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Assessment of the durability of corrosion-damaged prefabricated reinforced concrete structures

During exploitation, reinforced concrete structures are under the influence of an aggressive environment, as a result of which they are damaged and destroyed. The durability of structures depends on the nature of its interaction with aggressive environmental factors, which is described using general models of external influences, such as the action of loads, climatic, aggressive and other influences and their combinations. An example of calculating durability based on corrosion of concrete and reinforcement is given. The selected protective layer and the diameter of the reinforcement were checked in case of general corrosion. The average value of the service life in years was determined on the basis of carbonization of the protective layer of concrete and the formation of cracks in it. The methods for predicting the service life of reinforced concrete structures are analysed. The service life of the structure is determined taking into account the assessment of the technical condition of the structure according to external signs. In theoretical terms, the influence of various factors by the introduction of a multifactorial measure of damage is shown. The calculations for predicting the service life of structures and for determining the durability are analysed. The example of calculating the probabilistic service life is considered.


Introduction
The concept of durability in the Eurocode is considered as the result of the implementation of measures taking into account the purpose of the reinforced concrete structure, the form of structural elements and parts, the quality of performance, the level of control, measures to protect the influence of an aggressive external environment, measures to maintain the reinforced concrete structure in good condition for the expected service life.
Durability can be considered at three levels: material, element and constructions, object as a whole. Analysing the content of the concepts of longevity in Russian and foreign regulatory documents, we can distinguish two of the most important [1,2,3]: 1) sufficiency of initial quality characteristics to satisfy safety requirements and exploitational suitability not lower than the limits laid down in national standards; 2) economic rationale of the service life, including minimizing maintenance and repair costs. The design parameters for assessing the durability of reinforced concrete structures include: • material properties; • geometric dimensions of structures; • location of reinforcements. According to [4], reliability is defined as the ability of a technical object to perform specified functions under certain conditions for a given period of time. Reliability indicators are quantitative characteristics of one or more properties that make up the reliability of an object, such as: • reliability;

Methodology
Durability is one of the most important indicators of the quality of building structures. During exploitation, reinforced concrete structures are exposed to an aggressive environment, as a result of which they are damaged and destroyed. The use of special protective coatings and special materials in building structures ensure their durability [5].
According to the results of the surveys, it can be seen that the reinforced concrete structures designed without taking into account the requirements of durability are in disrepair after 5÷10 years, and there may be cases when the destruction occurred after only 3 years [6][7][8][9].
In [1], the basic requirements for materials of building structures that work in aggressive conditions are reflected.
Reinforced concrete structures have a finite service life, as they are subject to physical, chemical and mechanical changes, the result of which is their degradation and a decrease in their ability to perform the required functions [10]. The development of methods for predicting the durability of designed and evaluating the exploitation of reinforced concrete structures caused by [9,12]: • the increasing use of reinforced concrete structures in difficult climatic conditions or in aggressive environments; • the high cost of reinforcing, restoring and operating costs to maintain the required technical condition of structures and buildings; • the use of new types of concrete and reinforcement for which the boundaries of durability have not yet received sufficiently reliable experimental confirmation. The main issue in the field of durability is the prediction of the service life of new reinforced concrete structures, which is considered as a more guaranteed parameter than durability.
In practice, the durability and service life of reinforced concrete structures is controlled mainly by limiting the maximum allowable values of the water-cement ratio, concrete class, reinforcement and the thickness of the protective layer. New editions of domestic and foreign standards include the following factors: environment; protective layer; type and quality of materials, laying methods to ensure complete compaction; shape and dimensions of the structure. The limitation of the thickness of the protective layer is set depending on the class of aggressiveness of the environmental aggressiveness, diameter and location of reinforcement over the cross section of the element.
The following methods have been used to predict the service life of reinforced concrete structures: • assessments based on a generalization of experience in design, construction and long-term exploitation of structures; • deductive approaches, the conclusions of which are based on a comparison of the exploitational quality of similar reinforced concrete structures; • mathematical modelling based on knowledge of the mechanisms of degradation processes; • estimates obtained as a result of accelerated testing of materials and structures; • destruction mechanics and finite element methods. The general method is based on the use of expert assessments, which are based on general experience and knowledge obtained in laboratory and industrial tests of structures and materials, special studies.
The calculation of durability based on corrosion of concrete and reinforcement. We accept the specified (assigned) service life of the column tg=50 years Safety factor by service life for combined method y1=2.5 [13]. Then the estimated service life td : We accept the following formula parameters:

Results and discussion
We check the selected protective layer and the diameter of the reinforcement in case of general corrosion. On the outside, the column reinforcements are susceptible to damage due to concrete carbonation.
Defining reinforcements in relation to general corrosion in this case are clamps. Define the average value of the service life µ(tL) in years on the basis of carbonization of the protective layer of concrete and the formation of cracks in it: where С = C-Dh = 55 mm-10 mm= 45 mm h concrete thickness for transverse reinforcement; D h clamp Diameter; c = 1 env the factor, considering the influence of the environment; c = 0.7 air the factor, considering the air composition; rcorrosion rate of steel reinforcement before cracking (12 mm); coefficients depending on the type of binder: а=1800,b= -1.7 (for Portland cement). The first term characterizes the penetration time of the environment in concrete protective layer; the second cracking time in the protective layer due to corrosion of the reinforcement under the assumption of the maximum allowable loss of the radius of the cross section of the reinforcing bar. x хх year , which is longer than the estimated service life of 125 years. Therefore, the selected thickness of the protective layer is acceptable for this type of degradation. Duration and safety indicators of the column during exploitation t = 15year serv with prolonged action of the load in an aggressive environment are found with the following initial data: column resistance R and effort S obey the law of normal distribution: average resistance R = Assuming that actual service life obeys by the law of normal distribution and the depreciation rate during the service life is constant, then the law of conservation over time of reinforced concrete structures (value, reverse depreciation) can be expressed as:   Table. 1.

 =
, that is, the amplitude of the values of relative reliability, which also characterizes also stock level, for the 1st category of technical condition or in the initial period of operation of the structure in relation to its emergency condition is 35% .
In real operating conditions, structural elements of engineering structures are exposed to several effects in different sequences or at the same time [11,14,15,16,17]. As a result, the rates of degradation processes and the rate of damage accumulation increase. Theoretically, the influence of several factors can be taken into account by the introduction of a multifactorial measure of damage D , accepted a priori: As the most representative measure of damage, concrete strength may be suggested. The assessment of durability in such cases is based on the use of the residual strength of concrete, established experimentally or by calculation, using assumptions ( Figure 1). The method of direct probabilistic calculation of time can include several methods: applying a safety characteristic, constructing a distribution density, using the direct logarithmically normal law.
where , , , a ke av Кvrespectively, the expectation and coefficient of variation of the distribution of the thickness of the protective layer and random value К , The average value of the thickness of the protective layer is taken equal to the design in accordance with the requirements of regulatory documents. Other data obtained on the basis of statistical analysis of experimental data on the carbonization of the protective layer of concrete of exploited structures and manufacturing conditions ( Table 2). Table 2. The values of the coefficients for various operating conditions of reinforced concrete structures.

No.
Type of construction and conditions of its operation A quantitative characteristic of the corrosion process is the depth of neutralization of the concrete of the protective layer of the reinforced concrete structure with an aggressive substance for a certain period. The criterion for the onset of the limiting state is the condition, that destruction occurs at a moment of time, when the depth of the damaged layer exceeds the thickness of the protective layer, as further development of the process will be characterized by depassivation (depassivation -the release of metal from a passive state; depassivation is associated with the destruction of the passivating layer on the metal surface) reinforcement and its corrosion initiation.
In this formulation, the following problems are solved: a) for given parameters of the protective layer, find the distribution functions and the distribution density of carbonization time; b) what is the required thickness of the concrete protective layer, if the service life and the probability of destruction are specified. When solving the first problem in a probabilistic setting, the purpose of the calculation is to find the time during which with confidence probability Pn the protective layer will not be neutralized. The probability of complete neutralization of the protective layer. The probability of complete neutralization is 1-Pn and depends on the purpose of the construction.
Calculation of probable service life.  Table 3.

Conclusions
The work considers the existing methods for determining the technical condition and physical deterioration of the building as a whole and its individual structures using precast concrete structures as an example. Statistical data are presented, such as the average physical depreciation of buildings on the example of Moscow, the increase in physical depreciation over a decade, etc., graphs and functions of physical depreciation are given.
Factors affecting the performance of reinforced concrete structures, such as aggressive environments, are considered.
The calculations for predicting the service life of structures and for determining durability are analysed.