Assessment of the technical condition of large-panel buildings, due to their on-going use and covering resulting from thermomodernization works, is problematic. Results from non-destructive tests (NDT) are subjected to high uncertainty. Destructive tests, which give results with the highest level of confidence, are practically not used. Local sampling for testing gives only a partial image of the condition of a prefabricated building. In this type of construction, joints connecting the precast elements are the most vulnerable to degradation. Access to them is technically difficult. Demolition of this type of building is extremely rare. However, it is a unique opportunity to perform a full spectrum of both NDT and destructive testing. This gives an opportunity for large-scale demolition sampling to identify hidden defects and compare the results obtained by different methods. The comparison of results allows for the scaling of NDT methods and reveals the presence of typical relationships. The paper presents visual, non-destructive, and destructive tests’ results of an over 40-year-old large-panel building scheduled for demolition. The design of this building is repetitive and similar to solutions found in thousands of other buildings. The usefulness of particular research methods for evaluating the technical condition of prefabricated buildings has been determined. Reinforced concrete structures are influenced by various factors. Their destructive impact is important for a building’s durability ,2,3.Prefabricated buildings were commonly constructed in many European countries be-tween 1970 and 1990 ,5,6,7,8.The technical condition of structural elements made of reinforced concrete prefabricates depends on many factors. The most important ones are the quality control of prefabricates in the production plants, the way of transport and storage on the construction site, the accuracy of their assembly, the quality of filling the joints with concrete and, in the next stage, the proper maintenance of the building . NaturalIn the studied type of buildings, a typical prefabricated system was used for external curtain walls. It consisted of a concrete structural layer, thermal insulation from expanded polystyrene or mineral wool, and an external layer of textured concrete ,10. The The quality of the workmanship in these types of buildings during construction and after commissioning raised doubts regarding their durability, mainly in terms of corrosion protection. During several decades of use, the condition of both prefabricated elements and connections between elements was gradually degrading. Structural elements, as well as whole buildings, required regular repairs or modernization ,18,19,20Changes in energy costs and regulations concerning the limit values for heat transmission coefficients and thermal insulation of buildings have occurred in various countries. This resulted in the need to insulate external walls ,23,24,25Almost all large-panel buildings that have been constructed over the past few decades are still in use. Most of them are large multi-staircase and multi-storey residential buildings, but also office buildings, hotels, and public buildings. Buildings require condition assessment and ongoing monitoring for defects. Proper assessment also requires inspections inside dwellings. Such surveys are troublesome for residents, so they are rarely performed. The results of non-targeted scientific research on a group of buildings—using destructive  and onlyDespite thousands of prefabricated buildings, their demolition is a rarity and an extremely valuable research field. During the demolition of many unnecessary buildings in the former German Democratic Republic, most probably no detailed studies were performed. It was either a purely commercial activity or the results were published in the literature with limited scope. Mainly aesthetic and heat saving aspects were previously analysed . On the The present development of testing methods and equipment makes it possible to carry out more and more non-destructive tests. Sclerometric ,37,38, uTechnological innovations are also revolutionizing laboratory research, such as Digital Image Correlation (DIC), which allows for non-contact determination of selected concrete fracture mechanics parameters ,49,50. CResults obtained by non-destructive methods, due to economic and technical aspects, are rarely confirmed by a large number of destructive tests carried out on existing degraded objects.Demolished buildings are not only valuable research objects but also a source of materials that can be recycled. The construction of large-panel buildings was based on the use of large quantities of concrete. This provides an opportunity to recover aggregate, which can be successfully used to produce new structural elements. This is an increasingly common process in construction practice. Recycled aggregates are currently the subject of extensive scientific research. Results of research on combining recycled aggregates with geopolymer concrete are particularly interesting . IntriguThe authors of the present study participated in the demolition of a 12-storey building , where tThis paper aims to determine the suitability of particular testing methods for assessing the degradation state of structural elements of prefabricated objects. The places where destructive testing is necessary were located. Hidden defects, impossible to detect with currently used testing apparatus, were identified.The demolished two-segment building with 11 overground storeys and one underground storey was constructed in 1978 using OWT-67 technology  and served as an office building. The system was characterised by the size of the largest module being 5.4 × 4.8 m, with a storey height of 2.7 m [Due to the building’s location in the city centre and its proximity to the adjacent building, it was decided to demolish in a way that minimized the inconvenience to neighbouring buildings and their occupants. The use of explosives is characterized by considerable dust, vibration effects on the surroundings, enormous noise, as well as specific requirements regarding the size of the construction site and type of structure. Ultimately, mechanical demolition using demolition excavators b,c and lPrior to demolition works, visual assessment of the technical condition of the building was carried out. The assessment was conducted in the context of safety of people performing the survey and dismantling works, but also in the context of visible damage and signs of the building’s destruction. The survey was performed in accordance with the typical methodology of periodic inspections of buildings. At this stage of the study, there were no significant visible signs of degradation threatening the structure, which could raise suspicion of safety hazard during demolition. Also, the visible signs were not indicative of significant threats to the durability of the structure if it continued to be in use.In the horizontal joints of the floor slabs, cracks were visible. They were the evidence of vertical displacement of the edges of the adjacent slabs. In the upper parts of vertical joints of prefabricated walls, it was found that the fillings made during the building assembly were made without adequate precision. The concrete was very porous with an uneven surface and was not filling the entire wall joint. In the beam-walls, corroded rebar was inventoried where the window and door frames were supported . The corrosion could have been caused by a careless demolition of the window sills. During removal of window joinery, the workers damaged concrete layer and exposed reinforcing steel to environmental impacts. Elsewhere, no discoloration or cracks along the bar mesh, which could indicate an intensive corrosion process of the reinforcement bars, were found.For non-destructive testing, the authors used specialized testing equipment in the form of ultrasound a,b and fNon-destructive testing methods for the diagnosis of concrete structures have been widely described in the literature ,63,64. TIn the first stage, linear scans of longitudinal and transverse walls were made using ultrasound equipment with 54 kHz heads. Two-way access to the elements was used, thanks to doorways and openings created after excavations and core drilling. Five walls on each of floors −1, 3, 5, and 10 were examined. The diameter of the longitudinal wave transducer was 5 cm. The measurement points were marked with wax chalk at 10–15 cm intervals. Considering the dimensions of the transducers, in reality, the spacing between the edges of the heads was between 5 to 10 cm. A measurement grid with this spacing allows the surface under examination to be scanned accurately. Defects that could hide between the measuring points are negligibly small and would not affect the load-bearing capacity of the component. A total of 5 to 10 readings were taken on each wall. The number of readings was due to difficult access and the limited length of the cables feeding the transducers . Wave propagation velocities for all readings taken ranged from 3500 to 4000 m/s. According to the classification presented in , this inThe next stage was surface scans of walls and floors using a Pulse-Echo head. The thickness of the elements and the location of any voids and material discontinuities, as well as the propagation velocities of the shear waves, were analysed. The measurement grid ranged from 50 × 50 to 50 × 100 cm, with intervals of 10 cm. The thicknesses of the elements were within the execution deviations, i.e., ±0.5 cm. No areas indicating the presence of material discontinuities were located on the obtained images. The transverse wave velocity was within the range of 1800–2000 m/s, while the average wave velocity for concrete is usually 2000 m/s . Images Due to one-sided access to the vertical joints, the concrete quality was checked using indirect measurement mode, i.e., setting the heads on one surface. The tests were performed using 54 kHz transducers. In order to eliminate the uncertainty related to the length of the measurement path, one of the device functions was used. It allows to take readings by moving one of the heads while keeping the other one stationary. After taking 4 readings, with known path length, the device is able to draw a curve and estimate the longitudinal wave velocity. The obtained velocities ranged from 1700 to 2100 m/s. This may be an indication of poor concrete quality, but it should be noted that in surface measurement mode, the quality of the top layer of the element has a significant influence on the results. Micro-cracks and defects lead to under-estimation of wave velocity.The next stage was to perform B-scans of vertical joints and walls using the Pulse-Echo head. B-scan generates an image of the cross-section of the tested element, perpendicular to the scanning surface. The defects and discontinuities in the presented results are colour-coded from pink to purple. Heterogeneities in the concrete affect the propagation of the ultrasound pulse. They cause scattering of the signal coming from the Pulse-Echo head. The device measures the transit time of the wave and its amplitude. The colour in the image becomes darker as the amplitude of the wave increases. The local maximum of the amplitude results from the reflection of the wave at the boundary between the concrete and the air filling the structural defect. In the case of the darkest colour (purple), the wave amplitude was the highest. This indicates that the wave encountered significant material heterogeneity in the form of voids or delaminations. B-scans performed revealed the presence of voids and delaminations in the fillings a, which In order to identify the quality of the reinforcement work, line scans c and areThe results of the scans indicated insufficient cover thickness locally. The reinforcement distribution was consistent with the available original documentation.Using the sclerometric method, the concrete class of the precast elements was estimated based on the reflection number. It was determined that it is equivalent to the current class C16/20 according to EC2 .a = 100 mm from the crack, after which the ultrasound wave transit time was measured. The transducers were then moved apart to a distance of 2a = 200 mm and a second reading was taken. Based on the difference in transit times obtained from the two readings, the crack penetration depth was estimated. Debris was removed from the crack using an industrial vacuum cleaner. Cleaning is very important in penetration measurements using the ultrasound method, because wave propagation through the contamination can significantly under-estimate the readings relative to the actual condition. The depth of penetration was determined to be between 20 and 65 mm. An example of crack depth for a longitudinal wall and a transverse wall is shown in Locally, scratches were observed in the walls above the door openings. The cracks were measured with a Brinell magnifying glass and ranged from 0.3 to 1.1 mm. An ultrasound method was used to identify the depth of the cracks. The longitudinal wave transducers were placed on both sides of the crack. First, each transducer was placed at a distance of The accuracy of the non-destructive testing results was continuously verified by performing a large number of wall b, beam-wIn order to perform laboratory tests, core drillings were made in different parts of the structure a. The lod = 100 mm and d = 160 mm. The thicknesses of the precast wall and floor elements were 140 mm, so samples with l = 140 mm were taken. In order to determine the compressive strength, the samples with a diameter of d = 100 mm, were cut to l = 100 mm. This resulted in specimens with a ratio of l/d = 1. According to EN 13,791 [l/d = 1, the compressive strength corresponds to the strength obtained on cubic specimens with a side a = 15 cm. This eliminated the need for a strength correction due to the specimen dimensions. Samples of d = 160 mm were taken for future testing to determine Young’s modulus in compression and dynamic Young’s modulus. The calculation of the characteristic compressive strength of the concrete in the structure was also carried out in accordance with EN 13,791 [The samples were adjusted to standard dimensions using electric tools, their sur-faces were polished and then they were subjected to compressive strength tests under laboratory conditions c. SampleN 13,791 , for cylN 13,791 . The resN 13,791 , an averk,max = 0.4 mm. Cracks exceeding the opening width of 0.4 mm have been classified as not fulfilling standard recommendations.Drill holes were also made at the joints of the floor slabs c and walSamples taken from the floor slabs revealed missing rebar cover, which was only 3 mm in places. The pH tests of the concrete were also carried out . The rooPhenolphthalein solution was used for preliminary pH testing. The test is carried out by spraying the substance on a fresh break of the concrete sample. At pH values higher than approximately 9, the indicator turns the concrete red-violet. Only such coloured concrete shows alkalinity sufficient to ensure passivity of reinforcement steel. Testing was conducted in accordance with EN 14,630 . In the The level of concrete carbonatization in the texture layer was also determined in water extracts obtained by leaching the crushed concrete with distilled water. The samples taken from the core drillings were used for this purpose. Concrete was crushed in porcelain mortars, then coarse aggregate grains were removed, ground, and sieved. The sieved product was poured with distilled water and aqueous suspensions were obtained. The evaluation of the concrete’s suitability as a protective and load-bearing layer was carried out on the basis of tests using laboratory equipment with electrodes ensuring pH measurement accuracy within ±0.01. The results obtained for samples taken from the textured layer, within the pH range of 9.26–9.38, confirmed the occurrence of advanced carbonatization processes.Part of the research work was carried out during the disassembly of individual storeys. After removal of the floor slabs, photographic documentation of the welded joints of the precast elements was made . SubsequExamination of several dozen specimens collected showed localized missing welds b, both bThe increasing age of large-panel buildings in service requires numerous assessments of their technical condition. Structural defects caused during assembly, such as inaccurate steel connections or filling of joints, are implemented in the building’s structure from the beginning of its existence. The fact that such buildings are in operation for several decades despite these defects indicates that there are large reserves of capacity and redistribution of internal forces throughout the structural system. However, structures are constantly exposed to loss of durability due to environmental aggression and material ageing. This means that in spite of the relatively good condition of the buildings and the absence of confirmed failures, the structural condition of large-panel buildings must be continuously monitored.Demolition of such structures is rare and allows for a great number of tests, including destructive ones, which are most troublesome to the residents of the exploited structures.concrete carbonation and insufficient concrete cover thickness of prefabricated elements;careless filling of the joints between the prefabricated elements;workmanship defects in welded joints;local corrosion of reinforcing steel; andexceeding of dimensional tolerance both in the prefabricated elements themselves and during assembly.In the examined construction, the authors found both execution defects and those caused by environmental aggression. Degradation that can be dangerous to the safety of the structure manifests itself by:The results of ultrasound and destructive testing of vertical joints were consistent. B-scans and surface measurements of longitudinal wave velocity allowed to locate the areas of lower concrete quality, which was confirmed by examining samples obtained from core drillings. The change of concrete structure can significantly affect its material properties and it is reasonable to take it into account in terms of structural capacity.The ultrasound and sclerometric methods for testing the quality and strength of precast concrete, were found to be consistent with the results obtained in the testing machine. It is probably correlated with the good quality of concrete, without voids, in the precast elements. The good quality of concrete was confirmed by comparing the Pulse-Echo (B-scans) and longitudinal wave velocity measurements, which clearly excluded defects in the structure of the partitions.The lack of double-sided access to the tested elements  limits the usefulness of the ultrasound method for precise longitudinal wave measurement. Testing is reduced to performing measurements using the Pulse-Echo method, which immediately reveals discontinuities of materials and geometric deviations. It is also helpful in locating existing installations, which is important during repair works.The results of this study indicate that the ferromagnetic method is effective in evaluating the cover thickness and determining the distribution of reinforcing bars in precast large-panel elements, and can be helpful in selecting appropriate repair methods or locating sampling places for laboratory testing.Testing the pH of the concrete on both fresh splits and water extracts clearly showed the carbonation processes of the concrete. For buildings in use, the least invasive approach may be to take a sample using a small-diameter diamond core and then test it under laboratory conditions, which are the most precise.The removal of the floor slabs allowed to gain access to and assess the steel connections, which are not available for examination with current non-destructive testing equipment. The parameters of the steel used in the joints tested by correlating hardness and strength are sufficient to properly determine the strength characteristics, which was confirmed in the testing machine.The assessment of the structure’s technical condition should be based on a comparison of the results of initial visual assessment with the results of non-destructive and destructive tests. The number of destructive tests for large prefabricated structures may be reduced to a minimum, due to cost and damage to the structure in service.Demolition of a structure built of large-panel prefabricated system elements and according to system solutions is an invaluable source of knowledge about the technical condition of the examined building. It also allows to conclude on the technical condition of the whole category of similar buildings.to test the quality of concrete in vertical joints, it is recommended to use surface measurements of longitudinal wave velocity and B-scans;for one-sided access to the examined elements, surface scans using the Pulse-Echo method are helpful in detecting discontinuities and geometric deviations, as well as hidden installations;the quality and strength of concrete should be determined comparatively by sclerometric and ultrasound methods, and in case of discrepancies it is recommended to perform destructive tests;for testing the pH of concrete, it is recommended to take samples with small diameter diamond cores and then test under laboratory conditions;ferromagnetic testing is sufficient to determine the quality of the reinforcement work;the use of ultrasound method for crack penetration depth is helpful in monitoring crack propagation and making a possible decision to implement repair measures; anddespite the development of technology, in practice there is still a lack of equipment allowing for the assessment of the degradation of joints hidden in structural elements, such as wall joints in large-panel buildings.This paper presents a range of visual, non-destructive, and destructive tests that are helpful in determining the technical condition of reinforced concrete precast structures. On the basis of the research and correlation of the individual results, the following conclusions concerning the applied research methods of assessment of large-panel building’s degradation were drawn:The identification of defects in existing buildings with no signs of failure gives reasons for optimism about hidden capacity reserves. Nevertheless, further research is warranted to analyse their impact on structural performance. In the process of ageing of buildings, hidden safety reserves may be exhausted, especially under exceptional loads like gas explosions . TherefoA new contribution to the knowledge of large-panel buildings will also be the analysis of the impact of hidden defects, such as carelessly made steel joints of walls, on the safety of the structure. The authors identify a research gap here and intend to perform analyses using the materials obtained from the demolition.In order to make a reliable assessment of the condition of structural elements and their connections, it is necessary to perform static calculations of the elements that are most severely degraded. On this basis, further decisions related to the repair, reinforcement or demolition of the structure should be made. Actual values of physical and strength parameters of materials exploited for many years should be used for calculations. These parameters can be obtained from non-destructive testing.


Lactobacillus and their metabolites or components affect the function of the host’s immune system and result in microbiota-mediated restoration. Recent data have indicated that, by altering the composition and functions of gut microbiota, antibiotic exposure can also lead to a number of specific pathologies, hence, understanding the potential mechanisms of the interactions between gut microbiota dysbiosis and immunological homeostasis is very important. The Lactobacillus strategies for detecting the associations between the restoration of the relatively imbalanced microbiome and gut diseases are provided in this discussion. In this review, we discuss the recently discovered connections between microbial communities and metabolites in the Lactobacillus treatment of β-lactam antibiotic-induced dysbiosis, and establish the relationship between commensal bacteria and host immunity under this imbalanced homeostasis of the gut microbiota.A balanced gut microbiota and their metabolites are necessary for the maintenance of the host’s health. The antibiotic-induced dysbiosis can cause the disturbance of the microbial community, influence the immune homeostasis and induce susceptibility to metabolic- or immune-mediated disorders and diseases. The  Commonly, the gut microbiota of a human consists of 4 dominant phyla, including Firmicutes, Bacteroidetes, Protobacteria and Actinobacteria, and more than 500 species [The co-evolution of animals and symbiotic microbes has a history of at least 500 million years, forming a complex but relatively balanced gut microbiota which reach 10 species ,3. ThereDistinct antibiotics exhibited diverse inhibition spectra, and the inhibitor mechanism of the β-lactam antibiotic was more mild than the killed ones. Therefore, antibiotic treatment results in the enhanced susceptibility of the host to pathogen infection and the development of aberrant immunity, as the presence of a healthy microbiota is crucial for the prevention of the colonization of enteric pathogens ,5. The dLactobacillus strains are resident in the gastrointestinal tract and essential for the fermentation of cheese, wine, yoghurt and other brewing foods. The beneficial effects of Lactobacillus in the restoration of antibiotic-induced dysbiosis have been reported recently. A series of large-scale, double-blind, well-designed trials were conducted in validation of Lactobacillus’s effect on antibiotic usage. It is difficult to confirm the efficiency of Lactobacillus strains due to the diversity of strain-dependence, dosage of use and appropriate style. Therefore, we first discuss the effects of antibiotics on the host’s gut microbiota, homeostasis and the causes of dysbiosis. Second, we review studies elucidating the role of Lactobacillus under β-lactam antibiotics in gut dysbiosis and the related metabolic disease. Finally, we examine how Lactobacillus acts in dysbiotic microbiota and a disturbed immune system, as well as the interaction between metabolites and altered immunological pathways under antibiotic exposure , colorectal cancer, allergy, diabetes, obesity, autism and asthma, has extensively increased, and these diseases have been connected with the change in microbiome structure termed dysbiosis ,12,13,14Dysbiosis is a distinct microbial ecological state which is causally linked to the usage of exogenous materials or the treatment of disease . AlthougThe reduction or complete loss of a commonly resident bacterial community caused by microbial diminishing or decreased bacterial proliferation could induce gut dysbiosis . Such a Enterococcus blooms in an antibiotic-induced dysbiosis model, which may drive the pathogenesis of experimental gut inflammation and human IBD [Akkermansia was also proven to be a biomarker for an unhealthy gut [Escherichia coli in antibiotic-treated mice induced bacteremia and mortality was demonstrated [The altered abundance of some potentially beneficial microbes or opportunistic bacteria could affect the overall microbial community and lead to changes in particular microbiota-derived metabolites . This hauman IBD . In thislthy gut  and was nstrated .The reduction of α-diversity, which represents the species richness, is commonly associated with dysbiosis. Generally, the richness of gut microbiota increases during the first year for newborns , and is Clostridium difficile, particularly in the elderly population, and further causes diarrhea or fatal colitis [Antibiotics are conventionally bactericidal or bacteriostatic, and indiscriminately restrict or kill both pathogenic and beneficial bacteria. It has been documented that antibiotics affect the genomic, taxonomic and functional characteristic of the microbiome, and they could induce either short-term or everlasting consequences . The alt colitis .Antibiotics are classified into several types, including β-actin, fluoro-quinolone, glycylcycline, lincosamide, nitro-imidazole, a combination (β-lactam/amino-glycoside) and others. The β-actin antibiotic-induced dysbiosis of the gut microbiota has been listed in After antibiotic treatment, one of the common features of metabolic profiles in mice and humans is the decreased level of SCFA. SCFA, including acetate, propionate and butyrate, are normally produced by bacteria through undigested carbohydrates, then absorbed by host cells in the colon. After their consumption by colonocytes, the remaining SCFA are transported to the liver through the blood stream. They also play a role as signaling molecules in the regulation of glucose and lipid metabolism . AccordiTwo independent studies with cefixime- and cefoperazone-treated mice showed a reduced concentration of SCFA or free fatty acids, secondary bile acids and glucose, respectively, indicating that the microbe-related metabolic function was disrupted ,34. In aRecent studies have documented that antibiotics have short- or long-lasting alterations in the gut microbiota and related diseases. Antibiotic use for 1 week or less in healthy humans was reported to affect the diversity of bacterial taxa, deplete the specific families or genera and up-regulate the antibiotic resistance genes, and these effects persisted for 6 months to 2 years after the cessation of antibiotic intake. Furthermore, the antibiotic-induced microbiota perturbation may lead to disease in both infants and adults, especially those correlated to the allergic or metabolic syndromes. One study revealed a relationship between the usage of antibiotics during early life and the incidence of asthma by the seventh year based on data from thousands of children. As with the human study, mice studies also demonstrated that neonatal exposure to antibiotics induced more severe symptoms of asthma. Accordingly, penicillin administration during the gestation period resulted in more enhanced physiological changes than during the weaning period, indicating the important role of microbiota inmaintaining health during early life . TherefoDialister invisus, Bifidobacterium adolescentis, Clostridium cluster XIVa, Faecalibacterium prausnitzii and Ruminococcus gnavus in CD [Faecalibacterium prausnitzii is especially connected to the anti-inflammatory effects of the disease [As the most frequent forms of IBD, the mechanism of ulcerative colitis (UC) and Crohn’s disease (CD) remains unclear, however, extensive evidence shows that intestinal microbial dysbiosis acted as a major inducer of IBD ,39,40,41us in CD . Faecali disease . HoweverProteobacteria and Firmicutes were increased and the abundance of Lachnospiraceae was enriched in the diarrhea-predominant IBS (D-IBS) patients, indicating that significant differences exist in the specific bacterial taxa between D-IBS patients and healthy human [Bifidobacterium and Clostridium histolyticum; in addition, the level of IgA-coated bacteria was also linked to such gut dysbiosis [Apart from IBD, the dysbiotic state of the gut was related to other gut diseases such as coeliac disease, irritable bowel syndrome (IBS) and colorectal cancer. Earlier studies indicated significant differences in the diversity of the microbiotas between IBS and healthy individuals. Other studies demonstrated that the microbial communities of hy human . The dysysbiosis . The chaysbiosis . The relysbiosis . HoweverRecently, the gut microbiota dysbiosis has also been associated with metabolic diseases such as type 2 diabetes and obesity. The microbiota was changed in the obese mice, however, the incidence of metabolic endotoxemia and obesity-related parameters was reversed after antibiotic-conferred changes in the microbiota. Early-life antibiotic exposure can induce dysbiosis of the gut microbiome in fat-1 mice and lead to later-life obesity . Indeed,Citrobacter rodentium, where the microbiota lacked its ability to resist invasion by pathogenic bacteria (The activation of microorganism-associated molecular pattern (MAMP) recognition receptors is the crucial mechanism by which microbiota can affect host immunity, and these include Toll-like receptors (TLRs) and NOD-like receptors (NLRs). Some intestinal ligands modulated by microorganisms including LPS, peptidoglycan and lipoteichoic acid can activate pattern recognition receptors (PRRs). The activation of these ligand-engaged receptors on the epithelium can also result in proinflammatory responses . The micbacteria .In a normal microbiota environment, the homeostasis of the microbial community and host are sustained in relatively stability. The recognition of the microbiota signals through the NF-κB pathway and induces the production of AMPs. The expression of tight junction proteins and the levels of metabolite are relatively constant.In antibiotic-induced dysbiosis, the numbers of MAMPs are decreased as a result of the loss of microbiota, and the thinned mucus layer and damaged tight junction proteins lead to the infection of opportunistic pathogens. The levels of SCFA and related immune functions are altered after antibiotic exposure.H17 cell population [Furthermore, the studies of antibiotic use have proven that the gut microbiota also contributes to the maintenance of the immune system. Antibiotic treatment aggravated the DSS-induced colitis of mice by eliminating the microbial ligands which sustain the gut homeostasis through signaling TLRs. Amoxicillin administration resulted in the appearance of antigens and the decreasing secretion of antimicrobial defense molecules, such as phospholipase A2 and α-defensins. The loss of specific taxa of the microbial community with antibiotics could identify which microbial taxa may be responsible for the regulation of T cell activation. For example, the ampicillin treatment targeting the Gram-positive bacteria induced the depletion of the Tpulation .Escherichia coli and Klebsiella pneumoniae sepsis [The related immune impairment occurred after antibiotic-induced dysbiosis. After a low dose penicillin treatment, the expression of immune genes for Reg3γ, IL-17 and β-defensins was decreased in dysbiotic mice . Reg3γ ie sepsis . The ante sepsis . Similare sepsis ,60, indi+ T cell expansion, with levels of IgG, IFN-γ and TNF-α resulting in immunity injury in mice [Clostridium rodentium [+ T cells, IFNγ and IL-17, indicating that microbial signaling is essential in the maintenance of T cell effectors [In addition, antibiotic usage was shown to induce the reduction of CD8 in mice . The expodentium . A 10-daffectors .Lactobacillus strains and other probiotics has applied in the therapy or intervention of antibiotic-induced dysbiosis [Lactobacillus have investigated the mechanisms leading to the amelioration of antibiotic-induced dysbiosis in humans and animals  strain was proven to promote specific bacterial taxa including Citrobacter, Bifidobacterium and levels of SCFAs to attenuate ampicillin-induced dysbiosis while other two tested strains (LacP and LacG) had no such effect, suggesting strain-specific restoration of microbial community. Another study reported that L. reuteri, but not L. rhamnosus GG or nonpathogenic Escherichia coli, decreased the antibiotic-induced elevation of the Firmicutes: Bacteroidetes ratio. These differences and mechanism of Lactobacillus strains in enhancement of gut microbiota disorders and regulation of intestinal permeability could be further investigated.Importantly, the strain-specific effects on the antibiotic-induced dysbiosis were also evaluated amongst a number of studies. Recently, Lactobacillus strains are presumed to benefit intestinal health by their relatively direct actions on the composition and function of gut microbiota. Previous studies conducted effects of single strain either L. reuteri 6475 or L. rhamnosus GG, and L. reuteri but not L. rhamnosus reduced the ratio of Firmicutes: Bacteroidetes elevated after antibiotics use. Also, a modulated intestinal permeability and reduced femoral trabecular bone volume were observed [Lactobacillus strains mitigates both head and gut antibiotic-associated microbiota dysbiosis in adult bees by suppressing larval pathogen loads to near-undetectable levels [Lactobacillus consumption improved and recovered the gut microbiota composition toward pre-treatment state while some studied had no obvious results in alteration of the diversity of gut microbial community [Lactobacillus in microbial niches remains in need of discussion and exploration.Various studies have indicated that decreased diversity of gut microbial community could lead to the development of gut dysbiosis or other diseases ,81,82,83observed . Subseque levels . Despiteommunity ,85, the Lactobacillus during or after antibiotic administration. L. reuteri had been performed for 28 days in 31 patients receiving antibiotics, and decreased the incidence of AAD among hospitalized adults [L. plantarum has been proven to minimize the risk of developing loose or watery stools and a similar effect was observed in application of L. acidophilus NCFM [The antibiotic-induced dysbiosis in human body is frequently accompanied with antibiotic-associated diarrhea (AAD). Both clinical and routine interventions were investigated to analyze the effects of d adults . Consumplus NCFM ,73.Lactobacillus strains exhibited obvious alleviation in antibiotic-induced moderate or mild gut dysbiosis while these effects were weakened or disappeared in the severe cases. For example, when L. rhamnosus GG or L. casei Shirota was administered respectively in hospitalized severe patients receiving antibiotics, no restored improvement was observed in gut dysbiotic symptoms [L. rhamnosus GG or L. paracasei Lpc-37 were demonstrated that patients with an initial mild to moderate antibiotic administration could be alleviated in the gut dysbiosis [However, it should be emphasized that symptoms . On the ysbiosis Lactobacillus strains were broadly applied in the antibiotic-induced dysbiosis. Cocktail strains including Lactobacillus, Bifidobacterium, Lactococcus has been demonstrated to improve microbiological, clinical and immunomodulatory efficacy in case of post-antibiotic reconstitution of the gut mucosal [L. acidophilus and L. casei has been shown to be effective against antibiotic-associated diarrhea caused by Clostridium infections [L. helveticus and L. rhamnosus supplementation has also supported its efficacy in reducing the duration of diarrhea symptoms [L. reuteri strains could reduce the antibiotic-induced side effects, including dysbiosis and stomach problems [Lactobacillus was also broadly studied. Several reports including L. reuteri or L. plantarum 299v have been proven to alleviate antibiotic-induced dysbiosis, while a few studies revealed that L. casei Shirota or L. rhamnosus GG had no specific improvement on the antibiotic-induced side effects [Both single and cocktail  mucosal . Also, mfections . Meanwhisymptoms . Anotherproblems . On the  effects ,75.Lactobacillus cocktails in antibiotic-induced dysbiosis is more obvious than the effects of a single strain, however, the mechanism of these differences between single and mixture strains has not been investigated and remains unclear.Based on this research, the restoration of Lactobacillus strains on gut microbiota rebuilding is one of the essential aspects of modulation evaluation. Most studies focus on two points, which are the promotion of comparatively beneficial microbes and the suppression of unexpected bacteria or pathogens. For instance, the treatment of Streptococcus boulardii CNCM with a seven-day administration of amoxicillin-clavulanate reduced the levels of Escherichia coli overgrowth [L. acidophilus, Bacillus fragilis and Bifidobacterium longum treatment enhanced the relative abundance of Akkermansia [Lactobacillus could selectively protect against Bacteroides species but not against related pathogens in antibiotic-induced dysbiosis [Lactobacillus strategies for restoring the gut microbiota specifically and illuminating the activity spectra of β-lactam antibiotic in resident bacteria.The alterations of gut microbiota are relatively direct impacts caused by antibiotic administration. Therefore, the improved effect of ergrowth . Moreoveermansia . Especiaysbiosis . These fLactobacillus strains, the residual impact on microbial metabolism and the immune system may persist for a long time, and the negative effect of Lactobacillus strains could also exist in some severe antibiotic-induced cases [Lactobacillus treatment [Importantly, although an antibiotic-induced dysbiotic gut microbiome can be partly or totally reversed by ed cases . In a shreatment . Especiareatment ,93. The reatment . TherefoLactobacillus strains could colonize or promote the growth of beneficial microbes in the anaerobic intestinal tract via the formation of biofilms and the enhancement of gut mucosa. In addition, L. plantarum has been reported to secrete plantaricin EF and prevent damage to the gut barrier integrity through LPS-repairing [Lactobacillus strains can restore antibiotic-induced gut barrier disruption through the inducible expression of tight junction proteins. For instance, previous research announced that Lactobacillus cocktails may enhance the levels of tight junction proteins, including ZO-1 and occludin, and such a rearrangement leads to an immune response [Antibiotic exposure is one of the key factors that induces gut barrier disruption and immune dysfunction, and their overuse can affect gut barrier integrity and aggravate various health issues . The maiepairing . Furtherresponse .Lactobacillus or prebiotics, and their beneficial effects on intestinal microbiota and immunity. Summarily, the majority of research on this subject has proven that Lactobacillus administration can be impactful in antibiotic-induced dysbiosis. The administration of Lactobacillus can modulate and produce a positive impact on the microbial community through the regulation of the immune response, including the production of anti-pathogen inhibitory substances and the blockage of adhesion sites [Lactobacillus brevis could generate an effective immune response in breast cancer mice. Importantly, in L. casei, Zhang decreased the serum IL-1α level, indicating inflammation reduction after antibiotic treatment [Lactobacillus treatment.In recent years, several therapy strategies and alternative applications have been suggested to alleviate antibiotic-induced dysbiosis and mitigate the side effects of antibiotics. Several new therapies focus on the intervention of probiotics, including on sites ,100. It reatment , however+Foxp3+ T cell differentiation has been revealed by Lactobacillus strains, which suggests the importance of Treg cells in maintaining gut homeostasis. Relevantly, antibiotic-induced gut barrier disruption accelerated the translocation of pathogen evasion, and therefore increased pathoadaptive mutations of commensal intestinal microbiome genes and resulted in infective immune responses or bacterial dissemination [Lactobacillus species targeted regulated immunity cytokines and suppressed the potential invasion of pathogenic bacteria, which created a new strategy for restoring the pathways of beneficial species.Moreover, the regulation of Treg expression induction through CD4mination . LactobaLactobacillus strains release metabolic molecules such as short-chain fatty acids (SCFAs) that modulate immune responses and up-regulate the expressions of tight junction proteins. The SCFA have been reported to be involved in the development of colonic regulatory T (Treg cells) and engage the G protein coupled receptor GPR43 on neutrophils to decrease their infiltration into tissues and alleviate inflammation, thus SCFA could also enhance the expression of FoxP3 and drive the naïve T cell to Tregs [to Tregs ,104. It to Tregs , and theto Tregs . Indeed,to Tregs . The antto Tregs .reg cells [Escherichia coli O157:H7 infection, promote the IEC integrity and further inhibit the translocation of Shiga toxins [In addition to their activity as the ligands for G protein-coupled receptors (GPCRs), SCFA also act as the inhibitors of histone deacetylases (HDACs). HDAC inhibition produces anti-inflammatory response via the interaction of SCFA with DC and macrophages ,107. Heneg cells . Collecta toxins .Lactobacillus, Bifidobacterium and Clostridium. Most of these genera are safe and retain high adaptability to the gut ecosystem. In summary, the antibiotic-induced gut microbiota dysbiosis causes the decrease of SCFAs, which further results in the alteration of the regulation of the immune system. For instance, acetate acts as a substrate for fatty acid or cholesterol synthesis, which increases the rate of oxygen uptake and the movement of ileum by affecting its contraction [Lactobacillus by metabolites targeted two main aspects; one is the production or synthesis of functional SCFAs and other metabolites by specific Lactobacillus, and the other is the affection of relevant microbial species by Lactobacillus that enhance levels or ratios of beneficial metabolites.As reported, more than 70 gut bacteria have been investigated to produce SCFAs, most of which belong to the well-studied probiotic genera traction ,111. TheLactobacillus aimed at restoring the microbial community and gut barrier integrity are likely to directly reshape the gut ecosystem. Instead, we believe that intestinal metabolic alterations involving the consideration of SCFAs, immune regulatory factors and signaling receptors are more likely to show positive results in Lactobacillus-involved antibiotic dysbiosis regulation. However, we urgently need alternative strategies that substitute or supplement the currently widespread use of antibiotics.Here, we have highlighted the effect of antibiotic exposure in the disruption of commensal communities, and the resulting decreased interactions between gut microbiota and the mucosa. Based on a number of studies, we summarized the alteration of the appropriate metabolites and functional immune system in a host with antibiotic-induced dysbiosis. The antibiotic treatment has long- or short-lasting perturbations in microbiota that result in the potential risk of diseases including gut disorders, metabolic syndromes and immune-related defects. Therefore, treatments of Lactobacillus in antibiotic-induced dysbiosis. Furthermore, the effectiveness of Lactobacillus depends on the severity of the antibiotic-induced disease and specific function on the strain level. Indeed, the treatment between a single strain and mixture of different Lactobacillus genera or species should be urgently investigated in future studies.Recent advances should offer further evidence related to the unascertained association between animal experiments and the clinical efficacy of


Immunoglobulin (Ig) G4–related disease (IgG4-RD) reportedly has a strong relationship with adult-onset asthma and periocular xanthogranuloma (AAPOX) and may be linked to sclerosing mucoepidermoid carcinoma (MEC). We present a rare case of IgG4-RD and AAPOX occurring in a patient with resected eosinophilic or oncocytic MEC.A 52-year-old woman was referred to our rheumatology clinic in 2020 to be evaluated for suspected IgG4-RD.The patient had diagnoses of periorbital xanthelasmas, worsening glucocorticoid-dependent chronic rhinosinusitis and adult-onset asthma, and cervical lymphadenopathy persisting 2 years after resection of a low-grade MEC of a minor salivary gland.Because the patient’s symptomatic relief was glucocorticoid dependent, IgG4-RD was suspected, and she was referred to our medical center. Her amylase and lipase levels were elevated. Serum IgG4 levels were initially within normal limits, but IgG4-RD was diagnosed because of the presence of lymphadenopathy and evidence of pancreatitis, which was shown on positron emission tomography/computed tomography. Furthermore, the IgG4 levels later increased without explanation. After the patient began combination therapy with a glucocorticoid (prednisone) and methotrexate, her symptoms improved but recurred when the daily oral glucocorticoid dosage decreased below 10 mg. An excisional biopsy of her right submandibular gland in 2021 yielded results consistent with IgG4-RD. In addition, AAPOX was diagnosed, given the presence of periocular edema and plaques, adult-onset asthma, and rhinosinusitis.The patient was carcinoma free at last follow-up and was receiving medication to treat the other conditions.The diagnosis of these 3 concomitant, uncommon entities required approximately 7 years of medical investigations. Clinicians should know that IgG4-RD, AAPOX, and MEC may occur together. Adult-onset asthma and periocular xanthogranuloma (AAPOX) is a rare non-Langerhans histiocytosis that commonly presents with the triad of periorbital swelling and xanthelasmas, adult-onset asthma, and chronic rhinosinusitis. A strong relationship has been reported between IgG4-RD and AAPOX.Immunoglobulin (Ig) G4–related disease (IgG4-RD) is an autoimmune disease of lymphoplasmacytic infiltration and IgG4 deposition in multiple organ systems. Sclerosing MEC is associated with increased numbers of IgG4-positive plasma cells, a finding that raises speculation about a potential link between MEC and IgG4-RD.,6Primary malignant tumors of the salivary gland are rare. When they occur, most are mucoepidermoid carcinomas (MECs), which are composed of mucous, epidermoid, and intermediate cells. A rare variant known as sclerosing MEC also has unusual histologic characteristics consisting of abundant fibrous to hyalinized stroma and inflammatory cells.Herein, we describe the case of a patient with a combination of these 3 uncommon clinical entities: IgG4-RD, AAPOX, and MEC. The variant of MEC, however, was the eosinophilic or oncocytic subtype (not the sclerosing subtype).In 2020, a 52-year-old woman was referred to our rheumatology clinic by her otorhinolaryngologist to be evaluated for suspected IgG4-RD. In 2014, periorbital swelling and yellow plaques developed on her eyelids, and biopsy findings from her health care facility at that time were consistent with xanthelasma  and methotrexate, which have proved useful for treating both IgG4-RD and AAPOX.,15 Subsequently, she noted substantial improvement in her symptoms. Additionally, abdominal magnetic resonance imaging performed 2 months after initiation of treatment showed resolving pancreatitis  and an elevated IgG4:IgG ratio (up to 70%). Additional immunohistochemical tests for CD3, CD5, CD10, CD20, CD21, CD43, BCL2, BCL6, and cyclin D1 were performed, along with in situ hybridization of κ and λ light chains. Results of these studies highlighted a mixed infiltrate of small T cells and small B cells without aberrant antigen expression, along with a polytypic population of plasma cells without light chain restriction, offering no evidence of an underlying lymphoproliferative disorder. No evidence of neoplasia or cancer was seen . The decision regarding starting methotrexate treatment required a risk-benefit conversation with the patient, given her history of MEC. The potential for liver toxicity with higher doses of methotrexate and the recurrence of the patient’s symptoms after decreasing the methotrexate dose led us to confirm the IgG4-RD diagnosis with a salivary gland biopsy. After confirmation, we restarted treatment with glucocorticoids and are planning to start rituximab therapy.A limitation of our report is that it is a single case study. Although we cannot conclude from a single case that a link exists between IgG4-RD, AAPOX, and MEC, authors of previous reports suggested a possible association between salivary gland tumors and IgG4-RD when patients have atypical symptoms. Reports of additional cases and case series may help clarify whether a true association exists between IgG4-RD, AAPOX, and MEC or whether their occurrence in a single patient is merely coincidental. However, clinicians should know that IgG4-RD, AAPOX, and MEC may occur together. In this unusual case, the road to diagnosis and implementation of an effective, long-term treatment for this patient took 7 years.The Mayo Clinic Institutional Review Board does not require approval for a single case report. The patient’s written consent was obtained for the purpose of publication.Nikita Chhabra, DO: Substantial contribution to the acquisition of data, drafting of the manuscript, and revising it critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolveda: Substantial contribution to the acquisition of data, drafting of the manuscript, and revising it critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedJohn E. Cebak, DO, PhDb: Substantial contribution to the acquisition of data, drafting of the manuscript, and revising it critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedAlessandra Schmitt, MDc: Substantial contribution to the acquisition of data, revising the manuscript critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedDevyani Lal, MDd: Substantial contribution to revising the manuscript critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedAllison C. Rosenthal, DOe: Substantial contribution to revising the manuscript critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedCullen M. Taylor, MDe: Substantial contribution to revising the manuscript critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedRyan M. Thorwarth, MDf: Substantial contribution to revising the manuscript critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedAmi A. Shah, MDs: Substantial contribution to revising the manuscript critically for important intellectual content and final approval; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolvedAlicia Rodriguez-Pla, MD, PhDThe Scientific Publications staff at Mayo Clinic provided substantive editing, proofreading, administrative, and clerical support.


Antimicrobial resistance (AMR) is one of the biggest threats to global public health. Selection of resistant bacteria is driven by inappropriate use of antibiotics, amongst other factors. COVID-19 may have exacerbated AMR due to unnecessary antibiotic prescribing. Country-level knowledge is needed to understand options for action.To review AMR in Mexico and initiatives addressing it. Identifying any areas where more information is required will provide a call to action to minimize any further rises in AMR and to improve patient outcomes.Streptococcus pneumoniae and Haemophilus influenzae, were identified. National and international antibiotic prescribing guidelines commonly used in Mexico for specific CA-RTIs  were also reviewed, along with local antibiotic availability. Insights from a local clinician were sought to contextualize this information.National AMR initiatives in Mexico, antibiotic use and prescribing, and availability of susceptibility data, particularly the key community-acquired respiratory tract infection (CA-RTI) pathogens The Mexican national AMR strategy was published in 2018. This comprised similar objectives to the Global Action Plan from the World Health Assembly (2015) and was compulsory, requiring full compliance from members of the National Health System. Historically, antibiotic consumption in Mexico has been high, however, between 2000 and 2015, consumption fell, in sharp contrast to the majority of countries. Mexico lacks a national surveillance network for AMR, however there are several ongoing global surveillance studies providing local antibiotic susceptibility data. International and local antibiotic prescribing guidelines for CA-RTIs are used. A more standardized inclusive approach in developing local guidelines, using up-to-date local surveillance data of isolates from community-acquired infections, could make guideline use more locally relevant. This would pave the way for a higher level of appropriate antibiotic prescribing and improved adherence. This would, in turn, potentially limit AMR development in Mexico and improve patient outcomes. Even if new medicines are developed, without behaviour change, antibiotic resistance will remain a major threat’.3 The first paper in this Supplement included details about the multiple factors which can drive a rise in AMR along with the global initiatives that are in place to address this threat.2 Each country and/or region must also play their part through local initiatives.Antimicrobial resistance (AMR) is one of the biggest threats to public health throughout the worldStreptococcus pneumoniae and Haemophilus influenzae. National and international antibiotic prescribing guidelines for CA-RTIs, specifically community-acquired pneumonia (CAP), acute otitis media (AOM) and acute bacterial rhinosinusitis (ABRS), commonly used by healthcare professionals in Mexico were also reviewed, along with how these link to local antibiotic availability. Insights from a clinician in Mexico were sought to contextualize this information. In addition, we aimed to identify areas where more information is required and present a call to action to improve clinical outcomes for patients and to minimize further rises in AMR within Mexico.In order to identify how AMR can be addressed in Mexico in the future, it is necessary to review what is happening now. In this paper, we present the current situation in Mexico, determined by using published information  to ascertain any national initiatives to address AMR, antibiotic use and prescribing, and availability of susceptibility data, in particular for the key community-acquired respiratory tract infection (CA-RTI) pathogens 4 many countries began to develop their own National Action Plan (NAP). In 2018, the Mexican government published an agreement which announced that the National Strategy of Action Against Resistance to Antimicrobials was compulsory and required full compliance from members of the National Health System.5 The National Strategy is a comprehensive plan which has similar objectives to the GAP, namely to improve awareness of AMR through effective communication, to strengthen knowledge of AMR in the context of both humans and animals through study of surveillance and epidemiology, to reduce the incidence of infections in both humans and animals through preventative measures, to use antimicrobials rationally in both humans and animals and to develop an economic assessment of the problem in Mexico to ensure sustainable investment. Recent analysis by the WHO presented on their website showed that in 2020–21, the plan in Mexico was at the implementation stage.6Following the formulation by the World Health Assembly in 2015 of a Global Action Plan (GAP) for AMR7 A range of bacteria were investigated including S. pneumoniae isolates from respiratory tract infections (RTIs) where a decrease in resistance was observed against penicillin, cefotaxime, erythromycin and trimethoprim/sulfamethoxazole. Trends in antibiotic susceptibilities of bacteria tested were variable; for some bacteria, resistance to some antibiotics increased over the period of study. This highlights the need for regular monitoring of antibiotic susceptibilities. The study provided valuable information as it covered a wide geographic area with participation from multiple medical centres, however, the antibiotic susceptibility testing methodology was not standardized, and data were incomplete in some areas. This investigation had been preceded by a similar ‘snapshot’ study which investigated susceptibilities in 47 centres for a 6 month period. S. pneumoniae and H. influenzae were not included due to lack of information, but the authors particularly noted the multidrug resistance of Acinetobacter spp., Klebsiella spp. and Escherichia coli and the carbapenem resistance in specific groups of Enterobacterales. The main limitation was its duration (only 6 months), however, it reflects important information gathered from 20 Mexican states that will allow the definition of strategies for the control of resistance.8A recent review summarizes the progression of AMR in Mexico over a 10 year period (2009–18) through a retrospective analysis of both Gram-positive and Gram-negative bacteria by including data from 22 of the 32 Mexican states.9 This has been corroborated by a global study which showed that, unlike most of the countries throughout the world, between 2000 and 2015 the antibiotic consumption rate in Mexico fell by between 10 and 15 defined daily doses (DDD) per 1000 inhabitants per day, contrasting with the majority of countries . This resulted in an antibiotic consumption rate for Mexico for 2015 in DDDs per 1000 inhabitants per day that placed Mexico 73rd out of the 76 countries included in the study.10 This reduction in antibiotic consumption could be viewed as a promising trend towards rational prescription practices or, according to some authors, could alternatively be due to a decrease in economic growth during the last three decades and inequality of income leading to the reduced possibility of purchasing antibiotics, and hence reduced consumption.11Historically, antibiotic consumption in Mexico has been high. In 1997, Mexico was the leading country in Latin America for antibiotic clinical use but, by 2007, Mexico was in fourth place, behind Argentina, Venezuela and Peru.12 but in that year, Mexico (and Brazil), implemented a policy enforcing the prohibition of all systemic antibiotic sales without prescription.13 Analysis of the effect of this prohibition concluded that the trend in decreased antibiotic use in Mexico was boosted, in particular with the reduction in use of penicillins and fluoroquinolones, which is of importance since the penicillin antibiotics had been reported to be the most common group for self-medication in Mexico.Until 2010, over-the-counter (OTC) sale of antibiotics was allowable in Mexico. A systematic global study showed that pre-2010, 18% of antibiotics used in Mexico were obtained without a prescription11 In 2018, a non-governmental initiative launched the Network for Research and Surveillance of Drug Resistance [Red Temática de Investigación y Vigilancia de la Farmacorresistencia (INVIFAR)] in Mexico to include the surveillance of resistance in common pathogens such as Enterococcus spp., Staphylococcus aureus, E. coli, Klebsiella spp. and in collaboration with SIREVA/GIVEBPVac , S. pneumoniae but the number of isolates has been low.7Mexico lacks a national network for monitoring antibiotic resistance, so available data comprise local reports without uniform methodology.S. pneumoniae in Mexico stratified by patient age and sex are available from GIVEBPCVac.14 Two-hundred and fourteen isolates were from all age groups . The proportion of non-meningitis isolates susceptible to penicillin from patients of all age groups was 68.0% (≤2.0 mg/L); 17.8% were intermediately resistant (4.0 mg/L) and 14.2% were resistant (≥8.0 mg/L). Among 213 isolates, susceptibility to erythromycin  was 49.3% (105/213).Recent data (2019) on the susceptibility of Several ongoing global surveillance studies provide antibiotic susceptibility data in Mexico or will provide valuable data in the future.15 Two international organizations define breakpoint values: CLSI and EUCAST. Due to variation in criteria for their definition, there are some differences between CLSI and EUCAST in the clinical breakpoint values for certain bacteria for some antibiotics and this can impact the susceptibility interpretation of clinical isolates.16 EUCAST breakpoints are dose-specific and use the EMA-approved doses that are included in the Summary of Product Characteristics of an antibiotic. This means that by application of breakpoints for higher doses, the effect of using a raised dose on the clinical efficacy of a particular antibiotic can be predicted. Currently, most clinical microbiology laboratories in Mexico use CLSI breakpoints, however the international application of the EUCAST breakpoints is expanding17 so it is possible that dose-specific breakpoints could at some time also be applied in Mexico, as seen in Brazil. The EUCAST dose-specific breakpoints can also be used retrospectively to calculate the susceptibility of previously collected isolates to show the susceptibility levels that would have been achieved at higher doses.Clinical breakpoints in surveillance studies and antibiotic susceptibility testing are cut-off MIC values used to classify microorganisms into the clinical categories susceptible (S), intermediate (I) and resistant (R), in order to assist in the prediction of the clinical success or failure of a specific antibiotic.S. pneumoniae, the most commonly isolated respiratory pathogen19 for infections such as CAP, AOM and ABRS, has over time become less susceptible to amoxicillin/clavulanic acid in some countries20 since the MICs of some isolates have increased. When treating infections, it is important to be able to eradicate bacterial pathogens with raised MICs to optimize clinical outcome while at the same time minimizing the risk of selecting variants with even higher MICs. This is possible because β-lactams, unlike many other antibiotics, have time-dependent killing properties. Their efficacy depends on the amount of time the drug concentration is present at the site of action. Although increasing the concentration at the infection site above a particular concentration will not have any effect on the efficacy, the use of higher doses and/or more-frequent dosing allows for successful eradication of infections caused by pathogens with higher MICs because the time above the MIC is increased.21Use of the EUCAST dose-specific breakpoints shows the effect of increasing the antibiotic dose on the susceptibility of a pathogen, providing additional information so the prescriber can decide if a higher dose would be of benefit. For example, 22 Within ATLAS, susceptibility data for Mexico are available for a relatively small number of isolates. Figure S. pneumoniae isolates collected in 2014 to 2017. The susceptibility of S. pneumoniae to the antibiotics tested decreased overall during this period, apart from the fluoroquinolones, with isolates remaining fully susceptible to both levofloxacin and moxifloxacin. Results are available on the ATLAS database for H. influenzae isolates collected in 2015, 2017 and 2018 and are shown in Figure The Antimicrobial Testing Leadership and Surveillance (ATLAS) database is a global AMR surveillance programme which is fully searchable, available for general access and records the susceptibilities of a range of bacterial and fungal pathogens to a bank of antimicrobials.S. pneumoniae and H. influenzae isolates collected in Mexico although isolate numbers are again relatively low.23 The susceptibility rates, by CLSI breakpoints, from SENTRY shown in Figures The SENTRY Antimicrobial Surveillance Programme was initiated in January 1997 and was designed to monitor the predominant pathogens and AMR for nosocomial and community-acquired infections globally. Data are available for susceptibility of 24 to provide the supporting data required to ensure that countries can design cost-effective, evidence-based AMR response strategies. During the first four years, 91 countries or territories have enrolled in GLASS and data for over two million patients from 66 countries are included.25 Pathogens currently included in GLASS-AMR are: Acinetobacter spp., E. coli, Klebsiella pneumoniae, Neisseria gonorrhoeae, Salmonella spp., Shigella spp., S. aureus, and S. pneumoniae and a new and important component is the inclusion of antimicrobial consumption (AMC) surveillance at the national level.26In 2015, the WHO launched the Global Antimicrobial Resistance and Use Surveillance System (GLASS). GLASS is a global system that collects national AMR data for selected bacterial pathogens that cause common infections. The aim is to monitor the prevalence of AMR among major pathogens in clinical settingsWhilst Mexico is not currently one of the countries that has enrolled in GLASS, the WHO Regional Office for the Americas/Pan American Health Organization supports AMR surveillance, prevention and control in the Region through its AMR special programme and Mexico is one of the countries included in that initiative.For management of the common CA-RTIs, CAP, AOM and ABRS in Mexico, clinicians use several country-specific local antibiotic prescribing guidelines plus a range of international guidelines, examples of which are included in Table 35 For example, the first-line recommendation by the IDSA 2019 for treating adult outpatients with CAP35 is amoxicillin or doxycycline or a macrolide  but if the patient has comorbidities and CAP, the recommendations include amoxicillin/clavulanic acid, 500 mg/125 mg given three times daily, 875 mg/125 mg or 2000 mg/125 mg both given twice daily, or cephalosporins in combination with a macrolide or doxycycline, or monotherapy with a respiratory fluoroquinolone.35For the management of CAP in adults and paediatrics, the international guidelines referred to by clinicians in Mexico include those from the IDSA.34 and for ABRS in adults and paediatrics the international guidelines referred to in Mexico include those from the IDSA.33 For ABRS in adults the IDSA recommend first-line amoxicillin/clavulanic acid 500 mg/125 mg three times daily or 875 mg/125 mg twice daily.33For the management of AOM, the international guidelines used in Mexico include those from the American Academy of Pediatrics (AAP)27 in which amoxicillin 80–90 mg/kg/day given in two doses up to a maximum of 4 g per day is recommended as a first-line treatment and amoxicillin/clavulanic acid 90 mg/kg/day given in two doses up to a maximum of 4 g per day, is recommended as an alternative treatment for outpatients with bacterial CAP. In cases of penicillin allergy, azithromycin, clarithromycin or erythromycin are the recommendations. The prevention, diagnosis and treatment of CAP, update 201728 also recommends amoxicillin as a first-line, and amoxicillin/clavulanic acid as an alternative treatment in adults with low-risk CAP. For AOM, the national treatment guideline for prevention, diagnosis and treatment in paediatric patients recommends, for example, amoxicillin 80–90 mg/kg/day for 5–10 days in children with no previous antibiotic treatment, and amoxicillin/clavulanic acid 90 mg/kg/day for 5–10 days for children who have had previous antibiotic treatment.30For the management of CAP, AOM and ABRS, the commonly used national guidelines, which are mainly used in public institutions are, firstly for the treatment of CAP ‘Diagnosis and treatment of CAP in patients from 3 months to 18 years in the first and second level of care’, update 2015,38Access to antibiotics may be an issue for patients in low- and middle-income countries due to cost and insufficient government expenditure or support in this area. Drug supply chains may also contribute to the problem. Limited access to the most appropriate antibiotic to treat a specific infection may result in raised mortality from treatable bacterial infections, and the use of suboptimal amounts of antibiotic facilitates resistance development and allows resistant strains to persist.31 for outpatients with comorbidities, by the IDSA 2011 guidelines for the treatment of paediatric CAP in outpatients,32 by the IDSA 2012 treatment guidelines for sinusitis33 as initial treatment for children and in adults with risk of treatment failure, by the AAP 2013 AOM34 guidelines as initial or delayed treatment of AOM in children or after failure of initial treatment, and by the IDSA 2019 guidelines for adults with CAP35 who have comorbidities. The majority of the antibiotics recommended by CA-RTI guidelines are available in Mexico, e.g. most cephalosporins and macrolides. Generic antibiotics also play an important role.Antibiotic availability within a country is an important consideration when applying recommendations from guidelines. In Mexico, currently available formulations of amoxicillin/clavulanic acid are mentioned as first- or second-line recommendations by several of the RTI antibiotic prescribing guidelines. In particular, amoxicillin/clavulanic acid 90 mg/kg/day (high dose) is available in Mexico and is recommended by IDSA 2007 CAP treatment guidelines39 Since poor-quality antibiotics are unlikely to contain the full dose needed to eliminate all of the infecting pathogens, this will encourage resistance to develop and allow resistant strains to survive and be transmitted.40Substandard poor-quality or falsified antibiotics promote AMR and the spread of drug-resistant infections.40 The GSMS aims to work with WHO member states to improve the quality of reporting of substandard and falsified medical products, and, importantly, to ensure the data collected are analysed and used to influence policy, procedures, and processes to protect public health, at the national, regional and the global level. Use of substandard or falsified antibiotics not only compromises clinical outcomes but also risks increased AMR. The most recent summary (2013–17) reported substandard and falsified medicines in 46 member states, including Mexico, and antibiotics represent 16.9% of all products reported, second only to malaria drugs (19.6%).40The quality of medicines, specifically antibiotics, is an important consideration in Mexico. The WHO launched a Global Surveillance and Monitoring System (GSMS) for substandard and falsified products.The information included here will have an important relevance to the clinical practice of Mexican physicians because it raises awareness amongst physicians of AMR in common CA-RTI bacterial pathogens. The rational and conscious use of antibiotics by physicians will contribute in a significant way to the decrease of antibiotic resistance in Mexico.As mentioned previously, in order to tackle AMR, the Mexican government established a law making the sale of antibiotics possible only with a medical prescription. Unfortunately, the impact of this law has not been immediately obvious, however, in some hospitals antibiotic prescribing is controlled by an infectious diseases committee that applies strict criteria for the prescription and indication of antibiotics; this and other measures have decreased the antibiotic resistance of hospital-acquired infections.14 Data provided by the IDSA guidelines, AAP guidelines for acute otitis media and rhinosinusitis management, the AAO-HNS guidelines and the Pan-American clinical practice guideline for medical management of acute tonsillitis and adenoids hypertrophy could also be taken into account. To summarize, there is insufficient information in Mexico about AMR in pathogens causing upper airway infections. Guidelines are of benefit to the clinical practice, especially if they include local information.When considering the use of an antibiotic, the decision should be made by first considering the local and international surveillance data, but unfortunately in Mexico there is limited information regarding this topic. In terms of upper airway infections, the information is provided by the GIVEBPVac data related to invasive strains from 2019.In an era of rising AMR throughout the world, this paper aims to define areas where action is required to tackle AMR by analysing and understanding the current situation within Mexico. Information is presented concerning antibiotic use and prescribing, approach to AMR, availability of local susceptibility data, use of international and/or local management guidelines and how these link to antibiotic availability. To our knowledge this is the first time this information has been reviewed and presented in detail for a specific country.Inappropriate antibiotic prescribing in Mexico has been widespread and associated with antibiotic resistance. Historically, antibiotic usage in Mexico has been high, in fact in 1997 Mexico was the leading country in Latin America for antibiotic clinical use. However, unlike most of the countries throughout the world, the antibiotic consumption rate in Mexico fell between 2000 and 2015 by between 10 and 15 DDDs per 1000 inhabitants per day. A recent reduction could also be a reflection of the law banning the OTC sale of antibiotics, although, as mentioned in the clinician’s expert comment, the effect of this law may not have been felt immediately. Individual hospitals in Mexico may have their specific regulations concerning antibiotic use but similar restrictions relating to community medicine are not so obvious.2 regulatory bodies and other authors urge caution, restricting their use to limited situations due to serious safety concerns.41–44In terms of surveillance, there is a lack of a national network for measuring antibiotic resistance; available data comprises local studies only. Currently, results from the global surveillance studies ATLAS and SENTRY on the susceptibility of CA-RTIs reveal some lower levels of susceptibility amongst the common respiratory pathogens for some antibiotic classes, such as the macrolides. The fluoroquinolone antibiotics have so far retained activity, although as mentioned in the introductory paper to this Supplement,2 This would pave the way for improved adherence and a higher level of appropriate antibiotic prescribing in CA-RTIs which could, in turn, potentially limit AMR development and improve clinical outcomes for patients in Mexico.The Mexican clinician’s expert view is that more local susceptibility information is required. Surveillance studies are performed but they are either part of global studies and isolate numbers are relatively small, or local Mexican studies, such as GIVEBPVac data, only relate to invasive strains. While a range of international and local guidelines are utilized by clinicians in Mexico, a more standardized inclusive approach is needed to develop up-to-date local country-specific guidelines for CA-RTI management. These guidelines would be based on recent surveillance data of isolates from community-acquired infections which would make them more locally relevant for clinicians, reiterating the Consensus Principles as described in the introductory paper to this Supplement.


Reinforced concrete (RC) structures suffer from different types of loads during service life, and the corrosion characteristics of steel bars embedded in concrete under load are different from those under non-load. In this paper, when the interface between steel bars and concrete (IBSC) cracked and the concrete cover surface (CCS) cracked, the effects of load on the critical corrosion depth of steel bars were analysed based on the thick-walled cylinder model, and a prediction model for the corrosion-induced longitudinal cracking (CLC) time  of the CCS of RC structures under load was proposed. Finally, the influence of load on the CLC time of CCS was discussed on the basis of the proposed prediction model. The results showed that the load had a significant effect on the critical corrosion depth of steel bars when the IBSC cracked induced by corrosion, while the influence of load on the critical corrosion depth of steel bars when the CCS cracked induced by corrosion was not obvious. When the CCS cracks induced by corrosion under load, the influence of the rust-filling layer on the critical corrosion depth of steel bars was larger than that of the load. With the increase in load, the CLC time of CCS decreased. The calculated values of the proposed prediction model were in reasonable agreement with the experimental values, which can provide a reference for durability evaluation and service life prediction of RC structures and lay the foundation for the investigation of the corrosion depth of steel bars in concrete under load. In coastal, marine, or other chloride environments, chloride ion erosion is the main cause of steel corrosion in reinforced concrete (RC) structures, and corrosion further leads to the degradation of the durability of RC structures ,2. The cAfter the corrosion of steel bars embedded in concrete, the volume of corrosion products is approximately 2–4 times that of the original steel ,14. As aThe models for studying the corrosion-induced cracking process of the concrete cover can be divided into three types including the empirical model, numerical model, and analytical model. The empirical models are derived by fitting the experimental data under different experimental conditions, which mainly consider the effects of some parameters on corrosion-induced cracking, such as concrete cover thickness, the ratio of concrete cover thickness to the diameter of steel bars, concrete strength, mass loss of steel bars, and so on ,19,20. TThe three-stage model  has beenHowever, some researchers ,30,31,32Because the rust fills the pores of the concrete, a rust-filling layer is formed in the surrounding concrete. Some researchers have established corrosion-induced cracking models of the concrete cover considering the effect of the rust-filling layer, while the effect of the load was not considered in their models ,31. DuriTo fill this research gap, in this paper, based on the thick-walled cylinder model, the effect of the load was considered to analyse the critical corrosion depth of steel bars when the IBSC cracked and the CCS cracked. Then, a prediction model for the corrosion-induced longitudinal cracking (CLC) time  of the CCS of RC structures under load was proposed, and the rationality of the prediction model was verified by the experimental data of Wang et al. . FinallyThe bonding force between ribbed steel bars and surrounding concrete consists of three parts: the chemical adhesive force between the cement gel and the surface of steel bars, the friction force between steel bars and concrete, and the mechanical interlocking force between the transverse ribs of steel bars and concrete. Among them, the mechanical interlocking force is the main part of the bonding force. The oblique extrusion force between the transverse ribs of steel bars and the surrounding concrete forms slip resistance. The component force of the oblique extrusion force along the axial direction of the steel bars makes the concrete between two transverse ribs bend and shear as a cantilever beam, which causes oblique cracks inside the concrete. The radial component of the oblique extrusion force makes the surrounding concrete like a pipe wall under internal pressure, resulting in hoop tensile stress inside the concrete, which causes radial cracks in the surrounding concrete. After the ribbed steel bars are subjected to a force, the distribution of cracks near the transverse ribs is shown in p on transverse ribs, as shown in θ is the angle between the concrete sliding surface near the transverse ribs and the axis of ribbed steel bars, and μ is the friction coefficient between the ribbed steel bars and concrete. By decomposing the oblique extrusion force p and friction force μp on the concrete sliding surface along the axis and radial directions of the ribbed steel bars, the bond stress τ and the radial pressure q1 can be obtained, as written in Equations (1) and (2), respectively.After the ribbed steel bars are subjected to a force, the surrounding concrete produces oblique extrusion force θ as shown in μ between the ribbed steel bars and concrete increases with the corrosion degree of ribbed steel bars, and if the ribbed steel bars are not corroded, the friction coefficient can be taken as 0.3 /[Ec(2RC + C2)], M2 = n0n0 − 2, the following equations can be obtained:Substituting The algebraic cubic equation as shown in Equation (34a) has three roots. However, only one analytic real root for Equation (34a) is useful, which can be calculated as :(35a)x1=x1 through the MATLAB program, and then the critical mass loss of steel bars can be determined as:If Equation (34a) has no real roots, a real root can be approximately obtained as the value of Through the critical mass loss of steel bars, the critical corrosion depth of steel bars can be derived as:d = 18 mm and the yield strength is fy = 335 MPa, the thickness of the concrete cover is C = 30 mm, and the actual rust volume expansion ratio is n = 2. The parameters of concrete are tensile strength ft = 2.01 MPa, compressive strength fc = 20.1 MPa, elastic modulus Ec = 30 GPa, and Poisson’s ratio vc = 0.2. The range of n/n0 is about 1–1.2 [According to the calculation model of the critical corrosion depth of steel bars established above, to analyse the influence of load on the critical corrosion depth of steel bars when the IBSC cracks, an example is adopted. Assuming that the length of an RC beam is 2300 mm, its calculated length is 2100 mm, the size of its cross-section is 150 (width) × 250 (height) mm, the strength grade of concrete is C30, the tensile steel bars are two HRB335 ribbed steel bars with a diameter of ut 1–1.2 , and 1.0q1 occurs at the IBSC, which causes hoop tensile stress in the concrete cover, as shown in Equation (14). Assuming that the hoop tensile stress at the position rx = R of the concrete cover caused by the radial pressure is 0, 0.2, and 0.3 times the tensile strength of concrete, the tensile stress level δT = 0, 0.2, and 0.3, meanwhile, the IBSC does not crack [n/n0 = 1.0 and n/n0 = 1.2, when the tensile stress level δT increases from 0 to 0.3, the critical corrosion depth is reduced by 67.9% and 59.4%, respectively. Therefore, the influence of load needs to be considered when establishing the corrosion-induced cracking model of the concrete cover. n/n0 = 1.2, the critical corrosion depth is significantly larger than that of n/n0 = 1.0, which is because, the larger the n/n0, the more rust filled into the pores of concrete around steel bars, and the rust-filling layer does not produce squeezing force on the IBSC. As a result, the more serious the corrosion of steel bars, the larger the critical corrosion depth of steel bars.For the RC beam under a third-point concentrated load, the radial pressure ot crack . Figure Rc represents the radius of the interface between the cracked cylinder and the intact cylinder.After the IBSC cracks under the total radial pressure, the cracks propagate radially to the surface of the cylinder. Therefore, the thick-walled cylinder can be divided into two coaxial cylinders. The inside one is a cracked cylinder and the outside one is an intact cylinder, as depicted in The cracked concrete inside the thick-walled cylinder is assumed to be non-uniform orthotropic linear elastic material, and the outside intact concrete is assumed to be isotropic linear elastic material . Then, tStress components:Strain components:For the cracked cylinder, assuming that the cracks are uniformly distributed on the circumference of the cracked cylinder and propagate outwards radially, the damage degree of concrete along the direction of cracks is different, which near the steel bars is larger than that of concrete far away from the steel bars. The relationship between the radial stress and hoop stress at any point in the cracked cylinder is :(40)dσrdD is adopted to describe the damage degree of the cracked cylinder along the radial direction. Equation (40) can be rewritten as [φ is the internal friction angle of concrete, and c is the cohesive strength of concrete.Based on the Mohr–Coulomb failure criterion  and Mazaitten as :(41)σθ=σfc is the compressive strength of concrete.The internal friction angle of concrete decreases slightly with increasing concrete strength, which can be approximately taken as 37°, and the cohesive strength of concrete can be determined as ,58:(42)cD can be derived by Mazar’s damage equation [At and Bt are the coefficients of Mazar’s damage model, 0.7 < At < 1 and 104 < Bt < 105.The damage variable equation :(43)D=1−Substituting Equation (41) into Equation (40) yields ,30:(44)σThe boundary conditions are expressed as:Substituting Equation (45a) into Equation (44) yields:Substituting Equation (45b) into Equation (39) results in:Substituting Equation (45b) and Equation (46) into Equation (44), the total radial pressure can be derived as ,30:(48)qRc = R. Hence, the calculated results obtained at the non-cracking stage and partial cracking stage of the concrete cover are continuous. To simplify the calculation, the cracked cylinder is divided into N concentric rings of equal thickness, as depicted in D can be considered constant. The radial displacement ur in the cracked cylinder should satisfy the following equation [Equation (48) can be simplified to Equation (29) when equation ,60,61:, which is expressed as ,30:(50)uAccording to the geometric equation between displacement and strain, the radial and hoop strains at any point in the cracked cylinder can be derived as ,30:(51){R, the total thickness of the cracked cylinder is NΔR. For the Nth ring shown in DN is close to 0. Therefore, it can be assumed that the damage variable of the whole Nth ring is 0, and the constants rx = Rc. When the constants in Equation (50) are determined, the radial and hoop strains at any point in the Nth ring can be calculated according to Equation (51). Since the inner surface of the Nth ring is the outer surface of the (N−1)th ring, for the (N−1)th ring, the constants DN−1 of the (N−1)th ring can be determined by Equation (43), and the parameter εθ in Equation (43) takes the hoop strain on the inner surface of the Nth ring. As a result, the radial and hoop strains at any point in the (N−1)th ring can also be calculated according to Equation (51). Similarly, the constants and strains from the (N−2)th ring to the 1th ring can be calculated. The above-detailed analysis process can be referred to in the literature [D1 is the damage variable of 1th ring, which can be calculated according to Equation (43).Assuming that the thickness of each ring in the cracked cylinder is Δterature . AccordiRc = R + C, the critical corrosion depth of the steel bars can be determined as:At the partial cracking stage of the concrete cover, the mass loss and corrosion depth of the steel bars can be derived according to Equations (30)–(37). When the cracks develop on the surface of the thick-walled cylinder, i.e., φ of concrete is 37°, the cohesion strength c = 5.01 MPa of concrete is calculated according to Equation (42), ΔR is 1 mm, and the values of At and Bt are 0.7 and 104, respectively. Zhao et al. [Rc = R + 0.8C, the CCS cracks. Assuming that the hoop tensile stress at the position rx = R of concrete cover caused by the radial pressure is 0, 0.2, and 0.3 times the tensile strength of concrete, that is, the tensile stress level δT = 0, 0.2, and 0.3, and meanwhile, the IBSC does not crack [n/n0 is about 1–1.2 [To analyse the influence of load on the critical corrosion depth of steel bars when the CCS cracks are induced by corrosion, the same example as o et al.  found thot crack . The ranut 1–1.2 , and 1.0n/n0, the more rust filled into the pores of concrete around steel bars, resulting in a larger critical corrosion depth of the steel bars. For the three cases of δT = 0, 0.2, and 0.3, when the n/n0 increases from 1.0 to 1.1, the critical corrosion depth of steel bars increases by 30.8%, 30.2%, and 30.6%, respectively. For the three cases of n/n0 = 1.0, 1.05, and 1.1, when the tensile stress level δT increases from 0 to 0.3, the critical corrosion depth of the steel bars decreases by 4.6%, 4.8%, and 4.7%, respectively. Apparently, the value of n/n0, that is, the influence of the rust-filling layer on the critical corrosion depth of the steel bars is greater than that of the load. Compared with the analysis results of n0 described in According to the definition of the nominal rust volume expansion ratio Considering the thickness of the equivalent rust layer, when the CCS cracks, the total thickness of the rust layer is:t could be expressed as:αrust is the constant related to the type of corrosion products, which can be taken as 0.57 [ρrust is the density of corrosion products, which can be taken as 3600 kg/m3 [ρst denotes the density of the steel bars and is taken as 7850 kg/m3; Wrust (t) means the total amount of corrosion products at corrosion time t; and icorr (t) represents the current density at corrosion time t.Liu and Weyers  found th as 0.57 ; ρrust i00 kg/m3 ; ρst denWhen the current density of steel corrosion is a constant, the CLC time of CCS under load can be directly derived according to Equations (55) and (56):t from 0 to When the current density of steel corrosion changes with time, the relationship between current density and time needs to be determined according to the measured values. Then, the relationship is substituted into Equation (56), followed by integration with respect to αrust = 0.57, ρrust = 3600 kg/m3, ρst = 7850 kg/m3, R = 6 mm, n = 2, icorr = 0.01 mA/mm2, n/n0 = 1.1, C = 23 mm, b = 29 mm, vc = 0.2, φ = 37°, At = 0.7, Bt = 104, ΔR = 0.5 mm, and assuming Rc = 6 + 0.8 × 23 ≈ 25 mm when the CCS cracks.For the model established in n and n/n0, and the analysis results of n/n0 has an effect on the critical corrosion depth of steel bars, i.e., the larger the n/n0, the larger the critical corrosion depth, which in turn affects the CLC time of CCS under load, hence the n/n0 = 1.1 is taken in Equation (57), which may be greater than the actual value of n/n0 for the recycled concrete beams in the test of Wang et al. [The comparison of the calculated values from Equation (57) and the experimental values are listed in g et al. .n, and, n/n0 on the CLC time of CCS. The RC beam as shown in icorr = 0.01 mA/mm2. Assuming that the hoop tensile stress at the position rx = R of concrete cover caused by load is 0, 0.1, 0.2, and 0.3 times the tensile strength of concrete, the tensile stress level δT = 0, 0.1, 0.2, and 0.3. The range of the actual rust volume expansion ratio n is about 2–4 [n/n0 is about 1–1.2 [n, and n/n0 on the CLC time of CCS is shown in There are some factors affecting the CLC time  of CCS, such as concrete tensile strength, concrete cover thickness, reinforcement diameter, current density, rust property, etc. ,49,60. Bbout 2–4 ,14, and ut 1–1.2 , and 1.0n or load, the CLC time of CCS decreases, while the CLC time increases obviously with the n/n0. For example, in the case of n = 3 and n/n0 = 1.2, when δT increases from 0 to 0.3, the CLC time decreases from 397.42 h to 355.04 h; while in the case of n = 4 and δT = 0.30, when n/n0 increases from 1.0 to 1.2, the CLC time increases from 142.78 h to 243.80 h. In order to further investigate the effect of load on CLC time, the influence of load on the CLC time of CCS can be calculated quantitatively according to n and n/n0 affect the influence of load on the CLC time. When n/n0 remains unchanged, the greater the n, the more obvious the load can shorten the CLC time, that is, the percentage reduction in CLC time caused by load increases with n. For instance, in the case of n/n0 = 1.1, when n increases from 2 to 4, the percentage reduction in CLC time caused by the load is increased by 178.5%. However, when n remains unchanged, the percentage reduction in CLC time caused by the load decreases with the increase of n/n0. For instance, for the case of n = 3, when n/n0 increases from 1.0 to 1.2, the percentage reduction in CLC time caused by the load is decreased by 27.2%.(1)n/n0 = 1.0 and 1.2, when the tensile stress level increases from 0 to 0.3, the critical corrosion depth is reduced by 67.9% and 59.4%, respectively;The load has a significant effect on the critical corrosion depth of steel bars when the IBSC cracks induced by corrosion, and for the (2)n/n0 = 1.0, 1.05, and 1.1, when the tensile stress level increases from 0 to 0.3, the critical corrosion depth is decreased by 4.6%, 4.8%, and 4.7%, respectively;The load does not have an obvious effect on the critical corrosion depth of the steel bars when the CCS cracks induced by corrosion, and for the (3)When the CCS cracks induced by corrosion under load, the influence of the rust-filling layer on the critical corrosion depth of steel bars is larger than that of load, and the effect of load on the critical corrosion depth of steel bars when the IBSC cracks is larger than that of when the CCS cracks;(4)Considering the effects of load and the rust-filling layer, a prediction model of the CLC time of CCS is established, and the calculated values of the prediction model are in reasonable agreement with the experimental values;(5)n and n/n0 affect the influence of load on the CLC time. When n/n0 remains unchanged, the percentage reduction in CLC time caused by load increases with n. However, when the n remains unchanged, the percentage reduction in CLC time caused by load decreases with an increasing n/n0.The values of This paper studied the effect of load on the CLC time of CCS of RC structures, and the following conclusions can be drawn: