D p is in cm 2 /s and I 30kQ is the current corresponding to the coulomb charge passed in ma. By linear regression the correlation coefficient R 2 for the model [Equation (16)] is It appears that I 30kQ correlates linearly with D p regardless of concrete mixes. From Equations (7) (10) and (16) D n M and I 30kQ obtained from ACMT correlate linearly with D p from ponding test. Using D n or M to predict the D p need to measure the chloride concentration during the ACMT. Instead of measuring the chloride concentration during ACMT measurements of current and using Equation (16) provide a less expensive and easy way to obtain the diffusion coefficient (D p ) obtained from ponding test.

7 C.C. Yang / Materials and Structures 38 (2005) CONCLUSIONS The total chloride content and penetration depth of concretes were measured after the ponding test. The Fick s second law of diffusion was fitted to the data from experiment to determine the diffusion coefficient. The conusions derived from the experimental investigation and regression analysis are presented below. 1. The w/b ratio and the constituent materials of matrix influence the chloride diffusion coefficient of concrete. The diffusion coefficient obtained from ponding test increases with increasing w/b ratio. For concrete containing different type of mineral admixtures (fly ash and slag) the diffusion coefficient of fly ash concrete or slag concrete significant decreases. 2. The chloride diffusion coefficients obtained from ponding test and the non-steady-state diffusion coefficients from ACMT are linearly correlated regardless of concrete mixes. The non-steady-state diffusion coefficient obtained from ACMT in nonsteady-state condition is higher than the diffusion coefficient measured by ponding test. 3. The chloride diffusion coefficients obtained from ponding test and the migration coefficients from ACMT are linearly correlated. The diffusion coefficient measured by ponding test is higher than the migration coefficient obtained from ACMT in steady-state condition. 4. The good experimental correlation between the diffusion coefficient measured by ponding test and the current corresponding to the coulomb charge passed obtained from ACMT in steady-state condition was investigated. The measurements of current provide a less expensive and easy way to obtain the transport property of concrete. This study shows that the diffusion coefficients obtained from ponding test linearly correspond well with the nonsteady-state diffusion coefficient and the migration coefficient obtained from ACMT but the diffusion coefficient measured by ponding test is different from that measured by the ACMT in non-steady state and steady state. The further study is needed to find their reasons. ACKNOWLEDGEMENTS The financial support of National Science Council ROC under the grants NSC E is gratefully appreciated. REFERENCES [1] Andrade C. Sanjuan M.A. Recuero A and Rio O. Calculation of chloride diffusivity in concrete from migration experiments in non steady-state conditions Cement and Concrete Research 24 (7) (1994) [2] Tang L. and Nilsson L. Rapid determination of the chloride diffusivity in concrete by applying an electrical field ACI Materials Journal 89 (1) (1992) [3] Tang L. 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8 320 C.C. Yang / Materials and Structures 38 (2005) relationship to sand content and critical pore diameter Cement and Concrete Research 25 (4) (1995) [22] Andrade C. Calculation of chloride diffusion coefficients in concrete from ionic migration measurements Cement and Concrete Research 23 (3) (1993) [23] Yang C.C. and Cho S.W. The relationship between chloride migration rate for concrete and electrical current in steady state using the accelerated chloride migration test Mater Struct. 37 (271) (2004).