First, the development of civil structural engineering in China
Most civil structures are made of concrete. The durability of concrete structure is one of the most important frontier research fields in structural engineering. Strengthening durability research is the future development direction, and attaching importance to the durability of concrete structures is also the need of sustainable development.
For a long time, people have always thought that concrete should be a very durable material. It was not until the late 1970s that developed countries gradually discovered that the original infrastructure projects were damaged prematurely in some cases. According to a survey conducted by the Ministry of Construction in the 1980s, most industrial buildings in China need to be overhauled after 25-30 years of use, and the service life of buildings in harsh environments is only 15-20 years. The use environment of civil buildings and public buildings is relatively good, which can generally be used for more than 50 years, but the service life of outdoor components such as balconies and rain covers is usually only 30 ~ 40 years. The durability of infrastructure projects such as bridges and port projects is more serious. Because the concrete protective layer of steel bars is too thin and the compactness is poor, many projects are corroded and cracks appear in concrete after several years of completion.
The seriousness and urgency of the durability problem lies in that many of our projects under construction have not learned many painful lessons from the international and domestic, and they are still repeating the same mistakes along the old road. Some newly-built overpasses and expressway bridges in northern cities have not yet taken necessary comprehensive measures from the aspects of material properties and structural structure to prevent freeze-thaw and salt damage. Even for large-scale projects, such as the Lotus Bridge in Zhuhai, which was put into operation in 2000, C30 concrete with a 3 cm ~ 4 cm thick protective layer is still used in the splash zone. Some experts estimate that the climax of infrastructure construction in China can last for 20 years, but due to the neglect of durability, there will be a climax of "overhaul" for 20 years. This climax may not come for long, and its cost will be several times the original investment of these projects.
Second, the main problems and reasons of durability of civil engineering in China
(A) the durability of civil engineering in China.
① Chinese engineering designers and project managers have not paid enough attention to the safety and durability of civil structural engineering, especially the concept of system safety management has not yet been formed. Structural safety problems that violate the mandatory detailed regulations due to human error or human error occur from time to time, which directly affects the safety and durability of the structure and even endangers people's lives and property;
② The design requirements for earthquake resistance and fire protection of civil structures in China are relatively low, and there are few or no design requirements for resisting other disasters. There is a big structural safety hazard;
③ China's current design code basically only considers the limit state of the bearing capacity of the structure during the service period, while foreign countries focus on the economical and reasonable service life of the structure;
(4) The construction drawing issued by the design unit generally does not consider the construction method, and the construction unit generally does not grasp the design calculation, so some specific problems encountered in the construction process can only be decided by the technical personnel on the construction site according to experience, lacking scientific theoretical basis.
(B) the reasons for the further deterioration of concrete durability in civil structures
① The quality inspection of concrete is customarily based on a single strength index, which leads to improper pursuit of cement strength in the cement industry, increasing the fineness of cement and the proportion of early strength mineral components, which is not conducive to the durability of concrete.
(2) The construction unit improperly speeds up the construction progress, especially the improper intervention of the government administrative leadership on the construction progress. In particular, the durability and quality of concrete need sufficient construction and maintenance period to ensure, and the concept that premature delivery is harmful to life and health also applies to concrete. The so-called project of building roads, bridges and tall buildings in a few months, which is advertised in the domestic media, is likely to be a short-lived project, and it is destined to spend more money on overhaul in the future. Those who complete the construction period stipulated in the contract in advance will be punished abroad, because it means that the quality of the project may be damaged.
(3) Environmental deterioration caused by insufficient environmental protection, such as waste gas and acid rain (the area of acid rain in China has exceeded 30% of the whole country), is also an important reason affecting the durability of concrete.
Three, some views on improving the durability of civil engineering.
(a) write the corresponding design and construction technical documents, on the other hand, arrange systematic research projects and increase the support for durability research; The durability of concrete structure is one of the most important frontier research fields in structural engineering in the world. The durability research of concrete involves specific conditions such as raw materials and environment, and needs to consider the characteristics of China, but it cannot rely entirely on foreign research results.
(2) The durability of concrete structure is the need of sustainable development. It is necessary to increase the amount of high-performance concrete and reduce the amount of cement. Raw materials such as cement, sand and stone. The production of concrete needs to consume a lot of land resources and destroy vegetation and riverbed. China now produces more than 500 million tons of cement every year, accompanied by more than 2 billion cubic meters of sand and gravel, which is unsustainable in the long run. Extending the service life of the structure means saving materials, and durable concrete should generally be concrete with low cement content and high mineral admixture (industrial waste), so durable concrete meets the needs of environmental protection. The design service life of civil engineering such as bridges and tunnels in the world is mostly 100 years, and some in Britain are 120 years. Considering the huge economic loss and waste of resources caused by insufficient durability, there is a trend to further extend the minimum working life of these projects in recent years. For example, it is proposed that bridges in urban environment should be at least 150 years.
(3) Normal inspection and maintenance in the use stage of civil structural engineering. The concept of structural durability and service life cannot be separated from inspection, maintenance and repair in the use stage, especially for infrastructure projects in the open air and harsh environment.
In order to ensure the safety and durability of the structure, some projects should be tested and maintained regularly during the use after completion. At present, there are design specifications and construction specifications for structural engineering in China, but there is no specification on how to use them. Some countries stipulate that public works such as buildings, bridges and tunnels whose structural damage may affect public safety must be inspected regularly. Even the glass curtain wall, external wall brick and other building components are easy to hurt the public after falling, and there are also requirements for mandatory regular inspection. Due to the gap between China and foreign countries in construction management level and the quality of engineering operators, the quality control and quality assurance system is not perfect, and the standards of structural safety and durability are low, there are often some safety hazards in the projects already built or under construction. Therefore, it is more necessary to legally determine the requirements for normal use and regular inspection of civil engineering. Although the government has stipulated that the units and individuals responsible for design and construction should be responsible for it for life, it is not feasible to implement this requirement. Minimize structural safety and quality accidents, but also focus on prevention, and find problems in time through daily inspections.
(four) improve the technical standard system and management system, and give play to the role of societies and associations in the preparation, revision and management of technical standards; Gradually dilute the mandatory provisions of technical specifications; Encourage the preparation of local norms (standards) and enterprise standards that adapt to the differences in environmental geology, economy and technology in different regions, and encourage scientific and technological innovation and technological progress.
In a word, the safety and durability of civil engineering structures have always been a matter of great concern to designers and users, which is related to the coordination of safety and economy, the investment in infrastructure, the current national policies and regulations and the future economic development. This is a complex system engineering problem. The author thinks that on the basis of the national unified standard of reliability design, applying the new technical theory of reliability research and combining with the research of structural durability, appropriately improving the safety level, structural durability and service life of civil engineering structure design can make the reliability and durability of engineering structure better meet the needs of China's modernization and economic market.
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