Study on chloride binding of self-compacting concrete with different crack lengths: non-steady state migration test

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1 Study on chloride binding of self-compacting concrete with different crack lengths: non-steady state migration test S. Mu 1,2, G. De Schutter 2, and B. G. Ma 1 1 Wuhan University of Technology, Wuhan, People s Republic of China 2 Ghent University, Ghent,Belgium ABSTRACT: Concrete cracks, caused by mechanical loading, thermal stresses, shrinkage etc., can provide some shortcuts for aggressive substances to reach the surface of rebars and initiate corrosion. The present paper investigates the chloride binding of self-compacting concrete with different crack lengths using non steady state migration test. The obtained experimental results are further analysed. For the crack length, we adopted a non-destructive notch method to prepare different crack lengths from 20 mm to 100 mm, at a crack width of 0.2 mm and a crack depth of 20 mm. The chloride binding properties are characterized by experimental determination of the concentrations of water soluble chloride and acid soluble chloride. The relation between water soluble chloride and acid soluble chloride is further investigated. The results show that the non-destructive notch method is a good way to quantitatively study the influence of crack on concrete property. Secondly, Freundlich and Langmuir binding isotherms were better than the linear binding isotherms to match results in cracked concrete. Thirdly, the influence of crack length on chloride binding of concrete is not significant in this present research. 1 GENERAL INSTRUCTIONS Cracked concrete exposed to chloride salt environments has different mechanisms for chloride transport and binding. Most of literatures about this topic focused on the influence of crack width on chloride transport. Ismail et al (2008) studied the effect of crack opening on chloride transport in cracked mortar, and the result shows that the diffusion rate of concrete is depending on crack width. Audenaert et al (2009) reported that the penetration depth increased with increasing crack width from 0 to 0.1mm, but this increase is less clear and seems to be independent from the crack width when it is increased from 0.1mm to 0.2mm. Djerbi et al (2008) reported a bilinear relation between the crack width at the concrete surface, and the migration coefficient as determined in a steady state migration test. Win et al (2004) found the penetration depth around the crack and the penetration depth of the exposed face almost remained constant with increasing crack width from 100 µm to 500 µm. However, the penetration depth around a crack is slightly higher than that from the exposed surface. Castellotea et al (1999) indicated that almost no chloride binding occurred when chloride concentration in the pore solution was below 28 g/l. When the chloride external concentration is higher than 1 M in Cl, a similar phenomenon of chloride binding was observed both in diffusion test and migration test. As far as chloride binding is concerned, only few literature reports are available on the influence of cracks on chloride binding under non-steady state migration. Besides, self-compacting concrete (SCC) is widely used in construction because of the good flowability and the other

2 properties. Therefore, it is interesting to investigate the chloride binding of SCC with different crack lengths. 2 EXPERIMENT 2.1 Concrete composition One SCC mix (M1) was used for the tests: 360 kg/m³ CEM I 52.5 N, 240 kg/m³ filler calcite (Carmeuse MS), 853 kg/m³ river sand 0/4, 698 kg/m³ river gravel 2/8 and 165 kg/m³ of water. For the mix, the slump flow,v-funnel, L-box, sieve stability, air content and density were measured in fresh condition. Concrete cubes with a side length of 150mm for determination of the compressive strength, and concrete cylinders with a diameter of 100mm and a height of 50mm for non-steady state migration test, were manufactured separately. The concrete specimens were stored in a climate room at 20 C ± 2 C and more than 90 % R.H. until the testing age (approximately 60days). At the age of 28 days, the compressive strength was determined. At the testing age, chloride concentration, dry density and porosity of the hardened concrete were determined separately on the concrete cylinders separately after non-steady state migration test. 2.2 Testing methods Crack preparation and characterization A non-destructive notch method was adopted to prepare cracks by means of the positioning and removal after approximately 4 hours of thin copper sheets inside the specimen. The concrete samples with a diameter of 100 mm and a thickness of 50 mm were prepared by casting SCC into moulds directly. Afterwards, these copper sheets with a thickness of 0.2 mm but different lengths from 20 mm to 100 mm, were placed at a depth in concrete of 20 mm. After approximately 4 hours, the copper sheets were removed carefully before concrete totally hardened. After migration test mentioned below, all concrete specimens were sawn into two halves along the direction perpendicular to crack. Then a scanner was used to acquire the image of the cut surface, so that the crack pattern in concrete can be observed and analyzed. Finally, image analysis was conducted on the cut surface to measure crack width and depth. The average crack width was determined based on measuring 30 points with a uniform distribution along the crack Migration test A non-steady state migration test (CTH)-NT BUILD was carried out on the obtained concrete specimens. Prior to the migration test, the specimens were vacuum saturated with a saturated Ca(OH) 2 solution. Then an external electrical potential of 30 volts was applied on the specimens for 24 hours that forced the chloride ions from the 10% NaCl solution to migrate into the specimens (the exposed face is casting surface with crack). For each group of concrete, chloride profiles including water soluble chloride and acid soluble chloride were determined on three concrete specimens, which were the same specimens as used for the crack characterization. The mean value of three specimens was calculated Water soluble chloride After migration test and crack characterization, all concrete specimens were ground into powder layer by layer (Approximately 2 mm thickness per layer). The powder was passed through 160 μm sieve, and then was dried at 105 C to constant mass and cooled down to room temperature. Subsequently, 2.5 g powder and 50 ml de-ionised water were used to prepare a solution. This solution was stirred for 2 minutes, sealed and solved for 24 hours in a room at 20 C. Finally, 5 ml solution is extracted to determine the water soluble chloride content by titration.

3 The titration was conducted by means of a Metrohm MET 702 automatic titrator, and using 0.01 mol/l silver nitrate solution as titration solution (the same method as below). The water soluble chloride content can be calculated as: V C AgNO C Where C is the water soluble chloride content by the mass of sample, %; V is the consumed volume of silver nitrate solution, ml; CAgNO 3 is the concentration of silver nitrate solution after calibration, mol/l. For the good of understanding, the mass percentage unit of concentration for water soluble chloride can be transformed into molar concentration unit, mol/l: 1000 C C Where C is the water soluble chloride content by molar concentration, mol/l; ω is the water content of concrete by mass of sample Acid soluble chloride Using 5 ml of 0.3 mol/l nitric acid, 40 ml de-ionised water and 2.0 g the powder mentioned above, a solution was prepared to determine the acid soluble chloride content. This solution was stirred for 2 minutes, and then heated until boiling. After cooling down, the solution was pipetted into 100 ml flask with deionised water. At last, 10 ml of the filtered solution was pipetted for determination of the concentration of chloride. The acid soluble chloride can be calculated as: V C Ct AgNO3 Where C t is the acid soluble chloride content by the mass of sample, %; V is the consumed volume of silver nitrate solution, ml; CAgNO 3 is the concentration of silver nitrate solution after calibration, mol/l. (1) (2) (3) 3 RESULTS 3.1 Basic properties of concrete The basic properties, including flowability, mechanical and other properties, were determined on the SCC at different ages. The results of the basic properties are given in Table 1. Table 1. Basic properties of self-compacting concrete Flowablity Mechanical property The other properties Slump flow V- funnel (s) Sieve segregation L- box 28 days compressive strength (MPa) Air content Density (kg m -3 ) 60 days Dry density (kg m -3 ) 60 days Porosity

4 3.2 Crack characterization The crack lengths varied from 20 mm to 100 mm. In order to characterize these cracks quantitatively, the concrete specimens were sawn into two halves along the direction perpendicular to crack. In this present research, all six samples have same concrete mix M1, but different crack legnths from 20 mm to 100 mm. So, we use M1C6,M1C7,M1C2,M1C8 and M1C9 to stand for the samples with different cracks but M1means the sample without crack. The cut surfaces are shown in Fig.1 Calibration was made before scanning, and image analysis technology was applied on the concrete specimen to measure depth and width of the crack. Table 2 shows the results for different cracked concrete specimens. The variation coefficient of crack widths was acceptable for all concrete specimens, except for M1C8 sample. For the crack depth, the variation coefficients are lower than 10%. (a) M1C6 (b) M1C7 (c) M1C2 (d) M1C8 (e) M1C9 Figure 1. Cutting surface of cracked concrete Table 2. Results for crack width and depth Sample Mean Width SD a VC b Mean Depth SD VC Length M1C M1C M2C M1C M1C Note: a SD is the standard deviation b VC is the variation coefficient 3.3 Chloride binding isotherm Fig.2 shows that different fitting curves of chloride binding isotherm. The general trend was that the concentration of bound chloride increases with increasing water soluble chloride content. For the reference concrete M1, the correlation coefficient is about 0.9 for the fitting curves based on isotherms of linear binding, Langmuir binding and Freundlich binding (Fig.2 a). For the cracked concrete, the fitting curves of M1C2 and M1C6-9 (Fig.2 b and c-f) obtained a correlation coefficient of above 0.9 too. Considering the good correlation coefficient, Freundlich and Langmuir binding isotherms were better than linear binding isotherms to match results presented in this research. When the

5 correlation coefficient is higher than 0.9, the difference of fitting curves between Freundlich and Langmuir binding isotherms was almost neglected. All the fitting curves of Langmuir binding isotherm for the different concretes are shown in Fig.3. From this figure, it seems that M1C6 and M1C7 have higher bound chloride content than M1, M1C2, M1C8 and M1C9. Fig.4 a shows a comparison of the 95 % confidence band for M1 and M1 C6.The 95 % confidence band of M1C6 is higher than that of M1. In Fig.4 b, there is also a similar phenomenon about the 95 % confidence band for M1 and M1C7. So, the result shows that M1C6 and M1C7 have more significant chloride binding than M1. (a) M1 (b) M1C2 (c) M1C6 (d) M1C7 (e) M1C8 Figure 2. Isotherms of chloride binding in self-compacting concrete (f) M1C9

6 Figure 3. Langmuir isotherms of chloride binding in self-compacting concrete (a) M1 and M1C6 Figure 4. 95% of confidence band for M1, M1C6 and M1C7 (b) M1 and M1C7 4 DISCUSSION It is necessary to characterize the cracks when studying the properties of cracked concrete. From Table 2, we observed that a high variation coefficient is observed for crack widths in M1C8, even when regular cracks are prepared by a non-destructive notch method. This fluctuation of the crack width should have a close relation with the operation to take out the copper sheet from concrete. Compared with crack width, crack depth has a lower fluctuation. So, the nondestructive notch method is a good way to quantitatively study the influence of cracks on concrete properties. For chloride binding, the following discussion could be made: Firstly, the deviation of data points in M1C6 should be caused by some heterogeneity of the concrete. Actually, the fluctuation of data points for chloride binding isotherms was also mentioned in Castellotea et al (1999). Secondly, the influence of cracks on chloride binding of concrete is complicated under nonsteady state migration test. As we know, the driving force of the migration test is the electrical potential, and it is different from the driving force of a diffusion test. For CTH test, we assumed concrete located between cathode and anode to be homogenous, so the electric field applied on the concrete would be uniform. Based on the definitions of electric field, uniform electric field and current:

7 F E Q (4) U E d (5) Q I t (6) It can be derived I t U F (7) d Where E = electric field, F = electric force, Q = charge, U = voltage, d = distance between cathode and anode, I = current, t = time. In the present research, t, U and d are constant, so F is dependent on current I. In other word, the higher current value, the higher electric force. The higher electric force reduces the chloride ion s opportunity to contact cement matrix so that it is difficult for concrete to bind chloride by chemical adsorption, and maybe it is the reason why migration test has the lower bound chloride content than diffusion test in Castellotea et al (1999). However, the chloride binding is not only depending on electric force but also the surface concentration of chloride ions under non-steady state migration. Although the total chloride concentration of the migration test is constant, the surface concentration of chloride could be different for different testing samples. For example, table 3 gives us some information about current during CTH test: the initial and ultimate currents were increased with increasing crack length, which means the electric force was increased too. From fig.3, we can know that M1C6 and M1C7 have a higher concentration of bound chloride than M1C2, M1C8-9. Based on this explanation, M1 should have a higher chloride binding than M1C6-9 and M1C2, but the truth is that M1C6-7 had a significant chloride binding when compared with M1. The reason could be that M1 had a low surface concentration of chloride than M1C6 and M1C7. Yuan (2009) mentioned that a higher concentration of external chloride resulted in a high level of chloride binding. The further research of chloride binding in cracked concrete using CTH test will be conducted in future. Table 3. Current values before (initial) and after (ultimate) non-steady state migration Current (ma) Initial current Ultimate current M1 M1C6 M1C7 M1C2 M1C7 M1C CONCLUSION Firstly, the non-destructive notch method is a good way to quantitatively study the influence of cracks on concrete properties Secondly, Freundlich and Langmuir binding isotherms were better than the linear binding isotherms to match results in cracked concrete. Thirdly, chloride binding of cracked concrete is not only depending on crack length but also the surface concentration of chloride ion under non-steady state migration. The further validation is necessary in future.

8 6 ACKNOWLEDGEMENT Authors appreciate the financial support from China Scholarship Council, and from the Special Research Fund (BOF) of Ghent University. 7 REFERENCES Audenaert, K., De Schutter, G. and Marsavina, L. (2009) Influence of cracks and crack width on penetration depth of chlorides in concrete, European Journal of Environmental and Civil Engineering, 13(5): Castellotea, M., Andradea, C. and Alonsoa, C. (1999) Chloride-binding isotherms in concrete submitted to non-steady-state migration experiments, Cement and Concrete Research, 29(11): Djerbi, A., Bonnet, S. and Khelidj, A.,V. et al. (2008) Influence of traversing crack on chloride diffusion into concrete, Cement and Concrete Research, 38 (6): Ismail, M., Toumi, A. and François, R. (2008) Effect of crack opening on the local diffusion of chloride in cracked mortar samples, Cement and Concrete Research, 38(8-9): Win, P. P., Watanabe, M. and A. Machida. (2004) Penetration profile of chloride ion in cracked reinforced concrete, Cement and Concrete Research, 34(7): Yuan, Q., Shi, C. J., and De Schutter, G., et al. (2009) Chloride binding of cement-based materials subjected to external chloride environment: a review, Construction and Building Materials, 23 (1): 1-13