The influence of bending crack on rebar corrosion in fly ash concrete subjected to different exposure conditions under static loading

Construction and Building Materials 160 (2018) 293–307

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The influence of bending crack on rebar corrosion in fly ash concrete subjected to different exposure conditions under static loading Idrees Zafar a,⇑, Takafumi Sugiyama b a b

Department of Civil Engineering, College of Engineering, Al Imam Mohammad Ibn Saud Islamic University, P.O. Box 5701, Riyadh 11432, Saudi Arabia Division of Field Engineering for the Environment, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan

h i g h l i g h t s
Corrosion initiation period remained same for cracked fly ash concrete.
Crack width was found to be the governing factor for crack sealing ability of concrete.
The relation between corrosion depth and surface corroded area was introduced.

a r t i c l e

i n f o

Article history: Received 5 December 2016 Received in revised form 11 October 2017 Accepted 15 November 2017

Keywords: Corrosion Flexural cracks Fly ash concrete Exposure conditions Static loading

a b s t r a c t In the present study an effort was made to clarify the performance of pre-cracked fly ash concrete against corrosion under different exposure conditions. A total of twenty specimens from two different concrete mixes were tested against three different exposure conditions for 106 days. It was observed that the crack filling ability of concrete is more sensitive to crack width than fly ash replacement and exposure conditions. Under submerged conditions the fly ash concrete showed greater pitting corrosion, while under wet and dry cycle conditions, the corrosion damage was found to be less penetrating as compared to normal Portland cement concrete. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction The durability aspect of reinforced concrete is typically related to the concrete matrix, i.e. a dense microstructure will most likely show lower permeability and reduce the transport of corrosive agents to reinforcement. However, in real reinforced concrete structures, it is certain to have cracks, either in the form of the micro cracks between aggregate and cement paste or macro cracks encountered during the service life due to loading or degradation process. In addition, the increasing demands for greater loads, for example long-span bridges, make the structures prone to more cracking. Cracking adversely affects the serviceability and durability of a structure, particularly when exposed to marine environments because of the corrosion of rebars. Cracking has become a critical feature of reinforced concrete structures and significant efforts have been done throughout the world to minimize the cracking problem in reinforced

⇑ Corresponding author. E-mail address: [email protected] (I. Zafar). https://doi.org/10.1016/j.conbuildmat.2017.11.070 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

concrete structures. For the same reason, many codes and specifications have fixed the criteria of service life on the basis of allowable crack width [1,2]. Meanwhile, in last few years, the use of fly ash in concrete is becoming increasingly popular all over the world. The incorporation of fly ash in concrete can help to reduce the environmental impact of cement industry at a reduced or no additional cost [3]. Also, it is generally recognized that the inclusion of fly ash in concrete improves its resistance against chloride-induced corrosion of steel reinforcement by reducing its permeability, particularly to chloride ion transportation and increasing the resistivity of the concrete [4]. In addition, according to a recent study, provided enough curing, the fly ash concrete showed the same chloride threshold values as that of normal Portland cement concrete for corrosion initiation [5]. Moreover, it has also been shown that high volume of fly ash concrete has the ability to self-heal the cracks under moist conditions [6]. In addition, Na et al. [7] has shown that the self-healing performance in fly ash blended mixtures is dependent on the curing temperature, curing age and fly ash replacement ratio.

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In non-cracked concrete, the chloride induced corrosion initiates with a localized anodic reaction, limited to the area of steel where the chloride threshold value is exceeded. While in case of cracked concrete, the corrosion begins with the formation of a small anode at the crack and a large cathode consisting of passive steel in the non-cracked concrete around the crack [8]. It has also been reported that, for every active corrosion, the micro cell corrosion and macro cell corrosion normally co-exist [9]. The corrosion process of steel reinforcement embedded in cracked concrete is a complex phenomenon depending on exposure conditions, loading conditions, concrete composition, concrete resistance and crack width [10,11]. In the past, a lot of research has already been done to study the corrosion of steel reinforcement in cracked concrete. But most of the studies have focused on the effect of factors such as the crack width, crack depth, concrete composition, and loading conditions on the corrosion of reinforcement in cracked concrete, and relatively less research has been done on the corrosion of rebars with different exposure conditions, especially in fly ash concrete under cracked conditions. Furthermore, in most of the laboratory investigations, the cracked samples have been fully submerged in the salt solution in the unloaded state, which is quite different from the reality [12]. In addition, the mechanism of the corrosion reinforcement in cracked fly ash concrete with regard to its self-healing ability has not yet been made clear in the previous research studies. This study was conducted to evaluate the effect of bending cracks on the corrosion of rebars in fly ash concrete under static loading while exposed to different exposure conditions. Specially designed apparatus was employed to sustain the flexural moment in the reinforced concrete specimen along with the application of salt solution within the limited region of crack. This configuration will bring a severe corrosive environment with co-existence of salt solution application. Overall, the current study will be helpful for understanding of reinforcement corrosion in cracked fly ash concrete under different environmental conditions along with selfhealing abilities.

2. Experimental methodology 2.1. Material and specimen details The coarse aggregate (G) was crushed stone with a maximum size of 13 mm and the fine aggregate used (S) was river sand. This size of coarse aggregate was selected because of the experimental limitations i.e. the size of specimen, cover depth and the time to achieve certain amount of corrosion. The specific gravity of coarse and fine aggregate were 2.64 g/cm3 and 2.67 g/cm3 respectively. Japanese Industrial Standard [13] Type II fly ash was used. Properties of fly ash are summarized in Table 1. Deformed steel bar, having a diameter of 19 mm, was used in all the specimen series. Three different series of specimen, i.e. C, CB and CT, were prepared with constant water to binder ratio of 0.5. The notation C, CB and CT correspond to three different exposure conditions, i.e. wet and dry cycle, continuous application of salt solution from bottom side and continuous application of salt solution from top side of the specimen respectively. Wet and dry cycle correspond to the reinforced concrete structures exposed to dry and rainy seasons. The continuous application from the top represents the bridge slabs or RC structures in snowy areas like Hokkaido, Japan, that are exposed to snow for several months and to remove the snow salt is applied on the top surface. The continuous application from the bottom represents RC structures exposed to seawater like bridge piers. Fly ash concrete, for all three series, was made by replacing 30% ordinary Portland cement (C) with fly ash (FA). The

target air content and slump was 5 ± 0.5% and 12 ± 2 cm respectively. All specimen series were cured for 91 days. The mix proportions for all three specimen series (C, CT and CB) of respective normal and fly ash concrete were same and can be found in Table 2. Corrosive behaviour of rebar in concrete is normally involved in uncertainties resulting in a different manner even with duplicated specimens of the same batch being tested concurrently. Therefore, for the enhancement of the accuracy of the result, multiple duplicated specimens for each series were prepared and tested in this study. A total of twenty prisms (100  100  400) mm were prepared, and the detailed configuration of the reinforced concrete specimen is explained in Table 3. The cross section of the specimens is shown in Fig. 1(a). All specimens contain two reinforcing bars at a distance of 20 mm (UP) and 25 mm (DO) from the top surface. To avoid the bleeding water to accumulate underneath the rebars, fresh concrete was placed in the lengthwise direction. After the completion of curing period, all the surfaces of the specimens except top and bottom were sealed with butyl tape with outer alumina coating and top surface was gently cleaned with wire brush. The cracks were generated by using a specifically designed apparatus as shown in Fig. 1(b). In sustained loading system, the load tends to decrease due to the creep effect of concrete, however in our specimens the load was regularly monitored but it was noticed that almost negligible adjustment of load was required, so this effect was not considered. The loads of approximately 20 kN for normal concrete while 17 kN for fly ash concrete was applied. The apparatus was uniquely shaped to keep the specimens under constant bending moment along with the application of salt solution only in the limited region. This was intended to cover the limited area near the crack opening exposed to saline solution, so that maximum transport of the saline solution occurs through the crack, which, as a result, will also limit the anodic area on the embedded rebars. The target anode to cathode ratio for the current methodology is quite small allowing the corroding zone to approach the pure anodic behaviour. In this regard, the present methodology will allow major contribution from macro cell activity to the overall corrosion rate. A portable digital microscope was used to observe the crack width at the end of one complete cycle (wet and dry) for C series, while for CB series it was measured at regular interval of one month, and for CT it was measured twice, i.e. 60 days and after last cycle. The crack width in this study is defined as the distance between the two jaws of crack on the surface of concrete along its exposed side. The initial crack width, before the application of salt solution, for all the specimens was kept less than 0.1 mm. The crack width was not constant along the length of crack on the surface of concrete. It is common because concrete is not a homogenous material and response to same stress level can be different for different components of concrete. It was for the same reason that three points were selected along the length of crack and for each point the average of five measurement was taken. So the average crack width for each specimen refers to the average of fifteen points along the length of crack on the exposed surface of concrete as shown in Fig. 2. The average initial crack width for all the specimen series is shown in Table 4. After the generation of cracks, all three specimen series C, CB and CT were exposed to 10% NaCl solution through wet and dry cycle, continuous application from top and bottom respectively. The duration of each wet and dry cycle was kept as 7 days for a period of 56 days and the duration of last wet and dry cycle was increased to 25 days each to increase the degree of corrosion damage. During the wet cycle, the cracked specimens were exposed to 10% NaCl solution, and, during the dry cycle, the specimens were

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I. Zafar, T. Sugiyama / Construction and Building Materials 160 (2018) 293–307 Table 1 Properties of fly ash used in this research. Fly ash type

JIS standard (A6201) Type II

Fly ash used

5 (max) 45 (min) – – – –

Chemical properties

Loss on ignition SiO2 content Al2O3 content Fe2O3 content CaO content MgO content Others pH

(%) (%) (%) (%) (%) (%) (%)

–

2.4 66.4 18.88 3.63 0.90 0.54 9.63 10.1

Physical Properties

Absorption using Methylene Blue Absorption using Methylene Blue (Japan Cement Association) Density Fineness Blaine surface area Retain on 45 mm sieve Flow Activity coefficient (28 days) Activity coefficient (91 days)

(mg/g) (mg/g)

– –

0.60 0.53

(g/cm3) (cm2/g) (%) (%) (%) (%)

1.95 (min) 2500 (min) 40 (max) 95 (min) 80 (min) 90 (min)

2.20 3990 13 104 89 114

Table 2 Mix proportions of cracked concrete specimens. Type

N2 F30

Gmax*1

W/B*2

FA/(C + FA)*3

s/a*4

(mm)

(%)

(%)

(%)

13

50 50

0 30

43 43

Unit weight (kg/m3) W

C

FA

S

G

165 162

330 227

– 97

781 774

1024 1015

Compressive strength (N/mm2) 28 Days

91 Days

40.7 29.6

45.8 39.5

*1: Maximum aggregate size, *2: Water to binder ratio, *3: Fly ash replacement ratio, *4 Sand to aggregate ratio.

Table 3 Configuration of cracked concrete specimens. Specimen series

Fly ash replacement (%)

Exposure type

Number of specimens

N2C F30C

0 30

Wet and Dry

4 4

N2CB F30CB

0 30

Continuous application of salt solution from Bottom

3 3

N2CT F30CT

0 30

Continuous application of salt solution from Top

3 3

allowed to dry at room temperature. In case of CB and CT series, the specimens were continuously exposed to 10% NaCl at the crack region from top and bottom side for 106 days. The transport of the salt solution in the crack was mainly governed by the capillary suction phenomenon and then penetration into the non-cracked concrete through crack walls occurred under the diffusion mechanism.

2.2. Electrochemical measurements The corrosion monitoring of all the specimens was done at the end of one complete cycle (wet and dry) for C series, while for CB and CT series it was measured at regular interval of one month. The measurement of half-cell potential was done by using Lead-Lead oxide electrode (Pb/PbO2). An AC impedance spectroscope (hereafter called ‘‘corrosion meter”) was used to obtain the impedance data (apparent polarization resistance and apparent concrete resistance) along with half-cell potential (with reference to Ag/AgCl electrode) at the end of each cycle. The working principle of AC impedance measurement and distribution of current flowing lines using double counter electrode is referred in [14]. Measurements

were made at the location where the saline solution was applied on the surface of the specimen. In the current study the corrosion initiation shown by the half-cell potential values were later confirmed by the corrosion current values. As the corrosion current values were measured after a period of two weeks and indicated that the corrosion of reinforcement has already started, so the values of half-cell potential with regard to corrosion current values were considered for the corrosion initiation of reinforcement. The corrosion meter converts the apparent polarization resistance and apparent concrete resistance to true polarization resistance and specific concrete resistance, respectively. The details of the conversion process have been described elsewhere [5]. The corrosion current density, Icorr, is estimated by using the Stern and Geary equation and using the recommended Stern and Geary constant (B) value of 26 mV [15]. The area of corrosion used to calculate Icorr is defined as the target measurement area for reinforcement influenced by the central counter electrode of corrosion meter, and is 12.06 cm2 for this study. The mean corrosion depth was estimated using a formula based on Faraday’s law [15]:

Z ¼ a  0:0116 

Z

Icorr  t  dt

ð1Þ

where Z = Estimated mean corrosion depth (mm), a = pitting factor = 10 (maximum), 0.0116 = conversion factor (mA/cm2 to mm/year), Icorr = corrosion current density in mA/cm2 and t = propagation period in years. Chloride analysis was conducted at the end of the exposure period. Assuming the diffusion to be the same in the left (L) and right (R) side of the crack, either of sides was used for chloride analysis. In order to have a chloride ion distribution at the level of rebar, the entire salt application zone was used for chloride analysis, as shown in Fig. 3(a). The salt application zone was sliced in approximately 7 mm thick layers, as shown in Fig. 3(b). The sliced specimens were ground to obtain about 10 grams of powder for the chloride analysis. Japanese Industrial Standard [16] was used to

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20

25 25

20

DO 19

UP

100

(a) Anchorages Crack

Specimen in dry cycle Butyl Tape Specimen in wet cycle

10 % NaCl Reinforced Concrete Prism (100x100x400) mm (b)

Application of 10 % NaCl from Top

Crack

Reinforced Concrete Prism (100x100x400) mm

Butyl Tape

Application of 10 % NaCl from Bottom (c) Fig. 1. (a) Cross Section of the concrete Prism (Units in mm) (b) Apparatus showing the specimens in a three point bending mode along with the application of 10% NaCl solution during wet and dry cycle (c) Continuous application from top and bottom.

determine the total chloride ion concentration in each sample of the concrete. After the completion of all cycles, the specimens were cut to visually observe the rebar condition and measure the corroded surface area of rebar. In the current study, the corroded surface

area of the rebar refers to the surface area of the rebar being covered by corrosion products. A thick and transparent type scotch tape was wrapped carefully on the circumferential surface of corroded reinforcement after the cutting and splitting of the specimen. Then a permanent marker pen with a fine tip was used to

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Salt Solution Application Region

100 mm

a Point 1

b

Point 2

c d

Point 3

e Point 2

400 mm

(b)

(a)

Fig. 2. (a) Three measurement points on the surface of tension fiber along the length of crack (b) Five measurement locations for each measurement point.

Table 4 Average initial crack width for C, CB and CT series. Specimen No.

1 2 3 4

Initial Crack Width (mm) N2C

F30C

N2CB

F30CB

N2CT

F30CT

47 61 76 65

59 64 50 85

74 78 53 –*

69 61 59 –

64 69 55 –

71 64 57 –

*: 3x Specimens were casted for each series.

Crack

L

R

Salt Application Zone

L side of the Salt Application Zone 20mm 25mm D19 D19

Slice Thickness: 7mm Fig. 3. (a) Schematic diagram showing the location of the crack and salt application zone (b) Slicing of slat application zone for chloride analysis.

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1st Cycle

2nd Cycle

3rd Cycle

4th Cycle

5th Cycle (a)

6th Cycle

1st Cycle

2nd Cycle

3rd Cycle

150 µm 4th Cycle

5th Cycle (b)

6th Cycle

Fig. 4. Crack width variation at the end of specified cycles for (a) N2C2, (b) F30C2, (c) N2CB1, (d) F30CB1, (e) N2CT2 and (f) F30CT2 Specimen.

carefully sketch the corrosion area. Allowing sufficient time to dry the drawing, the tape was carefully transferred on a white paper and image of this paper was produced by scanning the paper. Computer image analysis software (Image J) was used to quantify the corrosion area. The corrosion products were removed from the surface of the rebar per Japanese Concrete Institute [17]. After cleaning of the rebar’s surface, maximum pit depth, number of pits and mean corrosion depth were measured by using a com-

puter operated three dimensional scanner (PICZA 3D scanner by Roland). The automated scanner gives the change in the depth with reference to a certain datum with a least count of 1 mm. The scanner measures the depths at an interval of 10 mm in both the directions of the plane, forming a grid. The values obtained are then used to draw the surface profiles and calculate the maximum pit depth, number of pits and mean corrosion depth by using commercially available software.

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At 0 days

299

At 30 days

At 60 days

At 106 days (c)

At 0 days

At 30 days

At 60 days

At 106 days (d) Fig. 4 (continued)

3. Results and discussion 3.1. Crack width variation The variation of the crack width, measured at the end of each cycle for N2C (2), F30C (2), N2CB (1), F30CB (1), N2CT (2) and

F30CT (2) are shown in Fig. 4(a–f) respectively. It was observed that average crack width remained almost constant for all the series. It was also observed that, after the generation of flexural cracks, the closure of the crack occurred at some points due to the falling of loose concrete into the crack. The fallen concrete particles were observed to have either faced out during the exposure

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At 0 days

At 60 days (e)

At 106 days

At 0 days

At 60 days (f)

At 106 days

Fig. 4 (continued)

Table 5 Crack closure with respect to crack width range for normal and fly ash concrete against different exposure conditions. Exposure conditions

Continuous submerged Wet and Dry conditions

Crack Width (mm) <50

50–100

>100

N2, F30 N2, F30

F30 F30

–* –

*: None of the cracked concrete specimen series showed crack closure.

period or have moved in the deeper parts by breaking down into more small particles. The results of crack mouth closure with regard to crack width variation and exposure conditions for normal and fly ash concrete are summarized in Table 5. Based on the crack width analysis at different points throughout the crack length at the end of each cycle, using the software of digital potable microscope, it was observed that, for the same concrete composition, irrespective of the exposure type, the crack filling effect of cracks was profound for the smaller crack width (<50 mm) as compared to larger crack width (>100 mm). Among the normal concrete specimen series, i.e. N2C, N2CT and N2CB, only one specimen in CB series showed the closure of crack mouth, as shown in Fig. 4(c). The closure of the crack is because of the smaller crack widths (around 50 mm) as compared to N2C and N2CT series, rather than the difference in the exposure type. The different exposure conditions at the crack mouth were found to have no effect on the crack filling ability of normal concrete. On the other hand, fly ash concrete under all exposure condition, F30C, F30CT and F30CB series, had shown the tendency to seal the cracks, as shown in Fig. 4(b), (d) and (f). However, it was noticed that the degree of closure of cracks relates to smaller crack widths, i.e. around 50 mm, than higher crack widths. In addition, F30C showed relatively less degree of crack filling ability as compared to F30CT

and F30CB series. The main reason for this behaviour might be the availability of more moist conditions for CT and CB as compared to C series. Furthermore, in comparison with the normal concrete series the fly ash concrete had shown higher degree of crack filling. Although the exact composition of the product was not verified, but Fig. 4(b), (d) and (f) clearly depict the closure of crack mouth. The crack filling did not occur at the full length of crack, but rather, was found at certain locations along the length of crack. The filling of the cracks in fly ash concrete might be because of the pozzolanic reaction, as reported previously that fly ash at low water to cement ratio has a tendency to heal the micro cracks under the water supply because of the delayed pozzolanic reactions, which ultimately can fill the crack space[18]. Sharman et al. also reported the self-healing of engineered cementitious composites containing fly ash when exposed to 3% NaCl solution [6]. Furthermore, the concentration of salt solution being used in this study is relatively high, causing the increase in the alkali content in the crack, which might have triggered the activation of fly ash. 3.2. Electrochemical measurements 3.2.1 Half cell potential The trend of the half-cell potential against time period for N2C, F30C, N2CB, F30CB, N2CT and F30CT series are shown in Fig. 5(a–f) respectively. The corrosion initiation for all the specimen series was detected based on ASTM C-876. The results of half-cell potential had shown that the corrosion had initiated for all the specimens within first two weeks of salt application period, irrespective of the fly ash addition and different exposure conditions. The short corrosion initiation period for all the specimen series has clearly shown that the cracks acted as a flow channel for the salt solution to reach the rebar and initiate corrosion, irrespective of the concrete composition. It

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Half Cell Potential (mV vs CSE)

UP1 UP3 ASTM

100

UP2 UP4 Half Cell Potential (mV vs CSE)

UP1 UP3 ASTM

100

-100

-100

-300

-300

-500

-500

-700

-700 0

30 60 90 Elapsed Time (Days)

0

120

30 60 90 Elapsed Time (Days)

(a)

UP1 UP3

120

(b)

UP1 UP3

100

UP2 ASTM

UP2 ASTM

Half Cell Potential (mV vs CSE)

Half Cell Potential (mV vs CSE)

100 -100

-100

-300

-300

-500

-500

-700 0

30 60 90 Elapsed Time (Days)

120

-700 0

UP1 UP3

60

90

120

(d)

100

UP2 ASTM Half Cell Potential (mV vs CSE)

100

30

Elapsed Time (Days)

(c)

Half Cell Potential (mV vs CSE)

UP2 UP4

-100 -300 -500

UP1 UP3

UP2 ASTM

-100 -300 -500 -700

-700 0

30 60 90 Elapsed Time (Days)

120

(e)

0

30 60 90 Elapsed Time (Days)

120

(f)

Fig. 5. Trend of half-cell potential against time for (a) N2C, (b) F30C, (c) N2CB, (d) F30CB, (e) N2CT and (f) F30CT series.

was found that crack filling has no beneficial effect for both cracked normal Portland cement concrete and fly ash concrete, with regard to corrosion initiation. In case of normal concrete, the crack filling ability was not to the extent which can affect the corrosion initiation of steel reinforcement. Although fly ash concrete showed the crack filling ability, the crack filling

was mostly observed at later stages of the exposure period, i.e. after the corrosion of steel reinforcement had initiated. It was also noticed that the values of half-cell potential measured for C series, for both normal and fly ash concrete, are on the lower side as compared to CT and CB series. This is because, for C series, the half-cell potential measurements were made

15

UP1

UP2

12

UP3

UP4

Corrosion Current Icorr (µA/cm²)

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Corrosion Current Icorr (µA/cm²)

302

9 6 3

15

UP1

UP2

12

UP3

UP4

9 6 3

0

0 0

30 60 90 Elapsed Time (Days)

120

0

(a)

UP1

UP2

15

UP3

UP1

12 9 6 3 0

UP2

UP3

12 9 6 3 0

0

30

60

90

120

0

Elapsed Time (Days) (c)

15

UP1

30 60 90 Elapsed Time (Days)

120

(d)

UP2

UP3

15

UP1

UP2

UP3

12

12

Corrosion Current Icorr (µA/cm²)

Corrosion Current Icorr (µA/cm²)

120

(b)

Corrosion Current Icorr (µA/cm²)

Corrosion Current Icorr (µA/cm²)

15

30 60 90 Elapsed Time (Days)

9 6 3 0

9 6 3 0

0

30 60 90 Elapsed Time (Days)

120

(e)

0

30 60 90 Elapsed Time (Days)

120

(f)

Fig. 6. Trend of corrosion current density values against time for (a) N2C, (b) F30C, (c) N2CB, (d) F30CB, (e) N2CT and (f) F30CT series.

at the end of one complete cycle, i.e. wet and dry, so the values correspond to the dry cycle in C series while, for CT and CB, measurements correspond to saturated state, as the salt solution was continuously applied at the crack mouth. As reported

previously, the value of half-cell potential is sensitive to the changes in the moisture content of concrete i.e. by wetting the concrete surface, the half-cell potential values in the wetted areas will shift to more negative values [15].

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Corrosion Potential (mV vs CSE)

N2C F30ZL* N2Z* -200

F30C N2C F30ZL*

N2Z* F30C

R² = 0.80

-300 -400 -500 -600

R² = 0.44 R² = 0.71

-700 0.1

R² = 0.69

1 Corrosion Current (µA/cm²)

10

(a)

Corrosion Potential (mV vs CSE)

N2CT F30ZL* N2Z* -200

F30CT N2CT F30ZL*

-300 R² = 0.98

N2Z* F30CT

R² = 0.95

-400 -500 -600 -700 0.1

1 Corrosion Current (µA/cm²)

10

(b) N2CB F30ZL* N2Z* -200

Corrosion Potential (mV vs CSE)

3.2.2. Corrosion current The trend of corrosion current values measured against the time period for each specimen series is shown in Fig. 6. The following order of corrosion current density (Icorr) measurements, CT > CB > C for both cracked normal and fly ash concrete, was depicted in Fig. 6. The variation in the corrosion current is a clear indication that different exposure conditions have an influence on the overall corrosion activity, affecting the oxygen diffusion and moisture state in the concrete matrix. After the initiation of the corrosion, the constant supply of the chloride ion is necessary to increase the corrosion rate along with other critical agents. For normal concrete, CT series has shown the highest corrosion current, this might be because no crack filling was observed for N2CT series as well as a high degree of saturation at level of rebar. As reported previously the corrosion current increases with an increase in the pore saturation of concrete, and maximum value was obtained for saturated concrete [19]. On the other hand, N2CB series showed some degree of crack filling ability, which might have hindered the supply of chloride ion to reach the rebar, causing the corrosion current to decrease relative to N2CT. Although N2C series has shown no crack filling ability, the corrosion current values are on the lower side as compared to CT and CB series. The main reason for this trend is because all the measurements correspond to the end of dry cycle, at which the degree of saturation is quite low, which will affect the supply of chloride ions to increase the corrosion activity. In the case of fly ash concrete, all the series have shown the crack filling effect, but the trend of corrosion current is different in all three series. In the case of F30CT and F30CB, they are continuously exposed to salt solution, and the crack space is fully saturated, but the dense microstructure of adjacent concrete matrix will restrict the diffusion in the perpendicular direction, causing the corrosion activity to enhance at crack steel interface. In addition, the crack filling will not allow the inside moisture to evaporate, and will thus increase the corrosion activity. While in the case of cyclic wetting and drying, increase in the corrosion rate is mainly because of the accumulation of chloride ions at the surface resulting from the evaporation of water and increase in the availability of oxygen during the drying cycle [20]. In addition, the drying rate is also dependent on the pore structure of the concrete, which causes the high performance concrete to dry at a slower rate as compared to normal concrete. Considering the inner walls of cracks also as the surface exposed to salt solution, the wet and dry cycle can cause an increase in the surface chloride ion concentration at the crack walls, and, ultimately, at crack steel interface, resulting in a high corrosion rate in the cracked concrete. The higher corrosion current of N2C as compared to F30C may be because of higher surface chloride ion concentration at crack steel interface and low resistance of normal concrete. For cracked concrete, it is normally considered that macro cell corrosion activity will govern the corrosion mechanism, and, subsequently, the corrosion rate will depend on the diffusion of oxygen through the un-cracked concrete to the cathodic sites or the electric resistance around the steel [21]. The experimental methodology used here also supports the formation of macro cell corrosion by limiting the application of salt solution only to crack region. From the results of Icorr, it was observed that fly ash concrete had shown the reduced values as compared to that of normal concrete under all exposure conditions. The superior behaviour of cracked fly ash concrete in terms of Icorr is mainly because of the restricted supply of oxygen at the cathodic sites due to dense internal structure of fly ash concrete, as compared to normal concrete. In addition, the high electrical resistance of concrete at the noncracked region (acting as cathode) will restrict the flow of electrons from anode to cathode, and will push the corrosion rate values towards the lower side. Another factor which may also influence

F30CB N2CB F30ZL*

N2Z* F30CB

-300 R² = 0.92 -400 R² = 0.97

-500 -600 -700 0.1

1 Corrosion Current (µA/cm²)

10

(c) Fig. 7. Semi-log plots of half-cell potential versus corrosion current for the steel embedded against N2Z and F30ZL in (a) N2C, F30C, (b) N2CT, F30CT (c) N2CB and F30CB series.

is that the closure of the cracks in fly ash concrete is greater compared to the normal concrete, which will affect the corrosion activity inside the crack.

N2C1 N2C4

N2C2 Fitting

N2C3

30 20 10 0 0

20

40

60

Chloride Ion Concentartion （kg/m3)

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Chloride Ion Concentartion （kg/m3)

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F30C1 F30C4

20 10 0 0

80

40

60

80

(b) N2CB2 Fitting

30 20 10

Chloride Ion Concentration (kg/m³)

Chloride Ion Concentration (kg/m³)

20

Distance from the Crack(mm)

(a)

F30CB1 F30CB3

F30CB2 Fitting

30 20 10 0

0 0

0

20 40 60 80 Distance along Crack (mm) (c)

20 40 60 80 Distance along Crack (mm) (d)

N2CT2 Fitting

30 20 10 0 0 20 40 60 80 Distance along Crack (mm)

(e)

F30CT1 F30CT3 Chloride Ion Concentration (kg/m³)

N2CT1 N2CT3 Chloride Ion Concentration (kg/m³)

F30C3

30

Distance along Crack(mm)

N2CB1 N2CB3

F30C2 Fitting

F30CT2 Fitting

30 20 10 0 0 20 40 60 80 Distance along Crack (mm) (f)

Fig. 8. Distribution of chloride ion concentration along the Crack for (a) N2C, (b) F30C, (c) N2CB, (d) F30CB, (e) N2CT and (f) F30CT series.

3.2.3 Ecorr vs Icorr plot The semi log graphs between the half-cell potential and corrosion current density for N2C and F30C, N2CB and F30CB and N2CT and F30CT are shown in Fig. 7(a–c) respectively. As a reference, the non-cracked specimen, having almost same mix proportions (N2Z and F30ZL), is also plotted in Fig. 7 [22]. The absolute

values of the slopes for the best fit plots of half-cell potential and corrosion current density were found to be 149, 115, 290, 207, 255, 247, 300 and 256 mV/decade for N2Z, F30ZL N2C, F30C, N2CB, F30CB, N2CT and F30CT respectively. There is a clear difference between the slopes of cracked and non-cracked specimens referring towards the difference in the corrosion mechanism of

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both. However, for cracked specimens, there seemed to be no significant difference between the slopes for fly ash concrete and normal concrete. For non-cracked specimens, the values of the slopes lie within the normal range of activation-controlled electrode process, i.e. 50–150 mV/decade [23], while, for cracked specimens, the values of gradients lie well above the mentioned range. This suggests that, for cracked concrete, the cathodic process of oxygen reduction might be subjected to concentration-controlled electrode process because of the protective film of the corrosion products formed at the rebar surface, which does not allow the aggressive agents to reach the naked rebar and accelerate corrosion.

3.3. Chloride ion concentration Fig. 8(a–f), shows the variation of measured and estimated chloride ion concentration along the crack for N2C, F30C, N2CB, F30CB, N2CT and F30CT, respectively. It was found that exposure conditions have an influence on the penetration of chloride ion into the concrete matrix. The highest chloride ion concentration for the same exposure duration was observed for the wet and dry exposure conditions for both normal and fly ash concrete. The high chloride ion concentration for wet and dry cycle is in accordance with the results of the crack filling ability of fly ash concrete. F30CT and F30CB series showed relatively high crack filling ability as compared to the F30C series, which may have hindered the penetration of chloride ion and led to low values at the rebar level. However, for each exposure condition, fly ash concrete showed lower values of chloride ion concentration at the level of rebar as compared to the normal Portland cement concrete. The average value of measured chloride ion concentration at a depth of 15 mm from the exposure surface for N2C, F30C, N2CB, F30CB, N2CT and F30CT was 12.5, 11.7, 12, 7.1, 12.1 and 8.8 kg/m3, respectively. It was observed that the overall corrosion rate for F30 series for all exposure conditions was on the lower side as compared to that of N2 series. For wet and dry cycle, although the average value of total chloride ion is almost same for both types of cracked specimen, the amount of free chloride ions, generally responsible for corrosion initiation, is less in fly ash concrete as compared to that of normal Portland cement concrete because of higher chloride binding for fly ash concrete [24]. While for CB and CT series, the values of chloride ion concentration for fly ash concrete are clearly on the lower side as compared to the normal ordinary Portland cement concrete, causing the reduction in corrosion current values. The difference in the chloride penetration is because, for CB and CT, the chloride penetration depended mainly on the presence of moisture in concrete matrix and fly ash concrete with a dense microstructure hinders the penetration, while, for C series, the chloride penetration, is governed by two phenomenon i.e. diffusion and capillary absorption. Diffusion occurs during the wet cycle, while capillary absorption is believed to take place immediately after the completion of the dry cycle resulting in higher values of chloride ion at rebar as compared to continuous application of salt solution. In addition, the high values of corrosion current for N2CT, N2CB and N2C correspond well with the high values of chloride ion concentration for their respective series. For fly ash series, although the values of chloride ion concentration are small as compared to normal concrete, the values of corrosion current for F30CT and F30CB are relatively on the higher side. This might be because fly ash replacement causes the chloride threshold value to decrease as compared to normal Portland cement concrete, and fly ash concrete can show high corrosion current values at even smaller chloride ion concentration values.

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3.4. Surface corroded area and mean corrosion depth Fig. 9 shows the plot between the corrosion depths estimated by using the Icorr’ values and measured by the scanner. Although the values are slightly scattered, still a good corelation between the two mean corrosion depths was still observed. The relation between surface corroded area (S) and estimated mean corrosion depth-scanner (u) for both the specimen series is shown in Fig. 10. In this study, the uniform corrosion and pitting corrosion has been defined in terms of surface corroded area and mean corrosion depth. Based on the experimental results, the approximate ranges for the pitting corrosion is found to exist when the S/u ratio is less than 0.7, while the uniform corrosion exists when S/u ratio is larger than 1.5, whereas in between these ratios both uniform and pitting corrosion coexist. Based on this definition, the dotted lines were plotted to represent the different corrosion types, as shown in Fig. 10. The region below line A represents the pitting corrosion, while the region above line B represents uniform corrosion, and the region in between these lines represents coexistence of uniform and pitting corrosion. It was observed that, for the same concrete mix, different exposure conditions have resulted in different corrosion mechanisms. The fly ash concrete showed higher surface corroded areas as compared to the normal concrete specimens when exposed to wet and dry cycle indicating to uniform corrosion. On the other hand, when fly ash concrete was continuously exposed to salt solution, i.e. CT and CB series, the corroded surface areas were reduced and corrosion depths were found to increase, indicating to the more localized corrosion. Tittarelli has also reported the corrosion attack to be more diffusive and less penetrating for the partial replacement of fly ash with ordinary Portland cement in recycled aggregate concrete when exposed to wet and dry cycles [25]. In addition, Jaffer also reported that the average spread of corrosion products on the rebar surface in ordinary Portland cement concrete in the vicinity of the intersecting concrete crack was less than that of high performance concrete when exposed to 3% salt solution under wet and dry cycle [26]. One of the reasons for this behaviour is that, for wet and dry cycle, the lower porosity for fly ash concrete prevented the diffusion of corrosion products into the neighboring concrete and limited them to the rebar-concrete interface, while, for normal concrete, the corrosion products can diffuse in the concrete matrix [24]. However, for the CT and CB series, the crack is fully saturated with the salt solution and the corrosion products can diffuse along the crack walls in the salt solution to the exposed surface. In addition, the crack filling ability in fly ash concrete also has an influence on corrosion depth of the reinforcement. It was observed, that for all the exposure conditions, the fly ash concrete had shown the crack filling ability. For wet and dry cycle, the crack filling is relatively less as compared to CT and CB series, which corresponds well with the corrosion depth results. The small crack filling in F30C series can allow the salt solution to evaporate and move in the lateral direction during the dry cycle, while, for the CB and CT series, the high degree of crack filling will not allow the salt solution to evaporate and stay in the crack, leading to severe pitting corrosion. Furthermore, it is generally observed that flexural cracking usually yields some local slip between the rebar and the concrete, resulting in additional skin separation between steel and concrete in the locality of crack, which can attract the chloride ions to move along the length of rebar. It is assumed the local slip between steel and crack, which can be different for normal and fly ash concrete, led to different corrosion behaviours under different exposure conditions, however, more detailed research is necessary to verify this phenomenon.

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N2C F30CB

F30C N2CT

N2CB F30CT

Estimated Mean Corrosion Depth-Icorr (µm)

80

1 cm

60 40

(a)

20 0 0

20 40 60 Estimated Mean Corrosion Depth-Scanner (µm)

80

(b)

Fig. 9. Plot between estimated mean corrosion depth-Icorr’ and estimated mean corrosion depth-scanner for all specimen series.

Corroded Surface Area (cm²)

N2C N2CT

F30C F30CT

N2CB Line A

F30CB Line B

100

(c) Uniform

50

Uniform + Ping

Ping

(d)

0 0

20 40 Estimated Mean Corrosion Depth-Scanner (µm)

60

Fig. 10. Plot between surface corroded area and mean corrosion depth.

3.5. Rebar surface analysis The corrosion products were removed from the surface of rebar to verify the extent of corrosion and measure the pit depths. Examples of the cleaned rebar specimens for (a) N2C4, (b) F30C4, (c) N2CB1, (d) F30CB1, (e) N2CT3 and (f) F30CT2 are shown in Fig. 11(a–f), respectively. The results of Fig. 10 are in accordance with the visual observation of the naked rebar surface. It is evident from Fig. 11 that the pitting effect was more prominent in N2C, F30CB and F30CT series as compared to F30C, N2CB and N2CT series. It was also observed that ribs were the most severely damaged part of the steel rebar. It was noticed that, for N2C series, the number of pits were fewer, but deeper and larger in diameter as compared to F30C, which showed a greater number of pits, but with shallow and small diameter, as shown in Fig. 11(a) and (b). On the other hand the pits were found to be more dispersed and shallow for N2CB and N2CT as compared to F30CB and F30CT. The large and deep pits indicate high corrosion rate while the shallow and disperse pits indicate low corrosion rate. These results of rebar surface analysis are in

(e)

(f) Fig. 11. Rebar surface after removal of corrosion products (a) N2C4, (b) F30C4, (c) N2CB1, (d) F30CB1, (e) N2CT3 and (f) F30CT2 specimen.

accordance with that of the corrosion current density, surface corroded area and attack penetration. In addition, the rebar surface analysis revealed that the number of pits is proportional to the surface corroded area, while the depth of a pit is inversely proportional to the surface corroded area.

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4. Conclusions In this paper, an experimental study was conducted on the cracked reinforced fly ash concrete specimens, which were exposed to wet and dry cycles under static conditions. The salient outcomes are as follows: 1. It was observed that the crack filling ability of concrete is sensitive to crack width as compared to the fly ash replacement and exposure conditions. Both ordinary Portland cement concrete and fly ash concrete had shown crack filling ability for the crack width less than 50 mm; however, with the crack widths in-between 50 and 100 mm, only fly ash concrete was found to heal the cracks. In addition, it was observed that, for normal Portland cement concrete, crack filling was not to the extent which could affect the corrosion behaviour; however, for fly ash concrete, the crack filling was found to be effective in retarding the corrosion rate of the steel reinforcement under all exposure conditions. 2. The results of half-cell potential indicated that the corrosion initiation period for cracked fly ash concrete appeared to be the same as that of cracked normal Portland cement concrete. In this regard no beneficial effect of the crack filling in the fly ash concrete was found. The presence of cracks, irrespective of the fly ash replacement as a part of cement, can adversely affect the corrosion initiation of rebars embedded in concrete when exposed to salt solution under static loading. 3. Exposure conditions were found to have a large influence on the corrosion mechanism of ordinary Portland cement concrete and fly ash concrete. It was observed that, under wet and dry cycles, the fly ash concrete showed decreased values of mean corrosion depth corresponding to high surface corroded areas as compared to the normal concrete, indicating the suppression of localized corrosion in cracked fly ash concrete. On the other hand increased values of mean corrosion depth corresponding to small corroded surface area were observed in submerged condition for fly ash concrete. In addition, the ratio of surface corroded area to mean corroded depth was introduced to depict the corrosion behaviour of steel reinforcement in concrete. 4. Fly ash concrete can effectively reduce the corrosion rate of steel reinforcement even in the presence of cracks when exposed to salt solution, but the dense micro structure and high electrical resistance can restrict the corrosion damage to be localized to the crack-rebar interface and result in severe pitting corrosion, depending on the exposure conditions.

Acknowledgement This research was partially funded by Japan Society for Promotion of Science (Research No. 23360187). References [1] Japan Society of Civil Engineers (JSCE), Standard specifications for concrete structures, 2012.

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