The accuracy of transmission ultrasonic tomography for the detection of brittle damage in concrete beams can be effectively supported by the graph theory and, in particular, by Dijkstra’s algorithm. It allows determining real paths of the fastest ultrasonic wave propagation in concrete containing localized elastically degraded zones at any stage of their evolution. This work confronts this type of approach with results that can be obtained from non-local isotropic damage mechanics. On this basis, the authors developed a method of reducing errors in tomographic reconstruction of longitudinal wave velocity maps which are caused by using the simplifying assumptions of straightness of the fastest wave propagation paths. The method is based on the appropriate elongation of measured propagation times of the wave transmitted between opposite sending-receiving transducers if the actual propagation paths deviate from straight lines. Thanks to this, the mathematical apparatus used typically in the tomography, in which the straightness of the fastest paths is assumed, can be still used. The work considers also the aspect of using fictitious wave sending-receiving points in ultrasonic tomography for which wave propagation times are calculated by interpolation of measured ones. The considerations are supported by experimental research conducted on laboratory reinforced concrete (RC) beams in the test of three-point bending and a prefabricated damaged RC beam. Concrete is one of the most commonly used materials in civil structures. From a scientific and technical point of view, it is a subject of interest both at the stage of designing a recipe, manufacturing various types of elements and during its operation. Ensuring safe and long-term use of concrete structures and elements requires, among other things, appropriate diagnostics. It can use destructive testing , semiThe tomographic imaging concrete elements available in the literature focus mainly on the identification of defects with much lower acoustic resistance than the surrounding concrete: e.g., artificially introduced defects, for research purposes, in the form of inclusions from foamed polystyrene ,22,23, eAnother important aspect of ultrasound tomography measurements is its accuracy which may be affected by the diffraction of waves when passing through and around areas of different acoustic resistance than the rest of the medium. It is commonly assumed in order to significantly simplify calculations that paths of the fastest wave propagation are rectilinear  running across the entire cross-section—e.g., as a result of simple bending or tension. Therefore, tomographic analyses were narrowed down to the identification of brittle damage in a flat longitudinal section of beam which will also be the plane of symmetry of this element, focusing on the assessment of changes in the speed of ultrasonic waves due to a local change in the stiffness of the cracked material. For this reason, in all computational examples and experimental studies presented in the paper, a system of opposite transmitting/receiving points of ultrasound waves in the transmission mode was used . LongituFrom the mathematical point of view, by indirect visualization of the material structure by means of maps of distributions of quantities characterizing the propagation of ultrasound waves, it is necessary to build an appropriate system of equations . The plδ1×δ2 . The funIn the problem, the longitudinal wave propagation times between the selected sending (ue (ART) . It was ue (ART) . On the ue (ART) ,46. For ue (ART) ).In this article, all tomographic images presented below were solved with the use of a randomized Kaczmarz algorithm. The final result was taken as In a given solution As mentioned in the introduction, another inconvenience of the presented method of tomographic imaging is the fact that ultrasonic waves are diffracted when avoiding areas with different acoustic resistance, so that the real paths of the fastest propagation are curvilinear. This is one of the basic sources of inaccuracy of the presented approach if there are sub-areas with significantly different acoustic resistances in the tested concrete element in relation to the rest of the element ,32,33. T3), 2); In concrete, due to its brittleness, the evolution of damage occurs in particular at the action of tensile stresses ,29,30,31.83·10−3 . In addiA separate issue related to the use of the discussed model is the adoption of appropriate material parameters that will enable the most accurate capture of the real behavior of concrete. The authors in paper  give theeference ,49,50). eference . These teference , becauseeference  concludespecimen . It shouThe tests included in Reference  were alse result b.First, ng phase a for elo2] is the minimum cross-sectional area of the specimen at the center of its length; hence, 2. This outcome corresponds to the global mean square relative error The calculations in formula (17) assumed eference . The autechanics ,49. The en model b with thGiven that the growth eference ,29,30,31It needs to be highlighted that the numerical results presented in this point can be directly used only in the case of pure tension in concrete. However, they can be also generalized usefully for the case of bent reinforced concrete (RC) beams when estimating the damage level in concrete of such beams with the following simplifying assumptions: the planar cross-sections of RC beam remain planar after loading (as in Bernoulli–Euler theory), failure of the concrete is determined by the normal cross-sectional stresses, and the average normal strains along beam axis are equal in the bonded reinforcing longitudinal bars and surrounding concrete. These assumptions are also commonly used in the design of RC beams, for example, as described in the EN-1992-1-1:2004 standard. Under the mentioned conditions, the results shown in i assigned to points S is one of the points A set In the set In the case of other nodes In this paper, Dijkstra’s algorithm  was usedeference ). Let a S, it reaches the point In the final effect of the algorithm operation, the vector components The primary purpose of the example is to show to what extent a change in stiffness in a localized, elastically degraded concrete zone can cause the paths of the fastest sound wave propagation to deflect. Another objective is to propose, on the basis of the performed analysis, a useful way to reduce the inaccuracies in tomographic imaging that may arise from this.The example presents Dijkstra’s method of calculating the paths and times of the fastest longitudinal ultrasound wave propagation between the assumed transmitting/receiving points in the longitudinal section of the concrete beam model. One damaged zone was assumed to be formed in the beam in its entire cross-section of The assumed scheme of the beam with transmitting/receiving points is shown in First, it was checked how the distance between the nodes of the graph . Using Selection of 2). Integral (26) was calculated in an approximate way using the rectangular method, taking the step of integration after In order to assess the correctness of the proposed approximation method, appropriate tomographic reconstructions of the longitudinal wave velocity maps in the beam model section from In this paper, it is proposed to calculate index A summary of The analysis of the presented results shows that the reconstruction error increases with the degree of development of localized brittle damage. If propagation times e defect , i.e., rpchip function in the MATLAB program environment was used). The results are shown in The purpose of the second calculation example is to show that the desired resolution of the image of changes in ultrasound wave velocity around localized damage can be obtained by using a reduced number of transmitting/receiving points. To this end, so-called fictitious transmitting/receiving points should be introduced for which propagation times will be interpolated on the basis of data from the original set of points . In In the same way as in the first example, the propagation times for the fastest ultrasound wave paths connecting opposite transmitting/receiving points were modified to bring them as close as possible to the propagation times of straight rays, but with the use of times Then, using formulas (23)–(25), the values of In order to assess the correctness of the proposed propagation time interpolation method, tomographic reconstructions of the wave velocity maps in the beam model longitudinal section from A summary of Analyzing the presented results one can draw general conclusions similarly to the reconstructions from the first example. In addition, it can be noted that reconstruction errors, compared to those in As part of the tomographic experiments, studies were carried out on RC elements after cracking initiation—three beams on a laboratory scale and one prefabricated beam on a natural scale. Since the calculation methodology presented in point 3 concerned the detection of elastically degraded zones with a course perpendicular to the beam axis, the experiments illustrating the possibilities of its practical application also focused on the evaluation of this type of defects. Hence, the laboratory beams were bent until the first perpendicular cracks were formed in the middle area. In the second case, a prefabricated industrially manufactured beam, that was damaged during transport to the construction site, was inspected. There were cracks perpendicular to the beam axis and visible to the naked eye. This beam was specially selected for an assessment to also test the presented calculation method in near-real conditions. An important aspect of this study was also the willingness to check whether ultrasound tomography in the presented approach could potentially be used for quality control of prefabricated RC elements in industrial conditions.The cross-sections of beams for tomographic imaging were selected so that the measurements were disturbed as little as possible by their reinforcement  ,53. In tOn the basis of theoretical considerations discussed in previous points, a detailed course of tomographic measurements and method of processing data obtained this way was also established and used during own experiments. The general scheme of this procedure is shown in A diagram of the beams is shown in eam axis . The loneam axis . The firTaking into account the measured longitudinal wave propagation times and basic frequency of ultrasonic pulses, the average wavelengths for beam No. 1, 2, and 3 were, respectively, ~1.7 cm, ~2.0 cm, and ~2.1 cm before the loading and ~1.7 cm, ~1.8 cm, and ~2.0 cm after the loading. At the same time, it allowed satisfying the basic requirement described in ASTM D2845-08 regarding the selection of frequency so that measurable longitudinal ultrasonic waves could be generated in the samples, i.e.,: dominant wavelength ≥ The first visible crack appeared in each case in the middle of the beam span as perpendicular to the beam axis . Measurehe study . In turn3), its relative decrease with the quoted range of changes in Another interesting issue that can be seen is that, despite the same period of care in the water of all beams, the speed of longitudinal waves in the intact configuration in beam No. 1  was lower by about 20% if compared to the speed in beams No. 2 and 3 (which were tested right after removing from water). The explanation for this may be the fact of the phenomenon of self-drying of young concrete as a result of hydration processes . On the y weight . Assuminhrinkage   The accuracy of transmission ultrasonic tomography for the detection of brittle damage in concrete can be effectively supported by the graph theory and, in particular, by Dijkstra’s algorithm. What is important, it allows the determination of real paths of the fastest ultrasonic wave propagation in concrete containing localized elastically degraded zones at any stage of their evolution. Thanks to the analyses conducted on this basis, the authors developed the method of reducing errors in reconstructions of longitudinal wave speed maps. In this approach, the errors are decreased which are caused by using a simplification of straightness of the fastest wave propagation paths assumed in the typical mathematical apparatus for ultrasonic tomography. The method is based on the appropriate elongation of the measured propagation times of the wave travelling between opposite transmitting-receiving transducers if the actual propagation paths deviate from straight lines.(2) Transmission ultrasonic tomography allows the estimation of the internal length of concrete defined in accordance with the methodology of damage mechanics. This problem is very important in the case of studies carried within this field of mechanics  Knowledge of the internal length of concrete allows rational determination of the appropriate resolution in ultrasonic tomography imaging and assessment of evolution of localized elastically degraded zones.(4) The use of fictitious transmitting-receiving points in ultrasonic tomography, for which wave propagation times are calculated by interpolation of measured times, can contribute to the reduction of the required number of transducers and possible costs in the considered approach to concrete beams assessment while maintaining proper resolution of tomographic images. This outcome was well-grounded in the case of numerical analysis conducted in the work and usefulness of this approach was pre-confirmed in the own experiments. However, due to the limited number of these tests, it needs further justification and experimental studies using different arrangements of distances between sending-receiving points for ultrasonic pulses on the same RC members and confrontation with tests based on, e.g., acoustic emission or X-ray tomography.


Meanwhile, the tension–compression ratio is increased by 22.29%, and the mechanical properties of concrete are remarkably enhanced. The research results were successfully applied to the construction of the Laoshan tunnel, and good engineering results were obtained.An important way to improve concrete performance is the use of alkali-resistant glass fibers (ARGFs) as reinforcement. This paper is based on the problems of the cracking of the partition wall and lining seepage in Laoshan Tunnel, Qingdao, China. Two types of ARGFs were selected as reinforcement materials for the partition wall and lining concrete: high dispersion (HD) and high performance (HP); and the compressive strength (CS), tensile strength (TS), flexural strength (FS), and impervious performance (IP) of concrete with different gradations of the two types of fibers were investigated. The results show that although the CS of graded glass fiber reinforced concrete (G-GRC) is slightly decreased, the TS, FS, and IP of G-GRC are significantly improved. When the densities of the ARGFs of HD and HP are 0.6 and 5 kg/m Good compressive strength (CS), tensile strength (TS), flexural strength (FS), and impervious performance (IP) are the inevitable requirements of engineering construction, and fiber reinforced concrete is one of the ways to improve these various factors of concrete under the premise of not improving the cement mark, which can reduce the construction cost to a certain extent. The idea for this article originated from the construction of the partition wall of the Laoshan Tunnel in Qingdao, China. As shown in Presently, commonly used concrete-reinforcement fibers include steel fiber and polypropylene fiber, among other materials; however, these have problems such as weak durability, precipitation, or clustering after mixing. The density of alkali-resistant glass fiber (ARGF) is equivalent to that of concrete, with excellent dispersibility in concrete, and ARGFs are a new type of green and environmentally friendly reinforcing material. On the physical level, owing to the high strength and strong bondability with concrete of ARGFs, the TS and FS of concrete can be greatly improved. At the chemical level, owing to the addition of alkali corrosion resistant zirconia in the auxiliary material, a layer of corrosion-resistant protective film forms on the surface of the fiber, which greatly improves its durability in concrete. The results of the China Research Institute of Building Materials show that after 360 days of an accelerated aging experiment, the retention rate of ARGF was still over 88.5%, and the fiber surface remained smooth, as observed via SEM. By analyzing the corrosion trend of fibers, it is inferred that the safe service life of ARGF cement products can exceed 100 years ,3,4 alsoARGF reinforced concrete research aspects include the following: Yang et al.  conducteThe literature shows that although many scholars have studied ARGF reinforced concrete and gained rich research achievements, they focused on a single type of ARGF (such as single high-dispersion (HD) fiber or high-performance (HP) fiber) and multiple types of hybrid fibers (such as glass fiber and polypropylene fiber) and their enhancement effect on concrete. There are few studies on the strengthening effect of a mixture of different types of ARGFs . Different types of ARGF mixtures are complementary in strengthening concrete, but their effects on the mechanical properties of concrete remain unclear. Therefore, it has important practical value to study the influence of different types of ARGF mixtures on the mechanical properties of concrete.The different types of ARGFs, mixed into concrete in a certain ratio, are called graded GRC (G-GRC). This study combined laboratory tests and engineering applications, through a compressive test, tensile test, flexural test, and impervious test, for six gradation cases of HD and HP ARGFs, and studied the mechanical properties of concrete under different fiber gradation cases. In addition, the effect of the gradation amount on concrete properties was analyzed. The research results were applied to the construction of the Laoshan Tunnel in Qingdao, China, and good engineering outcomes were achieved, which provide an important reference for the application of graded ARGF concrete.The cement used for the test was P.O42.5 benchmark cement produced by Yixing Tianshan Cement Co., Ltd., and two types of ARGF were used for the test, HD and HP, produced by Taishan Glass Fiber Co. . Images of the samples are shown in The two types of ARGF, HD and HP, were mixed during the production of ARGF concrete test blocks and were fully mixed to ensure their even and random distribution in the concrete test blocks, as shown in GRC is made by mixing cement, water, sand, stone, and ARGFs in certain proportions. To study the enhancement effect of HD and HP ARGFs on the mechanical properties of concrete under different grading cases, the concrete test set was classified and indicated using JZ30 for apparent concrete test blocks and FC30 for G-GRC test blocks; the concrete test block mix ratio design is shown in According to the requirements of the specification ,25,26,27After the concrete specimens were prepared and then formed, they were left in a room with a temperature of 20 ± 5 °C and relative humidity greater than 50% for 1–2 d. After demolding, they were placed in a standard curing room with a temperature of 20 ± 2 °C and relative humidity greater than 95%. When the curing requirements are met, the test is successful. There are more international test methods for fiber reinforced concrete ,30,31,32Based on the above test method, the results of the compressive, splitting tensile, flexural, and impervious tests of each experimental group were plotted as visual diagrams for data analysis and analyzed as follows.3, respectively.The CS increment ratio is the percentage of CS increment from that of normal concrete. To clearly demonstrate the degree of CS weakening, To express the relationship between the CS and the graded ARGF admixture, 3, respectively, which is 46.80%, an improvement of 3.91% compared with ordinary concrete.The TS increment ratio is the percentage of the TS increment to the TS of ordinary concrete and clearly presents the degree of TS enhancement, as shown in 3, respectively, and the corresponding TSs were 3.09 and 3.58 MPa. Compared with the ordinary concrete test group, the TS was increased by 14.87% and 15.86%, respectively.3, respectively, is the largest, which is 66.57%, higher than that of ordinary concrete by 10.26%.Concrete has the characteristics of a large CS, small TS, and high brittleness. Furthermore, in the long-term stress state, cracks easily appear without signs of crack damage. The tension–compression ratio (TCR) of concrete is the ratio of the TS to the CS of concrete, and it is an important index used to reflect concrete brittleness; the larger the TCR, the less brittle the concrete, and the stronger the toughness.3, respectively, exhibited the highest TCR, with a maximum value of 0.07956. Subsequently, with the continuous increase in the content of graded ARGF, the TCR began to decrease gradually, and the concrete became stronger.To reveal the relationship between the TCR and the dosage of graded ARGFs, 3, respectively, exhibited the largest increment in the TCR. Compared with ordinary concrete, the TCR was increased by 22.29%, and the effect of graded ARGF on the gain of the TCR of concrete is apparent.The increment ratio of the TCR is the percentage of the TCR increment compared with that of ordinary concrete. The increment ratio of the FS is the percentage of the increment in the FS compared with that of ordinary concrete. 3, respectively, had the highest flexural strength of 5.09 MPa. The FS increased by 14.38% compared with that of the ordinary concrete test group.PH is an important index for determining the impermeability of concrete; the smaller the PH, the better the impermeability. According to GB/T50082-2009 “Test Method for Long-term Performance and Durability of Ordinary Concrete,” only the IP of concrete test blocks at 28 d was studied and analyzed.3, respectively, displayed a minimum PH of 52 mm and the best permeability resistance. This is because the ARGFs improve the denseness of the concrete as well as the pore junction, thus reducing the infiltration channels and improving the permeability of the concrete.3, respectively, is the largest, which was reduced by 31.58%. However, the PH reduction ratio of the FC30-6 test group with HD and HP ARGF contents of 0.6 and 15 kg/m3, respectively, is the smallest, which is 11.84% higher than that of ordinary concrete. This demonstrates that the appropriate amount of gradation of ARGF can significantly improve the impermeability of concrete, but in the case of excessive mixing, it will have a countereffect and damage the IP of the concrete itself.The reduction ratio of the PH is the percentage of the reduction value of the PH compared with the PH of ordinary concrete. The Laoshan tunnel is located in Qingdao City, Shandong Province, China. The tunnel is a single-hole, double-track form. The tunnel lining was designed according to the principles of the new Austrian method. It adopts a composite lining structure and was constructed by drilling and blasting. The secondary lining partition wall works through the entire tunnel. As shown in 3, respectively, the performance is the best, and the TS and FS are improved most obviously. Therefore, this project selected HD and HP ARGF mixing contents of 0.6 and 5 kg/m3 for the construction site test.According to an analysis of the laboratory experiment results, when the gradation amounts of HD- and HP-type ARGFs in concrete are 0.6 and 5 kg/mAfter the construction of the middle partition wall with G-GRC, no large cracks appeared during the construction, and the performance of the middle partition wall improved significantly. A comparison of the partition wall with these different types of concrete is shown in crA) was calculated using Formula (1):crA is the nominal total area of the cracks, mm2. The crack area of the partition wall in the fiber concrete is denoted as fcrA, and the crack area of the partition wall in ordinary concrete is denoted as mcrA.To further verify the strengthening effect of ARGF on the middle partition concrete, an ordinary concrete middle partition section of K35 + 325 ~ K35 + 345 miles, and a G-GRC middle partition section of K35 + 345 ~ K35 + 365 miles were selected for testing at the construction site. To ensure the accuracy of the results, the length of the middle partition wall in the each section was 20 m and the age was 180 d. Calculations were performed and the results were analyzed with reference to the technical specifications for fiber reinforced concrete structures [i is recorded as iω, i.e., the crack width near the midpoint of the crack (mm).The nominal crack width of crack i is given by il, mm.The length of crack η) can be calculated by substituting the results of Formula (1) into Formula (2):The crack reduction coefficient  can be evaluated according to 2, and the nominal total crack area of the partition wall with G-GRC was 177.2 mm2; the total crack area decreased by 86.04%, and the partition performance in the tunnel improved significantly. ARGF reinforced concrete has also been widely used in the secondary lining of this project, and good engineering results were obtained.It can be seen from (1)3, the bridging and cracking effects of ARGFs are the most apparent, and the overall index performance of concrete is the best.Although the TS, FS, and IP of G-GRC were significantly enhanced, the CS decreased slightly. The grade ARGF admixture has less effect on the CS of concrete, and the decreasing value of CS remains stable, while with the augmentation of grade ARGF admixture, the TS, FS, and IP displayed trends of first increasing and then decreasing. When the contents of HD and HP ARGFs are 0.6 and 5 kg/m(2)The ESRs of the CS, TS, and FS of G-GRC were all improved. For TS and FS, the ESRs increased while the TS and FS of G-GRC at all ages increased. In contrast, for CS, the ESR increased while the CS of G-GRC at all ages was reduced by different degrees. As a result, the increase in the ESRs of concrete cannot represent the improvement in the mechanical properties.(3)According to the compression and tensile tests, owing to the substantial increase in the TS, the TCR of G-GRC was significantly improved. The TCR of G-GRC in the best-graded case is increased by 22.29% compared with that of ordinary concrete, which effectively avoids the problem of unforeseen crumbling damage of concrete due to brittleness.(4)Experimental research on ARGF concrete was successfully applied to the construction of the Laoshan Tunnel in Qingdao, China, and achieved a good application outcome, effectively improving the quality of the project while controlling the project cost.By mixing HD and HP, which are two types of ARGF, an experimental study on the CS, TS, FS, and IP of concrete with different fiber grades drew the following conclusions:This study investigated the advantages of G-GRC regarding the CS, TS, FS, and IP through indoor experiments. The proposed ARGF concrete was applied to the construction of the Laoshan Tunnel in Qingdao, China, and a good outcome was achieved. However, the action mechanism and constitutive relationship of its gradation advantages are not yet clear. Therefore, in the future, the enhanced constitutive model of G-GRC can be further studied by combining the micro-mirror method to reveal its action mechanism and to scientifically explain the test phenomenon of various properties of G-GRC.


In the course of experiments, changes in the development of cracks in individual series of concrete were also analyzed. In addition, the microstructures of all composites and the nature of macroscopic crack propagation in mature concretes were assessed. It was observed that the greatest increase in fracture toughness at shear was in the case of reference concrete during the first 28 days, whereas, in the case of concretes containing LCFA, in the period of time above 4 weeks. Furthermore, concrete without the LCFA additives were characterized by a brittle fracture. In contrast to it, concretes with LCFA additives are mainly characterized by a quasi-plastic process of failure. Moreover, most of the samples showed a typical pattern of the destruction that occurs as a result of shearing. The presented test results may be helpful in selecting the composition of concrete mixtures containing LCFA to be used in concrete and reinforced concrete structures subjected to shear loads.This study investigated the influence of the curing time on the fracture toughness of concrete produced with different content of low calcium fly ash (LCFA). During the study, the amounts of 20% and 30% of pozzolanic additive were used. In order to observe the effect of the applied pozzolanic additive on the analyzed concrete properties, the obtained results were compared with the values obtained for the reference concrete. Compressive strength— Concrete is the oldest artificial material with a history of 9000 years . Due to energy intensive,very expensive,2, NO, NO2,a process that produces harmful greenhouse gases (GHG), such as: COan activity that degrades natural mineral deposits, such as limestone.Unfortunately, the OPC manufacturing is ,7,8,9,10Therefore, it can be concluded that the production of OPC is non-ecological and has a negative impact on the natural environment, e.g., ,12,13.For this reason, the substitution of cement binder with other materials is recently becoming a more and more popular research topic, whereas OPC replacement materials, i.e., supplementary cementitious materials (SCMs) are increasingly used for concrete mix production, e.g., ,15,16,17The main group of SCMs is industrial wastes ,19. Takibinder production costs,energy consumption,2 emissions.COHowever, according to other reports, radiologically safe  LCFA as From the reasons mentioned above this activity is certainly environmentally friendly, e.g., ,28,29. IConcretes made with OPC have been used for almost 200 years, while composites with the addition of LCFA for almost 90 years. At that time, it was found that this type of materials has many beneficial characteristics, such as: increased strength after a long period of curing, increased corrosion resistance, high resistance to impact and dynamic loads, increased resistance to high temperatures, and increased electrical resistivity, e.g., ,36,37,38The curing time of the modified LCFA composites has a significant impact on improving their parameters. Due to relatively slow rate of pozzolanic reactions in LCFA concretes, in the initial period of their curing, a much less favorable effect of this binder substitute can be seen than it is the case in the subsequent curing periods of composites.Furthermore, based on previous research it is concluded that the most important features of LCFA are their high reactivity and favorable morphology of the particles. It manifests itself mainly in spherical shape of grains and fine structure. Exemplary LCFA grain with such parameters, i.e., fine and well-developed particle with a diameter of 10 μm, are shown in mechanical properties, e.g., ,47,48,49corrosion resistance, e.g., ,chloride penetration, e.g., ,51,drying shrinkage, e.g., ,porosity and pore volume distribution, e.g., ,heat development, e.g., ,water absorption and water permeability, e.g., ,hydration process, e.g., ,microstructure, e.g., ,52,53,crack propagation, e.g., ,fracture toughness evaluated in mortar tests with LCFA, e.g., .The review of studies analyzing various parameters of composites with the addition of LCFA depending on their curing time were presented in a tabular form in the papers ,43. AccoConcretes with the addition of reactive and fine-grained LCFA were also tested for fracture toughness. However, the vast majority of previous experiments assessed the parameters of fracture mechanics of composites under tension, i.e., mode I fracture, e.g., ,56,57. IHowever, one should not forget about the fracture toughness analysis for the 2nd and 3rd models of cracking. Especially in the case of modified concretes with a changed composition of the cement matrix, e.g., . SituatiFracture toughness of young and mature concrete with the additive of LCFA, under mode III fracture, were discussed in previous papers ,66, whilFracture behavior is an important issue to be taken into account in the analysis and design of concrete structure. This is due to the fact that the fracture of brittle or quasi-brittle material, such as concrete, begins at the weaker place and then, as a result of stress concentration at both ends of the crack, it spreads quickly without even increasing the load. On the other hand, the fracture toughness of concrete is strictly dependent on the input composition of the concrete mix and the materials used to make it . PartialKαc , e.g., [In structures made of unreinforced concrete there is a possibility of brittle failure due to tensile or shear stresses in the element’s sections, and microcracks in the material’s structure. Fracture mechanics deals with the problems of brittle failure, whereas the basic parameter of fracture toughness is a critical stress intensity factor—), e.g., ,79,80,81When analyzing the phenomena accompanying the destruction of materials with cement matrixes, one of the most important research problems is the connection of the structure of these materials with their strength considered in terms of fracture toughness ,83,84,85Fracture toughness is an extremely important parameter determining the properties of a given material, especially a construction material. The material constants determined in compressive and tensile tests are not enough because often materials with high mechanical properties (high strength) have low fracture toughness. In this case, such materials have limited usefulness as structural materials, especially in terms of fatigue loads in a given structure .Taking the above into account, the following are the issues important in the analysis of the fracture toughness of concrete composites modified with pozzolanic additives from the point of view of the microstructure of these materials. As these problems are rarely presented in the literature, they require a more detailed explanation. The fracture toughness of concrete under shear in the macroscopic approach has been presented in detail in several fundamental publications on this subject ,87,88,89First of all, it should be noted that, in most construction materials the mode I fracture is the dominant case that should be taken into account in the assessment of fracture toughness ,92. HoweGenerally, in the case of concrete composites modified with pozzolanic additives, such as LCFA, the situation becomes even more complex and more difficult to analyze. This is due to the fact that in such materials, the development of cracks may also propagate at the boundaries of additional phases formed in the modified composite structure. As a result of the intensification of the pozzolanic reaction, some of the new products formed in concrete, such as the C-S-H phase, definitely have a positive effect on reducing the development of intra-material cracks, and thus improving the fracture toughness of the material . On the Kc is lower for the CH phase compared to C-S-H, the mixture of C-S-H/CH [2 in this phase is 0.99. Mindess, in the conclusions presented in [The mechanical and strength parameters and the fracture toughness of the two main matrix phases are influenced by factors related both to their structure and the location of their occurrence in the matrix structure. The comparison of the fracture toughness test results for CH and C-S-H shows significantly worse values for portlandite compared to C-S-H. The authors of ,97 showeC-S-H/CH  and the C-S-H/CH  also revented in , stated As the CH phase is a weaker phase, with a lower fracture toughness, in relation to C-S-H, cracks in these zones of the paste most often appear along the cleavage planes of the CH phase . ClusterMoreover, at the boundaries of morphologically different phases, cracks are usually the result of complex stress states. Thus, it forces the analysis of the fracture toughness of such materials, which not only takes into account the first, but also the second and often the third mode fracture, e.g., .KIIc) of modified concretes mixtures as an important design parameter [Therefore, in such cases Mode II or in-plane sliding of crack faces is one of the possible fracture modes for concretes containing LCFA, which can often take place due to shear loading. Hence, civil engineers need to know the value of Mode II fracture toughness ,with 20% LCFA addition (LCFA-20),with 30% LCFA addition (LCFA-30).As in previous studies, which evaluated fracture processes of concrete composites with the addition of LCFA ,66,67,70fcm) andcompressive strength (KIIc).fracture toughness at shear  during numerical simulations,peak principal stress criterion for description of the crack grow.At this point, it should be noted that the specimens used for f 100 mm . TherefoMoreover, F)–displacement (f)  .On this basis, it was found that it is possible to use specimens of larger sizes in the fracture toughness tests of concrete composites under Mode II fracture.KIIc press  with maximum load of 100 kN was used.Compression strength tests were conducted using a compression machine  with a maximum load of 3000 kN, whereas, during investigations of fracture toughness a displacement controlled type of tests with the MTS head velocity equal to 0.25 mm/min,F) up to the final failure of the specimens.quasi-static increase of the loading force –displacement (f) relationship andload ; andconcrete with more LCFA will achieve higher KIIc and fcm were observed: in the case of reference concrete during the first 28 days, in the case of concretes containing LCFA in the time period above 4 weeks.the greatest increases in the greatest dispersion of results, represented by the highest values of ν, was observed in the concrete with the shortest curing time .The comparison of dependencies from both experiments indicates a clear similarity between the two analyzed mechanical parameters, in which changes in time took place in a very similar way. The main observations arising from the analysis of the graphs from fcm  technique were used ,56,57.quasi-straight crack, occurring mainly in the most brittle concrete, i.e., LCFA-00-00 a,slightly curved crack, which is related with the most common LCFA-20 concrete b, andclearly and strongly curved crack with small branches  which was observed mainly in the LCFA-30 concrete c.The example of crack shapes observed in the studies using DIC technique in the Mode II fracture for concretes after 28 days of their curing were presented in When analyzing the processes of cracking of cubes, it should be mentioned that the most of the specimens were destroyed as a result of typical shearing of concrete. A macroscopic crack, causing destruction of the material, appeared at the top of the modeled initial crack and then developed upwards to the place of force transfer through the steel plate. An example of a sample with a crack of this type is shown in At this point, it is worth noting that the above failure pattern is associated with the occurrence of compressive and tensile stress trajectories arising in the loaded element. On the other hand, the shear of the sample is along a broken line resulting from the action of variable directions of principal stresses. This phenomenon has been confirmed on the basis of elastoplastic tests carried out many years ago. As proof of this, The conclusion about the effectiveness of the used concrete modifier should be based on the results of the tests of mechanical parameters and the assessment of the structure of these materials. It is important as the changes in the composite phase system later influence its behavior under load, the manner of crack propagation, and, finally, the material’s fracture toughness; see 2 ratio in the locations of C-S-H phase, e.g., [2 ratios. In each of the concretes, the locations with clearly distinct C-S-H, CH phases, and ettringite (E) phases were analyzed.Qualitative changes in the concrete structure are best visible in the scanning electron microscope (SEM) images. Quantitative relationships between the individual components of the matrix can be identified additionally using an Energy Dispersive Spectroscopy (EDS) spectral analyses. A good indicator of the progress of the pozzolanic reaction and the degree of reaction of LCFA is the assessment of changes in CaO/SiOe, e.g., ,118. TheThe microstructural testing was carried out using a QUANTA FEG 250 . SEM pictures with EDS—EDAX analyses, at characteristic points  for each of the concretes are shown in The analysis of the phases occurring at the concrete interfaces showed that the composite with 20% LCFA additive had the most developed structure and these were mainly fibrous C-S-H (I) and honeycomb phases, i.e., C-S-H (II). Characteristically, the C-S-H phases in this material were so extensive that the reaction of CH crystals was clearly visible. This may indicate the reactions of transformation of the less favorable portlandite phase into the favorable C-S-H phase occurring in this concrete c.In the reference concrete LCFA-00, clusters of ettringite needles, occurring in the pores of the matrix and portlandite crystals, were visible. There were also small amounts of C-S-H (II) phase, but the main phase occurring in these composites were large hexagonal CH plates with overgrown ettringite needles a.In the concrete of LCFA-30 series, the unreacted grains LCFA with a diameter of a few micrometers were clearly visible (their position is shown by the blue dashed lines), while the structure of this concrete contained various types of slightly intense phases e.Additionally, the occurrence of microcracks inside the concrete structure of the LCFA-00 and LCFA-30 series was also observed. In the first material, the damage was in a characteristic location, i.e., on the border between the brittle plates of portlandite and the compact C-S-H phase a. In add2—phase C-S-H formed as a result of the pozzolanic reaction in concretes containing LCFA additive—is much lower than the same molar ratio of C-S-H in concrete LCFA-00  and (II), as well as the visible transformation processes of the phase CH in the phase C-S-H in the concrete LCFA-20, weakly developed phases, and unreacted LCFA grains and microcracks in concrete with 30% LCFA additive .Twenty percent LCFA additive causes a strong pozzolanic reaction in the cement matrix, thanks to which the molar ratio in this material is reduced to the level of 1.2, while, in concrete with a higher content of the additive, it is 2.0, and in non-modified concrete at 7.0.The presented test results may be helpful in selecting the composition of concrete mixtures containing LCFA to be used in concrete and reinforced concrete structures subjected to shear loads.Based on the obtained test results, the following conclusions can be drawn:


Immunoglobulin G4-related disease (IgG4-RD) is an immune-mediated fibro-inflammatory condition characterized by high serum IgG4 concentrations and tissue infiltration by IgG4-positive plasma cells. Reports have demonstrated that IgG4-RD affects various organs, including the pancreas, kidney, lung, thyroid, and lacrimal and salivary glands. In the nervous system, hypertrophic pachymeningitis and hypophysitis are mainly related to IgG4-RD; however, the peripheral neuropathy involvement is unusual.We report on a 69-year-old woman with multiple mononeuropathy, weight loss and kidney mass in the setting of IgG4-RD. Biopsies of the kidney mass showed lymphoplasmacytic sclerosing inflammation with numerous IgG4-positive plasma cells. IgG4 and IgG4/IgG ratios in the blood were elevated. The patient was treated with high dose methylprednisolone with improvement in her neuropathy.IgG4-RD is a relatively recently reported systemic fibrous inflammatory disease caused by the infiltration of IgG4-positive plasma cells in various organs. In the nervous system, symptomatic peripheral nerve invasion is very rare. However, as demonstrated in our case, IgG4-RD may present with primarily peripheral nerve disease. Immunoglobulin G4-related disease (IgG4-RD) is an immune-mediated fibro-inflammatory disease. Histologically, it is characterized by the infiltration of IgG4-positive plasma cells, storiform fibrosis, and obliterative phlebitis in the involved organs, with an increase in serum IgG4 . It was A 69-year-old woman presented with a 2 months history of progressive right hand weakness. A year prior, she was diagnosed with hypertension and diabetes. The patient had lost 8 kg unintentionally and had been experiencing generalized weakness in the six months prior to presentation. She frequently dropped objects and could not use a cooking knife because her right hand was weak. These challenges had lasted for approximately two months. Her condition gradually deteriorated over a two-week period to the point where she could not use chopsticks. Upon examination, she had right hypothenar and first dorsal interossei muscles atrophy. She had weakness in the right fingers abduction and adduction  and the left ankle dorsiflexion (MRC grade 4). Hypesthesia was observed in the 4th and 5th fingers of the right hand and the medial part of the palm. The rest of her examination, including deep tendon reflexes were normal.Nerve conduction studies and electromyography are summarized in the table. Tables  and 2. O3; normal range 4,000–10,800/mm3); neutrocyte 86.8 % and lymphocyte 6.1 %, thrombocytosis , and hypochromia . The erythrocyte sedimentation rate was increased to 120 mm/hr . The results of the serum test showed a slight increase in total protein  and a decrease in albumin . A reversed albumin/globulin ratio  and an increased C-reactive protein  were observed. Anti-myeloperoxidase and anti-proteinase 3 antineutrophil cytoplasmic antibodies (ANCA) were negative. Electrolyte, liver function, and renal function tests were normal. Serum protein electrophoresis showed multiclonal gammopathy. Serum immunofixation electrophoresis was normal.Blood tests showed leukocytosis  .Inoue et al. retrospectively evaluated 106 patients with IgG4-RD and found 21 peripheral nerve lesions in seven patients . InvasioWe believe this our patient has IgG4-RD manifesting as multiple mononeuropathy, weight loss and malaise. Our case is supported by the fact that the neurological symptoms as well as the serum IgG4 concentration improved with steroid treatment and by the fact that we rule out other causes of neuropathy. A nerve biopsy would have even supported our hypothesis further but was not performed due to patient’s wishes. In summary, physicians should consider IgG4-RD in patients with acute/subacute neuropathy in the setting of systemic symptoms and evidence of elevated serum IgG4 and/or IgG4-positive plasma cells in various organs.


Previous research reported that individuals prescribed antibiotics frequently develop antimicrobial resistance. The objective of this study was to evaluate whether frequent antibiotic use is associated with reduced hospital admissions for infection-related complications.Population-based cohort study analysing electronic health records from primary care linked to hospital admission records. The study population included patients prescribed a systemic antibiotic, recent record of selected infections and no history of chronic obstructive pulmonary disease. Propensity-matched cohorts were identified based on quintiles of prior antibiotic use in 3 years before.A total of 1.8 million patients were included. Repeated antibiotic use was frequent. The highest rates of hospital admissions for infection-related complications were observed shortly after antibiotic start in all prior exposure quintiles. For patients with limited prior antibiotic use, rates then dropped quickly and substantially. In contrast, reductions over time were substantially less in patients with frequent prior antibiotic use, with rates remaining elevated over the following 6 months. In patients without comorbidity comparing the highest to lowest prior exposure quintiles in the Clinical Practice Research Databank, the IRRs were 1.18 [95% CI 0.90–1.55] in the first 3 days after prescription, 1.44 [95% CI 1.14–1.81] in the days 4–30 after and 3.22 [95% CI 2.29–4.53] in the 3–6 months after.Repeated courses of antibiotics, although common practice, may have limited benefit and indicator of adverse outcomes. A potential mechanism is that antibiotics may cause dysbiosis , contributing to colonization with resistant bacteria. Antibiotics should be used judiciously and only periodically unless indicated. Antimicrobial stewardship should include activities focusing on the substantive number of patients who repeatedly but intermittently get antibiotics. Antimicrobial resistance (AMR) is a major public health concern resulting in increased morbidity, mortality and healthcare costs. Without additional action to tackle AMR, many common healthcare interventions and procedures could become too risky to undertake. Internationally, an estimated 700,000 deaths are attributed to AMR annually . In the The optimal level of antibiotic use or exposure characteristics (such as duration and repeated use over time) associated with the best clinical outcomes are not known. A study of common respiratory tract infections found that lower levels of antibiotic use were associated with a higher incidence of pneumonia and peritonsillar abscess . A syste. The CPRD and SAIL databases include broadly comparable study populations  using a similar healthcare system. Although there were many similarities between the two databases, different coding systems (versions of the Read coding dictionary), EHR software systems and data preparation procedures were used in CPRD and SAIL.This study used data from two sources which included the Clinical Practice Research Databank (CPRD) and the Secure Anonymized Information Linkage (SAIL) databases. CPRD contains longitudinal, anonymized, patient-level electronic health records (EHRs) from general practices in the UK with more than five million active patient records representing about 8% of the UK population . SAIL coThe study population consisted of patients who were prescribed a systemic antibiotic in their general practice . The CPRD study population included patients prescribed an antibiotic between January 2000 and June 2015; for SAIL, this time was between January 2000 and December 2017 (the end dates represented the date of the latest data collection). Analyses included all antibiotic prescriptions recorded in the EHR follow-up with a diagnosis of a common infection such as upper respiratory tract infection (URTI), lower respiratory tract infection (LRTI), UTI and otitis media and externa at the index date or month before. Patients with COPD or multiple common infections were excluded. To evaluate the long-term effects of antibiotics, the study population was restricted to patients with at least 3 years of prior history in the patient EHR (i.e. time between the prescription and start of EHR follow-up for a patient). The exposure of interest was the number of antibiotic prescriptions in the 3 years before divided into quintiles (five groups). Patients could move over time between different quintiles as prior history was evaluated at each individual antibiotic prescription . Four pairs of propensity-matched cohorts were then randomly selected in order to compare the higher quintiles to the quintile with the lowest history of antibiotic use. Propensity scores are the probability of assignment to one exposure conditional on a patient’s measured baseline covariates. Patients in the different quintiles of histories of prior antibiotic were matched by these propensity scores, initially by the rounded propensity score  followed by matching within the caliper (i.e. pre-specified maximum difference). The caliper was taken as 0.20 of the logit of the propensity score as recommended in the literature .The primary outcome of interest was hospital admission with infection-related complication (as recorded in HES) that occurred in the 30 or 182 days after the antibiotic prescription. Follow-up was reset to zero in case of a repeat antibiotic prescription in the follow-up period. The hospital admissions for infection-related complications were based on the primary admission diagnosis with the broad set of infections . The predefined ICD10 codes included A37-A41, A46, A48, A49, B95, B96, G00, G01, G04-G08, H60, H65-H68, H70, H71, I00-I02, I33, I38, I39, J00-J06, J13-J18, J20-J22, J36, J40-J42, J85, J86, K05, K35- K37, K61, K81, L00, L01-L05, L08, M00, M01, M86, N10, N30, N45, N70-N75, O23, O85, O86, O91, O98, P23, P36, and P37. Code lists used in this study are available on the Clinical Codes repository . Figure Four different analyses were conducted in this study. The first analysis compared the risk of clinical outcomes between patients with different levels of prior antibiotic prescriptions in the 3 years before. The risks of clinical outcomes were compared between quintiles with different histories of antibiotic use stratified by different time periods after the antibiotic prescription. The first 3 days after were considered, pragmatically, to reflect the severity of the underlying infection while more distant time was considered to reflect the treatment effects of antibiotics and that of the changes in infection severity. Incidence rate ratios (IRRs) were estimated for different periods comparing the quintiles of history of prior antibiotic use. The second analysis visualized the incidence rates of infection-related hospital admissions for each quintile of prior antibiotic use. IRRs were estimated for each time unit (day or week). The IRRs were then smoothed  . A serieP values were estimated in order to minimize the effects of multiple testing and the finding of false-positive statistical associations. All analyses were performed using SAS software version 9.4.The propensity scores for assignment to the quintile with the lowest prior antibiotic use were estimated comparing each of the four quintiles with higher histories of antibiotic use compared to the lowest quintile. Logistic regression was used to estimate the propensity scores for each patient. The risk factors included in the logistic models were age, sex, calendar year and season, ethnicity, socioeconomic class (IMD), comorbidity based on the Charlson Comorbidity Score (composite score of history of selected chronic conditions ), recordThe source population of patients with over 3 years of follow-up included 2.1 million patients from CPRD and 0.6 million from SAIL. Totals of 11.5 million antibiotic prescriptions in CPRD and 3.7 million in SAIL were provided to these populations. In CPRD, 5.1 million antibiotic prescriptions had a recent record of one of the selected infections , 2.0 million had a record of another infection and 4.3 million (37.8%) had no recorded indication. Table The incidence of infection-related hospital admissions was 2.3 in CPRD and 6.1 in SAIL per 1000 person-months in the 30 days after . The incidence in children < 5 years was 1.8 in CPRD and 9.0 in SAIL, in patients aged 20–29 1.2 in CPRD and 3.7 in SAIL and in patients aged 80+ 8.4 in CPRD and 17.3 in SAIL. In CPRD, 48.0% of the infection-related hospital admissions were for LRTI, 33.9% for pneumonia and 10.1% for sore throat. In SAIL, these percentages were 29.2%, 36.4% and 26.7%, respectively.Table Figure The adjusted IRRs of infection-related hospital admissions varied across the three different time periods after the antibiotic prescription and levels of prior antibiotic use Table . In the Table The sensitivity analyses of the IRRs of infection-related hospital admissions in months 3–6 after an antibiotic prescription are shown in Table P values were below 0.05 for all these clinical codes. Among the clinical codes with an OR > 1.5 in the quintile with highest history of antibiotic use compared to lowest quintile, there were no clinical codes describing conditions that can lead to immunosuppression.Comparisons of the clinical codes between patients with lowest and highest histories of antibiotic use showed considerably higher recording of clinical codes indicating infections. The ten clinical codes with highest ORs were recurrent urinary tract infection (OR of 57.88), hidradenitis suppurativa (34.34), myringoplasty (31.55), chronic prostatitis (30.61), history of recurrent tonsillitis (29.67), frontal sinusitis (19.96), tonsillectomy (19.14), recurrent UTI (19.09), recurrent cystitis (16.88) and history of recurrent cystitis (16.49). The false discovery rate-adjusted Many patients in UK primary care repeatedly receive antibiotics. The level of prior antibiotic use was found to be a strong predictor of the pattern of changes in the risk of infection-related hospital admissions (mostly due to LRTI or pneumonia) after the antibiotic prescriptions. Patients with limited prior antibiotic use showed rapid and strong reductions in the risks, with risks returning to low levels within 2 months. In contrast, patients with higher levels of prior antibiotic use showed more moderate reductions in risk, with sustained higher risks over time. A strong dose-response was found between the patterns of changes in risk and level of prior antibiotic use.There may be several potential explanations for the observed patterns of sustained higher risks of infection-related hospital admissions risks over time. The study findings could be related to confounding due to patients being immunocompromised and suffering more severe and more frequent infections, or colonization with resistant bacteria in patients previously admitted to the hospital. Another explanation could be a causal effect and involve gut microbiota. The intestinal commensal microbiota provides colonization resistance against pathogens. It may be possible that antibiotics cause dysbiosis , contributing to the loss of colonization resistance followed by an increment of the resistome in the intestinal microbiota , 18. A rThere is some evidence in the literature that suggests a direct link between changes in microbiota and antibiotic exposure. A randomized trial in preschool children reported increased antibiotic resistance in the gut microbiota after a single course of antibiotics . The posThis study has several strengths and limitations. A key strength was that two datasets were used including two very large populations of patients from both England and Wales and allowing replication of findings in independent datasets. The coding of clinical events may be suboptimal in UK primary care ; a largeIn conclusion, there is little evidence in the literature for the clinical effectiveness of repeated antibiotic use in primary care although is common practice. Repeated courses of antibiotics may have limited benefit and indicator of adverse outcomes. This is possibly due to dysbiosis of gut microbiota and resistance. Antibiotics should be used infrequently for common infections unless there is clear evidence of an infection with susceptible bacteria. Antimicrobial stewardship interventions should target these patients with high use of antibiotics but apparently limited value.


Pseudomonas aeruginosa biofilm-related infections are difficult to treat with antibiotics. Along the different layers of the biofilm, the P. aeruginosa population is heterogeneous, exhibiting an extreme ability to adapt his metabolic activity to the local microenvironment. At the deepest layers of the biofilm is a subset of dormant cells, called persister cells. Though antimicrobial failure might be multifactorial, it is now demonstrated that these persister cells, genetically identical to a fully susceptible strain, but phenotypically divergent, are highly tolerant to antibiotics, and contribute to antimicrobial failure. By eradicating susceptible, metabolically active cells, antibiotics bring out pre-existing persister cells. The biofilm mode of growth creates microenvironment conditions that activate stringent response mechanisms, SOS response and toxin-antitoxin systems that render the bacterial population highly tolerant to antibiotics. Using diverse, not standardized, models of biofilm infection, a large panel of antibiotic regimen has been evaluated. They demonstrated that biofilm growth had an unequal impact of antibiotic activity, colistin and meropenem being the less impacted antibiotics. Different combination and sequential antimicrobial therapies were also evaluated, and could be partially efficient, but none succeeded in eradicating persister cells, so that non-antibiotic alternative strategies are currently under development. This article reviews the molecular mechanisms involved in antibiotic tolerance and persistence in P. aeruginosa biofilm infections. A review of the antimicrobial regimen evaluated for the treatment of P. aeruginosa biofilm infection is also presented. While tremendous progress has been made in the understanding of biofilm-related infections, alternative non-antibiotic strategies are now urgently needed. Bacterial biofilms can survive antibiotics due to impaired antibiotic diffusion, antibiotic efflux, nutrient and oxygen limitation, expression of biofilm-specific genetic mechanisms, selection of resistant mutants, or survival of tolerant cells  despite antibiotic exposure at concentrations above MIC, without any change in the MIC  the burden of biofilm-related infections increased last decades . Even whnt cells . While r the MIC . In the  the MIC .Pseudomonas aeruginosa is an opportunistic pathogen frequently involved in biofilm-related infections. Though P. aeruginosa has largely been used as a study model to explore tolerance mechanisms and to investigate treatment strategies in biofilm, there is to date, no guidelines for the treatment of P. aeruginosa biofilm infections. Moreover, most of in vitro data on persister cells come from studies of Escherichia coli.P. aeruginosa biofilm-related models, the molecular mechanisms underlying tolerant phenotypes responsible for antibiotic failure, and the different antibiotic approaches proposed to overcome these mechanisms. Neither the non-antibiotic alternatives, nor the combinations of antibiotics and non-antibiotic compounds, to treat P. aeruginosa biofilms, were not voluntarily discussed in this article to focus on antibiotic strategies.In this context, we reviewed, in P. aeruginosa biofilms: a stalk-forming subpopulation situated at the substratum, and a cap-forming subpopulation on top  systems ppGpp metabolism, the key-component of stringent response. After 2 days, only 20% of mice survived, either infected by the wild-type or by the mutant strain. Interestingly, when mice were treated with ofloxacin, a 10% increase in survival was observed in mice infected by wild-type strain, while survival increased 50% in the ΔrelAΔspoT infected group. These results strongly suggest that a stringent response, regulated by RelA and SpoT, is involved in antibiotic tolerance. Biofilm environmental conditions could be a major trigger of stringent response, a mechanism contributing to the antibiotic tolerance of persister cells , but exhibited a much lower activity in a microtiter biofilm model    (Wang et12 mg/L) . Besidesacin MIC . In a flruginosa . Such hiruginosa . Neverthin vitro biofilm device model, the biofilm growth had a strong impact on ceftazidime and imipenem with a 10 to 40 times increase in their minimal biofilm inhibitory concentrations , whereas ciprofloxacin and tobramycin were less affected (2 to 4 times increase in their MBIC)  than on dormant cells . In an iir MBIC) . Among βc models . Quinoloc models . Antibiobiofilms  but did biofilms . Thus, ebiofilms , probablbiofilms .Since monotherapy did not eradicate mature biofilms, combination of antibiotics appeared as a therapeutic option for the treatment of biofilm-related infections.in vitro studies, colistin was used as the backbone of combination antibiofilm therapies. Indeed, colistin based combinations demonstrated efficacy against P. aeruginosa biofilms and seemed effective for the eradication of persister cells and to prevent the emergence of colistin resistant mutants  and/or meropenem (carbapenem-resistant strain) this combination may not be suitable for all biofilm-related infections (P. aeruginosa infections (P. aeruginosa using clinical strains (P. aeruginosa isolates under conditions simulating microaerobic or anaerobic environments that are present in biofilm-mediated infections (in vitro model of biofilm, the ciprofloxacin/amikacin combination had no benefit over monotherapy against laboratory and clinical P. aeruginosa strains in biofilm, regarding efficacy and persister eradication (In numerous  mutants . For exa mutants . Since,  mutants , a colisfections . Aminoglfections . Some amfections . Gentami strains . Fosfomyfections . In othedication . A lack dication .Finally, though some antibiotic combinations could prevent the emergence of resistant mutants in biofilm, none has proven a total efficacy in overcoming tolerance mechanisms or in eradicating persister cells.cinetobacter baumanii persister cells. In a flow-cell model of biofilm study, P. aeruginosa biofilm (Some authors proposed that antibiotic sequential exposure could eradicate persister cells. In planktonic cultures,  biofilm . Besides biofilm . OverallP. aeruginosa biofilm with reducing of bacteria virulence factors, adhesion abilities and thus biofilm formation by inhibiting the cell-cell communication system, quorum sensing. Azithromycin seems to be of particular interest in case of cystic fibrosis to decrease long-term lung inflammation. The potential of this antibiotic is less described, and seems less appropriate, in case of biofilm device-related infection, diagnosed when biofilm is already mature (Azithromycin showed potential inhibiting effects on y mature .P. aeruginosa biofilm, in combination with ciprofloxacin. Such targeted approaches, though promising, have long been in the pipe-line and are still to be developed. Recent reviews (As a result, the non-antibiotic therapeutic perspectives, which target the metabolic pathways potentially involved in biofilm persistence, could be more relevant strategies . In the  reviews  offer coNevertheless, there is a lack of high-quality clinical studies in the field of biofilm infections, both for cystic fibrosis patients and for tissue and device-related biofilm infections.in vitro and in vivo biofilm models to monitor treatment efficacy. There is a real need for standardization of in vitro biofilm assays to reduce the variability in experimental results and to allow more direct comparisons between studies. New biofilm methods are required to assess the activity of future therapies but also to determine PK/PD parameters of existing antibiotics on young and old biofilms including combination antibiotic therapy (For further investigation of anti-biofilm activity of future components, it would be necessary to improve  therapy .P. aeruginosa biofilm infection, we still lack standardized in vitro methods that would be representative of in vivo infections and that would allow a comparison of different regimens. Moreover, despite intense in vitro studies on biofilm tolerance, we are disappointed by the lack of therapeutic translation. To date, no antibiotic regimen has proven efficacy to overcome persistence mechanisms in P. aeruginosa biofilms, and non-antibiotic alternatives are highly expected.Finally, though efficient therapeutics are still far from current practice in the medical treatment of biofilm-related infections, tremendous progress has been made during the past decade regarding the knowledge on molecular mechanisms responsible for antibiotic failure. All recent studies confirm that persister cells, exhibiting a high level of tolerance to antibiotics, play a key-role in the ability of biofilms to escape antibiotics. The molecular mechanisms involved in persistence are still partially described, but dormancy, stringent and SOS response, are major contributors to persistence. The mechanisms involved in bacterial persistence seem to differ according to the bacterial strain, and to the nature of the biofilm, hence the therapeutic approach might also need to be adapted to the specificities of the biofilm considered. When it comes to treating AS reviewed the literature and wrote the manuscript. KA and ME reviewed the manuscript. All authors listed approved the work for publication.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Shape memory alloys (SMAs) have been widely used in civil engineering applications including active and passive control of structures, sensors and actuators and strengthening of reinforced concrete (RC) structures owing to unique features such as the shape memory effect and pseudo-elasticity. Iron-based shape memory alloys (Fe-SMAs) have become popular in recent years. Use of iron-based SMAs for strengthening RC structures has received attention in the recent decade due to the advantages it presents, that is, no ducts or anchor heads are required, friction losses do not occur and no space is needed for a hydraulic device to exert force. Accordingly, Fe-SMAs embedded in a shotcrete layer have been used for pre-stressing RC beams at Empa. The aim of this study is to present an approach to model and analyze the behavior of RC members strengthened and pre-stressed with Fe-SMA rebars embedded in a shotcrete layer. The lack of research on developing finite element models for studying the behavior of concrete structures strengthened by iron-based shape memory alloys is addressed. Three-dimensional finite element models were developed in the commercial finite element code ABAQUS, using the concrete damaged plasticity model to predict the studied beams’ load–displacement response. The results of the finite element analyses show a considerably good agreement with the experimental data in terms of the beams’ cracking load and ultimate load capacity. The effects of different strengthening parameters, including SMA rebar diameter, steel rebar diameter and pre-stressing force level on the beam behavior, were investigated based on the verified finite element models. The results were compared. The load-displacement response of an 18-m concrete girder strengthened and pre-stressed with iron-based SMA bars was examined by the developed finite element model as a case study. Shape memory alloys are a class of materials that have gained popularity in recent decades owing to unique features such as the shape memory effect (SME) and pseudo-elasticity. Numerous studies have addressed the use of these alloys in engineering applications. In civil engineering applications, Ni-Ti based alloys have been widely used in the confinement of columns, dampers, self-centering bridge components and pre-stressing of structural members . Iron-based shape memory alloys (Fe-SMAs) are an appropriate alternative for Ni-Ti based alloys in structural applications based on their lower cost, wider thermal hysteresis, good machinability, higher stiffness and the ability to develop high recovery stresses without thermomechanical training. It is noteworthy that the lower cost of Fe-SMAs stems from the lower cost of Fe and the less expensive production cost that can be conducted under atmospheric conditions ,7,8,9,10Shahverdi et al. experimentally investigated the application of a novel Fe-SMA developed at Empa for near-surface mounted (NSM) strengthening; their studies  showed tSMA-reinforced structures have been simulated through finite element models in few studies ,15,16. AAlthough many publications have numerically modeled Ni–Ti behavior in civil engineering structures using the inherent SME and super-elasticity of the Ni–Ti alloy, few researchers  have addThe main aim of this study is to detail the modeling of RC beams strengthened and pre-stressed with ribbed Fe-SMA bars embedded in a shotcrete layer. The Fe-SMA bars, with a diameter of 8 mm, were pre-strained and fixed to the bottom of the concrete beams. A shotcrete layer was applied to embed the bars. The recovery stress in the embedded Fe-SMA bars was activated by resistive heating . The resThree concrete beams with a cross-section of 160 mm × 250 mm and a span of 2 m were tested by Shahverdi et al.  Figure . Beam 9 To model the concrete behavior in ABAQUS, three methods are available: the smeared crack model, the brittle cracking model and the concrete damaged plasticity model. In the first method, as the name implies, cracks are not modeled individually on a macroscopic scale. The yield criteria are checked at each integration point and the corresponding stress and strain are calculated accordingly. However, defining damage parameters is not possible using this approach. The brittle cracking model is used when tensile cracking is the dominant behavior in members; compressive crushing is not considered. This model is capable of defining proper concrete behavior in the post-cracking stage but it simply assumes that concrete behaves elastically under compression. Concrete damaged plasticity is a combined model that accounts for both tensile cracking and compressive crushing of concrete. It is designed to predict the nonlinear behavior of concrete using the concepts of damage and plasticity, which are characterized by changes in elastic stiffness and irreversible deformations upon loading ,23. Numerous models have been proposed to define concrete behavior under uniaxial compression. The model suggested by , represeTo determine the direct tensile strength in this simulation, Equation (4) proposed in ACI-318  is used Equation (5) is used to convert flexural tensile strength to direct tensile strength  for shotTo define the tension stiffening in RC members, the contribution of concrete to the rigidity of RC members even after cracking should be considered . There ation (6) .(6)Gf=∫Equation (7) was proposed in CEB-FIP 90  to deterhotcrete . The behaviors of Fe-SMA bars with a composition of Fe–17Mn–5Si–10Cr–4Ni–1 (mass %), were modeled in this study. During pre-straining, the Fe-SMA bars were loaded to a tensile strain of 4% and unloaded to a stress-free state. The bars were anchored to the bottom face of the beam and the shape change was suppressed. The shape memory effect was activated in Fe-SMA bars by heating, resulting in the development of recovery stress due to the enforced deformation constraint. The generated recovery stress was used to pre-stress the concrete members. ABAQUS/Standard was used in this study as a finite element simulation tool to capture geometric and material nonlinearities. Only a quarter of the beam was modeled, due to symmetry in both the longitudinal and transverse directions, as shown in According to the experiments, no considerable slippage occurred between the Fe-SMA bars and the surrounding shotcrete or between the steel rebar and the concrete . Thus, tThe concrete–shotcrete interaction was a demanding step in this simulation, as the interfacial behavior had to transfer the pre-stressing force to the upper concrete section. To this end, cohesive behavior and rough contact options were used. Without defining a cohesive behavior, the pre-stressing force applied to shotcrete has no effect on the beam response because the concrete and shotcrete lose contact during loading. Rough frictional behavior was used, which assigns an infinite coefficient of friction to the concrete–shotcrete interface. As stated in the ABAQUS manual , combiniThe pre-stressing force was applied in the initial step before loading using the predefined field option in ABAQUS. The calculation for the upward deflection of the beam pre-stressed with two Fe-SMA bars activated with a recovery stress of 300 MPa is shown in Equation (8). The calculated deflection of 0.36 mm shows a good correlation with the FE result in b0/fc0, which was set to 1.16 according to the ABAQUS manual [Sensitivity analyses were conducted to determine the appropriate values for some concrete damaged plasticity model parameters. The first investigated parameter was the ratio of the initial biaxial compressive strength to the uniaxial compressive strength, fS manual  as therec and the plastic potential eccentricity e, were set as 2/3 and 0.1, respectively, as suggested by the ABAQUS manual [c is determined from the results of the full triaxial test of concrete. However, these data were not available and the default values were assigned [The shape of the loading surface kS manual . The kc assigned . The resThe concrete dilation angle ψ, which can be characterized as the internal friction angle of concrete, is an important parameter in the damaged plasticity model . As showViscosity or relaxation time is an optional parameter in the CDP model and helps to improve the convergence rate in the analysis . As statAs the CDP parameters were chosen based on the sensitivity analysis, the finite element models were fully developed. Pre-stressing concrete beams using Fe-SMA bars embedded in a shotcrete layer is state-of-the-art; the pros and cons have not been sufficiently studied. Thus, parametric studies are necessary to investigate the effect of variations in important parameters such as the diameter of the Fe-SMA bars on the response of beams, as shown in The displacement ductility index Based on the innovative nature of this strengthening technique, the feasibility of using ribbed Fe-SMA bars to strengthen the beams without activating the recovery stress is investigated. The results are compared to those for the pre-stressed beam and the beam reinforced by two steel rebars embedded in the shotcrete layer, as shown in ρ) of 0.34%, 0.75% and 1.3%, corresponding to rebar diameters of 8 mm, 12 mm and 16 mm, were analyzed. The analyses were conducted for different levels of pre-stressing, 100 MPa, 300 MPa and 400 MPa. A parametric study was conducted to investigate the effect of the recovery stress value on the beam flexural response. This study was conducted on the FE model of Beam 10, with different diameters assigned to the longitudinal steel reinforcement. Beams with reinforcement ratios  are 5, 3.5 and 2.2, respectively.2 cross-sectional area. A 1.25 m × 0.2 m upper concrete slab was also modeled. The cross-section schematic and longitudinal profile of the reference girder are illustrated in In previous sections, the feasibility of strengthening and pre-stressing reinforced concrete beams using ribbed Fe-SMA bars embedded in a shotcrete layer was studied by means of numerical analysis. The validated 3-D finite element models are used in this section in a case study on the load-bearing capacity of an 18m reinforced and pre-stressed concrete bridge girder strengthened by activated or non-activated Fe-SMA bars embedded in a shotcrete layer. The reference girder, used for comparison, is characterized by a double-tee section with a height of 1 m and is pre-stressed by three parabolic and two straight steel tendons, each with a 345 mmThe depth of the assumed bottom shotcrete layer was chosen as 40 mm. The shotcrete had the same material properties as the shotcrete applied to the beams in the previous sections. Key parameters defining the mechanical properties of concrete, longitudinal steel bars and tendons are presented in The simulated girders, strengthened by activated or non-activated Fe-SMA bars, were analyzed under static loading conditions. The results are compared to those for the non-strengthened girder (reference girder) in terms of the load vs. mid-span deflection curves. The results of the finite element analysis for girder G5 are illustrated in It can be concluded that application of activated Fe-SMA bars has a more considerable impact on cracking and yielding load of girders compared to the girders strengthened with non-activated bars. This advantage of activation must be evaluated considering the equipment and cost of activating bars for such massive structures. To recap, the case study in this paper aimed to investigate the feasibility of strengthening concrete bridge girders with Fe-based alloys. The finite element simulation results were promising; however, the applicability of the proposed strengthening method should be tested on several girder geometries with different reinforcement levels. Also, the proposed method’s applicability under different loading patterns could be an essential subject of further studies.c and the ratio of the biaxial compressive strength to the uniaxial compressive strength did not significantly affect the load–displacement response of the models; different concrete dilation angles yielded different results in the post-cracking stage. Adopting a proper viscosity parameter had a positive effect on the convergence and accuracy of the analysis results.In this study, numerical analyses of concrete beams strengthened and pre-stressed with Fe-based shape memory alloy bars were conducted. The constitutive models used for each material in the RC beam were explained along with the proper approach to implementing the pre-stressing effect on the beams in the 3-D finite element model. The results of the analyses compared to the experimental data indicate that the finite element models successfully predicted the load vs. mid-span displacement response of the beams. In addition, sensitivity analyses were performed to study the effect of variations in parameters in the concrete damaged plasticity model in ABAQUS. The results indicate that changes in eccentricity, k(ρ varies from 0.34–1.3%). Moreover, the variation in the recovery stress affects the cracking load more significantly than the yielding load and the ultimate load.Finite element analyses conducted for beams reinforced with different ratios of longitudinal rebars pre-stressed with three values of recovery stress activated in the Fe-SMA bars indicate that variation in the pre-stressing force produces a greater effect on a beam with a lower reinforcement ratio The flexural behavior of a similar beam strengthened by non-activated Fe-SMA bars was also investigated and compared to the pre-stressed and reference beams. The results indicate that activating the recovery stress increases the ultimate load by less than 5% compared to the non-prestressed beam; however, the cracking and yielding loads increase significantly. The feasibility of pre-stressing an 18-m concrete bridge girder by means of activated and non-activated Fe-SMA bars embedded in a shotcrete layer was studied. It was concluded that the load-bearing capacity of the bridge girders was significantly enhanced when using this method with Fe-SMA bars of a sufficient number and diameter.