# Flexural Behavior of Graphite Tailings RC Beams in Chloride Environment

Authors: Bo-Ya Feng; Xun-Guo Zhu; Hong-Chun Xia; Lin Yang; Te Xie
Journal: Advances in Civil Engineering
 (2023)

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## Abstract

This paper investigates the mechanical properties of graphite tailings concrete beams under various curing environments. The results show that the concentration of graphite tailings has an important influence on the mechanical properties of concrete. Finally, the bending characteristics of the beam and the modified bearing capacity calculation formula are obtained, which provides a theoretical basis and the reference value for the construction of green building materials. Through research, the main innovations and main findings of this paper are as follows: (1) In the three kinds of concrete experimental environment, the fracture load and ultimate load of concrete beams with graphite tailings replacement rate of 20% are the largest. (2) In the three kinds of concrete test environment, when the concrete test beams under the same load, with the increase of graphite tailings replacement rate, the mid-span deflection of concrete beams showed a trend of decreasing first and then increasing. When the replacement rate of graphite tailings sand is 20%, the mid-span deflection of the test beam is the smallest. (3) Through experiments, it is found that the normal section bearing capacity of graphite tailings concrete beams after chloride salt erosion conforms to the plane section assumption. After chlorine salt erosion, the strain of the longitudinal tensile steel bar of concrete beams with different graphite tailings sand replacement rates is the same as that of ordinary concrete beams, and the slope of the strain curve is the same, showing a linear trend. The experimental results show that the replacement rate of graphite tailings sand has an insignificant effect on the strain of steel bars. (4) The Gauss function is used to fit the experimental value of normal section bearing capacity of concrete beam, and the calculation formula of normal section ultimate bearing capacity of graphite tailings sand concrete beam under chloride environment is established. The research results of this paper have significant reference value for the study of mechanical properties of graphite tailings sand concrete components.


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## Body

## 1. Introduction

In recent years, with the development of the Chinese economy, the construction of coastal cities has developed rapidly [1]. Large-scale infrastructure construction needs a lot of concrete materials, which will consume a lot of natural stone and cause serious ecological problems [2–5]. To effectively reduce the use of natural stone, alternative raw materials must be found. After years of research, it has been found that graphite tailings sand can be used as a substitute for natural sand [6, 7]. A rational use of graphite tailings sand can not only effectively reduce the use of natural sand but also solve the problem of environmental pollution [8–10]. Coastal buildings are faced with salt-ion erosion due to their service environment, which leads to the durability failure of concrete structures [11–13]. Once the buildings are damaged, it will inevitably cause huge economic losses. To avoid the failure of concrete structures, it is necessary to study the mechanical properties of concrete structures in coastal environments.There are many factors contributing to the failure of concrete structures. Among them, chloride ion erosion is the main cause of steel corrosion, which directly leads to the durability failure of concrete structures [14, 15]. When chloride ions penetrate the concrete, they will destroy the steel passivation film, resulting in steel corrosion [16, 17]. Shang-Qin [18] conducted bending tests on concrete beams under chloride erosion conditions. The test results show that with the increase in service time, the ultimate bearing capacity of concrete beams increases first and then decreases. At the same time, based on the test results and the existing specifications, the formula for calculating the bearing capacity of concrete beams suitable for marine environment is obtained. Yin et al. [19] also conducted similar tests. The test results show that with the increase of the number of dry and wet cycles, the crack width and mid-span deflection of the test beam gradually increases, and the ultimate bearing capacity of the test beam gradually decreases.Chloride corrosion is the main cause of steel corrosion, and steel corrosion is the first factor affecting the durability of concrete structures [20–22]. Chloride penetrates concrete, destroys steel passive film, and causes steel corrosion [23–25]. In recent years, Hong-Bo et al. [26–30] studied the influence of graphite tailings on the properties of cement-based materials and concrete blocks. The results show that the mechanical properties of cement-based materials can be improved by adding an appropriate amount of graphite tailings. Compared with ordinary concrete, low-content of graphite tailings concrete has better compressive strength, compressive sensitivity, and impermeability. Sun [31] studied the influence of graphite tailings incorporation ratio on the mechanical properties of foamed concrete, and the results show that the appropriate amount of graphite tailings incorporation can improve its compressive strength.In summary, the research on the durability of graphite tailings concrete focuses on the permeability and resistance to chloride ion erosion of concrete test blocks under chloride salt erosion, while the mechanical properties of graphite tailings concrete components in a chloride salt environment such as bending resistance are less studied. The authors of this paper and their research team have conducted research on related issues and have achieved certain results [32]. To better improve the research paper, in this paper, the bending test of graphite tailings concrete beams with different substitution rates under three curing environments of air, water, and chloride salt is conducted. The failure mode, crack propagation law, load characteristic value, mid-span deflection, and strain of the concrete and steel bar of the test beams under three curing environments are analyzed. According to the above experimental values, a new calculation formula has been established. The above research results will provide good guidance value for similar research on graphite tailings concrete members.


## 2. Test Overview

### 2.1. Materials and Mixtures Ratio

(1)
Cement: P.O42.5 ordinary Portland cement.(2)
Stone: particle size of 5∼20 mm, good gradation, apparent density of 2650 kg/m3.(3)
Natural river sand: river sand with a fineness modulus of 2.49.(4)
Graphite tailings: graphite tailings from the graphite tailings reservoir.(5)
Poly carboxylic water reducer: MZ-10C poly-carboxylic high performance water reducer is selected from Table1.(6)
Steel bar: HRB400 bar is used. And its diameter is 10 mm, and its yield strength is 415 MPa.The physical characteristics of natural sand and graphite tailings is shown in Table1.Table 1
Physical properties of graphite tailings sand and natural sand.
Physical propertyGrain size (mm)Apparent density (kg·m−3)Bulk density (kg·m−3)Fineness modulusWater absorption (%)pH valueNatural sand0.16∼5262015402.4921.878Graphite tailings0.01∼3285516300.9030.1010


### 2.2. Proportion Relation of Concrete Component Materials

According to the mix proportion in Table2, graphite tailings concrete beams with substitution rates of 0, 10%, 20%, 30%, and 40% were prepared. It should be noted that the sand ratio in the table is 30%, which is significantly lower than that of the common pumping concrete in the project. This is because the fineness modulus of sand is small, and there are many particles in the sand, and the cohesion of the concrete is easy to be guaranteed. Therefore, small sand ratio is adopted, and this test is nonpumping concrete. The sand ratio of pumping concrete is 2%–5% higher than that of nonpumping concrete, so the sand ratio used in this test is lower than that of pumping concrete in the project.Table 2
Mix ratio and mechanical properties of 1 m3 graphite tailing sand concrete.
Test specimen numberVolume of concrete materials (kg/m3)Water cement ratioPercentage of sand (%)WaterCementNatural sandPebbleGraphite tailingsWater reducing admixtureSMK-0180.00409.09535.481294.45015.680.4430SMK-10180.00409.09481.931294.4553.5515.680.4430SMK-20180.00409.09428.381294.45107.1015.680.4430SMK-30180.00409.09374.841294.45160.6415.680.4430SMK-40180.00409.09321.291294.45214.1915.680.4430Note. Specimen number SMK-n, SMK represents test beam, n represents graphite tailings replacement rate value. For example, SMK-20 represents the test beam with graphite tailings replacement rate of 20%. Among them, SMK-0 is the ordinary concrete control beam.


### 2.3. Experimental Design

In this experiment, three groups of graphite tailings concrete beams (air, fresh water, and chloride environment, respectively, expressed by SMK, FE, and CE) were designed, and five concrete beams were made in each group. One of the beams was a natural sand concrete beam as the contrast beam, and the four roots were graphite tailings concrete beams, with substitution rates of 10%, 20%, 30% and 40%, respectively. The number of graphite tailings concrete beams in chloride environment was CE-0, CE-10, CE-20, CE-30, and CE-40. The number of graphite tailings concrete beams in air and freshwater environment was the same. The test beam size is length 1600 mm, section size is 120 mm × 180 mm, and net span is 1500 mm. To study the flexural performance of graphite tailings concrete beams in three environments, all test beams adopt the same reinforcement ratio. The reinforcement ratio of longitudinal tensile reinforcement is 1.257%, and the thickness of the longitudinal reinforcement protective layer is 20 mm. Specimen production and environmental erosion processes are shown in Figure1. The size and reinforcement of the beam are shown in Figure 2. Each reinforced concrete beam was made, and three cube test blocks were prepared at the same time. After the same condition of maintenance, the same environmental corrosion was conducted with the same group of test beams.Figure 1
Specimen production and environmental erosion process. (a) The specimen-making process. (b) Environmental erosion process.
(a)(b)Figure 2
Size and reinforcement diagram of the test beam (unit: mm).


### 2.4. Test Scheme and Measurement Content

Chlorine salt erosion test using 10% NaCl solution, natural immersion 120 d. The loading test adopts a three-point loading mode, forming a 500 mm pure bending section in the middle of the span. The loading device and the layout of the measuring points are shown in Figure3. According to the standard of concrete structure test method, the bending test of concrete beams after environmental action is conducted [33]. The concentrated load is applied by a hydraulic press, and the pressure sensor is placed in the span of the distribution beam to realize the loading mode of two concentrated forces and three points. Before the test loading, the test beam is preloaded to ensure the normal operation of the instrument and the normal contact of each point. At the beginning, the loading mode was according to each level of 3 kN loading, each loading stayed for 3 minutes, and then the change of cracks on the surface of the beam was observed, when the specimen’s first crack appeared, with each level of 5 kN for differential loading and gradual increase of load, the number of cracks gradually increased, cracks continued to extend upward and the crack width increased, with the pen to thicken the cracks of the test beam, in order to more clearly see the cracks of the test beam, and record the stress load and crack width.Figure 3
Layout of measuring points and loading device diagram. (a) Layout of measuring points. (b) Loading device.
(a)(b)The main measurement contents of the test are as follows: (1) Cracking load, ultimate load and crack development process of the test beam, and the crack development process is observed by the DJCK-2 crack width meter. (2) The strain of reinforcement and concrete is measured by sticking strain gauges in the middle of the tensile reinforcement, and five concrete strain gauges are evenly arranged along the height of the beam in the middle of the beam span to measure the strain of concrete; (3) Load-deflection changes, YHD-50 displacement gauges arranged at both ends of the beam and the middle of the beam to measure the deformation of the specimen.


## 2.1. Materials and Mixtures Ratio

(1)
Cement: P.O42.5 ordinary Portland cement.(2)
Stone: particle size of 5∼20 mm, good gradation, apparent density of 2650 kg/m3.(3)
Natural river sand: river sand with a fineness modulus of 2.49.(4)
Graphite tailings: graphite tailings from the graphite tailings reservoir.(5)
Poly carboxylic water reducer: MZ-10C poly-carboxylic high performance water reducer is selected from Table1.(6)
Steel bar: HRB400 bar is used. And its diameter is 10 mm, and its yield strength is 415 MPa.The physical characteristics of natural sand and graphite tailings is shown in Table1.Table 1
Physical properties of graphite tailings sand and natural sand.
Physical propertyGrain size (mm)Apparent density (kg·m−3)Bulk density (kg·m−3)Fineness modulusWater absorption (%)pH valueNatural sand0.16∼5262015402.4921.878Graphite tailings0.01∼3285516300.9030.1010


## 2.2. Proportion Relation of Concrete Component Materials

According to the mix proportion in Table2, graphite tailings concrete beams with substitution rates of 0, 10%, 20%, 30%, and 40% were prepared. It should be noted that the sand ratio in the table is 30%, which is significantly lower than that of the common pumping concrete in the project. This is because the fineness modulus of sand is small, and there are many particles in the sand, and the cohesion of the concrete is easy to be guaranteed. Therefore, small sand ratio is adopted, and this test is nonpumping concrete. The sand ratio of pumping concrete is 2%–5% higher than that of nonpumping concrete, so the sand ratio used in this test is lower than that of pumping concrete in the project.Table 2
Mix ratio and mechanical properties of 1 m3 graphite tailing sand concrete.
Test specimen numberVolume of concrete materials (kg/m3)Water cement ratioPercentage of sand (%)WaterCementNatural sandPebbleGraphite tailingsWater reducing admixtureSMK-0180.00409.09535.481294.45015.680.4430SMK-10180.00409.09481.931294.4553.5515.680.4430SMK-20180.00409.09428.381294.45107.1015.680.4430SMK-30180.00409.09374.841294.45160.6415.680.4430SMK-40180.00409.09321.291294.45214.1915.680.4430Note. Specimen number SMK-n, SMK represents test beam, n represents graphite tailings replacement rate value. For example, SMK-20 represents the test beam with graphite tailings replacement rate of 20%. Among them, SMK-0 is the ordinary concrete control beam.


## 2.3. Experimental Design

In this experiment, three groups of graphite tailings concrete beams (air, fresh water, and chloride environment, respectively, expressed by SMK, FE, and CE) were designed, and five concrete beams were made in each group. One of the beams was a natural sand concrete beam as the contrast beam, and the four roots were graphite tailings concrete beams, with substitution rates of 10%, 20%, 30% and 40%, respectively. The number of graphite tailings concrete beams in chloride environment was CE-0, CE-10, CE-20, CE-30, and CE-40. The number of graphite tailings concrete beams in air and freshwater environment was the same. The test beam size is length 1600 mm, section size is 120 mm × 180 mm, and net span is 1500 mm. To study the flexural performance of graphite tailings concrete beams in three environments, all test beams adopt the same reinforcement ratio. The reinforcement ratio of longitudinal tensile reinforcement is 1.257%, and the thickness of the longitudinal reinforcement protective layer is 20 mm. Specimen production and environmental erosion processes are shown in Figure1. The size and reinforcement of the beam are shown in Figure 2. Each reinforced concrete beam was made, and three cube test blocks were prepared at the same time. After the same condition of maintenance, the same environmental corrosion was conducted with the same group of test beams.Figure 1
Specimen production and environmental erosion process. (a) The specimen-making process. (b) Environmental erosion process.
(a)(b)Figure 2
Size and reinforcement diagram of the test beam (unit: mm).


## 2.4. Test Scheme and Measurement Content

Chlorine salt erosion test using 10% NaCl solution, natural immersion 120 d. The loading test adopts a three-point loading mode, forming a 500 mm pure bending section in the middle of the span. The loading device and the layout of the measuring points are shown in Figure3. According to the standard of concrete structure test method, the bending test of concrete beams after environmental action is conducted [33]. The concentrated load is applied by a hydraulic press, and the pressure sensor is placed in the span of the distribution beam to realize the loading mode of two concentrated forces and three points. Before the test loading, the test beam is preloaded to ensure the normal operation of the instrument and the normal contact of each point. At the beginning, the loading mode was according to each level of 3 kN loading, each loading stayed for 3 minutes, and then the change of cracks on the surface of the beam was observed, when the specimen’s first crack appeared, with each level of 5 kN for differential loading and gradual increase of load, the number of cracks gradually increased, cracks continued to extend upward and the crack width increased, with the pen to thicken the cracks of the test beam, in order to more clearly see the cracks of the test beam, and record the stress load and crack width.Figure 3
Layout of measuring points and loading device diagram. (a) Layout of measuring points. (b) Loading device.
(a)(b)The main measurement contents of the test are as follows: (1) Cracking load, ultimate load and crack development process of the test beam, and the crack development process is observed by the DJCK-2 crack width meter. (2) The strain of reinforcement and concrete is measured by sticking strain gauges in the middle of the tensile reinforcement, and five concrete strain gauges are evenly arranged along the height of the beam in the middle of the beam span to measure the strain of concrete; (3) Load-deflection changes, YHD-50 displacement gauges arranged at both ends of the beam and the middle of the beam to measure the deformation of the specimen.


## 3. Experiment Results and Analyses

### 3.1. Crack Propagation Distribution Law

Through the bending load test of the test beam, it is found that the failure process and crack development process of the graphite tailings concrete beam in the three environments are consistent with those of the ordinary concrete beam. The failure mode and crack development process of each test beam in the fresh water and chloride environments are also basically consistent with those in the air environment, but the characteristic load changes slightly. The bending failure mode and crack distribution of the test beam in the fresh water and chloride environment, are shown in Figure4.Figure 4
Bending failure mode and crack distribution map of test beam under water and chloride environment. (a) Fresh water environment and (b) chloride environment.
(a)(b)


### 3.2. Cracking Load and Ultimate Load of Beam

The test results of the cracking loadPcr and ultimate load Pu of each test beam under air, water and chloride environment are shown in Table 3. The effects of different variation parameters on cracking load and ultimate load of test beam are shown in Figures 5 and 6. It can be seen from Table 3 and Figures 5 and 6.Table 3
Test values of cracking load and ultimate load of test beams in three environments.
Graphite tailings sand replacement rate (%)Air environmentClear water environmentChlorine environmentPcr (kN)Pu (kN)Pcr (kN)Pu (kN)Pcr (kN)Pu (kN)012.1249.2512.4250.8511.0847.811013.2552.0113.3753.1712.4851.052013.3653.1614.2655.7512.8552.083012.4350.3313.1752.4211.5149.264011.2147.0112.1249.1710.1545.42Figure 5
Influence of graphite tailing sand replacement rate on cracking load and ultimate load of test beam. (a) Cracking load and (b) Ultimate load.
(a)(b)Figure 6
Influence of service environment on cracking load and ultimate load of test beam. (a) Cracking load and (b) Ultimate load.
(a)(b)


#### 3.2.1. Effects of Different Graphite Tailings Replacement Ratios on Cracking Load and Ultimate Load of Test Beams

It can be seen from Table3 and Figure 5 that with the increase of graphite tailings replacement rate, the variation laws of cracking load and ultimate load of test beams in air, water, and chlorine salt are the same, showing a trend of first increase and then decrease. Among them, in the chloride environment, the cracking loads of CE-10 and CE-20 increased by 12.64% and 15.97%, respectively, compared with CE-0, and the ultimate load increased by 6.78% and 8.93%, respectively. The cracking load and ultimate load of CE-40 decreased by 8.39% and 5.00% compared with CE-0. The results show that with the increase of graphite tailings replacement rate, the performance of test beams in different environments is better than that of ordinary concrete beams at the replacement rate of 10%∼20%, and the bearing capacity is lower than that of ordinary concrete beams at the replacement rate of 40%. The reason is that when the replacement rate of graphite tailings is 10%∼20%, the diameter of graphite tailings is smaller than that of natural sand, which makes the distribution of particles in concrete more uniform, increases the compactness of concrete materials and improves the bearing capacity of graphite tailings concrete beams.


#### 3.2.2. Influence of Different Erosion Environment on Cracking Load and Ultimate Load of Graphite Tailings Concrete Beam

It can be seen from Table3 and Figure 6 that the bearing capacity of the test beam under a clear water environment is the highest under the same replacement rate of graphite tailings, and the bending bearing capacity of the test beam after chlorine salt erosion is lower than that in an air (noncorrosion) environment. Taking the replacement rate of graphite tailings 20%, as an example, the cracking load and ultimate load of graphite tailings concrete beams in a clear water environment are increased by 6.74% and 4.87%, respectively, compared with those in an air environment. This is because the hydration reaction can continue to occur in the test beam in a clear water environment, and the hydration products are generated to make the concrete denser, thus enhancing the bearing capacity of the test beam. However, the cracking load and ultimate load of graphite tailings concrete beams after chloride corrosion are 3.82% and 2.03% lower than those in an air environment, respectively. It is also found that the cracking load is more affected by environmental erosion than the ultimate load, indicating that the bearing capacity of reinforced concrete beams immersed in a 10% NaCl solution for 120 days is more affected by concrete damage.


### 3.3. Mid-Span Deflection

The load-deflection curves of concrete beams with different graphite tailings replacement rates in air, water, and chloride environments are shown in Figures7(a)–7(c). To compare the change law of mid-span deflection of concrete beams with the same graphite tailings replacement rate after erosion in different environments with load, the load-deflection curves of concrete beams with a 20% graphite tailings replacement rate after erosion in air, water, and chloride environments are plotted, as shown in Figure 7(d). From Figure 7, it can be seen that the load-deflection curve trend of a graphite tailings concrete beam and an ordinary concrete beam is similar, which can be divided into three stages: (1) The noncracking stage of the test beam: at the initial stage of loading, the test beam is not cracked, the load-deflection curve is approximately linear change, and the deflection increment is small. (2) The test beam cracked at the yield stage of reinforcement. After the test beam cracked, the load-deflection curve showed a turning point, and the growth rate of deflection gradually accelerated. The load-deflection curve gradually developed nonlinearly after cracking. (3) The steel bar yield to the failure stage: after the steel bar yield, the load-deflection curve appears the second turning point, and the deflection of the beam increases sharply after yield. The load-deflection curve is also close to a horizontal straight line, and the final test beam is damaged.Figure 7
Load-deflection curve. (a) Air environment; (b) fresh water environment; (c) chloride environment; and (d) the replacement rate of graphite tailings is 20%.
(a)(b)(c)(d)Specifically, it can be seen from Figures7(a) and 7(b) that when the test beam bears the same level of load in an air and clear water environment, with the increase of the graphite tailings replacement rate, the mid-span deflection of the test beam decreases first and then increases. When the replacement rate is 20%, the mid-span deflection of the test beam is small. It can be seen from Figure 7(c) that after chlorine salt erosion of graphite tailings concrete beams, when the replacement rates of graphite tailings are 10% and 20%, the mid-span deflection growth rate of the test beam is slower than that of the ordinary concrete beam. When the replacement rate of graphite tailings is 40%, the mid-span deflection growth of the test beam is faster than that of the ordinary concrete beam, indicating that a small amount of graphite tailings can improve the flexural performance of reinforced concrete beams after chlorine salt erosion. It can be seen from Figure 7(d) that under the same load, the mid-span deflection of the graphite tailings concrete beam presents the law of clear water < air < chlorine salt.


### 3.4. The Variation of Mid-Span Concrete Strain

The mid-span concrete strain curves of concrete beams with different graphite tailings content under different loads in clear water and chlorine salt environment are shown in Figures8 and 9. It can be seen from Figures 8 and 9 that with the increase of load, the concrete strain of each test beam shows an increasing trend. When the load is 20 kN, the concrete strain of the test beam CE-10 is larger than that of the FE-10, indicating that the resistance to deformation of the graphite tailings concrete beam decreases after being corroded by chloride. Under the environment of clear water and chloride salt, when the test beam was subjected to the same load, the cross-sectional strain of concrete beams with different graphite tailings content was not significantly different, and the strain of concrete beams with 40% graphite tailings content was slightly higher. However, regardless of the amount of graphite tailings, the strain at each measuring point on the normal section is proportional to the distance from the point to the neutral axis, and the concrete strain of each measuring point on the mid-span section is linearly distributed. Therefore, in a clear water and chloride environment, the graphite tailings concrete beam still conforms to the plane section assumption. The test results show that the cross section of a graphite tailings concrete beam conforms to the plane section assumption under the three test environments listed above, which provides the experimental basis for the theoretical calculation of bearing capacity.Figure 8
Strain distribution of different section heights in the middle of the test beam in a freshwater environment. (a) FE-10; (b) FE-20; (c) FE-30; and (d) FE-40.
(a)(b)(c)(d)Figure 9
Strain distribution of different section heights in the middle of test beam in a chlorine environment. (a) CE-10; (b) CE-20; (c) CE-30; and (d) CE-40.
(a)(b)(c)(d)


### 3.5. Longitudinal Steel Strain in the Mid-Span

The variation trend of the longitudinal tensile reinforcement strain in the mid-span of the test beam under different environments with the increase of load is shown in Figure10. The load-steel strain curve of graphite tailings concrete beam can be divided into three parts: (1) In the early stage of loading, the test beam has not yet cracked elastic stage. In this stage, the tensile force is borne by concrete and steel, and the strain value of steel increases slowly and linearly. (2) From the cracking stage of the test beam to the yield stage of the steel bar, the tensile force of the test beam after cracking is transferred to the steel bar. In this stage, the steel bar strain is still linearly increased, but the growth rate is faster than that before cracking, and the slope of the steel bar strain-load curve is reduced. (3) When the steel bar yields to the failure stage, the load increases slightly and the strain increases sharply.Figure 10
Strain-load curve of the longitudinal tensile steel bar in the middle of each test beam. (a) Fresh water environment and (b) chloride environment.
(a)(b)It can be seen from Figure10 that after water and chloride erosion, the longitudinal tensile steel strain of concrete beams with different graphite tailings replacement rates is consistent with that of ordinary concrete beams, and the slope of the curve is the same, indicating that the graphite tailings replacement rate has an insignificant effect on the steel strain. In the chloride environment, when each test beam bears the same load, the strain of the steel bar of the graphite tailings concrete beam with the replacement rate of 20% is smaller than that of the ordinary concrete beam and is smaller than that of the other graphite tailings replacement rate test beams. The reason is that when the replacement rate of graphite tailings is 20%, the porosity of the test beam is the smallest and the densest, and the influence of chloride ions into the concrete on the steel bar is the smallest, so the steel bar strain is small. This shows that the pore structure of concrete beams with graphite tailings is better than that of ordinary concrete beams with a small amount of graphite tailings, which is more conducive to the resistance of concrete beams to chloride corrosion damage.Taking the replacement rate of graphite tailing sand of 20% as an example, the strain-load curve of the mid-span longitudinal tensile steel bar of 20% graphite tailing concrete beam in clear water and chloride environment is drawn, as shown in Figure11. It can be seen from the diagram that chloride salt erosion has a slight effect on the strain of steel bars of graphite tailings concrete beams, but due to the short immersion time and low corrosion degree, chloride salt erosion has no significant effect on steel bars.Figure 11
Strain-load curves of 20% graphite tailings concrete beams in different environments.


## 3.1. Crack Propagation Distribution Law

Through the bending load test of the test beam, it is found that the failure process and crack development process of the graphite tailings concrete beam in the three environments are consistent with those of the ordinary concrete beam. The failure mode and crack development process of each test beam in the fresh water and chloride environments are also basically consistent with those in the air environment, but the characteristic load changes slightly. The bending failure mode and crack distribution of the test beam in the fresh water and chloride environment, are shown in Figure4.Figure 4
Bending failure mode and crack distribution map of test beam under water and chloride environment. (a) Fresh water environment and (b) chloride environment.
(a)(b)


## 3.2. Cracking Load and Ultimate Load of Beam

The test results of the cracking loadPcr and ultimate load Pu of each test beam under air, water and chloride environment are shown in Table 3. The effects of different variation parameters on cracking load and ultimate load of test beam are shown in Figures 5 and 6. It can be seen from Table 3 and Figures 5 and 6.Table 3
Test values of cracking load and ultimate load of test beams in three environments.
Graphite tailings sand replacement rate (%)Air environmentClear water environmentChlorine environmentPcr (kN)Pu (kN)Pcr (kN)Pu (kN)Pcr (kN)Pu (kN)012.1249.2512.4250.8511.0847.811013.2552.0113.3753.1712.4851.052013.3653.1614.2655.7512.8552.083012.4350.3313.1752.4211.5149.264011.2147.0112.1249.1710.1545.42Figure 5
Influence of graphite tailing sand replacement rate on cracking load and ultimate load of test beam. (a) Cracking load and (b) Ultimate load.
(a)(b)Figure 6
Influence of service environment on cracking load and ultimate load of test beam. (a) Cracking load and (b) Ultimate load.
(a)(b)


### 3.2.1. Effects of Different Graphite Tailings Replacement Ratios on Cracking Load and Ultimate Load of Test Beams

It can be seen from Table3 and Figure 5 that with the increase of graphite tailings replacement rate, the variation laws of cracking load and ultimate load of test beams in air, water, and chlorine salt are the same, showing a trend of first increase and then decrease. Among them, in the chloride environment, the cracking loads of CE-10 and CE-20 increased by 12.64% and 15.97%, respectively, compared with CE-0, and the ultimate load increased by 6.78% and 8.93%, respectively. The cracking load and ultimate load of CE-40 decreased by 8.39% and 5.00% compared with CE-0. The results show that with the increase of graphite tailings replacement rate, the performance of test beams in different environments is better than that of ordinary concrete beams at the replacement rate of 10%∼20%, and the bearing capacity is lower than that of ordinary concrete beams at the replacement rate of 40%. The reason is that when the replacement rate of graphite tailings is 10%∼20%, the diameter of graphite tailings is smaller than that of natural sand, which makes the distribution of particles in concrete more uniform, increases the compactness of concrete materials and improves the bearing capacity of graphite tailings concrete beams.


### 3.2.2. Influence of Different Erosion Environment on Cracking Load and Ultimate Load of Graphite Tailings Concrete Beam

It can be seen from Table3 and Figure 6 that the bearing capacity of the test beam under a clear water environment is the highest under the same replacement rate of graphite tailings, and the bending bearing capacity of the test beam after chlorine salt erosion is lower than that in an air (noncorrosion) environment. Taking the replacement rate of graphite tailings 20%, as an example, the cracking load and ultimate load of graphite tailings concrete beams in a clear water environment are increased by 6.74% and 4.87%, respectively, compared with those in an air environment. This is because the hydration reaction can continue to occur in the test beam in a clear water environment, and the hydration products are generated to make the concrete denser, thus enhancing the bearing capacity of the test beam. However, the cracking load and ultimate load of graphite tailings concrete beams after chloride corrosion are 3.82% and 2.03% lower than those in an air environment, respectively. It is also found that the cracking load is more affected by environmental erosion than the ultimate load, indicating that the bearing capacity of reinforced concrete beams immersed in a 10% NaCl solution for 120 days is more affected by concrete damage.


## 3.2.1. Effects of Different Graphite Tailings Replacement Ratios on Cracking Load and Ultimate Load of Test Beams

It can be seen from Table3 and Figure 5 that with the increase of graphite tailings replacement rate, the variation laws of cracking load and ultimate load of test beams in air, water, and chlorine salt are the same, showing a trend of first increase and then decrease. Among them, in the chloride environment, the cracking loads of CE-10 and CE-20 increased by 12.64% and 15.97%, respectively, compared with CE-0, and the ultimate load increased by 6.78% and 8.93%, respectively. The cracking load and ultimate load of CE-40 decreased by 8.39% and 5.00% compared with CE-0. The results show that with the increase of graphite tailings replacement rate, the performance of test beams in different environments is better than that of ordinary concrete beams at the replacement rate of 10%∼20%, and the bearing capacity is lower than that of ordinary concrete beams at the replacement rate of 40%. The reason is that when the replacement rate of graphite tailings is 10%∼20%, the diameter of graphite tailings is smaller than that of natural sand, which makes the distribution of particles in concrete more uniform, increases the compactness of concrete materials and improves the bearing capacity of graphite tailings concrete beams.


## 3.2.2. Influence of Different Erosion Environment on Cracking Load and Ultimate Load of Graphite Tailings Concrete Beam

It can be seen from Table3 and Figure 6 that the bearing capacity of the test beam under a clear water environment is the highest under the same replacement rate of graphite tailings, and the bending bearing capacity of the test beam after chlorine salt erosion is lower than that in an air (noncorrosion) environment. Taking the replacement rate of graphite tailings 20%, as an example, the cracking load and ultimate load of graphite tailings concrete beams in a clear water environment are increased by 6.74% and 4.87%, respectively, compared with those in an air environment. This is because the hydration reaction can continue to occur in the test beam in a clear water environment, and the hydration products are generated to make the concrete denser, thus enhancing the bearing capacity of the test beam. However, the cracking load and ultimate load of graphite tailings concrete beams after chloride corrosion are 3.82% and 2.03% lower than those in an air environment, respectively. It is also found that the cracking load is more affected by environmental erosion than the ultimate load, indicating that the bearing capacity of reinforced concrete beams immersed in a 10% NaCl solution for 120 days is more affected by concrete damage.


## 3.3. Mid-Span Deflection

The load-deflection curves of concrete beams with different graphite tailings replacement rates in air, water, and chloride environments are shown in Figures7(a)–7(c). To compare the change law of mid-span deflection of concrete beams with the same graphite tailings replacement rate after erosion in different environments with load, the load-deflection curves of concrete beams with a 20% graphite tailings replacement rate after erosion in air, water, and chloride environments are plotted, as shown in Figure 7(d). From Figure 7, it can be seen that the load-deflection curve trend of a graphite tailings concrete beam and an ordinary concrete beam is similar, which can be divided into three stages: (1) The noncracking stage of the test beam: at the initial stage of loading, the test beam is not cracked, the load-deflection curve is approximately linear change, and the deflection increment is small. (2) The test beam cracked at the yield stage of reinforcement. After the test beam cracked, the load-deflection curve showed a turning point, and the growth rate of deflection gradually accelerated. The load-deflection curve gradually developed nonlinearly after cracking. (3) The steel bar yield to the failure stage: after the steel bar yield, the load-deflection curve appears the second turning point, and the deflection of the beam increases sharply after yield. The load-deflection curve is also close to a horizontal straight line, and the final test beam is damaged.Figure 7
Load-deflection curve. (a) Air environment; (b) fresh water environment; (c) chloride environment; and (d) the replacement rate of graphite tailings is 20%.
(a)(b)(c)(d)Specifically, it can be seen from Figures7(a) and 7(b) that when the test beam bears the same level of load in an air and clear water environment, with the increase of the graphite tailings replacement rate, the mid-span deflection of the test beam decreases first and then increases. When the replacement rate is 20%, the mid-span deflection of the test beam is small. It can be seen from Figure 7(c) that after chlorine salt erosion of graphite tailings concrete beams, when the replacement rates of graphite tailings are 10% and 20%, the mid-span deflection growth rate of the test beam is slower than that of the ordinary concrete beam. When the replacement rate of graphite tailings is 40%, the mid-span deflection growth of the test beam is faster than that of the ordinary concrete beam, indicating that a small amount of graphite tailings can improve the flexural performance of reinforced concrete beams after chlorine salt erosion. It can be seen from Figure 7(d) that under the same load, the mid-span deflection of the graphite tailings concrete beam presents the law of clear water < air < chlorine salt.


## 3.4. The Variation of Mid-Span Concrete Strain

The mid-span concrete strain curves of concrete beams with different graphite tailings content under different loads in clear water and chlorine salt environment are shown in Figures8 and 9. It can be seen from Figures 8 and 9 that with the increase of load, the concrete strain of each test beam shows an increasing trend. When the load is 20 kN, the concrete strain of the test beam CE-10 is larger than that of the FE-10, indicating that the resistance to deformation of the graphite tailings concrete beam decreases after being corroded by chloride. Under the environment of clear water and chloride salt, when the test beam was subjected to the same load, the cross-sectional strain of concrete beams with different graphite tailings content was not significantly different, and the strain of concrete beams with 40% graphite tailings content was slightly higher. However, regardless of the amount of graphite tailings, the strain at each measuring point on the normal section is proportional to the distance from the point to the neutral axis, and the concrete strain of each measuring point on the mid-span section is linearly distributed. Therefore, in a clear water and chloride environment, the graphite tailings concrete beam still conforms to the plane section assumption. The test results show that the cross section of a graphite tailings concrete beam conforms to the plane section assumption under the three test environments listed above, which provides the experimental basis for the theoretical calculation of bearing capacity.Figure 8
Strain distribution of different section heights in the middle of the test beam in a freshwater environment. (a) FE-10; (b) FE-20; (c) FE-30; and (d) FE-40.
(a)(b)(c)(d)Figure 9
Strain distribution of different section heights in the middle of test beam in a chlorine environment. (a) CE-10; (b) CE-20; (c) CE-30; and (d) CE-40.
(a)(b)(c)(d)


## 3.5. Longitudinal Steel Strain in the Mid-Span

The variation trend of the longitudinal tensile reinforcement strain in the mid-span of the test beam under different environments with the increase of load is shown in Figure10. The load-steel strain curve of graphite tailings concrete beam can be divided into three parts: (1) In the early stage of loading, the test beam has not yet cracked elastic stage. In this stage, the tensile force is borne by concrete and steel, and the strain value of steel increases slowly and linearly. (2) From the cracking stage of the test beam to the yield stage of the steel bar, the tensile force of the test beam after cracking is transferred to the steel bar. In this stage, the steel bar strain is still linearly increased, but the growth rate is faster than that before cracking, and the slope of the steel bar strain-load curve is reduced. (3) When the steel bar yields to the failure stage, the load increases slightly and the strain increases sharply.Figure 10
Strain-load curve of the longitudinal tensile steel bar in the middle of each test beam. (a) Fresh water environment and (b) chloride environment.
(a)(b)It can be seen from Figure10 that after water and chloride erosion, the longitudinal tensile steel strain of concrete beams with different graphite tailings replacement rates is consistent with that of ordinary concrete beams, and the slope of the curve is the same, indicating that the graphite tailings replacement rate has an insignificant effect on the steel strain. In the chloride environment, when each test beam bears the same load, the strain of the steel bar of the graphite tailings concrete beam with the replacement rate of 20% is smaller than that of the ordinary concrete beam and is smaller than that of the other graphite tailings replacement rate test beams. The reason is that when the replacement rate of graphite tailings is 20%, the porosity of the test beam is the smallest and the densest, and the influence of chloride ions into the concrete on the steel bar is the smallest, so the steel bar strain is small. This shows that the pore structure of concrete beams with graphite tailings is better than that of ordinary concrete beams with a small amount of graphite tailings, which is more conducive to the resistance of concrete beams to chloride corrosion damage.Taking the replacement rate of graphite tailing sand of 20% as an example, the strain-load curve of the mid-span longitudinal tensile steel bar of 20% graphite tailing concrete beam in clear water and chloride environment is drawn, as shown in Figure11. It can be seen from the diagram that chloride salt erosion has a slight effect on the strain of steel bars of graphite tailings concrete beams, but due to the short immersion time and low corrosion degree, chloride salt erosion has no significant effect on steel bars.Figure 11
Strain-load curves of 20% graphite tailings concrete beams in different environments.


## 4. Analysis of Bearing Capacity of Graphite Tailings Concrete Beam Normal Section Under Chloride Erosion Environment

After chlorine salt erosion, the internal damage of graphite tailings concrete makes the strength of graphite tailings concrete decrease. The degradation of concrete’s mechanical properties is the main factor affecting the bending performance of graphite tailings-reinforced concrete beams under the salt erosion environment. Therefore, in this section, the formula for calculating the ultimate bearing capacity of test beams after chlorine salt erosion is derived, and the influence of concrete strength changes with different graphite tailings contents on the bending bearing capacity of reinforced concrete beams after salt erosion is emphatically discussed. The theoretical value of the ultimate bearing capacity of the graphite tailings concrete beam after chloride corrosion is calculated by using the bearing capacity calculation formulas (1) and (2) of the normal section of the reinforced concrete beam [34]. The calculation results are shown in Table 4. In the table, Mcy represents the ultimate bending moment test value of the graphite tailings concrete beam after salt corrosion, and Muy represents the theoretical calculation value of the ultimate bending moment of the graphite tailings concrete beam after salt corrosion.(1)Mu=α1fcbxh0−x2,(2)α1fcx=fyAs,where, α1 is the simplified stress figure coefficient of the concrete in the compression zone, taking 1.0; fc is the measured compressive strength of graphite tailings concrete after chloride erosion, and takes the measured value, MPa; b is the beam width, mm; x is the height of the compression zone of the section, mm; h0 is the effective height of the section, mm; fy is the yield strength of longitudinal bars, and takes the measured value, MPa; and As is the effective cross-sectional area of the longitudinal reinforcement under tension, mm2. The table is the comparison of calculated and experimental values of ultimate bearing capacity of graphite tailings concrete beams after chloride salt erosion.Table 4
Comparison of experimental and computational results according to formulas (1) and (2).
Specimen numberfc (MPa)Mcy (MPa)Muy (MPa)Mcy/MuyCE-035.211.959.2061.298CE-1036.112.769.2181.384CE-2036.913.029.2291.411CE-3035.112.329.2041.339CE-4034.611.369.1971.235From Table4, the experimental values of graphite tailings concrete beams and ordinary concrete beams after chlorine salt erosion are greater than the calculated values, indicating that the ultimate bearing capacity of graphite tailings concrete beams after chlorine salt erosion is feasible to calculate according to concrete specifications. To obtain the ultimate bearing capacity of concrete beams with different replacement rates of graphite tailings after chlorine salt erosion more accurately, two different fitting functions are used, and the correction coefficient of graphite tailings replacement rate is introduced. The data of concrete beams with different replacement rates of graphite tailings after chlorine salt erosion are numerically fitted.


### 4.1. Polynomial Fitting

To express the ultimate bearing capacity of graphite tailings concrete beams more accurately, the test results in Table4 are fitted by polynomial function, and the fitting formula (3) and the fitting curve in Figure 12 are obtained. According to the fitting curve, the correlation coefficient of the fitting curve is 0.9879.(3)My=yMuy=Ax2+Bx+CMuy,where x is the replacement rate of graphite tailings sand, (%); y is the correction factor; Muy is the theoretical calculation value of the ultimate bearing capacity of the test beam after chloride corrosion, MPa; and My is the corrected ultimate bearing capacity of the test beam after chloride corrosion, MPa; A = −3.42143E−4, B = 0.01198, C = 1.29917. Table 5 is a comparison of calculated and experimental values from formula (3).Figure 12
Polynomial fitting curve of ultimate bearing capacity of concrete beams with different replacement rates of graphite tailings after chloride erosion.Table 5
Comparison of experimental and computational results according to formula (3).
The number of specimensMy (MPa)Mcy (MPa)Mcy/MyCE-012.0211.950.9942CE-1012.8212.760.9953CE-2013.0013.021.0015CE-3012.5012.320.9856CE-4011.4011.360.9965


### 4.2. Gaussian Function Fitting

The Gauss function fitting curve is shown in Figure13. According to the fitting curve, the formula for calculating the ultimate bearing capacity of a normal section of graphite tailings concrete beam after chloride salt erosion is (4) and (5), and the correlation coefficient of the fitting curve is 0.9985. Compared with the polynomial fitting curve, the correlation coefficient of the Gaussian function fitting curve is closer to one, and the degree of coincidence is higher, indicating that the calculation formula of ultimate bearing capacity modified by the Gaussian function is more suitable for the calculation of ultimate bearing capacity of graphite tailings concrete beams after chlorine salt erosion. The axial compressive strength in the calculation formula needs to use the measured compressive strength value of graphite tailings concrete block after chlorine salt erosion. In practical engineering application, the compressive strength can be determined according to the empirical data or through experiments. At the same time, the modified formula can estimate the ultimate bearing capacity of the graphite tailings concrete beams with other mass replacement rates after chloride attack.(4)My=yMuy,(5)y=y0+Aw×π/2×exp−2x−w0w2,where My is the corrected ultimate bearing capacity of the test beam after chloride corrosion, MPa; y is the correction factor; Muy is the theoretical calculation value of the ultimate bearing capacity of the test beam after chloride corrosion, MPa; and x is the replacement rate of graphite tailings sand, (%); y0 = 1.10078; A = 13.81231; w = 35.40572; w0 = 17.10824. Table 6 is a comparison of calculated and experimental values from formula (4).Figure 13
Gaussian function fitting curve of ultimate bearing capacity of concrete beams with different replacement rates of graphite tailings after chloride erosion.Table 6
Comparison of experimental and computational results according to formula (4).
The number of specimensMy (MPa)Mcy (MPa)Mcy/MyCE-011.9311.951.0017CE-1012.7812.761.0016CE-2012.9613.021.0046CE-3012.3312.320.9992CE-4011.3811.360.9984


## 4.1. Polynomial Fitting

To express the ultimate bearing capacity of graphite tailings concrete beams more accurately, the test results in Table4 are fitted by polynomial function, and the fitting formula (3) and the fitting curve in Figure 12 are obtained. According to the fitting curve, the correlation coefficient of the fitting curve is 0.9879.(3)My=yMuy=Ax2+Bx+CMuy,where x is the replacement rate of graphite tailings sand, (%); y is the correction factor; Muy is the theoretical calculation value of the ultimate bearing capacity of the test beam after chloride corrosion, MPa; and My is the corrected ultimate bearing capacity of the test beam after chloride corrosion, MPa; A = −3.42143E−4, B = 0.01198, C = 1.29917. Table 5 is a comparison of calculated and experimental values from formula (3).Figure 12
Polynomial fitting curve of ultimate bearing capacity of concrete beams with different replacement rates of graphite tailings after chloride erosion.Table 5
Comparison of experimental and computational results according to formula (3).
The number of specimensMy (MPa)Mcy (MPa)Mcy/MyCE-012.0211.950.9942CE-1012.8212.760.9953CE-2013.0013.021.0015CE-3012.5012.320.9856CE-4011.4011.360.9965


## 4.2. Gaussian Function Fitting

The Gauss function fitting curve is shown in Figure13. According to the fitting curve, the formula for calculating the ultimate bearing capacity of a normal section of graphite tailings concrete beam after chloride salt erosion is (4) and (5), and the correlation coefficient of the fitting curve is 0.9985. Compared with the polynomial fitting curve, the correlation coefficient of the Gaussian function fitting curve is closer to one, and the degree of coincidence is higher, indicating that the calculation formula of ultimate bearing capacity modified by the Gaussian function is more suitable for the calculation of ultimate bearing capacity of graphite tailings concrete beams after chlorine salt erosion. The axial compressive strength in the calculation formula needs to use the measured compressive strength value of graphite tailings concrete block after chlorine salt erosion. In practical engineering application, the compressive strength can be determined according to the empirical data or through experiments. At the same time, the modified formula can estimate the ultimate bearing capacity of the graphite tailings concrete beams with other mass replacement rates after chloride attack.(4)My=yMuy,(5)y=y0+Aw×π/2×exp−2x−w0w2,where My is the corrected ultimate bearing capacity of the test beam after chloride corrosion, MPa; y is the correction factor; Muy is the theoretical calculation value of the ultimate bearing capacity of the test beam after chloride corrosion, MPa; and x is the replacement rate of graphite tailings sand, (%); y0 = 1.10078; A = 13.81231; w = 35.40572; w0 = 17.10824. Table 6 is a comparison of calculated and experimental values from formula (4).Figure 13
Gaussian function fitting curve of ultimate bearing capacity of concrete beams with different replacement rates of graphite tailings after chloride erosion.Table 6
Comparison of experimental and computational results according to formula (4).
The number of specimensMy (MPa)Mcy (MPa)Mcy/MyCE-011.9311.951.0017CE-1012.7812.761.0016CE-2012.9613.021.0046CE-3012.3312.320.9992CE-4011.3811.360.9984


## 5. Conclusion

Graphite tailings are solid waste produced in the process of graphite mining, which has caused great damage to the natural ecological environment. Partial replacement of graphite tailings with construction sand is one of the effective ways to reuse solid waste. This paper studies the crack propagation law and bearing capacity of graphite tailings concrete beams under load. The following conclusions can be drawn from this study:(1)
Through experiments, it is found that the failure process and crack development process of graphite tailings concrete beams under three curing environments are the same, but the failure load of graphite tailings concrete beams under a chloride curing environment is small.(2)
In the three kinds of concrete curing environment, the fracture load and ultimate load of concrete beams with graphite tailings sand replacement rate of 20% are the largest. When the replacement rate of graphite tailings sand is the same, the fracture load and ultimate load of concrete beams under a chloride curing environment are the smallest.(3)
In three kinds of concrete experimental environment, when the concrete test beam under the same load, with the increase of graphite tailings replacement rate, the mid-span deflection of concrete beam decreases first and then increases. When the replacement rate of graphite tailings sand is 20%, the mid-span deflection of the test beam is the smallest; when the replacement rate of graphite tailings sand is 10% and 20%, the growth rate of the mid-span deflection of the test beam is slower than that of the ordinary concrete beam.(4)
Under the same load, the mid-span deflection of a graphite tailings sand concrete beam under the water experimental environment < the mid-span deflection of the graphite tailings sand concrete beam under the air experimental environment < the mid-span deflection of a graphite tailings sand concrete beam under the chloride experimental environment.(5)
The test results show that the cross section of a graphite tailings concrete beam conforms to the plane section assumption under the above three test environments, which provides the experimental basis for the theoretical calculation of bearing capacity.(6)
The experimental results show that the replacement rate of graphite tailings sand has negligible effect on the strain of steel bar. Comparing the strain values of the steel bars, it is found that the strain value of the steel bars in the graphite tailings sand concrete beam changes little than in the clear water experimental environment. In the chloride salt experimental environment, due to concrete and steel by chloride ion erosion, steel strain value changes large.(7)
Through the data fitting of the bearing capacity test results of graphite tailings concrete beams, the modified Gauss bearing capacity calculation formula is obtained. This conclusion has great reference value for the study of mechanical properties of concrete beams under chloride environment.



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*Source: 1427007-2023-01-31.xml*