1. Mechanism of concrete cracks in shield segments
The shield tunnel is a major project, and the shield segments used as the supporting structure are generally high-performance concrete and high-strength concrete, but even so, due to various reasons, micro-cracks or even visible cracks often occur in the segments during segment production, tunnel construction and operation. In recent years, many scholars have focused on a series of adverse effects of cracks on shield segments, systematically studied the causes and mechanisms of cracks in segments, and achieved some meaningful research results. Lin Nan [11] and Wu Zhenzhi [12] believe that according to the causes of formation, segment cracks can be divided into two categories: cracks caused by deformation and cracks caused by stress (including direct stress and secondary stress). Many scholars believe that the appearance of cracks is inseparable from the working process of segments, and cracks are generally divided into three categories: production cracks, construction cracks and operation cracks.
1.1 Cracks in the production process
In the production of segments in the prefabrication plant, due to the smallest density of water in the concrete constituent materials, after being subjected to the extrusion stress when the vibrating rod is working, the moisture in the concrete is easy to move to the joint between the side plate of the mold and the side of the segment and stay, resulting in plastic shrinkage of the surface and the appearance of surface microcracks when the concrete is solidified and hardened. Coupled with the action of self-weight and vibration force, the coarse aggregate in the concrete will sink and the cement slurry will float, which will deteriorate the uniformity of the concrete, which will also lead to the initiation and expansion of cracks [13]. For example, severe mesh cracks occurred in parts of Guangzhou Metro Line 1 and in a shield section of the Singapore Metro.
In addition, changes in the ambient conditions around the shield segments, such as temperature and humidity, can also cause cracks in the concrete segments. When there is a large temperature difference between the inner and outer surfaces of the concrete, it will affect the hydration rate of cement, so that the cementation force produced by cement hydration is not uniform enough, and it is easy to have large temperature stress. At this time, the concrete has not yet been finalized, and the connection force of the cohesive structure formed is small, and mortar cracks are easy to occur under the action of temperature stress. Similarly, when the humidity in the air is low, the evaporation rate of water on the surface of the concrete is much greater than the excretion rate of the water inside, resulting in more obvious shrinkage cracks [14]. For example, there are some segments in a shield section of Guangzhou Metro Line 2 and Singapore Metro that have serious grid-like cracking [15].
1.2 Cracks generated during tunnel construction
During transportation and installation, the edges of the shield segments are susceptible to damage and cracks caused by collisions, and the box-shaped segments are also easily cracked by the thrust of the shield jack. During the whole process of hoisting, transportation, shield machine tunneling, and segment assembly, the edge of the segment will inevitably be squeezed and collided by external forces and machinery, so the corners of the segment are damaged from time to time [16]. On the other hand, in addition to being the supporting structure of the tunnel, the shield segment also plays the role of providing reaction force for the march and turning in the shield construction, and when the jack thrust is at a certain angle with the segment, the segment will have additional bending moments, resulting in stress concentration in some parts and cracking of the segment [17-18]. In addition, the extrusion of the shield tail during the de-ring process may also cause segment misalignment and even cracking [19]. Lining damage caused by these construction factors may seem minor, but it can cause major problems after the lining deteriorates over time.
For example, the cracking of the concrete segment was found in the middle of the tunnel of Guangzhou Metro Line 1 and Line 2 in China, and the cracking at the top of the vault was more serious, and the distribution direction of 90% of the cracks was extremely related to the direction of shield propulsion. The shield segments of the gas tunnel in Sorenberg, Switzerland, are made of Bekaert steel fiber reinforced concrete, and the segments in the excavation section are severely cracked after the shield tail is assembled [20-21]. In 2009, during the excavation process of the shield tunnel in the 7th bid section of the first phase of the Shenzhen Metro, concrete cracks were found at the outer arc surface, corners and circumferential bolt holes of the segments. It can be seen that the influence of various external forces and mechanical actions on the crack propagation of concrete segments during the construction process of the tunnel cannot be ignored.
1.3 Cracks created during subway operation
Cracking of lining concrete is the most common form of disease in subway tunnels, and during the operation of subway tunnels, as the most important supporting structure of subway tunnels, concrete segments will also sprout various cracks under load [22]. In addition, the underground environment of the shield segment is relatively complex, and changes in the surrounding environmental conditions (including temperature, humidity, and pH) may cause the segment to bear certain additional stresses, and the foundation deformation, carbonization shrinkage, and steel corrosion expansion will cause cracks in the segment [23]. For example, in 2009, a section of the tunnel of Shanghai Rail Transit Line 11 was damaged, and it was found that the shield segment was broken down by the PC pile of the Beijing-Shanghai high-speed railway that was under construction [24]. In July 2011, the left line tunnel segment between Hubin Station and Longxiangqiao Station of Hangzhou Metro Line 1 was pierced by the geological exploration borehole of the West Lake Avenue Comprehensive Consolidation Project [25].
In view of the cracking problem of shield segments during subway operation, the relevant personnel used on-site measurement and theoretical analysis to study the cracking phenomenon of the segments. Yang Xu et al. [26] fitted the measured values of the Nanjing Metro shield tunnel section in operation, judged the stress state of each segment of the tunnel section, explained the internal reasons for the cracks in the tunnel segments, and provided a basis for the operation and maintenance of the shield tunnel. In view of the hazards of the water inrush accident in a section of the Xi'an Metro to the segments, the research of Lai Jinxing et al. [27] showed that the uneven stress of the segments caused by the water inrush and the stress concentration led to the initiation and rapid development of the segments. Based on fracture mechanics, Luo Yong [28] studied the cracking mechanism and control method of tunnel segment structure. Dong Fei [29] analyzed the influence of operation time on the cracking state of the tunnel according to the disease detection results of the Beijing subway tunnel. The results show that the source of the disease of the shield tunnel lies in the deformation of the segment joint, which changes the shape of the shield section, resulting in the collapse of the segment and the phenomenon of water leakage in the tunnel.
It can be seen from the above that compared with the attention to the cracks during the production and construction of segments, there are relatively few studies on the law of segment cracks in the operation process, and most of the existing studies focus on the combination of on-site detection and theoretical analysis of cracks, and there is still a lack of research on segment cracks caused by multiple additional loads. Therefore, the study of the damage to the shield segment structure that occurs during tunnel operation must be put on the agenda and more attention should be paid to it.
2. Crack control technology of shield segments
2.1 Structural design and construction control
Studies have shown that although the current technology cannot temporarily prevent cracks in concrete, cracks can be controlled to a certain extent [30-31]. Due to the intricate factors that cause the cracking of shield segments, the research on the control of concrete cracks in segments has always been a hot and difficult point in tunnel engineering. Based on this, according to the different use environments of concrete, corresponding design specifications for the corresponding maximum allowable crack width have been issued at home and abroad, and attempts have been made to inhibit concrete cracking to a certain extent through three aspects: regulating raw materials, precise design, and safe construction [32]. In this regard, Wang Xingang analyzed the causes of segment cracks, divided segment cracks into shrinkage cracks, temperature cracks and uneven settlement cracks, and put forward targeted measures and methods for crack control. Zhang Hui proposed that the management of all aspects of segment production and tunnel construction should be strengthened, appropriate raw materials should be selected, the quality of segment forming should be ensured, and the concrete curing time should be paid attention to; Strictly control the reloading and stacking of segments in the factory; Adjust the attitude of the shield machine in a timely manner according to the tunneling situation, pay attention to the segment assembly sequence, strengthen quality control, and avoid the phenomenon of segment misalignment.
Similar to the above studies, Peng Bo [33], Bentz [34], and Gao Zhongwei [35] believe that it is necessary to start with the optimal selection of raw materials and strengthen the mix ratio to optimize the design, so as to reduce the risk of concrete cracking at the source. It is believed that the use of cement with low hydration heat can effectively reduce the probability of cracks, and the appropriate addition of active mineral admixtures such as fly ash and slag can greatly reduce the early hydration heat, so as to achieve the purpose of reducing cracks. Some scholars [36-38] believe that we can start from the perspective of improving the structural design and improving the construction technology, and reduce the cracking of concrete from the aspects of reasonable setting of post-pouring belts, appropriately reducing the temperature of molding, and insulating and curing concrete. r.i.Gilbert [39] studied the control method of early cooling shrinkage confinement cracks in concrete, and gave a reasonable procedure for controlling the amount of reinforcement required for early cracking from the perspective of the actual constraint degree of reinforced concrete components.
2.2 Raw material control
The data show that many concrete cracks occur at a very early age [40], and non-structural cracks due to deformation changes account for about 80% [41-42]. In view of the problem of early cracking of concrete, Lee [43] and Ba Hengjing [44] compared the early cracking performance of concrete mixed with silica fume, fly ash and slag powder with that of benchmark concrete through crack resistance tests, and the research results showed that the good matching of mineral admixture particles and cement particles could improve the workability of fresh concrete, improve the compactness of concrete structure, and then reduce the risk of cracking. Yang et al. [45] studied the effect of fly ash content on the early crack resistance of concrete by using a temperature-stress testing machine, and found that when the fly ash content was 40%, the number of early cracks was the least. Some scholars have limited the development of shrinkage cracks by adding fibrous materials to concrete, and found that mixed fibers can effectively reduce shrinkage cracks [46]. Some scholars have found that the incorporation of admixtures can also improve the early crack resistance of concrete [47].
In view of the current temperature cracks of concrete, Wang Tiemeng [48] summarized the concrete cracking phenomenon of many years of engineering practice, and proposed a set of methods to solve the temperature stress—the combination method of "resistance" and "release", which has been applied to many large-scale projects in China. According to Ming Li [49], the temperature difference between the peak temperature generated by concrete hydration and the surrounding environment is the key factor leading to the early cracking of the segmented concrete main structure, and self-shrinkage will increase the risk of cracks. Jianda Xin [50] experimentally investigated the effects of temperature history and degree of constraint on the cracking behavior of early concrete, and the results showed that there were significant differences in the cracking potential of concrete under different temperature history conditions. He Zhu [51] has shown that the failure behavior of confined cracks caused by temperature and/or shrinkage is different from that of direct tensile failure, and that cracking will occur when the confined tensile stress exceeds 76% of the direct tensile stress.
Concrete is a typical brittle material, and many scholars have tried to incorporate fibers into concrete segments to improve their toughness, while the crack resistance of concrete has also been greatly improved. The theoretical and practical research on fiber-reinforced concrete technology was carried out earlier in Western countries, and fiber-reinforced concrete segments were widely used in metro engineering [52]. China's Beijing Metro Line 10 has tried to apply hybrid fiber reinforced concrete segmentation technology, and Shanghai M6 Metro Line has built a 50m steel fiber reinforced concrete segment test section [53-54]. Xu Yuan [55] studied the effect of basalt fiber on the performance of high-performance metro segments, and the results showed that basalt fibers had little effect on tensile and flexural strength when incorporated into concrete, but significantly improved the impact toughness of cement concrete, thereby improving the crack resistance of segments.
It can be seen that many scholars at home and abroad have explored the crack control technology from many aspects based on the load and service status of the shield segment based on the causes of cracks.
3. Crack control technology of shield segments
3.1 Crack repair technology
Although scholars at home and abroad have proposed a variety of methods to control cracking according to the causes of segment cracks, the cracking phenomenon of shield segments is still intensifying due to uncontrollable factors such as materials, pouring, and construction. Therefore, the construction of rail transit and subway in major cities has an urgent demand for concrete crack repair materials. In order to improve the service life of concrete segments, researchers at home and abroad have developed concrete crack repair materials, among which chemical grouting technology is favored by scholars and technical researchers. Chemical grouting is a concrete repair technology that uses pressure equipment such as chemical grouting pumps to prepare certain chemical substances into a real solution and inject them into formations or fractures to diffuse, coagulate, or solidify them [56].
In terms of segment crack repair, Huang Huimin [57] studied the chemical grouting materials according to the tunnel structure, cracks and leakage water, and believed that polyurethane resin has good water activity and penetration, and can resist the corrosion of various underground waters. Epoxy resin has higher strength than concrete itself and has good adhesion. Ye Jiaofeng [58] optimized the mix ratio of the repair material, and Wang Jianhui et al. [59] used polyurethane material to repair the shield segments, and the results showed that the impermeability of the repaired segments was better than that of the matrix, but the compressive and tensile strength were significantly reduced.
3.2 Segment reinforcement technology
In view of the segment damage that has occurred, Shi Taiwei [60] analyzed the tunnel damage mechanism based on the example of a subway shield tunnel damage project, and carried out the segment reinforcement design work on this basis. This paper analyzes the main causes of the disease of the subway shield tunnel, and proposes that the reinforcement design of the shield tunnel should follow the principle of "first urgent and then slow, both inside and outside", implement the repair standards of stiffness and durability, and meet the safety and durability requirements of the later operating structure. Liu Tingjin [61] implemented a bonded steel reinforcement scheme for the shield segment lining in a section of Shanghai Metro, and analyzed the bearing performance and failure mechanism of the bonded steel reinforced shield tunnel lining by using finite element software. On this basis, the reinforcement effect of shield segments in Guangzhou Metro was simulated and evaluated, and it was concluded that the reinforcement of bonded steel could significantly improve the bearing capacity of shield segments.
From the above, it can be seen that there are many studies on the repair of cracks in concrete segments, but there is still a common problem in the current research, that is, whether the cracked subway shield segments have durability deterioration in the complex underground service environment after repair or reinforcement, and what is the deterioration law, there are still few studies on this aspect. The subway segments have been in the underground humid environment for a long time, and the durability of the segments after crack repair directly determines the safety of the tunnel, so more attention should be paid to this aspect.
Conclusion
Due to the complexity of the service environment, the crack problem caused by the subway shield segment has attracted a large number of scientific researchers to study and explore it in many aspects, and some meaningful research results have also been obtained. At present, most of the research focuses on the analysis of crack generation mechanism, crack control and repair technology, but there are still some shortcomings, specifically in the following aspects:
1) Based on the complexity of the concrete material, cracks in the concrete are inevitable in the project. Although the shield segments are buried under the soil, the environmental effects in different areas are not the same, and the load effects on different parts of the segments are quite different. Therefore, it is necessary to analyze the segment cracks according to the specific engineering conditions, so as to make the proposed prevention and control measures more targeted and systematic.
2) Concrete mix design is the key link to ensure the crack resistance of segmental concrete, due to the complexity of concrete materials themselves, and the differences in the concerns and research objectives of different scholars, the current academic research depth on the cause, development and impact of segment cracks on the structure is not uniform. At present, the relevant research on the mechanism of anti-cracking is mainly analyzed through experiments, and it is far from rising to the theoretical level.
3) At present, the research on the crack repair technology of shield segments is still in the qualitative evaluation stage based on the combination of on-site detection and comprehensive system evaluation, and little attention is paid to the durability of the repaired segments. Therefore, a lot of work needs to be carried out in the quantitative evaluation of the structural performance and health state of shield tunnels under crack repair.