J. Cent. South Univ. (2019) 26: 196-206

DOI: https://doi.org/10.1007/s11771-019-3993-y

Bending performance of TRC-strengthened RC beams with secondary load under chloride erosion

YU Yu-lin(余玉琳)¹, YIN Shi-ping(尹世平)^{1, 2}, NA Ming-wang(那明望)²

1. State Key Laboratory for Geomechanics and Deep Underground Engineering, School of Mechanics and

Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China;

2. Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering, School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221008, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: Textile reinforced concrete (TRC) has good bearing capacity, crack resistance and corrosion resistance and it is suitable for repairing and reinforcing concrete structures in harsh marine environments. The four-point bending method was used to analyze the influence of the salt concentration, the damage degree and the coupled effect of the environment and load on the bending performance of TRC-strengthened beams with a secondary load. The results showed that as the salt concentration increased, the crack width and mid-span deflection of the beam quickly increased, and its bearing capacity decreased. As the damage degree increased, the early-stage crack development and mid-span deflection of the beam were less affected and the ultimate bearing capacity significantly decreased. In addition, the coupled effect of the environment and load on the beams with a secondary load was significant. As the sustained load increased, the ultimate bearing capacity of the strengthened beam decreased, and cracks developed faster in the later stage. In addition, the mid-span deflection of the beam decreased at the same load level because of the influence of the initial deflection due to the sustained load corrosion.

Key words: textile reinforced concrete; bending performance; secondary load; sustained load corrosion

Cite this article as: YU Yu-lin, YIN Shi-ping, NA Ming-wang. Bending performance of TRC-strengthened RC beams with secondary load under chloride erosion [J]. Journal of Central South University, 2019, 26(1): 196–206. DOI: https://doi.org/10.1007/s11771-019-3993-y.

1 Introduction

Due to the use of deicing salt and the seawater corrosion in coastal areas, some infrastructure is subjected to the combined action of factors such as chloride ions and load, which results in a decrease in the durability and reliability of these structures [1, 2]. Therefore, effective reinforcement is necessary to increase the service life of these structures and to reduce economic losses [3]. An economical and reliable reinforcement method is to use cement-based materials [4, 5]. Textile reinforced concrete (TRC) is a type of fibre reinforced cement-based composite material that contains textile and fine-grain concrete in a certain proportion, and TRC has good crack and corrosion resistance, good strain hardening characteristics, a high bearing capacity and other advantages [6, 7]. A textile is a corrosion-resistant material that does not require a protective layer as steel does; therefore, TRC can be very thin. When TRC is used to reinforce a component, the size and weight of the original component essentially do not change [8, 9].At present, scholars have extensively researched TRC-strengthened concrete beams. CONTAMINE et al [10] indicated that using TRC to reinforce concrete beams has a similar influence on the overall beam performance as using carbon fibre reinforced polymer (CFRP) in a conventional environment but that the overall performance of TRC-strengthened beams tends to be more stable. VERBRUGGEN et al [11] studied the influence of different reinforcement contact areas on the crack development in plain concrete beams. They found that TRC has a good bridging effect during crack development, and the reinforcement effect from covering the width of the beam limited the crack development and increased the bearing capacity. Furthermore, SHENG et al [12] studied the fatigue performance of TRC-strengthened concrete beams after the coupled effect of load and dry-wet cycles, and the results showed that the bond performance between the TRC and old concrete was affected by chloride erosion. However, TRC still improved the fatigue crack and fatigue life of the beam. YIN et al [13] studied the influence of the coupled effect of the environment and a sustained load on TRC- strengthened beams. The results showed that TRC can still act as a reinforcement in harsh environments by controlling cracks and improving the bearing capacity.

Previous studies have used TRC to reinforce unloaded concrete beams. A series of studies has been performed, but some components in an structure cannot be reinforced in unloaded conditions. Studies on beams with a secondary load that are reinforced via other methods have progressed further. ZHAO [14] studied beams with a secondary load that are strengthened with basalt fibre reinforced polymer (BFRP), and their experiments showed that the preload level increases the range of the maximum deflection for the same reinforcement amount. As the damage degree increases, the rate of increase of the maximum deflection range of the test beams decreases. A study by SHANG et al [15] indicated that using ferro-cement mortar to reinforce reinforced concrete (RC) beams with a secondary load can also improve the bearing capacity, crack resistance and stiffness of the reinforced component.

At present, studies on TRC-strengthened beams with a secondary load in marine environments have not been reported. In this paper, TRC-strengthened beams with a secondary load were studied in a marine environment using an experiment based on previous studies, and the influences of different salt concentrations, damage degrees and the sustained load levels on the strengthening effect were examined.

2 Experimental conditions

2.1 Specimen design

A total of 7 beams designated H1–H7 were produced for the experiment. The thickness of the TRC reinforcement layer was 10 mm, and the size of the beams prior to the reinforcement was 100 mm×190 mm×1200 mm. A schematic diagram of the beams after they were reinforced is shown in Figure 1. The experiment considered the salt concentration, the damage degree and the coupled effect of a sustained load and dry-wet cycles, and the details are shown in Table 1.

2.2 Specimen production and tests

2.2.1 Concrete

In this experiment, the concrete used for the RC beams was C40. The cement was 42.5R ordinary Portland cement. The coarse aggregate was gravel with a diameter of 5–10 mm, and its apparent density was 2720 kg/m³. The fine aggregate was medium sand with a fineness modulus of 2.7. The mix proportion was cement : water : medium sand : gravel: water reducer=415: 161:643:1181:2.85. The concrete test blocks, which were standard cubes with dimensions of 150 mm× 150 mm×150 mm, were cured for 28 d, and the average measured strength was 45.3 MPa.

2.2.2 Steel bars

Two main steel bars were arranged in the tensile region. The steel grade was HRB400, and the bar diameter was 12 mm. The yield strength was 518 MPa, as measured by a tensile test. The longitudinal bars were two HPB300 steel bars with a diameter of 8 mm, and the stirrup was an HPB300 steel bar with a diameter of 6.5 mm; the yield strength of these bars was 306 MPa, as measured by a tensile test. The stirrup spacing in the shear section was 60 mm and 150 mm in the pure bending section, as shown in Figure 1.

2.2.3 Textile

The textile used in the test is shown in the literature [13, 16, 17]. The carbon fibre bundle was used in the weft direction (reinforcing direction), and the glass fibre bundle was used in the warp direction, which had a fixed role. The spacing of the fibre bundle was 10 mm×10 mm. YIN et al [18, 19] showed that the adhesion sand on the surface of a textile allows the textile to exert its confining ability better and make full use of the crack limiting and reinforcing effect of the TRC reinforcement layer. In this paper, TRC made from a textile treated with an impregnated epoxy resin adhesive and adhesion sand was used as the reinforcement material. The physical and mechanical parameters are listed in Table 2.

Figure 1 Arrangement of measuring points and distribution of strain gauges (Unit: mm)

Table 1 Information of dry-wet cycle specimen

2.2.4 Fine-grain concrete

The mix proportion of the fine-grain concrete was provided by YIN et al [20], and the mix proportion was cement: fly ash : silica fume : water : fine sand : coarse sand : water reducer=475: 168:35:262:460:920:9.1. After 28 d, the average compressive strength was measured using a 70.7 mm×70.7 mm×70.7 mm cube and its value was 52.5 MPa.

2.2.5 Construction technology of reinforcing layer

The beams were roughened and reinforced after maintenance, and the reinforcement included two layers of the textile. First, the surfaces of the beams were roughened and cleaned. Second, the beams were loaded in different proportions using a jack. Third, the stirred fine-grain concrete was applied to the surface of the beams, which were then covered with a textile layer. Then, 2–3 mm fine-grain concrete was poured onto the surface of the textile. Fourth, the second layer of the textile was fixed to the surface of the fine-grain concrete. Finally, the 3 mm fine-grain concrete was poured, and the total thickness of the textile and fine-grain concrete layers was approximately 10 mm.

2.2.6 Erosion environment and sustained load mode

Currently, no specific specifications exist for TRC structures. Based on Refs. [12, 13], the corrosion test was performed for 28 d after reinforcement, and a jack was used for the four- point bending test. The pure bending section was 400 mm, and the coupling device for the load and environment is shown in Figure 2. In addition, due to stress relaxation, a sensor was placed on the jack, and the load was tested every three days. If the load decreased, the jack was adjusted to keep the load constant. The chloride salt dry-wet cycle was implemented as follows [17]: After soaking the specimens in a 5% sodium chloride solution for 12 h under room temperature and the humidity of 45%–75%, the chloride salt solution was removed, and the specimens were dried for 12 h (using fans for assistance, which is equivalent to simulate sea breeze). The whole dry-wet cycle lasted 24 h, and the cycle was repeated 270 times.

Table 2 Property parameters of hybrid fabric

Figure 2 Sustained load dry-wet cycle device

2.2.7 Determination of corrosion ratio of steel bars

First, after the bending test, two short steel bars were taken from the two longitudinal steel bars at the pure bending section of the beam. Second, soak the short steel bars in the diluted hydrochloric acid with a mass fraction of 12% for about 20 to 30 min. After the rust oxidation on the steel bar surface was completed, the samples were cleaned with water, then neutralized with lime water, and finally cleaned with water again. Third, the samples were placed in a dryer for 3 to 4 h. Finally, the mass of each sample was measured by an electronic scale with an accuracy of 0.01 g. The steel corrosion ratio was the average value of two samples. The corrosion rate of samples was calculated by solving formula (1):

                         (1)

where ρ is steel corrosion rate; m is the quality of non-corroded steel (g); m_I is the quality of steel bars after corrosion (g)

2.2.8 Loading mode and content test

The test was a four-point bending test which started after completely dry for specimens, and the loading points were symmetrically arranged on the beam. The span of the beam was 1000 mm, and the pure bending section was 400 mm. A steel strain gauge was attached to the middle of the longitudinal steel bars. Concrete strain gauges were attached to the mid-span of the beam: one on the top and four on the side at distances of 40, 80, 120 and 160 mm from the top of the beam. The deflection of the mid-span and the deformation of the fulcrum were measured via a displacement metre. The jack was used for manual graded loading. The upper part of the jack included a pressure sensor, and the load value was read using the monitor. The loading grade was 5 kN, and kept for 3–5 min after each loading, observing cracks and measuring the maximum crack width. When concrete cracked (Judged from observing and concrete strain) and steel bar yielded (Judged from steel strain), reduced properly the loading grade and recorded the data, until the beam was completely destroyed. Donghua 3816 static acquisition equipment was used for the data acquisition. The widths of the cracks were read via a crack width gauge produced by Beijing Earth Long Science and Technology Co., Ltd. with an accuracy of 0.02 mm.

3 Test results and analysis

The cracking load, yield load and ultimate load of the test beams are summarized in Table 3. Because the beams with a secondary load cracked under the coupled effect of the load and the environment, the cracking loads of those two beams were not considered in the loading experiment. The corrosion ratio of the steel bars is shown in Table 4.

3.1 Failure characteristics and crack analysis

According to the experimental results, there were two ultimate failure modes. The first was the textile snapping and the textile and old concrete cracking longitudinally along the steel bars together. This failure mode occurred due to the bond between the TRC and the old concrete being stronger than the bond between the steel bars and the old concrete. Additionally, the yielding of the steel bars also weakened the bond between the old concrete and the steel bars, as shown in Figure 3(a). The other failure mode occurred when the end of the TRC plate exfoliated, which probably occurred because the beam experienced more damage when it was reinforced under larger sustained load. Unloading after reinforcement resulted in deformation recovery in the beam, increasing the shear stress between the old concrete and the TRC reinforcement layer and weakening the bond between the interfaces. Additionally, the TRC limited the crack closure because the beam was more damaged, resulting in the presence of fine cracks. The chloride corrosion also increased, resulting in more crystallization and a decrease in the interfacial properties, which led to debonding, as shown in Figure 3(b). However, for both failure modes, the TRC worked well in the early stage and delayed the development of cracks, making the cracks denser and more. Thus, TRC can be used for reinforcement even with a sustained load and under dry-wet cycles. However, as the sustained load increased, the shear stress of the interface must be considered, and if necessary, some measures should be taken to improve the interfacial properties.

3.1.1 Different chloride concentrations

The beams H1, H2, and H3 experiences the failure modes shown in Figure 3(a). Figure 4(a) shows that the crack width in the three beams is small and that the crack development is slow before 120 kN, which indicates that the TRC has a good crack limiting effect. As the chloride concentration increases, the crack width increases for the same load level because salt water has a destructive effect on concrete. Additionally, with increasing chloride concentration, the crystallization product inside the concrete changes [21], which increases the internal pressure in the concrete and results in micro-cracks. Therefore, the concrete performance decreases, and the concrete damage and crack width increases with increasing load. It can be seen from Table 4 that as the chloride concentration increases, more chloride ions may intrude into the surface of steel bars, making the corrosion ratio of steel bars increase, but overall, the corrosion ratios are relatively smaller which have a helpful effect on the interfacial bonding properties of concrete and steel bars [22]. Therefore, at the initial stage of loading, the maximum crack width of beam H3 is smaller than that of beam H2, as shown in Figure 4(a).

3.1.2 Different damage degrees

Beam H4 has the failure state shown in Figure 3(a), and beam H5 has the failure state shown in Figure 3(b). Beam H5 experiences partial debonding failure, as mentioned in Section 3.1, and larger damage degree has a greater influence on the beam, which changes the failure mode of the beam and affects the crack width. Figure 4(b) shows that the maximum crack width in beam H5 is larger than that in the beams H2 and H4. This effect occurs because the weakening of the interface properties between the TRC and the old concrete weakens the protective effect of the TRC and affects its crack limiting ability. And at the beginning of loading, the maximum crack width of beam H4 is smaller than that of beam H2, as explained by Section 3.1.1, when the corrosion ratio of steel bar is small, as shown in Table 4, the increase of corrosion ratio is beneficial to the bonding between concrete and steel bars, delaying the development of crack, as shown in Figure 4(b). In general, the crack width development for the three beams is similar. The crack development shows that TRC has a good crack limiting effect, and the damage degree only has a small influence on the crack limiting ability of TRC.

Table 3 Summary of results

Table 4 Corrosion ratio

Figure 3 Ultimate failure mode of beam:

3.1.3 Different stress levels under a sustained load

Both beams H6 and H7 have the failure state shown in Figure 3(a). As shown in Figure 4(c), the crack development in the three beams is similar, which indicates that TRC has a good protective effect on concrete beams under the coupled conditions of the environment and load. The maximum crack widths in beams H6 and H7 are smaller than that in beam H2 before the yielding. Because of the sustained load, the beam has some cracks, and the cracks are filled with crystallization products after the dry-wet cycle, which results in crack closure. In addition, as explained in Section 3.1.1, because the corrosion ratios of steel bars are smaller relatively, and compared with beam H2, the corrosion ratios of steel bars of beam H6 and H7 are higher, as shown in Table 4, the interfacial properties of beam H6 and H7 are better. Therefore, the secondary loading after unloading causes the maximum crack width in the beam to develop slowly. However, the crack width development accelerates after the steel bars yielded. This is because on one hand, the long-term larger sustained load can induce and extend the internal micro cracks of concrete, which changes the internal transmission boundary conditions of chloride ions in the concrete, accelerating the penetration of chloride ions and reducing the performance of the concrete. On the other hand, steel bars yield earlier when the beam experiences a larger sustained load, and the TRC experiences the main force after the yielding, causing the maximum crack width to rapidly increase.

Figure 4 Load–maximum crack width diagram:

3.2 Analysis of bearing capacity

3.2.1 Different chloride concentrations

The cracking load, yield load and ultimate load of beams H1, H2 and H3 are shown in Table 5. As shown in Table 5, the cracking load, yield load and ultimate load decrease as the chloride concentration increases. As explained in Section 3.1.1, with the increase of chloride concentration, the performance of concrete decreases, which accelerates the damage of the beam, making the ultimate bearing capacity reduced. In addition, it is likely that the increase of chloride concentration promotes the permeation of chloride ions, which leads to the decrease of interfacial properties between TRC and concrete, making the reinforcement effect of TRC not play a better role and the bearing capacity decreases. The change in the cracking load is obvious, and compared with beam H2, the cracking load of beam H1 increases by 35.14%, and the cracking load of beam H3 decreases by 19.82%. This is due to the increase in the chloride concentration, which changes the hydration products and greatly affects the performance of the TRC, weakening the crack control.

Table 5 Load analysis of beam H1, H2 and H3

3.2.2 Different damage degrees

The cracking load, yield load and ultimate load of beams H2, H4 and H5 are shown in Table 6. As seen from Table 6, the beam with a high damage degree has a high cracking load, and compared with beam H2, the cracking load of beam H4 increases by 20.72%, and the cracking load of beam H5 increases by 27.93%. The deformation of the beam is likely larger when it experiences a dry-wet cycle in the unloading state after being reinforced with a sustained load, and the steel bar bears more force at the beginning of the loading, which reduces the force born by the TRC and causes the TRC to crack later. The yield load and ultimate load greatly decrease with the increase in the damage degree. This is because unloading causes the shear stress between the old concrete and the TRC reinforced layer to increase after reinforcement with a sustained load, which influences the interfacial properties and weakens the protective effect of the TRC. In addition, the beam itself has cracks when it is reinforced with a sustained load, and the limiting effect on the TRC decreases the crack closure ability. Then, the chloride erosion during the dry-wet cycle increases, which not only destroys the concrete structure but also increases the corrosion of the steel bar. Therefore, the overall performance of the concrete beam decreases. However, because of the stress lag and the premature yielding of the steel bars, the role of TRC is not complete.

Table 6 Load analysis of beam H2, H4 and H5

3.2.3 Different sustained stress levels

The cracking load, yield load and ultimate load of beams H2, H6 and H7 are shown in Table 7. Because the concrete beams experience a sustained load cycle after reinforcement and crack under the sustained load, the experiment do not consider the cracking load when loading again. As shown in Table 7, compared with beam H2, the yield load and ultimate load of beam H6 decreases by 1.56% and 1.41% respectively, while that of beam H7 decreases by 4.65% and 6.59% respectively. It indicates that TRC has a good protective effect on the concrete beams at a lower sustained stress level. A large sustained load has a large influence on the bearing capacity of a beam with a secondary load because a large sustained load causes the beam to have large cracks. In the presence of large cracks, chloride ions are more likely to erode the concrete and accelerate the corrosion of the steel bars inside the concrete. In addition, loading again results in a repeated shear force between the TRC and the old concrete, which decreases the interfacial performance and the reinforcement effect of the TRC. However, Table 7 shows that these decreases are within an acceptable range. Therefore, considering the bearing capacity, TRC is suitable in engineering practice for beam structures with large sustained loads.

Table 7 Load analysis of beam H2, H6 and H7

3.3 Analysis of load-mid-span deflection

3.3.1 Different chloride concentrations

The deflection curves of beams H1, H2 and H3 are shown in Figure 5(a). The figure shows that the deflection of the mid-span increases as the chloride concentration increases under the same load level. One of the reasons is that, as the chloride concentration increases, the deteriorating effect on concrete increases, resulting in a decrease in concrete performance, and during the loading, the cracks of the beam develop faster which accelerate the section damage process of the beam. Therefore, the stiffness of the beam decreases, and the deflection increases rapidly. Another reason is that with the increase of chloride concentration, the penetration of chloride ions is accelerated, and the interfacial properties between TRC and concrete decreases, affecting the limiting effect of TRC on crack development and deflection deformation.

3.3.2 Different damage degrees

The deflection curves of beam H2, H4 and H5 are shown in Figure 5(b). As seen in the figure, at the beginning of loading, the deflection of the beam decreases with the increase of sustained load. Possibly because of the larger sustained load, the beam has a larger deflection deformation, and the springback from deformation after the TRC reinforcement is limited; therefore, the beam has a corresponding deflection. At the time of the loading, a larger bearing capacity results in more stress lag, the steel bars are stressed earlier, and the deflection of the beam correspondingly decreases. And the deflection of beam H5 is greater than that of beam H4 because of the poor interfacial bonding performance, which makes the reinforcement effect of TRC not given full play, resulting in a smaller overall stiffness. It can be seen from the failure mode which is partial debonding, as shown in Figure 4(b). However, the change in the deflection increases with increasing sustained load after the steel bars yield. This is because the increase of the sustained load promotes the expansion of microcracks during reinforcement, promoting chloride erosion and reducing the performance of concrete. And because the TRC of beam H5 is damaged at the exfoliated end, the deflection also changes after the steel bars yield. However, as seen from Figure 5(b), the deflection curves of the three beams are close before the steel bars yield, and TRC has a good reinforcement effect on the beams with more damage under dry-wet cycles.

3.3.3 Different sustained stress levels

The deflection curves of beams H2, H6 and H7 are shown in Figure 5(c). The figure shows that the deflection of beam H7 with a larger sustained load is obviously smaller before the steel bars yield. This is because the deformation of the beam increases with the increase in the sustained load, and the residual deformation is larger after unloading. When reloading, the ductility of steel bars decreases because they had been stretched, making the overall deflection of beam H7 smaller. Section 3.2.3 shows that the yield load of beam H7 is small, the steel bars yield sooner, and the deflection rapidly increases after the yielding because the TRC control force is weakened after the steel bars yield. As shown in Section 3.1.3, larger cracks accelerate the beam damage due to the large sustained load and cause the deflection to quickly change. At the same time, the chloride ion corrosion increases because of the cracks from the sustained load, which weakens the performance of concrete; therefore, the deflection of the beam rapidly changes after the steel bars yield. The deflection change for a small sustained load is almost the same as that for no sustained load. However, for the same load level, the deflection for a sustained load is still slightly smaller due to the retention of some residual deformation. As shown by the curves in Figure 5(c), the deflection is controlled well for the three beams. Therefore, TRC is suitable for reinforcing beams under the coupled effect of environment and load based on its deflection control.

Figure 5 Load–mid-span deflection diagram:

4 Conclusions

1)As the chloride concentration increases, the crack width and deflection of beams with a secondary load increase, and the bearing capacity decreases. However, the overall change amplitude decreases, and in the early stage of loading, TRC works well as a reinforcement and limits the crack development.

2) Under dry-wet cycle conditions, TRC can act as a reinforcement to limit the damage, improve the crack development and beam deflection, and increase the bearing capacity. However, the overall performance of the beam with a stress ratio of 0.6 is greatly reduced, because the TRC is damaged at the exfoliated end and it cannot be fully used. When using TRC to reinforce beams with a lot of damage, the interfacial performance between the TRC and the old concrete must be considered, and some measures should be taken to enhance the interfacial performance if necessary. Regardless of the size of the sustained load, TRC controls the crack formation and beam deflection, and TRC is suitable as a reinforcement for beams with different degrees of damage.

3) Under the coupled effect of load and environment, the ultimate bearing capacity decreases as the sustained load increases, but the control of the cracks and beam deflection was better both before and after the steel bars yielded because the reinforcing effect of TRC improved the overall performance of the beam.

4) TRC is suitable for reinforcing beams with a secondary load, and TRC is effective for controlling the deflection and limiting the maximum crack width for beams with damage and beams subjected to the coupled effect of load and environment after damage.

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(Edited by HE Yun-bin)

中文导读

氯盐侵蚀作用下TRC加固二次受力RC梁的弯曲性能

摘要：纤维编织网增强混凝土（Textile Reinforced Concrete, TRC）具有良好的承载、限裂和耐腐蚀的能力，适用于海洋严酷环境下混凝土结构的修复加固。本文采用四点弯曲加载的方式，分析了盐水浓度、损伤程度、环境和荷载耦合作用等因素对TRC加固二次受力梁弯曲性能的影响。研究结果表明：随着盐水浓度的增加，梁的裂缝宽度和跨中挠度发展较快，承载力有所降低；随着损伤程度的增大，梁前期裂缝和跨中挠度的发展受影响较小，而其极限承载力有较大的降幅；此外，环境和荷载耦合作用对二次受力梁有明显的影响。随着持载水平的增大，加固梁的极限承载力有所降低，加载后期梁的裂缝发展较快。受持载腐蚀时的初始挠度的影响，在同级加载下梁的跨中挠度有所降低。

关键词：纤维编织网增强混凝土；弯曲性能；二次受力；持载腐蚀

Foundation item: Project(2017XKZD09) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2017-10-31; Accepted date: 2018-09-12

Corresponding author: YIN Shi-ping, PhD, Professor; Tel: +86-15262013916; E-mail: yinshiping2821@163.com; ORCID: 0000- 0001-8304-5914