sustainability

Article The Sustainability Performance of Reinforced Concrete Structures in Tunnel Lining Induced by Long-Term Coastal Environment

Zhiqiang Zhang 1,2, Ruikai Gong 1,2, Heng Zhang 1,2,* and Wanping He 1 1 School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China; [email protected] (Z.Z.); [email protected] (R.G.); [email protected] (W.H.) 2 Key Laboratory of Transportation Tunnel Engineering, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, China * Correspondence: [email protected]; Tel.: +86-028-8763-4386



Received: 25 March 2020; Accepted: 9 May 2020; Published: 12 May 2020


Abstract: At present, the damage caused by steel corrosion to structures has become a serious problem all over the world. In order to study the mechanical behaviors of tunnel lining structure system under the corrosive environment to rebars, first, the bending tests were performed to investigate the crack propagation behavior and structural bearing capacity of the reinforced concrete bending members degraded by corrosion. Secondly, the pull-out tests were performed to investigate the degradation of bonding strength between corroded rebars and the concrete. Finally, on the basis of the findings from the pull-out tests, a 3-D finite element bond-slip model of reinforced concrete lining structure has been established to simulate the changes of bearing capacity and durability of tunnel reinforced concrete lining under different corrosion degrees. The research has revealed: Rebar corrosion is the most important factor affecting concrete and steel corrosion. As the conversion rust rate increases, the ultimate drawing force continues to decrease. With the increase of the corrosion rate, the deflection of the specimen when it is destroyed becomes smaller, the cracking load becomes smaller and the bearing capacity also decreases. As the degree of corrosion increases, the overall deformation of the tunnel increases, and the overall safety of the lining structure decreases. The corner position is the most prone to problems after the lining structure is corroded, so pay more attention. As well, the safety of the lining structure will be basically lost when the final corrosion rate of the steel bars is greater than 30%. The findings of this research can be used to evaluate the corrosion degree of tunnel reinforced concrete lining structure and support the durability design of new tunnel concrete lining structure.

Keywords: coastal environment; tunnel engineering; lining structure; steel corrosion; bond-slip

1. Introduction With the social and economic development and the continuous improvement of people’s requirements for quality of life, the scale and quantity of traffic and the engineering construction projects in China tend to increase on the whole [1–4]. At present, China has developed into one of the countries with the largest scale, the largest number, the most complex structural forms and the most complex construction technologies of tunnels and underground projects in the world [5–9]. Both the tunnel construction process and the operation stage are facing great challenges [10–12]. Reinforced concrete structure has been the first choice for lining structures of tunnels in offshore areas due to their advantages such as high rigidity, high bearing capacity, easy placement, economical cost, and wide application. However, during the operation of the offshore tunnels, due to the combined action of environmental and load factors, the corrosion of the tunnel reinforced concrete lining structures

Sustainability 2020, 12, 3946; doi:10.3390/su12103946 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 3946 2 of 23 becomes more and more serious and the performance of the lining structures decreases continuously as the service life proceeds, and eventually the lining structures will be damaged, which leads to a substantial increase in the maintenance and operation costs of the lining structures and constitutes safety problems. The failures of concrete structures caused by rebar corrosion is a consequential and urgent problem to be solved, and also an important subject for durability research into reinforced concrete structures. Therefore, it is of great engineering significance to investigate the impact of the corrosion of the tunnel lining structures in offshore environment on their durability and bearing capacity [13–15]. The current research on corroded reinforced concrete structures mainly focuses on two aspects: The mechanical properties of rebars in corrosive environment and the overall performance of reinforced concrete structures in corrosive environment. The research on the impacts of corrosive environment on the properties of rebars can be divided into two directions: first, the experiments to investigate the mechanical properties of corroded rebars and the qualitative description and research of constitutive model; second, based on the findings from those experiments, the regression formula of the degradation of the mechanical properties of corroded rebars is analyzed and summarized. Yuan et al. [16,17] studied the mechanical properties of rebars with different corrosion conditions through comparative analysis of findings from laboratory tests and results from numerical simulation. Zhang et al. [18] revealed the regression formulas of such mechanical characteristics as the ultimate strength, elastic modulus, yield strength and ultimate strain, and the corrosion ratio of the of corroded rebars through mechanical experiments on 35 corroded rebars in natural state, 76 corroded rebars obtained from actual structure members and 156 corroded rebars of which the corrosion is accelerated by electric current in the room. Hui et al. [19] showed the statistical formulas of yield strength, tensile strength, and corrosion ratio of corroded rebars through a large number of mechanical tests on corroded rebars. Blomfors [20] modified the FIB model code based on the local bond stress-slip relationship to accommodate the corrosion effect, and further established a model to assess the anchorage capacity of corroded rebars. Liu et al. [21] clarified a statistical model of rebar corrosion through five years of outdoor tests, which takes into account the CI content, temperature, concrete resistance and corrosion time. Based on the principles of electrochemical reaction, mass conservation law and Fick diffusion law, Bazantl [22] proposed a Bzant model specific to reinforced structures in marine environment, which accommodates the effect of concentration polarization. The indoor rapid tests on corroded rebars can be completed by artificial simulation of corrosive environment or by electric current. When the corrosion ratio is small, the corrosion has little effect on mechanical properties of corroded rebars; when the corrosion ratio is large, the yield strength, ultimate strength, and ultimate elongation of corroded rebars decrease. Based on this research, this paper uses the method of electrochemical corrosion to accelerate rebar corrosion, and then to study the performance of rebars under different corrosion degrees. As for the overall structural performance of reinforced concrete structures, the laboratory test analysis and finite element simulation analysis are generally used [23–27]. The vast majority of scholars and engineering experts combine the two approaches and verify the results from one with that from the other, so as to obtain conclusions and measures that are more satisfactory in meeting the needs of the actual projects [28–31]. Fan et al. [32,33] adopted the "Replacing Structural Members" test method to conduct tests and draw the conclusion that the main factors affecting the bond strength between rebars and concrete are the corrosion ratio of rebar and the performance of concrete after corrosion. Yuan et al. [34] studied the degradation mechanism of structural properties of corroded reinforced concrete beams through tests and established the degradation models of stress-strain relationship of corroded rebars and bond stress-slip relationship between corroded rebars and concrete. In the laboratory, Oksada [35] conducted tests by applying accelerating corrosion to beam test pieces in laboratory until longitudinal cracks occurred. The results showed that the bearing capacity of corroded reinforced concrete beams decreased significantly, and the strength decreased sharply when repeated loads were applied. Castel [36] carried out mechanical experiments on corroded rebars and concluded that the prediction of bearing capacity of corroded reinforced concrete members should first based Sustainability 2020, 12, 3946 3 of 23 on the loss ratio of section of corroded rebars and the loss ratio of bonding force of two kinds of materials. Berra [37] applied temperature stress to realize the effect of the volume expansion of corrosion products based on the principle of thermal expansion and cold contraction and predicted the bond–slip relationship between rebars and concrete by synthesizing various constraints and the positions of rebars. Through a large number of tests and analysis, we can basically draw the main factors that affect the bond strength between rebars and concrete: When the rebars is corroded to a certain degree, its corrosion products remain in the gap between rebars and concrete. When the corrosion ratio is small, these chips are helpful to the bonding performance. However, a large amount of these chips will directly affect the friction coefficient between the two types of materials and are several times the volume of the original rebar, so that the concrete structure around the rebar is subjected to hoop tension. When the amount of corrosion products accumulates to a certain degree, and the expansion force generated exceeds the ultimate tensile strength of concrete, the concrete around rebars will have longitudinal cracks, resulting in the loss of the chemical bonding force between rebars and concrete. When the crack develops to penetrate the protective layer, the concrete of protective layer begins to fall off, so that the rebar loses its restraint and protection, and the mechanical bite force is also lost. Meanwhile, the rebar is also completely exposed to external environment, which accelerates the corrosion, and finally the reinforced concrete structure building loses its durability and is destroyed. The abovementioned activities indicate that the research on the properties of corroded reinforced concrete materials and the overall structure has achieved fruitful results, and the outcomes have found wide applications in practical projects [38–43]. However, few is on the durability and bearing capacity of reinforced concrete lining structure. The Yanlangshan Tunnel was taken as the research subject in this paper, which is located in Zhuhai Airport Expressway, Guangdong Province, in China. The tunnel is located in the offshore position. After long-term operation, the possibility of corrosive water appears in the surrounding strata is higher. The supporting structure is almost immersed in corrosive water, and the corrosion deterioration of concrete structure is very easy to occur. In view of this, firstly, crack propagation behavior and structural bearing capacity of reinforced concrete bending members degraded by corrosion were obtained through bending tests. Secondly, the degradation of bonding strength between corroded rebars and concrete is analyzed through the pull-out test of rebars. Based on this, a 3-D finite element bond-slip model of reinforced concrete lining structures is established. Finally, based the results from numerical simulations, the overall structural performance changes and ultimate corrosion ratio caused by rebar corrosion under different surrounding rock pressure loads were analyzed. The research results reveal the mechanical behaviors of tunnel lining structure support system degraded by corrosion, which are of important guiding significance and application value for the research of tunnel engineering durability. At present, the research on the influence of corrosion of steel bars on performance is mainly divided into two directions: First, the mechanical properties of corroded steel bars and the qualitative description and research of constitutive models; second, on the basis of experiments, the mechanical properties of corroded steel bars are analyzed and summarized deteriorated regression formula [44]. The focus is on studying changes in the properties of a single material. This article is based on the previous research, focusing on the effect of steel corrosion on the overall structure of the concrete structure, and applied the research to practical engineering problems. Based on indoor experiment, the numerical simulation calculations are mutually confirmed in this paper to obtain the general laws affecting the durability and bearing capacity of reinforced concrete structure. The specific research contents are: (1) Accelerated by electric current, the corrosion process and hazards of rebars are explored. The corrosion degree of reinforced concrete specimens caused by different concrete labels, protective layer thickness, and rebar specifications is also compared; Sustainability 2020, 12, 3946 4 of 23

(2) After conducting electrified accelerated corrosion on the test pieces of different concrete labels, protective layer thickness, and rebar specifications, the eccentric compression method is used to test structural bearing capacity, as well as flexural strength, deflection, and concrete adhesion. (3) Based on the indoor test law of corrosion effects, the change of structural bearing capacity of the tunnel secondary lining under different degrees of corrosion is simulated through the numerical simulation software such as ANSYS with the adhesive force degradation model. The law of lining structure bearing capacity with different corrosion ratios under the pressure of surrounding rock and the law of concrete protective layer crack development are obtained.

2. Compression-Bending Performance of Corroded Reinforced Concrete Structures The main structure in tunnel engineering is the reinforced concrete lining. In its long-term operation process, due to the corrosion of the external environment and the deterioration of the material itself, the corrosion damage of the internal steel bars will generally occur, which will cause serious damage to the structure itself and seriously affect the durability of the structure. Since reinforcement corrosion in reinforced concrete components is a complete electrochemical process, electrochemical testing is an effective way to accurately reflect its essential process. Its advantages are fast test speed, high sensitivity, continuous tracking, and in-situ testing. Therefore, it is of great significance to study the crack propagation and deformation of lining through laboratory tests under corrosive conditions. However, due to the large size of the lining structure, it is impossible to carry out full-size accelerated corrosion test, and it is difficult to meet the high requirements for the loading equipment. Comprehensively considering the action mode and mechanical state of the tunnel lining structure, as well as the existing test conditions and operation level, the reinforced concrete compression-bending members are chosen as the simulation test pieces to study the durability of the tunnel lining structure.

2.1. Materials The concrete composition of the experiment is based on the concrete of Zhuhai project, and the relevant data are all based on the actual data of Zhuhai project. The grade design of plain concrete is C30 (compressive strength is 30 MPa), and mixture proportions are listed in Table1. Cementitious materials made from P.O42.5R cement and Class I Fly Ash were employed in the test, and the water-cement ratio in the mix proportions was 0.45. The coarse aggregate is gravel with a maximum particle size of 31.5 mm and the fine aggregate is natural river sand.

Table 1. Mix proportions.

Unit Weight (kg/m3) Concrete Grade W/B Cement Water Fly Ash Fine Aggregate Coarse Aggregate C30 0.45 309 164 55 1048 824

2.2. Compression-Bending Test Scheme According to the cover depth and the size of reinforcing bar, the specimens are divided into four groups, 12 specimens in each group. The details of the specimens are listed in Table2. For each test specimens, the specimens no. is named after the cover depth, the reinforcing bar dimension, in which the C30 denotes the grade design of plain concrete, the number 35 (or 50) represents the cover depth, the second number 10 (or 14) represents the reinforcing bar diameter. For example, specimen C30-35-10 refers to a specimen with 35mm cover depth, 10mm reinforcing bar diameter. Sustainability 2020, 12, 3946 5 of 23

Table 2. Details of test specimens.

Reinforcing Bar Specimens Length L Specimens Width W Cover Depth C Beam no. Dimension Sustainability 2019, 11, x FOR PEER REVIEW(mm) (mm) (mm) 5 of 24 D (mm) C30-35-10 200 200 35 10 The sizes C30-35-14of the reinforced 200 concrete compression 200‐bending test 35 pieces used in 14 this research are as shown in FigureC30-50-10 1 and Figure 2, 200 and the concrete 200 casting mold 50is as shown in Figure 10 3. The concrete C30-50-14 200 200 50 14 beams are equipped with sufficient stirrups to ensure the bending failure of the beams instead of shear failure. After the curing of test piece is completed and the initial corrosion current density is The sizes of the reinforced concrete compression-bending test pieces used in this research are as determined,shown the in Figuresanode1 is and started2, and theto concretepass the casting current mold of is the as shownspecified in Figure strength3. The and concrete time. beams This process is implementedare equipped by with an suammeterfficient stirrups and toa ensureDC power the bending supply. failure The of the ammeter beams instead is used of shear to failure. measure the current. AfterThe concrete the curing specimen of test piece is is placed completed in a and container the initial containing corrosion current 5% NaCl density solution, is determined, so the bottom of specimenthe anode can isjust started be in to contact pass the currentwith water. of the specified The direction strength andof current time. This flow process is adjusted, is implemented so the rebar by an ammeter and a DC power supply. The ammeter is used to measure the current. The concrete in concrete beam act as anodes, and the stainless steel plates, which covers all sides of the test piece, specimen is placed in a container containing 5% NaCl solution, so the bottom of specimen can just be placed onin concrete contact with beams water. act The as direction cathodes. of current This arrangement flow is adjusted, makes so the rebar the current in concrete uniform beam act throughout as the entireanodes, length and of the rebar. stainless steel plates, which covers all sides of the test piece, placed on concrete beams In thisact as test, cathodes. the corrosion This arrangement of reinforced makes the concrete current uniform members throughout is accelerated the entire lengthby current of rebar. in groups, and the groupingIn this is test, based the corrosion on time of of reinforced applying concrete a constant members current. is accelerated The time by of current applying in groups, the constant and the grouping is based on time of applying a constant current. The time of applying the constant current is different, which leads to a big difference in the degree of component corrosion. In the test current is different, which leads to a big difference in the degree of component corrosion. In the test results, results,we assume we assume the the degree degree of component component corrosion corrosion which haswhich not been has accelerated not been by theaccelerated current is by the current iszero, zero, and and the required the required results are results obtained are by obtained comparing by them comparing with the results them of with each group the results [45,46]. of each group [45,46].

Figure 1.Figure Reinforced 1. Reinforced Concrete Concrete Compression Compression-Bending‐Bending MemberMember Elevation Elevation Size Size and Steeland Rebar Steel Rebar DistributionDistribution Diagram. Diagram.

Sustainability 2019, 11, x FOR PEER REVIEW 5 of 24

The sizes of the reinforced concrete compression‐bending test pieces used in this research are as shown in Figure 1 and Figure 2, and the concrete casting mold is as shown in Figure 3. The concrete beams are equipped with sufficient stirrups to ensure the bending failure of the beams instead of shear failure. After the curing of test piece is completed and the initial corrosion current density is determined, the anode is started to pass the current of the specified strength and time. This process is implemented by an ammeter and a DC power supply. The ammeter is used to measure the current. The concrete specimen is placed in a container containing 5% NaCl solution, so the bottom of specimen can just be in contact with water. The direction of current flow is adjusted, so the rebar in concrete beam act as anodes, and the stainless steel plates, which covers all sides of the test piece, placed on concrete beams act as cathodes. This arrangement makes the current uniform throughout the entire length of rebar. In this test, the corrosion of reinforced concrete members is accelerated by current in groups, and the grouping is based on time of applying a constant current. The time of applying the constant current is different, which leads to a big difference in the degree of component corrosion. In the test results, we assume the degree of component corrosion which has not been accelerated by the current is zero, and the required results are obtained by comparing them with the results of each group [45,46].

Figure 1. Reinforced Concrete Compression‐Bending Member Elevation Size and Steel Rebar DistributionSustainability Diagram.2020, 12, 3946 6 of 23

Sustainability 2019, 11, x FOR PEER REVIEW 6 of 24

Figure 2.2. ReinforcedReinforced Concrete Concrete Compression Compression-Bending‐Bending Member Member Section Section Size Size and and Steel Rebar Distribution Diagram.

Figure 3. Mold of Compression Compression-Bending‐Bending Test Pieces.

As an eccentric compressive stress needs to be applied to the piece in this test, to simulate the compression-bendingcompression‐bending modemode of of the the member, member, it it is is required required to to apply apply a linea line load load to theto the member. member. Based Based on theon the preliminary preliminary preparations, preparations, in order in order to ensure to ensure that that the the failure failure mode mode of the of membersthe members is mainly is mainly the yieldthe yield of steel of bars,steel itbars, is required it is required to apply to the apply large the eccentric large eccentric compression compression to the members. to the Therefore,members. theTherefore, compression the compression eccentricity eccentricity of the members of the is setmembers as 110 mm,is set and as 110 the mm, required and linethe required load is realized line load by theis realized shaft support. by the shaft The support. loading equipment The loading used equipment is 200 T used hydraulic is 200 loading T hydraulic test machineloading test as shown machine in Figureas shown4. Due in Figure to the 4. limitation Due to the of testlimitation conditions, of test this conditions, test is conducted this test is mainly conducted to study mainly the etoff studyect of corrosionthe effect ofof steelcorrosion bars on of deterioration steel bars on of deterioration the overall performance of the overall of theperformance members. of Therefore, the members. in the loadingTherefore, failure in the process loading of the failure members, process a large of the eccentricity members, is a applied large eccentricity to the members, is applied so that to when the reachingmembers, the so ultimatethat when state, reaching the members the ultimate will bestate, subjected the members to failure will resulting be subjected from to the failure tensile resulting yield of thefrom internal the tensile tensile yield steel of bars, the i.e.internal the large tensile eccentricity steel bars, failure. i.e. the Thus, large the eccentricity deterioration failure. of the Thus, concrete the materialsdeterioration caused of the by galvanicconcrete corrosionmaterials iscaused neglected by galvanic to ensure corrosion that the membersis neglected are to not ensure damaged that due the tomembers crushing are of not concrete damaged in the due compression to crushing zone. of concrete in the compression zone.

(a) (b)

Figure 4. Loading Device for the Test. (a): Side view of loading device; (b): Front view of loading device Sustainability 2019, 11, x FOR PEER REVIEW 6 of 24

Figure 2. Reinforced Concrete Compression-Bending Member Section Size and Steel Rebar Distribution Diagram.

Figure 3. Mold of Compression-Bending Test Pieces.

As an eccentric compressive stress needs to be applied to the piece in this test, to simulate the compression-bending mode of the member, it is required to apply a line load to the member. Based on the preliminary preparations, in order to ensure that the failure mode of the members is mainly the yield of steel bars, it is required to apply the large eccentric compression to the members. Therefore, the compression eccentricity of the members is set as 110 mm, and the required line load is realized by the shaft support. The loading equipment used is 200 T hydraulic loading test machine as shown in Figure 4. Due to the limitation of test conditions, this test is conducted mainly to study the effect of corrosion of steel bars on deterioration of the overall performance of the members. Therefore, in the loading failure process of the members, a large eccentricity is applied to the members, so that when reaching the ultimate state, the members will be subjected to failure resulting from the tensile yield of the internal tensile steel bars, i.e. the large eccentricity failure. Thus, the Sustainabilitydeterioration2020 of, 12 the, 3946 concrete materials caused by galvanic corrosion is neglected to ensure that7 ofthe 23 members are not damaged due to crushing of concrete in the compression zone.

(a) (b)

FigureFigure 4. 4Loading. Loading Device Device for for the the Test. Test. (a): (a Side): Side view view of loading of loading device; device (b):; ( Frontb): Front view view of loading of loading device device Since the linear polarization method in the electrochemical method for testing rebar corrosion in concrete components has strict requirements on the accuracy of instruments, the Corr Test (corrosion electrochemical test system) is selected for this test to complete the corrosion rate test [47]. In order to test the strain value in the middle of the concrete, five paper-based strain gauges are arranged at both sides of the middle part of the concrete, respectively, in the tension and compression zones of the concrete (as shown in Figure5). To test the lateral displacement of the members, three displacement meters are arranged at the front and back sides of the members, which are located at the upper and lower surface of the test piece at a distance of 10 cm and in the middle of the test piece, respectively, so as to study the deformation performance of the member under loading and to compare the changes before and after corrosion (as shown in Figure5). The measurement with the displacement meters is not only helpful to obtain the lateral displacement change of the member, but also helpful to accurately estimate the ultimate bearing capacity of the member, that is, if the load increases slowly or even decreases, while the displacement value still rises sharply, then this load can be considered as the ultimate value of the member. Also, the test shows the structural capacity under compression with large eccentricity. The compression eccentricity of the component is 110 mm, and the lateral displacement of the component is tested. Before the performance is officially loaded, the component is preloaded twice, graded to 60 kN, and then unloaded to zero step by step to eliminate the impact of the bearing offset and eccentricity on the bearing of the component in order to obtain more accurate test data. After pre-loading, the load is graded at a loading speed of no more than 200 N/s, each load is 20 kN, and each load is added to the first-level load for several minutes. Crack observation and mapping are performed, and the time when the load is added to an integer multiple tonnage. When approaching the ultimate load, continue to load slowly until failure. Sustainability 2019, 11, x FOR PEER REVIEW 7 of 24

Since the linear polarization method in the electrochemical method for testing rebar corrosion in concrete components has strict requirements on the accuracy of instruments, the Corr Test (corrosion electrochemical test system) is selected for this test to complete the corrosion rate test [47]. In order to test the strain value in the middle of the concrete, five paper‐based strain gauges are arranged at both sides of the middle part of the concrete, respectively, in the tension and compression zones of the concrete (as shown in Figure 5). To test the lateral displacement of the members, three displacement meters are arranged at the front and back sides of the members, which are located at the upper and lower surface of the test piece at a distance of 10 cm and in the middle of the test piece, respectively, so as to study the deformation performance of the member under loading and to compare the changes before and after corrosion (as shown in Figure 5). The measurement with the displacement meters is not only helpful to obtain the lateral displacement change of the member, but also helpful to accurately estimate the ultimate bearing capacity of the member, that is, if the load increases slowly or even decreases, while the displacement value still rises sharply, then Sustainabilitythis load can2020 be, 12 considered, 3946 as the ultimate value of the member. Also, the test shows the structural8 of 23 capacity under compression with large eccentricity.

Figure 5. Distribution Diagram of Stain Gauges and Displacement Meters.

2.3. AnalysisThe compression of Test Results eccentricity of the component is 110 mm, and the lateral displacement of the component(1) Analysis is tested. of Member Before the Deformation performance Deflection is officially loaded, the component is preloaded twice, gradedIn orderto 60 kN, to study and then the deformationunloaded to zero law of step bending by step members to eliminate with the di ffimpacterent corrosion of the bearing degrees offset as andthe load eccentricity increases, on the the load-deflection bearing of the curvescomponent of components in order to with obtain diff erentmore corrosionaccurate test degrees data. can After be predrawn‐loading, to explain the load intuitively. is graded The at load-deflection a loading speed curves of no of more the testthan pieces 200 N/ withs, each diff erentload specificationsis 20 kN, and Sustainabilityeachunder load diff 2019erentis added, 11 corrosion, x FOR to PEERthe conditions first REVIEW‐level areload extracted for several as shown minutes. in the Crack Figure observation6. and mapping8 of are24 performed, and the time when the load is added to an integer multiple tonnage. When approaching the ultimate load, continue to load slowly until failure. 14 14 C30‐35‐10 C30‐35‐10 C30‐35‐14 2.3. Analysis12 of Test Results 12 C30‐35‐14 C30‐50‐10 C30‐50‐10 ) 10(1) Analysis C30 of‐ 50Member‐14 Deformation Deflection) 10 C30‐50‐14 mm mm ( (

In8 order to study the deformation law of bending 8 members with different corrosion degrees as the load increases, the load‐deflection curves of components with different corrosion degrees can be 6 6 drawn to explain intuitively. The load‐deflection curves of the test pieces with different Movement specificationsMovement 4 under different corrosion conditions are4 extracted as shown in the Figure 6. 2 2

0 0

0 50 100 150 200 250 300 0 50 100 150 200 250 Load (kN) Load (kN)

(a) Standard Test Pieces (b) Test Pieces with 8‐day Corrosion (1–6)

FigureFigure 6. 6. LoadLoad-deflection‐deflection curves curves with with different different accelerated accelerated corrosion corrosion time. time.

AccordingAccording to to the the analysis analysis of of the the figures figures above, above, when when all all test test pieces pieces are are loaded loaded near near the the ultimate ultimate load,load, the the deflection deflection will will suddenly suddenly increase, increase, which which is mainly is mainly because because the failure the failure mode mode adopted adopted in this in testthis is test large is large eccentricity eccentricity failure, failure, that thatis, the is, thetest test pieces pieces are are damaged damaged due due to tothe the yield yield of of steel steel bars. bars. WhenWhen the steel rebarrebar yields, yields, there there will will be be large large tensile tensile deformation, deformation, which which will causewill cause the sudden the sudden change changeof the deflection of the deflection of the member. of the member. Besides, Besides, the deflection the deflection of the main of the tensile main rebar tensile test rebar piece withtest piece small withdiameter small is diameter relatively is large relatively when itlarge is damaged. when it Withis damaged. the increase With of the corrosion increase ratio, of corrosion the deflection ratio, of the the deflection of the test piece becomes smaller when it is damaged, which makes the ductility of the compression‐bending member smaller and makes the possibility of its brittle failure increase. This indicates that the steel bar has reached the yield, and the load corresponding to the steel bar yield decreases with the influence of corrosion. (2) Analysis of Overall Bearing Capacity of the Structure For the overall analysis of the test pieces, based on the initial cracking load and the ultimate load of the test pieces obtained in the compression‐bending test, the test data of the test pieces are shown in Figure 7, 8 and 9. Three sets of tests are conducted for each test piece, so the data presented is the average value of the data obtained from the three sets of tests. Among all the test pieces, the data of some specific test pieces is totally inconsistent with the test rules, especially the cracking load. Due to the construction technology, the internal concrete tamping degree and the errors of manual observation and record, there are some errors in the results, and such values are not taken into account.

90 C30‐35‐10 85 C30‐35‐14

) C30‐50‐10 kN

( C30‐50‐14 80 crack 75 first

at 70 Load 65

60 0123456789 Time (day)

Figure 7. Test Results of Cracking Load of C30 Concrete under Different Corrosion Degree. Conditions. Sustainability 2019, 11, x FOR PEER REVIEW 8 of 24

14 14 C30‐35‐10 C30‐35‐10 C30‐35‐14 12 12 C30‐35‐14 C30‐50‐10 C30‐50‐10 ) 10 C30‐50‐14 ) 10 C30‐50‐14 mm mm ( (

8 8

6 6 Movement Movement 4 4

2 2

0 0

0 50 100 150 200 250 300 0 50 100 150 200 250 Load (kN) Load (kN)

(a) Standard Test Pieces (b) Test Pieces with 8‐day Corrosion (1–6)

Figure 6. Load‐deflection curves with different accelerated corrosion time.

According to the analysis of the figures above, when all test pieces are loaded near the ultimate load, the deflection will suddenly increase, which is mainly because the failure mode adopted in this test is large eccentricity failure, that is, the test pieces are damaged due to the yield of steel bars. When the steel rebar yields, there will be large tensile deformation, which will cause the sudden Sustainabilitychange of the2020 ,deflection12, 3946 of the member. Besides, the deflection of the main tensile rebar test piece9 of 23 with small diameter is relatively large when it is damaged. With the increase of corrosion ratio, the deflection of the test piece becomes smaller when it is damaged, which makes the ductility of the test piece becomes smaller when it is damaged, which makes the ductility of the compression-bending compression‐bending member smaller and makes the possibility of its brittle failure increase. This member smaller and makes the possibility of its brittle failure increase. This indicates that the steel bar indicates that the steel bar has reached the yield, and the load corresponding to the steel bar yield has reached the yield, and the load corresponding to the steel bar yield decreases with the influence decreases with the influence of corrosion. of corrosion. (2) Analysis of Overall Bearing Capacity of the Structure (2) Analysis of Overall Bearing Capacity of the Structure For the overall analysis of the test pieces, based on the initial cracking load and the ultimate For the overall analysis of the test pieces, based on the initial cracking load and the ultimate load load of the test pieces obtained in the compression‐bending test, the test data of the test pieces are of the test pieces obtained in the compression-bending test, the test data of the test pieces are shown in shown in Figure 7, 8 and 9. Three sets of tests are conducted for each test piece, so the data presented Figures7–9. Three sets of tests are conducted for each test piece, so the data presented is the average is the average value of the data obtained from the three sets of tests. Among all the test pieces, the value of the data obtained from the three sets of tests. Among all the test pieces, the data of some data of some specific test pieces is totally inconsistent with the test rules, especially the cracking specific test pieces is totally inconsistent with the test rules, especially the cracking load. Due to the load. Due to the construction technology, the internal concrete tamping degree and the errors of construction technology, the internal concrete tamping degree and the errors of manual observation manual observation and record, there are some errors in the results, and such values are not taken and record, there are some errors in the results, and such values are not taken into account. into account.

90 C30‐35‐10 85 C30‐35‐14

) C30‐50‐10 kN

( C30‐50‐14 80 crack 75 first

at 70 Load 65

60 0123456789 Time (day) SustainabilityFigure 2019 7. Test, 11, Results x FOR PEER of Cracking REVIEW Load of C30 Concrete under Different Corrosion Degree. Conditions.9 of 24 Figure 7. Test Results of Cracking Load of C30 Concrete under Different Corrosion Degree. Conditions. 300 C30‐35‐10 C30‐35‐14 280 C30‐50‐10 ) 260 C30‐50‐14 kN (

240 load

220

Ultimate 200

180

160 0123456789 Time (day) Figure 8.8.Test Test Results Results of Ultimateof Ultimate Load Load of C30 of Concrete C30 Concrete under Di underfferent CorrosionDifferent DegreeCorrosion Conditions. Degree Conditions. Based on the data analysis of the test results in Figures7–9, it can be shown that: With the deepening of the corrosion degree of the steel bars, the bearing capacity of the members decreases continuously. The average ultimate load of C30-35-10 group test pieces is 227.33 kN under non-corrosion condition, and the ultimate bearing capacity of C30-35-10 group test pieces is only 166.67 kN under accelerated corrosion condition for 8 days, with a decrease by more than 26.63%. At the same time, it can be clearly found that the bearing capacity decline under corrosion condition for 4 days is significantly greater than that in the later period. The ultimate bearing capacity of C30-35-14 group test pieces decreases by 21.05% after four days of accelerated corrosion, decreased by 28.56% after six days of accelerated corrosion compared with that of non-corroded test pieces, and 35.38% after eight days of accelerated

Figure 9. Corrosion effect of steel bars.

Based on the data analysis of the test results in Figure 7, 8 and 9, it can be shown that: With the deepening of the corrosion degree of the steel bars, the bearing capacity of the members decreases continuously. The average ultimate load of C30‐35‐10 group test pieces is 227.33 kN under non‐corrosion condition, and the ultimate bearing capacity of C30‐35‐10 group test pieces is only 166.67 kN under accelerated corrosion condition for 8 days, with a decrease by more than 26.63%. At the same time, it can be clearly found that the bearing capacity decline under corrosion condition for 4 days is significantly greater than that in the later period. The ultimate bearing capacity of C30‐35‐14 group test pieces decreases by 21.05% after four days of accelerated corrosion, decreased by 28.56% after six days of accelerated corrosion compared with that of non‐corroded test pieces, and 35.38% after eight days of accelerated corrosion. According to the analysis of the causes, with the increase of days of accelerated galvanic corrosion, the corrosion degree of the steel bars increases gradually, the effective section and strength of steel bars will decrease gradually, due to which the bearing capacity of the steel bars will also decrease continuously. This process conforms to the rule that the bearing capacity of the reinforced concrete structure will gradually change with the corrosion of the steel bars. The cracking load also decreases with the increase of corrosion ratio. The initial cracking load of the members not under galvanic corrosion is about 80 kN; for the members with relatively low corrosion ratio, the initial cracking load is also about 80 kN; while the cracking load of the members with large corrosion ratio decreases obviously, which is 60–65 kN. Taking C30‐35‐10 group test pieces as an example, the cracking load is 84.67 kN under non‐corrosion condition, 82.67 kN under corrosion condition for 4 days, and decreases to 71.67 kN under the corrosion condition for 6 days, while the cracking load is only 64.33 kN under the corrosion condition for 8 days. This is mainly due to cracks in the concrete surrounding the reinforcement due to the expansion of the reinforcement, which reduces the cracking load. Sustainability 2019, 11, x FOR PEER REVIEW 9 of 24

300 C30‐35‐10 C30‐35‐14 280 C30‐50‐10 ) 260 C30‐50‐14 kN (

240 load

220

Ultimate 200 Sustainability 2020, 12, 3946 10 of 23 180

160 corrosion. According to the analysis0123456789 of the causes, with the increase of days of accelerated galvanic corrosion, the corrosion degree of the steel bars increases gradually, the effective section and strength of Time (day) steel bars will decrease gradually, due to which the bearing capacity of the steel bars will also decrease continuously.Figure 8. ThisTest processResults conformsof Ultimate to Load the rule of thatC30 theConcrete bearing under capacity Different of the Corrosion reinforced Degree concrete structureConditions. will gradually change with the corrosion of the steel bars.

Figure 9. Corrosion effect effect of steel bars.

The cracking load also decreases with the increase of corrosion ratio. The initial cracking load Based on the data analysis of the test results in Figure 7, 8 and 9, it can be shown that: With the of the members not under galvanic corrosion is about 80 kN; for the members with relatively low deepening of the corrosion degree of the steel bars, the bearing capacity of the members decreases corrosion ratio, the initial cracking load is also about 80 kN; while the cracking load of the members continuously. The average ultimate load of C30‐35‐10 group test pieces is 227.33 kN under with large corrosion ratio decreases obviously, which is 60–65 kN. Taking C30-35-10 group test pieces non‐corrosion condition, and the ultimate bearing capacity of C30‐35‐10 group test pieces is only as an example, the cracking load is 84.67 kN under non-corrosion condition, 82.67 kN under corrosion 166.67 kN under accelerated corrosion condition for 8 days, with a decrease by more than 26.63%. At condition for 4 days, and decreases to 71.67 kN under the corrosion condition for 6 days, while the the same time, it can be clearly found that the bearing capacity decline under corrosion condition for cracking load is only 64.33 kN under the corrosion condition for 8 days. This is mainly due to cracks in 4 days is significantly greater than that in the later period. The ultimate bearing capacity of the concrete surrounding the reinforcement due to the expansion of the reinforcement, which reduces C30‐35‐14 group test pieces decreases by 21.05% after four days of accelerated corrosion, decreased the cracking load. by 28.56% after six days of accelerated corrosion compared with that of non‐corroded test pieces, 2.4.and Compare35.38% after with Existingeight days Research of accelerated Results corrosion. According to the analysis of the causes, with the increase of days of accelerated galvanic corrosion, the corrosion degree of the steel bars increases gradually,In previous the effective studies, section Yuan etand al. strength [48] used of the steel artificial bars will climate decrease accelerating gradually, corrosion due to methodwhich the to studybearing the capacity failure form,of the ductility, steel bars bearing will also capacity decrease and continuously. structural properties This process degradation conforms mechanism to the rule of corrodedthat the reinforcedbearing capacity concrete of flexural the reinforced members with concrete different structure eccentricities, will gradually as well as thechange load-deflection with the relationshipcorrosion of the comparison steel bars. of the large and small bias components with different corrosion degrees. The researchThe cracking shows load that also the specimendecreases undergoeswith the increase three stages of corrosion under load: ratio. The The rising initial stage cracking before load crack of formation;the members the not crack under formation galvanic to the corrosion failure stage is about of the 80 specimen; kN; for andthe members the falling with stage. relatively The ductility low ofcorrosion reinforced ratio, concrete the initial members cracking decreases load is after also corrosion. about 80 kN; After while the tensile the cracking reinforcement load of reachesthe members yield, thewith concrete large corrosion in compression ratio decreases zone quickly obviously, reaches which the ultimate is 60–65 compressive kN. Taking strain,C30‐35 the‐10 ductilitygroup test of corrodedpieces as memberan example, is reduced, the cracking and the load brittleness is 84.67 kN is significantly under non‐corrosion increased. condition, 82.67 kN under corrosionIt can condition be seen fromfor 4 days, Figure and6 that decreases the variation to 71.67 of kN this under curve the is corrosion consistent condition with the for curve 6 days, of acceleratedwhile the cracking corrosion load method is only under64.33 kN artificial under climate.the corrosion At the condition same time, for 8 asdays. the This diameter is mainly of rebar due increases,to cracks in the the load concrete required surrounding for the curve the to reinforcement appear at a turning due to point the expansion has increased, of the mainly reinforcement, due to the yieldwhich stress reduces of tensile the cracking reinforcement load. increases with diameter. Fan et al. [49] used the ‘replacement member’ test method to conduct a flexural bearing capacity test on reinforced concrete members working in a chloride medium environment for a long time. Combined with the test, the mechanical properties of reinforced concrete member under load are compared before and after corrosion. After being corroded by chlorides, the reinforced concrete bending member is under load, and the internal stress effect of concrete expansion caused by corrosion causes the cracking load Pcr of the member to increase. However, if the concrete strength and bonding strength between rebar and concrete are reduced more after corrosion, the cracking load Pcr of reinforced concrete will be reduced after corrosion. Sustainability 2020, 12, 3946 11 of 23

In this paper, electrochemical corrosion is used to test corroded rebar of different diameters, and it is found that the components which do not participate in the energized corrosion increases as the corrosion rate of rebar increases. At the same time, it can be clearly seen from Figure8 that the reduction in bearing capacity after four days of corrosion is significantly greater than the latter.

3. Bond Performance between Corroded Steel Rebars and Concrete

3.1. Bond-Slip Pull-Out Test of Steel Rebars and Concrete The bond performance between steel bars and concrete is affected by various factors, and the interaction between these factors is extremely complex. There are various ways and methods to test and study the bond performance between steel bars and concrete, while they can be classified into three main categories: (1) Direct pull test; (2) simply supported beam or cantilever beam test; (3) local bond-slip test. For the first test method, which is the direct pull test method, the test pieces are easy to fabricate, it is simple to operate and the test results are easy to analyze; comprehensively considering all kinds of factors, such as test loading instrument, test instrument and the test piece fabrication standard, the direct pull test method is adopted for this test. (1) Fabrication of Test Pieces For the main reinforcement, the length of 420 mm is adopted for testing and measuring the slip value of the free end; as for the stirrups, the rectangular closed frame form is adopted, and the spacing is different based on different steel bars. In order to avoid corrosion, the stirrups and the main reinforcement are separated. The anchorage length is chosen to be 4 times of the diameter of the steel rebar, mainly to prevent the steel rebar from yielding under the action of the pull-out load. The specific sizes of the test pieces are as shown in Figure 10. The test piece numbers are C30-10 and C30-14,Sustainability respectively. 2019, 11, x FOR PEER REVIEW 11 of 24

Figure 10.10. Pull-outPull‐out Member Section Size and Steel Rebar Distribution Diagram.Diagram.

(2)(2) TestTest MethodsMethods TheThe loadingloading equipmentequipment used in thisthis uniaxialuniaxial pull-outpull‐out testtest isis 300300 kNkN microcomputermicrocomputer controlledcontrolled universaluniversal testing machine, and the testingtesting equipmentequipment is KB-150KB‐150 concreteconcrete pulloutpullout tester.tester. In order toto obtainobtain moremore accurate accurate test test data, data, KB-150 KB‐150 concrete concrete pull-out pull‐ testerout tester is especially is especially used to used test the to bondtest the strength bond betweenstrength steelbetween bars steel and concrete.bars and Thisconcrete. test instrument This test instrument not only meets not only the standard meets the for standard mechanical for performancemechanical performance test of ordinary test of concrete, ordinary but concrete, also meets but also the regulationmeets the regulation for test of for hydraulic test of hydraulic concrete. Theconcrete. test instrument The test instrument is as shown is inas Figureshown 11in. Figure 11.

(a) (b)

Figure 11. Uniaxial Pull‐out Test Loading and Test Equipment. (a): Overall structure drawing of pull test device; (b): Drawing test device details

3.2. Analysis of Test Results The pulling load of the test piece is continuously increasing, the bonding performance of the concrete and the reinforcing steel shear is damaged, and the concrete between the ribs is continuously compressed until the part of the concrete is crushed, so that some of the reinforcing steel is pulled out. According to the test, the load value of each test piece with certain pull‐out displacement is obtained, and the uniaxial pull‐out load displacement curves of each group of test pieces are obtained as shown in the Figure 12 and Figure 13. Sustainability 2019, 11, x FOR PEER REVIEW 11 of 24

Figure 10. Pull‐out Member Section Size and Steel Rebar Distribution Diagram.

(2) Test Methods The loading equipment used in this uniaxial pull‐out test is 300 kN microcomputer controlled universal testing machine, and the testing equipment is KB‐150 concrete pullout tester. In order to obtain more accurate test data, KB‐150 concrete pull‐out tester is especially used to test the bond strength between steel bars and concrete. This test instrument not only meets the standard for Sustainability 12 mechanical2020 performance, , 3946 test of ordinary concrete, but also meets the regulation for test of hydraulic12 of 23 concrete. The test instrument is as shown in Figure 11.

(a) (b)

Figure 11. Uniaxial Pull-out Test Loading and Test Equipment. (a): Overall structure drawing of pull Figuretest device; 11. Uniaxial (b): Drawing Pull‐out test Test device Loading details. and Test Equipment. (a): Overall structure drawing of pull test device; (b): Drawing test device details 3.2. Analysis of Test Results 3.2. AnalysisThe pulling of Test load Results of the test piece is continuously increasing, the bonding performance of the concrete and the reinforcing steel shear is damaged, and the concrete between the ribs is continuously The pulling load of the test piece is continuously increasing, the bonding performance of the compressed until the part of the concrete is crushed, so that some of the reinforcing steel is pulled out. concrete and the reinforcing steel shear is damaged, and the concrete between the ribs is According to the test, the load value of each test piece with certain pull-out displacement is obtained, continuously compressed until the part of the concrete is crushed, so that some of the reinforcing and the uniaxial pull-out load displacement curves of each group of test pieces are obtained as shown steel is pulled out. According to the test, the load value of each test piece with certain pull‐out in the Figures 12 and 13. displacement is obtained, and the uniaxial pull‐out load displacement curves of each group of test Uniaxial pull-out test mode is adopted for this test, assuming that the bond stress coefficient of the pieces are obtained as shown in the Figure 12 and Figure 13. contact surface between steel rebar and concrete is uniformly distributed in unit area, and the formula for calculating the bond strength can be assumed as:

P 3 τ = 10− (1) L a × × where τ is the bond stress (MPa); P is the drawing load (kN); L is the embedded length of steel bar Sustainability(mm); a is the2019 circumference, 11, x FOR PEER REVIEW of steel bar cross section (mm). 12 of 24

60 70 C30‐10 C30‐14

) 60 50 kN ( (kN)

50 40 force force

40 30 axial axial

30 20 20 Drawing Drawing 10 10

0 0 01234 01234 Reinforcement displacement (mm) Reinforcement displacement (mm) (a) (b)

FigureFigure 12. 12. C30C30 Standard Standard Test Test Piece Piece Pull Pull-out‐out Load Load Displacement Displacement Diagram. Diagram. (a ():a): C30 C30-10;‐10; (b (b): ):c30 c30-14.‐14 ;

60 40 C30‐10 50 C30‐14 ) ) kN ( kN

( 30 40 force

force

30 axial

20 axial

20

Drawing 10 Drawing 10

0 0 01234 01234 Reinforcement displacement (mm) Reinforcement displacement (mm) (a) (b)

Figure 13. C30 8‐day Corrided Test Piece Pull‐out Load Displacement Diagram. (a): C30‐10; (b): C30‐14

Uniaxial pull‐out test mode is adopted for this test, assuming that the bond stress coefficient of the contact surface between steel rebar and concrete is uniformly distributed in unit area, and the formula for calculating the bond strength can be assumed as:

P 3  =10 (1) La where  is the bond stress (MPa); P is the drawing load (kN); L is the embedded length of steel bar (mm); a is the circumference of steel bar cross section (mm). The obtained maximum pull‐out axial force of the uniaxial pull‐out test pieces and the corresponding corrosion degree are listed in Figure 12 and Figure 13, respectively. The bond strength results calculated by formula (2) are as shown in Figure 14 and Figure 15. Sustainability 2019, 11, x FOR PEER REVIEW 12 of 24

60 70 C30‐10 C30‐14

) 60 50 kN ( (kN)

50 40 force force

40 30 axial axial

30 20 20 Drawing Drawing 10 10

0 0 01234 01234 Reinforcement displacement (mm) Reinforcement displacement (mm) (a) (b) Sustainability 2020, 12, 3946 13 of 23 Figure 12. C30 Standard Test Piece Pull‐out Load Displacement Diagram. (a): C30‐10; (b): c30‐14 ;

60 40 C30‐10 50 C30‐14 ) ) kN ( kN

( 30 40 force

force

30 axial

20 axial

20

Drawing 10 Drawing 10

0 0 01234 01234 Reinforcement displacement (mm) Reinforcement displacement (mm) (a) (b)

FigureFigure 13. 13.C30 C30 8-day 8‐day Corrided Corrided Test Test Piece Piece Pull-out Pull‐out Load Load Displacement Displacement Diagram. Diagram. (a): C30-10; (a): C30 (‐b10;): C30-14. (b): C30‐14 The obtained maximum pull-out axial force of the uniaxial pull-out test pieces and the SustainabilitycorrespondingUniaxial 2019 pull, 11 corrosion, ‐xout FOR test PEER degreemode REVIEW is are adopted listed for in Figuresthis test, 12 assuming and 13, that respectively. the bond stress The bond coefficient strength13 of of24 theSustainabilityresults contact calculated 2019 surface, 11, x by FORbetween formula PEER REVIEWsteel (2) are rebar as shown and concrete in Figures is uniformly 14 and 15. distributed in unit area, and13 ofthe 24 formula for calculating the bond strength can be assumed as: 70 C30‐10 70 P C30‐1014 65 3 )  =10 C30‐14 (1)

kN 65 La ( ) 60 kN ( where  is the bond stress (MPa); 60 P is the drawing load (kN); L is the embedded length of steel bar

tension 55 (mm); a is the circumference of steel bar cross section (mm).

tension 55

The obtained maximum50 pull‐out axial force of the uniaxial pull‐out test pieces and the corresponding corrosion degree50 are listed in Figure 12 and Figure 13, respectively. The bond Maximum strength results calculated by formula45 (2) are as shown in Figure 14 and Figure 15. Maximum 45 40 40 0123456789 0123456789Time (day) Time (day) Figure 14. Bearing Capacity of Pull‐out Test Pieces. Figure 14. Bearing Capacity of Pull-out Test Pieces. Figure 14. Bearing Capacity of Pull‐out Test Pieces. 5.0

5.0 C30‐10 4.5 C30‐1014 ) 4.5 C30‐14 ) MPa (

4.0 MPa (

4.0 strength 3.5 strength 3.5

Bonding 3.0

Bonding 3.0

2.5 0123456789 2.5 0123456789Time (day) Time (day) Figure 15. BondBond Strength Strength of of Pull Pull-out‐out Test Test Pieces. Figure 15. Bond Strength of Pull‐out Test Pieces. According to the analysis of Figure 14 and Figure 15, the general trend of the pull strength beforeAccording and after tocorrosion the analysis is very of similar.Figure When14 and the Figure pull‐ out15, theforce general increases trend to theof themaximum pull strength value, beforethe pull and‐out after force corrosion will decrease is very rapidly similar. as When the ultimate the pull bond‐out forcestrength increases is reached. to the In maximum the descending value, thephase pull of‐out most force test will pieces decrease after the rapidly maximum as the loadultimate is reached, bond strength there will is reached.be a rebound In the of descending force, and phasethis is theof most process test in pieces which after the concretethe maximum between load the is ribs reached, of the theresteel barswill isbe thoroughly a rebound crushed.of force, Oneand thisof the is factorsthe process affecting in which the bond the concrete performance between between the ribs steel of rebarthe steel and bars concrete is thoroughly is the bond crushed. area of One the ofsteel the rebar, factors the affecting size of whichthe bond is determinedperformance by between the circumference steel rebar andof the concrete section, is whilethe bond the areasize of the steelsection rebar, is determined the size of whichby the istensile determined force of by the the steel circumference rebar. The oftest the pieces section, with while larger the diameter size of the of sectionsteel rebar is determined can bear larger by the axial tensile tensile force force, of the but steel its bond rebar. strength The test is pieces smaller. with The larger bond diameter stress will of steeldecrease rebar with can largerbear larger area of axial the tensilesteel rebar force, and but larger its bond relative strength bond is area. smaller. Taking The the bond standard stress willtest decreasepieces as withan example, larger area the bondof the strength steel rebar of C30 and‐10 larger group relative test pieces bond is area.4.66 Mpa, Taking while the thatstandard of C30 test‐14 piecesgroup astest an pieces example, is 3.80 the bondMPa. strengthThe rule of is C30 also‐10 true group for test corroded pieces istest 4.66 pieces. Mpa, whileAfter thateight of days C30‐ 14of groupaccelerated test piecescorrosion, is 3.80 the MPa.bond strengthThe rule ofis C30also‐10 true group for testcorroded pieces testis only pieces. 3.21 AfterMPa, decreasingeight days byof accelerated31.12%, while corrosion, the bond the strength bond strength of C30‐14 of groupC30‐10 test group pieces test decreases pieces is onlyby 27.89%, 3.21 MPa, which decreasing is only 2.74 by 31.12%,MPa. Corrosion while the of bond steel strengthrebar is theof C30 main‐14 factorgroup affecting test pieces corrosion decreases of byconcrete 27.89%, and which steel is rebar. only 2.74The MPa.ultimate Corrosion pull‐out of force steel is rebar decreasing is the main with factorthe increase affecting of corrosionthe corrosion of concrete ratio. The and ultimate steel rebar. pull ‐Theout ultimatebearing capacity pull‐out of force C30 ‐is10 decreasing group test withpieces the decreases increase by of 30.99%the corrosion after eight ratio. days The of ultimate constant pull current‐out bearingaccelerated capacity corrosion, of C30 while‐10 group the testbond pieces strength decreases decreases by 30.99% by 22% after after eight four days days of constant of accelerated current acceleratedcorrosion. Comparing corrosion, withwhile other the bondgroups strength of test pieces,decreases it is bynot 22% difficult after to four find days that theof accelerateddecreasing corrosion.rate of the Comparingultimate pull with‐out otherbearing groups capacity of test is decreasing pieces, it is with not thedifficult increase to findof corrosion that the degree.decreasing rate of the ultimate pull‐out bearing capacity is decreasing with the increase of corrosion degree. Sustainability 2020, 12, 3946 14 of 23

According to the analysis of Figures 14 and 15, the general trend of the pull strength before and after corrosion is very similar. When the pull-out force increases to the maximum value, the pull-out force will decrease rapidly as the ultimate bond strength is reached. In the descending phase of most test pieces after the maximum load is reached, there will be a rebound of force, and this is the process in which the concrete between the ribs of the steel bars is thoroughly crushed. One of the factors affecting the bond performance between steel rebar and concrete is the bond area of the steel rebar, the size of which is determined by the circumference of the section, while the size of the section is determined by the tensile force of the steel rebar. The test pieces with larger diameter of steel rebar can bear larger axial tensile force, but its bond strength is smaller. The bond stress will decrease with larger area of the steel rebar and larger relative bond area. Taking the standard test pieces as an example, the bond strength of C30-10 group test pieces is 4.66 Mpa, while that of C30-14 group test pieces is 3.80 MPa. The rule is also true for corroded test pieces. After eight days of accelerated corrosion, the bond strength of C30-10 group test pieces is only 3.21 MPa, decreasing by 31.12%, while the bond strength of C30-14 group test pieces decreases by 27.89%, which is only 2.74 MPa. Corrosion of steel rebar is the main factor affecting corrosion of concrete and steel rebar. The ultimate pull-out force is decreasing with the increase of the corrosion ratio. The ultimate pull-out bearing capacity of C30-10 group test pieces decreases by 30.99% after eight days of constant current accelerated corrosion, while the bond strength decreases by 22% after four days of accelerated corrosion. Comparing with other groups of test pieces, it is not difficult to find that the decreasing rate of the ultimate pull-out bearing capacity is decreasing with the increase of corrosion degree.

3.3. Compare with Existing Research Results Yuan et al. mixed rebar with chloride and exposed them to natural climates indoor and outdoor to approximate the actual effect of corrosion. From the results, it can be seen that the bond-slip curve of uncorroded reinforced concrete is composed of three parts, rising, approximate horizontal and falling. While the corroded reinforced concrete bond-slip curve consists of only two parts, rising and falling, and the gradient of corrosion decline curve increases with the degree of corrosion. It can be seen from Figure 13 that after corrosion accelerated by current, the reinforced concrete bond-slip curve is also composed of only the rising and falling sections, and the test results are similar to the corrosion effect under natural conditions. In addition, comparing the bonding strength under different rebar diameters, it is clear that as the rebar diameter decreases, the peak value of the curve also decreases. This is mainly because the bonding area of the contact between rebar and concrete is reduced as the diameter of rebar is reduced. This rule is also consistent with the experimental results made by Zhao et al. [50]. Since the main source of bonding strength between deformed rebar and concrete is the bite force between the deformed rebar rib and concrete, this is a more important influencing factor: As the amount of rebar corrosion increases, the area of ribs and concrete becomes smaller and smaller, which leads to the decline of bonding strength.

4. Numerical Analysis of Performance Degradation of Corroded Reinforced Concrete Lining Through the bending tests of corroded reinforced concrete members described in Section2, the mechanism governing structural durability loss of reinforced concrete materials degraded by corrosion has been roughly understood. When the mechanism found from the tests are applied to the actual tunnel lining structures, the numerical simulation code can be used to simulate and analyze the mechanical properties of the tunnel support system degraded by rebar corrosion. In this research, by referring to Dong et al ’s [51] traditional pull-out tests to measure the bond-slip behaviors of reinforced concrete structures, and based on the mechanism behind material strength loss found from pull-out tests described in Section3 to investigate the bond-slip behavior of reinforced concrete structures, the traditional formula for bond-slip model of lining structures is obtained. Based on the existing experimental results, an ANSYS-based 3-D finite element model for tunnel reinforced Sustainability 2020, 12, 3946 15 of 23 concrete lining structure was established. By choosing the appropriate material constitutive model, and by simulating the bond-slip effect between rebars and concrete through spring structural element, the change of overall bearing capacity of the structures caused by material degradation due to corroded rebars under different burial depth was analyzed.

4.1. Structural Elements Determination and Model Establishment At present, there are three main modeling methods for reinforced concrete structures: separate modeling, modular modeling, and integral modeling. Sinceedi it is the bond-slip effect between rebars and concrete that should be simulated, only separate modeling is adopted to simulate the relative slip between rebars and concrete by setting appropriate parameters of connecting elements. The concrete element used in this computation is solid65, and link8 element is used to simulate the rebars. Link10 bar element (set to be subject to compression only) is used to simulate the effect of surrounding rock on the main structure. Combin39 of 0 length is selected as the connecting element. This element can help the software to compute the sliding direction and the sliding distance from the corresponding curves, and finally obtain the stiffness coefficient of the connecting spring. It can effectively simulate the degradation of bonding performance and the change of bond stiffness of reinforced concrete structures. In order to save computation time and space and considering the actual mechanical state of the tunnel and the loading conditions in the model, the tunnel model is established as a half-edge model in this computation. The specific modeling is shown in Figure 16. Sustainability 2019, 11, x FOR PEER REVIEW 15 of 24

(a) (b)

FigureFigure 16.16. ANSYSANSYS numericalnumerical model:model: (a) RCS modelmodel withwith GroundGround SpringSpring Elements;Elements; ((bb)) Bond-SlipBond‐Slip SpringSpring Element.Element. TheThe twotwo legendslegends aboveabove areare allall frontfront viewsviews ofof thethe model. model.

4.2.4.2. Setting Material ParametersParameters andand WorkingWorking ConditionsConditions forfor ComputationComputation InIn thisthis computation,computation, SOLID65SOLID65 elementelement isis defineddefined byby W-WW‐W parametricparametric strengthstrength model: model: thethe shearshear transfertransfer coe coefficientfficient during during the the opening opening of concreteof concrete crack crack is set is to set 0.7, to the 0.7, shear the transfershear transfer coefficient coefficient during closingduring ofclosing crack of is setcrack to 1.0,is set the to uniaxial 1.0, the tensile uniaxial strength tensile is setstrength to 2.01 is and set theto 2.01 uniaxial and compressivethe uniaxial strengthcompressive is set strength to 31.4. is As set well, to 31.4. concrete As well, failure concrete under failure multiaxial under stress multiaxial is simulated stress is by simulated setting the by ultimatesetting the strength ultimate of concretestrength underof concrete tension under and tension compression. and compression. TheThe representationrepresentation ofof thethe bondingbonding performanceperformance betweenbetween rebarsrebars andand concreteconcrete isis thethe keykey toto thethe utilizationutilization ofof finitefinite elementelement simulationsimulation inin analyzinganalyzing thethe bondingbonding performanceperformance ofof corrodedcorroded reinforcedreinforced concrete,concrete, which which is is expressed expressed by by the the relationship relationship between between bond bond stress stress and and local local slip. slip. The The local local bond bond slip modelslip model is as is follows: as follows:  2 3 4 p τc = β 61.5S 693S 3140S 4780S fts c/d (2) − 23− 4 ( cts61.5SSS 693 3140  4780 Sfcd (2) 1 + 0.5625η 0.3375η2 + 0.055625η3 0.003η4...... η 7% β = 1.0369− − ≤ (3) 2.0786η− ...... η 7% 1 0.5625 0.3375234  0.055625   0.003 ...... ≥   7% where τ is the bond stress (MPa); β is the coefficient of bond strength reduction; S is the slip (mm); c    (3) 1.0369 = 0.75 fts is the splitting strength2.0786 of concrete ...... (MPa),...... fts 0.19...... fcu , fcu ...... is axial compressive......  7% strength (MPa);

where  c is the bond stress (MPa);  is the coefficient of bond strength reduction; S is the slip 0.75 (mm); fts is the splitting strength of concrete (MPa), fts=0.19 f cu , fcu is axial compressive strength (MPa); c is the cover thickness (MPa); d is the diameter of reinforcing bar (mm);  is the loss rate of steel bar section[52]. Based on the variation of bond strength between rebars and concrete with the corrosion degree of rebars obtained from bond‐slip pull‐out tests of reinforced concrete described in Section 3, the bond strength reduction factor, can be selected as shown in Figure 17. Sustainability 2020, 12, 3946 16 of 23 c is the cover thickness (MPa); d is the diameter of reinforcing bar (mm); η is the loss rate of steel bar section [52]. Based on the variation of bond strength between rebars and concrete with the corrosion degree of rebars obtained from bond-slip pull-out tests of reinforced concrete described in Section3, the bond Sustainabilitystrength reduction 2019, 11, x FOR factor, PEER can REVIEW be selected as shown in Figure 17. 16 of 24

1.4

1.2 factor 1.0

0.8 reduction

0.6 strength

0.4

Bond 0.2

0.0 0 4 8 1216202428323640 Corrosion ratio (%) FigureFigure 17. 17. VariationVariation of ofβ with with corrosion corrosion ratio. ratio. With the general situations of actual engineering taken into account, the selection of material With the general situations of actual engineering taken into account, the selection of material parameters is shown in Table3. parameters is shown in Table 3. Table 3. Parameters of Surrounding Rock and Supporting Material. Table 3. Parameters of Surrounding Rock and Supporting Material. Internal Modulus of Unit Weight Cohesion Materials Poisson Ratio Unit FrictionInternal Angle ElasticityModulus (GPa) of Poisson (kN/m3) Cohesion(MPa) Materials weight friction(◦) angle elasticity (GPa) ratio (MPa) V-Class 3 1.5 0.4(kN/m 18) 0.1 25(°) Surrounding rock V‐Class 1.5 0.4 18 0.1 25 SurroundingSecondary liningrock 30 0.2 23 — — SecondarySteel lining bar 21030 0.30.2 7823 —— —— Steel bar 210 0.3 78 — — With the actual engineering and research needs taken into account, the representative burial depth of 15With m and the 50 actual m is selected engineering in this and computation. research needs Then, taken the surrounding into account, rock the pressures representative under the burial two depthworking of 15 conditions m and 50 are m computedis selected byin this referring computation. to the Highway Then, the Tunnel surrounding Design Code, rock pressures JTG D70-2-2014. under theBased two on working the experimental conditions results are computed described inby Section referring3 and to thethe researchHighway results Tunnel from Design other Code, scholars JTG at D70home‐2‐ and2014. abroad, Based accordingon the experimental to Figure 17 results, the corrosion described ratios in Section of rebars 3 and selected the research for this computationresults from otherincludes scholars 0%, 1.5%, at home 3%, and 7%, abroad, 15% and according 30%. to Figure 17, the corrosion ratios of rebars selected for this computation includes 0%, 1.5%, 3%, 7%, 15% and 30%. 4.3. Analysis of Computation Results 4.3. Analysis(1) Deformation of Computation of Tunnel Results Lining Structure under Different Rebar Corrosion Conditions. (1)Under Deformation different of burial Tunnel depths, Lining with Structure the increase under Different of rebar Rebar corrosion, Corrosion the lining Conditions. deformation aggravates,Under different the vault burial of lining depths, structure with will the settleincrease to aof certain rebar extent.corrosion, The the extracted lining variationsdeformation of aggravates,settlement of the the vault vault of of lining lining structure structure will with settle the rebar to a corrosion certain extent. ratio are The shown extracted in Figure variations 18. of settlement of the vault of lining structure with the rebar corrosion ratio are shown in Figure 18. Sustainability 2020, 12, 3946 17 of 23 Sustainability 2019, 11, x FOR PEER REVIEW 17 of 24

2.540 2.39 2.535 Burial depth (15m) 2.38 Burial depth (50m) 2.530 2.37 ) ) 2.525 cm cm ( ( 2.36

2.520 2.35 2.515 volume volume

2.34 2.510 2.33 2.505 Settling Settling 2.500 2.32 2.495 2.31 2.490 2.30 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Corrosion ratio (%) Corrosion ratio (%) (a) (b)

Figure 18.18. Variation ofof vaultvault settlementsettlement withwith corrosioncorrosion ratioratio underunder didifferentfferent depth:depth: (a)) buried depth 1515 m;m; ((bb)) buriedburied depthdepth 5050 m.m.

As cancan be be seen seen from from Figure Figure 18, the18, settlementthe settlement of vault of increasesvault increases with the with increase the ofincrease steel corrosion of steel undercorrosion diff erentunder burial different depth. burial Compared depth. withCompared the scenario with the of 50-metre scenario burialof 50‐metre depth, burial the settlement depth, the of vaultsettlement is larger of vault in the is scenario larger in of the 15-metre scenario burial of 15 depth.‐metre The burial settlement depth. The of the settlement vault in theof the scenario vault ofin 15-metrethe scenario burial of depth15‐metre is 2.496 burial cm depth when is no 2.496 corrosion cm when occurs. no When corrosion the corrosion occurs. When degree the reaches corrosion 30%, thedegree displacement reaches 30%, of the the vault displacement reaches 2.536 of the cm, vault showing reaches an increase2.536 cm, by showing 1.6%, while an increase the displacement by 1.6%, ofwhile the vaultthe displacement in the scenario of ofthe 50-metre vault in burial the scenario depth increases of 50‐metre by 3.4%. burial Generally depth increases speaking, by it can 3.4%. be seenGenerally that the speaking, settlement it can of thebe seen vault that of lining the settlement structure causedof the vault by the of corrosion lining structure of rebars caused did increase, by the butcorrosion by a small of rebars margin. did increase, but by a small margin. (2) ConcreteConcrete Cracking Cracking of of Tunnel Tunnel Lining Lining Structures Structures under under Different Different Corrosion Corrosion Conditions Conditions of Rebars. of Rebars.In order to study the ultimate stress condition of the reinforced concrete when cracking occurs to it underIn order different to study corrosion the ultimate conditions, stress the condition stress-strain of curvesthe reinforced of inner concrete inwhen three cracking scenarios occurs were extractedto it under and different compared: corrosion without conditions, corrosion, the with stress a corrosion‐strain curves ratio of of inner 7% and concrete with a in corrosion three scenarios ratio of 30%,were asextracted shown inand Figure compared: 19. without corrosion, with a corrosion ratio of 7% and with a corrosion ratio Byof 30%, analyzing as shown the stress-strainin Figure 19. curves of concrete shown in Figure 19, it can be found that the concrete cracking stress can be determined to be the stress value where there is a big jump in the curve. After comparison among the curves, it can be found that the concrete cracking stress under different corrosion ratios is all 1800 KPa at which the lining structure cracks, during which the cracking stress has no obvious change. Figure 19 shows the crack damage evolution process of concrete by monitoring the change of stress state with time in the numerical calculation process, and the peak value represents the ultimate tensile strength. Since the ultimate tensile strength is the material characteristic parameter of concrete itself, the corrosion rate of reinforcement does not change this parameter, so the corrosion rate does not change the initial crack strength and cracking state of concrete. However, it will affect the distribution range and development trend of cracks, which will lead to the deterioration of the whole lining structure. The corrosion rate will affect the bearing capacity and safety greatly. (3) Relative Slip Between Rebar and Concrete under Different Corrosion Conditions of Reinforced Concrete. With the increasing corrosion of rebars, the relative dislocation between concrete and rebars in reinforced concrete lining structure will eventually lead to structural failure, which is also the focus of this paper. The relative slips of concrete and rebar at a series of key points (selection of key points shown in Figure 20) along the tunnel circumference are extracted and plotted, to analyze and study the slip state of various parts. The results are shown in Figure 21. Sustainability 2019, 11, x FOR PEER REVIEW 17 of 24

2.540 2.39 2.535 Burial depth (15m) 2.38 Burial depth (50m) 2.530 2.37

)

) 2.525

cm

cm

( ( 2.36 2.520 2.35 2.515 2.34 2.510 2.33 2.505

Settling volume volume Settling

Settling volume volume Settling 2.500 2.32 2.495 2.31 2.490 2.30 0 5 10 15 20 25 30 35 0 5 10 15 20 25 30 35 Corrosion ratio (%) Corrosion ratio (%) (a) (b)

Figure 18. Variation of vault settlement with corrosion ratio under different depth: (a) buried depth 15 m; (b) buried depth 50 m.

As can be seen from Figure 18, the settlement of vault increases with the increase of steel corrosion under different burial depth. Compared with the scenario of 50-metre burial depth, the settlement of vault is larger in the scenario of 15-metre burial depth. The settlement of the vault in the scenario of 15-metre burial depth is 2.496 cm when no corrosion occurs. When the corrosion degree reaches 30%, the displacement of the vault reaches 2.536 cm, showing an increase by 1.6%, while the displacement of the vault in the scenario of 50-metre burial depth increases by 3.4%. Generally speaking, it can be seen that the settlement of the vault of lining structure caused by the corrosion of rebars did increase, but by a small margin. (2) Concrete Cracking of Tunnel Lining Structures under Different Corrosion Conditions of Rebars. In order to study the ultimate stress condition of the reinforced concrete when cracking occurs to it under different corrosion conditions, the stress-strain curves of inner concrete in three scenarios Sustainability 2020, 12, 3946 18 of 23 were extracted and compared: without corrosion, with a corrosion ratio of 7% and with a corrosion ratio of 30%, as shown in Figure 19. 2000 2000 1800 1800 Corrosion rate 0% Corrosion rate 7%

) 1600 ) 1600

KPa 1400 KPa 1400

(

( 1200 1200 1000 1000 800 800 600 600 400 400

First principal stress First principal

First principal stress First principal 200 200 0 0

0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Time step Time step Sustainability 2019, 11, x FOR PEER REVIEW 18 of 24 (a) (b) 2000 1800 Corrosion rate 30%

) 1600

KPa 1400

( 1200 1000 800 600 400

First principal stress First principal 200 0

0.0 0.2 0.4 0.6 0.8 1.0 Time step (c)

FigureFigure 19 19.. StressStress-time-time step step curve curve of ofconcrete concrete structure: structure: (a) (Uncorroded;a) Uncorroded; (b) 7% (b) Corroded; 7% Corroded; (c) 30% (c) 30% Sustainability 2019, 11, x FOR PEER REVIEW 19 of 24 CorrodedCorroded (time (time step step means means calculation step).

By analyzing the stress-strain curves of concrete shown in Figure 19, it can be found that the concrete cracking stress can be determined to be the stress value where there is a big jump in the curve. After comparison among the curves, it can be found that the concrete cracking stress under different corrosion ratios is all 1800 KPa at which the lining structure cracks, during which the cracking stress has no obvious change. Figure 19 shows the crack damage evolution process of concrete by monitoring the change of stress state with time in the numerical calculation process, and the peak value represents the ultimate tensile strength. Since the ultimate tensile strength is the material characteristic parameter of concrete itself, the corrosion rate of reinforcement does not change this parameter, so the corrosion rate does not change the initial crack strength and cracking state of concrete. However, it will affect the distribution range and development trend of cracks, which will lead to the deterioration of the whole lining structure. The corrosion rate will affect the bearing capacity and safety greatly. (3) Relative Slip Between Rebar and Concrete under Different Corrosion Conditions of Reinforced Concrete. With the increasing corrosion of rebars, the relative dislocation between concrete and rebars in reinforced concreteFigure lining 20. structureSchematic will diagram eventually showing lead characteristic to structural section failure, selection. which is also the focus Figure 20. Schematic diagram showing characteristic section selection. of this paper. The relative slips of concrete and rebar at a series of key points (selection of key points shown in Figure 20) along the tunnel circumference are extracted and plotted, to analyze and study the slip state of various parts. The results are shown in Figure 21.

(a) (b)

Figure 21. Relative slips for characteristic section under different corrosion ratios: (a) 15 m burial depth; (b) 50 m burial depth.

Analysis of the curves in Figure 21 shows that: Under the same burial depth, with the increasing corrosion ratio of rebars, the relative slip between rebars and concrete increases continuously, and the change range is larger, up to 10 times that under the condition of no corrosion. Under the two burial depths, with the change of corrosion ratio of rebars, the change patterns of relative slips between rebar and concrete in lining structure are basically consistent. The maximum slip occurs at the footing of the tunnel lining, mainly because the lining at the wall footing bears greater concentrated stress. (4) Overall Safety Performance of Tunnel Lining Structures under Different Corrosion Conditions of Reinforced Concrete. To evaluate the overall safety of tunnel lining structure under different reinforcement corrosion conditions, the bending moments and axial forces of twelve key sections of tunnel lining under different corrosion ratios and burial depths are extracted respectively for this computation. The safety factors of different sections of tunnel lining are computed by referring to JTG D70‐2‐2014, which are used to analyze the safety performance of the whole structure. The selection of characteristic cross sections is shown in Figure 20. According to the above method, the safety factors of tunnel lining sections under different burial depths and corrosion conditions can be extracted and obtained through computation. The Sustainability 2019, 11, x FOR PEER REVIEW 19 of 24

Sustainability 2020, 12, 3946 19 of 23 Figure 20. Schematic diagram showing characteristic section selection.

(a) (b)

Figure 21. Relative slips for characteristic section under different corrosion ratios: (a) 15 m burial depth; Figure(b) 50 m 21. burial Relative depth. slips for characteristic section under different corrosion ratios: (a) 15 m burial depth; (b) 50 m burial depth. Analysis of the curves in Figure 21 shows that: AnalysisUnder the of same the curves burial depth,in Figure with 21 the shows increasing that: corrosion ratio of rebars, the relative slip between rebarsUnder and concretethe same increases burial depth, continuously, with the and increasing the change corrosion range is ratio larger, of up rebars, to 10 timesthe relative that under slip betweenthe condition rebars of and no concrete corrosion. increases Under thecontinuously, two burial and depths, the change with the range change is larger, of corrosion up to 10 ratio times of thatrebars, under the changethe condition patterns of ofno relative corrosion. slips Under between the rebar two burial and concrete depths, in with lining the structure change of are corrosion basically ratioconsistent. of rebars, The the maximum change patterns slip occurs of relative at the footing slips between of the tunnel rebar and lining, concrete mainly in because lining structure the lining are at basicallythe wall footingconsistent. bears The greater maximum concentrated slip occurs stress. at the footing of the tunnel lining, mainly because the lining(4) at Overall the wall Safety footing Performance bears greater of Tunnelconcentrated Lining stress. Structures under Different Corrosion Conditions of Reinforced(4) Overall Concrete. Safety Performance of Tunnel Lining Structures under Different Corrosion ConditionsTo evaluate of Reinforced the overall Concrete. safety of tunnel lining structure under different reinforcement corrosion conditions,To evaluate the bending the overall moments safety and of tunnel axial forces lining of structure twelve key under sections different of tunnel reinforcement lining under corrosion different conditions,corrosion ratios the bending and burial moments depths areand extracted axial forces respectively of twelve for key this sections computation. of tunnel The safetylining factorsunder differentof different corrosion sections ratios of tunnel and lining burial are depths computed are extracted by referring respectively to JTG D70-2-2014, for this computation. which are used The to safetyanalyze factors the safety of different performance sections of the of wholetunnel structure. lining are The computed selection by of referring characteristic to JTG cross D70 sections‐2‐2014, is whichshown are in Figure used 20to. analyze the safety performance of the whole structure. The selection of characteristicAccording cross to the sections above is method, shown in the Figure safety 20. factors of tunnel lining sections under different burial depthsAccording and corrosion to the conditionsabove method, can be the extracted safety factors and obtained of tunnel through lining computation.sections under The different safety burialfactors depths of the mostand corrosion dangerous conditions sections undercan be di extractedfferent working and obtained conditions through are analyzed,computation. with The the results shown in Table4 and Figure 22.

Table 4. Table of Most Dangerous Safety Factor for Each Working Condition.

Calculate Results Condition 15m (0%) 15m (1.5%) 15m (3.0%) 15m (7.0%) 15m (15.0%) 15m (30.0%) Safety factor 2.741 2.612 2.475 2.305 1.804 1.442 Section Number 1 9 8 10 8 9 Condition 50m (0%) 50m (1.5%) 50m (3.0%) 50m (7.0%) 50m (15.0%) 50m (30.0%) Safety factor 2.814 2.671 2.315 2.258 1.847 1.539 Section Number 8 9 8 9 9 9

It can be seen from Table4 and Figure 22 that under di fferent burial depths, the safety factor values of dangerous sections show small difference under different corrosion ratios of rebars, and with the increase of the corrosion degree of reinforcing bars, the change patterns of the structural safety performance is the same. Compared with Duan Shaoli’s research on the safety factor of tunnel lining structure under the condition of uncorroded steel bars, it can be ground, and the safety factor of the lining tunnel structure after corrosion of the steel bars is obviously reduced. Under the same Sustainability 2019, 11, x FOR PEER REVIEW 20 of 24

safety factors of the most dangerous sections under different working conditions are analyzed, with the results shown in Table 4 and Figure 22.

Table 4. Table of Most Dangerous Safety Factor for Each Working Condition. Sustainability 2020, 12, 3946 20 of 23 Calculate results Condition 15m (0%) 15m (1.5%) 15m (3.0%) 15m (7.0%) 15m (15.0%) 15m (30.0%) burial depth, when the corrosion ratio exceeds 7%, the safety factor of the structure decreases greatly. Safety factor 2.741 2.612 2.475 2.305 1.804 1.442 When the corrosion ratio reaches 30%, the safety factors of the sections cannot meet the requirements of Sectionthe Code Number (K = 1.7), which1 roughly show9 the loss of safety8 of the lining10 structure. Therefore,8 for the9 sake ofCondition structural safety,50m the (0%) allowable 50m corrosion (1.5%) of50m the tunnel (3.0%) reinforced 50m (7.0%) concrete 50m lining (15.0%) structure 50m should (30.0%) be lessSafety than factor 30%. By analyzing2.814 the position2.671 of the most2.315 dangerous 2.258 sections, it can1.847 be concluded that1.539 the Sectionwall footing Number of the lining8 structure is9 the most dangerous8 position9 and needs much9 attention. 9

3.0

2.8 Burial depth (15m) Burial depth (50m) 2.6

2.4

factor 2.2

2.0 Safety 1.8

1.6

1.4

0 5 10 15 20 25 30 Corrosion ratio of reinforcement (%)

FigureFigure 22. 22. SafetySafety factors factors of of the the most most dangerous dangerous sections sections under under various various working working conditions. conditions. 5. Conclusions It can be seen from Table 4 and Figure 22 that under different burial depths, the safety factor valuesThis of dangerous paper takes sections the Yanlangshan show small Tunnel difference of Zhuhai under Airportdifferent Expressway corrosion ratios as the of engineeringrebars, and withbackground the increase to study of the the corrosion influence degree of rebar of corrosion reinforcing on tunnelbars, the lining change structure. patterns Based of the on structural safetyperformance performance ‘bending-uniaxial is the same. Compared pullout’ test, with combined Duan Shaoli withʹs research finite element on the safety numerical factor simulation of tunnel lininganalysis, structure the general under laws the condition affecting the of uncorroded durability and steel bearing bars, it capacity can be ground, of reinforced and the concrete safety factor lining ofstructure the lining are tunnel obtained. structure The main after research corrosion contents of the andsteel achievements bars is obviously are as reduced. follows: Under the same burial(1) depth, From thewhen experimental the corrosion study ratio of theexceeds bending 7%, tests the ofsafety corroded factor reinforced of the structure concrete decreases members, greatly.it can be When found thatthe corrosion with the increase ratio reaches of corrosion 30%, ratio,the safety the deflections factors of of the the sections test pieces cannot become meet smaller the requirementswhen they are of damaged, the Code and (K the = 1.7), ductility which of theroughly bending show members the loss becomes of safety smaller of the and lining the possibilitystructure. Therefore,of brittle failure for the increases. sake of structural At the same safety, time, the the allowable thickness corrosion of the concrete of the tunnel cover reinforced has a certain concrete effect liningon the structure deflection. should The testbe less pieces than with 30%. thicker By analyzing concrete the cover position have of smaller the most deflection dangerous when sections, they are it candamaged, be concluded but the dithatfference the wall is relatively footing of small. the lining At the structure same time, is with the most the increase dangerous of rebar position corrosion and needsratio, themuch bearing attention. capacity of members decreases, and the cracking load decreases with the increase of corrosion ratio. 5. ConclusionsOne factor that affects the bonding performance of rebar and concrete is the bonding area, the size of which is determined by the perimeter of cross section, and the size of cross section is determined by This paper takes the Yanlangshan Tunnel of Zhuhai Airport Expressway as the engineering the tensile force of rebar. The test piece with a larger diameter can bear larger axial tensile force, but the background to study the influence of rebar corrosion on tunnel lining structure. Based on structural bonding strength is smaller. Due to the larger rebar and bonding area, the bonding stress is reduced. performance ‘bending‐uniaxial pullout’ test, combined with finite element numerical simulation Taking standard parts as an example, the bonding strength of C30-10 group test pieces is 4.66 Mpa, while strength of C30-14 group test pieces is 3.80 Mpa, and this rule is also the same in corrosion test pieces. After eight days of accelerated corrosion, the bonding strength of C30-10 group test pieces is only 3.21 Mpa, a decrease of 31.12%, and strength of C30-14 group test pieces is a decrease of 27.89%, only 2.74 Mpa. As the corrosion rate increases, the ultimate drawing force continues to decrease. After eight days of constant current accelerated corrosion, the C30-10 group test pieces’ ultimate pull-out Sustainability 2020, 12, 3946 21 of 23 bearing capacity decreases by 30.99%, and the adhesion drop rate is 22% after four days of acceleration. Comparing the other groups of test pieces, it is not difficult to find that the decrease in ultimate pull-out bearing capacity decreases continuously as the degree of corrosion increases. (2) From the pull-out tests of corroded rebar and concrete to investigate their bond-slip behaviors, it can be concluded that the general changing patterns of pull-out strength before and after corrosion are very similar. When the pull-out force increases to the maximum value, the pull-out force will decrease rapidly as the allowable bond strength is reached. One of the factors affecting the bonding behaviors between rebars and concrete is the bonding area of rebars. The test pieces of larger diameter rebars have larger axial pull-out force, but smaller bond strength. Since the surface area of rebars is larger, the relative bond area is larger, so the bond stress decreases. In practical engineering, the bonding performance of the structures can be improved by using rebars of appropriate diameter. Taking standard parts as an example, the bonding strength of C30-10 group test pieces is 4.66Mpa, while strength of C30-14 group test pieces is 3.80 Mpa, and this rule is also the same in corrosion test pieces. After eight days of accelerated corrosion, the bonding strength of C30-10 group test pieces is only 3.21 Mpa, a decrease of 31.12%, and strength of C30-14 group test pieces is a decrease of 27.89%, only 2.74 Mpa. (3) The numerical simulation results of tunnel lining reinforced concrete structure are basically consistent with the experimental results. With the aggravation of rebar corrosion, the structural deformation will increase but the extent is small, and the cracking of concrete will hardly be affected. However, the relative slip of rebars and concrete will increase significantly with the increase of corrosion ratio. With the aggravation of structural degradation, the overall safety of tunnel lining structure is greatly affected, and the safety of the lining structure will be basically lost when the final corrosion rate of the steel bars is greater than 30%.

Author Contributions: Z.Z. analyzed the calculation results. R.G. carried out the numerical simulation and wrote the article. H.Z. offered useful suggestions for the preparation and writing of the paper. W.H. processed the data. All authors have read and agreed to the published version of the manuscript. Funding: The study was supported by the National Key Research and Development Program of China (2016YFC0802202), the General Program of the National Natural Science Foundation of China (51878572) and Sichuan Science and Technology Program (2019YFG0460). Acknowledgments: We also highly appreciate the contribution of data collection from China Railway 12 Bureau Group Co., LTD. Finally, the authors would like to thank reviewers for useful comments and editors for improving the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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