Influence of external loading and loading type on corrosion behavior of RC beams with epoxy-coated reinforcements

Construction and Building Materials 93 (2015) 746–765

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Influence of external loading and loading type on corrosion behavior of RC beams with epoxy-coated reinforcements Xiao-Hui Wang ⇑, Bing Chen, Yang Gao, Jing Wang, Lu Gao Department of Civil Engineering, Shanghai Jiaotong University, Shanghai 200240, China

h i g h l i g h t s  Corrosion performance of epoxy-coated RC beam is investigated.  Both environmental attacks and external static and fatigue loads under serviceability state were considered.  Monitoring of open-circuit potential and electrochemical impedance spectroscopy is carried out.  Influence of factors on corrosion behavior of reinforcing bars in test specimens is discussed.  Fatigue loading causes great reduction of corrosion resistance of epoxy-coated bars in RC specimens.

a r t i c l e

i n f o

Article history: Received 21 November 2014 Received in revised form 16 April 2015 Accepted 2 May 2015 Available online 23 May 2015 Keywords: Epoxy-coat bar Corrosion resistance Fatigue load Static load Fatigue loading cycles Simulated seawater solutions

a b s t r a c t Corrosion performance of RC beam specimens with epoxy-coated reinforcing bars under in-service environments and external loads is experimentally investigated, where 14 kN static and fatigue loads (2.8– 14 kN) were applied as well as 500,000 and 1,000,000 fatigue loading cycles were considered to simulate loading conditions of RC elements under serviceability state. Test specimens were firstly subjected to the above-mentioned loads and then immersed in simulated seawater solutions. Monitoring of open-circuit potential and electrochemical impedance spectroscopy (EIS) was carried out. Results of actual coating thicknesses of the reinforcing bars in test specimens, crack monitoring after loading tests, first wet-cycle and dry-cycle open-circuit potentials and first wet-cycle EIS data of the reinforcing bars in test specimens are presented. Electrical equivalent circuits are used to model the EIS data. Influence of epoxy coating and coating thickness, external loads and loading type as well as fatigue loading cycles on corrosion behavior of reinforcing bars in test specimens is discussed. It is concluded that, under serviceability state of RC test specimens, one-time static load has little effect on corrosion behavior of uncoated and epoxy-coated reinforcing bars in specimens; while reduction of corrosion resistance of reinforcing bars is presented in test specimens subjected to fatigue load. When RC test specimens containing epoxy-coated reinforcing bars with 600 lm nominal coating thickness were subjected to fatigue load with longer loading cycles, larger reduction of corrosion resistance of epoxy-coated reinforcing bars is presented under the case of larger localized crack width and lower crack spacing in RC test specimen. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction To prevent corrosion of reinforcing bars in reinforced concrete (RC) structures exposed to aggressive environments, epoxy coatings have been applied to surfaces of reinforcing bars since mid-1970s. Accelerated tests were carried out to check the corrosion resistance of the RC members containing with epoxy-coated reinforcements in simulated seawater and/or chloride solutions [1–9]. Test results showed, although the corrosion resistance of ⇑ Corresponding author. E-mail address: [email protected] (X.-H. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2015.05.101 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

the epoxy-coated reinforcement may be influenced by the holidays and surface damage on the coating [9–10], the fusion-bonded epoxy coating did provide a significant improvement in corrosion resistance. In addition, field surveys on corrosion performance of RC members constructed with epoxy-coated reinforcing bars were also investigated [11–22]. Although the first evidence of unsatisfactory field performance of epoxy-coated bars emerged in 1986 in bridges of the Florida Keys [12], many field investigations reported good performance of the epoxy-coated bars. Investigation performed on 92 bridge decks, two bridge barrier walls, and one noise barrier wall located in the United States and Canada showed that, the use

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X.-H. Wang et al. / Construction and Building Materials 93 (2015) 746–765 Table 1 Details of the RC test specimens. RC test specimen

Length (mm)

Group 1: subjected to simulated seawater attacks

Group 2: subjected to 14 kN static load and simulated seawater attacks

Subjected to fatigue load and simulated seawater attacks

Group 3: 500,000 cycles and 2.8–14 kN

Group 4: 1,000,000 cycles and 2.8–14 kN

Cross section (mm) Width

Height

Average clear concrete cover (mm)

Coating thickness of the reinforcing bar in different positions (lm) Positive side 1

Middle 2

Opposite side 3

0 274.1 (106.2) 560.0 (128.1)

0 231.7 (54.4) 972.1 (163.7)

0 214.5 (94.1) 540.9 (83.7)

E1 E0.2E

1503 1500

300 301

121.5 121

40.5 40.5

E0.6E

1500

301

121

41

FE3

1500

301

122

41

0

0

0

E0.2FE4

1502

302

120.5

40.5

E0.2DE1

1501

301

120

40.5

E0.6FE6

1500

302

122.5

40

E0.6FE2

1500

300

121.5

40.5

315.1 (81.6) 332.4 (61.0) 1015.4 (145.5) 538.4 (155.7)

354.3 (101.8) 297.4 (82.8) 770.3 (194.4) 1024.5 (64.1)

347.8 (95.8) 303.8 (64.0) 596.3 (74.1) 1042.6 (95.2)

FE2 E0.2DE2

1504 1500

300 303

119.5 120

40.5 40.5

E0.2FE5

1500

300

120.5

40.5

E0.6FE5

1501

300

121.5

41

E0.6DE1

1503

302

119.5

41

DE2 E0.2FE1

1500 1501

304 301

120 120

40.5 40.5

E0.2FE2

1502

301

120

40.5

E0.6FE1

1502

300

121.5

40.5

E0.6DE2

1500

302

120.5

40.5

0 274.8 (183.7) 213.5 (42.0) 492.9 (59.1) 522.0 (122.7) 0 214.3 (69.0) 364.6 (73.1) 528.3 (124.0) 766.9 (109.7)

0 361.8 (83.8) 225.9 (60.5) 711.1 (145.2) 524.9 (161.7) 0 246.2 (61.1) 317.2 (50.7) 945.2 (105.0) 707.2 (161.1)

0 323.5 (66.8) 230.0 (76.0) 677.6 (121.3) 578.8 (72.3) 0 242.6 (77.1) 292.2 (53.8) 800.7 (147.6) 649.1 (154.9)

Fig. 1. RC beam test specimens.

of epoxy-coated bars has reduced, if not completely eliminated, the deterioration of the deck concrete resulting from the corrosion of

reinforcing steel. Epoxy-coated reinforcing steel has provided effective corrosion protection for three to 20 years of service [11].

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Fig. 2. Schematic loading test setup (opposite lateral surface of the test specimen).

Fig. 3. Schematic corrosion test for open-circuit potential and electrochemical impedance spectroscopy.

However, it should be pointed out that, for the in-service RC members and structures with epoxy-coated reinforcements under their in-service environments, concrete is cracked by natural occurring reactions during the hydration process and loading conditions. Thus, corrosion behavior of this kind of the RC members is influenced by not only the coating parameters, concrete parameters and the environment parameters, but also the loading parameters including the loading type, loading level and the loading history. Those cracks in concrete tend to influence the corrosion acceleration of the reinforcing steel, even in the presence of a protective epoxy coating. At the cracked areas, the damage was attributed essentially to the direct exposure of the epoxy coating to the wetting and drying cycles which caused the embrittlement of the epoxy coating and its subsequent debonding and breaking [8]. Investigation on effects of bridge-deck cracking on the performance of ECR bars in 80 bridges in the state of Iowa also showed that, the adhesion of the coating decreased at a faster rate for the reinforcing bars collected from cracked locations than that of reinforcing bars collected from uncracked locations [15]. For experimental research work carried out on the corrosion behavior of

the RC members with epoxy-coated reinforcements, only the environmental factors were emphasized [1–7]; while for field surveys on corrosion performance of RC members with epoxy-coated reinforcing bars, the lack or insufficient of the loading information makes it difficult to judge in what extent the corrosion behavior of the epoxy-coated reinforcing bars in the RC members is influenced by the external loads, although signs of corrosion of epoxy-coated reinforcing bars were exhibited in the cracked zones produced by the external loads [11,14]. In addition, it is also difficult to separate the contribution of the epoxy-coating to improved performance from other improvements in concrete quality and construction practices [12]. In order to evaluate how corrosion performance of RC members with epoxy-coated reinforcing bars was influenced by the in-service environments and external loads, a series of experimental research works were carried out. 14 kN static loading and two different fatigue loading tests (2.8–14 kN load range, 500,000 and 1,000,000 cycles, respectively) on RC beam specimens were performed firstly; then, the unloaded and loaded RC test specimens were subjected to simulated seawater solution immersion attacks.

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Electrochemical measurements of the reinforcing bars in the test specimens were carried out to evaluate the corrosion behavior of the uncoated and epoxy-coated reinforcing bars in the test specimens.

The cast surface is the top surface of the test specimen. For the test specimen to be loaded, the strain gages on the concrete were installed on one of the lateral surface after 28-day of curing. This lateral surface is called as positive lateral surface (see Fig. 1).

2.2. Material properties 2. Experimental program This extensive laboratory study was performed on small-scale RC test specimens containing epoxy-coated and uncoated reinforcing bars. Detailed information of the test specimens were presented in Table 1, where test specimens in Group 1 were prepared to be subjected to only simulated seawater solution attacks; test specimens in Group 2 were statically loaded to maximum load 14 kN for only one time; test specimens in Group 3 were subjected to 500,000 loading cycles and 2.8–14 kN fatigue load while test specimens in Group 4 were subjected to 1,000,000 loading cycles and 2.8–14 kN fatigue load. In the following sections, information about material properties in the test specimens and loading tests will be described briefly herein because details information are presented in Wang et al. [23] while information about exposure conditions and electrochemical measurements will be presented in details.

Two nominal coating thicknesses 200 lm and 600 lm were chosen for both 12 mm and 8 mm reinforcing bars, respectively. Coated and uncoated reinforcing bars of the same size were from the same heat. Actual coating thickness was measured by using a coating thickness gauge. For 1500 mm long 12 mm reinforcing bars with nominal 200 lm and 600 lm epoxy coating used in the test specimens, they were numbered consecutively with red numbers at the two ends and the coating thickness of each coated reinforcing bar was recorded (see Table 1). Commercial concrete from the same batch was used to cast all test specimens to ensure the same concrete properties of the test specimens. The concrete had target strength of 40 MPa and the actual concrete strength of standard cube is 47.5 MPa. Other detailed information about concrete can be referenced to Wang et al. [23].

2.3. Loading tests 2.1. Specimen details Test specimens in Table 1 were identified with letters and numbers designation. For test specimens with uncoated reinforcing bars, letters ‘‘FE’’ indicate test specimen subjected to fatigue load and environmental attack; letters ‘‘DE’’ indicate test specimen subjected to dead/static load and environmental attack. For test specimens with epoxy-coated reinforcing bars, the first letter ‘‘E’’ indicates epoxy coating, including both tensile and distribution bars; the second numbers indicate 200 lm or 600 lm nominal coating thicknesses; the following letters ‘‘FE’’ or ‘‘DE’’ indicate fatigue load and environmental attack or dead/static load and environment attack; the final number indicates the specimens number. For example, test specimen FE2 indicates uncoated RC specimen subjected to fatigue load and environmental attack; test specimen E0.6FE1 indicates epoxy-coated RC specimen subjected to fatigue load and environmental attack, both tensile and distribution bars having 600 lm nominal coating thickness. It should be pointed out, during the loading test, in order to ensure better conditions of the test specimens subjected to fatigue loads, some test specimens originally used in Groups 2 were exchanged to Groups 3–4, for instance, test specimens DE2, E0.2DE2, E0.6DE1 and E0.6DE2. All test specimens in Table 1 have a rectangular cross section of b  h = 300  120 mm, 1500 mm overall length and 1100 mm length between two supports (Fig. 1). Three 12 mm diameter deformed bars service as flexural reinforcements and 8 mm-diameter plain bars as the distribution reinforcements. The 8 mm-diameter plain bars were uniformly spaced at 50 mm at the beam ends and 100 mm at the beam span, see Fig. 1. The designed clear concrete cover of the tensile bars is 40 mm to meet the requirement of the minimum concrete cover of the RC members in the chloride-contained environment [24].

After 28-days of curing, test specimens were prepared for static and fatigue loading tests, where the maximum load is 14kN, which is about 32–35% of yield load of control RC test specimen [23]. Strain gages on the concrete of the positive and opposite lateral surfaces were installed, see Figs. 1 and 2, respectively. RC beam specimen was placed in the test set-up with both ends simply supported and a single, mid-span load point (Fig. 2). A 100 kN pulse fatigue testing machine was used to apply static and fatigue loading to the RC specimens and strains of steel and concrete were recorded by a DH3817 dynamic strain data measurement system. Midspan displacements of the test specimen and vertical displacements at the two supports were measured by displacement transducers (see Fig. 2). The detailed information about fatigue loading test can be referenced to Wang et al. [23]. During the loading test, developments of the cracks were observed and recorded at the cracking tips and the maximum cracking widths at two lateral surfaces were measured by using a Digital Concrete Crack Width Gauge. After static and fatigue loading tests, maximum crack width and average crack spacing of the bottom surface of each test specimen were also measured.

2.4. Exposure conditions After loading tests, unloaded test specimens in Group 1 and loaded specimens in Groups 2–4 in Table 1 were subjected to simulated seawater attacks. They were turned upside down and horizontally placed in the ground, see Fig. 3. The whole bottom surfaces of the test specimens were covered by sponges and 3.5–5% NaCl simulated seawater was sprayed on the sponges. Plastic sheets were used to cover all of them to keep the moisture of the bottom surface of test specimens for two

Table 2 Maximum crack width and average crack spacing of the bottom surface of the test specimens after loading tests. RC test specimen

Group 2: subjected to 14 kN static load and simulated seawater attacks

Subjected to fatigue load and simulated seawater attacks

Group 3: 500,000 cycles and 2.8– 14 kN

Group 4: 1,000,000 cycles and 2.8– 14 kN

Maximum crack width (mm)

Average crack spacing (mm)

Positive side

Opposite side

Maximum

Positive side

Opposite side

FE3 (+) E0.2FE4 (+) E0.2DE1 (+) E0.6FE6 (+) E0.6FE2 (+)

0.08 0.1

0.08 0.08

0.08 0.1

165 182

201 109

0.12

0.1

0.12

400

201

0.08

0.08

0.08

284

148

0.12

0.14

0.14

275

287

FE2 (+) E0.2DE2 E0.2FE5 E0.6FE5 E0.6DE1 (+) DE2 E0.2FE1 E0.2FE2 E0.6FE1 E0.6DE2 (+)

0.16 0.14 (0.5) 0.2 (0.6) 0.2 0.14 (1.2)

0.22 0.14 0.2 0.36 0.16

0.22 0.14 (0.5) 0.2 (0.6) 0.36 0.14 (1.2)

118 187 135 139 213

125 122 139 124 245

0.15 0.14 0.18 0.18 0.26

0.12 0.16 0.16 0.22 0.16

0.15 0.16 0.18 0.22 0.26

127 145 144 113 190

113 152 124 136 225

Note: (+) indicates test specimen with initial cracking resulting from transporting damage.

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Table 3 Open-circuit potential of the reinforcing bars in the test specimens during wet-dry cycles. RC test specimen

Group 1: subjected to only simulated seawater attacks

Group 2: subjected to 14 kN static load and simulated seawater attacks

Subjected to fatigue load and simulated seawater attacks

Group 3: 500,000 cycles and 2.8–14 kN

Group 4: 1,000,000 cycles and 2.8–14 kN

Wet-cycle potentials (mV)

Dry-cycle potentials (mV)

Corr+

Corr+

Corr

Corr

above-mentioned test specimens were mainly presented. In the present paper, due to the corrosion tests were performed in the bottom surfaces of the test specimens, the maximum crack width and average crack spacing of the bottom surfaces of the loaded test specimens, open-circuit potential results and EIS data of the uncoated and coated reinforcing bars in the unloaded and loaded RC test specimens are mainly reported. The reinforcing bars 1 and 3 shown in Fig. 1a were chosen and performed electrochemical measurements. The open-circuit potential of the reinforcing bar 1 was denoted as ‘‘Corr+’’ while that of the reinforcing bar 3 was denoted as ‘‘Corr’’. Due to the limitation of the manuscript length, only the first wet-cycle and dry-cycle open-circuit potentials as well as the first wet-cycle EIS data of the reinforcing bars 1 and 3 in test specimens of Table 1 are reported in the present paper. More test results of the open-circuit potentials and EIS data will be presented in the following papers.

E1

870

878

160

157

E0.2E E0.6E

218 207

223 218

282 309

297 394

FE3 (+)

1345

1138

1107

1100

E0.2FE4 (+) E0.2DE1 (+) E0.6FE6 (+) E0.6FE2 (+)

234

286

218

167

275

269

309

240

544

262

53

70

411

305

316

1126

FE2 (+) E0.2DE2 E0.2FE5 E0.6FE5 E0.6DE1 (+) DE2 E0.2FE1 E0.2FE2 E0.6FE1 E0.6DE2 (+)

671 413 526 415 455

678 491 438 422 388

959 374 337 53 258

354 414 331 196 98

The actual dimensions, average clear concrete cover and actual coating thicknesses of the reinforcing bars in different positions of the RC test specimens are summarized in Table 1, where reinforcing bars 1–3 in the test specimens are denoted in Fig. 1 and the numbers in the brackets are the standard deviations of actual coating thicknesses of the reinforcing bars.

788 513 443 385 340

789 549 445 382 380

521 381 296 351 229

511 344 373 350 355

3.2. Maximum crack width and crack spacing of the bottom surfaces of loaded test specimens

3.1. Detailed results of the actual test specimens

Note: (+) indicates test specimen with initial cracking resulting from transporting damage.

week. Then, two weeks later, the plastic sheets and sponges were moved away to make the specimens air-dried for one week. Thus three weeks constituted one cycle of wetting and drying. After four wetting-drying cycles and waiting for the repair of instrument PAR2263, a 4 m  5 m pool was built to accelerate the test. After immersion test specimens in 3.5–5% NaCl simulated seawater solution for two weeks, open-circuit potentials and EIS of the reinforcing bars 1 and 3 were measured and defined as the first wet-cycle potentials and EIS data. Following the another two-week immersion and nearly one-month air-dry, open-circuit potentials and EIS of the reinforcing bars 1 and 3 were measured again and defined as the first dry-cycle potentials and EIS data. 2.5. Electrochemical measurements The electrochemical system was composed of three electrodes: the working electrode was the two lateral reinforcing bars 1 and 3 in Fig. 1, the reference electrode was a KCl saturated calomel electrode (SCE) and the counter electrode was a 300 mm stainless steel bar (see Fig. 3). 3.5–5% NaCl solution was used to simulate sea water. The electrochemical measurements performed on the central 300 mm length of the test specimen with a portable potentiostat PARSTAT PAR2263, see Fig. 3. Open-circuit potentials of the two lateral reinforcing bars 1 and 3 in Fig. 1, Ecorr (mV), were determined from the potential versus time curve, after the potential having reached its equilibrium. Electrochemical impedance spectroscopy (EIS) was used for corrosion study of uncoated and epoxy-coated reinforcing bars embedded in unloaded and loaded RC test specimens, similar to most previous research work on the corrosion behavior of reinforcing bars in concrete [25–38]. EIS measurements were taken with a sinusoidal potential of 10 mV sine wave applied around the open-circuit potential Ecorr with a frequency range of 100 mHz–100 kHz to obtain Nyquist diagram of the reinforcing bars. All the experiments were performed using the same arrangement in order to minimize errors.

3. Test results In the previous paper of Wang et al. [23], rib parameters of the uncoated reinforcing bars, mechanical test results of uncoated and coated reinforcing bars, loading test results of the

For loaded test specimens in Groups 2–4, after finishing of loading tests, maximum crack width and crack spacing of the bottom surfaces were measured and recorded, see Table 2, where the numbers in the bracket in Group 3 indicate the local maximum crack widths due to the spalling of the concrete resulting from the nonhomogeneity of the concrete, the transporting damage and external loading. After finish of loading tests, each test specimen was moved from the test setup and turned upside down; then, locations of reinforcing bars 1 and 3 on the bottom surfaces was determined according to Fig. 1b, where the depth of lateral concrete cover is 60 mm. Space between two adjacent cracks was measured and average crack spacing was determined, see Table 2, where ‘‘Positive side’’ denotes the location of reinforcing bar 1 on the bottom surface while the ‘‘Opposite side’’ denotes the location of bar 3 on the bottom surface. For RC test specimens in Groups 2, due to they were subjected to static load for only one time, after loading test, maximum crack widths at the two sides are very small. Although maximum crack widths in some test specimens are larger than 0.1 mm, considering 0.05–0.1 mm initial cracks resulting from transporting damage, the load-induced crack width is still very small. However, compared with that of the uncoated test specimen with initial crack, the load-induced cracks are enlarged for epoxy-coated test specimens with initial cracks. For RC test specimens subjected to fatigue load in Groups 3, after 500,000 loading cycles, the difference in crack widths of the uncoated and epoxy-coated test specimens is quite little. However, for test specimens with thicker epoxy coating in Group 4, when the loading cycles increase to 1,000,000, the increase in crack width is obvious, especially for test specimen E0.6DE2 with initial cracking resulting from transporting damage. Compared with the good bond condition in the ordinary uncoated reinforcing bar, the comparatively ‘‘poor bond’’ of the epoxy-coated bar may result in longer distance between successive transverse cracks [39]. It can be seen from Table 2 that, for RC test specimens subjected to either static load or fatigue load, after the

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(a) Nyquist plot of real impedance Z´(Ω) versus imaginary impedance Z"(Ω)

(b) Bode plot phase angle versus frequency Fig. 4. Nyquist and Bode plots of reinforcing bar 1 in test specimens of Group 1.

loading test, average crack spacing of epoxy-coated RC test specimens is larger than that of the corresponding uncoated RC test specimens, as indicating by the Cairns [39] and Cleary and Ramirez [40].

3.3. Open-circuit potential results of the test specimens in the first wetting- drying cycle exposure The first wet-cycle and dry-cycle open-circuit potentials of the reinforcing bars 1 and 3 in test specimens are summarized in Table 3, where ‘‘Corr+’’ and ‘‘Corr’’denote the open-circuit potentials of the reinforcing bars 1 and 3, respectively. It can be seen from Table 3 that, on the whole, in the test specimens of Groups 1–4, the open-circuit potentials of the epoxy-coated reinforcing bars is higher than that of the ordinary reinforcing bars, indicating the corrosion protection of the epoxy coating. However, the open-circuit potential difference among the epoxy-coated bars with nominal 200 lm and 600 lm epoxy coatings in Groups 1–4 is not obvious. The reasons may be lie in (1) actual thickness of the epoxy coating of each epoxy-coated reinforcing bar in central 300 mm length of the test specimen; (2) the actual cracking pattern in the loaded test specimens and (3) the comparatively short open-circuit potential evolution with time. More monitoring of

open-circuit potentials of the reinforcing bars in test specimens is needed to show how the potential evolution with time. 3.4. Results of electrochemical impedance spectroscopy measurements of the test specimens in the first wet-cycle exposure In the following section, Nyquist plot of real impedance Z0 (X) versus imaginary impedance Z00 (X) and Bode plot—Phase angle versus frequency of the reinforcing bars 1 and 3 in Groups 1–4 in the first wet-cycle exposure will be presented. Similar to the signs used in Table 3, signs ‘‘+’’ and ‘‘’’ are used as superscripts to denote the reinforcing bars 1 and 3 in each specimen, respectively. 3.4.1. Electrochemical impedance spectroscopy results of Group 1 For test specimens E1, E0.2E and E0.6E subjected to only simulated seawater attacks in Group 1, the impedance spectra data obtained from the normal uncoated and epoxy-coated reinforcing bars are shown in Figs. 4 and 5, where in Fig. 4, Nyquist and Bode plots of the reinforcing bars 1 are presented while Nyquist and Bode plots of the reinforcing bars 3 are presented in Fig. 5. Similar trend in Nyquist and Bode plots of the reinforcing bars 1 and 3 are presented in Figs. 4 and 5 due to there are no load-induced cracks in the concrete cover: test specimens E0.2E

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(a)Nyquist plot of real impedance Z´(Ω) versus imaginary impedance Z"(Ω)

(b) Bode plot—phase angle versus Log (frequency) Fig. 5. Nyquist and Bode plots of reinforcing bar 3 in test specimens of Group 1.

and E0.6E present half arc with larger diameter as compared with that of the uncoated test specimen E1. These two specimens also have larger absolute phase angle, about 60–70°. The maximum absolute phase angle of the reinforcing bars 1 and 3 in test specimen E1 is about 52–55°.

3.4.2. Electrochemical impedance spectroscopy results of Group 2 For RC test specimens FE3, E0.2FE4, E0.2DE1, E0.6FE6 and E0.6FE2 subjected to 14 kN static load and simulated seawater attacks in Group 2, the impedance spectra obtained from the normal uncoated and epoxy-coated reinforcing bars are shown in Figs. 6 and 7, where in Fig. 6, Nyquist and Bode plots of the reinforcing bars 1 are presented while Nyquist and Bode plots of the reinforcing bars 3 are presented in Fig. 7. Although test specimens in Group 2 were subjected to 14kN static load before the simulated seawater attack, the 0.08– 0.14 mm load-induced and initial cracks (see Table 2) have limited influence on the corrosion performance of the reinforcing bars in test specimens. Although in Nyquist plots of reinforcing bars 1 and 3, a very small portion of flattened arcs appears in high frequency larger than 1 kHz (see Figs. 6a and 7a), the maximum absolute phase angles increased to 61–63° for uncoated reinforcing bars

in of FE3. For epoxy-coated bars with nominal 200 lm epoxy coating, in Nyquist plot, similar single arcs presents in bar 1 of E0.2FE4 and both bars of E0.2DE1 while an arc with a very small portion of arc appears in bar 3 of E0.2FE4. For epoxy-coated bars with nominal 600 lm epoxy coating, decreased single arcs presents in bars of E0.6FE2. In Nyquist plots of reinforcing bar 1 and 3 in test specimen E0.6FE6, a big arc with a small portion of flattened arc presents, indicating the penetration of the coating by the electrolyte in some zone [25]. Except for the increased absolute phase angle in bar 1 of E0.2DE1, the other absolute phase angles of the epoxy-coated reinforcing bars decrease (see Figs. 6b and 7b).

3.4.3. Electrochemical impedance spectroscopy results of Group 3 For RC test specimens FE2, E0.2DE2, E0.2FE5, E0.6FE5 and E0.6DE1 subjected to 500,000 cycles fatigue load (2.8–14 kN) and simulated seawater attacks in Group 3, the impedance spectra obtained from the normal uncoated and epoxy-coated reinforcing bars are shown in Figs. 8 and 9, where in Fig. 8, Nyquist and Bode plots of the reinforcing bars 1 are presented while Nyquist and Bode plots of the reinforcing bars 3 are presented in Fig. 9. Due to test specimens in Group 3 were subjected to 500,000 cycles fatigue load (2.8–14 kN) before the simulated seawater

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(a) Nyquist plot of real impedance Z´(Ω) versus imaginary impedance Z"(Ω)

(b) Bode plot—phase angle versus frequency Fig. 6. Nyquist and Bode plots of reinforcing bar 1 in test specimens of Group 2.

attack, where the load upper limit is the same as the static load limit used in Group 2, the load-induced cracks (see Table 2) is much larger than those of test specimens in Group 2 and the influence of load-induced cracks on corrosion performance is obvious. For uncoated reinforcing bar 1 in test specimen FE2, an arc with a small portion of flattened arc presents; in addition, the maximum absolute phase angle of bars 1 and 3 decreases to 51° and 48°, respectively. For epoxy-coated reinforcing bars with nominal 200 lm and 600 lm epoxy coatings, in Nyquist plots, arcs with very small portion of flattened arcs present in bar 1 of E0.2FE5 and E0.6FE5; decreased arcs present in bars 3 of E0.2FE5 and E0.6FE5 as well as bars 1 and 3 of E0.2DE2 and E0.6DE1. Decrease maximum absolute phase angles of the epoxy-coated reinforcing bars are shown in Figs. 8b and 9b. 3.4.4. Electrochemical impedance spectroscopy results of Group 4 For test specimens DE2, E0.2FE1, E0.2FE2, E0.6FE1 and E0.6DE2 subjected to 1,000,000 cycles fatigue load (2.8–14 kN) and simulated seawater attacks in Group 4, the impedance spectra obtained from the normal uncoated and epoxy-coated reinforcing bars are shown in Figs. 10 and 11, where in Fig. 10, Nyquist and Bode plots of the reinforcing bars 1 are presented while Nyquist and Bode plots of the reinforcing bars 3 are presented in Fig. 11.

Due to the increased fatigue loading cycles in Group 4 as compared with that performed in Group 3, reduction in corrosion performance of the reinforcing bars in test specimens is obvious. For uncoated reinforcing bars of test specimen DE2, although the maximum absolute phase angle of bars lies in 54°–59°, arcs with a small portion of flattened arcs present in Nyquist plots, see Figs. 10a and 11a. For epoxy-coated reinforcing bars with nominal 200 lm and 600 lm epoxy coatings, in Nyquist plots, decreased arcs with smaller diameter present in bars 1 and 3 of four test specimens, as compared with those of E0.2E and E0.6E. Decrease maximum absolute phase angles of the epoxy-coated reinforcing bars are also shown in Figs. 10b and 11b. 4. Electrical equivalent circuits fitting of electrochemical impedance spectroscopy data in the first wet-cycle exposure To explain EIS data, equivalent circuits are often used via electrical fitting [25–26,28–32,34–38]. Due to the complexity of the reinforced concrete system, the equivalent circuit that represents the physical model well is very important in order to provide a more reliable and realistic interpretation of the EIS data. According to Sagoe-Crentsil et al. [25], an equivalent electrical circuit should represent the concrete matrix and the steel/concrete

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(a) Nyquist plot of real impedance Z´(Ω) versus imaginary impedance Z"(Ω)

(b) Bode plot phase angle versus frequency Fig. 7. Nyquist and Bode plots of reinforcing bar 3 in test specimens of Group 2.

interface zone consisting of a layer of iron oxides and hydroxides in the form of a film in the passive stage and an interfacial film adjoined to the concrete matrix. So, electrical equivalent circuits (EEC) shown in Fig. 12a and b are used to model the corrosion system with uncoated reinforcing bars and epoxy-coated reinforcing bars, respectively. Here, RC represents the resistance of concrete matrix; Cf and Rf represent capacitance and resistance of the concrete/steel interfacial zone; CPEDL is the double layer capacitance; RP is the polarization resistance or charge transfer resistance of the corrosion reaction; CPEep is the coating capacitance; Rep is the pore resistance of coating. Application of the CPE in the EEC models is attributed to the nonhomogeneity of the system under study, where CPE is defined by two parameters Y and n. When n = 1, CPE resembles a capacitor with capacitance Y. When n = 0, CPE represents a resistor with resistance Y1 [18,36–38]. The effective capacitance was calculated according to the following equation [36]: 1

C ¼ Y nR

1n n

ð1Þ

where R is referred to Rep and RP when coating capacitance Cep and double layer capacitance CDL are calculated, respectively. Correspondingly, CPEep is represented by Yep and nep while CPEDL by YDL and nDL.

It is generally assumed that the elements of the equivalent circuit are correlated with the corrosion properties of the system [26]: the pore resistance Rep is a measure of the porosity and degradation of the coating; the increase of coating capacitance Cep with time is related to the water uptake of the coating; polarization resistance RP, and double layer capacitance CDL are two most direct parameters used to specify the delamination of the top coat and the onset of corrosion at the interface. They are related to the charge transfer during the corrosion process at the interface between the exposed reinforcing bars and the electrolyte inside mortar pore structure [36]. A constant CDL is an indication of a stable interface of the reinforcing bar. A change in CDL value can be associated with the competition between delamination and corrosion product accumulation at the interface, resulting in increase and decrease of CDL [26]. Tang et al. also pointed out [26], that a coated metal system which performs well in corrosion is characterized by high resistances Rep and RP, lower capacitances Cep and CDL, as compared with poor systems. The resulting parameters of Groups 1–4 are presented in Tables 4–7, respectively. ZsimpWin software was used to fit all electrochemical impedance spectroscopy data. The Chi-squared values of EEC models of all EIS data are presented in the brackets of the bar number, where they are in the range between 103 and 104

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(a) Nyquist plot of real impedance Z´(Ω) versus imaginary impedance Z"(Ω)

(b) Bode plot—phase angle versus frequency Fig. 8. Nyquist and Bode plots of reinforcing bar 1 in test specimens of Group 3.

except for that in test specimen FE3 in the order of 102. Figs. 13 and 14 show typical measured and fitted spectra using the above equivalent circuits for uncoated and epoxy-coated reinforcing bars in test specimens, respectively. The calculated coating capacitance Cep and double layer capacitance CDL are also presented in Tables 4–7. 5. Discussion 5.1. Influence of epoxy coating on corrosion behavior of the reinforcing bars in RC test specimens For uncoated reinforcing bars in test specimen E1 of Group 1 (see Table 4), polarization resistance RP and CDL are on the order of 105 ohm-cm2 and 105–104 F/cm2, respectively; while for epoxy-coated bars, polarization resistance RP is nearly four times of that of the uncoated bars while double layer capacitance CDL decreases to 107 F/cm2, showing lager corrosion resistance of the epoxy-coated bar. However, although bars of E0.6E has higher resistances RC, Rep and lower Cf and CDL, the difference between the resistances RP of reinforcing bars in two specimens E0.2E and E0.6E is not very obvious, indicating the

nonhomogeneity of the coating thickness and defects during handling for the coated bars. Larger standard deviations in Table 1 also reflect this phenomenon. For uncoated and epoxy-coated reinforcing bars in RC test specimens subjected to external loads, comparatively higher average polarization resistance RP and lower CDL are presented in epoxy-coated reinforcing bars, see Tables 5–7, indicating a comparatively higher corrosion resistance of epoxy-coated bars than that of uncoated reinforcing bars. However, decreased differences between the resistance RP and CDL of uncoated and epoxy-coated reinforcing bars are presented in Tables 5–7. For instance, in Tables 5–7, the average polarization resistances RP of epoxy-coated bars are 8.20  105 ohm-cm2, 4.38  105 ohm-cm2 and 1.8  105 ohm-cm2, respectively; while those values of uncoated bars are 4.30  105 ohm-cm2, 1.49  105 ohm-cm2 and 1.06  105 ohm-cm2, respectively. Although double layer capacitance CDL of epoxy-coated bars are lower than those of uncoated bars in Table 7, CDL of three epoxy reinforcing bars has the same order as that of the uncoated bars, i.e. 105 F/cm2. It can be concluded that, epoxy coating does improve the corrosion resistance of epoxy-coated bars in RC specimens; this improvement is greatly influenced by the external loads.

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(a) Nyquist plot of real impedance Z´(Ω) versus imaginary impedance Z"(Ω)

(b) Bode plot phase angle versus frequency Fig. 9. Nyquist and Bode plots of reinforcing bar 3 in test specimens of Group 3.

5.2. Influence of loading and loading type on the corrosion behavior of reinforcing bars in RC test specimens As stated above, compared with those of reinforcing bars in unloaded test specimens, for uncoated and epoxy-coated reinforcing bars in RC test specimens subjected to external loads (see Tables 5–7), their average polarization resistances decrease while double layer capacitances increase. It can be concluded that corrosion resistance of both uncoated and epoxy-coated bars in RC specimens is greatly influenced by the external loads; however, this influence is quite different in different situations. For uncoated bars in four test specimens E1, FE3, FE2 and DE2, the average polarization resistances are 1.90  105 ohm-cm2, 4.3 0  105 ohm-cm2, 1.49  105 ohm-cm2 and 1.06  105 ohm-cm2, respectively, indicating the influence of external loads on corrosion resistance of the bars. However, for corrosion systems in test specimen FE3 in Group 2, the one-time 14 kN static load seems help to increase the corrosion resistance of the reinforcing bars. It can be seen from Table 8 that concrete matrix resistance RC and resistance of the concrete/steel interfacial zone Rf as well as polarization resistance RP of reinforcing bars in Group 2 are larger than those of the reinforcing bars in Group 1, while capacitance Cf and CDL

in Group 2 are lower than the corresponding ones in Group 1. On one hand, the one-time 14 kN static load induces some cracks on concrete cover, which is about 0.08 mm (80 lm), see Table 2; on the other hand, due to the loading effect, porosities and voids at interfacial zone between reinforcing bar and concrete is reduced [41,42]. In addition, although all test specimens were cast by commercial concrete from the same batch, microstructures of the concrete cover in different test specimens are also different. This difference in concrete microstructures may also influence the measured values of the concrete resistance. Thus, it can be concluded that the one-time static load (under serviceability state) have little effect on corrosion behavior of reinforcing bars in concrete. Similar conclusion was also obtained, where the crack width <200 lm [43]. For corrosion system in test specimens FE2 and DE2 in Group 3 and 4, the situation differs from that in Group 2. Although average RC, Rf and RP of reinforcing bars in Group 3 are lower than those in Group 1, lower capacitance Cf and CDL are presented in Group 3 while higher Cf and lower CDL are presented in Group 4. Lower capacitance Cf in Group 3 may result from the reduction of porosities and voids at interfacial zone due to fatigue loading; with the increase of the fatigue loading cycles, fatigue load may induce

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(a)Nyquist plot of real impedance Z´(Ω) versus imaginary impedance Z"(Ω)

(b) Bode plot phase angle versus frequency Fig. 10. Nyquist and Bode plots of reinforcing bar 1 in test specimens of Group 4.

new cracks in interfacial zone, resulting in higher Cf in Group 4. Since the increase and decrease in CDL value is associated with the competition between delamination and corrosion product accumulation at the interface [26], change of CDL and lower Rf and RP of reinforcing bars in Group 3 and 4 may indicate active state of the steel surface. For epoxy-coated bars in unloaded and loaded test specimens, comparison of the model parameters in four groups is summarized in Table 9. For corrosion system in test specimens in Group 2, one-time 14kN static load induces limited effects on model parameters as compared with those in Group 1, just a slightly increases in CDL for both types of epoxy-coated bars. On the other hand, for corrosion system in test specimens subjected to fatigue loading, decrease in average RC and Rf is obvious. Due to the difference of actual coating thicknesses in reinforcing bars in Group 1, Groups 3 and 4, reduction trend in Rep of reinforcing bars in Groups 3 and 4 is not obvious while coating capacitances Cep greatly increase. Meanwhile, compared with those of reinforcing bars in Group 1, lower polarization resistance RP and higher double layer capacitance CDL are presented in bars in Groups 3 and 4, indicating the reduction of corrosion resistance of epoxy-coated bars resulting from the fatigue loading. The influence of external loading and loading type on the damages of epoxy coating is illustrated in Fig. 15. Gao et al. [44] used a wedge

model to discuss diffusion in metal films and Feng et al. [35] used a similar model to discuss the different load magnitudes on the corrosion behavior of uncoated reinforcing bars. In the present study, a similar model shown in Fig. 15 may explain the influence of external loading and loading type on the corrosion behavior of epoxy-coated reinforcing bars in RC test specimens. For RC test specimens subjected to one-time 14kN static load, compared with those of the fatigue-loaded test specimens, larger average crack spacing and smaller crack width are presented (see Fig. 15b). Due to the external loading effect, porosities and voids in interfacial zone between steel and concrete are also slightly reduced. Thus, although the external static loading may have some influence on corrosion behavior of the reinforcing bars, this effect is quite limited. On the contrary, for RC test specimens subjected to fatigue load, considering the advantage of reducedporositiesand voidsin interfacialzone, smalleraveragecrack spacing and larger crack width are adverse on corrosion resistance of reinforcing bars in test specimens. Moreover, due to the elastic and plastic deformation of the reinforcing bars during the loading test, where some steel strains even approach the yielding strain of the flexuralreinforcement [23],micro-cracksare producedin the epoxycoatings, see Fig. 15c and d. It is very difficult for the passive films to form again during the immersion period [35], thus, corrosion resistance of epoxy-coated reinforcing bars is greatly reduced by the fatigue loading.

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(a) Nyquist plot of real impedance Z´(Ω) versus imaginary impedance Z"(Ω)

(b) Bode plot phase angle versus frequency Fig. 11. Nyquist and Bode plots of reinforcing bar 3 in test specimens of Group 4.

Fig. 12. Equivalent electrical circuits for RC test specimens with: (a) uncoated reinforcing bars and (b) epoxy-coated reinforcing bars.

Table 4 Equivalent electrical circuit model parameters for reinforcing bars in Group 1: subjected to only simulated seawater attacks. RC test specimen

Reinforcing bar

RC (ohm-cm2)

Cf (F/cm2)

E1

1(0.004) 3(0.003)

783.8 1056

3.99  107 3.43  107

E0.2E

1(0.006) 3(0.006)

3054 2465

4.93  109 1.92  108

E0.6E

1(0.005) 3(0.005)

5338 2437

1.33  108 5.32  109

Rf (ohm-cm2)

nep

Rep (ohm-cm2)

Cep (F/cm2)

YDL (X1  cm2  snDL )

nDL

RP (ohm-cm2)

CDL (F/cm2)

– –

– –

– –

– –

2.17  105 4.58  105

0.70 0.66

1.25  105 2.55  105

3.33  105 1.62  104

1261 2346

1.37  107 5.09  107

0.86 0.80

1.65  104 0.63  104

5.08  108 1.21  107

7.33  107 4.35  107

0.84 0.86

9.14  105 7.77  105

6.80  107 3.65  107

1569 703.6

6.82  107 8.27  107

0.74 0.70

2.56  104 4.14  104

1.64  107 1.95  107

3.52  107 2.26  107

0.87 0.98

8.02  105 8.56  105

2.91  107 2.19  107

Cep (F/cm2)

YDL (X1  cm2  snDL )

nDL

RP (ohm-cm2)

CDL (F/cm2)

226.7 337.8

Table 5 Equivalent electrical circuit model parameters for reinforcing bars in Group 2: subjected to 14 kN static load and simulated seawater attacks. RC test specimen

Reinforcing bar

RC (ohm-cm2)

Cf (F/cm2) 7

Rf (ohm-cm2)

Yep (X1  cm2  sne )

nep

Rep (ohm-cm2)

5

– –

– –

– –

– –

1.24  10 2.22  105

0.84 0.77

4.20  10 4.39  105

1.70  105 4.38  105

961.5 1060

6.09  107 1.06  107

0.69 0.84

0.40  104 2.09  104

4.08  108 3.31  108

4.57  107 1.13  106

0.90 0.75

7.07  105 9.58  105

4.03  107 1.16  106

4.87  109 4.66  109

1194 1106

5.14  108 3.12  107

0.95 0.77

0.90  104 1.24  104

3.43  108 5.94  108

9.92  107 6.63  107

0.78 0.85

7.75  105 6.74  105

9.21  107 5.75  107

3152 6082

1.28  109 1.19  109

582.1 2139

1.59  107 8.0  107

0.68 0.46

4.84  104 3.71  104

1.61  108 1.29  108

2.22  106 6.43  107

0.66 0.83

9.79  105 1.55  106

3.31  106 6.43  107

1631 1989

2.68  108 7.50  1010

321.4 679.9

1.03  106 5.0  107

0.78 0.69

1.10  104 1.69  104

2.91  107 5.86  108

3.14  106 6.51  107

0.65 0.79

1.58  105 7.55  105

2.15  106 5.02  107

FE3

1(0.03) 3(0.01)

1185 790

5.48  10 1.42  107

944.4 364.6

E0.2FE4

1(0.002) 3(0.003)

2981 3196

1.49  109 6.31  109

E0.2DE1

1(0.008) 3(0.001)

2645 2635

E0.6FE6

1(0.0004) 3(0.002)

E0.6FE2

1(0.001) 3(0.005)

5

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Yep (X1  cm2  sne )

759

760

Table 6 Equivalent electrical circuit model parameters for reinforcing bars in Group 3: subjected to fatigue load (500,000 cycles and 2.8–14 kN) and simulated seawater attacks. RC (ohm-cm2)

Cf (F/cm2)

nep

Rep (ohm-cm2)

Cep (F/cm2)

YDL (X1  cm2  snDL )

nDL

RP (ohm-cm2)

CDL (F/cm2)

– –

– –

– –

– –

4.49  105 3.30  105

0.66 0.6

1.63  105 1.34  105

1.25  104 8.89  105

2250 1487

9.75  107 8.93  107

0.54 0.59

1.24  104 2.57  104

2.27  108 6.48  108

4.98  107 2.50  107

0.77 0.75

6.78  105 5.79  105

3.60  107 1.31  107

1263 464.1

2.52  107 1.31  106

0.75 0.74

0.43  104 0.17  104

2.59  108 1.53  107

2.29  106 2.84  106

0.74 0.68

6.0  105 5.04  105

2.56  106 3.36  106

7.08  109 5.95  109

856.1 670.3

2.19  107 1.12  106

0.80 0.69

0.81  104 0.84  104

4.49  108 1.38  107

6.57  106 4.86  106

0.58 0.70

5.20  105 0.83  105

1.60  105 3.29  106

3.52  108 4.59  109

191.9 503.7

6.02  106 6.50  106

0.61 0.63

2.76  104 1.17  104

1.91  106 1.43  106

5.59  106 2.70  106

0.76 0.78

2.22  105 3.15  105

5.98  106 2.58  106

Reinforcing bar

FE2

1(0.002) 3(0.002)

E0.2DE2

1 (0.0009) 3(0.002)

2934 2869

4.49  109 6.45  1010

E0.2FE5

1 (0.0009) 3(0.003)

1329 940.5

6.71  1010 7.43  109

E0.6FE5

1(0.009) 3(0.002)

1760 2079

E0.6DE1

1(0.002) 3(0.0006)

1185 2376

684.6 611.5

3.73  107 1.27  107

Rf (ohm-cm2) 268.1 123.9

Table 7 Equivalent electrical circuit model parameters for reinforcing bars in Group 4: subjected to fatigue load (1,000,000 cycles and 2.8–14 kN) and simulated seawater attacks. RC test specimen

Reinforcing bar

DE2

1(0.008) 3(0.009)

E0.2FE1

1(0.0009) 3(0.002)

E0.2FE2

RC (ohm-cm2)

Cf (F/cm2)

Rf (ohm-cm2)

Yep (X1  cm2  sne )

nep

Rep (ohm-cm2)

Cep (F/cm2)

YDL (X1  cm2  snDL )

nDL

RP (ohm-cm2)

CDL (F/cm2)

– –

– –

– –

– –

4.16  105 4.38  105

0.72 0.78

1.02  105 1.09  105

7.30  105 6.81  105

4.07  107 1.26  106

311.1 244.7

2536 2128

2.52  109 6.25  109

1040 401.6

1.69  106 1.66  106

0.61 0.54

1.64  104 2.94  104

1.71  107 1.27  107

2.75  107 1.82  106

0.88 0.71

2.92  105 2.06  105

1.95  107 1.26  106

1(0.002) 3(0.001)

916.1 1714

2.37  108 2.04  109

168.1 612.3

3.72  106 1.27  106

0.67 0.62

0.32  104 0.65  104

4.20  107 6.71  108

2.30  107 1.10  106

0.89 0.70

9.61  104 2.63  105

1.44  107 6.46  107

E0.6FE1

1(0.005) 3(0.002)

1326 2211

1.07  107 5.84  108

692 559.9

9.64  106 1.21  105

0.6 0.52

2.22  104 5.05  104

3.45  106 7.68  106

1.58  105 2.37  105

0.68 0.73

3.95  104 2.57  105

1.27  105 4.62  105

E0.6DE2

1(0.003) 3(0.002)

1634 1743

8.39  108 4.61  108

812.7 1173

4.81  106 1.33  106

0.70 0.74

2.75  104 2.10  104

2.02  106 3.79  107

1.63  105 3.25  106

0.67 0.61

1.16  105 1.74  105

2.23  105 2.26  106

702.5 623

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Yep (X1  cm2  sne )

RC test specimen

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761

(a) Measured and fitted impedance spectroscopy results of Nyquist plot

(b) Measured and fitted impedance spectroscopy results of Bode plots Fig. 13. Typical measured and fitted impedance spectroscopy results of uncoated reinforcing bars in RC test specimens.

5.3. Influence of fatigue loading cycles on corrosion behavior of reinforcing bars in RC test specimens As mentioned above, for the uncoated reinforcing bars in test specimens of four groups, the largest reduction of RP is presented in reinforcing bars of test specimen DE2, which was subjected to 1,000,000 cycles fatigue load (2.8–14 kN). However, compared with those of reinforcing bars in test specimen FE2 subjected to 500,000 cycles fatigue load with the same load range, double layer capacitance CDL of bars in DE2 decreases (see Table 8). Although CDL value may increase or decrease due to the competition between delamination or corrosion product accumulation at the interface [26], it seems the increase of fatigue loading cycles has limited influence on reduction of corrosion resistance of uncoated reinforcing bars. For model parameters for epoxy-coated reinforcing bars in Group 3 and 4, increase of the fatigue loading cycles results in reduction of average RC and increase of Cf. Due to the difference in epoxy coating thickness, trend of change in Rep is not obvious. However, for epoxy-coated reinforcing bars in test specimens of

Group 3, the range of the coating capacitance Cep is (2.27– 191)108 ohm-cm2 while this range increase to (6.71– 768)108 ohm-cm2 in Group 4, reflecting the increased water uptake and degradation of the coating in the latter case with the increased of the fatigue loading cycles. In addition, for epoxy-coated reinforcing bars with 600 lm nominal coating thickness, with the increase of the fatigue loading cycles, decreased resistance RP and increase double layer capacitance CDL are presented; while for epoxy-coated reinforcing bars with 200 lm nominal coating thickness, although decreased resistance RP is presented, the increase of CDL is not obvious in Group 4, mainly due to wider crack width in test specimen E0.2FE5. The wider crack widths in the two lateral surfaces (Wang et al. [23]) and bottom surface (see Table 2) of this specimen may cause the easier of water uptake. Also, more micro-cracks in epoxy coatings may be caused by elastic and plastic deformation of reinforcing bars in test specimens with the increased fatigue loading time (see Fig. 15d), making greater reduction of corrosion resistance of epoxy-coated bars in test specimens subjected to fatigue loading with longer cycles, especially for bars with thicker nominal coating thickness.

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(a) Measured and fitted impedance spectroscopy results of Nyquist plot

(b) Measured and fitted impedance spectroscopy results of Bode plots Fig. 14. Typical measured and fitted impedance spectroscopy results of epoxy-coated reinforcing bars in RC test specimens.

Table 8 Comparison of the model parameters for uncoated reinforcing bars in four groups. RC test specimens

RC (ohm-cm2)

Cf  109 (F/cm2)

Rf (ohm-cm2)

RP  104 (ohm-cm2)

CDL  106 (F/cm2)

Group Group Group Group

783.8–1056 790–1185 611.5–684.6 623–702.5

343–399 142–548 127–373 407–1260

226.7–337.8 364.6–944.4 123.9–268.1 244.7–311.1

12.5–25.5 42–43.9 13.4–16.3 10.2–10.9

33.3–162 17–43.8 88.9–125 68.1–73.0

1 2 3 4

5.4. Influence of different epoxy coating thicknesses on corrosion behavior of reinforcing bars in RC test specimens For epoxy-coated reinforcing bars in unloaded test specimens, it can be seen from Table 4, that, with the increase of the epoxy coating, higher coating resistances Rep and lower double layer capacitances CDL are presented in the epoxy-coated reinforcing bars with thicker coating. However, as mentioned above, due to the nonhomogeneity of the coating thickness and defects during handling for the coated bars, the difference between resistances RP of reinforcing bars in test specimens E0.2E and E0.6E is not very obvious. For epoxy-coated reinforcing bars in loaded test specimens, it can be seen from Table 8 that, for RC test specimens in the same

group, larger coating capacitances Cep, lower resistances RP and larger double layer capacitances CDL are presented in the reinforcing bars with 600 lm nominal coating thickness; and this trend becomes obvious when RC test specimens were subjected to fatigue load (see Tables 6 and 7). For test specimen with fatigue-load induced crack width large than 0.2 mm (200 lm), for instance, test specimen E0.6FE5 in Group 3 as well as specimens E0.6FE1 and E0.6DE2 in Group 4, values CDL reach the order of 105, where the largest value of CDL is presented in test specimen E0.6FE1. Although test specimen E0.6FE5 in Group 3 has the largest crack width in the bottom surface (see Table 2) after 500,000 cycles fatigue load, in the electrochemical measurement-performed central 300 mm length, nearly half of this length has no load-induced crack, see Fig. 16a; while for test specimen E0.6FE1

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X.-H. Wang et al. / Construction and Building Materials 93 (2015) 746–765 Table 9 Comparison of the model parameters for epoxy-coated reinforcing bars in four groups. RC test specimens Group 1 Group 2 Group 3 Group 4

200 lm 600 lm 200 lm 600 lm 200 lm 600 lm 200 lm 600 lm

RC (ohm-cm2)

Cf  109 (F/cm2)

Rf (ohm-cm2)

Rep  104 (ohm-cm2)

Cep  108 (F/cm2)

RP  104 (ohm-cm2)

CDL  106 (F/cm2)

2465–3054 2437–5338 2635–3196 1631–6082 940.5–2934 1185–2376 916.1–2536 1326–2211

4.93–19.2 5.32–13.3 1.49–6.31 0.75–26.8 0.6–7.43 4.59–35.2 2.04–23.7 46.1–107

1261–2346 703.6–1569 961.5–1194 321.4–2139 464.1–2250 191.9–856.1 168.1–1040 559.9–1173

0.63–1.65 2.56–4.14 0.4–2.09 1.1–4.84 0.17–2.57 0.81–2.76 0.32–2.94 2.1–5.05

5.08–12.1 16.4–19.5 3.31–5.94 1.29–29.1 2.27–15.3 4.49–191.0 6.71–42.0 37.9–768.0

77.7–91.4 80.2–85.6 67.4–95.8 15.8–155 50.4–67.8 8.3–52 9.61–29.2 3.95–25.7

0.37–0.68 0.22–0.29 0.4–1.16 0.5–3.31 0.13–3.36 2.58–16.0 0.14–1.26 2.26–46.2

(a)Unloaded test specimens

(c) Test specimens subjected to 500,000 fatigue load

(b) Test specimens subjected to one-time static load

(d) Test specimens subjected to 1,000,000 fatigue load

Fig. 15. Illustration for the influence of external loading and loading type on the damages of epoxy coating in RC test specimens.

(a)Test specimen E0.6FE5

(b) Test specimen E0.6FE1

Fig. 16. Cracking pattern at the bottom surface of test specimens E0.6FE5 and E0.6FE1 subjected to 500,000 and 1,000,000 cycles fatigue loading, respectively.

subjected to 1,000,000 cycles fatigue load, it has larger crack width and lower crack spacing as well as comparatively uniformly distributed cracks within the measurement-performed length of bottom surface (see Fig. 16b), resulting in the largest Cep and CDL.

Thus, it can be concluded that, for the test specimen containing epoxy-coated bars with thicker coating thickness, corrosion resistance of epoxy-coated reinforcing bars in RC elements is greatly reduced by the fatigue load: the thicker coating, larger localized

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crack width and lower crack spacing as well as increased loading cycles, the larger reduction of corrosion resistance of the epoxy-coated reinforcing bars in RC test specimens.

6. Conclusions Corrosion performance of RC beam specimens with epoxy-coated reinforcing bars under in-service environments and external loads is experimentally investigated. 14kN static load and fatigue load (500,000 cycles and 2.8–14 kN as well as 1,000,000 cycles and 2.8–14 kN) were applied on the RC test specimens containing uncoated normal bars and epoxy-coated bars firstly; then the test specimens were immersed in simulated seawater solutions to subject to chloride attacks. Monitoring of open-circuit potentials and electrochemical impedance spectroscopy (EIS) were used to evaluate corrosion behavior of the epoxy-coated and uncoated reinforcing bars in loaded and unloaded RC test specimens. Two nominal coating thicknesses 200 lm and 600 lm were chosen, respectively. The following conclusions are obtained: (1) For uncoated and epoxy-coated reinforcing bars in unloaded and loaded test specimens, epoxy-coated reinforcing bars show higher corrosion resistance than that of the normal uncoated reinforcing bars. However, compared with that of the uncoated bars, improvement in corrosion resistance of the epoxy-coated bars in RC specimens is greatly influenced by the external loads. (2) One-time static load have little effect on corrosion behavior of uncoated and epoxy-coated reinforcing bars in RC test specimens; while influence of fatigue loading on reduction of corrosion resistance of both kinds of reinforcing bars is obvious. (3) For uncoated reinforcing bars in RC specimens, increase of the fatigue loading cycles has limited influence on their reduction of the corrosion resistance; while for epoxy-coated reinforcing bars, with the increase of fatigue loading time, greater reduction of corrosion resistance of the epoxy-coated bars in test specimens is presented, especially for bars with thicker nominal coating thickness. (4) For epoxy-coated reinforcing bars in unloaded test specimens, higher coating resistances Rep and lower double layer capacitances CDL are presented in the epoxy-coated reinforcing bars with thicker coating. For epoxy-coated reinforcing bars in loaded test specimens, lower corrosion resistance is presented in the reinforcing bars with 600 lm nominal coating thickness, especially when RC test specimens were subjected to fatigue load: the thicker coating, larger localized crack width and lower crack spacing as well as increased loading cycles, the larger reduction of corrosion resistance of the epoxy-coated reinforcing bars in RC test specimens. Although the coated and uncoated reinforcing bars were bought from same company and it was reported that all bars of the same size were from the same heat and all 12 mm bars had the same deformation pattern, experimental scatter is still existed. This result scatter may result from the nonhomogeneity of the coating thickness and defects during handling for the coated bars, the difference in concrete microstructures resulting from the concrete casting. In addition, due to open-circuit potentials are affected by a number of factors, which include polarisation by limited diffusion of oxygen, concrete porosities and voids and the presence of highly resistive layers [28] as well as load-induced cracks in the concrete cover and different crack patterns within the

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