The advent of new antibiotics has helped clinicians to control severe bacterial infections. Despite this, inappropriate and redundant use of antibiotics, inadequate diagnosis, and smart resistant mechanisms developed by pathogens sometimes lead to the failure of treatment strategies. The genotypic analysis of clinical samples revealed that the rapid spread of extended-spectrum β-lactamases (ESBLs) genes is one of the most common approaches acquired by bacterial pathogens to become resistant. The scenario compelled the researchers to prioritize the design and development of novel and effective therapeutic options. Nanotechnology has emerged as a plausible groundbreaking tool against resistant infectious pathogens. Numerous reports suggested that inorganic nanomaterials, specifically gold nanoparticles (AuNPs), have converted unresponsive antibiotics into potent ones against multi-drug resistant pathogenic strains. Interestingly, after almost two decades of exhaustive preclinical evaluations, AuNPs are gradually progressively moving ahead toward clinical evaluations. However, the mechanistic aspects of the antibacterial action of AuNPs remain an unsolved puzzle for the scientific fraternity. Thus, the review covers state-of-the-art investigations pertaining to the efficacy of AuNPs as a tool to overcome ESBLs acquired resistance, their applicability and toxicity perspectives, and the revelation of the most appropriate proposed mechanism of action. Conclusively, the trend suggested that antibiotic-loaded AuNPs could be developed into a promising interventional strategy to limit and overcome the concerns of antibiotic-resistance. Bacterial pathogens produce a class of enzymes known as extended-spectrum β-lactamases (ESBLs) that impart an enhanced resistance towards conventional antibiotics . They arThe COVID-19 pandemic has given a strong lesson to the entire world to never neglect an impending threat, whether it is small or large. Regrettably, the warning raised by eminent scientists and the WHO about antibiotic resistance has been neglected for decades. Indeed, scientists have speculated antibiotic resistance as the next major issue after the COVID pandemic, and the WHO has listed antimicrobial resistance as one of the three most significant health threats to the worldwide population in the twenty-first century. According to the WHO, most of the resistance issue arises due to the inappropriate application of antibiotics. Thus, a better therapeutic strategy with applicable antibiotic stewardship programs is urgently warranted. The failure of conventional antibiotics, either by ESBLs or by other resistance mechanisms, demands the quest for ameliorated therapeutic options. Moreover, treating infections with the lowest possible dose could be the best possible approach for effective infection control . InteresAuNPs have been applied to deliver a variety of antibiotics in the past, such as ampicillin, ceftriaxone, cefotaxime, vancomycin, cefoxitin, delafloxacin, ciprofloxacin, levofloxacin, cefixime, cefotetan, colistin, amoxicillin, kanamycin, imipenem, meropenem, cefaclor, daptomycin, streptomycin, gentamycin, rifampicin, penicillin G, polymyxin B, gatifloxacin, norfloxacin, cephalexin, cefadroxil, and cefradine. These studies exhibited that the antibacterial potential of antibiotic-coated/or -conjugated AuNPs was significantly enhanced in comparison to the antibiotics alone (without AuNPs) against the tested bacterial pathogens ,31,32,33However, the major bottlenecks in the clinical transformation of these findings are a lack of information on mechanistic and toxicity aspects. The present review aimed to cover all the viewpoints pertinent to the applicability of antibiotic-loaded AuNPs against ESBL- producing strains. Moreover, the review is subdivided into the following sections to provide a gist from the plethora of information without compromising the intellectual insight of the topic.Antibiotic-loaded AuNPs as magic bullets to overcome resistancePlausible antibacterial mechanism of AuNPsClinical translation status and Toxicity aspects of AuNP-based drug delivery systemFuture prospects of antibiotic-loaded AuNPsChallenges associated with antibiotic-loaded AuNPsAuNPs are inorganic nanomaterials that have been widely applied in different biomedical applications due to their unique attributes . In addiIn the past, sodium borohydride has been used as a reducing agent for the synthesis of ampicillin, vancomycin, streptomycin, neomycin, gentamicin, and kanamycin AuNPs. However, conjugation for ampicillin was through Au-S interaction, and the authors reported that AuNPs were initially stabilized via citrate molecules, which were further replaced by a thioether moiety of ampicillin by mixing the AuNPs with ampicillin for 24 h . In anotInterestingly, in some reports, antibiotics themselves acted as reducing as well as capping agents b, such aBefore moving forward, it is important to discuss the influence of the sizes and shapes of AuNPs on their antibacterial activity. AuNPs could be developed in different sizes and shapes ,54,55,56To attain a perfect size and shape, manipulation can be performed in experimental conditions such as pH, temperature, and concentration of reducing and capping agents. Low or acidic pH is usually considered unfavorable for AuNP synthesis, as it forms unstable and aggregating bigger AuNPs. A pH between 6 and 10 is considered most appropriate for AuNP synthesis . Some ofThere are numerous reports on the use of AuNPs for antibiotic delivery that are cumulated in S. aureus (MRSA), E. coli, and P. aeruginosa. Both the stated nanocarriers played an imperative role in delivering antibiotics to the bacteria. Approximately a decline of 10 times and 20 times was reported in the effective quantity of amoxicillin against P. aeruginosa by using Au–silica core–shell mesoporous and silica mesoporous NPs, respectively [S. aureus, E. faecalis, and E. faecium [However, the next section elaborates on some of the major findings on antibiotic-loaded AuNPs. The formulation of Au–silica core–shell mesoporous NPs along with silica mesoporous NPs in conjugation with amoxicillin were evaluated for the bactericidal potential against methicillin-resistant ectively . In anot faecium .S. aureus, E. coli, and M. luteus. MIC was markedly reduced in the case of aminoglycosides after coating to AuNPs. MIC value of streptomycin was reduced from 14 to 7 μg/mL and 22 to 17 μg/mL against E. coli and M. luteus, respectively, after loading onto AuNPs. However, MIC values of kanamycin were reduced from 30 to 12 μg/mL, 32.5 to 23 μg/mL, and 9 to 5.8 μg/mL against E. coli, M. luteus, and S. aureus, respectively. In fact, a marked decline of 60% was observed in the case of kanamycin-loaded AuNPs in comparison to kanamycin alone [E. coli [S. aureus, E. coli, and K. mobilis [The single-step process to synthesize AuNPs was applied in an investigation to avert the interference of reducing chemicals by using antibiotics  as reducing and capping agents. The MIC concentrations of antibiotics-capped AuNPs and free antibiotics were evaluated against in alone . Ampicil[E. coli . Additio mobilis . Recentl mobilis ,25,26,63Trichoderma viride) bio-synthesized AuNPs, and compared its activity with vancomycin alone against vancomycin-resistant S. aureus, S. aureus, and E. coli [E. coli, S. aureus, and vancomycin-resistant S. aureus, respectively. In another study, vancomycin-capped AuNPs were synthesized and explored for their activity against vancomycin-resistant strains . There is a significant decrease observed in MIC, i.e., shifted from 128 μg/mL to 2 μg/mL [Rosa damascenes petal extract was also reported for AuNPs biosynthesis, and the AuNPs were subsequently conjugated with ceftriaxone (Cef-AuNPs). The efficacies of these synthesized nanoparticles were further evaluated for their anticancer (against human breast cancer cells) and bactericidal effects (against ESBL-producing bacteria). It was found that Cef-AuNPs exhibited relatively low anticancer effects; however, conjugation of ceftriaxone on AuNPs restored the activity of ceftriaxone in otherwise resistant bacterial cells [Lactobacillus plantarum, and their subsequent functionalization with ceftriaxone, ciprofloxacin, cefotaxime, and levofloxacin. These AuNPs exhibited considerable bactericidal effects against multi-drug resistant S. aureus, E. coli, and K. pneumoniae. Among all nano-formulations, the ciprofloxacin-conjugated AuNPs were most active against E. coli and K. pneumoniae, whereas levofloxacin-conjugated AuNPs were potent against S. aureus [S. aureus (MIC: 0.562 μg/mL), ciprofloxacin-loaded AuNPs were most effective against K. pneumoniae (MIC: 0.281 μg/mL) and E. coli (MIC: 0.140 μg/mL), and ceftriaxone-loaded AuNPs were most potent against K. pneumoniae (MIC: 0.281 μg/mL) [Fayaz and team attached vancomycin to fungal  .E. coli and K. pneumoniae [E. coli and K. pneumoniae, respectively. Recently, Alafnan et al. also applied cefoxitin-loaded AuNPs against ESBL positive cefoxitin-resistant strains of E. coli and K. pneumoniae, and observed a prominent shift of MIC from 19.5 to 1.5 μg/mL and 23 μg/mL to 2.5 μg/mL against E. coli and K. pneumoniae, respectively [Most of the antibiotics discussed above belong to the β-lactams family, and can be correlated with ESBL-associated resistance, as shown in eumoniae . Importaectively .All these interesting reports confirmed the potency of AuNPs as a tool to combat bacterial resistance, and prompted the deciphering of the plausible mechanism of action of these antibiotic-loaded AuNPs. However, this section mainly pinpointed that antibiotics, once loaded or attached to AuNPs, become significantly active against the bacterial pathogen that was earlier resistant to the same antibiotic. This was evident from a marked decrease in MIC values against resistant pathogens. In some reports, bacterial pathogens were completely resistant at the tested concentration of antibiotic; however, once the antibiotics loaded on AuNPs at the same concentration become resuscitated, they become potent antibacterials. In addition, the methods applied for the synthesis of these nano-antibiotics were quite simple and convenient. Thus, it could be stated that the nano-conversion of antibiotics through AuNPs could revive ineffective antibiotics into potent ones.Garcinia mangostana extract was used to bio-synthesize AuNPs, which was further conjugated with antibiotics. Biosynthesized AuNPs (without any antibiotic) did not show any significant antibacterial activity against Pseudomonas spp. and Staphylococcus spp. However, AuNPs conjugated with streptomycin and azithromycin exhibited enhanced bactericidal efficacy  against Staphylococcus spp., as compared to free streptomycin and azithromycin. On the other hand, the conjugation of azithromycin and penicillin with AuNPs augmented their bactericidal efficacy  against Pseudomonas spp. [The ability of AuNPs to interact with various biomolecules of the bacterial cell provides an added advantage to the antibiotic that is loaded onto them. In simple terms, AuNPs not only ease the entry of antibiotics into the bacterial pathogen, but also provide synergistic effects. There are several investigations that prove the synergism concept. In one study, nas spp. . Furthernas spp. . AuNPs cThere are four major mechanisms followed by bacterial pathogens to acquire resistance: i.e., reducing the uptake of antibiotics, effluxing antibiotics once they enter, antibiotics inactivation, and modifying antibiotic targets . ESBL enE. coli and K. pneumoniae. In addition, the authors investigated the role of porins (outer membrane proteins) in these ESBL-positive strains. The porin analysis confirmed that about 95.7% and 93.3% of strains of K. pneumoniae and E. coli either modified or lost their porins. Moreover, the genetic analysis revealed the reason as mutation (frameshift), which eventually translated small-size truncated porin proteins [K. pneumoniae and E. coli, and cephalosporins (ceftazidime and cefoxitin) resistance was strongly correlated with the loss of type Omp-K35 porins [K. pneumoniae. Interestingly, the loss of both these proteins was involved in the resistance of β-lactam antibiotics  as well as a non-β-lactam antibiotic (fluoroquinolones) [Usually, the β-lactam antibiotics enter bacterial pathogens through the porin channels ,75,76,77proteins . In a si5 porins . In 2008nolones) . Here, tE. coli. They observed that free DAPT is devoid of any antibacterial effects; however, when E. coli was exposed to DAPT-AuNPs, approximately 70% of the E. coli cells exhibited enhanced membrane permeability. Contrastingly, only 5% of the E. coli cells were found to be positive for such alteration of permeability in the control group. In addition, exposure to DAPT-AuNPs led to similar effects on P. aeruginosa. Moreover, it was observed that DAPT-AuNPs also facilitated the formation of vesicles from the outer membrane of bacterial cells. Eventually, the investigators concluded that DAPT-AuNPs exerted their bactericidal effects by altering the concentration of magnesium ions and subsequently chelating them [B. subtilis and E. coli [E. coli; however, when added to AuNPs, it showed a promising antibacterial effect [Tailored AuNPs have the ability to disrupt the cell membrane integrity and permeability. AuNPs could initially interact with the outer membrane proteins and lipopolysaccharides, and be deposited on the cell surface of the pathogens ,81. Furting them ,86. Anot E. coli . In one l effect . The autK. pneumoniae, observed the overexpression of efflux pumps (belonging to the RND family) in the tested strains. They concluded that ESBLs were not the only factor against β-lactam antibiotics; efflux pumps also effectively participate in developing resistance. Another investigation also confirmed these findings by applying an efflux pump inhibitor on an ESBL-positive strain. MIC values for cefotaxime, ceftazidime, and amoxy-clavulanic acid were markedly reduced after using the efflux pump inhibitor on resistant K. pneumoniae [Pseudomonas aeruginosa. They elucidated that AuNPs could reduce the expression of several efflux pumps on the bacterial cell surface by downregulating MexA and MexB efflux pump genes. On the other hand, Khare and his team [P. aeruginosa and E. coli.Once the antibiotic enters the bacterial cells, they use efflux pumps to reduce the antibiotic load. Interestingly, efflux pumps have shown a correlation to ESBL-producing strains . Maurya eumoniae . It is neumoniae  observedhis team  observedESBL enzymes are known for the inactivation of β-lactam antibiotics. Once β-lactam antibiotics enter the ESBL-producing pathogens, ESBL enzymes disarm them by hydrolyzing the β-lactam ring. There are several reports that showed β-lactam antibiotics, once attached to AuNPs, become potent in ESBL-producing resistant strains . HoweverE. coli, and showed an apoptotic-like cell death mechanism [E. coli that eventually induces ROS and SOS responses, leading to bacterial apoptosis [Apart from AuNPs’ interaction with bacterial enzymes/proteins, they have the ability to directly interact with bacterial DNA as well. AuNPs have been reported to enhance bacterial DNA fragmentation by 19.68% in echanism ,100,101.poptosis ,102. ConThis section deals with the mechanistic aspect of the antibacterial action of antibiotic-loaded AuNPs. The reports revealed that AuNPs not only deliver the antibiotic successfully to the resistant pathogen, but work in synergy with the antibiotic to increase the antibacterial effect. In addition, after the loading of antibiotics onto the AuNPs, the resistance mechanism of the pathogens could be overcome in various ways. In fact, AuNPs could work smartly on each antibiotic resistance mechanism of bacterial pathogens to provide the maximum antibacterial effect. Bacterial pathogens have the ability to develop resistance against the defined mechanism of action of any antimicrobial agent with time; thus, the multi-targeting ability of AuNPs or undefined mechanism of action could be considered a blessing in disguise. The present review tried to propose the best plausible mechanism of action of AuNPs (or antibiotic-loaded AuNPs) based on previous investigations. However, the work on the exact mechanism of action of antibiotic-loaded AuNPs is still obscure and needs to be deciphered.® system has already obtained approval and is used for genotyping different genes in DNA samples [Before discussing the clinical translation status, glimpses of some recent global market trends about AuNPs need to be shared. The global market of AuNPs has reached about USD 4.4 billion in the year 2021, and is further expected to reach around USD 8 billion by the year 2027 ,104. Sev samples . There a samples . Althoug2PC6H4SO3Na and P(C6H4SO3Na)3, showed cytotoxic effects against different cells. It was further shown that smaller AuNPs exhibited approximately 60-fold enhanced cytotoxic effects compared to 15 nm size AuNPs. During an extended study, the same research group demonstrated that AuNPs capped with triphenylphosphine mono-sulfonate (size of 1.4 nm) induced the production of reactive oxygen species (ROS), which subsequently augmented inflammatory mediators resulting in the onset of mitochondria-mediated apoptotic cell death in human cervical carcinoma HeLa cells [The scientific literature surveyed in the present review indicated that in the majority of investigations, AuNPs were found to be relatively less toxic chemically. Nevertheless, certain reports have also outlined the potential toxicity of these AuNPs, which is critically governed by the charge, shape, surface chemistry, and size of the synthesized AuNPs. A previously published report elucidated that AuNPs conjugated with ligands, namely PhLa cells . Indeed,La cells .Studies by Mironava et al. made it evident that AuNPs having a radius of 13 nm–45 nm fail to enter either the mitochondria or nucleus of human-derived dermal fibroblast cells. Furthermore, their report also indicated that AuNPs within the stated range are localized within the cytoplasmic vacuoles, and AuNPs of around 45 nm instigated increased cellular damage owing to their different uptake and release mechanisms in the cytoplasm . ContrasInterestingly, some literature has outlined the importance of AuNPs’ size and charge in regulating their biodistribution and absorption in different animal models. Several studies concluded that small AuNPs with a negative charge exhibit the highest absorption rate, and thus have distribution in an array of organs within the animals ,114,115.Drosophila melanogaster, thereby indicating that mutagenic effects of AuNPs can be further transmitted to progenies [It is to be noted that AuNPs genotoxicity has been evaluated in Zebra fish models, and administration of positively charged N,N,N-trimethylammoniumethane thiol stabilized AuNPs instigated disruption in development and pigmentation of the eyes that was concomitantly followed by impaired behavioral attributes with distinctive neuronal damage . Moreoverogenies ,122. It This section summarizes the clinical translation status and toxicity aspects of AuNPs. Companies are investing in AuNPs for different applications, and clinical trials are going- on in different phases. Most of the clinical trials have been conducted on fabricated AuNPs against non-infectious diseases, and some are conducted for viral infections. To date, antibiotic-loaded AuNPs have not reached any of the phases of clinical trials. Nevertheless, the results of fabricated AuNPs clinical trials have provided some strong optimism for AuNP-based therapy. Toxicity, biodistribution, and targeting are major obstacles in any drug development process. It is evident from the reports that AuNPs toxicity and distribution majorly depend on the fabrication or modification of the surface. However, antibiotic-loaded AuNPs have not been thoroughly studied for toxicity and biodistribution. Thus, the toxicity of antibiotic-loaded AuNPs could be considered one of the translational gaps in AuNP-based nano-antibiotic development.Till now, this review has substantially compiled and discussed the published research and progress pertinent to antibiotic-loaded AuNPs. However, the question still remains the same: ‘Could AuNPs be clinically applied to tackle the superbugs threatening our life?’. The answer to this question will not just be confined to simply saying yes or no. It has been clearly discussed in the above sections that AuNPs are versatile in their ways of interactions with bacteria and their cellular components, and further, their control over their resistance mechanisms. Interestingly, there are various reports suggesting AuNPs’ potential as an efficient tool to deliver antibiotics to the pathogen and significantly reduce its effective MIC concentration. However, the mode of action of antibiotic-loaded AuNPs still needs to be fully deciphered and validated. In addition, a substantial focus has to be shifted to deciphering the safety of AuNP-based antibiotic delivery systems. In fact, long-term in vitro and in vivo biosafety experiments have to be designed to translate and guide the successful transformation of research findings into clinical applications. Standardization of experimentations is another area that requires significant improvement in order to perform comparative studies. Nevertheless, it is also a fact that AuNPs are presenting new hope in the current scenario, where clinicians have become helpless against resistant bacterial infections due to the lack of availability of effective therapeutic options. It is strongly anticipated that AuNP-based therapeutic options against antibiotic-resistant pathogens have the capability to overcome resistance from society if utilized in an effective manner.It is quite clear from the earlier sections that AuNPs could be easily fabricated and loaded with the desired antibiotic(s) by various approaches. In addition, their complete characterization before and after the loading of antibiotics has become convenient nowadays. Moreover, there is a plethora of research that confirmed antibiotic-loaded AuNPs as smart and effective antibacterial agents against resistant pathogens. However, there are still certain challenges that have to be overcome before considering them as the best arsenal against resistant pathogens. The major challenges are as follows: (1) the safety of antibiotic-loaded AuNPs; (2) large-scale production and processing; and (3) cost-effectiveness over antibiotic treatment.There are several clinical trials going on to evaluate the potential of AuNP-based therapy, but none of them are on antibiotic-loaded AuNPs. The safety profile of antibiotic-loaded AuNPs is not fully investigated, and animal model evaluation studies are still undergoing. More biodistribution and pharmacokinetics studies on these antibiotic-loaded AuNPs have to be designed, as it has been observed that AuNPs have a tendency to aggregate at the site of inoculation. In addition, mistargeting is one of the aspects linked with nanoparticles; hence, this point also needs to be considered while designing antibiotic-loaded AuNPs.The scenario of large-scale production of nanomedicine is quite different from the laboratory level. Large-scale production requires cost-effective approaches, large equipment, skilled technicians, and a quality control/assurance unit. Although many big industries have started investing in AuNPs for various applications in the last decade, pharma giants also need to step up and show interest in the development of nano-antibiotics from antibiotic-loaded AuNPs. In addition, different approaches for the conversion of these AuNPs from lab scale to industrial scale in the best possible way have to be developed.The most important question is the cost effectiveness of AuNP-based antibiotics compared to available antibiotic options. AuNP development is itself a costly affair; hence, ways need to be developed to reduce the cost of AuNP-based nano-antibiotics formulation.Earlier, Zhang et al.  nicely rThe advent of nanotechnology has seen a surge in its advancement and usage to cater to the growing need for clinical interventions, primarily in cases of infectious diseases due to the increasing global prevalence of antibiotic resistance. Undoubtedly, conjugation/coating of several antibiotics onto AuNPs has demonstrated elevated antibacterial efficacy against resistant bacterial pathogens. Indeed, simple alteration in the attributes of AuNPs has allowed the investigators to develop novel antibacterial formulations that could plausibly be applied in clinical settings. The mechanistic aspect of the antibacterial action of AuNPs revealed that AuNPs could help to mitigate each and every perspective of antibacterial resistance. Importantly, preclinical findings on AuNPs are gradually moving towards clinical evaluations, and R&D sections of big industries are showing keen interest in these updates. Still, mechanistic details of its action, its fate inside the human body, and biosafety are the segments that need special attention. However, AuNPs are one of the most promising hopes against antibiotic resistance that require proper exploration to be translated from bench to bedside.


This paper reports on the flexural behavior of nine steel fiber reinforced-recycled aggregate concrete (SFRAC) beams through combined experimental and finite element analysis. The test parameters in this study include the steel fiber volume fraction, recycled aggregate replacement ratio, and concrete strength. The failure modes, crack development, load-deflection curves, and flexural bearing capacity of SFRAC beams are investigated in detail. The test results indicated that cracks and concrete crushing are formed in the compression zone of all specimens. The flexural bearing capacity of SFRAC beams increases with the increase of steel fiber volume fraction and concrete strength and the decrease of recycled aggregate replacement ratio. In addition, the results are compared with those of the finite element analysis. Based on the uniaxial compressive constitutive model of SFRAC, a new model for calculating the flexural bearing capacity of SFRAC beams is proposed. The prediction and test results are compared to evaluate the accuracy of the developed formula. The studies may provide a considerable reference for designing this type of structure in engineering practice. In comparison with natural aggregates, recycled aggregates reduce carbon emissions by 23–28%.Recycled Aggregate Concrete (RAC) is a new green building material that is made by partially or entirely substituting natural aggregates with recycled aggregates. Recycled aggregates are acquired by processing waste concrete with a high building waste content. The development of the construction industry has led to a large reduction of natural sand and gravel and the consumption of natural resources. The use of recycled concrete mitigates this situation . RAC hasHowever, compared with the natural coarse aggregate, the recycled aggregate will produce internal damage during the crushing process. With more significant porosity and crushing index, the old cement base usually wraps its surface. Therefore, there is a big gap between the performance of RAC and ordinary concrete. Existing studies show that the compressive strength, tensile strength, and flexural bearing capacity of RAC are lower compared to that of plain concrete with natural aggregates. Moreover, as the recycled aggregate replacement ratio increases, the macro mechanical properties of RAC decrease accordingly ,8,9. SimAlthough RAC itself has certain defects, existing studies have proved that the defects of recycled aggregate can be compensated to some extent by adding steel fibers. Adding steel fibers to recycled aggregates can improve the mechanical properties of RAC and the force and deformation properties of RAC structures ,18,19,20At present, there are relatively more studies on the mechanical properties of SFRAC and relatively fewer studies on the properties of SFRAC members. From the existing research results at home and abroad, the flexural bearing capacity of SFRAC beams is improved, the stiffness is increased, the deflection becomes smaller, and the crack resistance is improved compared with that of RAC beams. The force performance and reliability can reach the level of ordinary concrete beams , and the2, equivalent diameter of 63) were used to reinforce concrete beams. Polycarboxylic acid water reducer with a water-reducing rate of 25% was used to adjust the workability of concrete mixtures. Steel bars with diameters of 8 and 16 mm were used as longitudinal steel bars. The stirrup uses a steel bar with a diameter of 6 mm and a spacing of 150 mm. The mechanical properties of the steel bars are presented in Ordinary Portland cement (P.O 42.5) in terms of Chinese Standard GB 175-2007  was used3 with a clear span of 2700 mm. Steel bar 1 was used for the stirrup and erection bar in the compression zone, and steel bar 2 was used for the tensile longitudinal steel bar at the bottom, as shown in A total of nine SFRAC beams were designed to investigate the flexural behavior with the steel fiber volume fraction , recycled aggregate replacement ratio , and concrete strength  as test parameters. All beams were with the same dimensions of 150 × 300 × 3000 mmThe mixture proportions of SFRAC are summarized in All specimens were tested for flexural behavior according to Chinese Standard GB/T50152-2012 . The tesTo investigate the effect of the steel fiber volume fraction, concrete strength, and recycled aggregate replacement ratio factors on the flexural behavior of SFRAC beams, the flexural behavior tests on eight SFRAC beams were simulated see . The modc), the stress ratio (fb0/fc0), the eccentricity (ε), the viscosity (μ), and the dilation angle (ψ). According to the recommendation of ABAQUS (2014), these five parameters are 0.667, 1.16, 0.1, 0.005, and 30, respectively. The sensitivity of viscosity parameters was analyzed in the finite element analysis and the most appropriate μ value was selected to balance the accuracy and efficiency of the calculation. The value of the viscosity parameter is taken as 0.005 based on numerous simulations performed to investigate the effect of this parameter on the results. For the main steel bar, an ideal elastic-plastic bilinear model was employed.The concrete damage plastic (CDP) model available in ABAQUS was used to define the plastic properties of concrete. In addition to isotropic tensile and compressive plasticity, isotropic damage elasticity is also used to characterize the overall performance of concrete. The constitutive model of concrete under uniaxial compression stress–strain based on the relationship of SFRAC concrete in Gao et al.  is repreIn summary, the FE modeling approach, material constitutive model and boundary condition, as mentioned above, are proved to be reasonable and can be used to further investigate the flexural performance of SFRAC beams.The concrete in the compression zone (top) of the beams was crushed until the beam failed. Generally, the neutral axis depth kept reducing following crack development upon load application, so that a corresponding reduction of the compression zone. The steel bars yielded first, followed by a localized crushing of the concrete in the compression zone, as indicated in The variation law of the load-midspan deflection is basically the same for each beam. It can be seen from As can be seen from The addition of steel fiber can prevent crack development, increase strength, and increase toughness. When steel fiber volume fraction is 1.0% and 2.0%, the flexural bearing capacity of the SFRAC beams is 19.5% and 24.8% higher than that of RAC beams, respectively. It shows that under the same conditions, the flexural bearing capacity of SFRAC beams increased with the increase of steel fiber volume fraction. Moreover, the flexural bearing capacity of the initial crack increased with the increase of the ultimate compressive strain of concrete in the compression zone. The study also found that under the same conditions, the deflection of test beams decreased with the increase of steel fiber volume fraction. The reason is that steel fibers in the tensile zone after cracking can continue to bear part of stress, thereby preventing crack development and increasing the cracked section’s stiffness. To sum up, it can be seen that the addition of steel fibers increased the ultimate compressive strain of RAC, increased the toughness of RAC, and effectively reduced the deformation of the test beams and the development of cracks.The flexural bearing capacity of the normal section of SFRAC beams decreased with the recycled aggregate replacement ratio increased. The reduction of flexural bearing capacity of SFRAC beams is all smaller than that of steel fiber concrete beams with the same concrete strength and larger than that of plain concrete beams with the same concrete strength, as shown in The flat section assumption is generally applicable to continuous homogeneous elastic members. SFRAC beams are characterized by non-homogeneous material and the presence of cracks, and strictly speaking, the flat section assumption does not apply at localized locations of damage. The measured mean strain values for the midspan section are shown in Ordinary concrete beams generally do not consider the role of concrete in the tensile zone. The steel fibers in SFRAC make it possible for tensile stresses to remain in the specimen after cracking, which effect on the member’s breaking moment cannot be ignored. The steel fibers in the SFRAC provide tensile stress, which influences the ultimate moment of the specimen and cannot be ignored.The stress–strain relationship of reinforced concrete structural materials is shown in Stress–strain relationships for steel bar:In the existing literature, the equivalent rectangular stress graph method calculates the normal section flexural bearing capacity of SFRAC members. This method needs to determine two characteristic parameters. They are consistent, but the compressive stress and strain curves of concrete adopted by the national codes are the same. The method in this paper directly adopts the stress and strain curve instead of the equivalent rectangular stress graph method, which avoids the problem of determining the characteristic parameters by experiments. It provides a new calculation method for the design of the normal section of the beam. Stress and strain distributions of the strengthened beams under flexural failure is as shown in the The compressive stress–strain curve of SFRAC is expressed as:Simplify Equation (7) as:x, it is known from the theory of elasticity that the strain in the compression zone is:Assuming that the height of the concrete in the compression zone is 010-2010 :(11)εcu=From the section static equilibrium conditions, it follows that:Substituting Equations (9) and (10) into Equations (12) and (13):The formula for the flexural behavior capacity of a normal section of a flexural beam when damage to a reinforced beam is reached:The ultimate flexural bearing capacity of concrete beams reinforced with steel fibers depends to a large extent on their mode of destruction. ACI 544.4R-88  providesCalculation formula based on the Chinese code for the structural design of steel fiber concrete(JTG/T 465-2019) :(23)Mf=fAccording to the calculation formula of SFRAC normal section flexural bearing capacity proposed in this paper and ACI 544.4R-88  and the The stressing process of SFRAC beams is similar to that of ordinary concrete beams, which undergoes the elastic stage, the working stage with cracks, and the damage stage. However, the slope of the curve cannot change significantly after cracking due to the inhibition of the development of concrete cracks by the incorporation of steel fibers. With the increase of steel fiber volume fraction, crack development is inhibited so that the ductility of SFRAC beams is well improved.The flexural bearing capacity of SFRAC beams increases with the increase of steel fiber volume fraction and concrete strength, while the decrease of recycled aggregate replacement ratio. The flexural bearing capacity of the SFRAC beams is greater than that of the normal concrete beams and recycled concrete beam with the same concrete strength. FE results are established and verified by the experimental results. Reasonable agreement is achieved between experimental and corresponding FE results. It demonstrated that the FEA model can fairly predict the flexural behavior of the SFRAC beams, including the failure modes and crack development.A model for calculating the flexural bearing capacity of SFRAC beams is established with consideration of the influence of steel fiber volume fraction and recycled aggregate replacement ratio. What is more, this calculated model directly adopts the stress and strain curve instead of the equivalent rectangular stress graph method, which avoids the problem of determining the characteristic parameters by experiments. The predicted results exhibited good agreement with experimental results. Due to the limitation of time and the author’s level, this paper only studied the flexural behavior of SFRAC beams under static load, and further research is needed to study the flexural behavior and establish the analytical model of SFRAC beams under fatigue load.This paper aims to investigate and evaluate the flexural behavior of SFRAC beams through combined experimental, numerical, and theoretical studies in terms of the failure modes, crack development, load-deflection curves, and flexural bearing capacity. The parametric analysis is also conducted. Based on the limited results from the current study, the following conclusions can be drawn:


Anchorage to concrete is a recurring application in construction. For such applications, bonded anchors, formed by means of a polymer adhesive injection into a borehole, are a widely used product due to their flexibility in regards to the construction logistics and positioning of the attached element as well as high load capacities. At the same time, fibre-reinforced concrete is the material of choice for many engineering applications where anchors have to be installed. Moreover, the use of steel fibre-reinforced concrete is likely to increase, since it now falls in the scope of the second-generation Eurocode 2 (exp. 2023). Therefore, the condition of the anchor installation borehole—mainly the roughness and grip of its internal surface—is known to play a critical role in the stress transfer from the attached component, through the fastening and into the concrete, and, hence, to the load-bearing performance. At the same time, drilling through the steel fibre reinforcement, along with the accelerated wear of the drilling tools, can in turn influence the borehole’s roughness and the overall installation quality. Furthermore, steel fibre may lead to an additional local stiffening of the concrete where the anchor is installed. These complex elements are discussed herein on the basis of multiple tests on anchors in plain and steel fibre concrete, as well as numerical analyses. The results indicate particular aspects of bonded anchor design and product certification for different polymer-based construction adhesives. Steel fibre-reinforced concrete (SFRC) is currently the material of choice for a broad range of structures, such as industrial floors , prefabricated elements, thin shells, segmental and sprayed as well as cast final tunnel linings, special foundations and slabs on grade, watertight and containment structures , and protection and defence structures. Furthermore, its use has also been confirmed for many commonplace concrete component design situations, replacing the entire or a large portion of conventional rebar reinforcements in order to improve their load-bearing behaviour, but also their serviceability and durability characteristics . ThFollowing the extended use of SFRC, modern construction relies heavily on structural connection and assembly technologies. Prominent connection detail relies on post-installed, bonded anchors, which have been the focus of investigations in relation to various substrate materials and loading situations ,6,7,8,9.The focus of this paper lies in the influence of steel fibres in concrete with respect to the installation quality and the load-bearing performance of post-installed bonded anchors. To that end, the intersections of the drill hole surface and the steel fibres are qualitatively presented and pull-out tests with varying installation qualities and bonding polymer-based mortars are carried out. An overview of the engineering and design principles of bonded anchors in general and a brief presentation of previous studies for bonded anchors in SFRC are provided, disclosing the state-of-the-art and open questions handled by this study. The set-up and parameters of the tests on bonded anchors in plain concrete and SFRC by means of polymer mortars are reported and their results are discussed. Finally, the findings are summarised with the intention of contributing to practical engineering design and constructability issues. Rk as described by Equation (1) (assuming short-term quasistatic but not sustained loading). In Equation (1), d, efh are the nominal anchor diameter and embedment depth, respectively, in accordance with the uniform bond stress model for bonded anchors in uncracked concrete, as proposed in [The design and specification of bonded anchors, as with all post-installed fastenings, are primarily based on the “European Technical Product Specification” (ETPS) for each product. For concrete fastening products, the ETPS is the “European Technical Assessment” (ETA), which was prepared on the basis of a “European Assessment Document” (EAD). The EAD typically prescribes a testing campaign for a product’s assessment. The ETPS also confirms the “Declaration of Performance” through the product manufacturer and allows for the product’s CE marking. The “European Organisation for Technical Assessment” (EOTA) drives this standardisation procedure by endorsing and publishing the assessment principles and by coordinating the organisations carrying out the assessments . A similar procedure is followed in the U.S. with the International Code Council (ICC) and the Evaluation Service Reports (ESR). Once the product’s applicability is confirmed by observing the respective specification document, the design can be carried out based on a design norm . For theposed in , which sposed in , but alsposed in , moreoveposed in . In ordeposed in ,18). Morposed in , a propoposed in .(1)NRk,Focusing on sustained loads, a significant load-performance characteristic of bonded anchors is the point of the “loss of adhesion” on load-displacement curves from short-term quasistatic confined tests. From this point on, the slippage is not controlled by the mortar mechanical and bond properties, and the load-bearing mechanism begins to rely on friction and geometrical irregularities of the borehole. This is typically characterised by an abrupt and significant change in the load-displacement curve. The respective displacement is also set as the limit of long-term displacements to determine the maximum allowable displacements throughout a bonded anchor service life for various assessment approaches ,21,22, bAnother important aspect in the technological development of fastening products is the facilitation of a safe installation since installation defects can substantially decrease the fastening’s structural performance. This is discussed through the example of bonded anchors by ,26,27, w3 and a steel strength of up to 1050 Mpa. In these investigations, it was found that the expansion anchors could not be installed properly due to fibres crossing the borehole, but further problems with the drilling process have not been reported. Negligible differences in mean resistance values were found between plain and fibre concrete, while anchors in SFRC had a much larger scatter, a phenomenon attributed to the higher porosity of steel fibre concrete as well as the inhomogeneous distribution and orientation of the fibres in the mix. Ref. [3 and showed no influence on the pull-out capacity of the anchors, but a varying mode of failure was present between unreinforced and fibre-reinforced concrete. Ref. [3 in medium and high-strength concrete and showed that the bond strength increased notably in fibre concrete. In [Until now, various research studies have examined the performance of anchors in SFRC based on direct pull-out tests on various types of anchors. Ref.  reportedix. Ref.  performete. Ref.  tested dte. Ref. , on the rete. In , bonded rete. In , where arete. In  also tesrete. In ,45. Finarete. In ,47,48 adAs seen from the previous sections, research on the anchorage performance in SFRC by means of bonded anchors, particularly on the bond strength, is, thus far, limited, while previous investigations have been predominantly macroscopic without a specific focus on the critical interface of the borehole surface and the polymer adhesive agent. In this paper, the installation quality is specifically investigated as part of the bond performance assessment, and possible adverse influences of the drilling, setting and confinement of bonded anchors by the implementation of steel fibres are specifically discussed with respect to steel fibre reinforcement. The investigated variation parameters include plain and steel fibre concrete, different tool life states of the drill bit, and different polymer construction mortars. Furthermore, borehole surface anomalies and the applied confinement of the anchor by the fibres are discussed. The experimental results on the contribution of the fibres are complemented by three-dimensional FE analyses with a discrete modelling of the fibres in the concrete matrix. 3 during the concrete production in the concrete mixer. All concrete batches had an average compressive strength of 50.9 Mpa and a coefficient of variation of 2%, while fibre-reinforced samples had a consistently higher compressive strength by approximately 1.0 to 1.5 Mpa. The drilling equipment used in all cases included a dry rotary hammer machine and a drill bit with 4 cutting edges, nominal diameter of dd = 14 mm, and a working length of ld,w = 200 mm. The drilling machine applied 850 rpm rotary action and single impact energy of 3.6 Joules. The drill bit was composed of steel, while the cutting tip used a hard metal composite, composed of cobalt-wetted tungsten (wolfram) carbide. To assess the influence of steel fibres on the bond strength, confined pull-out tests were carried out on unreinforced concrete and steel fibre-reinforced concrete. The fibres used were Dramix 5D BG by NV Bekaert SA, which are filaments of cold-drawn wire with hook-ended shapes, with a length of 60 mm and a diameter of 0.9 mm. The material strength was 2300 Mpa and the rupture strain was 6.0%. For the tests, two different injections resin products were used, namely, (A) methacrylate-based and (B) epoxy-based, both with approval for concrete applications. The metal insert consisted of an M12 threaded rod of high-strength class 12.9. Casting of the specimens was carried out at the laboratory facilities of the TU Dortmund University, Dortmund, Germany. The concrete mix used CEM I 52.5 N Portland Cement and natural aggregates with a maximum size of 16 mm were used. The distribution for all concrete mixes was at 40%, 30%, and 30% for a sieve pass of 0/2, 2/8, and 8/16 mm, respectively. Plain and steel fibre concrete mixes were only varied in that either no fibre reinforcement or steel fibres were added at a dosage of 50 kg/mRegarding the behaviour of the steel fibres during the drilling process, it was evident that the fibres led to an increased drilling resistance. Moreover, temperature measurements were taken by means of a laser and contact thermometer, indicating a rise in temperature from 24.0 °C in the neutral condition to an average of 89.2 °C and 99.1 °C after 200 mm long drills in plain and steel fibre concrete, respectively. This 9.9 °C temperature increase also indicated a higher friction stress and respective thermal energy release in SFRC. An optical assessment of the fibres in the borehole indicated a local spalling of the concrete around the exposed fibre and an abrasive separation of the intersected steel fibres in the borehole and a widened end at the fibre separation due to crushing See . The visQuantitative data were retrieved from the internal surface of selected test drills by means of a microscopic investigation. The surface imaging was carried out utilizing the optical 3D coordinate measuring device Alicona Infinite Focus G5 with a lateral and vertical resolution of 0.44 and 10 nm, respectively. The image processing was carried out by means of the specialised surface analysis software μsoft by NanoFocus AG. Due to geometric limitations of the measuring device, only approx. 25 mm of the entire drilling length was recorded per borehole. The examined boreholes corresponded to unreinforced concrete and steel fibre-reinforced concrete.Following test drillings, cores were taken concentrically to hammer drills and they were cut open along their longitudinal axis for the surface analysis. These are indicatively presented in The tests were designed and performed at the laboratory of the TU Dortmund University to exhibit a pull-out type of failure, in which the bond and shear strengths of the injection mortar were the determining factors. The main features of the preparation and execution of tests are provided in Apart from the distinction of plain and steel fibre-reinforced specimens, and for each of the two mortar types, the boreholes were distinguished into two groups. Series 1 was cleaned following a procedure of blowing out the dust twice with compressed air, brushing twice with a fitting steel brush, and finally blowing out twice again. Series 2 did not undergo a particular removal of the drill dust from the borehole surface, and it was only ensured that the sediment did not reduce the intended anchoring length; this procedure understandably led to a substandard and potentially defective installation. The drilling was carried out by an existing hammer drill with a four-edge drill bit in all tests. The tests were displacement-controlled with a loading rate between 0.01 and 0.02 mm/s, so as to reach the peak load at approximately 1 to 3 min. The test rig is shown in Indicative failure modes of the anchors are shown in As derived from the load-displacement curves in The contribution of fibres in the load-bearing process of bonded anchors was additionally evaluated by means of three-dimensional finite element analyses with discrete modelling and stress measurements of the fibres in the matrix, in line with the modelling approach presented in . The geoThe results indicated that the fibres were activated under the proposed loading conditions both in compression and tension see . The imaIn order to evaluate the confinement of bonded anchors in relation to the borehole quality and addition of fibre reinforcements, an additional series of tests was carried out, particularly for product B. With the loading configuration remaining identical as described in the previous section, small-scale specimens were designed and used again under confined pull-out testing. In this case, the borehole depth was limited to 35 mm, allowing for an embedment depth of 30 mm. The specimens had a plan dimension of 150 by 150 mm, a thickness of 100 mm and at the mid-level of the embedment depth, i.e., at the 20 mm depth from the upper surface, they were equipped with a 6 mm confinement reinforcement stirrup. The specimens were produced by casting them in 150 mm cubes and cutting the upper 50 mm of the specimen, where the anchor would, subsequently, be placed in order to reduce the effects of a nonhomogeneous fibre distribution due to the casting boundary ,52. StraAs seen in The present paper contributes to the current technical discourse of the influence of steel fibre reinforcement in concrete on the installation quality and load-bearing performance of post-installed bonded anchors. It initially presented background technical information and relevant studies, which indicated a knowledge gap in understanding the influence of steel fibres in the performance of bonded anchors in concrete. A multilateral assessment of this effect was presented here, while two main parameters were varied, i.e., the inclusion or not of high-strength fibres in concrete and the use of two different types of polymer-based bonding mortars. The assessment included a high-level and microscopic qualitative investigation of the borehole surface roughness, full-scale pull-out tests to assess the bond strength of the anchors, an FE simulation of the tests and an assessment of fibre contribution in the load-bearing and confinement of the anchors, and, finally, a small-scale test with a measurement of the confinement and splitting stress retention through fibres with strain gauge measurements.The borehole roughness analyses indicated that the borehole roughness could be influenced by the presence of steel fibre reinforcement, mainly due to the protrusion of fibres in the borehole. This may be related to an increase in the bond strength of anchors in SFRC. Furthermore, this calls for a more insightful investigation of the drilling processes and drilling tool adjustments for the installation of anchors in SFRC.The pull-out results suggested that a mild increase in the bond strength could be expected in SFRC when high-strength mortars were used, i.e., when the load-bearing of the anchor relied strongly on the concrete–mortar interface. A substandard or generally inadequate borehole cleaning prior to the installation of the bonded anchor led to a significantly reduced pull-out resistance. This quality difference was by far more relevant to the performance of the anchors than the inclusion of fibres in the concrete mix. A slightly varying load-bearing performance under pull-out was observed between the plain concrete and SFRC, but this may have also been attributed to the overall variability related to the low installation quality. For anchors prepared and installed per the recommended procedures, those for Product A did not show a strong influence of the anchor performance by steel fibres in the concrete mix, while failures occurred predominantly at the mortar–thread interface through the shearing-off of the mortar. Anchors installed by means of Product B exhibited an approx. 10% higher overall resistance in SFRC at the mean value level. At the characteristic value, a load increase of more than 10% in SFRC was also observed at the characteristic value levels for both Products A and B. Additionally, the levels of deformation at the loss-of-adhesion for bonded anchors in SFRC were found to be higher as compared to plain concrete.The FE analyses indicated a contribution of fibres in the area around the bonded anchor, but this was of a rather inconsequential magnitude in regards to the load-bearing performance pull-out of the anchor. However, the confinement of the concrete close to the surface suggested that a possible influence could be present between the ratios of bond strengths measured by confined and unconfined test set-ups.Small-scale tests also contributed to the conclusions above since both bond strengths and confinement strains were found to be higher in SFRC as compared to plain concrete.In general, all studies converged to the conclusion that, for the tested configurations of materials and load set-ups, the resistance and robustness of bonded anchors in SFRC was not inferior to tests in equivalent plain concrete. This was also the indication in regards to the long-term performance of bonded anchors, since the deformation at the loss of adhesion was proportional to the qualification criterion of bonded anchors under sustained loads. The overall load–displacement performance of anchors in SFRC corresponded to typical patterns met in previous literature on bonded anchors in plain concrete. Moreover, some improvement was seen in terms of the displacement and load-bearing capacity, which was also in agreement with existing knowledge for bonded anchors in fibre-reinforced concrete.Further investigations on finer quality influences such as the roughness of the internal borehole surface, the drilling method, as well as variations in the fastening products and concrete/SFRC mixes could allow an improved insight on possible influences of steel fibre reinforcement on the performance of bonded anchors in concrete. Moreover, the comparison of confined and unconfined tests could lead to a clearer understanding of the combined bond and breakout failure mode and, consequently, the load capacity dependence on both the borehole quality and the concrete homogeneity and quality close to the free surface of SFRC components. Finally, a significant potential in the design service life of bonded anchors could be disclosed for bonded anchors in SFRC in long-term loading tests.


The present study is a detailed literal survey on the bond behavior of FRP (Fiber Reinforced Polymer) reinforcing bars embedded in concrete. There is an urgent need for the accurate assessment of the parameters affecting the FRP–concrete bond and quantification of these effects. A significant majority of the previous studies could not derive precise and comprehensive conclusions on the effects of each of these parameters. The present study aimed at listing all of the physical parameters affecting the concrete-FRP bond, presenting the effects of each of these parameters based on the common opinions of the previous researchers and giving reasonable justifications on these effects. The studies on each of the parameters are presented in detailed tables. Among all listed parameters, the surface texture was established to have the most pronounced effect on the FRP–concrete bond strength. The bond strength values of the bars with coarse sand-coating exceeded the respective values of the fine sand-coated ones. However, increasing the concrete strength was found to result in a greater improvement in bond behavior of fine sand-coated bars due to the penetration of concrete particles into the fine sand-coating layer. The effects of fiber type, bar diameter and concrete compressive strength on the bar bond strength was shown to primarily originate from the relative slip of fibers inside the resin of the bar, also known as the shear lag effect. Fiber-reinforced polymer (FRP) bars have been increasingly used in the field of structural engineering due to their significant advantages, such as high tensile and fatigue strengths, high corrosion resistance, lightweight, ease of transportation and handling, thermal and electrical insulating (GFRP only) properties and being unresponsive to magnetic fields ,2,3. On The previous research studies and field applications concentrated on four different types of FRP  that can be used in the form of concrete reinforcement. BFRP and GFRP bars are the most preferred types as internal reinforcement in concrete members owing to their lower prices and ease of supply than the CFRP and AFRP bars. BFRP bars have slightly higher modulus of elasticity and tensile strength values than GFRP bars, yet the respective values of both BFRP and GFRP are considerably lower than those of CFRP and AFRP ,7. GFRP CFRP has various superiorities over the other three types, including the highest tensile and fatigue strengths and being the least vulnerable FRP type to environmental effects , fatigue and creep rupture. Nevertheless, CFRP also has some major disadvantages, including the electric conductivity, high price, vulnerability to electrochemical corrosion when in contact with metal materials in a humid environment and the highly brittle nature ,4. RecenThe bond between an FRP bar and the surrounding concrete is the governing factor that determines the efficiency and suitability of the utilization of FRP bars as concrete reinforcement. In flexural RC members (slabs and beams), the compression forces in concrete are counterbalanced by the tension forces in the reinforcement and the development of these tension forces entails the adequacy of the reinforcement–concrete bond in the tension zone. The types of bond mechanism of FRP bars in concrete are similar to those of steel bars, which are the mutual adhesion, surface friction and shear interlock. Nevertheless, the mechanical properties of FRP bars are completely different from those of steel bars  14,15].,15.14,15In the literature, numerous experimental, analytical and numerical studies were conducted on identifying the prominent factors affecting the concrete-FRP bond; nevertheless, a great majority of these studies concentrated on monitoring the influence of only one or a few of all parameters. In the current study, on the contrary, all main parameters affecting the bond strength of FRP bars in concrete were investigated by conducting an extended literature survey. This survey indicated that there is consensus among the researchers on the effects of certain parameters on the FRP–concrete bond, while the effects of the other parameters have not been clearly unveiled yet. There is a variety of opinions on the effects of these parameters. The present literature review introduces the parameters one by one together with the findings of the previous researchers on this parameter. Besides, the related provisions from the structural FRP RC codes are also discussed throughout the manuscript. Accordingly, the main goal of the present study is to present the recent developments and challenges related to the utilization of FRP bars in concrete by underscoring the most influential factors affecting the FRP–concrete bond based on the previous works on this topic.d);Bar Diameter  and bar spacing (cs);Concrete cover (dl) or embedment length (el);Development ;Compressive strength  the inherent properties of FRP rebars; (ii) the arrangement and configuration of reinforcement; (iii) the inherent properties of concrete; and (iv) the method of testing. The complete list of parameters is as follows:A detailed section is devoted to each one of these parameters in the following discussion. A detailed table, which compiles the tests and studies used in that section, is given in each section to avoid any confusion and to clearly reveal the effects of each parameter on the FRP–concrete bond. The entire table of each section was discussed and debated in its entirety with additional comments of the authors.The previous researchers did not reach an agreement on the denominations of the surface textures of FRP bars. In other words, the same surface type was termed differently by different researchers. This discrepancy caused significant confusion among researchers. To avoid confusion in the present review, each table contains two columns for the bar surface notations. The first column corresponds to the original notations in the source papers, while the second one refers to the notations suggested and used in the present text. The bar surface notations of the present review are given in The graphs and test results in the majority of the previous studies cannot provide credential comments on the effects of a certain test parameter on bond strength. The previous experimental studies do not possess the merit of focusing on a single parameter by isolating the related experiments from the remaining test parameters. Consequently, the experiments, which are intended to unfold the effects of a certain parameter on bond strength, include the coupled effects of numerous parameters. Moreover, the average values of the test results were adopted in the previous works when explicating the scatter plots. However, the values in these plots are scattered in a broad range and reliable and precise results may not be inferred by using the average values. Unlike the other review studies, the coupling of effects of different parameters were taken into account in the present text and the findings were elaborated by avoiding controversial arguments.The effect of a single parameter (dependent variable) on the independent variable (the FRP–concrete bond strength in this case) can only be unfolded if all other dependent variables are kept fixed in the related experiments. Otherwise, the coupling between the effects of several parameters will not allow the researchers to isolate of the effect of a single parameter and set a relationship between the examined dependent variable and the independent variable. In the context of investigating the effects of the FRP material type on the FRP–concrete bond strength, for instance, the surface texture, diameter, clear cover and distance from the adjacent bar of the tested bars need to be kept identical in the related experiments as well as the concrete grade, concrete type and fiber content of the concrete mixture. In that respect, the existing experiments in the literature do not suffice for the development of specific relations between each test parameter and the FRP–concrete bond strength.The test data on FRP–concrete bond strength is extremely scattered. The wide dispersion of this data mainly stems from the coupling between the effects of several parameters in the previous experimental studies, which were designed without paying attention to all parameters affecting the FRP–concrete bond. The mathematical analyses on the data with such a dispersion do not generate meaningful and accurate expressions, since the deviation of the actual data from the mathematical curve remains high, meaning that the mathematical curve does not accurately represent the experimental data.The surface texture types of FRP bars have not been standardized with regulations, standards and previous experimental studies. For instance, the rib dimensions of the ribbed FRP bars and the grain sizes of the coating layer in the sand-coated bars are rather different in different studies. Hence, the surface type with the same notation can be excessively different in the related tests, which exacerbates the broad scatter of the test data and even results in opposing test data in different experimental studies.In this study, as much data as possible was compiled from the literature to prepare tables from which clear and accurate conclusions on the effects of a single parameter could be reached. In this way, the effects of the remaining parameters on the FRP–concrete bond were minimized, if not completely eliminated. Furthermore, each finding or conclusion was justified with sufficient reasoning. The authors did not utilize scatter graphs or curves. Furthermore, the authors avoided using precise statements on certain parameters due to significant discrepancies between the related experiments in the literature. The ambiguous and even sometimes opposing findings in different studies complicates to draw definitive conclusions on these parameters. These discrepancies have been completely ignored in the previous statistical review studies. The present paper leaves it to the readers on these parameters instead of deriving conclusions from the controversial data. Only obvious and well-explained conclusions were drawn according to the existing test results, presented in tables in each section. In the present study, the dependent and independent (bond strength) variables needed to be presented in the table format rather than conducting an analysis and presenting them in a mathematical form for three main reasons:The authors could not make separate analyses on each type of FRP bars  in each section due to the absence of an adequate number of studies in the literature. For instance, the number of studies on CFRP and AFRP reinforcing bars is so limited that conducting separate analysis and reaching specific conclusions on these two types is not possible at all. Strictly speaking, the majority of the studies in the literature pertain to the bond behavior of GFRP and BFRP reinforcing bars in concrete. Consequently, the authors tried to reach general conclusions on the effects of each parameter on the FRP–concrete bond without diving into special comments on different FRP types. The findings from a single type of FRP were not generalized to all types, but only the common conclusions on all FRP types were given in the manuscript.The present pertains solely to the short-term bonding performances of FRP reinforcing bars in concrete. The FRP–concrete interfacial bond strength is subject to degradation due to environmental factors, including but not limited to the temperature, corrosive environments and humidity. Furthermore, long-term effects, including creep and fatigue, are also responsible for the changes in the adherence of FRP bars to concrete. The long-term bonding performances of FRP bars and the durability issues are planned to be covered in a companion paper.Significant effort has been spent in the literature to determine the effects of FRP bar diameter on bond strength. There are three basic opinions on the influence of bar diameter on bond strength. The first of these opinions relies on the concept of the relative slip between the core and the fibers on the outer surface (shear lag effect) resulting from the low slip resistance within the epoxy resin and at the epoxy–fiber interface under the action of axial tension forces ,19,20,21In this respect, the main reason for the differences between the findings of different researchers are expected to be a result of the shear lag effect. Several inherent properties of an FRP bar, including the fiber density, resin type, resin density and the mechanical properties of the constituents, were found to impinge on the shear lag effect. The degree of this effect also changes with the manufacturing conditions of the bar and the persistence of these conditions. Hence, the effects of bar diameter on bond strength can only be identified by considering all these variables.The literature contains a lot of studies on the influence of fiber type  on FRP–concrete bond strength. The statistical, review and research studies on this very topic are presented in In order to determine the effect of fiber properties on bond strength, The only clear outcome from this comparison is that the GFRP bars is the least favorable polymer bars in terms of adherence with concrete among the four types of FRP. However, reaching a crystal-clear conclusion about the contribution of AFRP, BFRP and CFRP fibers to the adherence with concrete is impossible to reach based on the available test results. The improved bonding properties of CFRP, BFRP and AFRP bars originate from two main reasons. The first reason is the shear lag effect, just like the related discussion on the effect of bar diameter. The shear lag effect is lower in FRP bars with resin and fibers strong in tension as compared to those with resin and fibers weak in tension. Considering that AFRP, BFRP and CFRP bars generally possess higher tensile stiffness values in comparison to the GFRP rebars ,43, the In the literature, a limited number of studies have been conducted on the effect of the modulus of elasticity of FRP on the concrete–FRP bond. Although various views on the effect of the change in the modulus of elasticity on adherence are presented in the literature ,52,53,54A good number of research studies in the literature are devoted to the influence of surface texture on concrete–FRP bar bond strength. Statistical, review and research studies on this topic are summarized in Accordingly,R1 and R2 is the fine sand-coating (SCf).R3 and R4 shows the coarse sand-coating (SCr).R5 is the standard sand-coating (SC).R6 illustrates the helically wrapped (HW) surface.R7, R8 and R9 are the helically wrapped and sand-coated surface (HWSC).R10 shows the indented (In) surface.R11, R12 and R13 correspond to the ribbed (Rb) surface.As shown in Hence, the adherence behavior alters into the mechanical interlocking with increasing concrete strength in bars with fine coating. Moreover, the surface areas of the fine sand-coated bars are larger due to the presence of indentations on the surface, and therefore, the contribution of improving the concrete quality to the bond strength becomes more considerable in these bars, having greater contact surface with concrete.The other surface types, which have deeper surface deformations, convey the internal forces to the surrounding concrete through both friction and mechanical interlocking. The two-component transfer mechanism is the main reason for the higher bond strengths of these bars. The mechanical interlocking capacity changes with the rib height, rib spacing and rib thickness. From this point of view, with some exceptions, as the rib height increases and the rib spacing decreases, the bond strength increases. This increase stems from the increased surface area for the mechanical interlocking forces to develop. On the other side, significantly narrow and deep ribs might also lead to considerable reductions in the rigidity of ribs, and hence, lower limits for the spacing and upper limits for the rib height need to be established with the help of more detailed studies. In general, the bars with “In” surface type have larger rib spacings due to the increased rib thickness values and these bars possess lower bond strengths as compared to the bars with “Rb” surface texture. The lower bond strength values are caused by the fact that the force transfer in the indented bars rely mostly on the surface friction rather than mechanical interlocking. As in the case of indented bars, the bond strength values of the ribbed bars remain below the respective values of the helically wound bars.In HW bars, the proportion of the forces transferred by the surface friction and mechanical interlocking varies with the height of the ribs. The contribution of mechanical interlocking increases with increasing rib depth, resulting in the increased adherence. Additionally, the bar starts to behave similar to a wedge with increasing rib height. Consequently, the shear forces are transmitted to the HW bars gradually, dissimilar to the sudden transfer of the shear forces in the FRP bars with In and Rb surface types. At around the peaks of the ribs, the mechanical interlocking forces turn completely into friction forces. This gradual transfer might also delay the shear failure of the beam by providing the transmission of shear forces along numerous interlaminar shear surfaces on the FRP bar instead of a single surface. The bars with “Rb” type of surface have superior bonding strength values when compared to the bars with “HW” type of surface.The effects of concrete cover and bar spacing on FRP–concrete bond strength have also been subject to a variety of studies in the literature. The researchers have sought to identify the changes in the failure modes of FRP bars with changing bar spacing and concrete cover. Albeit an adequate number of studies were devoted to the effect of concrete cover, the bar spacing has caught the attention of only few researchers. d (bar diameter) in all concrete types, i.e., NSC , HSC (high-strength concrete), UHSC (ultra-high-strength concrete) .,63.60,63oncrete) ,65. Whenplitting , while ampletely . ACI 440The previous studies adopted a variety of surface textures and FRP mechanical properties. Furthermore, significant variations in concrete strength and embedment length makes it almost impossible to reach precise conclusions on the need for concrete cover for the complete prevention of concrete splitting. The concrete splitting failure is a result of the transfer of splitting forces to concrete , which iIn summary, the FRP–concrete bond strength generally increases with increasing concrete cover since the confinement around the bar is improved with increasing cover thickness. The general tendency of the change in the FRP–concrete bond strength with increasing cover is complicated to specify as the failure mode also changes with increasing thickness of the concrete layer around the bar. Strictly speaking, the bond strength undergoes sudden changes while increasing the concrete cover, particularly at the transition of the failure mode from concrete splitting to debonding or rupture. Furthermore, concrete cover cannot be considered alone in the evaluation of test results, since this parameter governs the failure mode of a rebar together with the bar embedment length. Hence, the previous studies refrained from focusing on the simultaneous effects of concrete cover and embedment length on the FRP–concrete cover. Instead, only one of these two parameters changed in the related tests while keeping the other parameter fixed.d, the bond strength was found to be unaffected by further increasing the bar spacing [The number of studies on the influence of bar spacing on the FRP–concrete bond is rather limited and these studies showed that increasing the spacing between bars can contribute to the bond strength up to 50% ,50. If t spacing .There has been a great deal of research undertaken in the literature on the effects of embedment or development length on the FRP–concrete bond strength ,76,77,78The studies on the effects of reinforcement location concentrated on two positions, namely the lower and upper portions of the member, according to the direction of concrete cast. These studies generally concluded that the bond strengths of the upper bars are smaller than their lower counterparts since the water, air and fine aggregates in the mixture move upwards and accumulate underneath the rebars  65,81,8,881,82.In two of these studies ,81, the A great majority of the previous studies in the literature consisted of pull-out tests for determining the FRP–concrete bond strength, and hence, the number of studies on the effects of transverse reinforcement on the FRP–concrete bond is rather limited. The confining effect of transverse reinforcement on the longitudinal bars can only be reflected with the help of beam tests. There are two common opinions on this confining effect. The first one is the contribution of transverse reinforcement to bond strength through limitation of the crack widths in the member . AccordiThe effect of concrete compressive strength on bond behavior may differ in steel and FRP reinforcing bars. Since steel rebars are homogeneous and isotropic as well as have a wholistic structure, bond failure patterns and stresses are governed by the shear strength of concrete . HoweverThe effects of fiber addition to concrete mixture, which aims at controlling the crack widths by increasing the tensile strength of concrete, on the FRP–concrete bond has been subject to various studies in the literature. Some of these studies are given in This cracking might also be affected by the fibers inside the mixture by improving the tensile strength of concrete and this effect will primarily depend on the density and length of fibers in the vicinity of the rebars . NonetheVarious types of concrete were employed in the previous studies on the FRP–concrete bond . HoweverThe present paper is a detailed literature review on all parameters affecting the bond behavior of FRP reinforcing bars embedded in concrete. The influence of each parameter is discussed in the light of the findings of previous researchers. Precise and clear comments are given throughout the manuscript. The controversial and opposing comments of the previous researchers are not mentioned in the manuscript, since most of these comments originate from the differences between the testing conditions and test methods in different studies and negligence of certain parameters affecting the FRP–concrete bond. With the aim of not listing the inconsistent and ambiguous findings, only the following unquestionable conclusions are given in the present text together with the justifications behind each finding.The bond strength of an FRP bar decreases with increasing bar diameter. This decrease is associated with three possible reasons. First, the slip of fiber layers within the resin, also known as the shear lag effect, is aggravated with increasing bar size and this effect has a negative impact on the FRP–concrete bond. Secondly, the amount of air voids and mixing water, accumulating underneath the bar, increases with increasing bar size and the weakness of the concrete around the bar results in the reduction of the bond strength. Finally, the mechanical interlocking and surface friction forces of a bar decrease as a result of the greater degree of Poisson’s effect on the bar with increasing bar diameter.The bond strength values of GFRP bars are lower than the respective values of their CFRP, AFRP and BFRP counterparts, embedded in a similar concrete mixture. The lower adherence of GFRP to concrete stems from the more considerable shear lag effect in the GFRP bars due to the lower axial stiffness than the other three types of FRP. The greater slip of fibers from the core in GFRP results in the reduced bond strength values of these bars. Furthermore, the increase in the radial thermal expansion of the BFRP, AFRP and CFRP bars due to the friction at the bar–concrete interface improves the mechanical interlocking and surface friction of these bars in concrete, as compared to the GFRP bars, which are known to have smaller thermal expansion coefficient.FRP bars with coarse sand-coating layer have higher bond strength values in normal-strength concrete than the bars with fine sand-coating layer. However, the bonding behavior of the fine sand-coated bars is improved to a greater extent with increasing concrete strength as compared to the coarse sand-coated bars. The better compaction and the lower amounts of air voids in high-strength concrete mixtures enable the fine particles of concrete to penetrate into the fine sand-coating layer and improve the bond behavior.The mechanical interlocking mechanism is improved in ribbed bars with increasing rib height and decreasing rib spacing. The increase in the surface area for the development of mechanical interlocking forces results in the FRP–concrete bond strength to increase when using deeper ribs. However, further studies on the topic are needed to determine the minimum spacing and maximum height limits of the ribs since too closely-spaced and/or too deep ribs might reduce the rib rigidity and have adverse effects on the bond strength.The thicker and more widely-spaced ribs in the bars with indented surface enables them to transmit greater surface friction forces as compared to the ribbed bars. Therefore, the bond strength values of the indented bars remain below the respective values of the ribbed and helically wrapped bars.The concrete cast depth underneath an FRP bar influences the bond strength to a significant extent. With increasing cast depths, the amount of air voids and water underneath a bar increases, resulting in the compressive strength of concrete surrounding the bar and the FRP–concrete bond strength to decrease.According to the existing studies in the literature, a clear concrete cover of at least three times the bar diameter is compulsory to avoid concrete splitting failure and to allow the debonding or tensile rupture failures to govern the specimen behavior. Increasing this spacing beyond seven times the bar diameter does not have a considerable effect on the FRP–concrete bond strength.The contribution of increasing the compressive strength of concrete to FRP–concrete bond strength is bounded by upper limits. Increasing this strength contributes to the shear strength of the concrete layers around the bar, yet beyond certain limits of concrete strength, the peeling of the outer bar surface from the core and/or slip of the fibers inside the resin can trigger the bond failure of the bar rather than the shear failure of the surrounding concrete.The bond strength tends to decrease with increasing embedment length of an FRP bar in concrete. The non-uniform stress distributions along the bar length and the reductions in the ability of a bar to convey the internal forces through surface friction are the primary reasons for the reduction in bond strength with increasing embedment length. This decrease follows a uniform path with increasing embedment length.The transverse reinforcement definitely affects the FRP-concrete bond strength. However, further studies are needed to unfold the degree of this effect due to wide range of variation of the other test variables in the existing studies.The maximum aggregate size and fiber length controls the initiation and spread of cracks in concrete based on the surface texture. Further studies are needed to uncover the effects of maximum aggregate size on the bond strengths of FRP bars embedded in the concrete mixtures with fibers.The existing studies are not sufficient to specify the concrete cover boundaries for the change in the type of failure of FRP bars in concrete due to wide variations in the surface types and mechanical properties of the tested bars as well as the wide ranges of concrete strength and bar embedment length in the related tests. The concrete splitting failure necessitates the transfer of adequate splitting forces in concrete , which i


Ambulatory antibiotic stewards, researchers, and performance measurement programs choose different durations to associate diagnoses with antibiotic prescriptions. We assessed how the apparent appropriateness of antibiotic prescribing changes when using different look-back and look-forward periods. Examining durations of 0 days (same-day), −3 days, −7 days, −30 days, ±3 days, ±7 days, and ±30 days, we classified all ambulatory antibiotic prescriptions in the electronic health record of an integrated health care system from 2016 to 2019  as chronic, appropriate, potentially appropriate, inappropriate, or not associated with any diagnosis. Overall, 16% percent of all prescriptions were classified as chronic infection related. Using only same-day diagnoses, appropriate, potentially appropriate, inappropriate, and not-associated antibiotics, accounted for 14%, 36%, 22%, and 11% of prescriptions, respectively. As the duration of association increased, the proportion of appropriate antibiotics stayed the same , potentially appropriate antibiotics increased , inappropriate stayed the same , and not-associated antibiotics decreased . Using the longest look-back-and-forward duration (±30 days), appropriate, potentially appropriate, inappropriate, and not-associated antibiotics, accounted for 18%, 44%, 20%, and 2% of prescriptions, respectively. Ambulatory programs and studies focused on appropriate or inappropriate antibiotic prescribing can reasonably use a short duration of association between an antibiotic prescription and diagnosis codes. Programs and studies focused on potentially appropriate antibiotic prescribing might consider examining longer durations. Owing to their size and clinical detail, large databases—including electronic health record and claims data—are commonly used to evaluate the appropriateness of ambulatory antibiotic prescribing. However, the indication for antibiotic prescriptions large databases is not always clear. Consequently, antibiotic stewards, researchers, and performance measurement programs using large data sets must choose how to associate prescriptions with diagnoses in a fair and clinically reasonable manner.Two choices include over what duration to associate antibiotic prescriptions with diagnosis codes and whether to look “backwards” or look “forwards” in time. The simplest approach might be to only consider diagnoses codes occurring on the same date as the antibiotic prescription. However, a patient could call and receive an antibiotic prescription during a telephone encounter with no same-day diagnosis, then make an in-person visit several days later with an antibiotic-appropriate diagnosis. In this situation, considering diagnosis codes during a look-forward period would allow the classification of antibiotic appropriateness. Alternatively, a patient could be seen in person for a potentially antibiotic-appropriate diagnosis, not receive a same-day antibiotic, send a follow-up electronic message days later, and receive an antibiotic. In this situation, considering diagnosis codes during a look-back period would similarly allow classification of antibiotic appropriateness. Owing in part to the lack of prior literature on the impact of these choices on assessments of antibiotic appropriateness, prior antibiotic prescribing studies and performance measurement programs have used widely varying look-back and look back-and-forward durations, such as 1 day ,3,4, 3 dWe conducted a retrospective cohort study to assess the appropriateness of all ambulatory, oral antibacterial antimicrobial prescriptions ordered in the EHR between 1 January 2016 to and 31 December 2019 by clinicians at Northwestern Medicine. Northwestern Medicine is a large, integrated academic health delivery system with hundreds of sites, about 4000 physicians, and 30,000 staff in the Chicago area  [We used an EHR-based medication grouper to identify all outpatient oral, antibacterial antimicrobial prescriptions. We excluded non-oral antibiotics and antibiotics most often used for urinary tract infection prophylaxis . The unit of analysis was the prescription. We included antibiotic prescriptions regardless of encounter type . Clinicians could order antibiotics with zero, one, or more than one diagnosis codes in the EHR. Diagnoses were not required for all encounter types. For example, at least one diagnosis was required for an in-person encounter, but not for telephone or patient portal encounters. Although antibiotics are often prescribed at the time of an encounter, clinicians have the option of opening a previously started encounter on subsequent days and prescribing antibiotics. Similarly, diagnoses are usually assigned at the time of an encounter but sometimes are assigned later. Clinicians can also assign diagnoses and prescribe related antibiotics in different encounters . In the EHR, there is no requirement that the same clinician prescribe the antibiotic and choose the diagnosis within or between encounters. At the time of the study, before the COVID-19 pandemic, Northwestern Medicine did not have synchronous video visits.Using diagnosis codes, we categorized all antibiotic prescriptions into five mutually exclusive groups: chronically used antibiotics , associated with an antibiotic-appropriate diagnosis , associated with a potentially antibiotic-appropriate diagnosis , associated with inappropriate diagnosis , or not associated with any diagnosis. To identify chronically used antibiotics, we examined diagnosis codes and problem list diagnoses present in the 6 months prior to antibiotic prescribing for 690 International Classification of Diseases 10, Clinical Modification (ICD-10-CM) codes that could be associated with a chronic infection or might be indications for chronic antibiotic use  ,17. If aTo assign the remaining antibiotics to one of the four other appropriateness categories, we classified all diagnosis codes within a defined duration (described below) as always, sometimes, or never justifying antibiotic prescribing using a previously developed scheme of all 94,249 ICD-10-CM diagnosis codes . If at lOther than antibiotics for chronic use, we initially classified the remaining antibiotics as appropriate, potentially appropriate, inappropriate, and not associated with any diagnosis based on diagnoses from the same day as the antibiotic prescription. We then examined the effect of expanding the look-back and look-back-and-forward durations between antibiotic prescriptions and diagnosis codes. We started with look back durations because this likely represents better clinical practice . We examined durations as long as 30-days because some analyses use within-month  to associate diagnoses and antibiotic prescribing, despite the fact that using these long durations may increase the likelihood of capturing problems unrelated to the antibiotic prescription” . We examined look-back periods of −3 days , −7 days, and −30 days. We examined look-back-and-forward periods of ±3 days , ±7 days, and ±30 days.Owing to large sample sizes, we did not conduct formal statistical significance testing. Rather, we considered absolute differences of 5% or greater to be clinically significant ,17.In the four-year study period from 2016 to 2019, there were 714,057 antibiotic prescriptions ordered for 348,739 unique patients by 2391 clinicians in 467 clinics. Patients had a mean age of 41, were 61% women, 78% white, and 52% had private insurance. Clinicians were 58% women and 59% staff physicians, 19% residents or fellows, and 21% nurse practitioners or physician assistants. In all analyses, by design, the proportion of prescriptions classified as chronic infection-related was 16%. Using different look-back and look-back-and-forward durations had modest effects on the distribution of prescriptions across the four remaining appropriateness categories . When onUsing the longest look-back-and-forward duration (±30 days), potentially appropriate diagnoses accounted for the largest share of prescriptions (44%), followed by inappropriate (20%), antibiotic-appropriate (18%), and not associated with any diagnosis (2%). Using different look-back and look-back-and-look-forward periods modestly changed the apparent appropriateness of antibiotic prescribing. Longer durations increased the likelihood of classifying an antibiotic prescription as potentially appropriate, did not change the likelihood of classifying a prescription as inappropriate, and decreased the likelihood of classifying a prescription as not associated with any diagnosis. Diagnoses that sometimes warrant antibiotics  are more common than those that always warrant antibiotics  ,18, so iThis analysis has limitations. As with other large data analyses of EHR or claims-based antibiotic prescribing appropriateness, we were dependent on clinician-assigned diagnosis codes and could not assess their validity. When feasible, smaller scale studies by front-line antibiotic stewards should verify clinician diagnoses through chart review. Our findings may not extend perfectly to claims data, which have delays between when the visit is billed, the prescription is filled, and when the filled prescription is billed. Other limitations include the use of a single EHR, conduct in a single US healthcare system with mostly white patients, and data that predate the COVID pandemic. We only examined the overall relationship between the antibiotic prescribing-diagnoses duration and apparent appropriateness as this relationship is a property of the data itself and we have no a priori reason for there to be differential relationships within patient, prescriber, or illness subgroups.Ambulatory antibiotic stewardship programs and research projects can select a duration of association that makes sense for their purposes. For example, if the intent is to identify and intervene on inappropriate diagnoses, then the use of a short interval–even same-day–is reasonable, as the proportion of prescriptions classified as inappropriate varies minimally based on the look-back and look-forward periods ,20,21,22