Experimental study on chloride permeability in concrete by non-contact electrical resistivity measurement and RCM

Construction and Building Materials 123 (2016) 27–34

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Experimental study on chloride permeability in concrete by non-contact electrical resistivity measurement and RCM Lianzhen Xiao a,b,⇑, Zheng Ren a, Wenchong Shi a, Xiaosheng Wei c a

School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, China c School of Civil Engineering and Mechanics, Huazhong University of Science and Technology, Wuhan 430074, China b

h i g h l i g h t s
A new technique (NC-ER) was proposed to test chloride diffusion coefficient.
There is a linear relationship between the results of pastes by NC-ER and RCM.
There are no errors caused by electrodes and about 5 min was taken to complete a test.

a r t i c l e

i n f o

Article history: Received 9 April 2016 Received in revised form 18 June 2016 Accepted 21 June 2016

Keywords: Electrical resistivity Slice Ion migration Chloride diffusion coefficient

a b s t r a c t A new non-contact electrical resistivity (NC-ER) measurement technique was proposed to measure the chloride diffusion coefficient of cement-based materials, and the results from testing were compared with the rapid chloride migration test, traditionally called RCM. Cement pastes with water cement ratios of 0.30, 0.35 and 0.40, and concrete with a water cement ratio of 0.4 were prepared for the chloride diffusion coefficient measurement by the two methods. The chloride diffusion coefficient results obtained by both approaches show a reasonable decrease with hydration age and increase with the increase of water cement ratio. The results for the pastes from the RCM test are in range of (0.693.15)  107 cm2/s at 3d80d, and the results from the NC-ER test are in the range of (0.531.71)  107 cm2/s., which are of the same order of magnitude and follow a linear relationship DRCM = 1.8821DNC-ER  0.3506 (R2 = 0.9205). From the NC-ER test, the chloride diffusion coefficient of concrete C0.4 is lower than that of paste P0.3 at the same ages and was verified by the porosity and formation factor analysis, while the RCM test obtained opposite trends. A better linear correlation between the chloride diffusion coefficient of the concrete and paste samples was obtained based on the NC-ER test. It can be deduced that the NC-ER set-up has advantages over the RCM set-up in measuring the chloride diffusion coefficient of concrete, such as there being no electrodes in the NC-ER setup which eliminates errors, and taking a much shorter time (about 5 min) to complete a test. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The chloride diffusion coefficient is an index of concrete durability. The methods to measure the diffusion of chloride ions for determining concrete permeability include standard methods, such as the test for the resistance of concrete to chloride ion penetration (Salt Ponding Test) [1,2], the electrical indication of the ability of concrete to resist chloride ion penetration [3,4], the rapid

⇑ Corresponding author at: School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430073, China. E-mail address: [email protected] (L. Xiao). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.110 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

chloride migration test (RCM) [5] and resistivity techniques [6,7]. The electrical methods are considered as rapid tests for evaluating concrete permeability [8]. Concrete is considered to be a solid electrolyte, and the transport of ionic species i in the pore solution of the solid electrolyte follows Nernst-Plank equation as Eq. (1) [9].

J i ¼ Di

@C i Z i F @E Di C i  Civ i þ @x @x RT

ð1Þ

where Ji is the ionic flux of species I (mol/cm2/s), Di is the diffusion coefficient of species i in the concrete (cm2/s), Ci is the concentration of species i in the pore solution as a function of location x

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L. Xiao et al. / Construction and Building Materials 123 (2016) 27–34

(mol/cm3), Zi is the valence and vi is the convection velocity of the ionic species I (cm/s). F is Faraday’s constant (96487 C/mol), R is the universal gas constant (8.314 J/mol.K), T is the absolute temperature (K), and E is the applied electrical potential (V). The each item of ion migration in Eq. (1) has a specific transportation mechanism, in which are the ionic movement under a concentration gradient is controlled by Fick’s first law, the ionic movement driven by electrical potential, and convection from the pressure gradient. If the pressure gradient is negligible, and the ion concentration in the concrete pore solution is constant in a steady state condition, and the ion migration is controlled only by electrical field as in Eq. (2), and the chloride diffusion coefficient can be obtained by Eq. (3), which is called the Nernst-Einstein equation.

J i ¼

Di ¼

Zi F @E Di C i RT @x RT ri

Z 2i F 2 C i

ð2Þ

ð3Þ

where ri is the partial conductivity of species i in the solution (S/ cm). The ri can be determined by the apparent conductivity of concrete and transfer number of species i in Eq. (4).

ri ¼ t i r

ð4Þ

where r is apparent conductivity of concrete (S/cm) and ti is transfer number of species i. The diffusion coefficient of chloride in concrete can be found by measuring the electrical conductivity of the concrete and the transfer number of chloride ti in the pore solution according to Eq. (5).

Di ¼

RTt i r Z 2i F 2 C i

ð5Þ

Test methods based on the Nernst-Einstein equation were proposed [6,10], and although great interest was aroused and many efforts have been made on electrical testing, the methods have not been widely accepted in the field yet. The experiments on chloride permeability were conventionally undertaken with electrode measurements, using either alternating current or direct current, where the polarization reaction on the electrodes consumes the ions in the solution, resulting in the ion concentration and the transfer number to vary during testing. The results and models presented by different researchers were not fully consistent due to the error caused by the electrodes, various measurement factors and the assumptions were not fully satisfied in regard to the theoretical equations used. A non-contact electrical resistivity method (NC-ER) [11–13] using alternating current (AC) is developed and used in this study to measure the electrical resistivity of the concrete mixtures saturated by NaCl solution, and hence to obtain the diffusion coefficient of chloride. The NC-ER eliminates the problems caused by the electrodes in conventional set-ups since there are no electrodes in this setup. In the non-contact electrical resistivity setup, there is no contact resistance between the test sample and the electrodes. No ions are consumed in the electrodes reaction and the total transfer number is truly equal to 1.0, compared to values smaller than 1.0 in the conventional electrode measurement method [14], and the transfer number of Cl (ti) in the used sodium chloride solution can be determined according to reference [15] as 0.604, and the chloride concentration Ci is equal to the saturated pore solution concentration. These parameters were used to calculate the diffusivity of the ions in the concrete according to NernstEinstein equation. Additionally, the RCM test was taken to measure the chloride diffusion coefficient of the same samples at the same

ages for comparison, and the results from the two methods were analyzed. 2. Raw materials and mixture preparation All tests were carried out using Ordinary Portland cement P.O42.5, meeting Chinese standard GB175-2007, and quartz sand of size 0.6 mm5 mm and crushed granite of size 5 mm10 mm were used in the concrete samples. The density and the Blaine specific surface area of the cement were 3.15 g/cm3 and 356 m2/kg, respectively. The chemical composition of the cement is shown in Table 1. Three paste samples were prepared with water cement ratios (W/C) of 0.30, 0.35 and 0.4, marked as P0.30, P0.35 and P0.40, respectively. A concrete sample was prepared with a sand/coarse aggregate weight ratio of 0.5 and a total aggregate volume fraction (Va) of 65%, at a W/C ratio of 0.40 marked as C0.40. Each sample was mixed for 2 min in a planetary-type mixer at 45 rpm and for a further 2 min at 90 rpm. The mixtures were respectively cast in a ring mould (1.67 L) for the NC-ER test and in a cylinder mould (diameter 100 mm  height 100 mm) for the RCM test. All the tests were conducted at 22 ± 2 °C. 3. Test methods 3.1. Slice sample preparation and non-contact electrical resistivity in AC field Slice samples were prepared by cutting the ring samples at the designated ages according to the dimensional requirement from the hardened mixtures for the chloride diffusion test. Each slice sample was vacuumed for 1 h and saturated with 10% (1.724 M) NaCl solution for 1 h using a vacuum pump. The electrical resistivity of each saturated slice sample was measured by a non-contact electrical resistivity setup (NC-ER) in alternating current (AC) field, as shown in Fig. 1(a) and (b), based on the transformer principle with 1 kHz frequency used. A transformer consists of two coils of wire wound on the same core. The primary coil is the input coil of the transformer and the secondary coil is the output coil. The ring sample acts as the secondary coil of the transformer in this setup. When an AC voltage is applied to the primary coil, a toroidal voltage (V) in Fig. 1(b) will be induced in the secondary coil. Subsequently, a toroidal current (I) will be generated inside the ring mould. There are two parts in the ring mould, a saturated slice sample inserted with sealant and the NaCl solution with the concentration of 10% (1.724 M) poured then. The saturated slice sample and the 1.724 M NaCl solution form a series model as secondary coil of the transformer in Fig. 1(b). The solution resistance Rsolution and the slice sample resistance Rslice can be calculated based on Ohm’s law in Eq. (6).

Rsolution þ Rslice ¼

V I

ð6Þ

The original pore solution inside the slice sample was fully replaced by the 1.724 M NaCl solution, and the conductivity of a slice sample results from the movement of conductive ions Cl and Na+ through the pore paths inside the slice sample under the electrical field to form current. The electrical resistivity of the slice sample (qslice) was then calculated by obtaining the electrical Table 1 The chemical composition of cement (wt%). CaO

SiO2

Al2O3

Fe2O3

MgO

SO3

K2O

Na2O

LOI

63.93

21.17

6.08

3.21

0.82

2.39

0.42

0.17

1.81

L. Xiao et al. / Construction and Building Materials 123 (2016) 27–34

29

Fig. 1. The set-up for non-contact electrical resistivity measurement of the slice sample.

resistance of the solution and the bulk electrical resistance of the setup, when the slice sample size and the height of the solution in the ring mould are known. The chloride diffusion coefficient (DNC-ER) can be calculated by Eq. (7), derived from Eq. (5).

DNC-ER ¼

RTt Cl Z 2Cl F 2 C Cl qslice

¼

9:22

qslice

ð7Þ

where DNC-ER: steady state chloride diffusion coefficient, 107 cm2/s;
qslice: the electrical resistivity of the slice sample, Ohm.m;
C Cl : chloride concentration, 1.724 M;
t Cl : the transfer number of Cl in the NaCl solution, 0.604;
Z Cl : the valency of Cl, 1.0.

The disc sample was removed and axially split after the measurement was done. 0.1 M AgNO3 solution was sprayed on to one of the fresh split sections and the area containing chloride ions appeared to be white due to the AgCl formation. The chloride penetration depth inside a sample was measured by a vernier caliper and the depth of the sample was determined by the average depth of ten points. The other parameters, such as the initial and final temperature, the specimen thickness, the voltage and time were recorded and the chloride diffusion coefficient was calculated by Eq. (8) [5,16].

DRCM

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 00:0239ðT þ 273ÞL ðT þ 273ÞLX d X d  0:0238 ¼ ðU  2Þt U2

ð8Þ

3.2. Disc sample preparation and rapid chloride migration test (RCM) Disc samples were prepared by cutting the cylinders into 50 mm thick sections at the designated ages from the hardened mixtures for chloride diffusion testing. Each disc sample was vacuumed for 3 h and saturated with saturated Ca(OH)2 solution for 1 h, then kept at the atmospheric pressure for more 18 h in the vacuum chamber. The electrical resistivity of each disc sample was measured by a rapid chloride migration setup (RCM), as shown in Fig. 2. In the RCM setup, 0.3 M NaOH solution was added in the anode chamber, and 10% NaCl solution was poured in the cathode chamber. The test period was set according to the electrical current in the first minute, and the voltage ranged 060 V.

where
DRCM: non-steady-state chloride diffusion coefficient, 1012m2/s;
U: absolute value of the applied voltage, V;
T: average value of the initial and final temperatures in the anolyte solution, °C;
L: thickness of the concrete specimen, mm;
Xd: average value of the penetration depths, mm;
t: test duration, hour. Three discs from each mixture, at each age, were measured in order to calculate the chloride diffusion coefficient, and the average value of the three discs was taken as the test result of DRCM.

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L. Xiao et al. / Construction and Building Materials 123 (2016) 27–34

Fig. 2. The set-up for RCM of hardened concrete disc.

3.3. Thermal analysis (TGA) and porosity Thermal gravimetric analysis (TGA) using TGA/DTA 92 (room temperature to 1000 °C, with increments of 10 °C/min, in N2) was performed for the quantification of the cement hydration products for calculating the degree of hydration [13]. The porosity of cement paste can be calculated by the hydration degree of the cement in the paste, according to Power’s law [17]. 4. Experimental results and discussion 4.1. The electrical resistivity and the chloride diffusion coefficient of the slice samples by the NC-ER test The electrical resistivity development during the first 12mins of the slice samples P0.30, P0.35 and P0.40 at 3d are shown in Fig. 3 as examples. Two slices from each mixture at each age were measured at intervals of 30 s and the average value of the electrical resistivity of the two slices was taken as the test result for

calculating the chloride diffusion coefficient by Eq. (7). The electrical resistivity of the slices for pastes P0.30, P0.35, P0.40 and concrete C0.40 at each age was measured and the results were obtained in the same way. It can be seen in Fig. 3 that the electrical resistivity does not change with time during the testing period, indicating that the slice samples have been fully saturated by the NaCl solution. The results of two slices from each mixture are very close, showing that this setup has high repeatability in measuring the resistivity. Additionally, the electrical resistivity of paste P0.30 is the highest, and decreases with increase of water cement ratio, which is reasonable due to the higher porosity in a higher water cement ratio paste for the same pore solution. The chloride diffusion coefficient (DNC-ER) of samples P0.30, P0.35, P0.40 and C0.40 was calculated from the measured electrical resistivity of the slice samples using Eq. (7) and the DNC-ER vs. ln(t) results at 1d, 2d, 3d, 14d, 28d and 80d are shown in Fig. 4, where the unit of time t is hours. It can be seen in Fig. 4 that the chloride diffusion coefficient (DNC-ER) of each sample decreases with age. The DNC-ER values of

9.0

P0.3

8.0 7.0

P0.35 P0.4

6.0 5.0 4.0 3.0 2.0 1.0 0.0 0.0

2.0

4.0

6.0

8.0

10.0

12.0

Chloride diffusion coefficient (×10-7cm2/s)

Electrical resistivity of slices (Ohm.m)

10.0

8.0

P0.4

7.0 6.0

P0.35

5.0 4.0 3.0

P0.3 C0.4

2.0 1.0 0.0

ln(t) (hour)

Time (min) Fig. 3. Repeatability of the electrical resistivity of the slice samples.

Fig. 4. The chloride diffusion coefficient (DNC-ER) of samples P0.30, P0.35, P0.40 and C0.40 at 1d, 2d, 3d, 14d, 28d and 80d.

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L. Xiao et al. / Construction and Building Materials 123 (2016) 27–34

4.3. The comparison of the chloride diffusion coefficient from the two measurements

the pastes reasonably increase with W/C at each age. The chloride diffusion coefficient of concrete C0.40 is clearly lower than that of paste P0.40 with the same W/C, and is the lowest of all the samples. Additionally, the chloride diffusion coefficient of the pastes or the concrete appears to have a higher slope before 3d and a lower slope after 3d. This is consistent with the cement hydration rate, which developed faster in earlier period. Additionally, the permeability of the samples decreases 812 times from 1d to 80d, and the concrete decreases more in this range. The DNC-ER values of sample C0.40 is much lower than that of sample P0.40 at each age. It is attributed to the aggregate without open pores in concrete sample C0.40, taking porous space by the volume fraction of 65%, greatly decreasing the ion migration path, which considerably exceeds the ITZ effect on ion diffusion.

4.3.1. The relationship of the chloride diffusion coefficient results (the DNC-ER vs. the DRCM) The chloride diffusion coefficient results from the two methods are of the same order of magnitude as mentioned earlier, however, the DRCM values in Fig. 5(b) are higher than the DNC-EM values in Fig. 4 by 1.072.54 times for the paste samples, and 3.405.52 times for the concrete sample at the same ages (3d, 14d, 28d and 80d). Additionally, the DRCM values of concrete C0.40 are clearly higher than that of paste P0.3 before 28d, higher than that of paste P0.35 at 3d and 7d as well, which differ from the results of the DNCER in Fig. 4. The relationship of the chloride diffusion coefficient of samples P0.3, P0.35, P0.4 and C0.4 obtained from the two measurements at 3d, 14d, 28d and 80d is shown in Fig. 6. It can be seen in Fig. 6 that there is a linear relationship between the chloride diffusion coefficient results from the RCM and the NCEM tests on the same mixture samples at the same ages, which has a similar development trend caused by either the water cement ratio or the hydration time. Both the RCM and the NC-ER are essentially based on chloride ions movement in an electrical field through the pores of the samples. It should be recognized that the electrical migration of chlorides in concrete and the rate of chloride diffusion both depend on the porosity, pore connectivity and tortuosity [18,19], which explains the correlation. The linear trend for all the samples is also shown in Eq. (9), however, a better

4.2. The chloride diffusion coefficient of the disc samples by RCM test

Chloride diffusion coefficient (h10-7cm2/s)

The Cl migration depth, seen as the white region in the split disc sample P0.35 at 3d, 7d and 28d in Fig. 5(a) is used, for calculating the chloride diffusion coefficient by Eq. (8). The chloride diffusion coefficient (DRCM) vs. ln(t) results are shown in Fig. 5(b). It can be seen in Fig. 5(b) that the chloride diffusion coefficient (DRCM) of the pastes with various W/C or concrete reasonably decrease with hydration age due to the hydration products filling the pore spaces, the DRCM value of the paste samples increases with W/C at each age, and the chloride diffusion coefficient of concrete C0.40 is clearly lower than that of paste P0.40.

Higher W/C paste has a

4.0

higher DRCM

P0.4

3.0

P0.35

2.0

C0.4 P0.3

1.0 0.0 0.0

2.0

4.0 ln(t) (hour)

6.0

8.0

(b) Fig. 5. The chloride diffusion coefficient (DRCM) of samples P0.30, P0.35, P0.40 and C0.40 at 3d, 7d, 14d, 28d, 56d and 80d.

L. Xiao et al. / Construction and Building Materials 123 (2016) 27–34

Chloride diffusion coefficient DRCM (×10-7cm2/s)

32

3.5 DRCM = 1.8821DNC-ER - 0.3506 R² = 0.920

Paste

3.0 Concrete

2.5

DRCM = 1.1935DNC-ER + 0.4381 R² = 0.5581

2.0 1.5 1.0 0.5 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Chloride diffusion coefficient DNC-EM (×10-7cm2/s) Fig. 6. The relationship of the chloride diffusion coefficient of the samples at 3d, 14d, 28d and 80d from the RCM test and the NC-EM test.

linear correlation, excluding concrete C0.4 can be obtained, as shown in Eq. (10). For paste and concrete samples,

DRCM ¼ 1:1935DNCER þ 0:4381; R2 ¼ 0:5581

ð9Þ

For paste samples,

DRCM ¼ 1:8821DNCER  0:3506; R2 ¼ 0:9205

ð10Þ

A poor linear trend in Eq. (9) from all the samples is due to that a higher contradiction of the two test methods arises in concrete C0.4. The DRCM values of concrete C0.40 are higher than these of paste P0.3 at the same ages, while the DNC-ER values of concrete C0.40 are lowest in all the samples, as mentioned earlier. It can be analyzed from the pore structure investigation and the formation factor, as described later. 4.3.2. The correlation between the chloride diffusion coefficient of concrete and paste The correlation between the chloride diffusion coefficient of concrete and paste from the two test methods are shown in Fig. 7. Two separate linear trends corresponding to the two test methods are observed in Fig. 7, the one from the RCM test is the upper line and a higher correlation coefficient (R2 = 0.9923) is obtained from the NC-ER test. The correlations between concrete C0.40 and paste P0.40 are shown in Eqs. (11), (12). From the RCM test,

DRCM ðC0:40Þ ¼ 0:6614DRCM ðP0:40Þ; R2 ¼ 0:9042

ð11Þ

From the NC-ER test,

DNCER ðC0:40Þ ¼ 0:3074DNCER ðP0:40Þ; R2 ¼ 0:9923

ð12Þ

The linear correlation in Eq. (12) shows that the chloride diffusion coefficient of a concrete with an aggregate volume fraction of 65% is a function of the chloride diffusion coefficient of the paste with the same W/C and paste matrix volume fraction of 35% inside the concrete. In the RCM test, a sample was saturated with Ca(OH)2 solution, and the Cl ions migrated inside the sample from the outside by the electrical driving force. The chloride diffusion coefficient (DRCM) was calculated by Eq. (8), ignoring ionic diffusion by the concentration gradient and convection migration in the concrete. However, the concrete at an early age probably has macropores or connected capillary channels, and there were additional Cl ions suction by capillary action, and concentration gradient as well, besides the electrical driving force. Therefore, the DRCM results calculated by Eq. (8) are over-estimated due to the height of the Cl migration from the combining effect of the electrical driving force and the capillary action. In the NC-EM setup, a sample was saturated with NaCl solution, the type and concentration of the pore solution inside the sample being the same as outside the sample in the electrical circuit, avoiding ionic diffusion driven by the concentration gradient and convection migration, really following the Nernst-Einstein equation. Moreover, the smaller sample size make a good saturation effect, and high accuracy can be reached. Additionally, each test takes 696 h to be completed, after sample preparation, in the RCM method. The electrodes reaction can change the solution concentration and the current was partially consumed in the electrodes reaction with the voltage of 060 V, not fully on Cl migration, which may cause the DRCM results variation. The NC-EM is newly proposed to measure the chloride diffusion coefficient, and there are no electrodes in this setup and the test can be completed in 5 min, with consistent results and better correlation, as in Fig. 7, then being obtained.

4.3.3. The porosity analysis by TGA and the formation factor by the NCER test The TGA results of pastes P0.3 and P0.4 at 3d are shown in Fig. 8. The non-evaporating water content in the hydrated pastes, as chemical bonded water, was obtained in the range 105 °C950 °C by TGA, considering the ignition loss of the original cement (LC). The hydration degree of the cement (a) in the pastes can be calculated by the equation [13] shown in Fig. 8, where W105 and W950 are the sample weights at temperatures 105 °C and 950 °C, respectively, and Wn/Ccomp. is chemical bounded water per gram completely hydrated cement.

Temperature (oC)

2.5

0

100 200 300 400 500 600 700 800 900 1000

-2

DRCM(C0.4)=

0.6614DRCM(P0.4) R² = 0.9042

-4

2.0 -6

from the NC-ER 1.5 1.0

= 0.3074DNC-ER(P0.4) R² = 0.99

DNC-ER(C0.4)

TG (mg)

Chloride diffusion coefficient of C0.4 (×10-7cm2/s)

0

-8 -10 -12

0.5

-14

0.0 0.0

-16

2.0

4.0

6.0

8.0

Chloride diffusion coefficient of P0.4 (×10-7cm2/s)

P0.4 P0.3

-18 -20

Fig. 7. The correlation of the chloride diffusion coefficient of paste P0.4 and concrete C0.4 from the RCM and the NC-EM tests.

Fig. 8. The TGA results of pastes P0.3 and P0.4 at 3d.

L. Xiao et al. / Construction and Building Materials 123 (2016) 27–34

The porosity (u) is determined by the ratio of the capillary volume (Vcapillary) to the total volume of water and cement in the paste (Vtotal), as shown in Eq. (13).

u¼

   V capillary W=C a a V total ¼   Dw V total Dh Dc

ð13Þ

where Dw, Dc, Dh represent the density of the pore solution, cement and hydrates, respectively. For Dw = 1.01 g/cm3, Dc = 3.15 g/cm3, Dh = 1.529 g/cm3 (based on assuming that one unit volume of cement produces 2.06 unit volumes of gel) [17]. The porosity of pastes P0.3 and P0.4 at 3d were 0.351 and 0.432 by Eq. (13), respectively. The porosity of sample C0.4 can be calculated by the porosity of sample P0.4 and the ITZ volume fraction in sample C40 according to the aggregate volume faction and the ITZ thickness. The aggregate volume faction of 65% was used in concrete C0.4, and the ITZ thickness of 0.05 mm is taken which is the highest value in the reference [20,21], the ITZ volume fraction of 17.6% can be obtained by the calculation [21], the possible total porosity of concrete C0.4 provided by the paste matrix (u of the matrix = 0.432  (1–65%) = 0.151) and the ITZ (u of the ITZ = 0.176) is 0.327, which is obviously lower than that of paste P0.3 (u = 0.351). It can be derived from the porosity values of samples P0.30 and C0.40 in that the chloride diffusion coefficient of sample C0.4 should be lower than that of paste P0.3 at the same ages. The formation factor (FF) as a global parameter characterizing the pore structure of concrete or paste materials is used to analyze chloride migration. The formation factor of the concrete and paste samples can be calculated by Eq. (14), and is equal to the electrical resistivity of a slice sample to the electrical resistivity of the solution inside the slice sample, as shown in Fig. 9.

FF ¼

qslice qsolution

ð14Þ

In the paste samples, sample P0.3 with the lowest W/C has the least free water space, resulting in the lowest connected pore network and the highest tortuous pore network highlighted by the highest formation factor values at each age. The concrete sample C0.4 has the highest formation factor in all the samples, indicating the lowest connected pore network and the most tortuous paths for ion migration. The formation factor results are consistent with the results of the chloride diffusion coefficient DNC-ER, confirming that the DNC-ER results are more reasonable, while the DRCM results appear to have clear errors, especially for the low permeability samples, such as concrete C0.40 and P0.30. Therefore, the porosity and the formation factor analysis verifies the reasonable order of the chloride diffusion coefficient results from the NC-ER test.

33

It can be concluded that the newly proposed measurement technique NC-ER has advantages in several aspects: (1) no electrode reaction consuming ions and no contact resistance causing error, (2) a smaller specimen is more easily saturated with solution, with the test completed in minutes, (3) using a higher concentration solution to saturate concrete specimen, preventing interference from the original ions in the concrete, (4) steady state chloride migration, avoiding interference from capillary action, concentration gradient and convection, which are hard to be exactly accounted for. 5. Conclusions (1) A new non-contact electrical method (NC-EM) is proposed to obtain the chloride diffusion coefficient of hardened concrete and paste by measuring the electrical resistivity of slice samples being saturated with 10% concentration NaCl solution. This technique eliminates the errors and problems caused in the traditional setups with electrodes. (2) The chloride diffusion coefficient results from the NC-EM test were compared with that from a rapid chloride migration test (RCM). Both measurements show that the chloride diffusion coefficient of the paste or concrete samples reasonably decrease with hydration age, and increases with increase of water cement ratio. (3) The chloride diffusion coefficient results of the paste samples at 3d80d by the RCM and the NC-ER tests are in range of (0.694.35)  107 cm2/s and (0.531.71)  107 cm2/s, respectively, there being linear relationships. All the results from the RCM tests are higher than from the NC-ER tests for the same samples at the same ages, due to the effect of the chloride migration into the samples in the RCM setup, possibly not only just by the electrical driving force which is only considered in the equation used. (4) There is better linear correlation between the chloride diffusion coefficient of the concrete and paste samples from the NC-ER test, and the chloride diffusion coefficient of concrete C0.4 is lower than that of paste P0.3 at the same ages from the NC-ER, which were verified by the porosity and formation factor analysis, while the RCM test showed an opposite trend.

Acknowledgements The authors gratefully acknowledge Funding by the National Natural Science Foundation of China (51348001 and 51478202), and the State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology) (SYSJJ2015-01).

Formation factor, FF

References

350 300 250 200 150 100 50 0

1d C0.40 23.73 P0.30 14.06 P0.35 10.25 P0.40 7.27

2d 3d 14d 43.64 134.73 210.00 23.32 56.07 83.15 16.90 38.79 73.71 14.68 33.68 63.20

28d 225.00 90.83 80.76 62.08

Fig. 9. The formation factor of the samples.

80d 292.66 106.35 94.08 91.56

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