Mechanism and control strategy of large-area concrete shrinkage cracks in the underground railway section

: The cracks of the large-area concrete structure in the underground railway section directly affect the structural safety and service life. How to take effective measures to control concrete shrinkage cracks is a difficulty to be solved in the project.In view of the crack control demand of large-area concrete structure cracks in a vehicle section of Wuhan metro, based on the existing engineering cases of large-area concrete structure , shrinkage crack mechanism and control strategies, the corresponding measures are proposed to prevent crack production from two aspects of concrete raw materials and construction technology.The results show that through the regulation of concrete materials,maintenance,casting,post-pouring belt, temperature monitoring and other links, the reduction of shrinkage cracks of large-area concrete structure in the underground railway section is realized.These results can provide references for similar projects to lessen cracks, ensure structural safety and construction quality.


Introduction
In recent years, with the increasing improvement of Chinese infrastructure construction, urban underground rail transit has also developed rapidly.Because of the advantages of unaffected climate conditions, safety, speed and environmental protection, rail transit has become the main force of the transportation network in major cities.Statistically, by the end of 2022,55 cities have opened urban rail transit operation in Chinese mainland , reaching 10287.45 kilometers.China's urban railway construction will stay in the golden stage of development for a long time in the future.At present, more and more subway projects widely use large-area concrete structures.The characteristics of small thickness,huge plane size [1],large concrete pouring volume, cement hydration temperature rise make it easy to produce crack, affect the function of the structure, reduce the durability of structure, and greatly impact operation safety. So in the urban railway engineering construction,how to effectively control cracks gradually attracts more attention from the scientific research workers. Based on the research results of control measures, mechanism and control technology in existing projects for large-area concrete shrinkage cracks, this paper takes a vehicle section of a subway project in Wuhan as the background, proposing preventive measures from two aspects of concrete material and construction technology, and discussing the technology for controlling large-area concrete cracks in subway section.

Application of large-area concrete cracks in the project
In the past few decades, with more and more projects of large concrete structures in China, the problem of large ground cracks has become more prominent.At present, Chinese engineers have had rich practical experience in large-area concrete structure projects, and put forward some effective measures to resist cracks, mainly from two aspects of construction materials and techniques.However, there is a lack of systematic theoretical research results on the control of such engineering cracks in China, and there is no clear criterion on the crack acceptance and repair methods of large-area concrete structures in the technical specifications and regulations of various industries in China.
The theoretical research of large-area concrete ground is not mature, but some effective practical methods have been obtained to solve the problem of concrete cracking.In recent years, concrete cracking has appeared in most large-area concrete structure cases in China. According to its crack situation, crack reasons and crack control measures, a simple summary (Table 1) is made, and several typical engineering cases are introduced in detail.

Case analyses of domestic large-area concrete structure (1) Front Parking Building Project of Terminal 2 of
Beijing Capital International Airport [2] Terminal 2 of Beijing Capital International Airport is a hub for temporary airport parking.The parking building is rectangular, stretching 262.8m from north to south, 133.8m from east to west and covering an area of about 167,000 square meters.After inspections by the construction department, it is found that the beams, plates and column components of the parts in the main structure have different degrees of defects.There are many cracks on the surface of the beam members.The crack width is mostly between 0.05-0.7mm.Most beam bottoms has transverse cracks, irregular vertical cracks (shown in Figure 1 (a)); The destructive form of the plate members is mainly manifested as lots of cracks around the transverse main joint.The maximum width of the main cracks reached 0.5mm (shown in Figure 1 (b)); The main form of the column member cracks is vertical cracks and circular cracks along the column.The interwoven mesh cracks have a maximum width of 1.5mm(shown in Figure  1 (c)).After the quality detection and safety identification of the testing mechanism, the conclusion is that the temperature effect, shrinkage deformation and live loads on the floor lead to multiple cracks in the structure. In terms of concrete structure design, the engineers give priority to different high-performance concrete, and control the temperature difference by mixing high-quality fine admixture and micro-expansion agent, strengthen the early construction and maintenance, setting up postpouring belt and other technical measures, which has a remarkable effect of project crack control. (2) Nanjing International Exhibition Center [3][4] Nanjing International Exhibition Center integrates exhibition, conference, trade, information, entertainment and catering service functions as one, and is renowned for its modern facilities, complete set-ups and unique modeling.The total construction area of the project is 89,000 m 2 .The building length is 292.8m, in width of 158.5m.The main structure of the project ( Figure 2) is divided into two parts. The roof is a large arc steel arch with a span of 75m, and the second floor is a large exhibition hall (240m x 117m). Expansion joints are arranged in both longitudinal and transverse sections. During the construction period, builders observe the G~J axis section of the 6 axis beam and the floor cracks, and see the cracks of prestressed and stressed concrete tension ( Figure 3 and 4 respectively). Nearly 10 cracks appear in H-J axis before stressed tension in width between 0.1-0.35mm; 9 cracks in G-H axis in width between 0.07-0.25mm.Most of the cracks are distributed near the bull leg of the steel truss, perpendicular to the beam length direction and extending upward. The distribution is relatively even, with no concentrated cracks. The shape of the cracks is wide in the middle and narrow on both sides.These cracks are typical temperature cracks. Due to the elevated thermal temperature of concrete, the weak part of the strong electric pipe produces stress concentration, the stress exceeds the tensile strength of concrete, and then the structure produces cracks. The construction department mainly strengthens the structural cracking resistance from the three aspects of concrete materials, concrete casting and maintenance measures.Concrete materials is low hydration thermal concrete with strength above C40.Moreover,it adds slow coagulation, water reduction and micro expansion into concrete. The pouring method is layer vibration, which shall be dense, the surface showing floating slurry and no longer subsidence; maintenance measures is taken for water storage.In the later process, after 4 months of observation, the temperature cracks were significantly reduced, and the construction scheme effect was obvious.  Combined with the above domestic large-area concrete structure projects,author can summarize the following main control measures:

Cracks and crack control measures in largearea structural projects
(1)Most of the large-area concrete structures adopt prestressed reinforced concrete as the main measure to resist the temperature deformation and shrinkage deformation.Prestress technology can improve the crack resistance of the concrete in the later stage, but also well close the early concrete cracks. For the shrinkage cracks and temperature cracks in the early stage, the crack width of the cracks is reduced after the prestress tension to a certain extent.
(2)The choice of materials exerts significant influence on large concrete structure cracking. Using a new type of concrete can reduce the early hydration heat,enhance concrete compaction and decrease contraction value.Another choice is adding high-quality admixtures into the concrete,such as water reducer, expansion agent, etc.More applied expander agent in projects is UEA expansion agent which ensures that the concrete produces cubical expansion during the hardening process.When the structure is limited by the boundary conditions, a pretensile stress ,which can effectively offset the tensile stress inside the concrete structure to guarantee no cracking, occurs within the concrete.
(3)Post-pouring belt or expansion reinforcement belt is also a common anti-crack method, often as a compensation measure.The post-pouring belt can overcome the shrinkage caused by the temperature difference, and the gap is generally reserved in advance.
After the internal hardening contraction of the concrete, the post-pouring belt part is done, which is conducive to protecting the integrity of the components.Combined with characteristics of the internal structure and the external environment, the distance between the rear pouring belt is usually controlled at 30-40m. If reasons of the building structure lead to no rear pouring belt, the expansion reinforcement belt is generally adopted.
(4)Construction technic is also a very important factor in the control of large-area concrete structure cracks.Generally, the production of cracks can be controlled from the aspects of concrete pouring methods, maintenance methods and construction methods. The common concrete construction technics to reduce cracks include layered pouring and layered tamping ,lowering the adverse impact of concrete temperature and shrinkage. The crack control measures mentioned above play different roles in different engineering projects, but there is a lack of theoretical researches of various practical methods, and the protection mechanism isn't clear. Therefore, in the future, how to explain large-area concrete cracking and control problems in principle is the core research direction.

Research status on shrinkage crack mechanism of large-area concrete
Combined with domestic and foreign research results [5][6][7][8] and construction engineering cases, cracks can be divided into two kinds according to the causes of cracks: one is caused by the main deformation effect of the construction structure, such cracks account for about 80% -85% of the main cracks of all construction structures, such as temperature deformation, expansion deformation, uneven settlement of concrete; the other is caused by the load.The cracks only accounts for 1% -20% .Therefore, crack control should focus on structural deformation.This paper discusses the factors caused by large-area concrete structure cracks and mainly starts from shrinkage deformation, internal and external temperature difference change, construction technic, quality of raw materials and so on.

Contraction crack caused by concrete shrinkage deformation
The shrinkage deformation of concrete refers to the phenomenon of volume shrinkage due to the deformation when the concrete is not subject to external forces. Influencing factors include the water to ash ratio, the amount of gel material, the cement dosage, etc.The shrinkage of concrete can be divided into temperature contraction and non-temperature contraction [9] . Temperature contraction refers to the contraction caused by thermal expansion and cold contraction, and nontemperature contraction refers to the contraction deformation caused by non-temperature factors, including chemical contraction, dry contraction, plastic contraction, carbide shrinkage, etc. In engineering practice, the main consideration is temperature contraction and dry contraction [10] . While several other contraction types account for relatively small proportions in concrete shrinkage.The general shrinkage rate of concrete is 0.04-0.06%, while the stretch limit of concrete is 0.01-0.02% [11] .Concrete is prone to crack when the tensile stress generated by the surrounding constraints exceeds the concrete limit tensile strength [12]. At present, scholars at home and abroad have studied many concrete shrinkage deformation prediction models, including ACl209 [13]、CEB 90 [14]、GL2000 [15]、B3 [16]、G-Z [17].The factors considered by each prediction model are different ,and hence the accuracy is also distinct.But all these do not determine the quality of a model.Factors considered by the various predictive models are listed in Table 2.As can be seen from Table 2, the component size, humidity, and concrete strength are the most considered factors in five models, while the other factors are considered by only a few models.Among them, the B3 model included the most comprehensive factors, and only the B3 model considered the component shape, but all models did not consider the effect of admixtures.After consulting the literature [18][19] [20],it is learned that from the model prediction accuracy, the B3 results are the best.The theoretical calculation value results of ACl209, CEB 90 model are close to the test value results, GL2000 is secondary and G-Z is worse.B3 model is the semiempirical and semi-theoretical formula, with the highest prediction accuracy compared to other models. For the large-area concrete structure in the actual engineering cases, concrete shrinkage and dry shrinkage of road surface can cause plastic deformation, resulting in crack shrinkage problems in the structure.For the underground railway section,about 5h after the completion of concrete pouring, in the process of concrete contraction and condensation, the wind speed, high external temperature and the violent reaction of concrete surface cement to water, large water evaporation, result in concrete plastic shrinkage [21]

Contraction cracks caused by internal and external temperature difference
Temperature deformation is also one of the main reasons for the shrinkage cracks in the concrete in the underground railway section.Large-area concrete has the characteristics of large section size, small thickness and more cement dosage. In general, a large amount of hydration heat gathering inside the concrete structure is not easy to emit, leading to the continuous rising temperature of the concrete center.However, the concrete surface heat dissipation is fast, so the soil layer inside and outside concrete forms a large temperature gradient [22].At present, the domestic theoretical research mainly focuses on finite element analysis, and many scholars combine with engineering practice, compare theoretical data and engineering results of practical projects, and take effective measures to achieve the purpose of crack control. Chen Yanzhuo [23]in the article applied ANSYS on the top of a city dig subway station to carry out temperature stress simulation.Through finite element simulation, temperature field analysis, stress field analysis,it shows that the temperature difference can lead to tensile stress in the plate when temperature is highest.Air heat exchange and large surface area lead to this phenomenon The core temperature is much lower than the temperature of the upper surface and is easy to prone to shrinkage cracks. Yuan Kang [24] established a finite element model of a single room on the basis of the substrate temperaturecontraction stress calculation formula through numerical simulation.It shows that the temperature-contraction stress is the main factor which causes the floor crack.When the temperature shrinkage stress is largest, the greater the temperature difference inside and outside the concrete, the greater the floor temperature contraction stress.So carrying out construction in the spring and autumn season is preferable,to achieve the purpose of temperature control. Wu Wenwu [25]conducted analyses of Wuhan Yangluo Yangtze River Highway Bridge project.Combined the finite element analysis and the field test data, it suggests that when the temperature difference inside and outside of the concrete structure is not less than 25℃, a special concrete temperature control design of anchorages must be carried out. Zuo Wenge [26]proposed that the temperature difference of concrete structure due to the environmental climate, structure, material hydration and other factors, uneven temperature difference is prone to make concrete crack in the early stage of concrete pouring. Chen Liling [27]suggested that the reasons for concrete cracks in large area construction, among which the most common is that the temperature difference between inside and outside concrete causes cracks.In the concrete pouring,errors resulted by hydration time control will lead to a large amount of heat gathered inside the concrete component.The temperature difference inside and outside the concrete structure is too large,which results in tensile stress.If the tensile stress exceeds the crack resistance of the concrete, it is easy to cause different degrees of temperature cracks. The above study shows that the studies of temperature control of the large-area concrete on temperature control have formed a series of mature results, mainly for test and demonstration from numerical simulation analysis, field monitoring and thermal calculation, but the results from the mechanism are less and not in-depth enough.

Contraction crack caused by construction technics
The construction technic is an important aspect of the research of large-area concrete cracks, which has a great impact on concrete shrinkage cracks, usually controlled and studied from several aspects of form work projects, maintenance measures, how to set post-pouring belt, and prestressing force technology.Aiming at the construction measures of large-scale areas of concrete, many scholars have carried out a lot of theoretical and experimental studies. Lin Haishan [28]starting from the engineering examples of the subway station in Xiamen coastal area, the insufficient concrete pouring will lead to uncompacted concrete structure, weak parts are easy to crack and form larger cracks when there are loads, and excessive vibration will cause the separation of aggregates and cement slurry layers, and produce dry shrinkage phenomenon. Wei Yulian [29]summarized previous results, and considering similar engineering construction experience for concrete structure crack control in Nanjing four bridge project.Through finite element analysis, combined with the structural characteristics of the anchor system, it shows that if the cooling pipe is not arranged in concrete,the cement hydration heat rise will eventually cause cracks. Wei Min [30]studied causes of structural cracks in the large-area cast-in-place building cover. Combined with engineering cases, the use of templates with poor thermal conductivity and unreasonable disassembly time will lead to the increase of hydration heat and cracks in the construction stage. Lin Min [31]summarized the typical engineering examples of guangdong Olympic sports center, Huawei scientific research center, Beijing east square.It shows that many large-area concrete projects exert prestressing force to resist the influence of shrinkage stress on cracks, when using prestressing force, post-pouring blocks provide the working surface with prestressing force to inhibit concrete from early shrinkage cracks. Chen Jinrong [32]pointed out that road cloth or sacks are often used in projects to make water maintenance. In the high temperature environment, the internal water of the concrete is greatly evaporated.The temperature between inside and outside the concrete is large, which will cause the production of plastic cracks or temperature cracks. Construction technics are the most important part of crack control.Not only because the construction technics can be controlled with greater flexibility, but also it has a decisive impact on the quality of its structure. Based on the actual engineering cases, most scholars analyze the impact of construction technics on concrete's characteristics.They put forward that insufficient vibration, backward maintenance measures, ignoring temperature control,wrong post-pouring belt location can lead to shrinkage cracks, so it is necessary to choose appropriate and effective construction measures,which can reduce the impact of cracks on engineering quality and life.

Quality of raw materials
In the large-area concrete structure, the type and dosage of cement directly affect the quality of the large-area concrete, the amount of the cement hydration heat, and the temperature stress [31]; Admixtures include water reducer, expansion agent, retarder, early strength agent and so on. The addition of concrete admixtures can significantly reduce the hydration heat of cement, prevent excessive temperature stress, and improve the early strength of concrete, and then improve the crack resistance of concrete [31]; Water cement ratio has a certain influence on the early self-contraction, hydration temperature rise and temperature deformation [33]. It can be seen that the type and dosage of cement, Water cement ratio, admixture variety and volume and so on,which have a great impact on the concrete cracks, so more and more scholars have also carried out related researches. Tao Jianqiang [7]put forward that cement hydration heat is the main factor of temperature stress, so the variety and dosage of cement are key factors in the control of largescale concrete cracks.Cement with high C3A 、 C3S produces more hydration heat,so it's better to choose low or medium heat cement; the more cement dosage, the more hydration heat of concrete.So that the temperature is higher inside the concrete. Li Zhanyin[34]demonstrated that the water cement ratio has a direct effect on the hydration rate.When the water cement ratio (W/C) increases from 0.3 to 0.6,28d hydration heat significantly increases.The water cement ratio of the most unfavorable plastic shrinkage area of concrete is about 0.5. When W/C> 0.5, the crack area decreases with the water evaporation rate increasing.When W/C<0.5, the crack area increases with the water evaporation rate increasing. Yang Changhui [35]pointed out that when the water cement ratio is high, the capillary pore diameter is large. The concrete mixture evaporates more water, the capillary develops at a small rate for a period of time and the heat liberation rate of hydration decreases;When the water cement ratio is low, the concrete integrity and cohesion are good, the plastic settlement is small, and the plastic shrinkage crack area is small.Therefore, plastic cracks caused by the rapid evaporation in the early stage should be avoided during construction. Pan Lisha [36]focused on the study of the influence of four water reducers on the cement hydration.The mechanism of admixture action is to change the rate of hydration to affect the cement hydration heat, and the water reducer can significantly disperse the cement particles, improve the contact degrees of cement particles and water, and affect the cement hydration rate. Wang Zilong [37]pointed out that the expanding agent can delay the condensation time of cement to compensate for the early shrinkage of concrete. Domestic researchers studied the influence mechanism of concrete shrinkage cracks through construction material quality in many aspects, and the remarkable results can be summarized as two points: Additives are used to influence the heat of hydration of cement by changing the rate of hydration.The type and dosage of cement can directly affect the hydration heat. The above study shows that the production of large-area concrete cracks is mainly caused by shrinkage deformation,difference changes of internal and external temperature, construction technics and raw material quality.A large number of scholars for large-area concrete cracks made a lot of theoretical analysis, field tests and engineering practice application.They made a lot of research results, but the overall theoretical level is not high and lack of comprehensive quantitative analysis and calculation.Most of the research results are based on projects and make qualitative mechanism analysis and put forward improvement measures,but most for bulk structure, less for large-area concrete.

Actual researches of large-area concrete shrinkage crack control in subway sections
Subway section is more difficult due to its complex construction technics, structural boundary conditions and complex geological conditions.The surface size of largearea concrete is large and the hydration heat produced after pouring is difficult to dissipate, so it's easy to produce greater temperature stress and deformation.Bottom plates, side walls, roof plates are constrained to varying degrees, concrete structure under loads and soil pressure.If the stress exceeds the structural tensile strength allowable value, it will harm the structural safety and building service life.Therefore, according to characteristics, it's necessary to control from the construction materials, construction methods, structural designs for different subway projects. How to conduct scientific analysis of concrete structure, how to take effective construction technics, how to choose better construction materials and how to reduce repair costs are important and difficult in projects.Solving these problems can bring huge economic and social benefits.Therefore, combined with numerical simulation, many researchers make in-depth analysis of the causes and distribution characteristics of cracks in subway sections and put forward effective measures to reduce shrinkage cracks from the perspective of construction designs. Zhu Xianfa [38]relied on the Changzhou Urban Rail Transit Line 1 project.He made the mixing proportion design and the application of anti-cracking in the concrete structure of main parts of the subway station.It shows that the HME-V efficient cracking agent with the dual control function of temperature and expansion can effectively reduce the generation of cracks. Ma Yongbo [39]put forward that the use of low thermal cement, fly ash of high quality , embedding the cooling water pipe in concrete, and controlling forming temperature is conducive to inhibiting the production of side wall cracks in the subway station. Xie Xiaoli [8]found that section pouring concrete will cause temperature contraction and humidity contraction is not synchronized which lead to crack generation through the super long span structure concrete beam temperature and strain test.After controlling the raw material temperature of concrete, optimizing the concrete mixing proportion, reducing the concrete pouring temperature, layered continuous pouring technology,it shows that there is almost no crack in the subsequent projects. Hu Dongkang and Hu Yixin [40][41][42][43]combined with a large number of practical cases from the basic technical requirements of the construction technics.They adopted new steel bar truss deck,sequence method, hoisting roof mesh and other technologies to reduce the generation of cracks and achieve the project goal. Liao Dongming [44]took an example of Meijin station in Shenzhen metro line 9.From structural designs, temperature, construction materials, construction technics, he analyzed the causes of concrete crack formation.And from materials, mixing proportion, templates, concrete pouring technics, empty positions, regular inspection of foundation and completion acceptance,he put forward methods to limit concrete cracks. Zhan Jianjun [45]took a station in jiangxi metro line 2 as the research object and built MIV-BP neural network models.For thin protective layers, large concrete blocks and unreasonable concrete mix proportion, after using steel mesh on the external wall, reducing concrete temperature,embedded cold water pipes in the side wall,adding admixture and other measures, the number of cracks in the station is significantly reduced. Li Zhaoping[46]explored the cause and deformation law of the concrete cracks in the joint part of the subway station, and then concluded that the mortise and groove parts in the improved Tongue and groove type connector can jointly resist the bending moment, which is beneficial to reduce the generation of cracks. Yu Yiming [47]made analyses of mixing proportion, cracking risk assessment, concrete pouring, vibration, mold removal and maintenance for concrete structure design of the side wall in a subway station.The test datas show that the use of cracking resistance of magnesium oxide with strong compensation performance can effectively reduce the number of cracks in the side wall. Fang Kejun [48]took a subway station of Beijing as an example.According to the law of cracks, width and time,he summarized the reasons for cracks and put forward choosing anti-seepage concrete, reasonable use post-pouring belt and section pouring to control the production of concrete cracks.It shows that there is no concrete cracks and decreasing number of shrinkage cracks, which reaches the engineering goal. Chen Yanzhuo [23]applied ANSYS to simulate the temperature stress of the roofs in a openly excavated subway station of a city.It shows that the difference of theoretical analysis and numerical simulation results is not obvious which means that preliminary design guidance of finite element software can be used to provide reference for crack control of similar projects. Fan Jiangsu [49]made the force analysis and the shrinkage measurement of the roof structure for a openly excavated subway station .It proposes that 425 slag cement, postpouring belt, layered pouring, layered tamping and other measures are taken to ensure the normal use of the roof. To sum up, scholars at home and abroad combined engineering practice verification and numerical simulation of concrete cracks for the subway station.They found that the optimized design of mixing proportion, reducing temperature of concrete pouring , reasonable reinforcement, using expansion agent can effectively control large-areas concrete cracks.Due to its high durability and safety requirements of large-area concrete, its structural designs, construction control and concrete material selection are more complex, and the performance of different measures in different engineering backgrounds is also different.At present, the research on the large-area concrete shrinkage cracks in the subway section is less, and the effect of the optimized design still needs to be explored.Therefore, on the basis of previous researches, this paper focuses on large-area concrete crack control from the aspects of concrete material quality and construction technology based on a subway project in Wuhan,which has practical guiding significance for similar projects.

Project Overview
The subway station is a second-story island-style station, in which the large-area concrete beam runs north-south, the total length is 220m,the standard width is 17.6m,the roof cover thickness in the center of platform is about 1.5-2.9 m, and the depth of foundation pit bottoms in the station of standard section is about 21.8m.It is a large-area and cast-in-place concrete structure.The station adopts the method of pouring in segments, with a post-pouring belt at the interval of 50-70m, and the enclosure structure adopts a continuous wall that is 700mm in thickness.The main body of the station is constructed by the method of open excavation and shield method at both ends, and both ends are originating technique for the shield structure. The construction joints or deformation joints are set in the roof plate, side wall and bottom plate for waterproof treatment.The main structure of groove section is constructed from small mileages to large mileages. In the same groove section, the construction sequence of each component is bottom plate, side wall, middle plate, roof plate and middle column. There was no crack in the early maintenance period. The age period of concrete is about 4 weeks. Some transverse cracks and vertical cracks appear on the roofs.There are a total of 18 cracks and the width of them is 0.3-0.7mm.They are mainly distributed in the four corners of the roof, extending around the station.Two floors are evenly distributed, roughly the same distribution.The platform side wall has 10 cracks, mainly including vertical cracks, through cracks.The crack width is 0.1-0.4mm and crack depth is between 142mm and 378mm.The shape is large in the middle and small at both ends, the tip mouth is at the top or bottom of the side wall.After the cracks in the structure are evaluated by relevant institutions, the conclusion is that the wall and plate cracks at the commuter station exceed the allowable range because interval of two concrete pouring is too long and the temperature difference during the temperature detection process exceeds 20℃. The large-area reinforced concrete ground in the vehicle section is exposed to the external environment for a long time. The ground temperature change is greatly affected by the external environment.The external environment impact on the concrete floor temperature manifests the seasonal temperature difference.Floor concrete pouring is mainly from July to August.The temperature in Wuhan is high.The average highest temperature is 32.5℃ and the average lowest temperature is 25.0℃.Concrete is affected by the external environment of frequent drying and wetting.Concrete condensation hardening causes a large temperature difference between the component surface and the internal concrete.Due to the area of the large-area concrete ground is large and the thickness is relatively small,the lower surface constraint effect is more obvious, which is more sensitive to contraction.It's easy to produce large deformation, and can lead to the production of shrinkage cracks.

Quality control of concrete materials
Before the construction operation, in view of the characteristics of the construction site, external environment climate change and other factors, in addition to strict sampling of raw materials for concrete mixes (cement, admixtures, sand, stone, etc.), it is also necessary to optimize the concrete mix ratio and choose the best mix ratio.Large-area concrete adopts the cement with low hydration heat and long condensation time, which can extend the time when the central point of the structure reaches the maximum temperature to reduce the hydration heat temperature in the center of the concrete structure [50]. At the same time, the cement admixture can be improved to enhance the concrete crack resistance ability.Mixed with selected quartzite aggregates, it is beneficial to reduce the probability of large-area cracks [51]. Under the condition that the strength of the concrete and the slump of the substrate can basically meet the design requirements, the content of admixture and aggregate increases and the amount of cement decreases.In addition,the water reducing agent, expansion agent, retarder can be added to large-area concrete according to the mix ratio, in order to improve the concrete performance.

Measures of conservation
The concrete construction area of the subway section is large, so the construction and maintenance work need to be carried out at the same time.With concrete strength meeting designing strength, it's necessary to ensure that the concrete surface remains wet.The two-sided framework maintenance method is adopted to ensure good moisturizing and heat preservation performance. The appearance quality of the concrete after mold removal is very high, and there is no crack, which effectively solves the problem caused by large temperature difference.

Arrangement plan for post-pouring belt
The post-poured concrete strip mixed with a small amount of aluminum powder is adopted and use highperformance concrete.The pouring area shall be divided according to the location of the post-pouring belt in this project.The"slant layering method"is adopted to carry out concrete layering pouring. That is, The pouring sequence follows from high to low, with each layer of 300-500mm thickness of pouring, so as to ensure the continuous operation of concrete and pouring uniform.The vibration method adopts the "secondary vibration method", and the concrete insertion depth of the vibration rod is about 5cm, which eliminates the concrete surface gap and strengthens the concrete anti-crack performance.

Temperature monitoring
Temperature monitoring is a key link in the construction of large-area concrete. In addition to the temperature detection in the process of concrete pouring, it is also necessary to strengthen the detection of environmental temperature and internal and external temperature difference in the maintenance process.The temperature difference between inside and outside of the general concrete structure shall be controlled to not exceed 20℃.The project uses internal water pipe and pours cooling water to reduce the internal temperature,which effectively reduces the risk of concrete cracking.Using concrete tank truck with insulation layers to effectively reduce the heating of concrete during transportation.In order to reduce the mold temperature, pouring concrete is carried out as much as possible at night. After taking the above series of anti-crack measures, there is no crack in the subsequent-poured concrete structure, and the ideal construction effect are achieved.The problem of shrinkage crack control of large-area concrete structure in underground railway section is successfully solved.

Conclusion
In this paper, on the basis of a large number of domestic and foreign references, the research results on the mechanism of shrinkage crack cracking in large-area concrete and its controlling methods at home and abroad are summarized; on the other hand, based on a vehicle section of the project of Wuhan city rail transit, the existing engineering experience and research results are flexibly applied to the actual project.Some conclusions are followed: (1)In the analysis of the mechanism of shrinkage cracks in large-area concrete,combined theoretical and engineering researches and analyses,different scholars come to conclusions that the factors affecting large-area concrete cracks include internal and external temperature changes, shrinkage deformation, construction techniques, raw material quality, etc.Although Many research results have been achieved, there is a lack of systematic analyses and calculation.And the mechanistic analysis is also mostly for specific engineering cases, without systematic results.
(2)In large-area concrete crack controlling measures of underground railway sections , domestic and foreign scholars found that optimising the mix ratio, reducing the temperature of concrete pouring, reasonable reinforcement and other measures can effectively reduce the generation of shrinkage cracks.At present, there is few domestic researches related to the construction controlling measures for large-area concrete cracks in underground railway sections.So the optimal design still are to continue to explore.
(3)Based on the large-area concrete structure of the vehicle section of the metro project in Wuhan city rail transit, through the use of concrete material quality controlling, double-sided with formwork maintenance, sloping layered pouring, temperature monitoring and other measures, the crack treatment effect is obvious and can ensure normal operation of the metro. The anti-cracking measures in this project can provide relevant guidance for the quality problems caused by the construction of large-area concrete structures in the future. (4)The causes of large concrete cracks in underground railway sections are complex, and there are many factors to be considered.So there are many issues that have not been noted or studied more systematically during analyses. How to carry out real-time monitoring of cracks in key sections of underground railway sections, how to apply fibre concrete impermeability technology to underground railway sections, and how to use induced joints to control crack development are all important directions for future research.