Rebar corrosion in coral aggregate concrete: Determination of chloride threshold by LPR

Journal Pre-proof Rebar corrosion in coral aggregate concrete: Determination of critical chloride content by LPR Zhangyu Wu, Hongfa Yu, Haiyan Ma, Bo Da

PII:

S0010-938X(19)30788-7

DOI:

https://doi.org/10.1016/j.corsci.2019.108238

Reference:

CS 108238

To appear in:

Corrosion Science

Received Date:

21 April 2019

Revised Date:

20 August 2019

Accepted Date:

22 September 2019

Please cite this article as: Wu Z, Yu H, Ma H, Da B, Rebar corrosion in coral aggregate concrete: Determination of critical chloride content by LPR, Corrosion Science (2019), doi: https://doi.org/10.1016/j.corsci.2019.108238

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Rebar corrosion in coral aggregate concrete: Determination of critical chloride content by LPR Zhangyu Wu, Hongfa Yu *, Haiyan Ma, Bo Da Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 211100, China *Corresponding author at: Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 211100, China. E-mail address: [email protected] (H. Yu).

Highlights 

Using the linear polarization resistance (LPR) method to test the steel bars them.

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embedded in the CAC prismatic specimen and obtain the corrosion rate of The anti-corrosion performances of different reinforcements in CAC were compared and ranked via corrosion rate measured by LPR method. 

The surface free chloride content of steel bar in CAC was obtained through the

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two-dimensional chloride diffusion model.

The critical chloride content of different steel bars in CAC were determined corrosion rate of them.

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based on the statistical relationship between surface free chloride content and

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Abstract: Critical chloride content (Ccr) is an important index to judge the initial corrosion of rebar in concrete structures, and a key parameter for durability analysis and service life design of reinforced concrete structures. In the present wok, the rust corrosion rate of steel bars embedded in coral aggregate concrete (CAC) and the chloride ion diffusion law of CAC were studied by the linear polarization resistance (LPR) method and two-dimensional chloride diffusion model. Finally, the Ccr values of different steel bars in CAC were determined by the relationship between the surface chloride content of steel bars and its corrosion rate. The results indicate that the rust corrosion rate of rebar in CAC gradually decreases with the increase of concrete strength grade and cover thickness and the anti-corrosion performance of them was rank as follows: epoxy resin coated steel (ERCS) ＞2205 duplex stainless steel (2205S) ＞316L stainless steel (316L)＞organic coated steel (OCS) ＞ordinary steel (OS). The Ccr values (% by weight of concrete) of these rebar in CAC were determined as follows: ＜0.15 % (OS), 0.21 % (OCS), 0.33 ~0.41 % (316l), ＞0.46 % (2205S) and ＞ 0.41% (ERCS). Keywords: A: Coral aggregate concrete B: Linear polarization resistance method C: Rebar corrosion C: Critical chloride content C: Chloride diffusion

1 Introduction

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Chloride induced corrosion of reinforcing steel is the main cause of deterioration of reinforced concrete structures [1]. It is assumed that depassivation of reinforcement initiates once the chloride content on the surface of the reinforcement exceeds a critical value. This critical chloride content is conceptually called Ccr [2]. For both the durability design and the service life prediction of reinforced concrete structures, Ccr is identified to be one of the most decisive parameters in probabilistic service life modelling [3]. Research results show that the Ccr value of different types of reinforcing steels are different. To determine the Ccr value of reinforcement in chloride environment, scholars at home and abroad have conducted extensive studies [4-7] on it using various measuring methods. Some researchers [8-11] measured the free chloride content, generally by analyzing the concrete or mortar powder near the reinforcement surface, and then determined the Ccr value range of the reinforcement based on the visual corrosion state of it. Through the above method, only lower [11] or upper [10] boundaries for Ccr were get, since no corrosion was detected or obvious corrosion had already appeared at the time of measurement, and the exact bounds for Ccr cannot be determined eventually. It is an effective method to monitor the rebar by electrochemical method, and then judge the corrosion state of rebar according to related electrochemical parameters. Alonso et al. [12] preliminarily established the relationship between Ccr and corrosion current density by constant current method, potentiostatic method and potentiodynamic polarization method. Li et al. [13] measured Ccr values of reinforcement by open circuit potential and cyclic polarization method. Song et al. [14] obtained the Ccr value in cement mortar with admixture by cyclic polarization method. Poupard et al. [15] determine Ccr values using electrochemical impedance spectroscopy (EIS). The linear polarization resistance method (LPR) and EIS were used to study the Ccr value of reinforcement in simulated pore solution by Zhang et al [16]. Above studies have obtained the Ccr value of cement mortar and simulated pore solution by electrochemical methods. Nonetheless, there are few literatures on the Ccr value of reinforcement in concrete by electrochemical techniques. Since the 1980s, coral aggregate concrete (CAC), a new type of "environmentally friendly" concrete, composed of coral aggregate, seawater and cementitious materials had gradually been applied for reef engineering construction such as ports and airports. Chloride induced corrosion in CAC has been the subject of great research attentions at home and abroad. Dempsey G [17] held that CAC structures exposed to atmospheric and humid conditions are prone to suffer corrosion damage. In 1950s and 1960s, the United States navy civil engineering laboratory published series of standards [18-19] for the performance of coral aggregate and the mix proportion of CAC, and some reports [20-21] also documented rebar corrosion in CAC structures. After that, the United States navy, army, air force and other relevant departments issued joint standards: coral aggregates used in CAC must be washed with freshwater to reduce chloride content and avoid steel corrosion. The investigation results reported by Howdyshell P A [22] of United States army civil engineering laboratory in 1974 indicates that the steel bar in CAC is easy to corrode, so it is necessary to use freshwater for mixing and curing, and to strictly control the Cl- content of coral aggregate and cover thickness. To solve the serious reinforcement corrosion problem in CAC, Bullen [23] adopted the technology of "fly ash + high-efficiency water reducer" in the mix proportion design of CAC to improve the anti-chloride diffusion permeability. In 2003, Wanchai et al. [24] the chloride ion diffusion coefficient (Da) of C40 coral concrete was twice than that of C40 ordinary Portland concrete (OPC). Tehada T [25] and Wattanachai P [26] found that the steel corrosion rate and Da value of CAC were significantly higher than that of OPC with the same mix proportion, which is mainly because of the Cl- in coral aggregate. Yu [27] investigated different CAC structures in South China Sea, the field work indicated that marine CAC structures were severely damaged in form of corrosion, dissolution, cracking, spalling, coral denudation and steel corrosion. Above research shows that steel corrosion is one of the main factors affecting the durability of marine CAC structures.

To solve the problems of high steel corrosion rate in CAC and short service life of CAC structure, it is important to study the Ccr of different types of steel bars in CAC for the durability design and service life analysis of the reef CAC structural applications. The present work determined the Ccr values of different steel bars in CAC by the LPR method and the two-dimensional chloride diffusion model [28], providing technical support for the development of steel anti-corrosion technology in reef CAC engineering structures. Firstly, the LPR method was adopted to test the steel bars embedded in the CAC prismatic specimen exposed to simulated seawater for 0~270d and obtain the corrosion rate (i) of them. According to the distribution rule of free chloride content (Cf), then the surface free chloride content (Csf) value of steel bar in CAC was obtained through the two-dimensional chloride diffusion model. Ultimately, the Ccr value range of steel bar in CAC was determined based on the statistical relationship between Csf and i.

2 Material and experiments

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2.1 Material

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Coral from the South China Sea islands were adopted as the coarse aggregate. The range of particle size of broken corals was 5 ~ 20 mm. The apparent density and bulk density is 2257 kg/ mm3 and 1062 kg/ mm3, respectively. The cylinder strength is 3.8 MPa, the porosity is 58.5%, the 1 h water absorption is 10.9%, the mud content is 1.3%, and the Cl- content is 0.0055%. Coral sand from the South China Sea islands was used as fine aggregate, and the Clcontent of it is 0.001%. The apparent density is 2500 kg/m3, the bulk density is 1115 kg/m3, and the fineness modulus of coral sand is 3.5 and it belongs to medium sand. Chinese standard 525# Portland cement (C) produced in Nanjing was used in CAC. The basic physical and mechanical properties are shown in Table 1, and the chemical composition is shown in Table 2. Class I fly ash (FA) and grade S95 slag (SG) produced in Nanjing were also used and the chemical composition is shown in Table 2. The PCA-I polycarboxylic high-performance water reducer and the calcium nitrate inhibitor (CN) were produced in Nanjing. The simulated seawater was substitute ocean water according to the ASTM D1141-2003. The content ratio was NaCl: Na2SO4: MgCl2·6H2O: KCl: CaCl2 = 24.5: 4.1: 11.1: 0.7: 1.2. The reinforcements adopted in the experiments included: ordinary steel (OS), organic coated steel (OCS), epoxy resin coated steel (ERCS), 316L stainless steel (316L) and 2205 duplex stainless steel (2205S), and the chemical composition of them is shown in Table 3. The coating thicknesses of OCS and ERCS were 240 μm and 280 μm, respectively. The reinforcement diameters were 10 mm, and the exposure length was 200 mm.

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2.2 Specimen preparation and mixture proportion

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Cutting a groove on the surface of steel bar, then the wire was fixed in the groove with AB glue (Fig. 2a), and the steel bar bundled with wires was fixed in the mold with PVC sleeves. The porous structure of coral makes it has the characteristics of “water absorption and return”, so some seawater needs to be pre-mixed with coral aggregates. Firstly, putting the coral (sand) into the mixer, and then mixing some seawater (5% of the total amount of coral aggregates) for 1 min, which make coral (sand) pre-absorbs the seawater. Then cementitious material was added into the mixture and mixed evenly. Finally, the mixture of inhibitor, water reducer and seawater was mixed for 3 min. The mixture proportion and basic mechanical properties of CAC (C35 and C60) are listed in Table. 4. Different strength grades of CAC specimens with dimension of 150 × 150 × 300 mm were casted and cured under sealed conditions for 24h. Then the specimens were demolded and cured under simulated sea conditions (20 ± 5 ℃) for 28d. The bottom surfaces of CAC specimen shouldn’t be sealed with epoxy resin until the specimen is dried, and then the sealed specimen needs to be immersed in artificial seawater for exposed test. The specimens list is shown in Table. 5 and

the positions of different types of steel bars embedded in CAC are shown in Fig. 2b and Fig. 3.

2.3 Experiment methods 2.3.1 Liner polarization resistance test

 E  Rp     I E 0

-p

Icorr 

（Eq. 1）

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The LPR method is an electrochemical detection method based on corrosion electrochemical theory and the Stern-Geary formula [29], and the test principle is to apply a constant potential (ΔE<10mV) at the steel bar to ensure that the disturbance signal is small enough to satisfy the linear relationship between voltage and current [30]. LPR test was conducted using three-electrode test system, with the saturated calomel electrode (SCE) as the reference electrode (RE), the stainless steel plate as the auxiliary electrode (AE) and the steel under testing as the working electrode (WE). The potential scanning range within the range of -10 mv ~ 10 mv with respect to the corrosion potential, the scanning speed is 0.1667 mV/s. The test instrument is the CHI600D electrochemical workstation produced by Hua Ke Pu Tian Technology CO., LTD., in Beijing. The schematic diagram of LPR test is shown in Fig. 4. The self-corrosion potential (Ecorr), polarization resistance (Rp), corrosion current density (Icorr) of the reinforcement could be obtained by the LPR method, and the specific formulas [31-32] are as follows:

babc 1 B  2.303(ba  bc) Rp Rp

（Eq. 2）

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Where Rp is the polarization resistance (Ω·cm2); Icorr is the corrosion currency density (μA·cm-2); and B is a constant. When the rebar is in the passivation stage, B = 52 mV, and when the rebar is in the corrosion stage, B = 26 mV. Here, ba and bc are the Tafel constants in the anode and cathode processes, respectively. The conversion relation (Eq 3) between corrosion rate (i) and Icorr could be deduced according to the Faraday's law [33]. M M 1μA/cm2 =3.6  10-3  nF  104 g / (m2  h) =3.27  103  nF mm/a

（Eq. 3）

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Where M is the atomic mass of iron (M= 55.85 g·mol-1); F is the Faraday constant (F= 96 485); and n is the number of electrons that iron loses per atom when it corrodes (n = 2). Therefore, Eq. 3 can be converted to 1 μA /cm2= 0.0104 g/ (m2·h) = 0.0116 mm/a. The reinforcement corrosion standard obtained by the LPR method is shown in Table. 6 [34].

2.3.2 Chloride ion diffusion test

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Using a hand-held drill to collect CAC powder at different depths, and the diameter of the drill is 6 mm. The drilling position is shown in the Fig. 5a. There are 12 holes on the two symmetrical sides of CAC specimen, the hole interval is 20 mm and the sampling depth is 0 ~5 mm, 5~10 mm, 10~15 mm, 15~20 mm, 20~25 mm, 25~30 mm, 35~40 mm and 45~50 mm. According to JTJ270-98, the Cf values at different depths of CAC were analyzed using the ZDJ-4A automatic potentiometric titrator produced by the INESA Scientific Instrument Co., Ltd in Shanghai. The calculation method and process of Csf and Da of CAC were shown detailed in the literature [35]. Since the bottom surfaces of CAC prismatic specimens immersed in the simulated seawater were sealed with epoxy resin coating, the Cl- diffused into the CAC is controlled by two-dimensional diffusion. The spatial rectangular coordinate system of two-dimensional chloride diffusion in CAC is shown in Fig. 5b. By substitution of the Cs, Da and coordinate (x, y) values into the two-dimensional chloride diffusion model (Eq. 4) [28], the Csf of reinforcements at different positions of CAC specimen could be calculated.

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Cf  C0  

 m n m n  16 sin( x ) sin( y ) exp  Da ( 2  )t  2 mn 2 L1 L2 L1 L2   2

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 (Csf  C0 )

m1,3,5 n1,3,5

2

2

2

(Eq. 4)

Where Cf is the free chloride content at the depth of x from the concrete surface (%); Csf is the surface free chloride content (%); C0 is the initial chloride content (%); Da is the apparent chloride diffusion coefficient (mm2·s-1); L1 and L2 are the depths in the directions of x and y (mm), respectively (Note: L1 = L2 = 150 mm; 1 mm = 0.0394 in).

3 Results and discussion 3.1 Corrosion rate of reinforcement in CAC

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Fig. 6 shows the linear polarization curves of different types of reinforcements in CAC immersed in simulated seawater for 270 d. The i values of reinforcements in CAC obtained from the Fig. 6 and Eq. 1 ~ 3 were presented in Table. 7. It can be seen from Fig. 6 that the Ecorr of rebar gradually increases with the increase of cover thickness, while the polarization curves gradually moves forward along the X-axis and shifts negatively along the Y-axis, indicating that the corrosion probability and corrosion rate of rebar decreases with the increase of cover thickness. This is due to the increase of cover thickness, the diffusion of harmful substance such as Cl- into concrete is hindered, then the passivation film on the surface of the steel tends to be stable, and the corrosion probability and corrosion rate of rebar gradually decrease. The Ecorr of OS in CAC-1 specimen is much lower than that of OS with the same cover thickness in CAC-2, and the corrosion rate of the former is obviously higher than that of the latter, indicating that the corrosion probability and corrosion rate of OS in C50 CAC is lower than C30 CAC. This is due to the improvement of concrete strength grade makes the hydration of cement in concrete more sufficient and reduces porosity inside concrete, which is beneficial to improve the density of concrete and reduce corrosion risk of chloride in CAC. There were differences of corrosion potential and corrosion rate of different types of reinforcements in CAC. For instance, the Ecorr of OCS in CAC-3 shifted positively relative to that of OS with the same cover thickness, indicating that the corrosion probability of OCS was lower than that of OS. Therefore, the anti-corrosion performance of OCS is better than that of OS under the same conditions. According to the analysis of polarization curves and corrosion rates of different steel bars in CAC, the anti-corrosion performance of them was ranked as follows: ERCS> 2205S> 316L> OCS> OS.

3.2 Chloride diffusion and diffusion parameter of CAC

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The distribution curve of Cf values over diffusion depth of CAC exposed for 270 d in simulated seawater is plotted in Fig. 7. It can be seen from Fig. 7 that: (1) the relationship between Cf and diffusion depth conform the Fick’s second law, which is similar to that of ordinary Portland concrete [35]; (2) The Cl- content of CAC at the depth of 40 ~50 mm ranges from 0.2 % to 0.3 %, which is much higher than that of ordinary high-performance concrete under the same conditions [36]. Although the cover can effectively reduce the chloride diffusion rate, the coral aggregate and mixing seawater make the initial chloride content of CAC (0.20% ~0.23%, by weight of concrete) is much higher than that of ordinary high-performance concrete (0% ~0.05%, by weight of concrete). According to the Cf values at different diffusion depths in Fig. 7, Da values of CAC and Csf values of different reinforcements were calculated by the two-dimensional chloride diffusion model and SAS analysis software, and the chloride diffusion parameters of CAC are shown in Table. 8.

3.3 Surface free chloride contents of different reinforcements in CAC

3.4 Ccr of different reinforcements in CAC

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Substituting the chloride diffusion parameters (Csf, C0 and Da) and coordinate value (x, y) of reinforcements into the two-dimensional chloride diffusion model, so as to calculate the Csf of different reinforcements in CAC, and the results are listed in Table. 9. As shown in Table. 9, it can be known that the Csf values of reinforcements in CAC gradually decrease with the increase of cover thickness. The main reasons are as follows: when the cover thickness is small, Cl- can easily invade concrete and accumulate on the surface of reinforcement, resulting in the increase of the Csf of reinforcements at shallow depths. With the increase of cover thickness, the chloride diffusion distance in CAC can be directly extended, so as to efficiently reduce the Csf of reinforcements at deep depths. Additionally, it can be seen that the Csf of ERCS in CAC was lower than that of other reinforcements under the same conditions, which is mainly due to 3% CN was mixed in CAC-4. The CN can fill the pore of coral aggregate or concrete and clog the chloride diffusion channel. In additional, the Fe2+ in concrete can be rapidly transformed into the compact passivation film on the surface of reinforcement under the action of the CN with strong oxidation [37], then the oxidation product will fill the pore and crack of steel/ concrete interface and enhance the density of interface zone. Through the mechanism analysis of CN, it can be known that the Da of CAC mixed with 3% CN is smaller than that of CAC without inhibitor (as shown in Table. 8), which is the fundamental reason that the Csf of ERCS in CAC was lower than that of other reinforcements.

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Fig. 8 plots the scatter diagram on relationship between Csf and i of different steel bars in CAC. As shown in Fig. 8a, it can be seen that the i values of OS were all bigger than 0.001 mm·a-1 when the Csf of them was bigger than 0.15 %. According to the corrosion standard by LPR (Table. 6), it is indicated that the OS in CAC was in the state of corrosion and the Ccr value range of OS is ＜ 0.15 %, which is consistent with the results in literature [38]. Simultaneously, according to the Fig. 8b ~ e, it is clear that the Ccr value range of OCS, 316L, 2205S and ERCS in CAC were 0.21 %, 0.33% ~ 0.41 %, ＞ 0.46 % and ＞ 0.41 %, respectively. Through the comparison of Ccr value range of different steel bars in CAC, the anti-corrosion performance of them could be ranked as follows: ERCS＞ 2205S＞ 316L＞ OCS＞ OS, which is coincided with the previous results (section 3. 1). The main reasons are as follows: (1) organic coating has good compactness, and the coating applied on the surface of steel bars can prevent Cl-, H2O, O2 and other corrosion substances from accumulating on the surface of the steel bars, which is equivalent to adding a protective layer on the surface of the steel bars to fully protect the rebar [39]. In addition, organic coating belongs to insulating material that can protect steel bars from stray current in concrete and plays a certain part on hindering electrochemical rebar corrosion. Therefore, the anti-corrosion performance of OCS is better than OS. (2) The Cr2O3 passivation film on the surface of stainless steel is more compact than the organic coating, so as to the possibility of pitting corrosion of stainless steel in lower than that of OCS, that is the anti-corrosion performance of stainless steel is better than that of OCS. What’s more, studies [40] have shown that duplex stainless steel has a duplex phase (α+γ) structure, endowing it with both excellent toughness and good machinability for austenitic stainless steel and the high strength and anti-chloride corrosion properties for ferritic stainless steel. Thus, the anti-corrosion performance (pitting corrosion, crack corrosion, stress corrosion and fatigue corrosion) of duplex stainless steel are significantly better than those of ordinary austenitic stainless steel and can even be comparable to high alloy austenitic stainless steel. Therefore, the anticorrosion performance of 2205S is better than that of 316L under the same conditions. (3) The epoxy resin coating has a strong adhesion and a high density, which can effectively cover the surface of the steel rebar and prevent Cl-, H2O, O2 and other corrosive substances from

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contacting the steel. According to the principle of corrosion cell, when there is little or no water and oxygen on the metal, it is difficult for the polarization and depolarization processes of the electrode to occur. Then, the electrode has a high polarization degree and a low cathode (anode) potential, leading to a significant reduction in the corrosion current and corrosion rate of steel bar [41]. Epoxy resin can efficiently inhibit the dissolution of anode metal ions and the cathode discharge phenomenon in concrete due to its excellent insulating properties. Thus, the resin can significantly prevent ion movement at the anode or cathode in concrete or solution, thus achieving the effect of adding resistance in the solution area of the corrosion cell circuit [41]. So the anti-corrosion performance of ERCS is stronger than that of the stainless steel rebar. The Ccr value range of different steel bars in different literatures [38, 42-44] and the Ccr value range obtained in this work are listed in Table. 10. Through the comparison of Ccr values of different steel bars, it is suggested that the Ccr values obtained in this paper are consistent with that in the literatures [38, 42-44]. This means that the research method of Ccr of steel bar in CAC is suitable for the research of that, and the Ccr values obtained in this paper are also proved to be reasonable.

4. Conclusions

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(1) The Ecorr of rebar in CAC gradually increases with the increase of cover thickness, while the polarization curves gradually moves forward along the X-axis and shifts negatively along the Y-axis, indicating that the corrosion probability and corrosion rate of rebar decreases with the increase of cover thickness. (2) The relationship between free chloride content and diffusion depth conform the Fick’s second law, which is similar to that of ordinary Portland concrete. The coral aggregate and mixing seawater make the initial chloride content of CAC (0.20% ~0.23%, by weight of concrete) is much higher than that of ordinary high-performance concrete (0% ~0.05%, by weight of concrete). (3) From the experimental results of the LPR method, including the linear polarization curve, corrosion potential and corrosion rate, the anti-corrosion performance of different rebar was ranked as follows: epoxy resin coated steel (ERCS) > 2205 duplex stainless steel (2205S) > 316L stainless steel (316L) > organic coated steel (OCS) > ordinary steel (OS). (4) Based on the statistical relationship between surface free chloride content and corrosion rate of rebar, the Ccr value range of different rebar in CAC were determined as follows: ＜0.15 % (OS), 0.21 % (OCS), 0.33 ~0.41 % (316l), ＞0.46 % (2205S) and ＞ 0.41% (ERCS).

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Acknowledgements

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The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under Grant No. 51878350 and No. 11832013, the China Postdoctoral Science Foundation under Grant No. 2018M630558, and the Natural Science Foundation of Jiangsu Province under Grants No. SBK2018041341.

Data Availability The data used to support the findings of this study are available from the corresponding author upon request.

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[23] F. Bullen, Coralline concrete in the Pacific, Proceedings of the Third International Colloquium on Concrete in Developing Countries, Beijing, China, May, 1990. [24] Y. Wanchai, O. Nobuaki, N. Takahiro, Study on strength and durability of concrete using low quality coarse aggregate from circum-Pacific region, Fourth Regional Symposium on Infrastructure Development in Civil Engineering (RSID4). Bangkok, Thailand, April 2003. [25] T. Tehada, M. Funahashi, Cathodic protection of building reinforcing steel. In: Triservice corrosion conference, NACE International: Orlando, Florida, USA, November 2005. [26] P. Wattanachai, N. Otsuki, T. Saito, A study on chloride ion diffusivity of porous aggregate concretes and improvement method, Doboku Gakkai Ronbunshuu E. 65(1) (2009) 30-44. [27] B. Da, H. Yu, H. Ma, Y. Tan, R. Mi, X. Dou, Chloride diffusion study of coral concrete in a marine Environment, Construction and Building Materials. 123(2016) 47-58. [28] H. Yu, Study on high performance concrete in salt lake: Durability mechanism and service life prediction. Ph. D. Thesis. Southeast University, Nanjing, 2004 (in Chinese). [29] M. Stern, A. L. Geary, Electrochemical polarization I. A theoretical analysis of the shape of polarization curves, Electrochem. Soc. 104(1957) 56-63. [30] S. Wu, L. Jiang, J. Xu, J. Xu, G. Huang, Effect of antirust agents on corrosion behavior of concrete steel reinforcement in the presence of chlorides, Materials Protection. 45(2) (2012) 11-14 (in Chinese). [31] B. Wei, Theory and application of metal corrosion, Chemical Industry Press. Beijing, 1984 (in Chinese). [32] C. Liang, Metal corrosion introduction, China Machine Press. Beijing, 1999 (in Chinese). [33] H. He, Y. Cui, M. Shi, Y. Gu, Real time monitoring of corrosion of rebar in concrete [J]. Journal of Building Materials. 16(01) (2013) 50-54 (in Chinese). [34] K. R. Gowers, S. G. Millard, On-site linear polarization resistance mapping of reinforced concrete structures, Corrosion Science. 35(1993) 1593-1600. [35] Y. Lu, H. Yu, H. Ma, H. Ma, Experiment on free chloride diffusion coefficient of concrete exposed to marine environment, Journal of Architecture of Civil Engineering, 28(04) (2011) 86-91 (in Chinese). [36] Z. Xu, H. Ma, H. Yu, M. Xu, Y. Xu, T. Feng, Time variation law of free chlorine ion content in the surface of marine structure and its influence on life, The Ocean Engineering, 35(4) (2017) 126-134 (in Chinese). [37] Z. Cao, P. Xiao, Application research review on nitrite-based corrosion inhibitors, Concrete. 10(2011) 49-54 (in Chinese). [38] O. E. Gjørv, Durability Design of Concrete Structures in the Severe Environments, Taylor & Francis Croup, Abingdon, 2009. [39] Q. Wang, G. Xu, Protective measurements for durability of concrete structures in chloride environment, Concrete. 7(2007) 96-100 (in Chinese). [40] Z. Liu, Research status of corrosion behavior of 316LSS and 2205DSS, Corrosion & Protection. 31(2) (2010) 149-153 (in Chinese). [41] Li W, EIS studies on degradation of epoxy heavy-duty corrosion protection coating systems, Master’s thesis. Beijing University of Chemical Technology, Beijing, 2007 (in Chinese). [42] B. Da, Research on preparation technology, durability and mechanical properties of concrete members of high strength coral aggregate seawater concrete, Ph.D. thesis. Nanjing University of Aeronautics and Astronautics, Nanjing, 2017. [43] A. Pachón-Montaño, J. Sánchez-Montero, C. Andrade, J. Fullea, E. Moreno, V. Matres, Threshold concentration of chlorides in concrete for stainless steel reinforcement: Classic austenitic and new duplex stainless steel, Construction and Building Materials. 186(2018) 495-502. [44] F. Lollini, M. Carsana, M. Gastaldi, E. Redaelli, L. Bertolini, The challenge of the performance-based approach for the design of reinforced concrete structures in chloride bearing environment, Construction & Building Materials. 79(2015) 245-254.

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[45] D. Hong, Corrosion and protection of steel bar in concrete, China Railway Press. Beijing, 1997 (in Chinese).

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Fig. 1. Different types of reinforcements. “A”, “B”, “C”, “D” and “E” represents ordinary steel (OS), 316L stainless steel (316L), 2205 duplex stainless steel (2205S), organic coated steel (OCS) and epoxy resin coated steel (ERCS), respectively.

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Fig. 2. The section diagram of CAC specimen. (a) Longitudinal section. (b) Cross section (AA)

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Fig. 3. Schematic diagram of the steel bars positions (mm). (a) Type a. (b) Type b. (c) Type c. The “OS”, “316L”, “2205S”, “OCS” and “ERCS” in the figure represents ordinary steel, 316L stainless steel, 2205 duplex stainless steel, organic coated steel and epoxy resin coated steel, respectively.

Fig. 4. The schematic diagram of LPR test. The SCE, RE, WE and AE in the diagram indicates the saturated calomel electrode, the reference electrode, the working electrode and the auxiliary electrode, respectively.

Fig. 5. The drilling position and the spatial rectangular coordinate system of two-dimensional chloride diffusion of CAC. (a) Drilling position. (b) Spatial rectangular coordinate system -0.40

(a)

-0.44

OS(70mm)

-0.52

OS(50mm)

-0.56

OS(50mm)

-0.52

E/V

E/V

OS(70mm)

-0.48

OS(40mm) OS(25mm)

-0.60

(b)

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-0.48

OS(40mm)

-0.56

OS(25mm)

-0.60 -0.64 -0.64

-0.68

-7

-6

-5

Log（ I/A）

-0.68

-4

-0.06

(c) -0.48

OCS(70mm)

-0.08

OCS(50mm)

-0.10

OCS(40mm)

-6

Log（ I/A）

-5

-4

(d)

ERCS(70mm)

ERCS(50mm)

re

-0.12

-7

ERCS(40mm)

E/V

E/V

-0.50

OS(15mm)

-p

OS(15mm)

-0.14

OCS(25mm)

-0.52

-0.16

ERCS(25mm)

lP

-0.18

OCS(15mm)

-0.54 -7

-6

-0.22

-4

-0.08

-0.08 2205S(40mm) -0.16 316L(25mm)

-0.24

na

(e)

-0.16

-8

-7

-6

Log（ I/A）

(f) 316L(40mm) 316L(25mm) 2205S(15mm)

-0.24

E/ V

2205S(15mm)

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E/ V

-5

Log（ I/A）

0.00

-0.32

ERCS(15mm)

-0.20

-0.32

OCS(70mm)

OCS(70mm)

-0.40 -0.48

OCS(50mm)

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-0.56

OCS(25mm)

-0.64 -8.0

-7.5

-7.0

-0.40 OCS(50mm)

OCS(40mm)

OS(50mm)

-0.56

OCS(15mm) -6.5

-6.0

Log(I/A)

-0.48

-5.5

-5.0

-4.5

OCS(25mm) OCS(15mm) OS(50mm)

-7.5

-7.0

OCS(40mm)

-6.5

-6.0

-5.5

-5.0

Log(I/A)

Fig. 6. Linear polarization curves of different types of reinforcements in CAC exposed for 270 d: (a) CAC-1. (b) CAC-2. (c) CAC-3. (d) CAC-4. (e) CAC-5. (f) CAC-6.

0.6

(a) CAC-1

0.6

(b) CAC-2

0.6

Test curve

(c) CAC-4

Test curve

Test curve

0.5

0.5

0.5 0.4

Cf / %

Cf / %

Cf / %

0.4

0.4

0.3

0.3

0.3

0.2

0.2

0.2

0.1

0

10

20

30

Diffusion depth x / mm

40

50

0

10

20

30

40

50

0

10

Diffusion depth x / mm

20

30

Diffusion depth x / mm

40

50

Fig. 7. The distribution curve of Cf values over diffusion depth of CAC exposed for 270 d in simulated seawater: (a) CAC-1. (b) CAC-2. (c) CAC-4. 0.014

0.014

(a)

CAC-1(270d) CAC-2(270d) Da B. (2017)[42]

0.012 0.010

i/ mm·a-1

i/ mm·a-1

0.010

(b)

CAC-3(270d) CAC-5(270d) Da B. (2017)[42]

0.008 0.006

0.008 0.006

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0.012

0.004

0.004

0.002 Rust 0.001 No rust 0.000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

0.002 Rust 0.001 No rust 0.000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Cf/ %

Cf/ % 0.0030

0.0025

CAC-5/6 (270d) CAC-5/6 (0-180d) Da B. (2017)[42]

(c)

0.0025

0.0020

i/ mm·a-1

i/ mm·a-1

0.0020

0.0015

0.0015

Rust

0.0010 No rust

Ccr

0.0005

0.0000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.0020

0.0016

i/ mm·a-1

0.0014

na

0.0012

0.0008

No rust

0.0006 0.0004

ur

0.0002 0.0000 0.10

Cf/ %

(e)

CAC-4(270d) CAC-4(0d)

Rust

0.0000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

lP

Cf/ %

0.0010

Ccr

No rust

0.0005

0.0018

(d)

Rust

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0.0010

CAC-5/6(270d) CAC-5/6(0~180d) Da B. (2017)[42]

-p

0.0030

0.15

0.20

0.25

0.30

Cf/ %

0.35

0.40

0.45

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Fig. 8. The scatter diagram on relationship between Csf and i of different steel bars in CAC. (a) OS. (b) OCS. (c) 316L. (d) 2205S. (e) ERCS. The mixture proportion, strength grade and exposure environment of CAC in literature [42] are same as the CAC in this paper.

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Table 1. Basic physical properties and mechanical properties of Portland cement. Flexural Setting time/ Compressive Specific strength/ Loss of h strength/ MPa surface area Consistency/ % MPa ignition/ % /m3·kg-1 Initial Final 3 d 28 d 3d 28 d 380 28.0 1.80 2.50 3.30 33.0 60.0 6.40 6.00

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Table 2. The chemical composition of cementitious material (mass fraction, %). Materials SiO2 Al2O3 CaO MgO SO3 Fe2O3 IL C 21.35 4.67 62.60 3.08 2.25 3.31 0.95 FA 54.88 26.89 4.77 1.31 1.16 6.49 3.10 SG 28.15 16.02 34.54 6.03 0.32 1.13 2.88

N 0.17

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Table 3. The chemical composition of reinforcements (mass fraction, %). Steel type C Si Mn P S Cr Ni Mo OS 0.20 0.56 1.42 0.021 0.039 316L 0.03 1 2 0.045 0.03 17 12 2 2205S 0.03 1 2 0.030 0.020 23 5.50 3.25

Material composition /kg·m-3 SG

FA

Coral

C35 C60

429 780

82.5 150

38.5 70

680 300

Coral sand 1020 700

C60

780

150

70

300

700

165 250

Water reducer 8.25 20

250

20

Water

f Total W/B

CN /%

Slump /mm

Flow /mm

Apparent density /kg·m-3

7d fcu /MPa

28d fcu /MPa

0.25 0.2

0.42 0.25

-

70 255

820

2170 2267

28.2 48.2

37.9 61.9

0.2

0.25

3

250

830

2245

53.5

65.4

e-

Cement

Net W/B

pr

Strength grade

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Table. 4. Mixture proportion and basic and basic mechanical properties of CAC

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Pr

Note: (1) The “fcu” in the table is the cubic compressive strength; (2) The content of calcium nitrite rust inhibitor (CN) in the table is the percentage of the cementitious materials.

Table. 5. The specimen list of CAC Strength grade

Steel type

CAC-1 CAC-2 CAC-3 CAC-4 CAC-5 CAC-6

C35 C60 C60 C60 C60 C60

OS OS OCS ERCS Multiple types Multiple types

Inhibitor Type Amount CN 3% -

Steel location type Type a Type a Type a Type a Type b Type c

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Table. 6. The reinforcement corrosion standard of the LPR method Rp /kΩ·cm2 Icorr /μA·cm-2 i /mm·a-1 Corrosion rate standard 2.5-0.25 10-100 0.1-1 Ⅰ 25-2.5 1-10 0.01-0.1 Ⅰ 250-25 0.1-1 0.001-0.01 Ⅰ Ⅰ ＞250 ＜0.1 ＜0.001 Notes: “Ⅰ”, “Ⅱ”, “Ⅲ” and “Ⅳ” in the table indicate the corrosion rate of steel bars is at the level of “Much higher”, “High”, “Medium, low” and “Now rust”, respectively.

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Table. 7. Corrosion rate (i) values of different types of reinforcements in CAC exposed for 270 d (×10-3 mm/a) Cover thickness/ mm Corrosion rate No 15 25 40 50 70 standard A B A B A B A B A CAC-1 2.24 2.84 3.11 1.22 3.52 3.35 3.29 5.25 1.25 Ⅱ、Ⅲ CAC-2 11.3 9.13 6.70 3.17 4.88 4.77 4.34 5.32 5.49 Ⅱ、Ⅲ CAC-3 6.98 9.06 2.66 5.67 2.92 3.26 0.454 0.282 0.448 Ⅲ、Ⅳ CAC-4 0.561 0.121 0.665 0.363 0.243 0.154 0.106 0.348 0.0552 Ⅳ CAC-5 5.16 0.720 2.64 0.504 2.74 0.184 3.55 0.209 0.234 Ⅲ、Ⅳ CAC-6 4.73 0.833 2.67 0.471 2.32 0.212 0.325 1.51 0.230 Ⅲ、Ⅳ Notes: (1) The “corrosion rate stand” in the table is consistent with that in Table. 6; (2) According to the type and position of reinforcements in CAC (Fig. 3), it could be indicated that: (a) There was only one type of steel bar in CAC-1, CAC-2, CAC-3 and CAC-4; (b) The “A” steel bars at different cover thicknesses in “CAC-5” and “CAC-6”were all OCS, and the “B” steel with the cover thickness of 15 mm, 25 mm, 40 mm and 50 mm are 2205S, 316L, 2205S and OS, respectively. Similarly, the “B” steel bars in “CAC-6” with the cover thickness of 15 mm, 25 mm, 40 mm and 50 mm are 2205S, 316L, 316L and OS, respectively.

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Table. 8. The chloride diffusion parameters of CAC exposed for 270 d in simulated seawater Strength grade No Inhibitor Csf /% C0/% Da/10-12mm2·s-1 C35 CAC-1 0.5770 0.2280 3.34 CAC-2 0.5918 0.2343 2.09 C60 CAC-4 CN-3% 0.7098 0.2192 1.15

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Table. 9. The Csf of different reinforcements in CAC exposed for 270 d in simulated seawater Cover thickness/ mm Strength grade Steel type 15 25 40 50 70 OS 0.43 0.33 0.23 0.19 0.16 C35 OCS 0.43 0.33 0.23 0.19 0.16 OS 0.44 0.32 0.22 0.19 0.16 OCS 0.44 0.32 0.22 0.19 0.16 C60 316L 0.32 0.22 2205S 0.44 0.22 ERCS 0.42 0.31 0.21 0.18 0.15

Table. 10. Ccr value range of different steel bars / %C Steel type OS OCS 1.4404 (316L) stainless steel 1.4462 (2205S) stainless steel ERCS

Ccr in literatures [38, 42-45] / %C 0.04~0.37 [38], 0.13 [42] 0.16 ~ 0.21 [42] 0.48 ~ 0.55 [43] 0.70~1.03 [43], 0.55% ~ 2.38% [44] 0.18%~0.73% [45]

Ccr in this paper /%C ＜0.15 0.21 0.33~0.41 ＞0.46 ＞0.41

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Notes: The unit (%C) of Ccr value is by weight of concrete.