The Corrosion Resistance of Reinforced Magnesium Phosphate Cement Reactive Powder Concrete

Magnesium phosphate cement-based reactive powder concrete (MPC-RPC) is a cement-based material with early strength, high strength and excellent durability. The slump flow and setting time of steel fibers reinforced MPC-RPC are investigated. Meanwhile, the flexural strength, the compressive strength, the ultrasonic velocity and the electrical resistivity of specimens cured for 3 h, 1 day, 3 days and 28 days are determined. Moreover, the corresponding corrosion resistance reinforced MPC-RPC exposing to NaCl freeze-thaw (F-T) cycles and dry-wet (D-W) alternations is researched. In this study, the steel fibers used are 0%, 0.5%, 1.0%, 1.5%, 2.0%, 2.5% and 3.0% by the volume of MPC-RPC. The corrosion of the inner reinforcement is reflected using the mass loss, electrical resistivity, ultrasonic velocity, and the AC impedance spectrum. Researching findings show that the steel fibers lead to decreasing the slump flow and setting time. The flexural strength, the compressive strength and ultrasonic velocity of MPC-RPC cured for 3 h are higher than 45% of the MPC-RPC cured for 28 days. Moreover, when the MPC-RPC is cured for 7 days, the flexural strength, the compressive strength and ultrasonic velocity of MPC-RPC are higher than 85% of the specimens cured for 28 days. The electrical resistance decreases in a quadratic function as the volume ratio of steel fibers increases. The corrosion resistance of the internal reinforcement can be improved by adding steel fibers at appropriate dosages. The reinforcement inner MPC-RPC corrodes more seriously under the NaCl D-W alternations than NaCl F-T cycles.


Introduction
Cement concrete is a type of material with high mechanical properties and good durability, which has been used in civil engineering industry for many years [1,2]. Recently, the frequent construction of large marine concrete structures provides a platform for the development and application of cement concrete [3]. The sea crossing bridge is the hub of coastal cities, which usually encounters complex application environment similar to scouring, dry-wet (D-W) alternations and freeze-thaw (F-T) cycles of seawater [4][5][6]. The expansion joint anchorage zone of sea crossing bridge encounters the action of complex loads. Due to the erosion by sea water and the complex loads, the cement concrete of the anchorage zone of bridge expansion joint materials is frequently damaged.
Sulphoaluminate cement matrix, the Portland cement matrix, the magnesium phosphate cement matrix and compound cement matrix are usually applied in the rapid repairing of cement concrete constructions [7][8][9][10][11]. Sulphoaluminate cement-based materials have been proved to possess quite high mechanical strength at early curing age (less than 1 d) [3]. However, the mechanical strength at later curing age is lower than that of Portland cement concrete. Meanwhile, the durability of magnesium phosphate cement matrix in marine environment is inferior [12]. Portland cement-based materials with early strength agent and the compound cement matrix show better later mechanical strength and durability than    Table 3 is the mix proportion of MPC-RPC per unit volume, which is used for making the MPC-RPC. The percentages of admixtures are obtained from prior studies [9,17,19], which are based on the maximum density theory. In addition, it is also convenient for comparison with previous studies. The specimens are manufactured by the following steps. Firstly, the weighed dried raw materials (MgO, MgCl 2 , Borax, K 2 HPO 4 , FA and BFS) are added to the Hobart A200C mixer and combined at the stirring speed of (107 ± 5) rpm for 2 min. During the mixing, the steel fibers are scattered into the mixing pot in batches and combined at the mixing speed of (198 ± 5) rpm for another 1 min, finally the mixed liquids with water and water reducer are added to the mixing pot and 5 min stirring with the mixing speed of (361 ± 5) rpm provided for stirring the MPC-RPC mixture. The fresh MPC-RPC is used to measure the slump flow and setting time after the mixing is completed. NLD-3 electric jumping table cement mortar fluidity tester is used for the measurement of  [33,34]. After the above testing, the fresh MPC-RPC is poured into molds with sizes of 40 × 40 × 160 mm 3 and 50 × 50 × 50 mm 3 to form specimens.

Mechanical Strengths
YAW-300E cement mortar compressive and flexural machine is used to measure the flexural and compressive strengths of specimens with size of 40 × 40 × 160 mm 3 . After the specimens are cured in the environment of 20 • C and relative humility of 99% for the needed curing ages, the specimens are moved to the bending fixture with the flat and smooth surface of test piece on the fixture. Then, load rate with 0.05 kN/s is provided for the flexural strength. After the flexural specimen is broken, two fault blocks are moved to the compressive fixture and the load with the loading rate of 2.4 kN/s is used [35]. The measuring process of mechanical strengths is shown in Figure 1.
× 50 mm to form specimens. YAW-300E cement mortar compressive and flexural machine is used to measure the flexural and compressive strengths of specimens with size of 40 × 40 × 160 mm 3 . After the specimens are cured in the environment of 20 °C and relative humility of 99% for the needed curing ages, the specimens are moved to the bending fixture with the flat and smooth surface of test piece on the fixture. Then, load rate with 0.05 kN/s is provided for the flexural strength. After the flexural specimen is broken, two fault blocks are moved to the compressive fixture and the load with the loading rate of 2.4 kN/s is used [35]. The measuring process of mechanical strengths is shown in Figure 1.

Ultrasonic Velocity Test
Specimens with size of 50 × 50 × 50 mm 3 are applied in the determination of ultrasonic velocity. The Jinghong CJ-10 intelligent nonmetal ultrasonic detector manufactured by Cangzhou Jinghong Engineering Instrument Co., Ltd., Cangzhou, China is used for the measurement of ultrasonic velocity. The vaseline is coupled with the surface of the specimens before testing. Figure 2 shows the measurement of the ultrasonic velocity.

Ultrasonic Velocity Test
Specimens with size of 50 × 50 × 50 mm 3 are applied in the determination of ultrasonic velocity. The Jinghong CJ-10 intelligent nonmetal ultrasonic detector manufactured by Cangzhou Jinghong Engineering Instrument Co., Ltd., Cangzhou, China is used for the measurement of ultrasonic velocity. The vaseline is coupled with the surface of the specimens before testing. Figure 2 shows the measurement of the ultrasonic velocity.

AC Electrical Parameters Test
The electrical parameters of specimens are tested by TH2810D LCR digital electric bridge (AC electrical resistance measurement) and PARSTAT 3000A electrochemical workstation (Determination of AC impedance spectrum). The testing frequency, the voltage and the sampling frequency of TH2810D are 10 4 Hz, 1 V and 10 Hz. The testing fre-

AC Electrical Parameters Test
The electrical parameters of specimens are tested by TH2810D LCR digital electric bridge (AC electrical resistance measurement) and PARSTAT 3000A electrochemical workstation (Determination of AC impedance spectrum). The testing frequency, the voltage and the sampling frequency of TH2810D are 10 4 Hz, 1 V and 10 Hz. The testing frequency and voltage of PARSTAT 3000A electrochemical workstation are 10 5 Hz~1 Hz and −10 mV~10 mV. Two pieces of 316 L stainless steel mesh with aperture's diameter of 4 mm serve as two electrodes. The distance between two electrodes is 40 mm. The measurement of the electrical parameters are shown in Figure 3. The experimental details are described in Wang's paper [36,37].

AC Electrical Parameters Test
The electrical parameters of specimens are tested by TH2810D LCR digital ele bridge (AC electrical resistance measurement) and PARSTAT 3000A electrochem workstation (Determination of AC impedance spectrum). The testing frequency, the v age and the sampling frequency of TH2810D are 10 4 Hz, 1 V and 10 Hz. The testing quency and voltage of PARSTAT 3000A electrochemical workstation are 10 5 Hz~1 Hz −10 mV~10 mV. Two pieces of 316 L stainless steel mesh with aperture's diameter of 4 serve as two electrodes. The distance between two electrodes is 40 mm. The measurem of the electrical parameters are shown in Figure 3. The experimental details are descri in Wang's paper [36,37].

Corrosion of Steel Bars Inner MPC-RPC under NaCl Eroded Environment
The steel bars are embedded the center position of the MPC-RPC. The specimens under the NaCl F-T cycles' environment with the NaCl concentration of 3%. Fully a matic control concrete rapid freezing and thawing test box with the temperature rang −25 °C~50 °C and unit operating power of 5.5 kW is used for the freeze-thaw experim 24 days' standard curing condition is provided for the specimens. After this, some sp mens are immersed in the solution containing 3% NaCl for 4 days and are moved to rapid freezing and thawing test box with the working temperature of −15 °C~8 °C. So other specimens are used for the experiment of D-W alternations of NaCl solution. Fir the specimens are immersed in the NaCl solution for 10 h, then the specimens are d in the DHG series vertical 300 °C blast drying oven with the temperature of 60 °C fo h. Finally, the specimens are moved from the drying oven and cooled in the tempera of 20 °C and relative humility of 40% for 2 h, until the next D-W cycle. The mass loss r the ultrasonic velocity, the electrical resistance and the AC impedance spectrum of sp mens during the corrosion process are obtained. The measuring methods are the sam RPC specimens without steel fibers. A 316 L stainless steel mesh serves as an electr

Corrosion of Steel Bars Inner MPC-RPC under NaCl Eroded Environment
The steel bars are embedded the center position of the MPC-RPC. The specimens are under the NaCl F-T cycles' environment with the NaCl concentration of 3%. Fully automatic control concrete rapid freezing and thawing test box with the temperature range of −25 • C~50 • C and unit operating power of 5.5 kW is used for the freeze-thaw experiment. 24 days' standard curing condition is provided for the specimens. After this, some specimens are immersed in the solution containing 3% NaCl for 4 days and are moved to the rapid freezing and thawing test box with the working temperature of −15 • C~8 • C. Some other specimens are used for the experiment of D-W alternations of NaCl solution. Firstly, the specimens are immersed in the NaCl solution for 10 h, then the specimens are dried in the DHG series vertical 300 • C blast drying oven with the temperature of 60 • C for 36 h. Finally, the specimens are moved from the drying oven and cooled in the temperature of 20 • C and relative humility of 40% for 2 h, until the next D-W cycle. The mass loss rate, the ultrasonic velocity, the electrical resistance and the AC impedance spectrum of specimens during the corrosion process are obtained. The measuring methods are the same as RPC specimens without steel fibers. A 316 L stainless steel mesh serves as an electrode, meanwhile, the embedded steel bar is used as another electrode. The experimental measurements of corrosion of steel bars inner RPC are exhibited in Figures 4 and 5. In this study, 3 specimens are used for the measurement of mechanical strength, and 6 specimens are applied in the test of electrical parameters and ultrasonic velocity.

The Workability of MPC-RPC
The slump flow and setting time of the MPC-RPC are shown in Figure 6. Figure 6 shows that when the ratio of steel fibers increases, the slump flow and the setting time decrease. This is mainly because the steel fibers' networks can prevent the flow of fresh MPC-RPC paste [38]. Therefore, the slump flow of fresh MPC-RPC slurry will be decreased due to the addition of steel fibers. Moreover, as depicted in Figure 6, the setting time of MPC-RPC decreases with an increase in steel fibers ratio as the steel fibers may absorb some free water, leading to decreasing the setting time of MPC-RPC. Moreover, the addition of steel fiber will affect the physical state of MPC-RPC, thus decreasing the setting time of MPC-RPC [39]. The values of error bars are lower than 0.096, which ensures the experiment's accuracy. The setting time of MPC-RPC ranges from 33.2 min to 56.1 min, which provides sufficient operation time for construction. Meanwhile, the slump flow of fresh MPC-RPC is 121.4 mm~181.3 mm, which ensures the sufficient fluidity during pouring.

The Workability of MPC-RPC
The slump flow and setting time of the MPC-RPC are shown in Figure 6. Figure 6 shows that when the ratio of steel fibers increases, the slump flow and the setting time decrease. This is mainly because the steel fibers' networks can prevent the flow of fresh MPC-RPC paste [38]. Therefore, the slump flow of fresh MPC-RPC slurry will be decreased due to the addition of steel fibers. Moreover, as depicted in Figure 6, the setting time of MPC-RPC decreases with an increase in steel fibers ratio as the steel fibers may absorb some free water, leading to decreasing the setting time of MPC-RPC. Moreover, the addition of steel fiber will affect the physical state of MPC-RPC, thus decreasing the setting time of MPC-RPC [39]. The values of error bars are lower than 0.096, which ensures the experiment's accuracy. The setting time of MPC-RPC ranges from 33.2 min to 56.1 min, which provides sufficient operation time for construction. Meanwhile, the slump flow of fresh MPC-RPC is 121.4 mm~181.3 mm, which ensures the sufficient fluidity during pouring.

The Workability of MPC-RPC
The slump flow and setting time of the MPC-RPC are shown in Figure 6. Figure 6 shows that when the ratio of steel fibers increases, the slump flow and the setting time decrease. This is mainly because the steel fibers' networks can prevent the flow of fresh MPC-RPC paste [38]. Therefore, the slump flow of fresh MPC-RPC slurry will be decreased due to the addition of steel fibers. Moreover, as depicted in Figure 6, the setting time of MPC-RPC decreases with an increase in steel fibers ratio as the steel fibers may absorb some free water, leading to decreasing the setting time of MPC-RPC. Moreover, the addition of steel fiber will affect the physical state of MPC-RPC, thus decreasing the setting time of MPC-RPC [39]. The values of error bars are lower than 0.096, which ensures the experiment's accuracy. The setting time of MPC-RPC ranges from 33.

The Mechanical Strengths of MPC-RPC
As reported in References [3,9], the curing ages of 3 h, 1 day, 3 days and 28 days are usually used for reflecting the mechanical strength of magnesium phosphate cementbased material. In order to facilitate comparison with previous studies, the curing ages of

The Mechanical Strengths of MPC-RPC
As reported in References [3,9], the curing ages of 3 h, 1 day, 3 days and 28 days are usually used for reflecting the mechanical strength of magnesium phosphate cement-based material. In order to facilitate comparison with previous studies, the curing ages of 3 h, 1 day, 3 days and 28 days are selected. The flexural and compressive strengths of MPC-RPC are depicted in Figure 7. It can be observed in Figure 7, the flexural and compressive strengths increase with the increasing steel fibers ratio and the curing age. The flexural strength of MPC-RPC cured for 3 h increases by 156.9%, when the steel fibers ratio varies from 0% to 3%. While, when the curing ages are 1 day, 3 days and 28 days, the increasing rates are 119.4%, 61.4% and 50%, respectively. Moreover, the increasing rates of compressive strength by steel fibers of specimens cured for 3 h, 1 day, 3 days and 28 days are 0~30.4%, 0~32.7%, 0~64.8% and 0~50%, respectively. When the curing age ranges from 3 h to 1 d, the increasing rate of flexural strength of MPC-RPC is 3.8~21.5%. Meanwhile, when the curing age ranges from 3 h to 28 days, the increasing rate of flexural strength of MPC-RPC is 28.2~119.6%. Additionally, the maximum increasing rate of compressive strength by curing age is 102.1%. Meanwhile, the maximum increasing rate of compressive strength by the steel fibers ratio is 49.4%. This is attributed to the reason that the magnesium oxide will react with potassium dihydrogen phosphate to form hydrated magnesium phosphate rapidly [40][41][42], besides the mechanical strengths of MPC-RPC at low curing age is enough high. The addition of steel fibers can bridge cracks inner MPC-RPC, thus improving the mechanical strengths, especially the flexural strength. The values of error bars are lower than 0.073, indicating the accuracy of experimental results. Compared with the RPC prepared with sulphoaluminate cement, the flexural strength of MPC-RPC is reduced by 13.2~25.4%, and the compressive strength is increased by 10.6~31.3% [43]. The flexural strength and compressive strength of MPC-RPC before 28 days are 25% higher than those of RPC prepared with ordinary Portland cement. Meanwhile, when the curing age is 28 days, the mechanical strengths of MPC-RPC are lower than that of RPC with Ordinary Portland cement [44].

The Ultrasonic Velocity of MPC-RPC
The ultrasonic velocity (v) of MPC-RPC after curing for 3 h, 1 day, 3 days and 28 days, is shown in Figure 8. It can be noticed in Figure 8, as the curing age increases and steel fibers are added, the ultrasonic velocity also increases. This is explained by the fact that when the curing age increases, the amount of hydrated magnesium phosphate increases and MPC-RPC become more compact, which increases the ultrasonic velocity [43]. Moreover, the increased dosages of steel fibers can improve the compactness of steel fibers' networks, which results in increasing the ultrasonic velocity. In comparison to 80% of the specimens that are cured for 28 days, the ultrasonic velocity of specimens cured for 3 h is higher. Additionally, the ultrasonic velocity of blank specimens is higher than 81.7% of the specimens with 3.0% steel fibers. The flexural strength, the compressive strength and ultrasonic velocity of RPC cured for 3 h are higher than 45% of the MPC-RPC cured for 28 days. The flexural strength, the compressive strength and ultrasonic velocity of MPC-RPC

The Ultrasonic Velocity of MPC-RPC
The ultrasonic velocity (v) of MPC-RPC after curing for 3 h, 1 day, 3 days and 28 days, is shown in Figure 8. It can be noticed in Figure 8, as the curing age increases and steel fibers are added, the ultrasonic velocity also increases. This is explained by the fact that when the curing age increases, the amount of hydrated magnesium phosphate increases and MPC-RPC become more compact, which increases the ultrasonic velocity [43]. Moreover, the increased dosages of steel fibers can improve the compactness of steel fibers' networks, which results in increasing the ultrasonic velocity. In comparison to 80% of the specimens that are cured for 28 days, the ultrasonic velocity of specimens cured for 3 h is higher. Additionally, the ultrasonic velocity of blank specimens is higher than 81.7% of the specimens with 3.0% steel fibers. The flexural strength, the compressive strength and ultrasonic velocity of RPC cured for 3 h are higher than 45% of the MPC-RPC cured for 28 days. The flexural strength, the compressive strength and ultrasonic velocity of MPC-RPC cured for 7 days are higher than 85% of the specimens cured for 28 days. The values of error bars are all lower than 0.085, ensuring the precision of researching results. The ultrasonic velocity of MPC-RPC is 5.4~11.3% lower than that of RPC with Ordinary Portland cement [44,45]. The detailed data of Figures 6-8 have been provided by the graphs be presented in the form of tables at the end of the article (Appendix A).
fibers are added, the ultrasonic velocity also increases. This is explained by the fact that when the curing age increases, the amount of hydrated magnesium phosphate increases and MPC-RPC become more compact, which increases the ultrasonic velocity [43]. Moreover, the increased dosages of steel fibers can improve the compactness of steel fibers' networks, which results in increasing the ultrasonic velocity. In comparison to 80% of the specimens that are cured for 28 days, the ultrasonic velocity of specimens cured for 3 h is higher. Additionally, the ultrasonic velocity of blank specimens is higher than 81.7% of the specimens with 3.0% steel fibers. The flexural strength, the compressive strength and ultrasonic velocity of RPC cured for 3 h are higher than 45% of the MPC-RPC cured for 28 days. The flexural strength, the compressive strength and ultrasonic velocity of MPC-RPC cured for 7 days are higher than 85% of the specimens cured for 28 days. The values of error bars are all lower than 0.085, ensuring the precision of researching results. The ultrasonic velocity of MPC-RPC is 5.4~11.3% lower than that of RPC with Ordinary Portland cement [44,45]. The detailed data of Figures 6-8 have been provided by the graphs be presented in the form of tables at the end of the article (Appendix A).

Electrical Resistivity of MPC-RPC
The electrical resistivity (ρ) of MPC-RPC is shown in Figure 9. As illustrated in Figure 9, the electrical resistivity of MPC-RPC decreases in the form of quadratic function. The fitting results of the relationship between ρ and V are listed in Table 4, and it can be seen that the fitting degree is higher than 0.99, which verifies the rationality of the fitted equation. When the volume ratio (V) of steel fibers increases from 0% to 1.5%, the electrical resistivity of MPC-RPC drops obviously. This is ascribed to the fact that the conductive fiber networks come into being, therefore, the electrical resistivity is very sensitive to the increase of steel fibers' content [46]. While, when the steel fibers' volume ratio is higher than 1.5%, the electrical resistivity of MPC-RPC tends to be stable, due to the complete conductive fibers' network. Hence, the increasing steel fibers' ratio has little influence on the conductive performance of MPC-RPC.

Electrical Resistivity of MPC-RPC
The electrical resistivity (ρ) of MPC-RPC is shown in Figure 9. As illustrated in Figure  9, the electrical resistivity of MPC-RPC decreases in the form of quadratic function. The fitting results of the relationship between ρ and V are listed in Table 4, and it can be seen that the fitting degree is higher than 0.99, which verifies the rationality of the fitted equation. When the volume ratio (V) of steel fibers increases from 0% to 1.5%, the electrical resistivity of MPC-RPC drops obviously. This is ascribed to the fact that the conductive fiber networks come into being, therefore, the electrical resistivity is very sensitive to the increase of steel fibers' content [46]. While, when the steel fibers' volume ratio is higher than 1.5%, the electrical resistivity of MPC-RPC tends to be stable, due to the complete conductive fibers' network. Hence, the increasing steel fibers' ratio has little influence on the conductive performance of MPC-RPC.

Corrosion Resistance of Steel Bars Inner MPC-RPC
The mass loss rate (∆m/m) of reinforced MPC-RPC during NaCl F-T cycles(N) is illustrated in Figure 10. The fitting results of the relationship between (∆m/m) and the N are shown in Table 5. Figure 10 shows that as the number of NaCl F-T cycles (N) increases, so does the mass loss rate of reinforced MPC-RPC. This is explained by the fact that frozen-heave stress can make the surface on the cement concrete spall [47]. Moreover, the chloridion can corrode the steel bars and steel fibers inner MPC-RPC thus causing cracking of MPC-RPC and reducing the mass of MPC-RPC. When the steel fibers' ratio varies from 0% to 2.0%, the mass loss increases up as the dosage of steel fibers is increased, due to the fact that the steel fibers enhance the loss of electronic capability. When the dosage of steel fibers is 2.0%~3.0%, the mass loss rate of reinforced MPC-RPC decreases with the increasing steel fibers' ratio. This is explained by the ability of the steel fibers to bridge internal cracks in MPC-RPC, which decreases the mass loss of MPC-RPC [48].  The variation rate of ultrasonic velocity of MPC-RPC is illustrated in Figure 11. The ultrasonic velocity decreases with the increasing number of NaCl F-T cycles, as shown in Figure 11. This is explained by the fact that the NaCl F-T cycles can accelerate the MPC-RPC's crack propagation which blocks the propagation process of ultrasound [49,50]. Additionally, the chloride ions will penetrate into MPC-RPC through the cracks developed by NaCl F-T cycles thereby corroding the passive film of reinforcement and steel fibers thus accelerating the following corrosion [28,29]. Consequently, the cracks increase and the ultrasonic velocity decreases. Moreover, as observed in Figure 11, in some conditions, the addition of steel fibers lead to decreasing the ultrasonic velocity, due to the increasing Freeze-thaw cycles Mass loss rate (%) 0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0% Figure 10. The mass loss rate of reinforced MPC-RPC during NaCl F-T cycles. The variation rate of ultrasonic velocity of MPC-RPC is illustrated in Figure 11. The ultrasonic velocity decreases with the increasing number of NaCl F-T cycles, as shown in Figure 11. This is explained by the fact that the NaCl F-T cycles can accelerate the MPC-RPC's crack propagation which blocks the propagation process of ultrasound [49,50]. Additionally, the chloride ions will penetrate into MPC-RPC through the cracks developed by NaCl F-T cycles thereby corroding the passive film of reinforcement and steel fibers thus accelerating the following corrosion [28,29]. Consequently, the cracks increase and the ultrasonic velocity decreases. Moreover, as observed in Figure 11, in some conditions, the addition of steel fibers lead to decreasing the ultrasonic velocity, due to the increasing dosages of steel fibers can improve the electrical conduction of MPC-RPC, which accelerates the electrochemical corrosion of the inner reinforcement. On the other hand, the increasing dosage of steel fibers can limit the cracking of cracks, which increases the ultrasonic velocity [51]. The values of error bars are lower than 0.1, which exhibits accurate experimental results. Decreasing rate(%) 0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0% Figure 11. The ultrasonic velocity of MPC-RPC during NaCl F-T cycles. Figure 12 presents the electrical resistivity of MPC-RPC. It is apparent from Figure  12 that the electrical resistivity increases with the increasing NaCl F-T cycles(N). The relationship between electrical resistivity and the number of F-T cycles is deduced as quadratic function. The fitting results are exhibited in Table 6. As shown in Table 6, the fitting degrees are higher than 0.99, thus proving the accuracy of the fitting equations. This is mainly because the NaCl F-T cycles can increase the F-T cracks which blocks the migration of conductive particles and increases the electrical resistivity of MPC-RPC [49,50]. Moreover, the NaCl F-T cycles can accelerate the corrosion of reinforcement and steel fibers. The rust inner MPC-RPC can prevent the electron transferring though reinforcement and steel fibers. Moreover, the rust can block the channel of pore solution, thus increasing the electrical resistivity [52,53]. Furthermore, the electrical resistivity of MPC-RPC is decreased by the increase in the dosage of steel fibers due to the improved steel fibers' network.   Figure 11. The ultrasonic velocity of MPC-RPC during NaCl F-T cycles. Figure 12 presents the electrical resistivity of MPC-RPC. It is apparent from Figure 12 that the electrical resistivity increases with the increasing NaCl F-T cycles(N). The relationship between electrical resistivity and the number of F-T cycles is deduced as quadratic function. The fitting results are exhibited in Table 6. As shown in Table 6, the fitting degrees are higher than 0.99, thus proving the accuracy of the fitting equations. This is mainly because the NaCl F-T cycles can increase the F-T cracks which blocks the migration of conductive particles and increases the electrical resistivity of MPC-RPC [49,50]. Moreover, the NaCl F-T cycles can accelerate the corrosion of reinforcement and steel fibers. The rust inner MPC-RPC can prevent the electron transferring though reinforcement and steel fibers. Moreover, the rust can block the channel of pore solution, thus increasing the electrical resistivity [52,53]. Furthermore, the electrical resistivity of MPC-RPC is decreased by the increase in the dosage of steel fibers due to the improved steel fibers' network. Decreasing rate(%) 0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0% Figure 11. The ultrasonic velocity of MPC-RPC during NaCl F-T cycles. Figure 12 presents the electrical resistivity of MPC-RPC. It is apparent from Figure  12 that the electrical resistivity increases with the increasing NaCl F-T cycles(N). The relationship between electrical resistivity and the number of F-T cycles is deduced as quadratic function. The fitting results are exhibited in Table 6. As shown in Table 6, the fitting degrees are higher than 0.99, thus proving the accuracy of the fitting equations. This is mainly because the NaCl F-T cycles can increase the F-T cracks which blocks the migration of conductive particles and increases the electrical resistivity of MPC-RPC [49,50]. Moreover, the NaCl F-T cycles can accelerate the corrosion of reinforcement and steel fibers. The rust inner MPC-RPC can prevent the electron transferring though reinforcement and steel fibers. Moreover, the rust can block the channel of pore solution, thus increasing the electrical resistivity [52,53]. Furthermore, the electrical resistivity of MPC-RPC is decreased by the increase in the dosage of steel fibers due to the improved steel fibers' network.    The mass loss rate of reinforced MPC-RPC during NaCl D-W alternations (n) is exhibited in Figure 13. As can be seen in Figure 13, the mass loss rates of all curves increase as a quadratic function with the NaCl D-W alternations. The fitting results are shown in Table 7. It can be found in Table 7, the fitting degrees are higher than 0.99, therefore, the fitting equation is reasonable. This is due to the fact that the NaCl D-W alternations can increase crystallization stress' effect thus increasing the spalling of MPC-RPC and decreasing the following mass. Moreover, the NaCl D-W alternations lead to accelerating the corrosion of reinforcement and steel fibers, the rust by corrosion can induce the spalling on the surface of MPC-RPC, which decreases the mass [54,55]. The mass loss rate of reinforced MPC-RPC during NaCl D-W alternations (n) is exhibited in Figure 13. As can be seen in Figure 13, the mass loss rates of all curves increase as a quadratic function with the NaCl D-W alternations. The fitting results are shown in Table 7. It can be found in Table 7, the fitting degrees are higher than 0.99, therefore, the fitting equation is reasonable. This is due to the fact that the NaCl D-W alternations can increase crystallization stress' effect thus increasing the spalling of MPC-RPC and decreasing the following mass. Moreover, the NaCl D-W alternations lead to accelerating the corrosion of reinforcement and steel fibers, the rust by corrosion can induce the spalling on the surface of MPC-RPC, which decreases the mass [54,55].   Figure 14 demonstrates the ultrasonic velocity of MPC-RPC during NaCl D-W alternations. It can be noticed from Figure 14 that the ultrasonic velocity of MPC-RPC decreases with the increasing NaCl D-W alternations due to the increased inner cracks by NaCl D-W alternations. Moreover, the increased steel fibers' dosages can form dense networks inner MPC-RPC, which increases the ultrasonic velocity of MPC-RPC.    Figure 15 shows the electrical resistivity of MPC-RPC during NaCl D-W alternations. As depicted in Figure 15, the electrical resistivity of MPC-RPC increases with the increasing number of NaCl D-W alternations. This is due to the fact that the NaCl D-W cracks induced by NaCl D-W alternations can reduce the transmission speed of conductive particles, therefore, the electrical resistivity of MPC-RPC is increased by the NaCl D-W alternations [55]. Table 8 illustrates the fitting results. As illustrated in Figure 15 and Table 8, the relationship between electrical resistivity of MPC-RPC and the number of NaCl D-W alternations fits well with quadratic function. Moreover, the corrosion degree of steel fibers and steel bars are increased by NaCl D-W alternations, resulting in a higher electrical resistivity of MPC-RPC [56,57]. Furthermore, the electrical resistivity of MPC-RPC is decreased with the increase in the increasing amount of steel fibers, due to the improved conductive networks by steel fibers.    Figure 15 shows the electrical resistivity of MPC-RPC during NaCl D-W alternations. As depicted in Figure 15, the electrical resistivity of MPC-RPC increases with the increasing number of NaCl D-W alternations. This is due to the fact that the NaCl D-W cracks induced by NaCl D-W alternations can reduce the transmission speed of conductive particles, therefore, the electrical resistivity of MPC-RPC is increased by the NaCl D-W alternations [55]. Table 8 illustrates the fitting results. As illustrated in Figure 15 and Table 8, the relationship between electrical resistivity of MPC-RPC and the number of NaCl D-W alternations fits well with quadratic function. Moreover, the corrosion degree of steel fibers and steel bars are increased by NaCl D-W alternations, resulting in a higher electrical resistivity of MPC-RPC [56,57]. Furthermore, the electrical resistivity of MPC-RPC is decreased with the increase in the increasing amount of steel fibers, due to the improved conductive networks by steel fibers.    Figure 15 shows the electrical resistivity of MPC-RPC during NaCl D-W alternations. As depicted in Figure 15, the electrical resistivity of MPC-RPC increases with the increasing number of NaCl D-W alternations. This is due to the fact that the NaCl D-W cracks induced by NaCl D-W alternations can reduce the transmission speed of conductive particles, therefore, the electrical resistivity of MPC-RPC is increased by the NaCl D-W alternations [55]. Table 8 illustrates the fitting results. As illustrated in Figure 15 and Table 8, the relationship between electrical resistivity of MPC-RPC and the number of NaCl D-W alternations fits well with quadratic function. Moreover, the corrosion degree of steel fibers and steel bars are increased by NaCl D-W alternations, resulting in a higher electrical resistivity of MPC-RPC [56,57]. Furthermore, the electrical resistivity of MPC-RPC is decreased with the increase in the increasing amount of steel fibers, due to the improved conductive networks by steel fibers.    Figure 16 depicts the AC impedance spectrum curves of reinforced MPC-RPC. The AC impedance spectrum curves consist of real part and imaginary part. The real part represents the electrical resistance. Meanwhile, the imaginary part refers to the electrical reactance. The imaginary parts of all curves firstly decrease and then increase with the increasing real part. As illustrated in Figure 16, the values of extreme point move from right to the left when the amount of steel fibers is increased, indicating enhanced electrical conduction. Moreover, it can be found in Figure 16, the NaCl F-T cycles and NaCl D-W alternations lead to increasing the real parts' values of the extreme point. This is due to the increased electrical resistance by NaCl F-T cycles and NaCl D-W alternations, reflecting that the corrosion degree of reinforcement has been accelerated [55]. Furthermore, the increasing rate the values of extreme point by NaCl D-W alternations are higher than that by NaCl F-T cycles.  Figure 16 depicts the AC impedance spectrum curves of reinforced MPC-RPC. The AC impedance spectrum curves consist of real part and imaginary part. The real part represents the electrical resistance. Meanwhile, the imaginary part refers to the electrical reactance. The imaginary parts of all curves firstly decrease and then increase with the increasing real part. As illustrated in Figure 16, the values of extreme point move from right to the left when the amount of steel fibers is increased, indicating enhanced electrical conduction. Moreover, it can be found in Figure 16, the NaCl F-T cycles and NaCl D-W alternations lead to increasing the real parts' values of the extreme point. This is due to the increased electrical resistance by NaCl F-T cycles and NaCl D-W alternations, reflecting that the corrosion degree of reinforcement has been accelerated [55]. Furthermore, the increasing rate the values of extreme point by NaCl D-W alternations are higher than that by NaCl F-T cycles.  Figure 17 shows the equivalent circuit diagram of reinforced MPC-RPC. The electric circuit of reinforced MPC-RPC is consisted of three parallel electrical components (the parallel electrical resistance and reactance of passive film, steel fibers and pore solution), as detailed in Figure 17. The corresponding Chi-squared is lower than 0.01, indicating the rationality of equivalent circuit diagram.  Figure 18 shows the electrical resistivity calculated by the equivalent circuit diagram of Figure 17. As can be seen in observed from Figure 18, the electrical resistivity of passive film increases when the addition of steel fibers increases from 0% to 0.5%, the electrical resistance increases with amount of steel fibers used, due to the increased electrochemical corrosion of inner steel bars [55]. However, when the dosages of steel fibers increase from 0.5% to 3.0%, the electrical resistivity of passive film decreases with the increasing steel fibers. This is attributed to the improving effect of steel fibers on the corrosion resistance of steel bars. Finally, it can be found that the electrical resistivity of the passive film of the specimens after 30 NaCl D-W alternations is higher than that after 300 NaCl F-T cycles. Therefore, the steel bars inner MPC-RPC corrode more seriously after 30 NaCl D-W alternations than that after 300 NaCl F-T cycles.  Figure 17 shows the equivalent circuit diagram of reinforced MPC-RPC. The electric circuit of reinforced MPC-RPC is consisted of three parallel electrical components (the parallel electrical resistance and reactance of passive film, steel fibers and pore solution), as detailed in Figure 17. The corresponding Chi-squared is lower than 0.01, indicating the rationality of equivalent circuit diagram.  Figure 17 shows the equivalent circuit diagram of reinforced MPC-RPC. The electric circuit of reinforced MPC-RPC is consisted of three parallel electrical components (the parallel electrical resistance and reactance of passive film, steel fibers and pore solution), as detailed in Figure 17. The corresponding Chi-squared is lower than 0.01, indicating the rationality of equivalent circuit diagram.  Figure 18 shows the electrical resistivity calculated by the equivalent circuit diagram of Figure 17. As can be seen in observed from Figure 18, the electrical resistivity of passive film increases when the addition of steel fibers increases from 0% to 0.5%, the electrical resistance increases with amount of steel fibers used, due to the increased electrochemical corrosion of inner steel bars [55]. However, when the dosages of steel fibers increase from 0.5% to 3.0%, the electrical resistivity of passive film decreases with the increasing steel fibers. This is attributed to the improving effect of steel fibers on the corrosion resistance of steel bars. Finally, it can be found that the electrical resistivity of the passive film of the specimens after 30 NaCl D-W alternations is higher than that after 300 NaCl F-T cycles. Therefore, the steel bars inner MPC-RPC corrode more seriously after 30 NaCl D-W alternations than that after 300 NaCl F-T cycles.  Figure 18 shows the electrical resistivity calculated by the equivalent circuit diagram of Figure 17. As can be seen in observed from Figure 18, the electrical resistivity of passive film increases when the addition of steel fibers increases from 0% to 0.5%, the electrical resistance increases with amount of steel fibers used, due to the increased electrochemical corrosion of inner steel bars [55]. However, when the dosages of steel fibers increase from 0.5% to 3.0%, the electrical resistivity of passive film decreases with the increasing steel fibers. This is attributed to the improving effect of steel fibers on the corrosion resistance of steel bars. Finally, it can be found that the electrical resistivity of the passive film of the specimens after 30 NaCl D-W alternations is higher than that after 300 NaCl F-T cycles. Therefore, the steel bars inner MPC-RPC corrode more seriously after 30 NaCl D-W alternations than that after 300 NaCl F-T cycles.

Conclusions
This paper aims to develop the rapid repairing cement-based material named MPC-RPC. The corrosion resistance of reinforced MPC-RPC exposed to the environment of NaCl F-T cycles and D-W alternations is systematically studied. The working performance and mechanical properties of MPC-RPC are investigated. The corresponding corrosion resistance of reinforced MPC-RPC exposed to the environment of is obtained. The conclusions are derived as follows.
The slump flow and the setting time of fresh MPC-RPC are decreased by an increase in the dose of steel fibers. The lowest slump flow and the setting time are 33.2 min and 121.4 mm.
The addition of steel fibers demonstrates positive effect on the flexural and compressive strengths of hardened MPC-RPC. The flexural strength, the compressive strength and ultrasonic velocity of RPC cured for 3 h are higher than 45% of the MPC-RPC cured for 28 d. The flexural strength, the compressive strength and ultrasonic velocity of MPC-RPC cured for 7 d are higher than 85% of the specimens cured for 28 d. The increasing rate of flexural strength by steel fibers of MPC-RPC cured for 3 h is 0%~156.9%, meanwhile, when the curing ages are 1 d, 3 d and 28 d, the increasing rates are 0%~119.4%, 0%~61.4% and 0%~50%, respectively. Additionally, the increasing rates of compressive strength by steel fibers of specimens cured for 3 h, 1 d, 3 d and 28 d are 0%~30.4%, 0%~32.7%, 0%~64.8% and 0%~50%, respectively. The maximum increasing rate by steel fibers is 18.3%.
When the reinforced MPC-RPC is exposed to the NaCl F-T cycles and NaCl D-W alternations, the mass loss rate and the electrical resistivity increase in the form with the numbers of NaCl F-T cycles and NaCl D-W alternations. The electrical mechanism can be explained by an equivalent circuit, which is tandem parallel electrical resistance and reactance of pore solution, steel fibers and passive film. As obtained from the results of the ultrasonic velocity, the mass loss rate, and the AC impedance spectrum, the steel fibers can improve the corrosion resistance of reinforced MPC-RPC, except 0.5% steel fibers. Moreover, the reinforced MPC-RPC corrodes more seriously exposed to NaCl D-W alternations than NaCl F-T cycles.
Appendix A