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REPORT
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Publication Number:  FHWA-HRT-11-060    Date:  November 2011
Publication Number: FHWA-HRT-11-060
Date: November 2011

 

Multiple Corrosion-Protection Systems for Reinforced Concrete Bridge Components

APPENDIX A. DISBONDMENT OF CONVENTIONAL EPOXY-COATED AND MC BARS IN RAPID MACROCELL TEST

In addition to the rapid macrocell tests described in the body of the report, 24 ECR and 24 MC rapid macrocell tests using 6.04 molal ion (15 percent) concentration sodium chloride solution were used to compare the disbondment characteristics of conventional ECR and MC bars under the severe exposure conditions produced in the test as a function of corrosion loss and time. The specimens consisted of a single ECR or MC bar as the anode and two bare steel bars as the cathode. Every 5 weeks, three specimens were pulled from testing and a disbondment test was performed on the anode. The schedule of testing is shown in table 53. In addition, five control specimens of each type were placed in simulated pore solution with no chlorides to track disbondment in the absence of chlorides. Disbondment was performed using the procedures described in chapter 2 for the cathodic disbondment test. Measurements are expressed in terms of the total area of disbonded material at each test point.

Table 53. Rapid macrocell test program.

ECR

MC

Specimen Designation

Weeks Tested

Specimen Designation

Weeks Tested

M-ECR-1

5

M-MC-1

5

M-ECR-2

5

M-MC-2

5

M-ECR-3

5

M-MC-3

5

M-ECR-4

10

M-MC-4

10

M-ECR-5

10

M-MC-5

10

M-ECR-6

10

M-MC-6

10

M-ECR-7a

15

M-MC-7

15

M-ECR-8

15

M-MC-8

15

M-ECR-9

15

M-MC-9

15

M-ECR-10

20

M-MC-10

20

M-ECR-11

20

M-MC-11

20

M-ECR-12

20

M-MC-12

20

M-ECR-13a

40

M-MC-13

25

M-ECR-14

25

M-MC-14

25

M-ECR-15

25

M-MC-15

25

M-ECR-16

25

M-MC-16

30

M-ECR-17

30

M-MC-17

30

M-ECR-18

30

M-MC-18

30

M-ECR-19a

40

M-MC-19

35

M-ECR-20

30

M-MC-20

35

M-ECR-21

35

M-MC-21

35

M-ECR-22

35

M-MC-22a

40

M-ECR-23

35

M-MC-23a

40

M-ECR-24

40

M-MC-24a

40

Control specimens in simulated pore solution (no salt)

M-ECR-A

10

M-MC-A

10

M-ECR-B

20

M-MC-B

20

M-ECR-C

25

M-MC-C

30

M-ECR-D

30

M-MC-D

35

M-ECR-E

35

M-MC-E

40

a Linear polarization resistance tests performed on this specimen.

TEST RESULTS

The duration of the test ranged from 5 to 40 weeks. The purpose of the test program was to establish a relationship between corrosion loss, time, and disbondment of the epoxy coating for conventional ECR and MC reinforcement.

Corrosion Rate, Loss, and Potentials

The average macrocell corrosion rates based on total area for macrocell specimens with ECR and MC reinforcement are shown in figure 204. During the first five weeks of testing, the specimens with MC reinforcement exhibited significantly higher corrosion rates than the specimens with conventional ECR. A slight increase in corrosion rate was observed for the MC specimens between weeks 15 and 20. The corrosion rates were otherwise comparable to those observed in conventional ECR.

During the first 5 weeks of testing, the specimens with multiple-coated (MC) reinforcement exhibit significantly higher corrosion rates than the specimens with conventional epoxy-coated reinforcement (ECR). A slight increase in corrosion rate is observed for the MC specimens between weeks 15 and 20. Corrosion rates are otherwise comparable to those observed in conventional ECR.

1 Mum = 0.0394 mil

Figure 204. Graph. Rapid macrocell test, average corrosion rate based on total area of ECR and MC reinforcement.

The average corrosion losses based on average corrosion rates for the macrocell specimens with ECR and MC reinforcement are shown in figure 205. The increased corrosion rate observed for MC reinforcement from weeks 0 to 5 and weeks 15 to 20 resulted in a greater overall corrosion loss at 40 weeks (7.55 Mum (0.297 mil) compared to 5.43 Mum (0.214 mil) for ECR) as well as greater losses throughout the test.

The increased corrosion rate observed for multiple-coated (MC) reinforcement from weeks zero to 5 and weeks 15 to 20 results in a greater overall corrosion loss at 40 weeks of 7.55 µm (0.297 mil) compared to 5.43 µm (0.214 mil) for epoxy-coated reinforcement (ECR), as well as greater losses throughout the test.

1 Mum = 0.0394 mil

Figure 205. Graph. Rapid macrocell test, average corrosion loss based on total area of ECR and MC reinforcement.

The average anode and cathode potentials with respect to an SCE are shown in figure 206 and figure 207, respectively, for specimens with conventional ECR and MC reinforcement. At the anode, the corrosion potentials of both ECR and MC were more negative than -0.500 V for most of the test. The MC specimens showed a more negative anode potential than the ECR specimens for the first 27 weeks of testing. After 27 weeks, the corrosion potentials of the specimens with MC and ECR were comparable.

The corrosion potentials of both epoxy-coated reinforcement (ECR) and multiple-coated (MC) are more negative than -0.500 V for most of the test. The MC specimens show a more negative anode potential than the ECR specimens for the first 27 weeks of testing. After 27 weeks, the corrosion potentials of the specimens with MC and ECR are comparable.

Figure 206. Graph. Rapid macrocell test, average anode potential of ECR and MC reinforcement.

 

The corrosion potentials of the specimens with both epoxy-coated reinforcement (ECR) and multiple-coated (MC) reinforcement are approximately -0.200 V (SCE) at the start of the test. For the ECR specimens, the corrosion potential gradually decreases to approximately -0.350 V by week 40, whereas the corrosion potential of the MC specimens remains approximately -0.200 V until week 36 when it decreases to -0.300 V over a 4-week period.

Figure 207. Graph. Rapid macrocell test, average cathode potential of ECR and MC reinforcement.

At the cathode, the corrosion potentials of the specimens with both ECR and MC reinforcement were approximately -0.200 V at the start of the test. For the ECR specimens, the corrosion potential gradually decreased to approximately -0.350 V by week 40, whereas the corrosion potential of the MC specimens remained at approximately -0.200 V until week 36, when it decreased to -0.300 V over a four-week period.

Linear polarization resistance measurements were performed on selected specimens on a monthly basis. The total corrosion losses calculated from the linear polarization readings are compared to the macrocell corrosion losses in table 54. On average, the total corrosion losses were 4.0 times greater than macrocell corrosion losses, with the ratio of total to macrocell corrosion loss ranging from 1.45 (M-ECR-7) to 8.41 (M-ECR-13).

Table 54. Total and macrocell corrosion loss for selected rapid macrocell

Specimen

Age (weeks)

Corrosion Loss (Mu m)

Microcell

Total

M-ECR-7

15

2.83

1.95

M-ECR-13

40

55.2

6.56

M-ECR-19

40

19.1

4.95

M-MC-22

40

18.2

7.64

M-MC-23

40

35.4

7.83

M-MC-24

40

31.0

9.04

1 Mum = 0.0394 mil

Visual Observations

For the ECR specimens, corrosion products were visible at some damage sites after 5 weeks of testing (see figure 208), with moderate amounts of disbondment observed at some damage sites (see figure 209). Other damage sites on the ECR bars tested at 5 weeks showed no disbondment. The ECR bars tested for 10 weeks or longer showed disbondment at all damage sites. For the ECR specimens tested for 30 weeks or longer, corrosion products were observed at all damage sites (see figure 210), with the disbonded region covering much of the area of the bar (see figure 211). The ECR bars removed after 30 to 40 weeks of testing showed severe disbondment at all damage sites (see figure 211).

Corrosion products were visible at some damage sites after 5 weeks of testing.

Figure 208. Photo. Rapid macrocell test, M-ECR-1, 5 weeks, before disbondment test.

Moderate amounts of disbondment were observed at some damage sites.

Figure 209. Photo. Rapid macrocell test, M-ECR-1, 5 weeks, after disbondment test.

Corrosion products were observed at all damage sites.

Figure 210. Photo. Rapid macrocell test, M-ECR-13, 40 weeks, before disbondment test.

Disbonded region covers much of the area of the bar.

Figure 211. Photo. Rapid macrocell test, M-ECR-21, 35 weeks, after disbondment test.

For the MC specimens, those removed after 5 or 10 weeks of testing showed no signs of iron corrosion products (see figure 212) in contrast to specimens with ECR, which showed visible corrosion products after 5 weeks. Disbondment tests on the MC bars revealed minimal disbondment on specimens removed from testing after 5 or 10 weeks (see figure 213). Regions where disbondment did occur on the MC bars showed a ring of darkened metal around the damage site, indicating the zinc in this region has been consumed (see figure 214). The MC specimens began to show dark orange iron corrosion products by week 15, with all three of the MC specimens removed at week 40 showing dark iron corrosion products (see figure 215). The disbondment tests performed at 40 weeks on the MC bars showed moderate disbondment at all damage sites (see figure 216). The corrosion products in the disbonded regions of the MC bars consist of a central circle of dark iron corrosion products around the damage site surrounded by a larger region of light gray corrosion products, most likely from corroding zinc.

Those specimens removed after 5 or 10 weeks of testing show no signs of iron corrosion products.

Figure 212. Photo. Rapid macrocell test, M-MC-4, 10 weeks, before disbondment test.

Disbondment tests on the multiple-coated (MC) bars reveal minimal disbondment on specimens removed from testing after 5 or 10 weeks.

Figure 213. Photo. Rapid macrocell test, M-MC-4, 10 weeks, after disbondment test.

Regions where disbondment occurs on the multiple-coated (MC) bars show a ring of darkened metal around the damage site, indicating the zinc in this region has been consumed.

Figure 214. Photo. Rapid macrocell test, M-MC-5, 10 weeks (zinc depletion in regions surrounding damage sites).

The multiple-coated (MC) specimens begin to show dark orange iron corrosion products by week 15, with all three of the MC specimens removed at week 40 showing dark iron corrosion products.

Figure 215. Photo. Rapid macrocell test, M-MC-23, 40 weeks, before disbondment test.

. The disbondment tests performed at 40 weeks on the multiple-coated (MC) bars show moderate disbondment at all damage sites. The corrosion products in the disbonded regions of the MC bars consist of a central circle of dark iron corrosion products around the damage site surrounded by a larger region of light gray corrosion products, most likely from corroding zinc.

Figure 216. Photo. Rapid macrocell test, M-MC-23, 40 weeks, after disbondment test.

In general, damage sites with visible corrosion products tended to exhibit greater disbondment than damage sites with no visible corrosion products for both ECR (figure 209) and MC (figure 216). Both the disbonded area and visible corrosion increased with time. The corrosion products at the damage sites were dark orange or brown in color. Corrosion products underneath disbonded epoxy were initially black (figure 209), but turned dark orange with exposure to air as the corrosion products oxidized.

Disbondment Results

Disbonded area and corrosion loss for the ECR and MC bars are summarized in table 55 and table 56, respectively. Figure 217 and figure 218 show average disbonded area at a damage site as a function of time for ECR and MC reinforcement, respectively. A best fit line is plotted for each set of data. Based on the best fit lines, disbondment of ECR progressed at about twice the rate of disbondment of MC reinforcement, with ECR showing increases in disbonded area averaging 10.8 mm2/week (0.0167 in2/week) compared to 5.32 mm2/week (0.0082 in2/week) for MC reinforcement. It should be noted that disbondment at a given age varies widely for individual specimens of both types, especially at later ages.

Table 55. Disbonded area and corrosion loss for rapid macrocell specimens with ECR.

Specimen Week Average Disbonded Area, mm2 Corrosion Loss,
Mu m
Disbondment
Rate,
mm2/week
Disbondment Rate, mm2/ Mu m

M-ECR-1

5

35.5

0.359

7.10

98.8

M-ECR-2

5

16.1

0.318

3.23

50.7

M-ECR-3

5

37.1

0.233

7.42

159.2

M-ECR-4

10

74.2

0.638

7.42

116.3

M-ECR-5

10

158.1

0.513

15.8

308.1

M-ECR-6

10

133.9

1.29

13.4

103.8

M-ECR-7

15

116.1

1.92

7.74

60.5

M-ECR-8

15

154.8

0.428

10.3

361.8

M-ECR-9

15

187.1

3.35

12.5

55.8

M-ECR-10

20

175.8

4.99

8.79

35.2

M-ECR-11

20

338.7

4.59

16.9

73.8

M-ECR-12

20

140.3

4.71

7.02

29.8

M-ECR-13

40

456.5

4.94

11.4

92.4

M-ECR-14

25

567.7

3.73

22.7

152.2

M-ECR-15

25

187.1

1.04

7.48

179.9

M-ECR-16

25

408.1

2.16

16.3

188.9

M-ECR-17

30

232.3

3.35

7.74

69.3

M-ECR-18

30

162.9

2.37

5.43

68.7

M-ECR-19

40

261.3

4.64

6.53

56.3

M-ECR-20

30

198.4

5.75

6.61

34.5

M-ECR-21

35

874.2

6.24

25.0

140.1

M-ECR-22

35

222.6

5.41

6.36

41.1

M-ECR-23

35

598.4

3.27

17.1

183.0

M-ECR-24

40

241.9

5.38

6.05

45.0

1 mm2 = 0.00155 in2
1 Mum = 0.0394 mil

Table 56. Disbonded area and corrosion loss for rapid macrocell specimens with MC reinforcement.

Specimen

Week

Average Disbonded Area, mm2

Corrosion Loss,
Mu m

Disbondment Rate, mm2/week

Disbondment Rate, mm2/ Mu m

M-MC-1

5

9.7

1.11

1.94

8.7

M-MC-2

5

4.8

1.73

0.968

2.8

M-MC-3

5

21.0

1.3

4.19

16.1

M-MC-4

10

41.9

1.97

4.19

21.3

M-MC-5

10

43.5

3.42

4.35

12.7

M-MC-6

10

50.0

3.17

5.00

15.8

M-MC-7

15

77.4

4.97

5.16

15.6

M-MC-8

15

54.8

1.99

3.66

27.6

M-MC-9

15

74.2

2.98

4.95

24.9

M-MC-10

20

133.9

6.12

6.69

21.9

M-MC-11

20

125.8

4.19

6.29

30.0

M-MC-12

20

69.4

3.03

3.47

22.9

M-MC-13

25

162.9

3.58

6.52

45.5

M-MC-14

25

137.1

2.86

5.48

47.9

M-MC-15

25

146.8

6.61

5.87

22.2

M-MC-16

30

116.1

5.48

3.87

21.2

M-MC-17

30

98.4

3.13

3.28

31.4

M-MC-18

30

156.5

3.93

5.22

39.8

M-MC-19

35

117.7

6.89

3.36

17.1

M-MC-20

35

129.0

6.73

3.69

19.2

M-MC-21

35

308.1

8.64

8.80

35.7

M-MC-22

40

127.4

7.64

3.19

16.7

M-MC-23

40

177.4

7.83

4.44

22.7

M-MC-24

40

301.6

9.04

7.54

33.4

1 mm2 = 0.00155 in2
1 Mum = 0.0394 mil

Disbondment of epoxy-coated reinforcement (ECR) progresses at about twice the rate of disbondment of multiple-coated (MC) reinforcement, with ECR averaging 10.8 mm2/week (0.0167 inches2/week). Disbondment at a given age varies widely, especially at later ages.

1 mm2 = 0.00155 in2

Figure 217. Graph. Rapid macrocell test, disbonded area versus time for specimens with ECR.

Disbondment of epoxy-coated reinforcement progresses at about twice the rate of disbondment of multiple-coated (MC) reinforcement, with MC reinforcement showing disbonded area averaging 5.32 mm2/week (0.0082 inches2/week). Disbondment at a given age varies widely for individual specimens, especially at later ages.

1 mm2 = 0.00155 in2

Figure 218. Graph. Rapid macrocell test, disbonded area versus time for specimens with MC reinforcement.

Figure 219 and figure 220 show disbonded area versus macrocell corrosion loss for ECR and MC reinforcement, respectively. The ECR showed greater disbondment at a given corrosion loss than the MC reinforcement. On average, the disbondment of the ECR increased by 56.3 mm2 (2.21 in2) for every 1 Mum (1 mil) of corrosion loss, compared to an average increase of 26.5 mm2 (1.04 in2) for every 1 Mum (1 mil) of corrosion loss for the MC bars. The difference in disbondment rate between ECR and MC reinforcement was more pronounced in specimens with low corrosion losses. For epoxy-coated bars with less than 2.0 Mum (0.079 mil) of corrosion loss, the disbondment rate ranged from 50.7 to 308.1 mm2/Mum (2.00 to 12.1 in2/mil) of loss, with an average rate of 159.9 mm2/Mum (6.30 in2/mil). For the MC bars with less than 2.0 Mum (0.078 mil) of corrosion loss, the disbondment rate ranged from 2.80 to 27.6 mm2/Mum (0.11 to 1.09 in2/mil) of loss, with an average rate of 15.3 mm2/Mum (0.60 in2/mil).

The epoxy-coated reinforcement (ECR) shows greater disbondment at a given corrosion loss than the multiple-coated (MC) reinforcement; on average, the disbondment of the ECR increases by 56.3 mm2 (0.0873 inches2) for every 1 µm (0.04 mil) of corrosion loss. For epoxy-coated bars with less than 2.0 µm (0.079 mil) of corrosion loss, the disbondment rate ranges from 50.7 to 308.1 mm2 (0.0786 to 0.478 inches2) per 1 µm (0.039 mil) of loss, with an average rate of 159.9 mm2/µm (0.248 inches2/µm).

1 mm2 = 0.00155 in2
1 Mum = 0.0394 mil

Figure 219. Graph. Rapid macrocell test, disbonded area versus corrosion loss for specimens with ECR.

The epoxy-coated reinforcement shows greater disbondment at a given corrosion loss than the multiple-coated (MC) reinforcement. On average, the disbondment of the MC bars increases by 26.5 mm2 (0.0411 inches2) for every 1 µm (0.04 mil) of corrosion loss. For the multiple-coated bars with less than 2 µm (0.079 mil) of corrosion loss, the disbondment rate ranges from 2.80 to 27.6 mm2 (0.0043 to 0.0428 inches2) per 1 µm (0.039 mil) of loss, with an average rate of 15.3 mm2/µm (0.0237 inches2/µm).

1 mm2 = 0.00155 in2
1 Mum = 0.0394 mil

Figure 220. Graph. Rapid macrocell test, disbonded area versus corrosion loss for specimens with MC reinforcement.

In addition to the rapid macrocell specimens, five epoxy-coated bars and five MC bars with damaged regions identical to those used in the rapid macrocell test were exposed to a simulated pore solution without salt to determine the disbondment of the coatings with time in the absence of chlorides and chloride-induced corrosion. The results are shown in figure 221. At 10 weeks, the MC bars had a disbonded area of 25.8 mm2 (0.0394 in2), whereas ECR exhibited no disbondment. However, at 20 weeks, the ECR and MC reinforcement exhibited the same degree of disbondment in the absence of corrosion, and at later ages, specimens with ECR exhibited greater disbonded areas than specimens with MC reinforcement. For both ECR and MC bars, disbondment at 40 weeks in the absence of chlorides was 20-25 percent of the values observed at 40 weeks in bars subjected to chloride-induced corrosion.

At 10 weeks, the multiple-coated (MC) bars show a disbonded area of 25.8 mm2 (0.0394 inches2), whereas epoxy-coated reinforcement (ECR) shows no disbondment. However, at 20 weeks, the ECR and MC reinforcement exhibit the same degree of disbondment in the absence of corrosion. At later ages, specimens with ECR exhibit greater disbonded areas than specimens with MC reinforcement.

1 mm2 = 0.00155 in2

Figure 221. Graph. Disbonded area versus time for ECR and MC reinforcement in simulated pore solution without salt.

Summary

For the rapid macrocell test, the specimens with MC reinforcement showed greater corrosion losses than the specimens with conventional ECR. However, MC reinforcement showed significantly less disbondment than ECR, both as a function of corrosion loss and as a function of time.

 

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