Then, a working hypothesis was adopted to find optimal and effective combinations of these components in the concrete composition. In the course of experiments, the most effective combination of additives for obtaining the best RHC composition was deduced by modeling the average strength values of samples in the early ages of curing. Further, RHC samples were tested for frost resistance in an aggressive environment at the ages of 3, 7, 28, 90, and 180 days to determine operational reliability and durability. The test results showed a real possibility of accelerating concrete hardening by 50% at the age of 2 days and achieving up to 25% strength gain by using both microsilica and calcium chloride (CaCl2). The best frost resistance indicators were observed in the RHC compositions with microsilica replacing part of the cement. The frost resistance indicators also improved with an increase in the amount of microsilica.This article presents the results of research conducted to determine the possibility of accelerating strength gain and enhancing the operational reliability of concrete. The study tested the effects of modern modifiers on concrete in order to select the composition of rapid hardening concrete (RHC) with better frost resistance characteristics. A basic composition of RHC grade C 25/30 was developed using traditional concrete calculations. Based on the analysis of previous studies by other authors, two basic modifiers (microsilica and calcium chloride (CaCl Modern construction requires accelerated curing of concrete, especially in monolithic construction . In monolithic house-building, the main economic indicator is the reduction of time needed for the construction of one floor to 3 days. This technical requirement is dictated by commercial necessity since the rapid attainment of the design strength of monolithic structures is necessary to increase the turnover of molds and formwork, make more efficient use of equipment, and enhance the productivity of construction activities. Such fast construction pace can be achieved by using concrete with accelerated strength gain, commonly known as rapid hardening concrete (RHC) ,3. The k2 + SiO2 + mH2O = CaO*SiO2*nH2O) [To accelerate the hydration of clinker minerals and intensify the concrete curing process, several technological approaches can be used. At low water–cement (W/C) ratios, there is an oversaturation of the aqueous medium by the products of hydration and hydrolysis, i.e., cement minerals, which causes accelerated strength gain . The mosO2*nH2O) ,9.2), but it should be used with caution in reinforced concretes [To obtain high-strength RHC, the pore size can be reduced by adding ultradispersed active silica, a waste product of ferrosilicon production . The useoncretes .-High technological properties of the obtained concrete mixtures by reducing their water content and increasing their homogeneity and resistance to stratification;-Acceleration of cement hydration rate at the early stage of curing, which reduces the construction time while ensuring high quality, as described by ;-Use of effective modifiers and hyperplasticizer additives, allowing the achievement of high rates of strength gain without reducing the operational properties.Obtaining modified RHCs of superior quality offers various benefits, including:The goal of this study was to select rapid hardening concrete compositions with over 70% compressive strength gain on day 2, equivalent to grade C 25/30 strength, and frost resistance of at least 180 cycles (where 1 cycle is 1 day of freeze–thaw). -2) and laboratory testing of their strength characteristics according to [Calculation of RHC compositions using two modifiers  and chemical additive based on polycarboxylates to the composition of RHC;Stage 3: Addition of various amounts of modifiers (microsilica and CaCl-Stage 4: Comparison of the kinetics of compressive strength gain of samples in the early ages of curing (2 and 7 days);-Stage 5: Study of the microstructure of obtained samples of RHC under the ZEISS Axio Vert.A1 microscope  via reflection shooting at a magnification of ×500;-Stage 6: Selection of optimum compositions in terms of early strength and cement consumption, and testing for frost resistance in 10% salt solution (NaCl) at the ages of 3, 7, 28, 90, and 180 days.-Stage 7: Comparison of compressive strength of samples obtained after frost resistance tests at the ages of 28 and 180 days;-Stage 8: Analysis and selection of the superior compositions based on the test results of the modified RHC.The research was conducted in 8 stages, each of which was aimed at solving specific tasks:Below is a description and characteristics of the raw materials used.Grinding fineness: The cement used had a grinding fineness of 94.4%.Normal density and setting time of cement dough: The cement used had a normal consistency of 27.30%. The beginning of setting occurred after 2 h 11 min, and the end of setting occurred 4 h 10 min from the moment of mixing. These values were consistent with ,20.To confirm the compliance of the selected cement with the norms and requirements of , severalThe fine aggregate used was sand from the manufacturer Giyada, LLP . This aggregate met the requirements of the standard . Table 3fM) of the sand used was 2.6 . This aggregate was found to meet the requirements of the standard . Table 4According to , to obta3. According to its chemical composition, MCU-95 is represented mainly by non-crystalline silica with a content of about 97%. It has a specific surface area equal to 3980 cm2/g, according to [MCU-95 contains spherical particles with a diameter of 0.1 microns. Its bulk density ranges from 150 to 250 kg/mrding to .Comparing the chemical composition presented by the manufacturer to the quality standard , it foll2), taken according to [rding to .The effectiveness of additives in regulating the strength gain kinetics of concretes and mortars was evaluated, according to , based o2) proposed in previous studies [Further studies aimed to calculate the composition of RHC with Portland cement for the first test mixes and approve the reference composition. After that, experimental confirmation was used to improve the compressive strength indicators  in the e studies ,30,31. I studies  within 3When selecting the RHC composition with Portland cement for the first test mixes and approval of the reference composition, Equations (2) and (3)  were use-Increase in the strength of concrete at the ages of 2 and 7 days as a result of the use of a particular method of acceleration of curing;-Increase in the strength of concrete at the ages of 2 and 7 days as a result of using several methods is not a direct sum of the increases in the strength of concrete achieved by each method separately;-Rheological properties of RHC should be taken as an average within the workability range corresponding to the slump of 10–15 cm .Experimental verification and correction based on the results of the preliminary calculations of the concrete composition are mandatory , and the-Experiments on the acceleration of concrete curing by replacing part of the cement with MCU-95 ;-Experiments on the acceleration of concrete curing with no silica by adding more cement to the composition than in the reference composition;-Experiments on the acceleration of concrete curing by simultaneously increasing the amounts of cement and chemical additive based on polycarboxylate, AR Premium;-2.Experiments designed to accelerate early curing with the addition of various amounts of CaClThen, based on the RHC reference composition obtained by the calculations, several experiments were carried out:After that, test samples of concrete were made from the experimental compositions, which were tested for compressive strength at the ages of 2 and 7 days. R, MPa) was calculated with an accuracy of 0.01 MPa using Equation (4).F—breaking load, N; A—the area of the working section of the sample, mm2; α—scaling factor for converting concrete strength to concrete strength in cubic samples with rib size of 100 mm.As part of the compressive strength test, cubes with a rib length of 100 mm were molded using a mixture of the reference composition and each subsequent modified composition. Three samples were taken per test. After the samples had reached the ages of 2 and 7 days, tests were conducted at a relative humidity of at least 95% and a temperature of 20 ± 5 °C, in accordance with . The comIn addition, the microstructure of RHC compositions with high early strength characteristics (2 and 7 days) was studied using a microscope. After the microstructure analysis, the compositions were tested for frost resistance.To study the microstructure of concrete, small pieces of concrete were cut from the area of interest and prepared for microscopy. First, any loose debris or dirt from the surface of the concrete was removed. Then, a diamond saw was used to cut small pieces of concrete. Next, progressively finer grits of sandpaper were used to polish the surface of the concrete until it was smooth and flat. Finally, a metallographic polishing machine was used to polish the surface to a mirror finish.The polished concrete samples were then mounted onto a glass slide using a small amount of adhesive, ensuring that the area of interest was in the center of the slide.To enhance the contrast of the concrete microstructure, the sample was stained using a 1% aqueous solution of methylene blue. The stain was applied to the sample and allowed to sit for a few minutes before being rinsed off with water.The stained concrete sample was then placed onto the stage of the ZEISS Axio Vert.A1 microscope . The objective lens was selected for magnification at ×500 to visualize the microstructure of the concrete. The illumination and focus settings were adjusted to obtain clear images of the microstructure, and imaging software was used to capture and analyze the images.The images were analyzed to identify and quantify the various components of the concrete microstructure, such as the cement paste, aggregate particles, and any voids or cracks.After examining the microstructure of the RHC samples, the compositions with the best characteristics were tested for frost resistance.The test to determine frost resistance was carried out using the second accelerated method of alternate freezing and thawing . The essTests were conducted on samples  using an LGPv laboratory freezer with the Profi electronic system . The samples were saturated in 10% NaCl solution and then thawed in a 10% NaCl solution. The accelerated tests by the second method were carried out according to the conditions and modes in During the frost resistance tests, the main indicator was the mass loss of the samples. Samples that lost more than 2% of their original weight were considered to have failed the frost resistance test and were not further tested .After obtaining the data from the frost resistance tests, the dependence of the effect of MCU-95 on the frost resistance at early ages  and the compressive strength of RHC at the ages of 28 and 180 days was determined. Based on the tests performed, comparative calculations were made, and the composition of RHC with the best performance characteristics was determined.2 created conditions for the formation of a structure with the densest packing of crystals, consisting mainly of low-base calcium silicate hydroxide (Ca5(OH)2Si6O16-4H2O ⇒ (CaO/SiO2 ˂ 1.5)), which confirmed the theory presented in [ented in .2, significantly affected the fundamental physical and technical properties of concrete, resulting in improved characteristics. Specifically, microsilica contributed to a more efficient use of the chemical energy of clinker by creating additional centers of crystallization. Additionally, an increase in the amount of microsilica in the composition led to an increase in the amount of stable low-base calcium silicate hydroxide (Ca5(OH)2Si6O16*4H2O ⇒ (CaO/SiO2 ˂ 1.5)), which reduced the most unstable component of cement stone—Ca(OH)2 crystals. This is important for producing dense and durable concrete [concrete .2SiO2-H2O ⇒ (CaO/SiO2 ˃ 1.5)), which was consistent with the opinion of the authors of [thors of .2 as a percentage of the cement (No. 2–5).2 to the concrete mixture led to a general tendency towards an increase in early strength gain, up to a certain amount equal to 2% of the weight of cement. Furthermore, the experiments conducted suggested that an increase in the amount of CaCl2 to 3% of the weight of cement and more was accompanied by the same dynamics of strength gain increase at the ages of 2 and 7 days as when the amount of CaCl2 was equal to 2%. This complemented information on the acceleration of hardening stated in [From tated in .2, obtained with the ZEISS Axio Vert.A1 microscope via reflection shooting at a magnification of ×500.2-3Ca(OH)2-12H2O shown in 2 with CaCl2, and it can change the microstructure of concrete as well as the properties of its pore solution. The formation of calcium oxychloride (as an expansive phase) can alter the transport properties of concrete by causing pore blockage and severe failure due to internal expansion. The chemical reaction is described by the formation of a calcium oxychloride compound using the equation 3Ca(OH)2 + CaCl2 + 12H2O → CaCl2-3Ca(OH)2-12H2O. The reaction between calcium chloride and calcium hydroxide is rapid and can weaken the structure if formed in the concrete matrix due to internal hydraulic pressures that can occur. The calcium oxychloride compound is relatively unstable at room temperature and low levels of relative humidity. Calcium oxychloride can exist in different molar ratios, including CaCl2-3Ca(OH)2-12H2O, CaCl2-Ca(OH)2-xH2O, and CaCl2-Ca(OH)2. These phases can coexist and are interchangeable in the compound Ca(OH)2-CaCl2-H2O. Hence, the compound CaCl2-3Ca(OH)2-12H2O, as seen in 2-Ca(OH)2 or CaCl2-Ca(OH)2-xH2O with changes in temperature or ambient humidity [The calcium oxychloride compound CaCl2, where SD stands for standard deviation.2 in the amount of 2% of the weight of cement affected the curing acceleration by 9.4 MPa at the early age of 2 days and by 7.5 MPa at the age of 7 days, which was consistent with [ent with ,31.After comparing the data on strength during the early hardening period at 2 and 7 days, examining the microstructure of the obtained RHC samples, and studying previous works in this research area ,11, it w2) [2) .Based on the data collected on the strength of RHC samples during the early hardening period at 2 and 7 days, it can be noted that almost all compositions, except those using pure cement, achieved the target of reaching a compressive strength of 70% or more of concrete grade C 25/30 at 2 days. Interestingly, increasing the amount of cement beyond the reference composition did not lead to higher early strength, which was an unexpected result. However, this finding was partially explained by previous studies .2 in compositions with the replacement of some of the cement with microsilica facilitated the formation of an early structure with the densest packing of crystals, as evidenced by [5(OH)2Si6O16*4H2O ⇒ (CaO/SiO2 ˂ 1.5)). The use of polycarboxylate plasticizer in compositions with an increase in the amount of cement up to 15% and hyperplasticizer AR Premium up to 20% resulted in good early strength gain dynamics of RHC up to 110% at the age of 2 days, accompanied by a decrease in the B/C ratio. The addition of CaCl2 in the concrete mixture led to an increase in early strength gain due to early bonds between portlandite Ca(OH)2 and calcium chloride CaCl2, resulting in the formation of gel-like calcium oxychloride CaCl2*3Ca(OH)212H2O, up to a certain amount equal to 2% of the weight of cement [2 had no additional effect on the strength gain at the ages of 2 and 7 days. The frost resistance tests on the samples of RHC showed that the best indicators of frost resistance were observed in the samples where the cement part was replaced by microsilica in the quantity of up to 50 kg per 1 m3. The performance was better with a higher content of microsilica, indicating the higher operational reliability of products made of concrete with active microsilica. The results confirmed the theory of the creation of additional centers linking hydrolyzed lime into an additional amount of strong, cementitious calcium hydrosilicates and showed the compliance of the obtained results with the theoretical assumptions about the creation of additional centers of crystallization and the reduction of pore space in the concrete body when using reactive pozzolanic additives.The results of the compressive strength tests conducted at 2 and 7 days of hardening for the remaining three types of samples were as expected, with all of them exhibiting a compressive strength of at least 70%. The presence of active SiOenced by , which cf cement ,31. FurtThe results of the tests confirmed the purpose of the research, which was to select RHC compositions that achieved a strength gain of 70% or more in the first 2 days of hardening. Out of the four series of samples tested, three samples with different modifier and binder contents were able to achieve the required early strength indicators.Based on the calculations and laboratory tests, it can be concluded that the use of modified RHC is justified both from technical and scientific points of view. Compared to ordinary cement concrete compositions, RHC with modifiers can accelerate the construction process. RHC with MCU-95 microsilica accelerated the strength gain rate and exhibited the best frost and strength indicators at 90 and 180 days after placement, indicating the absence of destructive processes in concrete due to the use of microsilica and good performance characteristics. 2). The process of binding Ca(OH)2 by the active mineral additive SiO2 into the low-solubility compound—calcium hydrosilicate—occurred according to the equation: Ca(OH)2 + SiO2 + mH2O = CaO*SiO2*nH2O. Experiments with accelerated curing using a specific amount of CaCl2 as well as compositions with simultaneously increased amounts of cement and hyperplasticizer based on the polycarboxylate AR Premium demonstrated an improvement in the early strength gain of RHC. However, microscopic examination of the concrete samples revealed a heterogeneous and weakly crystallized structure, indicating the poor quality and durability of the resulting RHC, especially since chlorides should be used with caution in reinforced structures.The test results demonstrated the consistency of the obtained data in RHC with the addition of microsilica with the theoretical assumptions regarding the emergence of additional centers of crystallization and the reduction of pore space in the concrete body during the use of reactive pozzolanic additives (active SiOThe compositions of RHC (No. 4–6) with MCU-95 from


This study investigated the fracture characteristics of plain concrete and polypropylene fiber-reinforced concrete (PFRC) using pre-notched three-point bending beam tests with the digital speckle correlation method (DSCM). Then, the fracture instability behavior of the two types of beams was simulated in finite elements based on the plastic damage model and the cohesion model, for which the applicability was assessed. Furthermore, the stability of the Big Gang Mountain Dam made from plain concrete or PFRC subjected to the earth-quake loading was simulated with the plastic damage model. The results show that the limiting length of the non-local deformation zone can be used as an indicator of instability damage in a concrete structure. The simulation results of the plastic damage model agreed well with the local deformation in the pre-notched three-point bending beam test obtained from the DSCM. The plastic damage model was found to be capable of describing the residual strength phenomenon, which the cohesive model was not capable of. The damage evolution regions of the PFRC dam are strictly constrained in some regions without the occurrence of the local deformation band across the dam, and PFRC can dramatically reduce the failure risk under earthquake loading. The numerical solution proves that PFRC is an advisable material for avoiding failure in concrete dams. The meso-cracks existing in concrete structures always dramatically influence the safety and durability of the structure and reduce the loading capacity of the structure due to the quasi-brittle essence of the concrete material . It is cIn order to identify concrete fracture behavior, various equipment has been fabricated for testing the fracture resistance capacity of concrete. The most sensitive fracture index of concrete mainly concerns the tensional fracture resistance capacity . IronicaHillerberg et al.  and PeteGenerally, the three-point bending beam test is of importance in obtaining fracture information during crack initiation and propagation to understand the mechanism of the fracture phenomena . The chaIn summary, all the studies mentioned above not only focused on the evaluation of concrete fracture energy but also paid attention to describing the concrete fracture phenomena in the three-point bending beam tests. Many damage models and fracture models were also developed based on the three-point bending beam test results. Actually, more and more research has been undertaken to discover the mechanism underlying the local deformation of concrete materials in the last two decades ,26. Wellp-CMOD) curve and the evolution regulation of the local deformation zone for plain concrete and PFRC in the three-point bending beam tests with a digital speckle pattern system. The numerical results with the plastic damage model were compared with the experimental data to determine the rational parameters. Subsequently, catastrophe theory was adopted for the damage evolution process to determine the instability of the concrete fracture behavior by analyzing the energy evolution regulation. Finally, concrete dams suffering earthquake loading were simulated with a plastic damage constitutive model. This research is of vital importance in determining the rationality and accuracy of the parameters as the input of the plastic damage constitutive model.The studies mentioned above have shown that plastic damage models can reasonably describe the fracture behaviors of concrete. However, few studies have described the fracture instability behaviors of plain concrete and polypropylene fiber-reinforced concrete by comparing the size of the fracture process zone, crack initiation and instability fracture between experimental and numerical results. In this paper, we investigated the pressure crack mouth opening displacement (C25 concrete was fabricated. Crushed stone with a maximum size of 19 mm was used as coarse aggregate. River sand with a fineness modulus of 2.6 was used as fine aggregate. The cement used was 42.5 grade ordinary Portland (P.O) cement and the properties are shown in After the specimens had been cured for 28 days, the specimens were tested for three-point bending. The three-point bending test equipment used for this test is shown in The digital speckle correlation method (DSCM) was employed to monitor the fracture behaviors of the concrete beam, including the crack mouth opening displacement, displacement and strain distribution contour, because DSCM can record the deformation of the material without interference from the equipment ,34. PrioThe concrete plastic damage model proposed by Lee and Fenves  to accouAdditive strain incremental decomposition:A scalar damage factor D has linked non-damage status with damage status for an incremental form of the elasticity stress-strain relation as:Yield function Non-associated potential flow rule:The model is based on two sets of uniaxial data and five additional parameters. The uniaxial data correspond to the stress-strain behavior (in compression and tension separately) after reaching yielding.The effective stress tensor For any cross-section, the coefficient (1 − D), being 0 and 1 corresponding to the total damage condition and non-damage condition, respectively, represents the loss of the effective cross-section bearing loads to the total cross-sectional area. The effective stress and Cauchy stress are equal in the non-damage condition.t and Dc corresponding to tensional and compressional circumstances, respectively.Evolution regulation of the damage factor D is determined by the two factors Dbσ0 biaxial compressive yield stress to cσ0 uniaxial yield stress, adopting the yield surface in the biaxial stress state. The second parameter cK changes the shape of the deviatoric cross-section from its circular shape into a smooth triangular shape. The following paragraph discusses the yield function used in the concrete damaged plasticity model, including an explanation of each parameter. The yield function is defined by Equation (7):The classical Drucker-Prager yield surface is modified by two independent parameters and two sets of data. The data sets are the uniaxial stress-strain relationship independent of tension and compression. The first parameter is the ratio of Parameter G is given by Equation (9):The potential flow is a hyperbolic Drucker-Prager function influenced by 3 parameters. The first is the dilatation angle al plane . The sectential) . This ec−6, and the reason can be attributed to the stress intensity characteristic of the material in front of the crack tip, as shown in The resolution ratio of the industrial video camera is fixed, and the shooting coverage dramatically influences the accuracy and sharpness of the result, decreasing with the increase in the coverage area. Therefore, the camera only captures the region in front of the initial crack tip because the deformation phenomena changes precisely in front of the initial crack. −4 to 1 × 10−5, indicating that the balance of the beam had already formed. With the loading increasing to about 3 mm, the local deformation zone began to extend dramatically and the maximal strain was about 1.7 × 10−3, indicating that the material in front of the crack tip was not performing in an elastic state . a0, t, and h are the length of initial crack, width and height of the beam specimens, respectively. h0 is the distance from the bottom of the beam with the displacement measurement gauge.Nine hundred four-node plane stress reduced integral elements were adopted to establish the finite element model of the pre-notched beam, as shown in 8 Pa, was calculated by equation, and Poisson’s ratio was 0.3. It should be noted that the initial limit strengths and ultimate limit strengths, In the finite element model, the C25 plain concrete and C25 PFRC were both investigated with the plastic damage model and cohesion model. The value of elastic modulus for concrete, being 5 × 10Meanwhile, the cohesive model was also employed in the simulation for comparison, and the fracture energy utilized in the cohesive model was defined as 150 N/M and 200 N/M, respectively, to represent the fracture behavior of concrete with or without fiber reinforcement .The results of the experimental tests for each group of beams are listed in p-CMOD curves of the plain concrete beam from the experiment and simulation results of the adopted cohesive model and damage model are plotted in p-CMOD curve was slightly different from the experimental curve in the softening stage after the loading reached the maximal value. The concrete difference was in that there was a residual strength in the numerical result. It can be seen that the cohesive model exhibited a more rational simulation on the fracture behavior of plain concrete, as shown in The p-CMOD curve. An obvious phenomenon was that the plastic damage model exhibited a residual strength, being about 40% of the maximal strength of the material after the softening stage. For plain concrete, the fracture initiated and then extended rapidly due to the brittle nature of the material. In this process, the cohesive model reflected the cohesive stress distribution in the fracture process zone appropriately. Meanwhile, plastic deformation was not the main part in the fracture. For the PFRC, crack propagation is hindered by the fiber in the cement matrixes, as shown in p-CMOD curve. Definitely, the plastic damage model describes these phenomena appropriately for the failure features of PFRC.For PFRC, the plastic damage model provided more rational numerical results e compareBased on the above discussion, it can be concluded that the plastic damage model can, to some extent, describe the residual strength of the structure after the stress peak, compared to the cohesion model. Therefore, the plastic damage model can be used to investigate the effect of residual strength on the seismic performance of large concrete structures.For a brittle material, the occurrence of a macrocrack always indicates that the structure will fail immediately. However, the detection of crack initialization is usually complicated, as well as the evaluation of the safety of the structure. In fact, the fracture process zone appears and propagates before the macrocrack, providing a method for monitoring the failure process of brittle materials, especially concrete. Prediction of the evolution regulation of the fracture process zone contributes to an understanding of the mechanism underlying instability failure induced by a macrocrack. The cohesive model is not an advisable choice for simulating the local deformation phenomenon since the fracture process zone is modeled with a non-depth interface in the cohesive model. Oppositely, the plastic damage model can reflect the local deformation phenomenon accurately by means of the damage distribution contour. In this part, the plastic damage model is employed to simulate the local deformation zone to illustrate the failure behavior of plain concrete. In the initial loading process, the damage value in the entire beam was 0. When the loading scale reached about 27% of the ultimate loading displacement, a small damaged region, with the damage value being almost 0.9, appeared in front of the crack tip, as shown in p-CMOD curve in p-CMOD curve, the CMOD at this spot exceeded the limit strength slightly.These phenomena also indicate that damage behavior usually accompanies the stress intensity status and the macrocrack is already generating in the damage evolution region. This stage corresponds to the circle spot of the p-CMOD curve in The damage distribution contours of the pre-notched bending beam with different loading scales are shown in With the increase in the loading scale, the evolution regulation of the damage region can be summarized as two main features: (1) the region with the damage behavior, being above 0.9, increases in the vertical direction; (2) the other regions without the damage behavior will manifest the elastic unloading conditions. Finally, a narrow damage band, in which the damage values are all above 0.9, will cross through the whole beam forming an obvious failure surface in the beam, as shown in To judge the fracture type, the horizontal and vertical strain distribution contours at the ultimate stage of the loading process are also plotted in The stress and stress tensor distribution contours of the pre-notched bending beam at the ultimate stage of the loading process are presented in Various kinds of energy evolution curves including damage energy, external energy, internal energy and plastic energy are plotted in It should be emphasized that the demarcation point in various kinds of energy curves just corresponds to the step in which the damage behavior begins to generate in the numerical model. Therefore, it is exquisitely rational to believe that the appearance of the damage can also be adopted to calibrate the boundary between the elastic and nonelastic stages of the energy curves. In particular, the evolution regulation in the second stage is of vital importance in determining the instability of the beam structure; thus, it would be worth investigating more carefully. We just focused on the damage energy curve and plastic energy curve since the instability fracture is relevant to the evolution regulation of the two kinds of energies. The slopes of the curves manifest a sharp change regulation ranging from a large value to a constant value with the increase in the loading scale.W represents the nonelastic energy which can be chosen as plastic energy, pW, damage energy, dW or the sum of plastic energy and damage energy. a and λ are the length of the local deformation zone and loading scale factor, respectively. k is defined as Cusp catastrophe theory was adopted to investigate the stability of the dissipation behavior occurring in the simulation for determining the instability demarcation point. Firstly, the loading scale, being 27% of the entire loading, was regarded as the initial point of the non-elastic stage (second stage). In the numerical simulation, the displacement loading type was adopted on the indenter. The assumption that the length increment of the local deformation zone is proportional to the displacement scale is proposed to obtain Equation (11).In the Taylor expansions, Equation (12) can be written as:Due to the regulation of catastrophe theory for determining the governing equation, only the first four terms are retained as Equation (14).Through the coordinate transformation regarding When the balance condition is satisfied, Equation (15) is equal to 0:To judge the stability of the damage energy, a three-order polynomial is employed to match the damage energy evolution curve. When the equation loses instability, the sign of the discriminant format, It is necessary to divide the nonelastic stage into two sub-stages to determine the bifurcation point of Equation (17). For this simulation result, the point with 45% loading scale of the entire loading was chosen since the slope of the damage energy curve is maintained as a constant value, indicating that a steady circumstance of the damage evolution occurs after that.The first sub-stage and second sub-stage of the damage energy evolution curve were fitted with Equations (18) and (19):The polynomial fitting curves for the first and second sub-stages are plotted in m Zeeman ,58. FromA finite element model for the double curvature arch concrete dam, located on the Big Gang Mountain was performed with a dynamic earthquake loading of 7.0 magnitude. The plain concrete and the PFRC were considered as the materials for the dam, separately.The body of the dam and surrounding rock are meshed with hexahedron elements (C3D8R), and the interface model  was adopIn the numerical simulation process, the C25 plain concrete and C25 PFRC were both investigated with the plastic damage model. The earthquake loading was sustained for 20 s. The damage parameters for the definition of the plain concrete and PFRC are listed in The damage contours after the earthquake for the numerical tests for plain concrete and PFRC are listed in For the PFRC dam, the maximal damage value increased to 1.5, indicating the damage region of the dam structure was much more vulnerable. Fortunately, there was no local deformation zone in the dam, and the region with a higher damage factor was just a corner. Meanwhile the damage region areas in the PFRC dam heel were much smaller than those in the plain concrete dam heel, as shown in The pre-notched three-point bending beam tests were performed to investigate the fracture features of plain concrete and PFRC in the laboratory and in numerical simulation. Then, the bifurcation criterion proposed from the cusp catastrophe theory was used to determine the fracture stability of concrete. Finally, a dam with plain concrete or PFRC subjected to earthquake loading was simulated with the plastic damage model to evaluate the safety and stability of the dam. The conclusions are as follows.The test results show that the ultimate length of the nonlocal deformation zone can be regarded as an indicator of instability damage in a concrete structure.The plastic damage model and cohesive model were found to be accurate and rational for predicting the fracture features of concrete, especially for the Type I fracture feature.The plastic damage model can suitably describe the residual strength phenomenon of PFRC, which the cohesive model is not capable of. However, the cohesive model can better reflect the cohesive stress distribution in the fracture process zone of plain concrete.The DSCM results indicate that when the fracture in the beam begins to propagate unstably, the length of the local deformation zone will reach a threshold. The corresponding numerical result also exhibits a similar regulation.The cusp catastrophe theory was adopted to investigate stability with the damage energy curve and the demarcation points in which damage evolution starts to lose stability were also identified and similar to those obtained from the experiment.The damage evolution regions of the PFRC dam were strictly constrained in some regions without the occurrence of the local deformation band across the dam and the PFRC could dramatically reduce the failure risk under the earthquake loading.In conclusion, the plastic damage model developed in this study can successfully simulate the fracture performance of PFRC. This provides a theoretical basis for the application of PFRC in dam engineering.


In this study, the effect on the flowability, compressive strength, absorption, sorptivity, and carbonation resistance of concrete with different copper slag (CS) replacement ratios was investigated. For this research, four concrete mixes with different percentages of CS were made . In addition, the microstructure was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), and thermogravimetric analysis (TG–DTG). The results shows that the incorporation of CS reduces the workability and compressive strength of the mixtures, being more significant in concrete with 30% CS. The carbonation depth of concrete with CS increases monotonically with increasing CS. In addition, the compressive strength of the carbonated (20% and 30% CS) concretes show a loss of compressive strength at 90 days of exposure when compared to their water-cured counterparts. The use of low percentages of CS does not generate a decrease in workability and its mechanical effect is not significant at prolonged ages, so the use of this waste as SCM in percentages close to 10% is a viable alternative to the sustainability of concrete and the management of this residue. Amissions ,7, prima2, Al2O3, and Fe2O3 must be at least 50% and 70%, respectively. However, this standard guide limits the classification of SCMs to two materials , so an alternative for evaluating new SCMs is the process described in ASTM C1709 [The use of SCM in concrete can improve the performance of this composite material in terms of mechanical properties, workability, and durability . This imTM C1709 . 2) [2 is due to SCM reacting with Ca(OH)2 and forming secondary C–S–H (Equation (2)) [In general, SCMs with pozzolanic activity are used as a partial substitute for cement to generate pastes. In this binder, SCMs generate the pozzolanic effect and the dilution effect, which leads to a reduction in calcium hydroxide (Ca(OH)2) ,12, whicion (2)) ,13.Ceme2O3 and SiO2 in percentages between 35–60% and 25–40%, respectively, in addition to few amounts of Al2O3, CaO, and CuO [Copper slag (CS) is a solid waste with high hardness produced from the production and refining process of copper  and is  and CuO . In copp and CuO . The hig and CuO , and if  and CuO .To reduce the environmental impact generated by CS, several studies have been carried out to take advantage of this by-product. CS has been used as an aggregate in the production of asphalt mixtures for several years. For example, Rojas-Pardo et al.  incorporResearch on CS as a supplementary cementitious material (SCM) has garnered attention recently due to the importance of decarbonizing the cement industry . For exa2 in the environment has increased in the last years, reaching a monthly mean CO2 concentration of 416 ppm in March 2021 [2 from the atmosphere is considered to be one of the aggressive substances for reinforced concrete, since it can diffuse through the pore network of concrete, dissolve in the pores and react with the portlandite or C–S–H. This phenomenon is known as carbonation [2, which are becoming increasingly common in our environment.The present research seeks to fill the gaps that exist in the use of CS as a SCM, by evaluating the mechanical performance and carbonation resistance of concrete. This last property is becoming more and more relevant due to the fact that the presence of COrch 2021 . The CO2bonation . This st3. This by-product was crushed and milled to be used as a supplementary cementitious material; the morphology and particle size distribution of the CS and cement are shown in 2O3: 55.58% and SiO2 20.43%). Hydraulic cement of general use of the company Argos, type GU according to ASTM C1157 , was useNatural river sand was used as fine aggregate and crushed gravel with a nominal maximum size of 12.7 mm was used as coarse aggregate. The physical characterization of aggregates is given in The concrete mixtures were designed according to ACI 211 guidelines. Four levels of replacement of CS by cement were made (including the reference concrete). The concrete specimens were cast using a revolving drum mixer. The mixing process remained constant in all the concrete mixes carried out. Before using the mixer, a manual homogenization of the fine material (cement–CS) was carried out for concretes that contained CS in their composition. First, the aggregates (coarse and fine) were mixed with approximately a quarter of the total water for two minutes. Then, the binder (cement only (reference mix) or cement with CS) was added with half the water and mixed for two minutes, without interrupting the mixing process, before the remaining water mixed with superplasticizer was added and mixed for two more minutes.2, 23 ± 2 °C, 65 ± 2% RH). At 28, 42, 56, 70, and 90 days of exposure, each cylinder was cut and the depth of carbonation was measured by applying phenolphthalein to the surface and the depth of carbonation was measured (six measurements were taken per specimen). The cylinders were returned to the carbonation chamber for continued accelerated CO2 exposure after resealing the top of the remaining cylinder.Four tests were performed on the concrete to study the influence of slag content in the mixtures. The slump flow test  was carrResults of the slump test of the four concrete mixtures are shown in The compressive strengths of all the mixtures at different ages of curing are shown in The water absorption, apparent density, and void result for 28 days of aging of the reference and concrete with CS are presented in 2, temperature of 23 ± 2 °C, and relative humidity of 65%. The depth of carbonation was determined using phenolphthalein, as shown in the upper images of All concretes exposed to accelerated carbonation conditions were previously cured for 28 days in water. After this period, specimens of each concrete were placed in the carbonation chamber with a concentration of 1.2% CO2 for 28, 56, and 90 days. At 28 days of exposure, there is a gain in compressive strength of 37.8%, 27.2%, 52.7%, and 48.3% of the reference, 10% CS, 20% CS, and 30% CS mixtures, respectively, compared to the compressive strength developed of the concretes before to the beginning of the exposure in the carbonation chamber. This behavior is attributed to the carbonation reaction of CO2 with Ca-rich minerals, especially portlandite [2 to penetrate the samples. However, at 90 days of exposure in the carbonation chamber of the different mixes, the compressive strength decreased when compared to that developed at 28 days of exposure to CO2. Despite this loss in compressive strength of the different concretes, the strength at 90 days of exposure of the different concretes is not lower than the strength of the mixtures at 28 days cured in water . The loss in compressive strength of the different mixtures at long periods of exposure to CO2 is attributed to the decomposition of calcium silicate hydrate (C–S–H) and the formation of micro-cracks [tlandite , as thiso-cracks . 3A and C4AF hydration products. From 200 °C to 450 °C corresponds to the dehydration of calcium hydroxide Ca(OH)2 [ Ca(OH)2 ,45. Comp Ca(OH)2 . After 2 Ca(OH)2 . 2 has when in contact with cementitious matrix materials. 3A with gypsum and the hydration of C3S and C2S, respectively, as well as the non-hydrated phase C2S, and quartz. After 28 days of accelerated exposure in the carbonation chamber, the CH peak becomes less intense due to the carbonation process and generates a higher amount of CaCO3. It is important to note that no diffraction peaks of the C–S–H gels were identified due to their glassy nature. 2S and C3S that generates C–S–H and portlandite. With respect to the carbonation process, it is evident that the 20% CS sample in the carbonation chamber (CO2) generates a higher amount of CaCO3, which can be generated from the CH and Ca of the C–S–H [The XRD patterns of the reference mix (100% cement) and 20% CS at 28 days of curing and 28 days after exposure in the carbonation chamber are shown in he C–S–H .In In this study, the effect of the use of copper slag (CS) on the slump, compressive strength, porosity, sorptivity, and carbonation resistance, and its effect on microstructure, were investigated. Based on the results, the following conclusions were drawn:The use of CS as a replacement for cement in concrete mixtures has been investigated, and it has been found that the workability of the concrete decreases as the percentage of CS increases. The irregular particle morphology of CS can affect the cohesion of the concrete, and the use of superplasticizers may be necessary to achieve similar consistency between concretes.The replacement of high percentages of CS (20% and 30%) by hydraulic cement generated a loss in the slump compared to the reference concrete (100% cement).The increase in CS in the concrete designs monotonically reduces the compressive strength, with the mix with 30% CS having the lowest strength. This behavior is attributed to the dilution effect generated by substituting CS for cement2 with Ca-rich hydration products.The concrete mixture with CS presented a greater susceptibility to carbonation, due to the lower alkaline reserve (presence of portlandite) attributed to the dilution effect, indicating that the CS used does not provide a good performance in the concrete mixtures in this environment. The use of CS generated a higher carbonation rate, which increased monotonically as a higher CS content was used. However, the incorporation of CS can improve the compressive strength of the concrete specially at prolonged curing ages, due to the denser microstructure generated by the carbonation reaction of COThe incorporation of CS in low percentages as a SCM (around 10%) improves flowability, enabling the production of concrete mixtures with a lower water–cement ratio while achieving the same consistency as a concrete mixture with 100% cement. This could compensate for the slight reduction in compressive strength at early ages, which is not very significant at longer curing ages.


Transverse myelitis is a rare but documented sequela of heroin use. While the underlying etiology is not clearly elucidated, the prevailing pathophysiologic mechanism amongst existing literature suggests an immune-mediated hypersensitivity reaction due to heroin insufflation following a long period of abstinence. Outcomes vary among the limited reports, but prognosis tends to be poor due to an acute and rapidly progressive disease course. Here, we describe a case of extensive transverse myelitis in a chronic heroin user following heroin insufflation. This report hopes to provide greater insight into the underlying cause of this rare phenomenon due to our patient’s discrepancy from the documented norm of heroin abstinence preceding disease onset. Transverse myelitis (TM) is a rare neurological condition characterized by inflammation and damage to the spinal cord with potentially severe long-term neurological morbidity and mortality. The etiology of TM varies, including a large spectrum of underlying causes including, but not limited to, drug/toxin, infectious, paraneoplastic, idiopathic, systemic autoimmune disorders, and acquired demyelinating diseases -2. QuickA 50-year-old man with a past medical history of heroin use and hypertension presented to the Emergency Department (ED) with acute onset right-sided focal weakness. The patient reported smoking heroin earlier that day and then napping for 4 hours. Shortly after waking, the patient noticed right-sided weakness with difficulty moving the right upper and lower extremities. The severity of the weakness became progressively worse, at which point the patient came to the ED. The symptom onset was roughly 5 hours after heroin insufflation. The patient denied any similar problems in the past and endorsed being a consistent heroin user. Past medical history was significant for consistent heroin use, hypertension, obesity, diabetes mellitus type 2, and hepatitis C. Vital signs were within normal limits. The initial physical exam showed weakness in the right upper and lower extremities with no sensory changes. Re-evaluation two hours after admission showed progressive weakness now spreading to the left extremities. Widespread loss of sensation and rapidly progressing paralysis soon occurred, ultimately requiring intubation due to decreased respiratory effort. Diagnosis of TM was confirmed with an MRI of the cervical spine revealing abnormal T2 signaling extending multiple levels down the central spinal cord tracts from C2-C7 with increased cord expansion suggestive of transverse myelitis Figure . This waNeurology was consulted for continued management. Our patient received a 5-day course of high-dose steroids with 1g IV methylprednisone. Unfortunately, his symptoms did not improve following high-dose steroids. During a neurological exam with the patient not on sedation, there was complete paralysis and loss of sensation in all extremities. The patient was able to blink and stick out his tongue in response to commands. Before intubation, the patient had a limited verbal response of grunting. The patient’s course was complicated by persistent rhabdomyolysis with elevated creatine kinase (CK) causing worsening renal failure, ultimately leading to the patient requiring hemodialysis. Eventually, a tracheostomy and percutaneous endoscopic gastrostomy tube were placed, given his continued ventilator dependency and inability to swallow. The patient was eventually discharged to a long-term acute care facility for extensive rehabilitation.To our knowledge, roughly 10 cases of heroin-induced TM are available in the scientific literature over approximately the last 50 years. Strong similarities have been noted between cases including most being secondary to heroin insufflation, disease onset following use after a period of abstinence, and the presence of rapid onset of symptoms often resulting in permanent neurological deficits and death. While high-dose steroids have been successful in halting progression in some patients, the prognosis is usually poor -9. A key difference in drug use history was present in our patient which may shed light on various theories as to why heroin-induced TM occurs. The greatest consistency amongst previous cases is heroin insufflation in a previous user following a period of non-use. This documented timeline strongly suggests an immune-mediated hypersensitivity reaction as being the underlying cause of heroin-induced TM. Ivanosvki et al clearly demonstrated this association by testing for opioid use through hair sampling to support a period of previous abstinence in their patient . ThroughMany different hypotheses have been proposed for the underlying mechanism behind heroin-induced TM, including vascular damage leading to ischemia, compression, direct/contaminant toxicity, and immunological effects from heroin or associated contaminants -8. WhileIschemia due to poor vascular supply has also been proposed as a possible mechanism ,7,10. CoOur report highlights the difficult diagnostic challenge of heroin-induced transverse myelitis and furthers our understanding of clinical features, diagnostics, treatment, and prognosis. As heroin-induced TM progresses rapidly, fast diagnosis and treatment are imperative. Obtaining a detailed patient history, thorough neurologic exam, and imaging is essential to diagnosis. Although there is limited efficacy, treatment with high-dose steroids is the recommended plan of action. Due to our case’s unique spontaneous onset of transverse myelitis during consistent heroin use, we would like to emphasize the impact this case report may have on the current understanding of the pathophysiology behind heroin-induced transverse myelitis as we compare autoimmune, vascular, and other etiologies.


Escherichia coli, Staphylococcus pseudintermedius, and Enterococcus faecium. Eight experimental beagle dogs were treated with ~5 mg/kg of nitrofurantoin macrocrystal PO 8qh for 7 days. The urine samples were collected via cystocentesis at 2, 4, and 6 h after administration on day 2 and day 7 and used to quantify nitrofurantoin concentrations by ultra-high performance liquid chromatography. The results showed that 26.13–315.87 μg/mL nitrofurantoin was detected in the dogs’ urine with a mean and median concentration of 104.82 and 92.75 μg/mL, respectively. Additionally, individual dogs presented with urinary nitrofurantoin concentrations greater than 64 μg/mL for at least 50% of the dosing intervals. This concentration efficiently killed E. coli, and S. pseudintermedius, but not E. faecium strains carrying an MIC90 value equal to 16, 16, and 128 μg/mL, respectively. Taken together, these results suggest that the value of 64 μg/mL may be set as a breakpoint against UTI pathogens, and nitrofurantoin could be an effective therapeutic drug against E. coli and S. pseudintermedius for canine UTIs.Nitrofurantoin, a broad-spectrum nitrofuran class antibiotic, is applied as a first-line antibiotic in treating human urinary tract infections (UTIs) due to its great efficacy and high achievable concentration. The interest in using this antibiotic in companion animals has increased due to the growing demand for effective antibiotics to treat UTIs caused by multidrug-resistant bacteria. Currently, the susceptibility interpretations for nitrofurantoin are based on the breakpoints set for humans, while the canine-specific breakpoints are still unavailable. In this study, we assessed the concentration of nitrofurantoin reaching the dog’s urine using the recommended oral dosing regimen. In addition, we examined the efficacy of this breakpoint concentration against the common canine UTI pathogens,  Escherichia coli as the most common among Gram-negative bacteria, and Staphylococcus pseudintermedius and Enterococcus sp. among Gram-positive bacteria (Lower urinary tract infections (UTIs) are one of the most common diseases in companion animals , 2. The bacteria . These ubacteria . Additiobacteria . Thus, iS. pseudintermedius (MRSP) and extended-spectrum beta-lactamase (ESBL) producing E. coli  determine the in vitro antibacterial activity of nitrofurantoin against these pathogens in canine urine; (4) assess the side effects of treatment.To assess the potential efficacy of nitrofurantoin in treating canine UTIs, our study objectives are to (1) measure the urinary concentrations of nitrofurantoin in dogs that can be achieved through a 7-day oral administration of macrocrystalline nitrofurantoin and establish a clinically relevant breakpoint; (2) define the MICTo establish achievable nitrofurantoin concentration in the urine of normal dogs, eight adult purpose-bred female spayed beagle dogs with a weight range of 8.75–10.9 kg and a median weight of 9.95 kg were included in the study . All dogThe minimum inhibitory concentration (MIC) breakpoints have been applied as a universal standard for susceptibility interpretation. To determine the potential MIC breakpoint for canine urine, nitrofurantoin concentrations in each dog were further analyzed. Traditionally, drug concentrations for time-dependent drugs should exceed the MIC of the organisms for at least 50% of the dosing interval . Thus, tThe concentration of native-form nitrofurantoin in each of the urine samples was measured by high-performance liquid chromatography (HPLC) , 25.2 = 0.9993). This validated technique met all procedural requirements in terms of specificity, sensitivity, and repeatability, thereby allowing precise and accurate quantification of nitrofurantoin in canine urine.To generate a standard curve, drug-free sterile  urine from one of the experimental beagle dogs was spiked with nitrofurantoin to create solutions containing final concentrations of the drug ranging from 0.05–300 μg/mL. A final concentration of 20 μg/mL of furazolidone was also included in each standard and sample as an internal standard. The linearity of the detection range (0.05–300 μg/mL) of nitrofurantoin was adequate  collected from a 4-year-old healthy female spayed German Shorthaired Pointer  containing 64 μg/mL of nitrofurantoin (the breakpoint established in the previous experiments). After 0, 4, 8, 16, and 24 h of incubation, the bacterial suspension was diluted by 10-fold serial dilutions and plated on LB agar in triplicate to determine the surviving bacterial numbers.Five isolates of Statistical analyses were conducted by using the STATA/IC software . For each dog, median and mean concentrations of nitrofurantoin were calculated.A linear regression model was constructed that accounted for the repeated measures of dogs to compare the concentration of nitrofurantoin on day 2 and day 7. Linear predictions for nitrofurantoin concentration and their 95% confidence intervals on day 2 and day 7 were calculated and illustrated in a graph.As shown in Additionally, we found that nitrofurantoin concentrations equal to or greater than 64 μg/mL were achieved in at least 50% of the dosing intervals in each dog . These rE. coli, S. pseudintermedius, and E. faecium. The MICs of nitrofurantoin ranged between 1 and 128 μg/mL for Escherichia coli, between 4 and 16 μg/mL for S. pseudintermedius, and between 32 and 512 μg/mL for E. faecium  of E. coli and 100% (108/108) of S. pseudintermedius carried a MIC less than the proposed breakpoint of 64 μg/mL for nitrofurantoin in dog urine. In contrast, E. faecium had a MIC90 of 128 μg/mL, and only 6% (6/106) of the isolates possessed a MIC less than the proposed breakpoint.To evaluate whether nitrofurantoin could be an effective antibiotic against common UTI pathogens in dogs, we also examined the MIC distribution of nitrofurantoin in at least 100 clinical isolates of each of the following:  faecium . The MICin vitro bacterial killing assay to mimic in vivo conditions. We used urine containing 64 μg/mL of nitrofurantoin and added the E. coli, S. pseudintermedius, or E. faecium strains carrying the MIC value equal to the MIC90. As shown in 10 reduction in bacterial numbers for E. coli and S. pseudintermedius was observed. For E. faecium, in contrast, the bacterial counts remained high in the urine  also efficiently killed E. coli and S. pseudintermedius strains carrying MICs equal to the species’ MIC90 in an in vitro cystitis simulation. Taken together, these results suggest that nitrofurantoin could be an effective antibiotic for these two common pathogens causing UTIs in dogs.This study assessed the urinary concentration of nitrofurantoin in experimental beagle dogs after administration of a ~5 mg/kg dose of nitrofurantoin orally, every 8 h for 7 days. Our results indicate that nitrofurantoin urinary concentrations exceed 64 μg/mL for greater than 50% of the dosing interval, suggesting that this concentration may be established as the breakpoint for treating susceptible urinary pathogens. The MICS. pseudintermedius , extended-spectrum β-lactamase-producing E. coli (2), MDR E. coli (1), Enterococcus spp. (1), MDR E. coli with MRSP (1), and E. coli with Enterococcus spp. (1)  . Nitrofuspp. (1) . In our Nitrofurantoin is commonly utilized as prophylaxis treatment for recurrent UTIs in humans . HoweverE. faecium isolates in our study and could not kill the organism after a prolonged incubation period of 24 h (90 of this drug for our isolates of this bacterial species was higher than the concentration used. These results indicate that nitrofurantoin should not be used empirically against E. faecium and only be used for treatment if the isolates are susceptible to nitrofurantoin. In contrast to E. faecium, our previous study showed no resistance in the other most common Enterococcal UTI pathogen, E. faecalis (Enterococcus isolate is important, as the species-associated resistance pattern may lead to distinctive nitrofurantoin treatment outcomes. In addition, studies have shown that some UTI pathogens can be resistant to this drug. Proteus, Pseudomonas, Serratia, and Morganella are known to be intrinsically resistant to nitrofurantoin (Klebsiella, Citrobacter, and Enterobacter (Our results showed that nitrofurantoin did not inhibit the growth of  of 24 h . The MICfaecalis . Thus, kurantoin . A high robacter . Based orobacter , 32. Thurobacter . AvoidinE. coli isolates showed resistance to nitrofurantoin (E. coli, this activation requires nitroreductase, NfsA, and NfsB (ribE, which is involved in the biosynthesis of the co-factor of NfsA and NfsB, could confer resistance to nitrofurantoin (OqxAB also contributes resistance by reducing the toxic intermediate inside the bacterial cytoplasm (Enterobacteriaceae (We showed that a small percentage of urantoin . To exerurantoin , 34. In and NfsB . Nonsensurantoin , 34. Theytoplasm . These reriaceae . Future E. coli and S. pseudintermedius induced UTIs in dogs. Administering macrocrystalline nitrofurantoin with a dosing regimen of ~5 μg/mL PO q8h, this antibiotic presented and maintained good concentrations in the dogs’ urine during a 7-day treatment course. A 64 μg/mL concentration of nitrofurantoin was detected in individual dogs’ urine for at least 50% of the dosing intervals. This concentration is considered to be the threshold with clinical efficacy in dog UTI cases, and can further be applied as the MIC breakpoint for the interpretation of nitrofurantoin susceptibility in canine urine specimens.Nitrofurantoin is a safe and effective antibiotic to treat The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.The animal study was reviewed and approved by Institutional Animal Care and Use Committee at the University of Illinois at Urbana-Champaign with protocol no. 20242.C-CH, JR, CV, CM, and LF: conceptualization. C-CH, JR, CV, CM, and AS: investigation. C-CH, CM, and RD: methodology. C-CH and CV: formal analysis and visualization. C-CH, CV, CM, and JR: writing-original draft. C-CH, JR, CV, CM, LF, RD, RF-G, NP, and AS: review and editing. C-CH, RF-G, NP, and AR: resources. C-CH: supervision and project administration. All authors contributed to the article and approved the submitted version.


The relationship between the macroscopic mechanical properties of lightweight aggregate concrete and its microstructure is a hot topic in the discipline of concrete materials. It is very meaningful to provide an efficient numerical analysis method to conduct a meso-level analysis. This study proposes an automatic dissection algorithm and adapts the calculation program of the base force element method to conduct a non-linear damage analysis. In the numerical simulation, three groups of 100 mm × 100 mm × 100 mm specimens were selected for the uniaxial compression experiment and uniaxial tensile experiment, respectively. The average tensile strength of the numerical simulation for the uniaxial compression test was 21.86 MPa. The stress–strain softening curve, stress contour plot, strain contour plot, and damage plot of the light aggregate concrete were also analyzed. These research results provide data for analyzing the failure mechanism of light aggregate concrete and reveal the failure mechanism of light aggregate concrete. At the same time, the reliability of the proposed algorithm is verified. Our aim is to provide a more efficient and accurate analysis of meso-damage in lightweight aggregate concrete, which benefits industries involved in production, construction, and structural engineering. Lightweight aggregate concrete, especially artificial lightweight aggregate concrete, has the characteristics of being lightweight and multi-functional as well as having a high level of strength. It is a green energy-saving building material as well as a wall material for the promotion and application of prefabricated buildings. At present, the mechanical properties and damage mechanism of lightweight aggregate concrete are hot topics in the engineering field, and the relationship between its meso-structure and macroscopic mechanical properties is also a cutting-edge research topic in the academic community . Many scIn addition, numerical analyses and analysis methods for lightweight aggregate concrete and other concretes have also been studied. For examples, Chen et al.  analyzedThe meso-damage analysis of many concrete materials is mostly carried out by conventional finite element software. The research of different analytical methods needs further exploration. The base force element method (BFEM) utilizes either the potential energy principle or the complementary energy principle as its foundation and employs base force elements to represent the behavior of intricate structures ,18,19. TMoreover, when it comes to the discussion of the methods of fracture mechanics at the meso level, the challenges associated with using recycled aggregates in concrete construction were addressed by proposing a novel equation to predict the shear strength of recycled aggregate concrete (RAC) Z push-off specimens. The aim of the study was to provide a reliable method for determining the shear strength of RAC structures. The proposed equation was developed based on experimental testing and an analysis, demonstrating improved accuracy compared to that of existing methods. This research enhances our understanding and practical applications of recycled aggregate concrete in structural engineering . In a seIn this study, a novel approach using the BFEM to analyze meso-damage in lightweight aggregate concrete is introduced. The objective of this study is to develop a random aggregate model for lightweight aggregate concrete and utilize the Delaunay triangulation method to create an algorithm for automatically subdividing layered meshes within the lightweight aggregate. Additionally, this study aims to develop a program for the automatic subdivision of lightweight aggregate concrete and adapt the calculation program of the base force element method to conduct a non-linear damage analysis to investigate the mechanical characteristics and failure mechanisms of this concrete variety. The significance of this study to related industries is noteworthy. By proposing an automatic dissection algorithm based on the Delaunay triangular mesh algorithm, this study offers an efficient solution for pre-processing in finite element calculations. This advancement can significantly improve the efficiency and accuracy of analyzing lightweight aggregate concrete, providing valuable insights into its mechanical behavior and failure mechanisms. The findings from this study can have practical implications for industries involved in lightweight aggregate concrete production, construction, and structural engineering, enabling them to optimize material properties, enhance structural designs, and ensure the durability and safety of concrete structures.In a Cartesian coordinate system, when a plane strain problem occurs, the stiffness matrix tion (1) :(1)KIJ=EThe main program algorithm for the two-dimensional potential energy method using the basic element force method is shown in Dmax represents the maximum particle size of the aggregate; and n of aggregate particles with sizes In this study, circular shapes were used to simulate lightweight aggregates in the numerical specimens of lightweight aggregate concrete. The grading curve of aggregates refers to a smooth curve plotted with the maximum size of sieves of different sizes through which particles with different particle sizes in the aggregate pass. In this study, the lightweight aggregates were all modeled using Fuller’s maximum density curve, also known as the Fuller grading curve, which can be used to describe the particle size distribution of aggregates, as expressed in Equation (4) .(4)P :(5)Pc{Xn+M is calculated according to Equation (8) [i.Determine the boundary and coordinate system based on the known dimensions of the lightweight aggregate concrete specimen.ii.nR and nE of the nth aggregate both falling within the range of , following a uniform distribution.Utilize the Monte Carlo method to acquire random numbers iii.Use these two random numbers to determine the coordinates for placing the aggregate b is the width of the specimen section, and h is the height of the specimen section.Equation (7) represents a recursive equation used for generating pseudo-random numbers uniformly distributed within the  range, where tion (8) .(8)M=2SAccording to the obtained coordinates, the bone center coordinates are used for the bone placement, ensuring that the lightweight aggregate and its surrounding old mortar and interface are within the range of the specimen’s side length. In order to prevent bone overlap, the distance between adjacent bone center points is set to be greater than 1.2 times the sum of the two bone diameters.When establishing a refined numerical model of concrete material, finite element meshing of the specimen is the first step. When using digital image technology for modeling, the mesh can be divided based on pixel points . HoweverReferring to the method of Peng et al. , this stIn 1934, the Russian mathematician Delaunay proved that there is only one triangulation method that maximizes the sum of the minimum interior angles of all triangles in a given region. Therefore, the Delaunay triangulation method avoids the appearance of pathological triangles as much as possible and is more suitable for the automatic meshing of finite element meshes. The Delaunay triangulation is the dual graph of the Voronoi diagram , and itsiV. For a given set X of N discrete points in a plane, X, iX whose closest distance is to point iX is called the Voronoi region iX. The Voronoi diagram is composed of a series of polygons ion (10) .(10)V, with x and y specifying the range of points in the x and y directions. X and Y are the resulting vectors of the point coordinates that meet the target spacing.Where [To produce scattered points in the circular aggregate and bond interface layer of the concrete, this article uses a self-made function ‘txyqd’ in MATLAB to produce a series of concentric circles inwardly from the circular aggregate with a certain radius difference and distribute points on each circle according to a certain interval to facilitate the subsequent formation of the mesh. Its usage is as in Equation (11) .x and y are the center coordinates of each concentric circle, r1 is the radius, and the variable ‘ms’ is the spacing value for point distribution in the concrete matrix. The output is the horizontal and vertical coordinates of the points placed on the circular circumference and the number of points on each circle, where sL = round(2*pi*r1/ms). It should be noted that the value of ‘ms’ should be suitable for the thickness of the bond interface layer. Otherwise, slender triangles may appear excessively.ii.Perform Delaunay triangulation.tri = Delaunay , where the row number of the returned value represents the element number of each triangle, and the three values in each row represent the node numbers of the vertices of each triangle.The Delaunay triangulation command is iii.Property determination.After meshing, the mesh is projected onto the random aggregate model region, and material properties are assigned to each element based on the relative positions of its nodes. The element type is determined by judging the relative positions of the nodes of the Delaunay triangles according to the following principles. An element is classified as a lightweight aggregate particle element if all three vertices of its Delaunay triangle are within the coordinate range of the lightweight aggregate particles. An element is classified as a cement mortar element if all three vertices of its Delaunay triangle are within the coordinate range of the cement mortar matrix. Otherwise, the element is classified as an interface element. The water–cement ratio is a key factor affecting the strength of concrete, mainly affecting the mechanical properties of the cement mortar and bond interface in concrete. The empirical equations for the water–cement ratio and cement mortar and bond interface are summarized by Watson , as showThe basic mechanical parameters of lightweight aggregates cannot be measured by standard test methods, such as those for conventional coarse aggregates, making the selection of mechanical parameters difficult. During compressive strength tests of lightweight aggregates, there are large gaps between particles, which are filled with cement mortar with a higher strength in actual lightweight aggregate concrete specimens. Therefore, the compressive strength of the lightweight aggregate particles themselves should be higher than the measured compressive strength. This study integrates ‘GBT 17431.1-2010 Lightweight Aggregates and their test methods Part 1: Lightweight Aggregates’ to determine the strength of lightweight aggregates in numerical simulations. The tensile strength of lightweight aggregate concrete is commonly estimated as approximately 1/10 times the value of its compressive strength based on empirical observations when specific test data are unavailable . The elaion (16) .(16)Ea=ferences ,28, the σ of the damaged material can be represented by utilizing the strain of the effective stress in the undamaged material. σ can be calculated according to Equation (17) [According to the strain equivalence principle proposed by Lemaitre , the nomion (17) .(17)σ=EE following damage can be determined based on the initial elastic modulus. E can be calculated according to Equation (18) [Assuming that damage does not impact the Poisson’s ratio, the elastic modulus ion (18) .(18)E=ETherefore, the damaged elastic modulus of the three-phase medium in lightweight aggregate concrete can be represented as Equation (19) (mortar (m), aggregate (a), and interfacial transition zone (ITZ)).η represents the coefficient of residual strain, which falls within the range of 1 to 5. ξ represents the ultimate strain coefficient, where ξ is greater than η. λ denotes the elastic strain coefficient, which takes values between 0 and 1. ꞵ represents the coefficient of elastic compressive strength, ranging from 0 to 1. γ represents the coefficient of residual compressive strength, and α represents the coefficient of residual tensile strength. The subscripts “t” and “c” signify the tension and compression states, respectively.Many scholars have concluded that the reason for the nonlinear macroscopic stress–strain curve of concrete under external loads is mainly the expansion of micro cracks ,18,19. TTo provide a more intuitive demonstration of the higher computational efficiency of the second meshing method proposed in this study, this section establishes a mesoscale model using the proposed two meshing methods. When performing finite element calculations, the two models corresponded completely in terms of size, random aggregate distribution, material parameters, constitutive models, boundary conditions, and loading methods.The stress–strain curves obtained using these two meshing methods are shown in From From λ = 0.1, and residual compressive strength coefficient of In order to verify the correctness of the program and algorithm developed in this study, this section takes a two-dimensional specimen with dimensions of 100 mm × 100 mm under both tension and compression as examples, respectively, and performs uniaxial displacement loading. A homogeneous material is used with the following microscale parameters: an elastic modulus of 30 GPa, tensile strength of 3 MPa, compressive strength of 30 MPa, Poisson’s ratio of 0.2, residual tensile strength coefficient of The comparative analysis between the numerical solution and the theoretical value is as follows:The calculated results are shown in It is noted that the stress–strain curves of the specimen obtained by the nonlinear BFEM program introduced in this study are consistent with the theoretical curves. The analysis result proves the correctness and effectiveness of the program and provides a reliable theoretical basis and simulation conditions for our research.The specimen used in this numerical simulation is a cubic specimen with a size of 100 mm × 100 mm × 100 mm, and the calculation model is simplified to a two-dimensional model with a cross-sectional size of 100 mm × 100 mm. The mesh size of mortar and aggregate is 1 mm. The thickness of the bonding interface in the microstructure calculation simulation of concrete is usually between 0.2 and 0.8 mm , and thiThe coarse aggregates used in the numerical simulation of this study were all between 5 and 15 mm. In order to make the aggregate grading in the model meet the maximum density curve proposed by Fuller , the agg3 and an element volume usage of 507 kg/m3. The proportion of the volume occupied by the coarse aggregate can be determined by Equation (24) [Consequently, in the experiment , the coaion (24) .(24)Pk=kP can be calculated as shown in Equations (25)–(27).In this numerical simulation, only coarse aggregate particles were used with a volume fraction of 40%. The above data are substituted into Equation (5)  for calcThen according to Equation (5), the number of According to the empirical equation, the material parameters and constitutive parameters selected for this numerical simulation experiment are shown in The three numerical specimens of lightweight aggregate concrete used in this simulation were obtained by selecting different random numbers for aggregate placement, as shown in This section utilizes a micro-damage calculation program based on the principle of potential energy and the BFEM. The numerical simulation of uniaxial compression tests was conducted using the three aforementioned randomly generated aggregate specimens, with the criterion for element failure determined by the maximum tensile strain. The calculation results of the three specimens and the experimental data are presented in Using periment , as showFrom After the peak stress point, weaker elements in the specimen start to suffer damage successively, cracks in the specimen expand and penetrate rapidly, and the stress value starts to decrease. Finally, the stress of the specimen tends to be a lower value.The stress–strain relationship curve obtained by numerical simulation in this study matches well with the experimental results , which dAdditionally, it is noted that the stress–strain curves obtained from the above three randomly generated specimens for uniaxial compression are basically the same, indicating that the stress of lightweight aggregate concrete under uniaxial compression is not significantly affected by the random distribution of aggregates.To analyze the stress characteristics and damage failure process of lightweight aggregate concrete under uniaxial compression, the calculation results are visualized. Using the graphics display module in the Fortran program, the damage and failure diagram of Specimen 1 under uniaxial compression is drawn. MATLAB-R2022a is used to plot the maximum principal strain contour plots and the maximum principal stress contour plots of the elements during the uniaxial compression process of lightweight aggregate concrete, as shown in i.Comparing ordinary concrete and recycled concrete ,18,19, tii.Initially, the stress and strain distributions in the specimen are relatively uniform. As loading continues, stress concentration phenomena start to appear due to the heterogeneity of lightweight aggregate concrete, for which the mechanical properties of each phase’s medium differ. This phenomenon of the stress concentration is the same as that of ordinary concrete and recycled concrete ,18,19. Wiii.As the displacement load continues to increase, the stress values of the lightweight aggregate particles and their upper and lower elements become larger, as indicated by the stress contour plots. Cracks in the specimen begin to expand and propagate along the darker-colored areas of the stress concentration zone in the contour plots. In the later stage of loading, the specimen is severely damaged, with numerous cracks appearing inside. These cracks release stress, reducing the stress concentration zone and making the overall stress distribution more uniform, corresponding to the descending segment of the stress–strain curve. This is same from the damage of ordinary concrete and recycled concrete ,18,19.iv.By analyzing the damage and failure diagrams and stress or strain contour plots, the complete process of crack appearance and propagation in the specimen during compression was obtained. The characteristics of the stress and strain distribution during compression correspond to the crack distribution in the damage and failure diagrams, validating the correctness of the damage and failure diagrams.Based on the damage and failure diagrams and the stress or strain contour plots, the following analysis can be made:In summary, the shape and characteristics of the stress–strain curve of lightweight aggregate concrete are the external manifestation of changes in the internal element structure. The macroscopic failure of lightweight aggregate concrete is the result of the gradual damage of the internal micro-cracks and the progressive failure of each phase’s material, eventually resulting in a loss of bearing capacity.In this numerical simulation, a two-dimensional cross-sectional size of 100 mm × 100 mm was used for the specimen. During loading, all nodes on the bottom edge were vertically constrained, and the center node on the bottom was horizontally and vertically constrained. The static displacement was loaded step by step along the vertical direction, and the displacement value of each loading step was The three numerical specimens of lightweight aggregate concrete used in this uniaxial tensile simulation are the same as those used in uniaxial compression.Using the micro-scale damage calculation program based on the principle of potential energy, this section conducts uniaxial tensile tests on the three randomly graded aggregate numerical specimens mentioned above and judges the failure of the element based on the maximum tensile strain criterion. The stress–strain curve of the numerical simulation is plotted with stress as the ordinate and strain as the abscissa, as shown in Using the micro-damage calculation program based on the principle of potential energy, this section conducts uniaxial tensile tests on the above three random aggregate numerical specimens and judges the failure of the element based on the maximum tensile strain criterion. The stress–strain curves of the numerical simulations are plotted with stress as the ordinate and strain as the abscissa, as shown in −6. The stress–strain curves of the three random aggregate specimens show a similar trend, and the rising section of the curve is approximately a straight line, indicating that the specimens are basically in an elastic state in the initial stage of loading and have not yet produced cracks.The average peak stress values of the three numerical specimens calculated are 2.05 MPa, and the corresponding strain values are 130 × 10As the displacement load increases, some weak elements in the middle of the specimen begin to get damaged, the rate of the increase in stress slows down, more and more elements are damaged successively, and the specimen enters the softening stage. Afterward, the stress decreases rapidly and eventually goes to zero, which align with the actual situation.Therefore, the numerical model established in this study and the failure criterion adopted can conduct a finite element calculation and analysis of lightweight aggregate concrete specimens. The method of the BFEM can describe their mechanical behavior under uniaxial tension and obtain the overall stress–strain curve. The results of the BFEM show the rising section, peak stress point, and descending section corresponding to damage and softening.Due to the difficulty of realizing uniaxial tensile tests, there are currently few comparable experimental data. The final failure mode of the lightweight aggregate concrete specimens obtained in this section is listed in Using the graphic display module in the Fortran program, the damage and failure diagram of the specimen under uniaxial tension was plotted using specimen 1 as an example. The maximum principal strain cloud map and the maximum principal stress contour plots of the elements during the uniaxial tension process of the lightweight aggregate concrete were drawn using MATLAB programming, as shown in i.During axial tension of the numerical specimen, as displacement is continuously applied, the local elements of the numerical simulation specimen begin to undergo damage and gradually develop cracks, which then extend to different locations and interconnect until the entire specimen is destroyed. From the shape of the crack extension, it can be seen that the cracks first appear inside the lightweight aggregate and are not uniformly distributed throughout the entire specimen, but are mainly concentrated at the upper and lower ends and middle part of the lightweight aggregate in the numerical specimen. When cracks appear in nearby lightweight aggregates, they penetrate the mortar and interfacial layers, eventually resulting in a main crack that penetrates the entire specimen to reach a failure state. This is consistent with the conclusions of previous studies ,3,4,5,6.ii.During the early stages of loading, the stress and strain distributions of the specimen are relatively uniform. With the increase in displacement load, the stress of the lightweight aggregate particles at the left and right ends of the specimen is much greater than that of other areas of the lightweight aggregate and cement mortar, and the stress concentration occurs in the same direction as the loading. In the later stages of loading, the damage to the specimen is more serious, and the cracks inside the specimen release stress, resulting in fewer concentrated stress regions and a more uniform stress distribution, which correspond to the descending segment of the stress–strain curve. This is the same as the damage of ordinary concrete and recycled concrete .iii.The extension of cracks during loading shows that the damage to the lightweight aggregate is most severe in the three-phase medium, resulting in the weakening of the local mechanical properties of the surrounding area of the lightweight aggregate, making it easier for the cement mortar in this area to undergo damage. A small amount of bonding interface also undergoes damage, but the cracks do not propagate along the bonding interface layer, but rather penetrate through the lightweight aggregate particles to form a main crack. This suggests that the presence of the bonding interface has little effect on the crack extension behavior of the lightweight aggregate concrete specimen, which significantly differs from that of ordinary aggregate and recycled aggregate concrete specimens .iv.The strain distribution during the numerical simulation of the tensile process is completely consistent with the crack distribution in the damage and failure map, which verifies the correctness of the damage and failure map.Combining the failure process diagram and the strain contour plots, the changes in the damage state of the specimen during uniaxial tensile loading can be analyzed to reveal the tensile failure behavior of lightweight aggregate concrete. The following observations can be made:In this study, an automatic dissection algorithm of lightweight aggregate based on the Delaunay triangular mesh algorithm is proposed. The damage analysis based on the BFEM and the corresponding calculation analysis program can numerically simulate the uniaxial tension and compression tests of lightweight aggregate concrete. Our self-developed post-processing software can obtain the stress–strain curve, micro-damage failure process diagram, maximum principal stress contour plots, and maximum principal strain cloud map during loading.i.The corresponding automatic dissection program can be developed by stratifying the internal area of the lightweight aggregate. This method can effectively improve the efficiency of pre-processing in finite element calculation.ii.The numerical analysis results in this paper are in good agreement with the experimental results. This proves the effectiveness of the proposed algorithm.iii.The study on lightweight aggregate concrete shows that the failure mode of lightweight aggregate concrete significantly differs from that of ordinary concrete. It is found that the light aggregate of light aggregate concrete is damaged first in the loading process, and the cracks are broken inside the light aggregate. This indicates that the light aggregate is a weaker area of light aggregate concrete.iv.The research results provide effective analysis methods and data for engineering applications of lightweight aggregate concrete materials.In the future, in-depth and systematic numerical research works will be carried out on the failure mechanism of three-point curved beams, the application of image processing technology, the mechanical properties of high-strength lightweight aggregate concrete, and the failure mechanism of fiber lightweight aggregate concrete.