Effects of lightweight sand and steel fiber contents on the corrosion performance of steel rebar embedded in UHPC

Construction and Building Materials 238 (2020) 117709

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

Effects of lightweight sand and steel fiber contents on the corrosion performance of steel rebar embedded in UHPC Liang Fan a, Weina Meng b,⇑, Le Teng a, Kamal H. Khayat a a b

Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO 65401, United States Department of Civil, Environmental and Ocean Engineering, Stevens Institute of Technology, Hoboken, NJ 07030, United States

h i g h l i g h t s
Steel bar reinforced UHPC with different lightweight sand (LWS) and steel fiber content were prepared for electrochemical tests.
The feasibility of ASTM C876 standard in the determination of corrosion state of steel bars was discussed.
Tafel test was conducted to measure the real bc and ba values for UHPC specimens to calculate the corrosion current density.
The effects of LWS and steel fiber contents on the corrosion performance of steel bars and electrical resistance of UHPC matrix were discussed.

a r t i c l e

i n f o

Article history: Received 27 September 2019 Received in revised form 18 November 2019 Accepted 24 November 2019

Keywords: UHPC Lightweight sand Steel fibers Electrochemical impedance spectroscopy Corrosion Steel rebar

a b s t r a c t Ultra-high-performance concrete (UHPC) is able to provide better protection for embedded steel rebar from corrosion due to its superior impermeability characteristics. Lightweight sand (LWS) and steel fibers are utilized in mixing UHPC to promote internal curing and improve flexural strength, respectively. But the effects of different contents of LWS and steel fibers on the corrosion performance of steel rebar and resistance of UHPC matrix are rarely investigated. In this study, steel bar reinforced UHPC specimens with different LWS and steel fiber contents were prepared for electrochemical tests, involving open circuit potential (OCP), Tafel polarization, linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS). The experiment results indicated OCPs values based on ASTM C876 could not be applied to decide the corrosion state of steel rebar since lower content of oxygen in UHPC reduced the cathodic reaction, which led to lower OCP values. Chloride ions did not penetrate into UHPC matrix and steel rebar were in passive state in the whole test period of 147 days (d). Corrosion resistance of steel rebar, and resistance of UHPC matrix and passive film displayed an increasing trend with time. Generally, the resistance decreased when the LWS and steel fiber contents were increased. This is because LWS increased the permeability of UHPC and steel fibers led to shortcut of current. For better electrical resistance and compressive strength of UHPC, it is recommended to limit the LWS replacement ratio to 25%. It is also concluded that, steel fiber with the volume content up to 3% will not lead to corrosion of steel rebar and can be safely used in UHPC. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction UHPC is a cementitious composite which has a higher compressive strength (>150 MPa), a very low water to binder ratio, and superb durability in freeze/thaw, salt-scaling, and abrasion environment [1–3]. The flexural strength is improved through incorporating fibers in the mixture [2]. Some fine supplementary materials, like fly ash, silica fume, limestone and quartz flour can be used to improve the performance of UHPC [4]. Such materials ⇑ Corresponding author. E-mail address: [email protected] (W. Meng). https://doi.org/10.1016/j.conbuildmat.2019.117709 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

can contribute to the refinement of the microstructure that can reduce the permeability of harmful chemicals including chlorides [5]. Since the superhigh strength and durability of UHPC are able to extend the service life and reducing maintenance of bridges, bridge elements made of UHPC such as precast girders [6], deck panels [7], segmental bridge columns [8], and in-fill deck joints [9] are increasingly used in accelerated bridge construction. UHPC has also been employed in the rehabilitation of structures. For example, steel bridge girder with reduced cross-section due to corrosion was encased with UHPC to restore the lost bearing capacity [10]. UHPC shell is applied in retrofit of the damaged body of the marine piles due to corrosion induced concrete cracking and loss

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L. Fan et al. / Construction and Building Materials 238 (2020) 117709

of reinforcement [11]. Besides, at the plastic-hinge zone of pre-cast columns, UHPC has been used as a grout material to enhance the seismic performance of bridges [12,13]. The superior durability of UHPC indicate that steel rebar in UHPC can be better protected from corrosion than those in conventional or high performance concrete (HPC) [5]. Linear polarization experiment was conducted to assess the corrosion rates of steel rebar in UHPC and HPC after they were acceleratedcorroded [5]. The experiment results illustrated that steel rebar in UHPC had lower corrosion current compared with those in HPC. But the corrosion resistance of rebar corroded through accelerated corrosion test cannot represent the actual corrosion resistance of steel rebar in natural corrosion condition. UHPC and conventional concrete were used as repair materials for the damaged reinforced concrete [14,15]. Macro-cell current generated between the part of the steel rebar embedded in new repair material and the other part of the same bar in the existing concrete with vestigial chloride ions. The measured macro-cell current was much smaller for the specimen repaired with UHPC [14,15]. Because of the low permeability and oxygen availability of UHPC, cathodic reaction on the surface of rebar in the existing concrete could not be sustained and thus the electrochemical corrosion was mitigated. However, in the above studies, the advantageous protection of UHPC to steel rebar is qualitatively compared with conventional concrete or HPC, the fundamental parameters of corrosion behaviors of steel rebar embedded in UHPC has not been quantificationally studied. Saturated lightweight sand (LWS) was successfully used in HPC and UHPC for effective internal curing [16,17]. The small presaturated LWS porous particles can be homogenously dispersed in the UHPC matrix and effectively store water during mixing and setting, and then progressively release water during cement hydration, which helps reduce shrinkage [18] and potential cracking [19]. With the optimum amount, it can also increase mechanical properties [20]. From the authors’ previous studies [16–17], autogenous shrinkage was decreased from 490 to 195 mm/m, when the LWS content in the mixture rose from 0 to 75% (LWS/(river sand + LWS), vol%). Compressive strength rose from 130 to 158 MPa when the LWS content was increased from 0 to 25%, due to the promoted cement hydration and reduced porosity (i.e., the hydration product can fill the pores of the LWS, when LWS was limited to 25%). However, when the LWS content was increased from 25% to 75%, compressive strength decreased and total porosity significantly increased. The increment of interconnected pores can increase the permeability of oxygen and water to UHPC so the rebar has a higher chance to be corroded. It is necessary to study how the LWS content affects the UHPC resistance and the corrosion resistance of rebar in UHPC. Steel fibers are widely used for mixing UHPC to increase its flexural/tensile strength as well as crack tolerance [21–23]. Recent studies pointed out that they can also change the microstructure of the UHPC [24] and thus influence the corrosion resistance of the embedded rebar [23,25]. Test results indicated that the total permeable voids showed a deceasing tendency with the increment of steel fiber content in UHPC (i.e., 0–6%) [24]. This can be due to the large quantity of steel fibers that tend to interrupt the continuity of pore network and lead to greater tortuosity, which reduces the number of permeable voids. The low permeability reduces the intrusion of corrosive ions like chloride ions into UHPC and increases corrosion protection for embedded steel rebar. However, more air is introduced into the matrix during mixing when more steel fibers are used, which could increase the porosity of the UHPC [20]. Besides, oxygen is necessary for the steel passive film formation, but the vast surface area contributed by the addition of steel fibers is likely to take the oxygen away from the surface of steel rebar as well as weaken the passive film formation [26,27]. There-

fore, the impact of steel fiber content on the corrosion resistance of embedded rebar remain unclear. In this study, UHPC cylindrical specimens were cast with steel rebar in the middle. Two types of UHPC mixtures were used: UHPC with different contents of saturated LWS and UHPC with different contents of steel fibers. Electrochemical tests including OCP, LPR, Tafel polarization, and EIS were conducted in the NaCl solution to examine the corrosion performance of steel rebar for 147 d. Powder was collected from UHPC cylindrical specimens to measure the chloride content after the immersion tests. Rapid chloride permeability test (RCPT) was performed to quantify the amount of electric charge that passed through 100-mm diameter cast cylinders. Through these tests, the effects of LWS content and steel fiber content on the corrosion performance of steel rebar and electrical resistance of UHPC were discussed. This study improves the understanding of corrosion performance of rebar in the UHPC and provides the guidance for determination of LWS content and steel fiber content in UHPC mixture design when considering the mechanical strength of UHPC and corrosion resistance of rebar.

2. Experimental setup 2.1. Cylindrical specimen preparation UHPC mixtures developed by the authors [16,21] were used in this study, and mixture designs of the UHPC with different LWS and steel fiber contents are listed in Table 1 and 2, respectively. For each mixture design, the superplasticizer content, air detrainer content, and the w/b were fixed at 1%, 0.8% and 0.20%, respectively. The volume replacement ratio of lightweight sand to concrete sand were 12.5%, 25%, 37.5%, and 50% for UHPC designated as LWS-12.5, LWS-25, LWS-37.5, and LWS-50, respectively. Based on composite volume, the volume fractions of steel fibers were 0, 0.5%, 1.0%, 1.5%, 2.0%, and 3% for UHPC codified as ST-0 (Ref.: without LWS and steel fibers), ST-0.5, ST-1, ST-1.5, ST-2.0, and ST-3, respectively. Straight steel fibers with copper coating were used. The fibers had the diameter of 0.2 mm and the length of 13 mm; the tensile strength and Young’s modulus were 203 GPa and 1.9 GPa, respectively. The compressive strength, flexural strength and autogenous shrinkage of different kinds of UHPC at 28-d are summarized in the Table 3. Cylindrical specimens as shown in Fig. 1 (a), had the height of 101.6 mm and diameter of 38.1 mm, and were cast for electrochemical tests. The steel rebar had the length of 76.2 mm and the diameter of 12.7 mm and was placed in the middle of the cylindrical specimen. Table 4 lists chemical composition of steel rebar. Before UHPC casting, steel wire brush was utilized to eliminate the mill scale on the steel rebar surface. A copper wire was soldered on one end of the rebar, which was utilized to connect the steel bar to the instrument during electrochemical measurements. To make sure that the middle of the rebar was the electrochemical response site, both ends of the steel rebar were encased by PVC tube with epoxy to fill the gap. And steel rebar has a length of 50.8 mm exposed to the UHPC matrix and a UHPC cover of 12.7 mm. Fig. 1 (b) and 1 (c) show that a PVC pipe (inside diameter 38.1 mm), was fitted into the pre-cut groove on the plyboard and used as the mold for casting UHPC specimen. Standard cylindrical specimens with the diameter of 100 mm and the height of 200 mm were also cast for rapid chloride permeability testing (RCPT). Before mixing UHPC, the water absorption value of LWS after soaking in water for 24 h and the relative desorption of the LWS using centrifuge method were determined to be 17.6% and 96.4%, respectively, in accordance with ASTM C1761 [28]. The 72 h water absorption was also measured as 18.4%. 96% of water in the LWS was lost at a 92% relative humidity, implying that water can be effectively transported from the LWS to cement paste at a high rel-

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L. Fan et al. / Construction and Building Materials 238 (2020) 117709 Table 1 LWS UHPC mixture design table (no steel fiber, unit: kg/m3). Denotation

Cement

Fly ash-C

Silica fume

Masonry sand

River sand

LWS

Air detrainer

Superplasticizer

Water

LWS-12.5 LWS-25 LWS-37.5 LWS-50

661 661 661 661

413 413 413 413

42.1 42.1 42.1 42.1

305 305 305 305

639 547 456 365

58 116 174 231

8.9 8.9 8.9 8.9

33.5 33.5 33.5 33.5

178.9 181.6 184.4 187.2

Table 2 Steel fiber UHPC mixture design table (unit: kg/m3). Denotation

Cement

Fly ash-C

Silica fume

Masonry sand

River sand

Air detrainer

Superplasticizer

Water

Steel fiber

ST-0 ST-0.5 ST-1.0 ST-1.5 ST-2.0 ST-3.0

661 658 654 651 648 641

413 411 409 407 405 401

42.1 41.9 41.7 41.5 41.3 40.8

305 304 302 301 299 296

708 726 723 719 715 708

8.9 8.9 8.8 8.8 8.8 8.7

33.5 33.3 33.2 33.0 32.8 32.5

176 175 174 173 172 170

0 39 78 117 156 234

Table 3 Compressive strength, flexural strength and autogenous shrinkage of different types of UHPC at 28-d. Code

Compressive strength (MPa)

Flexural strength (MPa)

Autogenous shrinkage (mm/m)

LWS-12.5 LWS-25 LWS-37.5 LWS-50 ST-0 ST-1.0 ST-2.0 ST-3.0

140 158 142 140 140 143 153 158

23 24 16 15 9.6 12.5 21.3 22.4

430 375 325 280 500 410 350 320

mixed liquid and blend them at 120 rps for 6 min. For UHPC mixtures with steel fibers, these four steps were the same, but the time required for step three and step four were 5 min and 1 min, respectively. The additional step was to gradually add steel fibers to the mixture for 1 min with the mixer operating at 60 rpm, which was then increased to 120 rpm to homogenize the mixture for 2 min. UHPC was cast into the PVC mold in one lift without consolidation. Wet burlap and plastic sheet were used to cover the cylinders to reduce the potential shrinkage. The samples were demolded after 24 h and then cured at room temperature (21 °C ± 1 °C) in saturated lime-water for 28 d. 2.2. Electrochemical measurements

ative humidity for internal curing. The moisture content of the bulk LWS was measured in accordance with ASTM C128 [29]. The rest amount of water to be added in the LWS was calculated by subtracting the water content in the LWS from the total water demand of the LWS to secure a saturated-surface-dry (SSD) condition. After adding the rest amount of water to the LWS, the LWS was homogenized with water and then placed in a sealed plastic bag for 24 h before batching to secure the SSD condition. The UHPC mixtures with LWS were prepared in four steps: (1) mix all the sand at the speed of 60 rpm for 1 min (min); (2) add cementitious materials and homogenize them with sand at the same speed for another 2 min; (3) blend water, air detrainer, and superplasticizer together, put 80% of mixed solution in the mixer and blend them at 60 rpm for 3 min; (4) put in the remaining

All the cylinders were immersed in the 3.5 wt% sodium chloride electrolyte for 147 d and tested at room temperature. At ages of 1, 21, 42, 63, 84, 105, 126, and 147 d, OCP, Tafel polarization, LPR, and EIS tests were conducted for monitoring the corrosion performance of steel rebar and electrical resistance of UHPC. Gamry, 1000E Potentiostat/Galvanostat was utilized for electrochemical tests with saturated calomel electrode (SCE) as the reference electrode, and a graphite rod as the counter electrode [23,30]. When OCP values stabilized, LPR test was carried out through scanning the potential ± 15 mV around OCP at a rate of 0.125 mV/s. The slope of the potential vs. current curve around zero current was determined as the polarization resistance [31,32]:

Rp ¼ DE=DI

Fig. 1. UHPC cylinders: dimensions (unit: mm) (a), plyboard with pre-cut groove (b), and steel rebar in the center of PVC mold (c).

ð1Þ

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L. Fan et al. / Construction and Building Materials 238 (2020) 117709

Table 4 Composition of steel rebar (mass percentage). Fe

Mn

C

Cu

Ni

Si

P

Cr

Mo

S

Sn

V

Co

97.4

1.00

0.38

0.37

0.20

0.18

0.12

0.10

0.07

0.06

0.03

0.02

0.01

where DI and DE represent the current and potential increments around i = 0, respectively. Corrosion current density was determined as [32]:

icorr ¼ B=ðARp Þ

ð2Þ

where icorr is the corrosion current density, B denotes Stern-Geary constant, and A represents the exposed surface area of rebar in UHPC. Subsequently, the EIS test was conducted at a small sinusoidal potential (10 mV in amplitude) around OCP. The experiment was run at five points per decade with the frequency from 105 Hz to 5  10-3 Hz. After EIS test was finished at 147 d, Tafel polarization test scanned the potential ± 75 mV around OCP with a small scanning rate of 0.1 mV to decide Stern-Geary constant B. Small overpotential 75 mV was selected to extract the anodic and cathodic Tafel coefficients without introducing damage due to the large overpotential values [33]. B was obtained from Eq. (3), as follows:

B ¼ ba bc =½2:303ðba þ bc Þ

ð3Þ

where ba and bc represent anodic Tafel coefficient and cathodic Tafel coefficients, respectively.

2.4. Rcpt The RCPT was conducted based on ASTM C1202 [35]. This test is used to monitor electrical current that passed through the specimens at certain time under a constant potential. The specimens prepared for this test were small cylindrical specimens with the diameter of 100 mm and thickness of 50 mm, which were sectioned from the standard cylindrical specimens with the diameter of 100 mm and a height of 200 mm. RCPT was performed according to the following procedures: (1) place specimens in the container and vacuum it for 3 h; (2) introduce water into the container to immerse all the specimens and keep the vacuum state for 1 h; (3) enable air to enter the container and keep the specimens under water for 20 h; (4) sandwich the specimen between the anodic cell and cathodic cell, apply a 6-h stable potential of 60 V between the cells, and record the cumulative charge (in Coulomb) going through the cells. The anodic cell was filled with 0.3 N (12 g/l) sodium hydroxide solution and the cathodic cell was filled with 3.0% (by mass) sodium chloride solution. 2.5. Scanning electron microscopy images

2.3. Rapid chloride test (RCT) After 147 d of immersion in NaCl solution, the specimens were removed and sectioned into two halves with a diamond blade. The cross-sections were prepared to collect UHPC powder at two locations: adjacent to the surface of steel rebar and close to the outside surface of UHPC specimen as illustrated in Fig. 2. RCT involves: (1) collecting 1.5 g powder sample with a masonry drill bit at different locations, as indicated in Fig. 2; (2) introducing the 1.5-g powder sample into a vial with acid liquid and shaking the vial for 5 min; (3) keeping the vial over-night for the chloride from the sampled UHPC to dissolve into the acid solution; (4) connecting the RCT chloride electrode to the electrometer with its chamber filled with wetting agent; (5) immersing the chloride electrode tip in the calibration liquids and recording the mV readings that are then plotted on the calibration sheet as the calibration curve; and finally (6) immersing the electrode tip in the sample solutions and determining the mV readings on the calibration curve and converting the readings to the chloride content in percent of UHPC mass [34]. The calibration curve is a straight line with the X axis representing mV readings, and the Y axis describing the % of Clby mass of UHPC.

After all the immersion tests, one small specimen with the diameter of 16 mm and height of 8.0 mm was cut from one cylindrical sample of each type of UHPC specimen with a diamond saw. Silicon carbide papers were used to abrade the small cylinders following the sequence of 80, 180, 320, 600, 800, and 1200 grits, cleansed with water after grinding and dried in furnace at 50 °C for 24 h. A scanning electron microscopy (SEM) was used to examine the microstructures of interface between steel rebar and UHPC matrix. 3. Results and discussion 3.1. Ocp OCP is the potential between saturated calomel electrode and steel rebar when there is no external potential applied. It indicates corrosion situation of the embedded steel rebar. The variations of OCPs with time for the different specimens are shown in Fig. 3. Generally, the OCPs did not follow any changing trend through the test period regardless of the LWS content and steel fiber content. The OCP values of all the specimens with LWS fluctuated in the range of 0.14 to 0.57 V while the OCP values of the cylindrical specimens with steel fibers varied from 0.33 to 0.65 V. According to ASTM C876 [36], there is 90% probability that the steel rebar was in active corrosion state since their OCP values were below 0.276 V/SCE [31,37]. However, this standard may be not suited to rebar embedded in UHPC. UHPC has fewer permeable voids compared with conventional concrete, and the oxygen ingress to UHPC matrix and oxygen supply to the surface of steel rebar were substantially reduced [24,38]. According to mixed potential theory as shown in Fig. 4, line 1 changes to line 2 when oxygen supply is reduced [38]. As the intersection point of anodic and cathodic reaction lines represents the OCP value [38], Eocp of 0

Fig. 2. Collection locations of UHPC powder for chloride content analysis.

steel rebar embedded in UHPC is reduced to Eocp :Therefore, in non-corrosion state, the steel rebar embedded in UHPC have lower OCP values compared with those in normal concrete. One more important observation is that the steel rebar embedded in speci-

L. Fan et al. / Construction and Building Materials 238 (2020) 117709

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Fig. 3. Variations of OCPs of UHPC cylinders with different LWS (a) and steel fiber contents (b) for 147 d.

mens with LWS have higher OCP values compared to those in specimens with steel fibers. This can be because LWS increased the number of interconnected pores of UHPC. The increased porosity allows more oxygen and water to penetrate into the UHPC to sustain the cathodic reactions, which increase the OCP values.

3.2. Lpr LPR is inversely proportional to the current density, which is able to decide corrosion state of rebar based on Durar Network Specification: passivity with corrosion current density smaller than 0.1 mA/cm2, low corrosive state with corrosion current density in the range of 0.1 mA/cm2 and 0.5 mA/cm2, high corrosive state with corrosion current density in the range of 0.5 mA/cm2 and 1.0 mA/ cm2, and very high corrosive state with corrosion current density >1.0 mA/cm2 [37,39]. Fig. 5 shows that LPR gradually increased with time but decreased with the increment of LWS or steel fiber contents. Fig. 6 shows the calculated corrosion current densities of steel rebar based on Eq. (2), in which the widely accepted B = 52 mV is used [40]. The corrosion current densities showed declined tendency with time, indicating that the rebar transformed from high corrosion state to low corrosion state. However, the steel rebar is less likely to be in the high corrosion state on the first day of immersion since it takes time for Cl- ions to destroy the passive film and corrode the steel rebar. As B of 52 mV is obtained by setting the cathodic coefficient (bc ) equal to 120 mV/decade and the

Fig. 4. Evans diagram of current density (Log (i)) vs. mixed potential (E). Line 1 represents reduction reaction with sufficient oxygen supply; line 2 represents reduction reaction with insufficient oxygen supply.

anodic coefficient (ba ) to infinity [41], it is necessary to measure the real bc and ba values for UHPC specimens in order to figure out the actual corrosion current densities. Tafel polarization curves were measured and are shown in Fig. 7. The bc and ba values were obtained by drawing tangential lines to the cathodic and anodic curves. The cathodic reaction and anodic reaction at the steel bar surface are respectively represented by:

1 O2 þ 2H2O þ 2e ¼ 2OH 2

ð4Þ

Fe  2e ¼ Fe2þ

ð5Þ

The absolute values of Tafel coefficients represent the ease of occurrence for an electrochemical process in response to the small polarization (±75 mV around OCP). Table 5 summarized the coefficients bc and ba and the calculated B values of different specimens. According to Table 5, the cathodic Tafel coefficients are from 21 to 30 mV, and the anodic Tafel coefficients are from 23 to 34 mV. Based on Eq. (3), the calculated B values ranged between 5.0 and 6.9. These values are used to deduce the current densities of steel rebar, as shown in Fig. 8. The current densities gradually decreased with time, which is different from the increasing trend of current densities of rebar in ordinary cement mortar [42]. The calculated corrosion current densities were lower than 0.1 mA/cm2 after 42 days for the samples with steel fibers, while the samples with LWS reached this value after 84 days. All the steel rebar transformed from low corrosion state to passivity, and the corrosion densities gradually stabilized after 126 days. The reduced corrosion current density can be due to slowly formation of passive film which helps increase polarization resistance of rebar [43]. Besides, Cl- ions induced corrosion is irreversible since the passive film is already destroyed. The transition from low corrosion state to passivity is less likely to occur. Therefore, rapid chloride test is conducted to observe the chloride content at two spots adjacent to the surface of steel rebar and outside surface of UHPC specimens. According to Table 6, the chloride contents for all the specimens at the two spots were lower than 0.01%, which were much lower than the threshold values (0.07–1.06%) [37,42] for the active corrosion. Besides, the chloride content at the two spots are quite close, which means additional chloride did not penetrate into the UHPC matrix during the test period. The examined chloride in UHPC matrix is likely introduced during UHPC mixing. Therefore, the steel bars were in passive state and corrosion did not happen on the surface of rebar embedded in UHPC from the beginning of the immersion test.

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L. Fan et al. / Construction and Building Materials 238 (2020) 117709

Fig. 5. LPR of the rebar in UHPC with different LWS content (a) and steel fiber content (b).

Fig. 6. Evolution of corrosion current densities of rebar in UHPC cylinders with different LWS (a) and steel fiber contents (b) when B = 52 mV.

Fig. 7. Representative fitting results of a Tafel polarization curve.

3.3. Eis EIS is a non-destructive technique that is able to provide timedependent electrochemical properties of reinforced UHPC. Fig. 9 represents the impedance spectra of UHPC samples in the Bode

diagram form for 147 d. The impedance is obtained by applying a small sinusoidal potential to the tested electrochemical cell and simultaneously measuring generated sinusoidal current flowing through the cell. The electrochemical impedance is the ratio of the applied sinusoidal potential and measured sinusoidal current. In Bode diagrams, left Y axis and right Y axis denote impedance and phase angle respectively; X-axis means tested frequency range in the logarithmic format. Generally, spectra of all the cylinders show increasing trend with time despite of the different LWS and steel fiber contents. In high frequency range (>104 Hz), impedance represents the UHPC; in middle frequency range of 1–104 Hz, impedance denotes the total response of UHPC and passive film; in low frequency range of 10-3-1 Hz, impedance accounts for the total response of charge transfer resistance, passive film, and UHPC [42,43] as illustrated in Fig. 10. The gradual increase of impedance and phase angle in the high frequency mean the growing resistance of UHPC against intrusion of corrosive ions and rebar corrosion [44,45]. The advantage of EIS technique is to distinguish the electric properties of each component under investigation. Equivalent electric circuit model (EEC) in Fig. 11 was applied to decide electrochemical behaviors of rebar reinforced UHPC individually: steelelectrolyte interface, passive film and matrix [37,42]. In the Fig. 9, the dotted points and solid lines denote experimental data fitting curves with the EEC model, respectively, as illustrated in Fig. 9. ZsimpWin are used to fit all the data and Chi-squared values

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L. Fan et al. / Construction and Building Materials 238 (2020) 117709 Table 5 Tafel coefficients bc and ba (mV/decade) and B (mV) of different samples.

ST-0 ST-0.5 ST-1 ST-1.5 ST-2 ST-3

bc

ba

B

23 21 30 22 27 25

23 28 34 26 28 28

5.0 5.3 6.9 5.1 5.9 5.7

LWS-12.5 LWS-25 LWS-37.5 LWS-50

bc

ba

B

22 27 24 26

32 31 32 31

5.6 6.3 6.0 6.1

Fig. 8. Evolution of corrosion current densities of steel rebar with different LWS content (a) and steel fiber content (b) when B values were obtained from Tafel polarization test.

Table 6 Chloride contents of collected powders from different specimens (wt. % of powder). Cl- content

ST-0 ST-0.5 ST-1.0 ST-1.5 ST-2 ST-3

Cl- content

Spot 1

Spot 2

0.0011 0.0043 0.0048 0.0041 0.0035 0.0037

0.0021 0.0041 0.0037 0.0045 0.0037 0.0032

LWS-12.5 LWS-25 LWS-37.5 LWS-50

were smaller than 10-3 with satisfactory fitting result. In the model, Rs denotes resistance of NaCl solution, Rc and CPEc denote the resistance and capacitance of UHPC matrix, Rf and CPEf are the resistance and capacitance of passive film, and Rct and CPEdl mean charge transfer resistance and double layer capacitance at the interface between rebar and electrolyte. The pure capacitor was replaced with constant phase element CPE because of the nonhomogeneity of steel rebar -passive film-UHPC matrix system under consideration [37,42]. CPEc considers the non-uniform distribution of composition in the UHPC, CPEf accounts for the irregularity of passive film thickness, and CPEdl takes into account the nonhomogeneous distribution of potential on the rebar surface [37,42]. The CPE is denoted by:

n

ZCPE ¼ Y1 ðjwÞ

ð6Þ

where n is an index that denotes deviation away from the pure capacitor, w (rad/s) is angular frequency, and Y is proportional to the capacitance of pure capacitor [43–45]. Detailed discussion on the parameters are elucidated in Section 4.

Spot 1

Spot 2

0.0050 0.0051 0.0058 0.0060

0.0048 0.0054 0.0060 0.0050

3.4. SEM images SEM images at the interfaces between UHPC and steel rebar of representative specimens (LWS 4 and ST-3) after immersion tests are shown in Fig. 12. Fig. 12 (a-1) and (b-1) show that large sand particles are distributed in the matrix. In Fig. 12 (b-1), steel fibers are distributed in the UHPC and they do not contact the rebar. Good bond between steel rebar and UHPC matrix can be observed in Fig. 12 (a-2) and (b-2) and corrosion products are not seen at the steel rebar-UHPC interface.

4. Discussion on the resistance of UHPC and passive film, and charge transfer resistance Basically, higher UHPC resistance means that the microstructure of UHPC matrix becomes denser and UHPC is more impermeable to the corrosive ions like water, oxygen and chloride, and is able to provide better and longer protection to the embedded steel rebar from corrosion. Fig. 13 displayed that the resistance of UHPC for all the samples had an increasing trend with time throughout the test period regardless of the LWS content and steel fiber content. This is because hydration and pozzolanic reaction continued

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L. Fan et al. / Construction and Building Materials 238 (2020) 117709

Fig. 9. Evolution of impedance spectra with testing time in the format of Bode diagram for UHPC samples: LWS-12.5 (a), LWS-25 (b), LWS-37.5 (c), LWS-50 (d), 0 fiber (e), fiber 0.5% (f), fiber 1% (g), fiber 1.5% (h), fiber 2% (i), and fiber 3% (j).

during immersion test and the products generated from them slowly filled the capillary pores, which reduced ionic flow through the UHPC matrix [46–48]. Fig. 13 (a) indicated that the electrical resistance of UHPC gradually decreased when LWS content increased. The saturated LWS gradually released water to the matrix to support hydration process. However, LWS can increase the number of interconnected

pores of UHPC. With the increase of LWS content, the reduction in porosity due to the fill of hydration products is compromised by the introduction of porosity by LWS [16,17]. The additional interconnected pores increases the ease of ion movement across the matrix, which tends to lower the electrical resistance of UHPC matrix. As the authors have observed that when LWS content increased from 25% to 75%, compressive strength indicated a

L. Fan et al. / Construction and Building Materials 238 (2020) 117709

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Fig. 9 (continued)

Fig. 11. Equivalent electric circuit model (EEC) used to fit the spectra in Fig. 9.

Fig. 10. Illustration of Bode plots with impedance components in each frequency range.

decreasing trend due to the considerable increase in total porosity. Considering about the compressive strength and electrical resistance, it is better to limit the maximum light weight sand content to 25%.

The resistance of UHPC matrix in Fig. 13 (b) showed a declined tendency with the increment of steel fiber content. Basically, measured electric current travels in the UHPC matrix through ionic flow in the inner-connected capillary or C-S-H gel pores [48]. When steel fibers are in the UHPC, the current passes through steel fibers instead of ionic flow as shown in Fig. 14 [26]. Since steel fibers have zero resistance, the current encounters less resistance when going through the UHPC matrix, which reduces the measured electrical resistance of UHPC matrix. The results of the total charge passing through specimens after 6 h of testing are shown in Fig. 15. Generally, the higher the total charges going through the specimens, the lower the resistance

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L. Fan et al. / Construction and Building Materials 238 (2020) 117709

Fig. 12. SEM images at the interfaces of steel rebar and UHPC matrix with LWS (a) and steel rebar and UHPC matrix with steel fiber (b).

Fig. 13. Electrical resistance of UHPC with different LWS content (a) and steel fiber content (b).

against the chloride penetration. With the increase of LWS content, more charge can passed through the specimens, and the resistance of UHPC against the penetration of chloride ions decreased, which is due to the increment of interconnected pores introduced by the LWS [16,17]. The result can explain the changing trend of the electrical resistance of UHPC matrix in Fig. 13(a). Charges passing through the specimens increased with the increment of steel fiber, which is due to the shortcuts of current by steel fibers [26]. This also demonstrates the changing trend of resistance of UHPC matrix in Fig. 13 (b). However, it needs to be mentioned that steel fibers

are able to interrupt the connections of capillary pores and impede the chloride movement across the specimens [24]. The increase of charges across the specimens with more steel fiber content cannot represent the decrease of UHPC resistance against chloride intrusion. Therefore, the RCPT is not appropriate for testing the electrical conductivity of steel fiber reinforced UHPC, and electrical resistance test is not suitable to decide the durability of UHPC with steel fibers. The passive film resistance shows an increasing trend with immersion time, as shown in Fig. 16. Oxygen is a necessity for

L. Fan et al. / Construction and Building Materials 238 (2020) 117709

Fig. 14. Schematic view showing electrons and ionic flow through porous network of UHPC matrix.

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the passive film formation [29]. Due to the low oxygen content in UHPC, it takes longer time for passive film to grow [49]. Besides, calcium hydroxide (Ca(OH)2) gradually precipitated near the surface of steel rebar due to the slow hydration process, which supports the continuous growth of the resistance of passive film [50]. With the increase of LWS content, the resistance of passive film showed a decreasing trend. UHPC has very low w/b, which indicates that there is limited space for the precipitation of hydration products [16]. With the increase of LWS, more space is available for the hydration products and the microstructure of the precipitated Ca(OH)2 near the steel rebar becomes less compact, which tends to reduce the passive film resistance. Confirmation of this theory will be further studied. Besides, steel fibers in UHPC have a large amount of surface area and it is possible for the fibers to attract the oxygen to its surroundings and reduces the oxygen content around the steel rebar [26]. As oxygen is necessary for the generation of passive film, passive film resistance (Rf) was reduced with the increment of steel fiber. Charge transfer resistance (Rct) decides the ease of electrons transferring between the steel rebar and the electrolyte, which is in reverse proportion to the corrosion rate of rebar [37,42]. The

Fig. 15. Total charge passed through UHPC samples with different LWS content (a) and steel fiber content (b).

Fig. 16. Passive film resistance of steel rebar in UHPC with the increase of LWS content (a) and steel fiber content (b).

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L. Fan et al. / Construction and Building Materials 238 (2020) 117709

Fig. 17. Rct of the steel rebar in UHPC with increase of LWS content (a)and steel fiber content (b).

higher charge transfer resistance indicates the better corrosion performance of steel rebar. Fig. 17 illustrates that Rct of the steel rebar embedded in UHPC had an increasing trend with time but decreased with the increment of LWS content or steel fiber content. Compared to Fig. 5, the difference between Rct and linear polarization resistance is smaller than 10%. As both parameters have the same meaning indicating the resistance at the interface between steel bar and electrolyte, it proves that the fitting results are good with the proposed EEC model. Passive film is critical to the corrosion resistance of steel rebar and higher passive film resistance results in higher Rct [42]. Resistance of passive film increased with immersion time as shown in Fig. 16, which led to the increase of charge transfer resistance. Besides, with the increment of LWS or steel fiber contents, resistance of passive film showed a decreasing trend, which led to the decrease of charge transfer resistance. Different from the decreasing trend of Rct of steel rebar in ordinary mortar, passive film resistance and Rct of steel rebar in UHPC show an increasing trend with time regardless of steel fiber content, indicating that steel fibers with the volume content up to 3% will not lead to the corrosion of steel rebar and can be safely used in UHPC. In the future, long-term corrosion tests need to be carried out to observe the influence of higher contents of steel fibers and LWS on the corrosion performance of steel rebar.

harder for the ingress of chloride ions. As both electrical resistance and compressive strength of UHPC demonstrated a declined trend when the LWS content was increased from 25% to 50%, it is recommended to limit the maximum LWS content to 25%.
The presence of steel fibers can lead to the shortcut of current, which hinders the applicability of the RCPT and electrical resistance test for evaluating the electrical conductivity of UHPC with steel fibers.
The gradual growth of charge transfer resistance of steel rebar in UHPC indicates that steel rebar in UHPC become more corrosion-resistant with time, which shows opposite changing trend compared to the steel rebar embedded in ordinary concrete.
The use of steel fibers with volume of up to 3% did not promote corrosion of embedded steel rebar in UHPC. Author contributions L.F. and W.M. conceived and designed the experiments; L.F., W. M., and L.T. performed the experiments; L.F., W.M. and L.T. analyzed the data; L.F. W.M., L.T. and K.H.K prepared the manuscript.

5. Conclusion

Declaration of Competing Interest

In this paper, the effects of LWS and steel fiber contents on the corrosion performance of embedded steel rebar and electrical resistance of UHPC were studied through electrochemical tests. Based on experiment results and detailed analysis, the following conclusions can be made:

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.


OCP values of steel rebar embedded in UHPC were lower than 0.276 V/SCE but steel rebar cannot be determined to be in the corrosion state according to ASTM C876. This is because the lower supply of oxygen in UHPC is able to reduce the cathodic reduction of oxygen, which leads to lower OCP values.
Combining the experiment results from LPR, Tafel polarization test and RCT, it can be determined that the rebar is in the passive state in UHPC through the test period of 147 d. The chloride ions did not penetrate into UHPC matrix given its dense microstructure that can reduce the ingress of chloride ions.
The electrical resistance of the UHPC matrix showed increasing trend with time regardless of the UHPC type indicating that the microstructure of UHPC becomes denser, which makes it even

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