Discussion on Improving the Self-waterproofing Quality of Civil Air Defense Structure

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From the internet; hope to share it with everyone! ! ! 　　 　　1 Introduction The 21st century is one in which underground spaces are developed as important resources. Civil defense facilities not only serve as an essential part of the defense system during wartime, but they also play a crucial role in economic development in peacetime. However, leakage in underground construction projects has long been a major problem that plagues builders. Leakage to varying degrees in basements limits the utilization of these existing civil defense facilities, requiring significant investment of human, material, and financial resources each year for repairs. Therefore, improving the waterproofing performance is one of the key issues that need to be addressed in the construction of civil defense facilities today. The newly revised \"Code for Waterproofing Technology of Underground Structures\" (GB50108－2001) from 2001 specifies that structural self-waterproofing is \"required\", rather than \"recommended\" as in the previous version, reflecting the principle of giving priority to structural self-waterproofing. Structural self-waterproofing is the fundamental measure for ensuring the waterproofing of civil air defense projects, and improving the quality of this self-waterproofing is key to achieving optimal waterproofing results in such projects. Based on my own experience in participating in the design, quality supervision, and construction of civil air defense projects, I will discuss several key aspects that should be taken into account to improve the self-waterproofing capabilities of civil air defense structures. It aims to serve as a starting point to contribute to the current development of civil air defense. 　　 　　2 Main factors affecting the self-waterproofing quality of civil air defense project structures 　　　2.1 Engineering geology aspects. Engineering geology has a significant impact on the uniform settlement of foundations, and it is one of the key factors affecting the waterproofing performance of civil air defense projects. In some urban areas of Hunan, the geological conditions are quite complex, with karst formations and soil caves present to varying degrees. These areas are prone to geological disasters. If the engineering geological conditions are not properly understood, if the depth of geological drilling is insufficient, or if adjacent geological reports are copied without proper analysis, then the engineering geological report will not accurately reflect the properties of the soil layers, groundwater conditions, and results of geotechnical tests. This can lead to suboptimal design of structural solutions and inadequate construction measures, potentially resulting in uneven settlement of foundations and water leakage in underground shelters. 　　2.2 Design aspects. In terms of understanding, the design concept that emphasizes the self-waterproofing properties of concrete structures as the foundation of waterproofing has not been truly established; in practical work, emphasis is often placed on waterproofing materials rather than waterproof concrete. During design, emphasis is placed on the strength grade of waterproof concrete, while insufficient attention is paid to its crack resistance. The detailed structure and reinforcement are unreasonable; the waterproofing design is not well integrated with the structural design, and the structural form design is overly complex. At the same time, there is a shortage of specialized design firms for civil air defense projects, and the design expertise possessed by professionals at non-specialized firms varies greatly, leading to numerous design issues in such projects and affecting the quality of the work. 　　2.3 Construction aspects. Poor quality control of raw materials, inadequate control of the slump value, improper handling of detailed structures such as construction joints, and failure to carry out curing of the concrete in accordance with the requirements of construction specifications. The construction of self-waterproofing concrete structures is a delicate process; it is necessary to select appropriate values for parameters such as mix ratio, water-cement ratio, and slump. Proper attention must be paid to the pouring and vibration of the concrete, as well as to the curing time and conditions. Otherwise, voids may form within the concrete, leading to cracks on its surface. 　　2.4 Supervision and management. Insufficient supervision and inspection were carried out on the quality of waterproofing projects; the entire construction process was not supervised and inspected in strict accordance with relevant regulations. The drawing design is not standardized enough, and the design review is inadequate. Before construction, no proper technical briefing of the drawings was carried out, and the contractor was not familiar with the key points of waterproofing work. In many flood control projects, on-site supervision was not adequate at the critical stages and key areas of construction. The supervision and management of supervision companies are insufficient; their quality and technical capabilities are low, and there are even supervisors who lack the necessary qualifications. 　　 　　3 Measures to Improve the Self-waterproofing Quality of Civil Air Defense Structure 　　　3.1 Selecting a suitable structural design. Structural self-waterproofing relies on the density, water-repellency, and stiffness of the structure itself to enhance its impermeability; it requires that the structure possess a certain degree of stiffness, and an appropriate structural design is key to improving the overall stiffness of the structure. Therefore, in terms of structural design, the choice should be determined comprehensively based on factors such as protection requirements, normal and wartime usage functions, as well as engineering and hydrogeological conditions. It is necessary to avoid sudden changes in the structure’s plan view or cross-sectional stiffness, and efforts should be made to ensure a regular plan layout for the structure in order to enhance its overall stiffness. 　　3.2 Scientifically design plans to prevent concrete cracking. The key to improving the self-waterproofing performance of concrete structures is to control the formation of cracks in reinforced concrete. In the waterproof design of underground projects, special attention should be paid to design solutions aimed at preventing concrete cracking; cracks in concrete should be reduced and controlled through aspects such as concrete strength design and material selection, the arrangement of rebar and tie bars, and the prevention of uneven settlement. 　　3.2.1 Select an appropriate strength for the main structural materials. When it comes to the quality of waterproof concrete, people naturally think about how to improve its strength and water resistance. The higher the designed strength and water resistance of the concrete, the more cement is required, and the heat generated by the hydration of cement is also greater, leading to increased shrinkage and deformation, which in turn causes cracks in the concrete structure. Therefore, the strength of the structural materials for the civil air defense basement must be determined based on factors such as its intended use, waterproofing level, and depth of burial. The concrete strength grade should not be lower than C30, but it should also not be too high. The impermeability grade should be selected from the range of S6 to S12, depending on the depth of burial of the structure. 　　3.2.2 Select concrete raw materials and mix proportions appropriately. First, choose the correct type of cement and the appropriate amount to use. Cement with a lower heat of hydration is generally chosen; by selecting an optimal concrete mix, the amount of cement used is reduced as much as possible, while still ensuring the strength and other properties of the concrete. Second, strict control should be exercised over the quality of aggregates; particular attention should be paid to the selection of coarse aggregates. It is advisable to use stones with a rough surface, high hardness, good gradation, low porosity, and low sand content. Thirdly, fly ash that meets **standard requirements is used to replace part of the cement in concrete, thereby producing fly ash concrete. This reduces the heat of hydration, increases density, enhances the strength of the concrete over time, and improves its resistance to admixtures and cracking. Fourth, an appropriate amount of expansion agent is added to prepare shrinkage-compensating concrete. Compensating shrinkage waterproof concrete not only reduces the amount of shrinkage that occurs in concrete at various ages, but also delays the onset of shrinkage during the hardening process, thereby enhancing the concrete’s ability to resist shrinkage stresses and reducing the number of shrinkage cracks. 　　3.2.3 Design of reinforcement layout. First, appropriately increase the horizontal structural reinforcement in the walls. Excessive stress-reinforcing bars in the wall and insufficient horizontal structural bars are among the reasons why walls tend to crack. To prevent the formation of these cracks, threaded rebar can be used, and the spacing of horizontal reinforcement bars can be appropriately reduced along with an increase in the number of such bars, in order to enhance the ultimate tensile strength of the concrete. Second, the tie rod structure should be designed reasonably. When setting the tie bars, a \"plum blossom\" arrangement is used to keep their number as low as possible while ensuring effectiveness. For double-sided reinforcement, a uniform modulus is used to determine the spacing of the rebar, ensuring that the lines connecting the intersection points of the rebar on both sides are perpendicular to the rebar mesh. A water-stop ring is welded in the middle of the tie bars, or the tie bars and the formwork tie rods are combined to form a single water-stop ring. 　　3.2.4 Prevent uneven settlement of the foundation. Before starting the design, it is necessary to obtain comprehensive and accurate engineering geological data. During design, attention should be paid to the balanced layout of the superstructure in order to reduce settlement differences caused by uneven loads on it. The design of the foundation focuses on controlling deformation levels, and the design team must carry out calculations regarding the final settlement of the foundation as well as the eccentricity distance. In civil air defense projects where karst and soil caves are present to varying degrees, appropriate measures must be taken to address these phenomena within the range of the bearing strata. Settlement joints should be installed in areas where there are significant differences in the compressibility of the foundation soil, or where different methods are used for foundation treatment. 　　3.3 Improving the construction quality of waterproof concrete 　　3.3.1 Ensuring the quality of raw materials. The raw materials for concrete must comply with the current **standards, construction and acceptance specifications, as well as relevant provisions of the design. Before construction begins, materials arriving at the site must be sampled and inspected on-site; those that do not meet the required standards shall not be used. Special attention should be paid to controlling the amount and strength of cement, as well as the mud content and grading of sand and gravel. The use of high-quality fly ash can help reduce the amount of cement needed, thereby preventing the actual strength of the concrete from exceeding the designed strength and improving its crack resistance. 　　3.3.2 Ensure proper concrete pouring and vibration. Concrete should be poured in layers and vibrated in each layer. The time interval between successive layers of pouring should be determined appropriately based on the temperature conditions, to ensure a strong bond between the upper and lower layers of concrete before they set. When pouring concrete into the formwork, it should be poured in horizontally and evenly, and the height of its free fall should be controlled. Before vibrating the concrete, it is necessary to determine the vibration points based on the design of the specific structure. The vibration time is generally 10 to 30 seconds, with the criterion being that concrete begins to flow out and no bubbles appear; this helps to avoid under-vibration, insufficient vibration, or over-vibration. 　　3.3.3 Select an appropriate concrete slump. Practice has shown that, under identical conditions, the lower the concrete slump, the less the early shrinkage of the concrete, and the fewer cracks that appear in the main structure after construction. For ready-mixed concrete used for waterproofing purposes, the slump value when it is poured into formwork should be controlled at 120±20 mm. Currently, concrete pumping is commonly used in underground shelters; to control the slump value while still ensuring pumpability, it is necessary to use concrete pumps of high quality, with imported concrete pumps being the best choice. 3.3.4 Set up and handle the detailed construction properly. Concrete should be poured continuously as much as possible, with few or no construction joints left. The placement of construction joints is primarily aimed at ensuring the strength of the concrete poured in one go and effectively controlling cracks caused by concrete shrinkage. Before pouring more concrete at the construction joint, the surface of the joint should be roughened, and water-swelling seal strips or embedded seal belts should be applied. In areas where post-cast strips are required due to engineering design considerations, the construction quality must be improved by using compensating shrinkage concrete; its mix ratio should be determined through testing. Before construction, the joint surface must be carefully cleaned with a wire brush, the surface mortar layer removed, and fresh concrete exposed before pouring takes place. 　　3.3.5 Pay attention to concrete formwork removal and curing. Due to the addition of large amounts of mineral admixtures, crack-resistant waterproof concrete generally shows a relatively slow increase in strength in the early stages, but experiences a higher rate of continuous strength growth later on. Therefore, the timing for removing formwork and the curing procedures differ from those for ordinary concrete; the formwork for this type of concrete is usually removed 2 days later than in the case of ordinary concrete, and it is strictly prohibited to remove the formwork too early. After the concrete has reached final setting, it should be cured for a period of not less than 14 days to prevent cracking due to drying during the hardening process. 　　3.4 Strengthen quality supervision and management, and ensure proper control at the stages of design review, construction supervision, and completion inspection. 3.4.1 Adhere to standards and enforce strict controls during design review. It is necessary to select qualified design firms to improve the quality of the design. It is necessary to ensure proper review of construction drawings; for those that do not comply with the design specifications for civil air defense projects, the mandatory provisions, and industry standards, review comments should be provided so that the design unit can make the necessary revisions. Only after approval following such reviews can construction proceed. Conduct design briefings and drawing reviews to ensure that the construction party is familiar with the design drawings, understands the characteristics of the project and the design intent, as well as the quality requirements for the construction of civil air defense projects. 　　3.4.2 Ensure proper tracking and maintain strict oversight of construction activities. The quality supervision departments for civil air defense projects are responsible for overseeing and inspecting the legality of the quality-related actions taken by construction, survey and design, construction, and supervision agencies during the project development process. They formulate quality supervision plans for each project that is submitted for supervision, and carry out ongoing inspections of key processes, critical areas, and essential stages. Legal, economic, and administrative measures are employed to address issues promptly, thereby preventing substandard work quality from emerging or progressing to the next stage of the project. Construction and contracting units must establish and improve quality management systems, set up control points at various stages of the construction process, ensure that construction materials meet the required standards upon arrival, strengthen quality control during construction, and prevent arbitrary changes to the design or construction without following the approved design drawings. This is necessary to ensure that the design plans, construction methods, and quality assurance measures are properly implemented in practice. 　　3.4.3 Implement standardized management and strictly control the completion acceptance process. By utilizing the mandatory procedure of filing for completion inspection of civil air defense projects, supervision and management can be strengthened; the effectiveness of work in areas such as design and construction can be assessed, and any existing problems must be rectified promptly. This ensures that projects that do not meet the standards cannot be filed for approval, let alone put into use. 　　 　　4 Engineering Practice 　　　The building in question has 12 floors above ground and 2 floors below ground; its height is approximately 56 meters, with a total construction area of around 104,100 m2. The first and second basement floors are used as parking lots and for housing supporting equipment. On the second basement floor of the main building, there are also some civil defense facilities, with a water resistance rating of grade one. In recent years, as the issue of structural cracks affecting the proper use of underground facilities in civil air defense projects has become increasingly prominent, this project paid close attention to the problem of self-waterproofing of the structure from the very beginning. The principle of \"comprehensive treatment, with emphasis on the self-waterproofing capabilities of the concrete structure\" was adopted, and the following measures were taken to ensure the structure’s self-waterproofing: 4.1 Conducting thorough investigations of the geological conditions and using a suitable design for the main structural elements. Based on the geological conditions of the site where the project is located, appropriate measures were taken to address karstification and soil caves within the bearing stratum. The foundation design for the main building of the civil air defense basement involved the use of composite foundations, as well as adjustments to the length, number, and diameter of the piles, in order to control the settlement of various parts. The basement is well-organized and features a cast-in-place reinforced concrete structure, which enhances the overall stiffness of the structure through proper structural design. 　　4.2 Optimize the mix proportion design. The concrete mix ratio is carefully determined after repeated trial mixes with the concrete manufacturer. For shear walls such as C45 and S8, the amounts of various materials per 1 m3 of concrete are as follows: 340 kg of cement, 649 kg of river sand, 1058 kg of crushed stone, 73 kg of fly ash, 73 kg of slag, 7.29 kg of retarding high-efficiency water reducer, and 180 kg of water; thus, the mix ratio is 1:1.909:3.112:0.215:0.215:0.021:0.529. The main advantage of this mix design is that by incorporating high-quality fly ash and slag (at a dosage of 146 kg per m3 of concrete), the amount of cement used is reduced, which lowers the maximum absolute temperature rise of the concrete and simultaneously saves costs. 　　4.3 Adhere to strict construction techniques and methods. Strictly control the concrete slump; when the slump loss results in it no longer meeting the construction requirements, add cement slurry with the original water-cement ratio or use a secondary water reducer during mixing – adding water directly is strictly prohibited. Strictly control the temperature of the concrete when it is placed in the formwork, and avoid pouring concrete during high-temperature periods. During the construction of the main structure, appropriate vibration methods should be used; the formwork should not be removed too early. Proper concrete curing must be ensured, and an automatic sprinkling system for concrete curing should be employed. 　　4.4 Strengthen the supervision of project quality. Review and modification suggestions were put forward for the problems existing in the design, and construction was carried out after further approval. A quality supervision plan for this project was formulated, with focused supervision, random inspections, and acceptance checks carried out on key processes. 　　Nearly a year since the construction of this civil air defense basement was completed, the quality of the work has remained excellent, with no signs of leakage, achieving good results. 　　 　　5 Conclusion 　　　The above are my personal insights gained over the years from theoretical to practical work in the design, construction, and quality supervision of civil defense projects. They represent my research and discussions on the issue of self-waterproofing in civil defense structures. In general, a solution framework for improving the self-waterproofing quality of such structures can be summarized as \"one concept, one focus point, and four methods\". That is, to adopt the concept of \"prioritizing self-waterproofing in concrete structures, combined with water prevention and drainage measures\" ； Focusing on preventing concrete cracking to improve the waterproof durability of civil air defense projects ； Adopt “four measures”: First, select a regular structural layout. Second, develop a scientific plan to prevent concrete cracking by choosing the appropriate concrete strength, optimizing the mix ratio, arranging rebar and tie bars properly, and taking steps to avoid uneven settlement; especially given the abundance of high-quality fly ash in Longyan, reduce the amount of cement used in the concrete while maintaining its strength, and increase the use of fly ash. Third, improve the quality of waterproof concrete construction – particularly by reducing the slump of the concrete as much as possible within allowable limits, and ensuring proper pouring, vibration, and curing processes. Fourth, strengthen quality supervision and management by overseeing the entire process of waterproof concrete construction, from before construction begins to after it is completed. I hope this can help improve the self-waterproofing capability of civil air defense structure designs; please feel free to offer any corrections if there are any mistakes.