The concept of durability of concrete structures
The traditional durability design of concrete structures [1e] is defined as comprehensively considering material quality, construction processes, and structural construction, so that the structure can work normally in a certain environment and does not require maintenance within the required period of time. The definition of durability of concrete structures in the Unified Standard for Reliability Design of Building Structures (GB 50068-2001) is that the deterioration of material properties of a structure in a specified working environment within a predetermined period of time will not result in an unacceptable probability of failure. Under normal maintenance conditions, the structure can be used normally for the specified design service life. The above definition of structural durability is limited to the long-term effects of external environment (non load action) on the structure, resulting in certain limitations in the changes of structural performance (degradation or enhancement). In fact, the cumulative damage caused by loads to structures should also belong to the category of durability. The author believes that the durability of a structure is the comprehensive performance of the structure, reflecting the changes in structural performance over time. Therefore, the study and definition of the durability concept of concrete structures should start from the factors that affect changes in structural performance; There are roughly three factors that affect changes in structural performance: load effects, environmental effects, and internal factors of structural materials. The impact of loads on changes in structural performance is mainly reflected in the cumulative damage of the structure. Cumulative damage is divided into static cumulative damage and dynamic cumulative damage. Static cumulative damage refers to the accumulation of structural damage over time under static loads. Dynamic cumulative damage refers to the accumulation of structural damage over time or the number of load applications under dynamic loads (repeated loads, repeated loads). Fatigue under dynamic loading is a typical dynamic cumulative damage. Structures subjected to repeated loads may experience fatigue failure when the load level is much lower than the normal failure load. The consequence of cumulative damage is a decrease in structural performance, thereby reducing the reliability of the structure. The impact of environment on structure can be divided into natural environment and usage environment. The main degradation effects of corrosive media on structures in natural environments include carbonation of concrete, chloride ion erosion, sulfate corrosion, freeze-thaw cycles, etc. The adverse effects of the usage environment on the structure mainly include corrosion of the structure by chemical media. The infiltration of corrosive media into the interior of reinforced concrete structures can cause corrosion of the steel bars, reduce their strength, and affect the bonding force between the steel bars and concrete, thereby reducing the performance of structural components. The main effect of internal materials is that the materials gradually age over time, leading to a decrease in material properties and strength. Active materials undergo slow chemical reactions with other constituent materials, such as alkali aggregate reactions in concrete. The degradation of material properties will inevitably lead to a gradual decline in structural performance. The reasons for the decrease in structural performance mentioned above are also the real reasons for the durability of structures. They all have a common feature, that is, damage accumulates over time and is a dynamic and gradual process. When such damage accumulates to a certain extent, it will affect the applicability, safety, and other performance of the structure.
Therefore, from the perspective of structural cumulative damage and changes in structural performance, the author redefined the durability of structures that include factors such as loads [28], which refers to the ability of structures to resist long-term performance degradation under various effects that may cause changes in their performance (loads, internal factors of environmental materials, etc.), within a predetermined service life and appropriate maintenance conditions.
Research content on durability of concrete structures
The research on the durability of concrete structures can be divided into four levels: environment, materials, components, and structure. Relatively speaking, the research on materials and components is more in-depth; Further in-depth research is needed on the relationship between environment and structural hierarchy [29-31]. In order to illustrate the research content involved in the durability of concrete structures more intuitively, Figure 1-13 is drawn for illustration.
Reference
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[28] Jin Weiliang, Zhong Xiaoping. The relationship between durability, safety, and applicability throughout the entire life of a structure. Journal of Architectural Structures, 2009, 30 (6): 1-7
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[30] Jin Weiliang, Yuan Yingshu, Wei Jun, et al. Durability Theory and Design Method of Concrete Structures in Chloride Environments. Beijing: Science Press, 2012
[31] Jin Weiliang. Corrosion Concrete Structures. Beijing: Science Press, 2012
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