Experimental studies of non-centrally compressed corrosion-damaged reinforced concrete elements under dynamic loading

. In the scientific literature, there is practically no analysis of the effect of corrosion damage on the operation of compressed reinforced concrete elements, especially on the stress-strain state of such structures under dynamic loading. For experimental studies, 37 reinforced concrete samples were made – columns of square cross-section with dimensions of 100x100mm, height of 700mm, extensions of 100x200mm were made in the supporting parts to create off-center compression. In the manufactured samples of reinforced concrete, local corrosion damage of concrete and reinforcement was created for accelerated corrosion of elements, a concentrated solution (37%) of hydrochloric acid (HCl) was used as an aggressor. The article describes experimental studies of non-centrally compressed reinforced concrete elements damaged by corrosion under dynamic loading. According to the load cell readings, it was recorded that corrosion damage leads to a decrease in the height of the compressed concrete zone, due to a decrease in the cross-section of the stretched reinforcement, as well as the lack of joint reinforcement with concrete. The obtained deformation diagrams of transverse, non-centrally compressed, corroded and undamaged samples based on glued strain gages on concrete and stretched reinforcement showed that these deformation diagrams fundamentally differ in shape. The deformations of reinforcement and concrete obtained as a result of a full-scale study made it possible to assess the stress-strain state of damaged and uncorroded structures according to the parameter Ne-1/r (curvature). The effect of corrosion damage on the nature of destruction of non-centrally compressed elements has been established.


Introduction
Concrete and reinforced concrete structures can be subjected to both operational loads and various aggressive environments.This can lead to changes in the strength and deformation properties of materials, the formation of longitudinal cracks in concrete caused by corrosion products of reinforcement, as well as to the accumulation of damage in structures [1,2,3].
In the field of corrosion of reinforced concrete, many scientists have conducted and are conducting experimental studies related to the diffusion of aggressive media, the kinetics of corrosion processes, which are aimed at studies related to the rate of corrosion processes in concrete and reinforcement [4,5,6,7].
Experimental studies by other authors [8] aimed at analyzing the impact of the concrete class, which showed that a higher concrete class reduces the corrosion rate of reinforcement in the body of the structure.In addition, the dependence of the adhesion forces of the corroded reinforcement on the concrete class was revealed.
Corrosion damage leads to a violation of the coupling of reinforcement with concrete, which requires changing and adapting existing methods for calculating reinforced concrete structures, taking into account the peculiarity of the work of such elements [9,10,11,12].
The work [13] carried out studies aimed at assessing the effect of corrosion damage to the reinforcement on its anchoring, the analysis of cracking and peeling of the protective layer of concrete depending on the percentage of damage, the effect of corrosion of transverse reinforcement on the adhesion of concrete to the longitudinal anchor reinforcement was established.
When exposed to aggressive environments of various nature on reinforced concrete elements, not only degradation of concrete and reinforcement occurs, but also affects the relationship between them, the authors note in [14] experimental tests of reinforced concrete beams with variable coupling, longitudinal reinforcement with concrete was performed, depressurization was performed using plastic tubes laid on longitudinal rods.The results of these studies have shown a negative effect of the lack of adhesion of reinforcement with concrete on the bearing capacity, the moment of cracking, the width of crack opening and deflection of reinforced concrete beams.
When scientists conduct experimental studies, in most cases, the experimental samples are reinforced concrete beams [19,20,21,22], at the same time, there are practically no studies of non-centrally compressed structures with corrosion damage under the influence of dynamic loads.In this regard, the analysis of the features of the work of structures in such conditions leads to the need for experimental and theoretical studies.

Experimental research methods
The main purpose of experimental research is to analyze the work and determine the features of the stress-strain state of corrosion-damaged non-centrally compressed reinforced concrete elements under dynamic loading that do not have reinforcement coupling with concrete.
To conduct experimental studies, 37 reinforced concrete samples were made -columns of square cross-section measuring 100x100mm, 700mm high, extensions of 100x200mm were made in the supporting parts to create off-center compression.
To compare the results of the influence of various factors on the static, dynamic strength and deformability of centrally and non-centrally compressed reinforced concrete elements, the reinforcement of the prototypes was assumed to be the same: µ=2.01%.The longitudinal reinforcement of the elements was assumed to be 4Ø8 A500, As =2.01 cm 2 .Transverse reinforcement of the samples was carried out with closed clamps made of class A240 rods with a diameter of 6mm, while in experimental studies the most unfavorable variant of damage location is considered.To do this, the pitch of the transverse reinforcement in cross-sections of 100x100mm is taken so that it does not affect the section of concrete damaged by corrosion and the rods of the longitudinal reinforcement that do not have adhesion to concrete, but the calculated location of the working reinforcement during concreting.
Due to the different corrosion rates of concrete and reinforcement, they were immersed separately in a solution of concentrated hydrochloric acid.Thus, the reinforcing bars were covered with anti-corrosion paint, except for areas where it was necessary to create corrosion damage.Then the rods were immersed in baths with a solution of concentrated (37%) hydrochloric acid and kept for 3-5 days.It took this time to reduce the diameter of the rod from 8mm to 6mm.
Manufactured reinforced concrete samples with corroded reinforcement were coated with anticorrosive paint according to the type of damage.Then the samples were immersed in baths with a solution of concentrated (37%) hydrochloric acid and kept for 1-2 days.The specified time was sufficient for corrosion damage to the sample in unprotected places with paint, the aging process continued until the complete removal of the concrete layer that provides adhesion to the reinforcement.Two types of local corrosion damage were provided for conducting experimental studies (Fig. 2).Local locations of corrosion damage are selected based on the most loaded zones of non-centrally compressed elements.The samples were tested under static loading on the Instron 1000 HDX installation, under dynamic loading on the Instron 8802 machine, according to the schemes shown in Fig. 3.The sample was installed in the supporting places of the press, on metal plates with a hemisphere to provide a hinge support and prevent local destruction of concrete.Next, a load of 0.5-2kN was applied and, according to the readings of strain gages glued at the edges, the sample was centered.After that, deflection sensors were installed.During dynamic tests, the average speed of application of the destructive load was 0.1s.With static load application, the sample was loaded in stages with five-minute exposures between stages.Loading at the last stage was carried out before the destruction of the sample.
To determine the height of the compressed zone, as well as the resulting deformations in concrete and reinforcement at each stage of load application during static and dynamic tests, strain gauges were glued to the samples according to Fig. 3.

Results
The height of the compressed concrete zone was determined by the constructed deformation diagram according to load cells attached to concrete and stretched reinforcement.Fig. 4 shows the obtained deformation diagrams for corrosion-damaged and undamaged sections.
Experimental studies have shown that the height of the compressed zone of noncentrally compressed corrosively damaged and undamaged elements differs significantly among themselves.Based on the presented results, a characteristic feature of the sections of non-centrally compressed elements tested by dynamic loading is an unambiguous decrease in the height of the compressed zone from 9% to 30%.
To determine the effect of corrosion damage on the nature of destruction of experimental non-centrally compressed samples (brittle or plastic) with relative eccentricity, the graphical dependencies "moment-curvature" presented in Fig. 5, 6 are performed.
The curvature values are obtained on the basis of experimental data of deformations of the extreme fiber of the concrete of the compressed zone e b and deformations of the stretched reinforcement e s in each section under study according to the formula: (1) где, 0 h -the working height of the section of an off-center compressed element.Based on the results of the analysis of the "moment-curvature" graphical dependencies, the following conclusions can be drawn: the nature of curvature graphs under dynamic and static loads of intact and corrosion-damaged samples is similar;; -the graphs of deformation of intact with increasing and abruptly ending sections, presented in Fig. 5, indicate the brittle destruction of the samples.Therefore, the condition ξ>ξ R is satisfied.
the deformation graphs of damaged samples (type No. 1) correspond to the principle of destruction of non-centrally compressed elements with a large eccentricity, that is, after the formation of cracks in the cross section and the subsequent development of plastic deformations in the stretched reinforcement, the compressed zone of the sample collapsed, as evidenced by the horizontal sections on the Ne-1/r graphs (Fig. 6).The destruction of such samples is plastic in nature.Therefore, the condition ξ<ξ R is satisfied.
Also, in the course of experimental studies, the deflections of samples were measured, both under static loading with a clock deflection meter and under dynamic loading with an electronic deflection meter, which is due to the high speed of application of the destructive load.
Table 2 shows the coefficients of increasing the deflection of non-centrally compressed samples under dynamic loading.According to the results of Table 2, it can be concluded that with dynamic loading of non-centrally compressed samples, the deflection relative to the statically applied load increases, therefore, the coefficient of increase in deflections for damaged and undamaged elements ranges from 1.35-1.93.

Discussion
The experimental studies carried out are of great importance in the field of the stress-strain state of reinforced concrete structures, since there are practically no studies in this direction.Therefore, data on the height of the compressed concrete zone, the curvature and deflection of corrosion-damaged elements under dynamic loading will allow us to more accurately assess the operation of non-centrally compressed elements and, as a result, develop new ones or adapt existing ones.methods of calculation of such constructions.
The obtained cross-sectional deformation graphs (Fig. 4) of non-centrally compressed corrosion-damaged and undamaged samples based on glued load cells on concrete and stretched reinforcement showed that these deformation graphs fundamentally differ in shape.At the same time, the peculiarity of the formation of sites on damaged sites is the absence of a hypothesis about flat sites, which is observed on undamaged sites.
In addition to the analysis of deformation plots, experimental studies have revealed a significant difference in the values of the heights of compressed concrete zones in damaged and undamaged areas.So, in the damaged section, the height of the compressed zone under dynamic loading was 1.0cm, and in the intact section 4.0cm, which corresponds to a difference of 75%, while the reduction in the diameter of the stretched corrosion-damaged reinforcement is 25% (from 8mm to 6mm).
Such a difference in the formation of deformation areas and the height of the compressed zones is due to the redistribution of forces due to the heterogeneity of concrete in cross-section (damaged and undamaged concrete), as well as the work of corrosiondamaged stretched reinforcement rods without adhesion to concrete.
Experimental studies aimed at studying changes in the nature of destruction of damaged non-centrally compressed samples by analyzing the obtained curvatures have shown that intact samples have a brittle nature of destruction due to the absence of plastic deformations in the stretched reinforcement, on the contrary, since samples with the first type of corrosion damage are destroyed after reaching plastic deformations in the stretched reinforcement, as evidenced by horizontal plots on the Ne-1/r graphs (Fig. 6).The reason for achieving plastic deformations with stretched corrosion -damaged reinforcement is a decrease in its cross-sectional area, as well as a decrease in the rigidity of the element due to corrosion damage to concrete.Consequently, the formation of local corrosion damage leads not only to a decrease in the bearing capacity, but also to a change in the stress-strain state of the elements.
In addition to studies aimed at studying the effect of damage on the stress-strain state of non-centrally compressed reinforced concrete elements, deflections of the studied samples were measured and compared when static and dynamic loads were applied.Based on the results of these measurements, a table2 has been compiled, in which there is a clear trend towards an increase in deflections during dynamic loading of samples, which may be due to a delay in plastic deformations during dynamic loading and, as a consequence, an increase in deflection at the moment of destruction relative to statically loaded samples.
It is also worth noting that the deflection values for samples with damage type No. 1 and No. 2 do not agree with respect to the deflection of an intact sample, namely, for elements with the first type of damage, they increase by 83%., and for elements with the second type of damage, the deflection is reduced by 45%.
An increase in deflection in samples with the first type of damage is quite logical, since a decrease in the stiffness of the section led to an increase in deformations.And the reduction of deflections in samples with the second type of damage to relatively intact elements is due to the lack of adhesion of stretched and compressed reinforcement with concrete, as well as sufficiently massive corrosion damage to concrete, which led to an excessively large decrease in the stiffness of the section and, as a consequence, a decrease in the strength and deformability of the element.

Conclusions
Based on the conducted experimental studies of corrosion-damaged and undamaged noncentrally compressed reinforced concrete elements for static and dynamic load, the following conclusions can be drawn: 1.The diagrams of deformations in the sections of non-centrally compressed corrosion-damaged elements with broken adhesion of stretched reinforcement with concrete differ fundamentally in outline with the diagrams of deformations in the sections of intact elements.2. The height of the compressed zone of concrete of non-centrally compressed elements under the influence of an aggressive environment can change significantly, so with 25% corrosion damage to the stretched reinforcement, the value of the height of the compressed zone will decrease by 75%; 3. When testing non-centrally compressed undamaged and corrosion-damaged elements with a dynamic load, a decrease in the height of the compressed zone from 9% to 30% relative to static loading tests is manifested in the cross sections; 4. The nature of the formation of "moment-curvature" graphs indicates that corrosion damage to non-centrally compressed reinforced concrete elements leads not only to a decrease in load-bearing capacity, but also to a change in the case of fracture from brittle to ductility, which occurs as a result of a decrease in the rigidity of the element due to corrosion damage to concrete and reinforcement; 5.The deflections of the experimental non-centrally compressed samples under dynamic loading are greater than under static loading.

Fig. 1 .
Fig. 1.The process of corrosion: a) reinforcing bars immersed in an acid solution; b) samples immersed in an acid solution; c) a corrosion-damaged part of the element section.

Fig. 2 .
Fig. 2. Two types of corrosion damage to samples.

Fig. 3 .
Fig. 3. Diagram of the location of strain gages on the surface of concrete and reinforcement.The test scheme and the design scheme of off-center compressed samples.

Fig. 4 .
Fig. 4. Diagrams of deformations of a corrosion-damaged (a) and undamaged cross-section (b) of an off-center compressed element under dynamic loading.The experimentally obtained values of the height of the compressed zone for undamaged elements and elements with damage type No. 1 and No. 2 are averaged and presented in Table1.

Fig. 5 .
Fig. 5. Graph of the dependence of the curvature on the bending moment of intact samples under static and dynamic loads.

Fig. 6 .
Fig. 6.Graph of the dependence of the curvature on the bending moment of samples having the first type of corrosion damage under static and dynamic loads.

Table 1 .
1. Heights of compressed zones, non-centrally compressed intact and corrosion-damaged samples under static and dynamic loads.

Table 2 .
Coefficients of dynamic deflection of out-of-center compressed damaged and undamaged samples.
This work was financially supported by the Ministry of Science and Higher Education of Russian Federation (grant # 075-15-2021-686).Tests were carried out using research equipment of The Head Regional Shared Research Facilities of the Moscow State University of Civil Engineering E3S Web of Conferences 410, 02020 (2023) https://doi.org/10.1051/e3sconf/202341002020FORM-2023