Critical conditions and life prediction of reinforcement corrosion in coral aggregate concrete

Construction and Building Materials 238 (2020) 117685

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Critical conditions and life prediction of reinforcement corrosion in coral aggregate concrete Ditao Niu a,b, Lu Zhang a,b,⇑, Qiang Fu a,b,⇑, Bo Wen a,b,⇑, Daming Luo a,b,⇑ a b

State Key Laboratory of Green Building in Western China, Xi’an University of Architecture & Technology, China Department of Civil Engineering, Xi’an University of Architecture & Technology, China

h i g h l i g h t s
The critical chloride concentration in coral aggregate concrete was obtained.
The time-dependent denaturation of the corrosion current in the coral aggregate concrete were analyzed by LPR and EIS. 2


The rebar corrosion rate in the coral aggregate concrete was 0.25 mA/cm at the beginning of steel corrosion, and the corrosion rate ratio in the two

concrete types was 1.67.
The life prediction equation of the steel bars at the beginning of corrosion in the coral aggregate concrete was obtained.

a r t i c l e

i n f o

Article history: Received 23 June 2019 Received in revised form 15 November 2019 Accepted 22 November 2019

Keywords: Coral aggregate concrete Rebar corrosion initiation conditions Critical chloride concentration Electrochemical impedance spectroscopy Time-dependent denaturation Life prediction

a b s t r a c t As a new type of cement-based composite material, coral aggregate concrete exhibits excellent performance in terms of using local materials, high efficiency and economy, and broad application prospects in the construction of ocean-going island reef projects. In this study, based on the corrosion mechanism of steel bars, the critical chloride concentration in coral aggregate concrete was obtained, and the model of the rebar corrosion initiation conditions was established. Combined with electrochemical test and power loading device, the stainless steel bar replaces an electrode in energized devices and it is the counter electrode in electrochemical test. A rapid determination method was proposed for chloride ions accelerated by the electric field by improving the anode energization acceleration method. The linear polarization resistance method and electrochemical impedance spectroscopy were used to analyze the time-dependent denaturation of the corrosion potential and corrosion current in the coral aggregate concrete. The corroded layer on the steel reinforcement surface was observed using scanning electron microscopy. The rebar corrosion rate in the coral aggregate concrete was 0.25 mA/cm2 at the beginning of steel corrosion, and the corrosion rate ratio in the coral aggregate concrete and common aggregate concrete was 1.67. Based on the similarity theory of life test, an accelerated life test was used to predict the durability life of the coral aggregate concrete structures. Ó 2019 Published by Elsevier Ltd.

1. Introduction With economic developments, marine advancement offers great strategic significance for the country’s future, owing to the depletion of land resources. The majority of islands and reefs are far away from land. The building materials and freshwater resources of islands are extremely limited, and construction on islands is restricted by steep upfront costs. Therefore, it is logical to use local coral aggregate instead of traditional building materi⇑ Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (Q. Fu), [email protected] (B. Wen), [email protected] (D. Luo). https://doi.org/10.1016/j.conbuildmat.2019.117685 0950-0618/Ó 2019 Published by Elsevier Ltd.

als for this purpose. Coral aggregate concrete is widely used in offshore reef construction. Without damaging the natural ecological environment of the island, coral aggregate is used to replace natural sandstone in preparing coral aggregate concrete. At present, no clear definition of coral concrete exists. It refers to a new type of building material, with coral sand as the fine aggregate and coral reef rubble as the coarse aggregate. The cement, water, and water-reducer are mixed in a certain proportion. The main mineral components of coral aggregates are aragonite, dolomite, and calcite, in which the main ingredient is calcium carbonate and a small amount of chlorinated salt [1,2]. Therefore, the concrete durability is seriously threatened by the inorganic salt ions. Research has demonstrated that coral aggregates can be used to replace ordinary

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D. Niu et al. / Construction and Building Materials 238 (2020) 117685

aggregates for preparing concrete in the case of a shortage of ordinary aggregates. However, the durability problems caused by the chloride ions of coral aggregates must be addressed [3–5]. Since 1970, several roads, airports, and other buildings have been built using coral concrete on the Western Pacific islands (Midway, Kwajalein, Eniwetok, Bikini, Johnston, Wake, Saipan, and Guam) [6]. Moreover, coral concrete has been used in breakwaters, sand dikes, revetments, retaining walls, sidewalks, and building foundations [7], as well as for the emergency restoration of island reef projects [8]. A certain degree of basic research and application of coral aggregate concrete has been carried out in several countries with tropical coastal areas. However, research on steel corrosion in coral aggregate concrete has rarely been conducted. Wattanachai et al. [9] studied the two-dimensional diffusion of chloride ions in coral concrete. Kakooei et al. [10–11] studied the oxygen permeability and corrosion behavior of rebar in coral aggregate concrete. Wang et al. [12] conducted a feasibility study on the application of coral concrete in a steel tube, based on the chloride ion corrosion mechanism. The theoretical analysis proved that oxygen could not diffuse through the coral concrete when it was poured into the steel pipe, so the problem of chloride ion corrosion could be effectively avoided. Howdyshell et al. [13] suggested that the corrosion protection of the steel in the coral concrete must ensure sufficient thickness of the protective layer, while the chloride ion content in the concrete must be strictly controlled. To sum up, coral aggregate concrete is widely concerned and applied in reef construction, but its durability is rarely studied. Reinforced concrete structures in the marine environment undergo long-term exposure to chlorine salts. If the durability design, testing, and protection of the structure do not receive sufficient attention, it will be difficult for the service life to meet the design requirements, thereby resulting in huge economic losses and safety hazards in national infrastructure construction [14,15]. Therefore, durability studies of reinforced concrete structures in the marine environment have become an important topic, and one important parameter in the prediction, detection, and protection of the durability life is the critical chloride ion concentration [16–18]. The chloride ion migration mode includes capillary absorption and infiltration, in addition to diffusion in the ocean wave splash and tidal zones, which results in a high intrusion rate. Moreover, it is affected by the alternation of wet and dry conditions, wind wave loads, and microbial corrosion. Therefore, steel reinforcements experience severe corrosion in these areas [19]. Numerous researches have been conducted on the critical chloride concentration of concrete in the marine environment, but the conclusions have differed substantially. Therefore, evaluating the critical value of steel corrosion is the key factor influencing the research results. The steel bar forms a dense passivation film under the concrete alkaline environment and the physical barrier of the protective layer. When an external corrosive medium (such as Cl) is transmitted to the steel bar surface and reaches a certain concentration, or the pH value of the steel bar surrounding environment decreases sharply, the passivation film gradually begins to destabilize, and the steel bar surface starts to rust. Therefore, the corrosion mechanism and corrosion process of steel bars in coral aggregate concrete were studied by comparing the initial steel corrosion conditions in coral aggregate concrete and ordinary concrete. In this study, according to the characteristics of coral aggregate concrete, the linear polarization resistance (LPR) method and electrochemical impedance spectroscopy (EIS) were used to monitor the steel corrosion in concrete under different voltage values. This paper discusses the method for judging the critical value of the steel corrosion. The effect of the electric field on the critical chloride concentration in the coral aggregate concrete was analyzed. Moreover, the corroded layer on the steel reinforcement surface

was observed by a scanning electron microscope (SEM). The similarity theory of life test is put forward and the durability life of coral aggregate concrete structure is predicted by accelerated life test method. Experimental parameters were provided for the durability design and life prediction of the coral aggregate concrete structures. 2. Accelerated corrosion and electrochemical test 2.1. Raw materials and mix ratio The cementing materials used in the experiment were PO42.5 ordinary Portland cement and Grade II fly ash, as displayed in Table 1. The ordinary concrete fine aggregates were made of river sand with a fineness modulus of 2.4. The coarse aggregates were basalt gravel with a particle size of 5 to 20 mm. The coral aggregate concrete consisted of coral sand with a fineness modulus of 2.4 and a coarse aggregate of coral gravel with a particle size of 5 to 20 mm. The physical properties of coral coarse aggregate and coral sand are presented in Table 2. Both the fine and coarse aggregates met the requirements of the aggregate grading curve. The water-reducing admixture was polycarboxylic acid superplasticizer with a water reduction rate of 25% to 30% (mass fraction), and the mixing water was tap water. The mixture ratio of the C30 concrete is presented in Table 3. The Ø10 steel bar was polished, and the grease was removed with acetone. A 500 mm wire was connected at one end, and the end of the steel bar was sealed with a heat-shrinkable tube with a 10 diameter. The effective length of the working electrode was 20 mm and the exposed area was 6.28 cm2. A stainless steel bar of Ø12 was embedded in the lower part of the steel bar as a counter electrode. Two plastic end plates were fixed in a steel mold of 200  200  200 mm to ensure that the thickness of the concrete protective layer was 20 mm. The concrete was poured, formed, and removed after 24 h, and then cured under standard conditions for 56 d. There were two upper and lower parallel faces, as the chloride ion erosion surface and other concrete surfaces were sealed with epoxy resin. Two types of concrete were used, namely coral aggregate and common aggregate concrete, as shown in Fig. 1(a). The compressive strengths of the test pieces were 35.1 and 37.3 MPa in 56 d, respectively. Moreover, 100  100  100 mm cube specimens were prepared for the concrete porosity and resistivity testing. The composition and mechanical properties of the reinforcement are displayed in Table 4. 2.2. Accelerated chloride migration test Several methods are available for studying the corrosion behavior of steel bars in concrete, as follows: (1) Simulated concrete pore fluid method [20,21]: The concrete alkaline environment is simply simulated by mixing several chemical reagents to prepare an alkaline solution with a certain pH value, which is quite different from the actual concrete conditions. The test result can only qualitatively reflect the electrochemical characterization of the steel bars in different alkaline environments, and it cannot be used to determine the chloride threshold. (2) Natural soaking method or dry and wet cycle method [22,23]: This method is consistent with the actual conditions, but the experimental process is time consuming and the data are more discrete. (3) Acceleration method of adding chlorine salt [24,25]: Although the corrosion process of the steel bar can be obviously accelerated, it does not reflect the transmission process of the external corrosion medium in the concrete, and the initial passivation process of the steel bar in the highalkali medium is neglected [26]. (4) Anode energization acceleration method: By adjusting the anode current value, the steel bar

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D. Niu et al. / Construction and Building Materials 238 (2020) 117685 Table 1 Chemical composition of cement and fly ash (kg/m3). Material

CaO

SiO2

Al2O3

Fe2O3

MgO

SO3

Na2O

K2O

TiO2

Other

Cement Fly ash

57.7 21.14

18.48 35.71

4.82 16.57

3.26 8.92

1.51 1.41

1.83 1.94

0.33 –

1.07 –

0.23 –

– –

Table 2 Physical properties of coral coarse aggregate and coral sand. Material characteristics

Coral coarse aggregates

Coral sand

Bulk density (kg/m3) Apparent density (kg/m3) Void content (%) Cylinder compressive strength/MPa Fineness Modulus (Mx) Chloride ion content

904 1667 45.8 2.27 – 0.025

1181 1538 23.2 – 2.8 0.05

corrosion rate can be controlled within a short time. However, the corrosion of steel bars caused by an electric current is very different from that induced by chloride in the natural environment [27,28]. The steel bar corrosion products are obviously different, the distribution of the corrosion products varies, and hydrogen or oxygen evolution reactions occur in the steel–concrete interface zone [29]. It can be concluded that all of the above methods exhibit certain defects, and therefore, a rapid determination method for chloride ions based on the electric field acceleration method is proposed by improving the shortcomings of the fourth method above. Considering the defects of the natural soaking method or dry and wet cycle method, the acceleration method of adding chlorine salt, and the anode energization acceleration method, the accelerated chloride migration method under an external electric field has been used extensively in the study of the chloride ion diffusion coefficient in concrete, and the penetration of the migration-type rust inhibitor. In this study, the electro-migration method was improved to accelerate the transport of chloride ions in the concrete and combined with the electrochemical methods to study the corrosion behavior of the steel bars in the coral aggregate concrete. The electro-migration chloride ion device included a DC stabilized power supply, test bar, stainless steel bar, and 3.5% NaCl solution, as illustrated in Fig. 1(b). A plexiglass container with 3.5% NaCl solution was mounted on the surface of the concrete test piece. A stainless steel plate was placed on the NaCl solution to connect the negative pole of the power supply. The stainless steel bar in the concrete was connected to the positive pole of the power supply. The test piece was immersed in distilled water. Direct voltages of 0.6, 1.0, 2.0, and 3.0 V were applied between the stainless steel plate and stainless steel bar by a DC stabilized power supply. To ensure the stability of the steel surface and steel–concrete interface area, the electrochemical tests were performed after the power was turned off for 24 h. The accelerated chloride migration test lasted for 77 d. 2.3. Steel corrosion monitoring method An electrochemical workstation was used to monitor the steel corrosion in the concrete. The electrochemical test was carried

out by the three-electrode method used in the PARSTAT 2273 electrochemical workstation. An Ag-AgCl electrode (saturated KCl) was used as the reference electrode, stainless steel bar was served as the auxiliary electrode, and the steel bar in the concrete specimen was used as the working electrode. Firstly, the test piece was placed in a 3.5% NaCl solution, and the open circuit potential of the system was tested. If the open circuit potential did not exceed ±2 mV within 5 min, the test system was determined to be in a stable state, and the LPR and EIS tests could be performed. Thereafter, according to the electrochemical test results, the corrosion state of the steel bar in the concrete was evaluated. When the steel bar corrosion current density was less than 0.1 mA/cm2, the electromigration chloride ion test was continued. When the steel bar corrosion current density was greater than 0.1 mA/cm2, the EIS test could be performed to evaluate the steel bar corrosion status further. The scanning potential of the linear polarization test corresponded to 0.1 ~ +0.1 V vs. OCP, and the scanning rate corresponded to 0.15 mVs1. In the EIS test, sinusoidal voltage excitation signals with disturbance amplitude corresponding to ±10 mV vs. OCP were used, and the frequency range approximately corresponded to 102–104 Hz. The potentials of all electrochemical tests were relative to the saturated Ag-AgCl electrode. The determination of the steel bar corrosion current density is illustrated in Fig. 2. 3. Corrosion condition of reinforcement 3.1. Corrosion potential value The time-varying curve of the steel bar corrosion potential during 77 d is presented in Fig. 3.The corresponding voltage values are indicated by the curves in Fig. 3. The curves 1 and 2 represent the voltage value of 0.6 V; the curves 3 and 4 represent the voltage value of 1.0 V; the curves 5 and 6 represent the voltage value of 2.0 V; and the curves 7 and 8 represent the voltage value of 3.0 V. The potential values of the two batches of steel bars under different voltage values were basically the same; however, they exhibited small fluctuations. When the coral aggregate concrete was electrified for 21 d, the potential curve dropped sharply and the corrosion potential value changed rapidly, indicating that the steel bar had been corroded. Under a voltage of 3 V, the rebar corrosion potential value in the coral aggregate concrete fluctuated between 280 mV and 300 mV at 17 d, while the voltage value of the common aggregate concrete was less than 276 mV at 21 d. According to the ASTM C876 standard [30], the corrosion potential value is less than 270 mV (vs. Ag/AgCl), which indicates that the steel bar had a corrosion probability of 90%. It can be concluded that the rebar in the coral aggregate concrete began to rust at 17 d, while the common aggregate concrete began to rust at 21 d. Similarly, it can be concluded that the rebar in the coral

Table 3 Concrete mixture ratio (kg/m3). Number

Cement

Fly ash

Water reducer

Water

Water-binder ratio

Fine aggregate

Coarse aggregate

O30 S30

370.8 446.25

– 78.75

1.48 2.63

163.0 183.8

0.44 0.35

658.4 900

1222.7 600

Note: O30 represents common aggregate concrete, S30 represents coral aggregate concrete.

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D. Niu et al. / Construction and Building Materials 238 (2020) 117685

(a) Concrete sample

(b) Electrochemical test

Fig. 1. Electro-migration chloride ion device.

Table 4 Composition and mechanical properties of reinforcement. Steel type

HRB400

Composition/%

Mechanical properties

C

Si

Mn

P

S

Fe

Yield strength /Mpa

Extension strength /Mpa

Elongation

Yield Strength Ratio

0.25

0.31

1.08

0.015

0.025

other

480

630

1.31

1.20

(a) Electromigration sample

(b) Electrochemical test

Reference electrode 3.5%NaCl Electrochemical workstation

Test bar Concrete specimen Stainless steel bar

WE

RE

CE

Distilled water

(c) Schematic diagram Fig. 2. Determination of reinforcement’s corrosion current density.

aggregate concrete began to rust at 63 d when the voltage was 0.6 V. When the voltage was 1.0 V, the steel bar started to rust at 49 d, and when the voltage was 2.0 V, the steel bar started to rust at 35 d. The effects of the different voltage values on the initial rebar corrosion in the concrete were analyzed. For the coral aggregate concrete, when the voltage values were 0.6 and 1.0 V,

the corrosion potential value of the steel bar was stable within 0 to 7 d, and the value was less than 126 mV (the steel bar had a 50% corrosion probability). The steel bars formed a complete passivation film and no corrosion occurred. However, when the coral aggregate concrete was electrified for 7 to 21 d, the steel bar corrosion potential values decreased sharply in the coral aggregate

D. Niu et al. / Construction and Building Materials 238 (2020) 117685

5

Fig. 3. Corrosion potential value.

concrete. This implies that the steel passivation film was being destroyed by the chlorine ions (0.08% free chlorine ion content, accounting for the cementing material weight) contained in the coral aggregate. The interior chloride ions of the coral aggregate migrated to the reinforcement surface to participate in the steel corrosion reaction under the action of the electric field, which accelerated the steel corrosion. When the voltage values were 2 and 3 V at 7 to 21 d, the steel bar corrosion potential values decreased sharply in the coral aggregate concrete, indicating that the steel passivation film was being destroyed by chlorine ions. The ‘‘stable period” refers to the reinforcement voltage value being stable at a certain value under the action of an electric field. For common aggregate concrete, a stable period of 28 d occurred under the action of 0.6 V, a stable period of 21 d occurred under the action of 1.0 V, and a stable period of 14 d occurred under the action of 2.0 or 3.0 V. However, the steel bar corrosion potential value in the coral aggregate concrete exhibited a shorter stable period, as the chloride ions in the coral aggregate concrete migrated to the reinforcement surface to participate in the steel corrosion reaction under the action of the electric field. As in the coral aggregate concrete, the voltages of the ordinary concrete continued to decrease until the passive film of the steel bar was destroyed, and this period was defined as the ‘‘drop period” of the corrosion potential value. The steel bar drop period in the coral aggregate concrete was longer than that in the common aggregate concrete, mainly because the coral aggregate concrete not only had its own chlorine ions, but also exhibited large porosity and small resistivity. A given voltage is applied to a concrete specimen of 400  300  100 to measure the current flowing through it. Concrete resistivity is the resistance of concrete per unit length and per unit area to the current. Concrete resistivity is an indirect measurement of concrete transmission performance. The porosity of the coral aggregate concrete sample was 18.95%, but the value of the common aggregate concrete sample was 10.2% according to a vacuum-filled test. Meanwhile, the resistivity of the coral aggregate concrete sample was 14.7 kXcm, but the value of the common aggregate concrete sample was 19.1 kXcm based on the four-electrode method [31]. Under the action of the electric field, chloride ions continued to gather on the steel bar surface and participate in the corrosion reaction, until the steel bar passive film was destroyed. Thereafter, the corrosion potential values existed in the ‘‘plummeting period”. Compared to the coral aggregate concrete, the voltage value of the ordinary concrete decreased rapidly within a short time, but the

corrosion potential values of the coral aggregate concrete exhibited little change in the ‘‘plummeting period” and ‘‘drop period”. In summary, the steel bar in the coral aggregate concrete could form a complete passive film, although chlorine ions existed in the coral aggregate. The corrosion potential steady period in the coral aggregate concrete was relatively short, but the durations of the drop period and plummeting period were not far apart. There was a long stability period in the ordinary concrete, but the plummeting period was long and varied significantly. 3.2. Corrosion current density The testing of the corrosion potential is affected by numerous factors, such as the concrete resistivity, oxygen content, and relative humidity in the concrete. Therefore, the corrosion current density (icorr) was adopted to characterize the corrosion degree of the steel bar quantitatively. The corrosion density can be obtained according to Faraday’s law by using the LPR to measure the polarization resistance of the steel bar (Rp), as indicated in Eq. (1).

icorr ¼ B=Rp

ð1Þ

In the above formula, B is the Stern–Geary constant. Considering its influence, the value of B was set to 26 mV according to corrosion hanging test [32–34]. The time-varying curves of the steel bar polarization resistance and corrosion current density value during 77 d are presented in Figs. 4 and 5. At the beginning of the experiment, the polarization resistance of the coral aggregate concrete was smaller than that of the ordinary concrete, and the ratio was 2.0 to 2.2. Moreover, the reinforcement corrosion current density of the coral aggregate concrete was higher than that of the ordinary concrete. Different voltage values had little effect on the steel bar polarization resistance, and the change trend of the curve was relatively consistent. According to reference [35], when the corrosion current density of the steel bar is greater than 0.1 mA/cm2, the passive film of the steel bar will be destroyed. When the voltage of the coral aggregate concrete was 3.0 V, the passive film of the steel bar was destroyed after 17 d. In the same manner, the passive film of the steel bar was destroyed after 35 d under the accelerating electric field of 2.0 V, after 49 d under 1.0 V, and after 63 d under 0.6 V. In the ordinary concrete, the passive film of the steel bar was destroyed after 21 d under the accelerating electric field of 3.0 V, after 42 d under 2.0 V, after 63 d under 1.0 V, and after 77 d under 0.6 V. The results are

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D. Niu et al. / Construction and Building Materials 238 (2020) 117685

Fig. 4. Polarization resistance value.

Fig. 5. Corrosion current density value.

consistent with the corrosion potential of the steel bars in the two types of concrete. The corrosion current density in the two types of concrete at the beginning of the corrosion stage exhibited a steady current phase (‘‘stable period”) and a rising stage (‘‘rising period”). After the rebar passivation film ruptured, the corrosion current density of the steel bar in the ordinary concrete increased continuously, but the value tended towards 0.15 mA/cm2 under the action of different voltages. The change trend of the rebar corrosion current density in the coral aggregate concrete was the same as that in the ordinary concrete. The corrosion current density in the coral aggregate concrete was 0.25 mA/cm2 under different voltage values, and the ratio of the corrosion rate in the two types of concrete was 1.67. The resistivity of the coral aggregate concrete was less than that of the ordinary concrete. The porosity of the coral aggregate concrete sample was 18.95%, and the porosity ratio in the two types of concrete was 1.85. Meanwhile, the resistivity of the coral aggregate concrete sample was 14.7 kXcm, and the resistivity ratio in the two types of concrete was 1.30 according to the four-electrode method. In summary, the initial corrosion time of reinforcement in coral aggregate concrete is less than that of ordinary concrete. The

corrosion current steady period in the coral aggregate concrete was relatively short, and the corrosion rate increases rapidly after the passivation film is destroyed. The corrosion rate in the coral aggregate concrete was 0.25 mA/cm2, and the ratio of that in the two kinds of concrete is 1.67. 3.3. Reinforcement performance when corroded The weak polarization curves when the reinforcement began to corrode are presented in Fig. 6. EIS fitting results are shown in Table 5. The open circuit potential value of the polarization curve and corrosion potential value were similar when the steel bar began to rust. The times at which the reinforcement started to corrode under different voltages are illustrated in Fig. 6. The time to the start of rusting of the coral aggregate concrete was obviously shorter than that of the ordinary concrete. At the beginning of the corrosion, the current densities of the steel bars under different voltages were close, which indicates that the critical condition of the reinforcement corrosion is independent, and only related to the steel bar types and the interface between the steel bar and concrete.

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D. Niu et al. / Construction and Building Materials 238 (2020) 117685

Fig. 6. Weak polarization curve when the reinforcement begins to corrode.

Table 5 EIS fitting results. Number

Rs Xcm2

Rct Xcm2

Rf Xcm2

CPE1 7

Y0/(10 O30-0.6V O30-1.0V O30-2.0V O30-3.0V S30-0.6V S30-1.0V S30-2.0V S30-3.0V

0.049 0.052 0.067 0.001 0.017 0.0025 0.388 0.042

2739 6109 8033 4201 2798 3636 8344 8726

)X

1

2

cm s

808 70.1 40.7 621 54.92 494 32.8 32.7

In order to accurately present the corrosion process of steel bars, the EIS parameters of each component in the fitted circuit can be obtained via fitting. The EIS of rebar in concrete is analysed and fitted via the equivalent circuit shown in Fig. 7. Specifically, Rs denotes the resistance of simulated seawater erosion solution, CPE1 denotes constant phase angle element of the double layer capacitance at the rebar/concrete interface, and Rct denotes charge transfer resistance in the electrochemical corrosion reaction process. Additionally, CPE2 denotes the constant phase angle element of the passivation film layer capacitance, and Rf denotes the resistance of steel passivation film layer. The polarization resistance of reinforcement includes charge transfer resistance Rct and passivation film resistance Rf. The n is diffusion coefficient (0 < n < 1), and Y is normal phase angle element admittance. The Nyquist plots for when the reinforcement began to corrode are presented in Fig. 8. The low-frequency capacitive reactance arc became an oblate arc, which demonstrates that the interface microstructure between the reinforcement and concrete tended to be complicated as the corrosion progressed. The double layer capacitance gradually deviated from the ideal state owing to the uneven surface of the reinforcement and concrete polytropism. However, compared

Fig. 7. EIS equivalent circuit diagram.

n

n 0.473 0.572 0.579 0.482 0.412 0.464 0.432 0.597

223,618 186,692 158,815 171,368 196,940 162,483 170,012 164,753

CPE2 Y0/(104) X1cm2sn

n

14.56 7.05 10.77 8.93 18.67 18.55 7.46 9.63

0.683 0.628 0.637 0.744 0.713 0.735 0.556 0.532

with initial curve, the curvature of the capacitive and reactance arc in the low-frequency region has changed and evolved into a large arc, and the corrosion tendency has increased. Therefore, the double layer capacitance was replaced by a special constant phase angle element to analyze the steel reinforcement corrosion. The polarization resistance (electron transfer resistance of the interfacial pore liquid double layer between the steel and concrete) was less than 230 X/cm2. According to the references (Rp  250 kX/cm2) [35], the steel bar had rusted.

3.4. Life prediction of coral aggregate concrete The similarity theory of life test is a basic theoretical problem for predicting concrete life in the laboratory. According to the second similarity theorem, if two phenomena are similar, the relations between several parameters describing the phenomenon can be transformed into the functional relations between similarity criteria, and the similarity criterion function relations of the similar phenomena are the same. The similarity criterion for degraded performance between the experimental simulation condition and real environment was simulated by studying the similarity theory and determining the influencing factors through various basic tests. The basic guiding ideology of the accelerated test is predicting the average life under normal influencing factors by the average life span under high-impact factors, thereby obtaining an arbitrary accelerated simulation test method by changing the numerical value of the influence parameters, and predicting and evaluating the material property attenuate law in the real environment by means of the accelerated test results. In the natural environment, three mechanisms exist by which chloride ions enter the concrete, including penetration, diffusion,

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D. Niu et al. / Construction and Building Materials 238 (2020) 117685

Fig. 8. Nyquist plots when the reinforcement begins to corrode.

and capillary adsorption [36]. Chloride penetration involves chloride ions entering into the concrete under pressure. Chloride diffusion refers to chloride ions being transferred from a high to low concentration because of the concentration difference. Capillary adsorption involves chloride ions being transferred into the concrete under capillary action, which is related to the concrete pore structure and humidity. The concrete structure is usually invaded by chloride ions by means of a combination of the above several methods, and one of these is dominant. Chloride diffusion is the main means of chloride ion transport in most cases. For concrete buildings in seawater, the concrete surface is considered to be in a wet condition at all times, as the concrete is in contact with seawater. In the interface between the seawater and concrete, the concentration of chloride ions in the seawater is higher than that in the concrete. Therefore, the concentration difference causes chloride ion diffusion into the concrete; that is, this concentration difference is the chloride ion diffusion driving force in underwater concrete. For chloride ions, the negative logarithm of the molar concentration (pCl) is linearly related to the concentration potential, as illustrated in Fig. 9 [37]. The chloride ion concentration in seawater is presented in Table 6. After the above conversion, the concentration difference voltage of the chloride ion diffusion in underwater concrete is 4.99 mV. As mentioned, the concentration difference is the driving force of the chloride ion diffusion in underwater concrete. Therefore, according to the similarity principle of life prediction, impressed voltages were applied as the influencing factor. Xu et al. [38] con-

cluded that high voltages break down the steel bar passive film. Considering the time required for the test, the acceleration test applied voltage values of 3, 2, 1, and 0.6 V. Under different voltages, the passive film of the steel bar broke down when the polarization current value of the steel bar reached 0.10 mA/cm2. The times at which the reinforcement began to corrode under different voltages are presented in Table 7. The relationship between the impressed voltages and conduction time in the concrete is illustrated in Fig. 10. As can be observed, with a decrease in the impressed voltage, the time at which the reinforcement began to corrode gradually increased. The life prediction curve was obtained by fitting each point in the diagram, where the fitting equation was the life prediction equation of the C30 coral aggregate concrete under the action of the electric field. The concentration difference voltage of 4.99 mV was plugged into the equation above, and it could be determined that the lives of the C30 ordinary concrete and coral aggregate concrete were 10.3 and 6.95 y, respectively. The life prediction curves of the steel bars at the beginning of corrosion in the C30 ordinary concrete and coral aggregate concrete with a 20 mm concrete cover are presented in Eqs. (2) and (3).

y ¼ 59:01x0:783

ð2Þ

y ¼ 46:85x0:753

ð3Þ

In the above formulae, x is the impressed voltage and y is the time at which the reinforcement begins to corrode. 3.5. Corrosion morphology of steel reinforcement and concrete

Fig. 9. Relationship of pCl and concentration potential.

The concrete specimen was placed on a press machine to split the concrete along the steel bar when the steel bar began to rust. The rebar surface was observed using a high-definition camera to obtain the corrosion morphology, as illustrated in Fig. 11(a). The interface between the corrosion and non-corrosion of the steel bar surface was observed by an SEM, as illustrated in Fig. 11(b). The passive film at the steel bar surface was destroyed and corrosion occurred on the steel bar, as indicated in Fig. 11(a). The SEM morphology of the steel bar surface at the interface between the corrosion and non-corrosion is presented in Fig. 11(b). The triangle region was black rust, the elliptical area was red rust, and the rectangular area was the passive film on the steel bar surface. The steel rust was formed in the rupture of the reinforcement passivation

9

D. Niu et al. / Construction and Building Materials 238 (2020) 117685 Table 6 Composition of sea water (g/L). Cl

Na

SO4

Mg

K

Ca

HCO3

Br

BO3

19.13

10.75

1.89

1.37

0.38

0.32

0.08

0.06

0.04

Table 7 Concrete life at different voltages. Impressed voltage/V

3

2

1

0.6

Conduction time in common concrete/d Conduction time in coral concrete/d

21 17

42 35

63 49

77 63

80

60 50 40 30 y = 46.854x-0.753 R² = 0.8993

20 10 0 0

0.5

1

100 90 80 70 60 50 40 30 20 10 0

Conduction time/d

Conduction time/d

70

1.5

2

2.5

3

3.5

4

y = 59.009x-0.783 R² = 0.9179

0

0.5

1

Impressed voltage /V

1.5

2

2.5

3

3.5

4

Impressed voltage /V

(a) Common aggregate concrete

(b) Coral aggregate concrete

Fig. 10. Life prediction curves of steel bars in concrete.

(a) Corrosion morphology of reinforcement

(b) SEM morphology in concrete

Fig. 11. Corrosion morphology.

film, but the undamaged part of reinforcement passivation film was strongly combined with a cement hydration product (ettringite acicular crystal). Most of the rust on the surface of the steel bars was black rust (Fe3O4), while only a small part was red rust (Fe2O3). In conclusion, the steel bar had been in the initial corrosion stage.

3.6. Critical threshold of chloride ion in concrete The concrete specimen was placed on the press machine to split the concrete along the steel bar when the steel bar began to rust. The two ends of the split specimen were cut off to eliminate the boundary effect, and only concrete sections with a 150 mm length were retained. The coarse aggregate was removed, and then placed in the pulverizer to mill it to powder at the interface between the steel and concrete. The free chloride ion content in the powder sample was measured by chloride ion selective electrodes. The test process for the chloride ion concentration is illustrated in Fig. 12.

The chloride content (mass fraction) at the protective layer depth of 20 mm was used as the critical chlorine ion concentration (CCl) for the beginning of corrosion in the steel bar. The critical chloride ion concentration in the common aggregate concrete was 0.62% (accounting for the cementing material weight), while that in the coral aggregate concrete was 0.49% (accounting for the cementing material weight). Meanwhile, chloride threshold values in terms of Cl/OH is 2.85. The critical chloride concentration of coral aggregate concrete is smaller than that of ordinary concrete.

4. Conclusion This study has proposed a rapid determination method for chloride ions accelerated by the electric field, by improving the anode energization acceleration method. A small voltage was applied to simulate the chloride ion diffusion propagation in the marine environment.

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D. Niu et al. / Construction and Building Materials 238 (2020) 117685

(a) Sice up

(d) Shock solution

(b) Ground powder

(e) Filter solution

(c) Seal preservation

(f) Test concentration

Fig. 12. Chloride ion concentration test.

(1) Corrosion of steel bar in concrete was determined by electrochemical test. The critical chloride ion concentration in the coral aggregate concrete was 0.49% (accounting for the cementing material weight). The critical chloride concentration of coral aggregate concrete is smaller than that of ordinary concrete. (2) Corrosion behavior of reinforcement in coral aggregate concrete is studied. The rebar was rapidly corroded in the initial stage of steel corrosion on the coral aggregate concrete, owing to existing chloride ions in the coral aggregate, as well as greater porosity and lower resistivity than those of common concrete. (3) The rebar corrosion rate in the coral aggregate concrete was 0.25 mA/cm2 at the beginning of steel corrosion, and the corrosion rate ratio in the two types of concrete was 1.67. (4) The life prediction equation of early corrosion of reinforcement in coral aggregate concrete is obtained. The time at which the reinforcement begins to rust in coral aggregate concrete is estimated in the marine environment. CRediT authorship contribution statement Ditao Niu: Data curation. Lu Zhang: Writing - original draft. Qiang Fu: Writing - review & editing. Bo Wen: Methodology. Daming Luo: Validation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors wish to acknowledge their gratitude towards the Major Program of the National Natural Science Foundation

(No.51590914) (in China), the National Natural Science Foundation Project (No.51578450, No.51608432, No.51808438) (in China), Program for Innovative Research Team in University of Ministry of Education of China (Grant No. IRT_17R84) (in China), International cooperation projects of Shaanxi province (No.2019KW047) (in China), and State grid shaanxi province electric power company science and technology project (SGSNJY00SJJS1900025). Declaration of Competing Interest The authors declare that they have no conflict of interest. References [1] B. Da, H.F. Yu, H.Y. Ma, Chloride diffusion study of coral concrete in a marine environment, Constr. Build. Mater. 123 (2016) 47–58. [2] C.Y. Chen, T. Ji, Y.Z. Zhuang, Workability, mechanical properties and affinity of artificial reef concrete, Constr. Build. Mater. 98 (2015) 227–236. [3] P.B. Dale, Influence of internal curing using lightweight aggregates on interfacial transition zone percolation and chloride ingress in mortars, Cem. Concr. Compos. 31 (5) (2009) 285–289. [4] J. Liu, Z. Ou, W. Peng, Literature review of coral concrete, Arab. J. Sci. Eng. 43 (4) (2017) 1529–1541. [5] A.E. Rick, Coral concrete at bikini ato II, Concr. Int. 13 (1) (1991) 19–24. [6] Q.K. Wang, T. Peng, Mechanical properties and microstructure of portland cement concrete prepared with coral reef sand, J. Wuhan Univ. Technol. 31 (5) (2016) 996–1001. [7] W.C. Yodsudjai, N. Otsuki, T. Nishida, Study on strength and durability of concrete using low quality coarse aggregate from circum pacific region 8(4), in: Fourth Regional Symposium on Infrastructure Development in Civil Engineering (RSID4), 2003, pp. 8–10. [8] P. Wattanachai, A study on chloride ion diffusivity of porous aggregate concretes and improvement method, Adv. Mater. Res. 65 (1) (2013) 30–37. [9] W. Pitiwat, O. Nobuaki, S. Tsuyoshi, N. Takahiro, A Study on chloride ion diffusivity of porous aggregate concretes and improvement method, Doboku Gakkai RonbunshuuE 65 (1) (2009) 30–44. [10] S. Kakooei, H.M. Akil, A. Dolati, The corrosion investigation of rebar embedded in the fibers reinforced concrete, Constr. Build. Mater. 35 (10) (2012) 564–570. [11] S. Kakooei, H.M. Akil, M. Jamshidi, The effects of polypropylene fibers on the properties of reinforced concrete structures, Constr. Build. Mater. 27 (1) (2012) 73–77. [12] F. Wang, X. Zha, Experimental and theoretical study on coral concrete filled steel tube, J. Build. Struct. 34 (s1) (2013) 288–293.

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