| The Importance of Durability Issues in Concrete Structures |
As is well known, concrete structures combine the advantages of steel reinforcement and concrete, with low cost, and are the preferred form in civil engineering structural design. Their application range is very wide. Although many new structural forms will emerge in the future with the emergence of new structural calculation theories and new building materials, it can be affirmed that concrete structures are still one of the most commonly used structural forms.
Of course, this does not mean that concrete structures are perfect. In fact, in the nearly 200 years since the application of concrete in construction projects, a large number of concrete structures have failed prematurely due to various reasons and have not reached their predetermined service life. Some of this is due to insufficient resistance in structural design, some is due to adverse changes in load usage, but more is due to insufficient durability of the structure. Especially for concrete structures in marine and nearshore areas, early damage to the structure is caused by the corrosion of the marine environment, especially the rusting of steel bars, resulting in the loss of structural durability. This has become an important issue of practical engineering failure. Early damaged structures require a significant amount of financial resources for repair and reinforcement, and may even result in significant economic losses due to work stoppages. A port in a southern city of China was built in 1956 for a pier. During an investigation in 1963, it was found that there were rust cracks along the steel bars at the bottom of the beam. Although repairs were carried out the following year, it was found that the steel bars had corroded more severely after 20 years of use. The concrete on the bottom plate had fallen off extensively due to the corrosion of the steel bars, and the exposed steel bars accounted for 21% of the bottom plate. After multiple discussions, the upper structure had to be dismantled. Therefore, durability failure is the main reason for the failure of concrete structures in normal use.
Through further analysis, it can be found that the causes of structural durability failure exist in various stages of structural design, construction, and maintenance. Firstly, although many countries' regulations clearly stipulate that reinforced concrete structures must have safety, applicability, and durability, the issue of structural durability has not been fully reflected in specific design provisions. Instead, environmental and durability issues are considered in construction measures, resulting in a widespread emphasis on strength design over durability design in structural design. Taking the design specifications issued by China in 1989 as an example, in addition to some structural measures to ensure the durability of concrete structures, only some parameters related to durability design, such as crack width of concrete structures, are controlled in the normal use limit state verification. However, the control of these parameters does not play a decisive role in the durability design of structures, and these parameters will also change over time. Secondly, unqualified construction can also affect the durability of structures. Common construction problems such as unqualified concrete quality and insufficient thickness of steel reinforcement protective layer may lead to premature corrosion of steel reinforcement. In addition, the decrease in structural durability caused by improper maintenance during the use of the structure cannot be ignored, such as collisions, wear, and deterioration of the usage environment, all of which will prevent the structure from reaching its predetermined service life.
Domestic and foreign statistical data indicate that the losses caused by durability defects in concrete structures are enormous, and durability issues are becoming increasingly severe. According to surveys, the losses caused by corrosion in the United States reached 70 billion US dollars in 1975 and 168 billion US dollars in 1985. Currently, the value of the entire concrete project is about 6 trillion US dollars, and the annual cost of maintenance or reconstruction is expected to reach 300 billion US dollars in the future. The construction cost of 11 concrete elevated bridges on the circular expressway in the central part of England Island was 28 million pounds, and by 1989, it had cost 45 million pounds for maintenance, which is 1.6 times the original cost. It is estimated that it will cost 120 million pounds in the next 15 years, which is nearly 6 times the original cost. This reflects that the losses caused by structural durability far exceed people's estimates. Foreign scholars have vividly described the importance of durability design for concrete structures using the "five fold law", which means that saving $1 on steel reinforcement protection during the design phase means that taking measures to prevent steel corrosion will result in an additional $5 in repair costs; When the concrete surface cracks along the reinforcement, measures will be taken to increase the repair cost by $25; Measures taken in case of severe damage will result in an additional repair cost of $125. In China, the durability problem of concrete structures is also very serious. According to a survey conducted by the National Bureau of Statistics and the Ministry of Construction in 1986 on 323 cities and 5000 towns in 28 provinces, municipalities, and autonomous regions in urban and rural areas, the current urban housing construction area in China is 4.676 billion square meters, accounting for 60% of the total housing construction area. There are about 500 million square meters of industrial plants, covering more than 500 billion yuan of state-owned fixed assets. Among these buildings, about 2.3 billion square meters need to be evaluated and reinforced in stages and batches. More than half of them urgently need maintenance and reinforcement before they can be used normally. According to the 1994 railway autumn inspection statistics, there were 6137 railway bridges in China with varying degrees of damage, accounting for 18.8% of the total number of railway bridges.
It can be seen that the durability issue of concrete structures is a very important and urgent problem that needs to be solved. Given the importance of this issue, scholars both domestically and internationally have conducted extensive research in the field of durability of concrete structures. The progress of domestic and international research will be detailed in section 1.3 of this chapter. Here, it is emphasized that the study of durability of concrete structures has both temporal and spatial scales. For concrete structures, the durability failure process should include the entire life cycle of structure construction, use, and aging. The durability research should also involve every link in the entire life cycle of the structure (Figure 1-1), and the durability research of concrete structures should be carried out based on the entire life cycle of the structure. At the same time, traditional research often attributes the durability failure of concrete structures to material issues, while ignoring the "structural" properties that concrete structures should possess. The study of concrete structure durability must be based on the research results at the material level, comprehensively considering the "structural" characteristics of the research object (Figure 1-2). From the interdisciplinary fields of materials engineering, structural engineering, and heterogeneous material mechanics, research on concrete structure durability should be conducted, and a research system for the entire life cycle of concrete structure durability consistent with time effects should be established. This plays an important role in improving the theoretical system of concrete structure durability and has important application value in guiding the design, construction, and maintenance of actual concrete engineering.
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