TIME DEPENDENCY OF CHLORIDE MIGRATION COEFFICIENT IN SELF-COMPACTING CONCRETE

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1 TIME DEPENDENCY OF CHLORIDE MIGRATION COEFFICIENT IN SELF-COMPACTING CONCRETE Katrien Audenaert and Geert De Schutter Magnel Laboratory for Concrete Research, Department of Structural Engineering, Ghent University, Belgium Abstract The chloride diffusion coefficient of concrete is strongly decreasing in time. Neglecting this aspect in the modelling of the diffusion process ill overestimate the chloride penetration depth and thus lead to an uneconomic design. In order to get an accurate insight in this decreasing process, 16 self-compacting concrete mixes ere made. Four types of cement (portland cement and blast furnace slag cement), three types of filler (fly ash and to types of limestone filler ith a different grading curve) and to types of coarse aggregate are used and the influence of the amount of poder and the amount of ater is studied. The chloride migration coefficient as experimentally determined ith the non-steady state migration test, developed by Tang et al. and described in NT BUILD 492, and making use of an electrical field. This migration coefficient as determined at different concrete ages, up to 5 year. From the migration tests, the diffusion coefficients ere deduced. The decrease in time of the diffusion coefficients is studied and modelled. Both the experimental and modelling results ill be presented and discussed in the paper. 1. INTRODUCTION In a marine environment, chloride penetration into concrete is the determining factor for the service life of concrete structures. This implies that a good insight in the chloride penetrating process is of utmost importance. In saturated conditions, diffusion is the primary penetration process of chlorides into concrete. In this process, the chloride ions diffuse in presence of a chloride concentration gradient, hich is created hen at least one face is continuously exposed to ater and salt. This process is described by Fick s second la, hich is ritten as follos for one dimensional problems: C t C = D x x (1) here C is the total chloride content, t time and D the diffusion coefficient. If the folloing boundary conditions are considered: a single spatial dimension x, ranging from 0 to for the semi infinite case, C = C 0 at x = 0 and t > 0 (boundary condition), 325

2 C = 0 at x >0 and t = 0 (initial condition). and under the assumptions of 1) homogenous concrete, 2) constant chloride concentration at the exposure surface 3) constant diffusion coefficient 4) linear chloride binding (the line has to pass through the origin) and 5) constant effect of co-existing ions, the analytical solution of the equation (1) has the form: C = C 0 1 erf 2 x Dt in hich C 0 is the surface concentration, and erf( ) represents the error function. From equation (2) follos that the diffusion process of chloride depends on to factors: 1) The intrinsic permeability of the concrete, hich is changing during the process of cement hydration ith time and 2) the surface chloride concentration, influencing the chloride concentration level in the pore solution, hich is also changing due to the continuous chemical reaction of chlorides ith the cement hydration products. On the other hand, the variation of the pore structure depends on the W/C ratio, degree of hydration, cement type, etc. and is also changing ith time at various locations. As a result, both the chloride ion concentration and diffusivity are time and space variables. This paper deals ith one of these dependencies, more specifically the time dependency of the diffusion coefficient created by the ongoing hydration of the concrete. For the experimental determination of this time-dependency, non-steady state migration tests, folloing NT BUILD 492 [1] ere carried out on 16 self-compacting concrete compositions. Due to the short duration of these tests, approximately 24 hours, they determine the instantaneous migration coefficient. The tested concrete ages vary beteen 28 days and 5 years. 2. MATHEMATICAL EXPRESSIONS As concrete matures, additional hydration occurs hich serves to reduce the diffusion coefficient. The time dependency of diffusion through concrete has been observed to be an exponential function [2, 3, 4]: m t ref ( ) = D (3) ref D t t Where D(t) is the diffusion coefficient at time t, D ref the diffusion coefficient at some reference time t ref and m a constant, normally referred to as the age factor hich is depending on the concrete composition. Some other relations are also given in literature [5, 6, 7], but ill not be considered in this paper. Hoever, this equation could not be inserted in equation (2) because that one is derived based on the assumption of a constant diffusion coefficient. Hoever, in many researches this method is applied, leading to large errors as discussed in [8]. In literature, some empirical relationships are described for the age factor m. Mostly they are depending on the W/C ratio [2, 9, 10], undoubtedly a very important parameter. Surprisingly, the influence of the W/C ratio is different: in the relationships of [2] and [10] an increasing W/C ratio leads to a higher age factor. In [9], a higher W/C ratio leads to a loer m. This is illustrated in Fig. 1. (2) 326

3 1.4 m (-) Mangat et al. Tang et al. Frederiksen et al W/C (-) Figure 1 : Age factor in function of W/C ratio 3. EXPERIMENTAL PROGRAM During 5 years, chloride migration tests ere carried out at different ages on concrete specimens of 16 self-compacting concrete mixes and 4 traditional concrete mixes. With a chloride migration test (NT BUILD 492), an instantaneous value for the chloride migration coefficient is obtained. With the migration coefficients on different concrete ages, a value of the age factor for each concrete mix as determined. 3.1 Concrete mix design At the Magnel Laboratory for Concrete Research, 16 self-compacting concrete mixes (SCC). In the first 9 mixtures a constant amount of poder materials (cement and filler) is considered: 600kg/m³, as ell as a constant amount of ater, sand and gravel, respectively 165k/m³, 853 kg/m³ and 698 kg/m³. Four types of cement are used (Portland cement CEM I 42.5 R, CEM I 52.5, CEM I 52.5 HSR and blast furnace slag cement CEM III A 42.5 LA), three types of filler (fly ash and to types of limestone filler BETOCARB P2 and Superfine S, the last one having a finer grading). In the next three mixes, the amount of poder is varied (500 kg/m³, 700 kg/m³ and 800 kg/m³). In the folloing three mixes, the amount of ater is varied (144 kg/m³, 198 kg/m³ and 216 kg/m³). In SCC16 crushed limestone gravel as used instead of river gravel. The amount of superplasticizer as determined in order to obtain a suitable floability ithout segregation. Also the floing time in the V-funnel as measured (values beteen 5s and 10s), air content (values beteen 1% and 3%) and the U-box requiring self levelling. In Table 1 the mix composition is given together ith the compressive strength at 28 days measured on concrete cubes ith side 150 mm. From the mixes described above, cubes 150 x 150 x 150 mm³ ere made and ere stored in a climate room at 20 C ± 2 C and more than 90 % R.H. At an age of 21 days, three cores ith a diameter of 100 mm and a height of 50 mm ere drilled from each cube. 327

4 Table 1: Mix composition CEM I 42.5 R [kg/m³] CEM I 52.5 [kg/m³] CEM III/A 42.5 LA [kg/m³] CEM I 52.5 HSR [kg/m³] limestone filler S [kg/m³] limestone filler P2 [kg/m³] fly ash [kg/m³] ater [kg/m³] sand 0/5 [kg/m³] gravel 4/14 [kg/m³] Limestone gravel 2/14 [kg/m³] W/C [-] C/P [-] compressive strength [MPa] SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC SCC Test method As described in section concrete mix design, cores ith a diameter of 100 mm and a height of 50mm ere drilled from each cube. Afterards the concrete cores ere placed back in the fog room until the testing date. On these cores a non-steady state migration test as performed folloing the method of Tang et al.[11] as described in NT BUILD 492 and shon in Fig. 2. The correspondence of this method ith the natural ingress of chlorides by diffusion as validated in previous research carried out at the Magnel Laboratory for Concrete Research, Ghent University [12, 13, 14]. Firstly the specimens are vacuum saturated ith a saturated Ca(OH) 2 solution. Afterards, an external electrical potential (for the tests described in this paper beteen 25 V and 40 V) that forces the chloride ions from the 10% NaCl solution (catholyte) to migrate into the specimens, is applied across the specimen for a limited duration. The test duration as 24 hours, as prescribed by NT BUILD 492. Three specimens ere tested simultaneously. After the test, the specimens are axially split. On the freshly split sections, a 0.1 M AgNO 3 solution is sprayed and the chloride penetration depth is measured on each part at 7 points from the visible hite silver chloride precipitation. This colorimetric method is described in [15]. For each concrete core, to penetration profiles are obtained. In this ay, 6 penetration profiles are obtained for each composition and concrete age. 328

5 Figure 2: Test method developed by Tang [12] From the mean penetration depth, the non-steady state chloride migration coefficient D nssm can be calculated, as described in NT BUILD 492, ith: ith: D nssm RT x α x = (4) zfe t U 2 E = and L α 2 RT C erf zfe Co = D nssm : non steady state migration coefficient, m²/s z : absolute value of ion valence, for chloride: z = 1 F : Faraday constant, F = 9.648x104 J/(V.mol) U : absolute value of the applied voltage, V R : gas constant, R = J/(K.mol) T : average value of the initial and final temperatures in the anolyte solution, K L : thickness of the specimen, m x : average value of the penetration depths, m t : test duration, s C d : chloride concentration at hich the colour changes, c d = 0.07 N for Portland cement concrete C o : chloride concentration in the catholyte solution, C o = 2N 3.3 Test results Tests ere performed at the ages of 28, 56, 90 days, 1, 2 and 5 years. Based on these experimental results, age factors ere derived ith a regression analysis. In Fig. 3, the experimental results of SCC 16 are given. In Table 2 the values for D 1 (non-steady state migration coefficient at 1 year concrete age) and age factor m are given, together ith the correlation coefficient R² for all concrete mixes. d 329

6 12 D (10-12 m²/s) y = 3,9881x -0,3867 R 2 = 0, Time (year) Figure 3: Non-steady state migration coefficient in function of time (SCC16) Table 2: Experimental D 1, m and R² D 1 [10-12 m²/s] m [-] R² [-] SCC1 5,55 0,28 0,89 SCC2 3,31 0,35 0,97 SCC3 1,66 0,43 0,93 SCC4 6,59 0,17 0,75 SCC5 8,39 0,22 0,94 SCC6 5,38 0,19 0,95 SCC7 3,09 0,34 0,93 SCC8 4,95 0,37 0,95 SCC9 0,15 1,38 0,97 SCC10 5,09 0,38 0,90 SCC11 6,11 0,30 0,94 SCC12 7,42 0,22 0,98 SCC13 3,65 0,40 0,90 SCC14 9,16 0,22 0,94 SCC15 13,52 0,22 0,96 SCC16 3,99 0,39 0, Discussion By studying the variation of the cement type of SCC, CEM III/A leads to the loest migration coefficient. The cement type has also a large influence on the age factor, hich is in correspondence ith literature [4]. By changing the amount of cement for a constant amount of ater, the migration coefficient is decreasing for an increasing amount of cement (SCC5 SCC1 SCC6 SCC7), 330

7 hich is explained by a decreasing porosity. Hoever, for the age factor this relationship is not clear. By changing the total amount of poder P (cement + filler), keeping the W/C ratio and the C/P ratio constant (SCC10 SCC1 SCC11 SCC12), the migration coefficient is increasing and the age factor decreasing ith an increasing amount of poder, an increasing amount of cement and an increasing amount of ater. But for a constant W/C ratio. This leads to the conclusion that for self-compacting concrete, the W/C ratio is certainly not the only parameter to describe the migration of chlorides. By keeping the amount of cement constant, together ith a constant amount of poder but ith a changing amount of ater (SCC13 SCC1 SCC14 SCC15), the migration coefficient is increasing and the age factor is decreasing for an increasing amount of ater and an increasing W/C ratio. From this conclusion a decreasing m for an increasing W/C ratio the same result is obtained as predicted by the relationship of Tang illustrated in Fig. 1. Hoever if the mixes SCC5 SCC1 SCC6 SCC7 are studied, this is not clear. This could be explained by the fact that an increasing amount of cement leads also to other changes in the concrete: more paste, other pore structure of the paste, etc. These to conclusions could be combined to one clear conclusion if the capillary porosity is used as parameter instead of the W/C ratio, as ill be illustrated in the next section. Three other aspects are also studied. Firstly, by changing the limestone filler by an other limestone filler ith a finer grading curve (SCC8 vs. SCC1), the migration coefficient is decreasing and the age factor increasing. Secondly, the use of fly ash as studied in one mix (SCC 9). The migration coefficient is very lo and the age factor very high. This illustrates the very different behaviour of concrete ith fly ash. In SCC16, broken limestone aggregate as used instead of river gravel. The rest of the composition is identical as SCC1. This leads to a stronger contact beteen the gravel and the cement matrix, hich leads to a loer migration coefficient and a higher age factor. 3.5 Influence of capillary porosity on migration coefficient and age factor In this paper the capillary porosity ill be calculated based on the model of Poers [12, 16, 17]: V cap = capillary pores + free ater = capillary pores + ater gel ater bounded ater C α W 0.28 C α 0.23C α 0.23C α = ( ) c c (5) C α 1 = ( W C α ) c W αc = ith V cap the volume of capillary pores [m³], C the amount of cement [kg], W the amount of ater [kg], α the degree of hydration [-], c and the mass density of respectively cement and ater [kg/m³]. 331

8 V concrete = V W = ater + V C + c cement + V A + S + F + agg coarse aggregate + V sand + V filler (6) Vcap capillary porosity = ϕ cap = (7) V concrete ith V concrete the volume [m³], A the amount of coarse aggregate [kg], S the amount of sand [kg], F the amount of filler [kg] and agg the mass density of aggregate [kg/m³]. The parameters W, C, A, S and F are knon from the mix proportions. For the mass densities, a value of 1000 kg/m³ is used for ater, 2625 kg/m³ for the aggregates, sand and filler and 3115 kg/m³ for portland cement. In order to plot D 1 (non-steady state migration coefficient at the age of 1 year) and the age factor m as a function of the capillary porosity, the ultimate degree of hydration is used to calculate φ cap. This ultimate degree of hydration is calculated by Mill s formula (8) [18]: W h C ultim = W C In figure 4 and 5, respectively D 1 and m are plotted in function of the capillary porosity. The regression lines ere calculated based on the results of the pure Portland cement concretes (thus excluding SCC3, SCC4 and SCC9), but all mixes are given in the figures. From these figures, it is clear that the non-steady state migration coefficient is increasing ith an increasing capillary porosity. The age factor shos a decreasing tendency if the capillary porosity increases. Hoever, the age factor is less influenced by the concrete composition than the migration coefficient itself. For Fig. 4, the equation of the regression line, ith a correlation coefficient of 0,84, is given by: D 1 = 1,4502 φ cap 5,1934 (9) (8) D1 (10-12 m²/s) φ cap (%) Figure 4: Migration coefficient at 1 year (D 1 ) in function of capillary porosity (φ cap ) 332

9 In Fig. 5, the equation describing the relation beteen the age factor m and the capillary porosity φ cap has a correlation coefficient of 0,76 and is ritten as: m = -0,0318 φ cap +0,5544 (10) m (-) φ cap (%) Figure 5: Age factor (m) in function of capillary porosity (φ cap ) 4. CONCLUSIONS By studying the time dependency of the chloride migration coefficient of 16 self-compacting concrete mixes, the folloing conclusions are made: - Equation 3, generally used to describe the time dependency of the chloride diffusion coefficient, can also be used to describe the time dependency of non-steady state migration coefficients, determined ith NT BUILD In literature, it is not clear hether the age factor is decreasing or increasing if the W/C ratio is increasing. In this research project, an increasing W/C ratio leads to a decreasing age factor. - With the experimental part of the paper, it is illustrated that the non-steady state migration coefficient and the age factor are depending on many factors in the concrete composition such as the amount and type of cement, amount and type of filler, amount of ater. In this paper, these factors ere combined in one parameter: the capillary porosity. This parameter seemed a good tool to describe the migration coefficient and the age factor. If the capillary porosity is increasing, the migration coefficient is increasing and the age factor decreasing. ACKNOWLEDGEMENTS The financial support for the post-doctoral project of the FWO-Flanders is greatly acknoledged. 333

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