According to a Cochrane review, continuous support during childbirth increases the mother’s chances of a vaginal birth without identified adverse effects. However, this evidence-based practice is not universally implemented. The objective of the study was to identify challenges encountered in implementing continuous support during childbirth in public hospitals in the North West Province of South Africa.An explorative, descriptive and contextual qualitative approach was used. The data were collected during 2013 by conducting focus group interviews with 33 registered midwives who had worked in maternity units in three selected public hospitals in the North West Province for at least two years.Midwives identified challenges that negatively impacted the implementation of continuous support during childbirth at organisational and interpersonal levels. At the organisational level, challenges included human resources, policies and guidelines as well as the architectural outlay of the maternity units. The personal challenges related to communication and attitudes of nurses, patients and their families.Organisational and personal challenges had a negative impact on the provision of continuous care during childbirth. Continuous support during childbirth enhances the chances of a normal vaginal birth and according to a Cochrane review, such support has no identified adverse effects   were selected to represent the different sizes of hospitals in the North West. Annually, the district hospital conducts 2270 births, the regional hospital 3100 births and the large hospital 5427 births. The population comprised all midwives working in the maternity units in the three selected public hospitals. Knowledgeable participants were selected purposefully, as they could provide rich data. The inclusion criteria specified that participants had to be registered with the South African Nursing Council (SANC) as registered midwives and should have worked in these maternity units for at least two years.The research question was the following:What challenges impact the implementation of continuous support during childbirth in the selected three public hospitals in the North West Province of South Africa?Focus group interviews were conducted from August to October 2013. At least one focus group was conducted in each public hospital until data saturation occurred. The focus groups were recorded and transcribed. Group dynamics in the groups and non-verbal communication cues were recorded as field notes  had more than five years’ experience, were women , and aged 30–55 . Of the participants, 40% (n = 13) had an additional qualification in advanced midwifery.Six focus groups interviews involving 33 registered midwives were conducted. Most of the participants These views of the participants in the current study seem to be confirming findings of other authors. McLachlan et al.  demonstr‘If we are admitting many patients in labour rooms, we cannot just focus on that one as the others have to be helped also.’ (K2)The ability to provide continuous support during childbirth depends on the staff-patient ratio. The National Institute for Health and Care Excellence (NICE) guidelines on safe midwifery staffing for maternity settings in the United Kingdom , stresseThe midwives in this study reported that they sometimes had to attend to four patients during the active stage of labour:‘…the staff …… you have to see of [sic] can manage two patients at the same time, but when it comes to now you have to see three patients and you are alone on that patient, the other sister is also seeing four patient [sic] alone and then there is a foetal distress in those four patients of yours and you have to go to theatre, when you come back the patient, your patient then delivered on her own [sic].’ (K2)In the smaller district hospital, the midwives on duty were responsible for providing care to different types of maternity patients:‘Another point is that the nurse and patient ratio is not balancing, really we are having more patients than nurses and we end up with so many complications; eh another one … the second point is that there is a shortage of staff in our hospital, really we need more staff more midwives, really there is a shortage. On night duty we have two registered midwives who are responsible for a ward of plus or minus fifty. There are labour patients, antenatal clients, post-natal deliveries, post-caesarean section, premature unit, neonatal and high and normal deliveries.’ (T1)The participants reported that other responsibilities, such as the burden of supervision of interns, student nurses and agency nurses, could also influence midwives’ implementation of continuous support during childbirth:‘… Most of our doctors, they are interns, they are doing their internships. They are working also in our department and the nurses are the ones helping guiding them so it is another burden on us because all responsibility will be left on the sisters and sometimes you will find the students also you know there is not always that they will have companionship from the college side or even from university. So most of the time you know development, it is only the sisters so it is an extra responsibility also on the sisters.’ (K2)‘… The hospital make use of moonlight staff midwives, they come in and they are still inexperienced or they don’t know the environment so the care of the patient is also still at risk and then the person who is there working with them, the experienced person had also to help this people because you be there for all the patients, you have to cover for all the patients, so at the end you work the whole night.’ (K2)The heavy workload has consequences for the midwives as well as the quality of care they can provide to women:‘Our workload is too much for us because there is much shortage of staff, we are ending up with burnout syndrome because of this shortage of staff, and really we really do need more staff in our hospital.’ (T1)‘They end up having burnout syndrome because of the pressure and most of them also suffer from ill health because of overloading of the work that they performed and sometimes we end up not providing quality care, especially on records; our records really are very poor. But not poor in the sense that we don’t want to eh to provide quality care but poor because we want to try but because of not enough staff we end up having poor records that could lead us to jail.’ (T1)Igboanugo and Martin  reportedIf the midwife herself or another person is unavailable to provide continuous support during childbirth, the quality of care will be negatively affected:‘And then also the companionship is not there, the relatives they are also not there to support, because if you as a midwife you are alone and there is somebody or some relative who is supporting that woman through labour that can also help; but if there is no one, then the woman becomes anxious, she pushes prematurely and all those things.’ (P1)The staff shortage and the heavy work load can also lead to a loss of trust of the community in the midwives and the maternity service:‘… and then I must say this has been a continuous ongoing thing of shortage of staff because we have been working like that throughout the years and yet nothing has been done and to our people in the community also don’t understand that most of the things are happening and it is due to shortage of staff. So we can’t give proper nursing care, you know, to our patients and if they have the knowledge to know that these people are working very hard maybe they could understand. In impedes really our nursing care.’ (P1)The second sub-category under human resources concerns challenges related to doulas.Gilliland  reportedOne of the hospitals which participated in the current study has appointed doulas, although the midwives do not think there are enough of them and that they are not optimally utilised. The other two hospitals did not have any doulas:‘… We have no doulas available to provide continuous support…’. (T1)The doulas are sometimes expected to do tasks not primarily part of their scope or role. Although breastfeeding and the accreditation as a baby-friendly hospital is important, it must not be seen as the doulas’ work to assist the new mothers if they are primarily appointed as doulas to provide continuous support during childbirth.‘We as a hospital are striving for baby-friendly accreditation; so the doulas must also go to the theatre for C-sections that they can accompany the mother and baby back from theatre after the Ceaser.’ (K1)‘The doulas are there to accompany them to the theatre and to remain with the mother, when she reaches recovery room she helps them to breast feed.’ (P1)Even where doulas are appointed, they do not work during the nights and over the weekends:‘At the moment they are of big help, but they are only working during the day, that is our problem they are not really working Sundays or public holidays and also night duty because they don’t get night duty allowance, they don’t get the extra money for public holidays or for Sundays.’ (K1)The absence, formulation or lack of implementation of various policies and guidelines present challenges for the implementation of continuous support during childbirth.The first sub-category concerns policy regarding companions.Policies and guidelines deal with the management of pregnant patients and also specify what type of management should be provided at each level of care The Guidelines for Maternity Care in South Africa ‘… People must be informed that this is the expectation of the person who is coming into labour room that we are expecting somebody who is going to give labour support.’ (P1)Beksinska et al.  also recWell-designed hospital environments, including the size and scale of hospital buildings, the layout, lighting and landscape, could impact women’s childbirth experiences And:‘we will need a space for it because at the moment our space for it is not even enough and I think in most of the government hospitals the space is not enough. We find that patients are overcrowded and I think a lot has to be done when coming [sic] to infrastructure.’ (T2)The second sub-category concerns privacy. Women in labour (and their companions) also need privacy:‘I would say about the structure, it would be nice if like every [sic], we would have only single rooms like for privacy for all the patients like if maybe a patient is having a supporter so that they can have their own room you know.’ (K2)Lothian  emphasisLewis  reportedThe midwives in this research understood the importance of communication:‘I also think that as much as we are short staffed ne, I think we have to have this relationship with the patient, continuous communication. As much as I am alone at the back, let me let the patient know I can’t be with you all the time throughout because now I am alone there are admissions there are other things that I should take care of, so I won’t uhm I won’t be able to sit next to you for the whole delivery; if maybe by also communicating with the patients maybe they will also understand because sometimes we find ourselves things will be happening, tomorrow it will be said the patient was neglected.’ (P1)The first sub-category under communication concerns language. Participants reported communication barriers if the support provider and the woman did not understand each other’s languages:‘We had this specific one last week here; a Shangaan and we were not able, but so luckily we were able to get an interpreter from another ward who could come and help us. And that is why sometimes it is better to get one of the family members who can also stay behind and support the patient.’ (P1)‘Clients come from Korea, China and Pakistan, so there is a difficulty in communication.’ (T1)Research conducted among women originally from Africa but currently staying in Australia  are from different cultural backgrounds, then the implementation of continuous support could be a challenging task. According to Berman et al. , caregiv‘Cultural factors also are a challenge because we experience when the white patients come they usually bring a lot of people following them, even the Indian people.’ (K1)‘Cultural factors also are a challenge because we experience when the white patients come in they usually bring a lot of people following them, even the Indian people. They are always there, they are always on your feet … and this people, especially the whites, and the Indians they are always nagging and demanding and you know wanting answers, wanting this and wanting that to be done fast and it is only three people being allocated per shift per day, night or day, but they are always in demands.’ (K2)Taniguchi and Baruffi  and NoblAkhavan and Edge  reportedIf the support providers display caring attitudes, they re-assure the woman and thereby enhance the experience of continuous support during childbirth. Disrespect and abuse in maternity care are attracting much attention: both globally ‘… but I also think to link it again with short staffed, because we are overworked, we are tired and we are irritated; so the staff morale is very low, so it reflects also on the patients; because we are irritated, so we get irritated with the patients; we are tired, so we [are] not nice to the patients; because we are overworked, the morale is extremely low and we are just negative.’ (K2)Kruger and Schoombee  explaineThis bad attitude can have serious consequences. In addition to negatively affecting continuous support during childbirth, Igboanugo and Martin  commenteThe second sub-category concerns the attitudes of the women and their families. Midwives also considered patient’s attitudes to be a challenge for providing continuous support:‘And problematic patients or problematic families you know sometimes patients eh patients from the community they come with the what, a negative attitude you know like eh they come here thinking that they are going to be shouted at, yelled at and already when they come in here they speak to us like we are nothing.’ (K1)‘Okay it means then that it is the problematic family which are uninformed, they do not know what is expected of them; if they are stepping into labour room, then they come being misinformed at some stage and it results in negative attitude, then it inhibits the care provided to be able to achieve maintain the care towards the client which she is supposed to be getting [sic].’ (K1)Kruger and Schoombee  concludeThe study participants indicated that continuous support during childbirth would be ideal but was not implemented in their maternity units because of organisational and personal challenges. The main challenges seem to be staff shortages and bad attitudes.A midwife (meaning ‘with woman’) is the ideal person to provide continuous support (Aune et al. Antenatal health care providers should inform pregnant women about the benefits of continuous support during childbirth and about their choice of preferred labour companion.Companions should be informed about their expected role in providing intrapartum support.Interested community members should be identified and receive doula in-service training to provide continuous support during childbirth.Family members selected as companions should receive information as to how to provide continuous support during childbirth.A friendly, welcoming and enabling environment in labour rooms where the patient can be surrounded by her choice of preferred companion for continuous support during childbirth should be provided.There must be enough posts for advanced midwives, registered midwives and doulas to provide optimal care – including continuous support during childbirth.Agency staff should be oriented regarding continuous support during childbirth.The importance of continuous support during childbirth should be emphasised in both the undergraduate and postgraduate midwifery curricula.Clinicians should also be competent in terms of communication. This goal can be achieved with cross-cultural training or the use of interpreting services to enhance continuous support during childbirth.The proposed curriculum content for doulas should include:non-pharmacological management of pain such as massage techniques, using different positions and hydrotherapyphysical and emotional supportcultural sensitivity.Antenatal policy and guidelines should explicitly mention health education regarding continuous support during childbirth and pregnant women should be encouraged to identify a companion of their choice.Each maternity unit should develop a policy about the presence of persons who can provide continuous support during childbirth.Culturally sensitive language policies should be available to ensure that midwives work in cross-cultural situations with backup interpreters to improve continuous support during childbirth. A list of interpreters should be available in maternity units.The following topics could enhance continuous support during childbirth:Developing a model of continuous support during childbirth appropriate to specific South African contexts.Establishing a model for patient:midwife ratios to determine how many midwives and doulas are needed in maternity units to render continuous support during childbirth.The study was conducted in the public sector in the North West Province, thus limiting the findings only to these hospitals. Some midwives were working in shifts, including night duty, which meant that some could not keep their focus group interview appointments. Midwives with less than two years’ experience in labour wards were excluded from the study. Their perceptions about challenges affecting the implementation of continuous support during childbirth might have added value. Only midwives participated in the focus group interviews. Consequently, women’s experiences and perceptions about continuous support during childbirth were not addressed.


Polyhedral oligomeric silsesquioxane (POSS) has been considered as one of the most promising nanofillers in academic and industrial research due to its unique multifunctional nanostructure, easy functionalization, hybrid nature, and high processability. The progress of POSS has been extensive, particularly applications based on single- or multiple-armed POSS. In polymer hybrids, in order to enhance the properties, bifunctional POSS has been incorporated into the backbone chain of the polymer. This review summarizes recent developments in the synthesis, modification, and application of bifunctional POSS-containing composite materials. This includes amino-POSS, hydroxyl-POSS, aromatic ring-POSS, ether-POSS, and vinyl groups-POSS and their applications, exemplified by polyurethanes (PUs) and polyimides (PIs). In addition, the review highlights the enhancement of thermal, mechanical, and optical properties of the composites. In recent years, inorganic–organic hybrid materials with carbon nanotubes ,2,3,4,5,Tg) were greatly enhanced. High-molecular-weight copolymers will be obtained by incorporating POSS moieties, which will result in improved mechanical properties. Due to this excellent property, these polymeric materials have found wide application in both academic and industrial fields. For example, hybrid POSS/polyurethane (PU) material has been applied for the construction of synthetic heart valve leaflets  Fi Fi73] Fi8) and 4OH-B-POSS (9) were synthesized. 2OH-B-POSS was formed by the reaction between 2H-B-POSS and a 1.3-fold excess amount of di(ethylene glycol) vinyl ether by means of a direct hydrosilylation reaction (1H-NMR. Through surface pressure-area (π-A) isotherms and Brewster angle microscopy, the amphiphilic B-POSS monolayer at the air–water interface was observed.Due to the strong aggregation tendency of the super-hydrophobic POSS moiety , we revireaction . Similar1H-NMR and FT-IR. The 2DEGNH-B-POSS with a urethane group at the end exhibited a strong hydrogen bond interaction in the 2DEGNH-B-POSS rod-like assemblies. A uniform liquid-like monolayer was formed by the mixture of 2DEGNH-B-POSS and full name 2DEG-B-POSS (1:1 molar ratio). Monolayer properties could be improved because of the hydrogen bonds between the urethane groups in 2DEGNH-B-POSS and the hydroxyl groups in 2DEG-B-POSS, which guaranteed the successful separation of L-B film.To further probe the hydrogen bonding effect on the monolayer properties, they also synthesized amphiphilic B-POSS with core-corona amphiphile full name 2-di(ethylene glycol) urethane B-POSS (2DEGNH-B-POSS) . Ethyl i10) was synthesized by Wei et al.  catalyst. Then, B-POSS-2Si-decenyl was obtained by the condensation reaction between B-POSS-4OH and dichloro(dec-9-enyl)methylsilane under triethylamine (R3N) and THF. B-POSS-2-Si-Allyl (n = 1), B-POSS-2-Si-hexenyl (n = 4), and B-POSS-OH (n = 8) were also obtained by the same reaction. Walczak et al. . Fi. Fi87]. k et al.  also repAlong with silylative coupling, cross-metathesis provides efficient and complementary synthetic routes, leading to B-POSS hybrids of great practical importance. Zak et al.  producedMw) of copolymers improved with the elongation of reaction time, which was possibly due to the steric hindrance.After that, silylative coupling and metathetic copolymerization (ADMET) were further used to synthesize a new class of vinylene-arylene and B-POSS copolymers in another work . DiffereIt was found that the activity of the catalyst was affected by the feeding ratio of B-POSS in the copolymerization. Groch et al.  synthesiIncorporating B-POSS in the polymer chain provides a more effective retardation of the chain motion of the linear polymer, which might allow higher effectiveness in property modulation. However, it is noteworthy that B-POSS bridging different segments or chains on each side may offer an interesting new class of materials.4 and (CH3)(R)SiCl2, (CH3)SiCl3, as shown in 15 (AB) with asymmetric functionality. However, significant symmetric byproduct waste would be generated and it required differences in polarity between the byproducts and the desired asymmetric material to be effectively separated.Vogelsang et al.  reported16) by the selective protection of silanols with boronic acid  b3)]) as efficient and selective catalysts [3)] complexes were obtained by ligand exchange between tricyclohexylphosphine (PCy3) and N-heterocyclic carbene (NHC). The high catalytic activity exhibited by the new catalyst allowed the bifunctionalization of three divinyl-substituted B-POSS cage compounds in one pot. Tian et al. -bridged B-POSS with hexamethyltrisiloxane by Sodkhomkhum et al. . Phosphoyte film . First,  (POCl3) . 1H-NMR g et al. . Novel s3·Et2O and siloxanolate in the synthesis of an amide product. The difluorosilane, bearing a vinyl- or BF3-complexed amino group, was used as the raw material to synthesize polycyclic silsesquioxanes under mild conditions. This general strategy enabled the synthesis of B-POSS where various functional groups could be introduced.With a special substituted difluorosilane as a precursor, Tanaka et al.  reported13) was synthesized by Wang et al. via a hydrosilylation reaction between 3,13-dihydrooctaphenyl B-POSS (2) and allyl glycidyl ether [13, Zhang et al. [Tg value was higher than the PBZ and increased with the increased content of di-functional POSS macromere. With various amounts of 3,13-diglycidyl B-POSS, the nanocomposites containing 30 wt % of POSS showed the highest Tg value. The Td increased with the increased compound 13 content.3,13-Diglycidyloxypropyloctaphenyl B-POSS  terpyridine complexes exhibiting stereoisomerism was reported by Au-Yeung et al. . The cisLinear polymers refer to molecular chains that are linear and irregular. Due to the simplicity of materials processing, good solubility, high chain regularity, and POSS content in the hybrids ,105, morIn recent years, PU has been applied in various areas such as packaging, coatings, footwear, and consumer care products . The app8OH) and polyols through a one-step method. DMA analysis suggested that the enhanced Tg was due to the presence of B-POSS in the main chain. SEM images showed the presence of nano- and micro-sized B-POSS aggregates because of the heterogeneous dispersion of B-POSS in liner PU, which was further confirmed by the presence of a nanocrystalline phase of B-POSS by XRD analysis. The hydrophobicity and mechanical performance of the liner PU with B-POSS was obviously enhanced as well. Xu et al. [Tg had no influence on the relaxation of α′. The incorporation of POSS in the polyurethane structure affected the glass transition, the crystallinity of the soft phase, and the order–disorder transitions [Hebda et al.  reportedu et al.  also invu et al.  reportednsitions .Polyimides are high-performance materials that display many advanced properties, such as for instance good resistance at high temperature, low water absorption, and alkali resistance. Thus, they have been widely used in many fields such as microelectronics and the aerospace industry . Many ef4.Liu et al.  synthesiThis review focuses on the functional methods of bifunctional POSS and composites with B-POSS in the main chain. B-POSS has great prospects for the development of large numbers of groups in organic–inorganic hybrid copolymers with B-POSS as the main component. Therefore, a number of breakthrough studies were done on the structures and properties of hybrid materials with B-POSS. Nanocomposites with excellent properties were constructed with the incorporation of POSS into linear polymers. Good thermal stability and mechanical properties and exceptional dielectric properties and solubility were reported through this modification. Moreover, in the future, POSS-containing monomers will be applied in various fields that require outstanding properties of polymers. Without a doubt, many exciting developments await POSS-containing hybrid materials, with the possibility for exciting new discoveries in the future.


Bisphenol A is highly debated and studied in relation to a variety of health outcomes. This large variation in the literature makes BPA a topic that is prone to selective use of literature, in order to underpin one’s own findings and opinion. Over time, selective use of literature, by means of citations, can lead to a skewed knowledge development and a biased scientific consensus. In this study, we assess which factors drive citation and whether this results in the overrepresentation of harmful health effects of BPA.A citation network analysis was performed to test various determinants of citation. A systematic search identified all relevant publications on the human health effect of BPA. Data were extracted on potential determinants of selective citation, such as study outcome, study design, sample size, journal impact factor, authority of the author, self-citation, and funding source. We applied random effect logistic regression to assess whether these determinants influence the likelihood of citation.One hundred sixty-nine publications on BPA were identified, with 12,432 potential citation pathways of which 808 citations occurred. The network consisted of 63 cross-sectional studies, 34 cohort studies, 29 case-control studies, 35 narrative reviews, and 8 systematic reviews. Positive studies have a 1.5 times greater chance of being cited compared to negative studies. Additionally, the authority of the author and self-citation are consistently found to be positively associated with the likelihood of being cited. Overall, the network seems to be highly influenced by two highly cited publications, whereas 60 out of 169 publications received no citations.In the literature on BPA, citation is mostly driven by positive study outcome and author-related factors, such as high authority within the network. Interpreting the impact of these factors and the big influence of a few highly cited publications, it can be questioned to which extent the knowledge development in human literature on BPA is actually evidence-based.The online version of this article (10.1186/s41073-019-0065-7) contains supplementary material, which is available to authorized users. Bisphenol A (BPA) is a chemical substance, which is used in plastics of, for example, food containers and can linings. It is considered a potential endocrine disruptor, as it might bind to estrogen receptors in the body and mimic estrogen’s function . Most reScientific knowledge development to a large extent is driven by citations. Due to the large and growing number of scientific publications in the biomedical domain and the limitation of the maximum number of references in many journals, it is often not feasible to refer to all available relevant literature . In manyThe objective of this study is to assess the prevalence and determinants of selective citation in human studies on BPA in a quantitative manner.https://bit.ly/2kiDK4Z). The protocol is also available as Additional file The design of this study was described in a study protocol, which was finalized and published online prior to the data collection  AND (“Human*”). No limitations with regard to the health outcomes studied were applied.The search strategy was very broad and not specific, in order to avoid missing relevant publication. This led to a large number of publications, namely 3412. The article selection was carried out in two phases. The first selection round was based on the publication title, to limit the number of publications. The second selection round included studying abstracts, figures, and tables, to finalize the network of human BPA studies. Many publications discussed BPA together with many other chemical compounds. By looking only at the abstract, it was not always clear to which extent the publication included information on BPA. By looking also at the figures and tables, we could make sure the publication contained sufficient information on BPA to be part of the network. The complete article selection was conducted individually by two researchers, MJEU and BD, followed by several consensus meetings. In case no consensus could be reached, a third researcher (GMHS) was asked to take a decision . The reproductive outcomes studied included, among others, polycystic ovary syndrome, miscarriage, sperm quality, and in vitro fertilization implementation failure. The study designs presented in this network were observational studies , systematic reviews, and narrative reviews. The network contains 126 publications that reported empirical data, which were summarized in 43 review publications. None of the systematic reviews included a meta-analysis. Since the network contained only one experimental study, this publication was classified as a cohort study. Looking at the evidence on adverse effects of BPA on human health, 40 publications reported statistically significant results, 36 publications reported non-significant results, and 47 publications reported mixed results. The authors of 92 publications concluded that there was a harmful effect of BPA on human health. A mixed or unclear conclusion was drawn in 49 publications, against 28 publications that concluded there was no harmful health effect of BPA. None of the studies was funded solely by for-profit organizations, which made it impossible to assess the effect of funding source as a determinant of citation. This underrepresentation of private parties in BPA research is also visible in the affiliation of the corresponding authors. Corresponding authors of 136 publications are affiliated with university whereas only one corresponding author is affiliated with industry.Table Although the first human BPA studies were published in 2002, the majority of the literature is published from 2010 onwards. Nevertheless, it seems that some of the early studies attract a high number of citations. Two publications, a narrative review published in 2007 and a cross-sectional study published in 2008, received more than 50 citations , 26. On Study outcome, measured both as statistical significance and as author conclusion in line with the hypothesis that BPA is harmful to health, shows a significant positive association with the likelihood of citation. Significant and positive studies are approximately 1.5 times as likely to be cited compared to negative and non-significant studies, an effect that remains after the adjustment for study design. The concordance analysis showed that the study outcome was not likely to be concordant between the cited and citing publication (OR 1.06 0.79–1.42), Additional file , AdditioContrary to our expectation, systematic reviews were not more frequently cited than narrative reviews in the full network. Sample size, number of affiliations, and journal impact factor showed a moderate positive association with the likelihood of being cited, with ORs between 1 and 2. These effects could partly be explained by study design. The type of affiliation of the corresponding author, gender of the corresponding author, report of funding, and number of references showed no association with citation. Authority of the author and self-citation was found to have the strongest association with the likelihood of being cited. High authority, which was measured by a combination of the number of publications and the number of earlier citations in this field, increased the likelihood of citation by approximately three times. Authors were five times more likely to cite their own work compared to that of others.Knowing that the number of citations per publication is very skewedly distributed, we tested to which extent the results are driven by the two highly cited studies. As a data-driven, post hoc analysis, we excluded these two studies, which received more than 50 citations Table . The sigWith this citation network analysis, we aimed to quantify the occurrence of citation bias in the human BPA literature and the determinants that influence the citation behavior in this field. Based on the finding that positive studies have an approximately 1.5 higher likelihood of being cited compared to negative studies, we conclude that citation bias is present in the BPA literature, although its magnitude might be limited. This effect was not confounded by study design and remained after excluding the most highly cited studies. Also, based on the results from the concordance analysis, citation bias does not appear to be influenced by the study outcome of the citing publication.These results are in line with the findings of a recent meta-analysis on citation bias in various scientific fields, of which most were biomedical . This syThe second aim of this study was to assess the effect of other potential determinants of citation. In the complete network of 169 publications, sample size, journal impact factor, the number of affiliations involved, the authority of the author, and self-citation were found to affect the likelihood of being cited. This was in line with our expectations, based on previous research in different research areas , 31. HowIn addition to the citation bias found, we should be aware that a large proportion of the literature seems to be completely ignored. More than one third of the publications receive zero citations, and even though these are both positive and negative publications, it means that part of the evidence is being left out of the picture and researchers are not appreciated for their work. Looking at the distribution of the number of citations per publication over time, it seems that the highly cited publications are early publications in the field. With the growing amount of literature, the chance of not being cited at all seems to increase. Although it is logical and acceptable that founding publications are often mentioned to describe the research field, we should be aware that the more recent evidence is less often referred to. Especially because BPA is a research field that is highly debated in risk assessment and risk management procedures, it is important to have a complete overview of all available evidence. The finding that a big part of literature is not valued in terms of citations has also been found in other research fields –36. For If we look, on the other hand, at the highly cited publications, it is remarkable that these studies have a narrative review and a cross-sectional study design, both of which are study designs that are typically not very highly valued. Although we did not look into the content of the publications in this study, we should be aware that both studies do not give a complete overview of the literature, as a systematic review would do, and thereby improve the chance of skewed knowledge development.One of the limitations of the current study is that the search strategy was only applied to the Web of Science Core Collection, making it quite possible that some relevant publications have been missed. The search was limited to this database because Web of Science is the only database that has the option to download the publications together with their reference lists. This information was necessary to set up the database and perform the statistical analysis. Nevertheless, we have no reason to believe that the identified determinants of selective citation would be different if literature from other sources would have been included in the network. A related limitation was the fact that we did not check reference lists for missing publications. This might have led to missing relevant publications. However, we believe that checking of reference lists might have interfered with our research question. Checking of reference lists would only identify publications that were actually cited within the network, while still missing relevant publications that did not receive any citations.Concluding, we found proof that citation bias is present in the human literature on BPA. Publications that concluded a harmful health effect of BPA are 1.5 times more likely to be cited compared to negative publications. The association between other determinants and the chance of being cited is found to be hard to quantify since our analysis was highly influenced by a low number of highly cited publications. Nevertheless, journal impact factor and author-related factors such as author’s authority and self-citation show a consistent positive association with the chance of being cited. With these findings, we could conclude that the development of the available knowledge on BPA seems to be mostly driven by authority-related factors, instead of by the best available evidence.Additional file 1: Research protocol. (DOCX 436 kb)Additional file 2:Table S1. Effect of concordance between the cited and citing article on the likelihood of being cited. Reference list bisphenol A network. (DOCX 80 kb)


Escherichia coli O157:H7 was induced to the VBNC state at two different temperatures (-20°C and 4°C) and its resuscitation and morphological changes under different nutritional conditions were studied. The initial inoculum of 2.1 × 107 CFU/mL E. coli O157:H7 cells were induced into the VBNC state in normal saline, distilled water, LB broth at -20 °C after 176, 160, 80 days, respectively. The results showed that E. coli O157:H7 reserved at -20°C, and LB culture medium were easier to enter VBNC state than others conditions, the cells still had metabolic activity and the cell morphology changed from the typical rod shape to short rod and the cell size decreased. The resuscitate ways including the direct warming resuscitation, gradual warming resuscitation, adding chemical substance resuscitation, and adding nutrients resuscitation were studied. The results showed that the optimal conditions of 5% Tween 80 and 3% Tween 80 acculated the resuscitation of E. coli O157:H7 VBNC state cells induced by low temperature LB medium and low temperature saline. E. coli O157:H7 VBNC state failed from resuscitation when incubating in LB broth, respectively using direct warming and adding nutrients substance. This study provides new insights into induction and resuscitation of VBNC E. coli O157:H7 and offers an approach for investigating the formation mechanism of VBNC foodborne pathogens in food safety.Viable but non-culturable (VBNC) cells are alive bacteria cells, but lose their culturability in conventional culture media, usually escape detection by the plate count method and pose a serious threat to food safety and public health. Therefore, it is urgent to study the VBNC status, and to provide theoretical basis and scientific basis for food processing and safety control caused by pathogenic microorganisms. In this study,  Escherichia coli O157:H7, Staphylococcus aureus, Salmonella enterica, Listeria monocytogenes, Campylobacter jejuni, and so on , such as hemorrhagic enteritis, hemolytic uremic syndrome, and thrombotic thrombocytopenic purpura  state and formation of biofilms and so on  was mainly used as the target strain, and the ability of the cells to enter the VBNC state under different environmental conditions was studied according to the possible environment of food storage in the low temperature. The metabolic activity of VBNC E. coli O157:H7 was detected by AODC and DVC methods. And the resuscitation methods on the VBNC cells induced by low temperature were also explored.Low temperature refrigeration is the most common way of food preservation. However, many research reported that low nutrient and low temperature are main causes of the induction of VBNC stages of pathogenic bacteria . TherefoTM Tryptic Soy Broth  containing 20% glycerol.Bacterial strains were stored at -80°C in BactoE. coli O157:H7 medium  for 24 h at 37°C. The bright red single colonies were used to identify presumptively the presence of E. coli O157:H7 .All the strains were cultured on chromogenic E. coli O157:H7 was diluted to 2% in 100 mL fresh Luria Bertani broth  then inoculated at 37°C in shaking bed with 190 r/min. Then the absorbance value (at 600 nm) of LB broth was determined every 0.5 h until 12 h. Duplicate experiments were performed, and final absorbance values were obtained as means of three determinations in each experiment. Therefore, the logarithmic growth phase of E. coli O157:H7 can be determined by growth curve.Overnight suspension of E. coli O157:H7 bacterial suspension was inoculated into LB broth at 37°C, 190 r/min until the E. coli was cultured into mid-logarithmic phase. And then the initial number of CFU/mL about phase bacterial suspension was confirmed by plating serial decimal dilutions in sterilized saline water followed by overnight incubation at 37°C. Exponential phase cells were harvested by centrifugation at 8,000 × g for 5 min, washed three times with 0.85% (w/v) sterile saline solution remove nutrients. Cells were resuspended in the 0.85% (w/v) sterile saline solution at a final density of approximately 107 CFU/mL. According to the environment and temperature of food preservation, the objective of this study was to induce E. coli O157:H7 under the conditions of physiological saline at -20°C, sterile distilled water at -20°C, LB broth at -20°C, sterile distilled water at 4°C, and LB broth at 4°C, respectively. Bacteria were counted every 4 days to determine their culturability. In order to avoid the influence of the repeated freezing and thawing by plate counting, we dispensed the mixed bacteria suspension 1 mL into several 1.5 mL tubes and placed at low temperature for induction.The overnight culture of E. coli O157:H7 were determined by the direct viable count (DVC). One milliliter of each sample was centrifuged at 4000 r/min for 5 min, washed three times with ultrapure water. The pellets were incubated with final concentration of 0.025% yeast extract  in the presence of 0.002% nalidixic acid  at 37°C for 12 h. After that, the incubated suspension with the dye mixture of E. coli O157:H7 were determined according to the above procedure of AODC. All the numbers of total and viable cell counts were obtained as means of counts visualized in 10 randomly selected microscopic fields  according to c fields .E. coli O157:H7 (ATCC 43895), in VBNC state for 3 days, in the mid-exponential phase, and the killed by boiled water for 5 min, were centrifuged at 8000 r/min for 5 min and washed three times with sterile phosphate-buffered saline . The harvested bacteria pellets were placed in a fixative 0.2 mol/L phosphate buffer solution containing 2.5% (wt/vol.) glutaraldehyde for 4 h at 4°C. After the fixed completion, the samples were rinsed three times with additional PBS to remove fixative and were further dehydrated for 10 min in graded ethanol series solutions , and then rinsed in 100% ethanol at 20 min intervals for two times. At last, all the samples were replaced with 100% tert butanol for 15 min and dried in freeze-drying machine. For scanning electron microscopy (SEM) assay, the samples were coated with gold palladium for 90 s in an IB-3 vacuum evaporator coating apparatus (Eiko) and were then examined with scanning electron microscope .Cells of E. coli O157:H7 chromogenic medium.The resuscitate ability of any bacterium is critical, if the VBNC state is the real survival strategy. To investigate the recovery culturability processes of the bacterium, a series of independent experiments was conducted. In this study, we took four methods such as improving temperature up at 37°C directly, increasing temperature from 15°C to 37°C, adding Tween 80 with improving temperature at 37°C and adding nutrients with improving temperature at 37°C to explore the resuscitation of VBNC bacteria. In this work, samples containing no supplements and without any treatment were designated as controls. At the same time, all experiments were carried out in three replicates, and the recovered cells would be validated on E. coli O157:H7 that remained in the microcosms, in this study, all the samples were diluted tenfold in sterilized distilled water as described previously studies  was diluted with distilled water to desired concentrations and sterilized. Each sample added sterilized Tween 80 to make the final concentration was 8, 5, 3, 2, 1, 0.5% (w/v), respectively, and incubated at 37°C with shaking. Each sample of colonies was enumerated after 24 h of incubation.Resuscitation of the VBNC cells was performed by improving temperature at 37°C in the 25% (w/v) sterilized yeast extract  broth with shaking. After 24, 48, and 72 h incubated, 0.1 mL sample were spread on plate count agar. At same time, samples containing no supplements were designated as control.E. coli O157:H7 grew rapidly in the geometric progression within 1 to 4 h. After 6 h, the growth rate slowed down and the absorbance value increased slowly. According to the relevant studies, the bacteria morphology and biological activity in the logarithmic phase are more typical, which are very sensitive to the external environment, and easier to access the VBNC state under certain induction conditions  during the first 10 days. The initial inoculum of 2.1 × 107 CFU/mL declined to below 0.1 CFU/mL to undetectable levels on the 80th day. Continuous cultivation for 3 days, the plate counts were always below 0.1 CFU/mL. At this stage, it could be considered that E. coli O157:H7 may enter to the VBNC state. However, the culturable number variation of E. coli O157:H7 in normal saline was similar to that in distilled water at -20°C. Both colony counts declined rapidly within first 10 days and then gradually decreased to undetectable levels at 176 and 160 days, respectively.The culturability of E. coli O157:H7 at -20°C are much less than that at 4°C. Beyond that, E. coli O157:H7 in eutrophic condition has poor tolerance to low temperature than in oligotrophic condition, and are more susceptible to loss of the culturability. This study also reveals that the E. coli O157:H7 in the physiological saline is more stable than in the distilled water environment at -20°C. It is well known that physiological saline has a balanced osmotic pressure on cells, which means that it has a certain protective effect on cells. Thus, the results may be due to the physiological saline that has a balancing effect on osmotic pressure of bacteria compared with sterile distilled water . Thus, we performed experiments by adding Tween 80 with improving temperature at 37°C, and the results showed that E. coli O157:H7 in the VBNC state could resuscitate in the presence of Tween 80 with temperature upshift. The resuscitation results are listed in Table E. coli O157:H7 cells produced a number of colonies with 7.2 × 102 CFU/mL and 1.3 × 108 CFU/mL on the plates at 24 and 48 h, respectively, while no colony was detected on the non-amended plates. The VBNC E. coli O157:H7 cells incubated with 3% Tween 80 for 48 h at 37°C, no colony was detected on the plates, but VBNC E. coli O157:H7 cells produced a number of colonies with 7.3 × 106 CFU/mL after 72 h. Meanwhile, all the recovered cells were validated on E. coli O157:H7 chromogenic medium, and found that the colonies were bright red and consistent with the characteristics of the normal state of the cell, indicating that the resuscitation of VBNC bacteria restored its culture features. But, the bacteria cannot resuscitate with same conditions when E. coli O157:H7 were induced into VBNC state for 10 days. VBNC E. coli O157:H7 were incubated in other concentrations of Tween culture, but no colony was observed on the plate. At the same time, all the sterile colonies (-) shown in Table Viable but non-culturable cells were conducted in triplicate by different measures to examine the recovery potential and characteristics. When resuscitation of the VBNC cells was performed by increasing temperature from 15°C to 37°C, the results showed that the method was unable to induce resuscitation of E. coli O157:H7 cells were induced to the VBNC state at -20°C and 4°C in different conditions. In the first 8 days, all the organisms maintained at 4°C showed the culturable colony counts increased especially for LB broth. We conjectured that E. coli O157:H7 of mid-exponential growth phase had strong growth and reproductive ability, a low temperature of 4°C did not inhibit its reproductive growth in a short time. The LB broth contains essential nutrients for growth and reproduction of E. coli O157:H7, thus the culturable colony counts increased especially. Meanwhile, E. coli O157:H7 at 5°C was better than at 20°C or 30°C. The cells incubated at -20°C were induced to the VBNC state in three conditions , which needed 80, 160, and 176 days, respectively. These results are similar with E. coli O157:H7 was induced to enter the VBNC state in LB broth at low temperatures (-18°C) for a shorter time than in physiological saline. While the reason for this result is not clear at the moment, we suspect that this may be related to the adaptability of bacteria in different conditions. However, the cells were incubated at 4°C about 190 days, culturable numbers were maintained at a bacterial concentration greater than 107 CFU/mL. Our results were similar to those of E. coli O157:H7 ATCC 43895 remain 107 CFU/mL and failed to enter VBNC state at 4°C with limited nutrient. Compared with previous literatures, S. aureus spp.  were induced into the VBNC state after 210 days under filtered-sterilized seawater at 4°C. V. vulnificus in artificial sea water at 4°C incubated into VBNC state need 70 days. 8 cells/mL of C. jejuni 241 and C. jejuni ATCC33291 in fresh water at 4°C were induced to VBNC state after 48 and 46 days, respectively. Therefore, these results suggested that the cells incubated at 4°C may spend more time for VBNC state formation than the cells incubated at -20°C or fail to enter this state until death. In addition, it was found that the ability and the speed of the VBNC state induction were mainly related to the incubation conditions and species of the cells and species. This study used fluorescence microscopy and scanning electron microscopy to observe the viability status and morphological changes in VBNC samples. It suggested that the VBNC state E. coli O157:H7 may exist in the qualified frozen foods detected by the national standard culture method and had threat to the food safety and also provided the basic data for the microbial detection standard. The non-culturable cells of E. coli O157:H7 were detected by the DVC method, most of cells morphology had obvious growth and elongation compared to the cells by the AODC method detection. It suggested that the VBNC state E. coli O157:H7 was still alive. Morphological and characteristics of VBNC cells were observed in this work. Compared with mid-exponential-phase cells, the cells entered into VBNC which morphological changed from the typical rod shape to short rod and the cell size decreased, except that the cells were relatively rough, shrunken, the shape irregular, and some of them tended to be clustered. This state was between logarithmic and dead cells and caused by bacteria to resist environmental pressure. And the morphological of VBNC cells were consistent with other researchers reported , which is food processing conditions. It can be obtained that bacteria are subjected to different environmental stresses, the ability and the speed of the VBNC state induction were also different. At the same time, it also can indicate that the foodborne pathogens may enter the VBNC state during food processing techniques, such as high pressure, low temperature storage high temperature, and they have become a potential risk for food safety. If the VBNC state is a real strategy to survive the pressure of the environment, the resuscitate ability of any bacterium is critical. Therefore, we attempted variety of methods to resuscitate E. coli O157:H7. Our resuscitation experiments showed that E. coli O157:H7 in LB and physiological saline at -20°C induced VBNC state can resuscitate in 5% Tween 80 and 3% Tween 80, respectively, at 37°C, which has not been recognized previously. However, the VBNC states were not resuscitated by temperature upshift treatment, adding nutrients with improving temperature and gradient warming. There was clear difference between some previous literatures reported  are commonly used to preserve food products in our life. In this study, reported . In our reported . But, adreported . VBNC E.7 CFU/mL E. coli O157:H7 cells were induced into the VBNC state in normal saline, distilled water, LB broth at -20°C after 176, 160, 80 days, respectively. Through detecting VBNC cell by the DVC methods and AODC method and SEM, the E. coli O157:H7 VBNC cells still have metabolic activity and the cell morphology changed from the typical rod shape to short rod and the cell size decreased. VBNC E. coli O157:H7 cells could regain the ability to divide after adding Tween 80 at 37°C. This study indicates that E. coli O157:H7 can be induced into VBNC state with active metabolism by low temperature and also can resuscitate to fully culturable. The formatting of VBNC and the potential resuscitative strategy described in this study may have meaning to the risk assessment in food safety and pathogenic bacteria detection method.In this study, the initial inoculum of 2.1 × 10XZ conceived and designed the experiments. XZ and CW performed the experiments and contributed to the writing of the manuscript.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


To investigate the transport mechanism of the multispecies, complex scientific methods and accurate mathematical models are needed. The purpose of this study is to develop a more robust mathematical model and better computational technique to characterize the coupled effect of ionic transport mechanisms as well as the influence of interaction of ionic species. The new mathematical model was developed based on the Nernst–Planck equation and null current condition to solve the ionic-induced electrostatic potential, and the model was implemented by a parallel finite element algorithm. The verification of mathematical model was done by comparing the model prediction with experimental results for ionic transport in saturated concrete. The comparisons showed good results. The model prediction of the multispecies transport in partially saturated concrete demonstrated that the ionic species dissolved in pore solution could be carried by the moisture movement and pressure gradient. Therefore, the multispecies transport model based on the parallel finite element method is effective, accurate, and can be used for solving the partial differential equations for ionic species transport in concrete.The chloride-induced corrosion of steel reinforcement embedded in concrete is undoubtedly one of the most important durability problems of reinforced concrete structures. The chloride ions as well as other ionic species (Na Frequently, different types of deicers can be used in the same area at the same time and the interactions among the ions have significant effects on the corrosion behavior of the steel in concrete and thus on the service life of reinforced concrete structures [Concrete is the most widely used construction material in the world because of its availability and good performance. Relatively low tensile strength and low ductility of concrete lead to the necessity of steel reinforcement in concrete structures. Reinforced concrete structures have a long-term durability problem that is the corrosion of steel reinforcement in concrete. This problem is especially severe under the service conditions such as roadways and parking structures where deicing chemicals are used in the winter and marine structures where the structures are in direct contact with seawater. In these reinforced concrete structures, the corrosion of steel has been considered to be caused mainly by the chloride ions in the deicers or seawater, called chloride-induced steel corrosion. However, the chloride ion is not the only ion penetrating into the concrete, several other ionic species in the environment penetrating into the concrete together with the chloride. For examples, sodium chloride (NaCl), calcium chloride  model have been developed with the Nernst–Planck equation related to the ionic interaction of multi-ions in concrete pore solution. This model can be incorporated with influences of boundary conditions, current density, and transport parameters of concrete. In electrical field, the concentration profile of ionic species charged by various current densities can be demonstrated by considering the activity of ions in the solution ,9. For tTo evaluate the electrical coupling between ions in porous media and to consider the chemical activity effects in non-ideal solution, Nernst–Planck equation can be solved together with Poisson’s equation. Lately, the transport model of ions in nonsaturated cement-based materials was researched in relation with external sulfate or calcium and hydroxide leaching. A mathematical model was developed based on a set of Nernst–Planck and Poisson equation by taking into account the diffusion, electrical coupling, chemical activity, and advection mechanisms in the isothermal condition ,16,17. Recently, a multispecies transport in a multiphase medium has been developed by considering the effects of aggregates, interface transition zones (ITZs), and ionic binding on chloride penetration in concrete to provide a more accurate prediction of the complex interaction between ions ,19,20.As mentioned above, the transport equation of multispecies has been developed with mathematical theories for obtaining analytic solutions. It turns out that finding explicit solution is very difficult due to the complexity of equations and limits of analytical tools. Thus, computational techniques such as finite element method (FEM), finite difference method (FDM), and so on need to be employed ,21,22,23The phenomenon of multi-ions transport occurs usually in a thin layer of concrete structures covering steel bars . In order to capture the distribution profiles of multi-ions in non-steady state in the concrete cover, very small finite elements must be adopted. For predicting the service life of large-scale concrete structures with different environmental conditions, enormous number of elements are needed and each element consists of several nodes and each node includes lots of information . To makeIn this study, a mathematical model for the transport of multispecies deicing salts in concrete is presented. The model involves not only the effect of transport mechanism but also the influence of interaction of the ionic species. Furthermore, the coupled effect of moisture and multi-ions diffusion is incorporated in the model. The numerical results are obtained by the parallel finite element method and are verified by available experimental results in saturated concrete. Finally, multispecies transport is predicted in partially saturated concrete.−, Na+, Ca2+, K+, OH−) in concrete mainly occurs in the cement paste or mortar and in the pore solutions. Total chloride is the sum of bound and free chloride. The free chlorides are water-soluble and they can attack the passive film on the surface of reinforced rebar. In traditional methods to predict the service life of concrete structures, only chloride ions are taken into account. Even though this way is simple and easy to implement in the computation, it fails to characterize realistically the concrete deterioration mechanisms due to ionic and mineral interactions. To better describe the ionic transport process in a porous media, the governing equation of multispecies transport in partially saturated concrete affected by moisture gradient can be written as follows.Concrete can be considered as a composite material with three phases including aggregate, cement paste/mortar, and porosity. The transport of ionic species is negligible in the aggregate because the transport parameters of aggregate are much lower than those for cement paste and mortar. However, a pore system provides the diffusion path to migrate ionic species. Therefore, the transport of multispecies , iZ is the charge number, F is the Faraday’s constant, R is the gas constant, T is the temperature, Ф is the electrostatic potential, and index i represents for i-th species.For the flux of each species in the pore solution of saturated concrete, the Nernst–Plank equation isФ.Under an internally induced electric field , all the ionic species in the pore solution have effect on the electroneutrality condition. In order to solve the system of equations, a supplemental relation is needed to consider the electrical potential caused by the movement of all species. There are three methods to determine the magnitude of ElectroneutralityNull currentε is the dielectric constant. These three equations are taken over all ionic species in the solution. Nguyen et al. found that an electrostatic potential could be evaluated in terms of electroneutrality which can simplify the numerical simulation shown in Equation (2) [Poisson equationtion (2) . The eletion (2) ,10,14. Ption (2) ,17. In tFinally, the flux of multispecies in nonsaturated concrete is,The mass balance equations for each ionic species can be expressed asBy substituting Equation (1) into Equation (4), yieldsHJ is the flux of relative humidity, HD is the diffusion coefficient, and H is relative humidity.The moisture transport can be described,In the same way, the mass balance equations for moisture is+, Ca2+, K+, OH−, Cl−). Total degree of freedom in this matrix form is 6 unknowns.Finally, Equations (8) and (10) can be combined and simplified into matrix form,In order to numerically solve the multispecies transport problem, material parameters shall be needed as shown in Equations (12) and (13); the moisture capacity (nt (DCl) .+, Ca2+, K+, OH−) capacity is employed as 1 in this study.The moisture capacity i et al. ,27,28,29d Bazant . The othThe moisture diffusion coefficient nt paste . The difd Bazant . It depeThe influence of chloride and moisture was described by Ababneh and Xi and Abarr ,33. AbabHowever, there have been no experimental data or material models of coupling parameters for other species, Na, Ca, OH, and K. As the first approximation, the coupling parameters of these ionic species can be evaluated proportionally based on the previous research ,33. As s+, Ca2+, K+, OH−), and relative humidity  and (20) are defined in terms of nodal values, The nodal relative humidity, free chloride, and other species concentrations are solved by substituting the approximated values of Equations (19) and (20) into governing equations of Equations (8) and (10), and applying the Galerkin procedure to the weak forms, then the finite element matrix can be obtained as shown in Equation (24):In detail, the components in element matrices are as Equations (28)–(33),Finally, Equation (34) is also discretized in time space with time interval The value of parameter Equation (34) is simplified as linear system equation.The parallel program developed in this study is designed to be divided into multiple fragments that can be executed simultaneously on its own processor with applying a domain decomposition method.The domain decomposition (DD) method is attractive in parallel finite element computational algorithm because it is usually used for solving large-scale system equations and allows individual subdomain operations to be performed concurrently on separate processors .There are two types of domain decomposition methods, overlapping and nonoverlapping methods. In this study, the overlapping method was applied to solve the linear sparse matrix because of easier implementation in algebraic approach as well as faster convergence than nonoverlapping DD method. Furthermore, the boundaries of extended subdomains are smoother than nonoverlapping subdomains. The domain decomposition method needs to use iterative solver. In this study for the iterative solver, GMRES  was mainly adopted because of its ability to solve nonsymmetric linear system of global and local matrix. To improve the convergence of multispecies transport problem, the additive Schwarz preconditioner was applied as well.The details of preconditioned Schwarz method as the mathematical formulations of the overlapping domain decomposition (DD) method can be demonstrated as in the following ,35. To oHere, Recall Equation (35) and it can be simplified as below,The additive Schwarz preconditioned system can be written asIn order to form the algorithmic structure of overlapping DD method, there are four components: subgrid preparation component, subdomain solver component, data exchange component, and global administration component. In detail, the starting point of the overlapping DD method is a set of overlapping subdomains, which can be its subdomain solvers. Then for updating the boundary condition during each iteration, data must be exchanged between neighboring subdomains. In order to coordinate the subdomain solvers and data exchange, a global administration component would play a role in controlling the progress of DD iterations, monitoring the global convergence behavior, starting the subdomain solvers, and invoking data exchange .For the implementation of a parallel finite element (PFE) program for the multispecies transport problem, various programs were used such as Triangle for mesh generation, Parmetis, PETSc, and MPI ,39,40,41In order to provide the numerical codes involving the implicit numerical solution of PDEs, PETSc (3.0.0 p8) was employed. PETSc library is written in C, but it may be accessed from user codes written in C, Fortran, and C++ . MPI  was employed. This is an open-source multiplatform application for scientific visualization developed to analyze extremely large datasets using distributed memory computing resources. For large-scale information, VTK file format was used because it is easy to be read and written by hand or programmatically. VTK input file is automatically generated once the program runs. For verifying the parallel finite element algorithm, the test results of multispecies penetration into saturated concrete were employed, which were obtained by Damrongwiriyanupap . The mixThe Nernst–Planck equation can be used to characterize the penetration of multi-types of deicing solutions into concrete. In order to verify the numerical model, total chloride profiles corresponding to As shown in These figures indicated that the numerical results have a good agreement with the experimental results of total chloride concentrations. However, the numerical results with 0.55 water–cement ratio fit better with the corresponding experimental results than the case of 0.65 water–cement ratio. That is because the material models used in the numerical analysis were more accurate to predict the penetration of chloride into concrete made by 0.55 water–cement ratio.+) and potassium (K+) were plotted in comparison with numerical results at 15 and 30 days of exposure periods. The trends of sodium (Na+) and potassium (K+) profiles resulting from the prediction model have a good match with the experimental results as shown in The present numerical model can be used to simulate not only the chloride transport but also the transport of other chemical species. As illustrated in 2, and the other species, Na+, K+, and OH−, were initialized to be dissolved in the concrete pore solution. Particularly, the concentration of alkali ions in pore solution, Na+ and K+, was obtained from cement manufacturer which provided the chemical composition of cement [2 was used as the boundary condition. All other sides except the top surface were assumed to be sealed. The moisture condition inside the concrete specimen was assumed to be partially saturated with 60% relative humidity (RH). This study focused on the chloride penetrating into concrete specimen from the top surface. The dimension of the specimen was 15 cm high and 30 cm wide as shown in f cement . In realtol = 10−4, and absolute tolerance, Atol = 10−10, respectively. As previously mentioned, Krylov Subspace Method (KSP) GMRES with additive Schwarz preconditioner was used to solve the large linear sparse matrix.Obviously, this numerical model is used to simulate the concrete deterioration with multispecies under service condition in which the concrete is not saturated, regarding to a long-term pond test. With these initial and boundary conditions, one can speculate that the chloride and the moisture transports were in the same direction. Total analysis time was 400 days and the time step was 0.5 day. There were two kinds of stopping tolerance conditions used in the program; relative tolerance, RIn order to implement the parallel finite element computation for coupled moisture and multispecies transport problem, large sparse matrix should be derived and the continuous variables, relative humidity , and the time taken for the same code to execute on p processors, Tp(n), as below Equation (42). Parallel efficiency can be defined as the ratio between speed-up and the number of processors as shown in Equation (43).To verify the performance of parallel finite element algorithm, the concept of speed-up and efficiency is necessary. The definition of speed-up is the ratio between the time taken by the code to execute on a single processor, For the parallel effect of this algorithm, the meshes composed of the numbers of nodes, 3000, 6000, 12,000, and 24,000, were used with increasing the number of processors up to 64. In 2 was used as a boundary condition. Moreover, the ions in concrete pore solution, K+, Na+, and OH−, were considered as initial conditions. The material parameters and input data for numerical simulations were the same as used in 2 solutions on the top surface.In the nonsaturated concrete, the effect of chloride penetration on moisture transport is significant . For obsThe profile of total and free chloride concentration at different exposure times is shown in −, Na+, and Ca2+). That is, the ionic transport can occur through saturated and partially saturated concrete. Moisture transport in saturated condition can be controlled by a pressure gradient into concrete. The ionic species dissolved in pore solution also can be moved by permeation of moisture or water.As plotted in +) and hydroxyl (OH−) ion were from inside to outside of concrete because the inside concentration was higher than the concentration of the exposed surface, which means that any source of potassium and hydroxyl solution was not supplied on the top surface. The variations of potassium and hydroxyl concentration with depth at different exposure periods were shown in On the contrary of chloride, sodium, and calcium concentration profiles, the concentration gradient of potassium  come from deicing salt and are transported from the environment into concrete. The phenomenon of multi-ions transport can be observed in a thin layer (concrete cover depth) of concrete structures. Parallel computing is one of effective techniques to save computational time. To investigate the transport mechanism of multispecies in this study, complex scientific methods and more accurate mathematical models are needed.The parallel finite element model was implemented with higher level library, PETSc  and MPI (Massage Passing Interface), based on C++ program language. For the easier implementation in algebraic approach as well as faster convergence, the overlapping DD method was employed to solve the linear sparse matrix. To improve the convergence of multispecies transport problem, the additive Schwarz preconditioner was applied and GMRES  as an iterative solver was mainly adopted because of its ability to solve nonsymmetric linear system of global and local matrix. The new mathematical model was developed based on the Nernst–Planck equation and null current condition to solve the ionic-induced electrostatic potential. This model included the material models of transport mechanisms in cement paste and aggregates. The model can be used to simulate the multispecies penetration into concrete structures by considering the effect of moisture gradient. Moisture transport has significant effect on penetration of deicing salts into concrete by accelerating the penetration rate of chloride, sodium, and calcium ions. The coupling parameter was assumed because of no experimental data or material models of coupling parameters for other species. The parameter considering the effect of moisture on ionic transport was assumed by the same as used for the chloride ions due to carrying any other ions as it carries the chloride ions. The coupling parameter of each species can be estimated by the ratio between a specific ionic species and the chloride ions.In order to verify the new mathematical model, the parallel finite element program was developed and the numerical results were compared with the experimental results about the penetration of multi-types of deicing solutions into saturated concrete. The comparisons showed good results and the accuracy of the new mathematical model for the transport mechanism of the multispecies was proven effectively. The model prediction for the ionic transport in partially saturated concrete was conducted to investigate the transport of ionic species such as chloride, sodium, calcium, hydroxyl, and potassium. The penetration of chloride, sodium, and calcium increased over time because the initial concentration outside concrete was much higher and the moisture gradient had an effect on accelerating the transport rate of ions. However, the concentration of hydroxyl and potassium was decreased due to opposite reason. This model can be used satisfactorily to predict the penetration of aggressive chemicals, such as deicing salts into nonsaturated condition.For more robust verification, additional deicing ponding tests under nonsaturated conditions are needed in the future. More material models are needed to take into account the effects of mineral additives such as fly ash and silica fume on chloride penetration and ion transport in concrete. Additionally, the temperature effect is to be considered as one of the crucial parameters.The chloride-induced corrosion of steel reinforcement is undoubtedly one of the most important causes and has an effect on the integrity and the service life of concrete structures. The chloride as well as other ionic species (Na


Although considerable effort has been taken regarding concrete damage, the physical mechanism of concrete damage under compression remains unknown. This paper presents, for the first time, the physical reality of the damage of concrete under compression in the view of statistical and probabilistic information (SPI) at the mesoscale. To investigate the mesoscale compressive fracture, the confined force chain buckling model is proposed; using which the mesoscale parameters concerned could be directly from nanoindentation by random field theory. Then, the mesoscale parameters could also be identified from macro-testing using the stochastic damage model. In addition, the link between these two mesoscale parameters could be established by the relative entropy. A good agreement between them from nano- and macro- testing when the constraint factor approaches around 33, indicates that the mesoscale parameters in the stochastic damage model could be verified through the present research. Our results suggest that concrete damage is strongly dependent on the mesoscale random failure, where meso-randomness originates from intrinsic meso-inhomogeneity and meso-fracture arises physically from the buckling of the confined force chain system. The mesoscale random buckling of the confined force chain system above tends to constitute the physical mechanism of concrete damage under compression. Concrete, a mixture of Portland cement, water, sand and aggregate, hydrated to form cementitious material with micro-crack, void and inhomogeneous , exhibitTo disclose the physical mechanism, the physical experiment could always be considered as a direct and effective way. Although it is generally realized that the nonlinearity and randomness of the mechanical properties for concrete are directly related to the fracture of the meso-element and its accumulation, the direct access to the knowledge regarding the local properties has never been provided until the 1970s. Hereafter, Beaudoin and Feldman systematically studied the properties of the autoclaved calcium silicate systems including elastic modulus, micro-strength, micro-hardness and the relationships between the properties by micro-hardness testing, and additionally, suggested the linear relationship between microscale compressive strength and micro-hardness . From IgIn this research, the damage of concrete is investigated when subjected to compressive loading. Based on the previous research achievement, it is hypothesized that the fracture of “concrete fiber” originates at the mesoscale. On the one hand, nanoindentation tests are conducted on each constituent of concrete: hardened cement paste (HCP), interfacial transition zone (ITZ) and aggregate. By application of the confined force chain buckling model proposed in this paper, reconstruction technique  and randThe material prepared here was ordinary concrete with water: cement: sand: with an aggregate ratio of 0.4:1:2:5. The bars were made measuring 0.1 m × 0.1 m × 0.3 m, and hydrated with the humidity of 95% at the room temperature for three months, which were used for uniaxial compression testing at the macroscale. Then, the prisms were sliced into small specimens with approximate dimensions of 0.02 m × 0.02 m × 0.005 m, which were prepared for nanoindentation. After embedding into the epoxy resin, grinding and polishing with silicon carbide papers, and ultrasonically cleaning, the samples for nanoindentation  were preH and indentation modular M were obtained from loading-unloading curves. Furthermore, Young’s modulus linking to the elastic constants of specimen and indenter [The equipment conducting nanoindentation in the present paper is the NanoTest Vantage system  of cement paste in the surrounding area of the indenter, when applying compression by a strain Substituting Equations (6)–(8) into Equation (5), one obtains the potential energy density:It is evidently observed that the normal strain and the tangential strain could be decoupled.According to the principle of resident potential energy, the partial derivative of the averaged potential energy density with respect to each degree of freedom should be zero, we getSubstituting Equation (9) into Equation (10), and solving Equation (10), the relationship between the stress and the normal strain yields Equation (11).From Equation (11), an apparently stable path for the confined force chain could be achieved when the force chain structure keeps straight from the beginning of loading, which corresponds toSolving Equation (11), Another stable path could be acquired when To conduct Taylor expansion for Equation (16) and Equation (17) at As easily seen from Equation (19), a mixed-mode of shearing , the hardness and the yield stress relationship of the form of 20–30 . Similarof 20–30 , the ratBased on the previous researches ,29 and EWith the assumption of elastic-brittle property of mesoscale element aforementioned ,7,8, theDue to the attribute of a sufficient large degree of disorder, the knowledge of the probabilistic characteristics of concrete is fundamental to understanding the intrinsic random microstructure, even the nanostructure. Generally, random field theory deals effectively with the complex distributed disordered system . As the The random field could be defined as homogeneous, as long as the mean (1)Choose a sample (2)Compute the observed cumulative distribution function (CDF) (3)Estimate the parameters of the hypothesized distribution as described below based on the observed data and determine the theoretical CDF (4)Form the differences (5)Select a value of (6)Accept or reject the testing hypothesis by comparing To investigate the statistical characteristics, Kolmogorov–Smirnov test (referred to as the K–S test) could be adopted to acquire the probabilistic density function (PDF) of each point of the random series (containing six sections). To execute the K-S test, the main procedure is outlined as follows:The procedure stated above is the classic K-S test process. However, since the critical value is approximate, the null hypothesis is usually rejected or accepted by comparing the returned In this study, the hypothesized PDF commonly used in civil engineering could be made including normal distribution, lognormal distribution, Weibull distribution and gamma distribution. Then, the estimated PDF could be acquired by executing the K-S test. According to Ref. , there iBy conducting the probabilistic and statistical modeling on the constrained SFS, the 1D PDF of constrained SFS for each constituent of concrete could be obtained.Generally, complex global behaviors could be captured on the basis of the fiber bundle model whose individual element is endowed with a simple response  could be modeled as a random field using the observed values from nanoindentation tests. Performing “maximum possibility criterion” , the kno (22) in . Then, esults in . Finallytropy in .Specifically, to investigate the SPI of concrete damage, the phase SPI could be generated firstly due to the obvious bound among each constituent of concrete. Combined with the phase SPI, concrete SPI could be reproduced by the reproduction technique . To characterize the probability distribution of properties from nanoindentation, three zones including HCP, ITZ and aggregate were selected randomly shown in By performing “maximum possibility criterion” , the 1D As mentioned above, each constituent of concrete materials exhibits randomness, which constitutes the complex, distributed disordered system. The PDF of concrete, regarded as a random media , could bd result , a volumOne may argue that the optimal distributions listed in E = 37.00 GPa, λ = 7.62 and ξ = 0.52. The theoretical and experimental results are displayed in To validate the PDF of SFS from nanoindentation, the behaviors of the total seven prism specimens measuring 0.1 m × 0.1 m × 0.3 m subjected to uniaxial compressive loading were investigated by MTS in this research. In the present paper, the focus would be on the mean stress and strain relationship, from which the parameters including the mean value and the standard deviation of the distribution could be recognized. However, the standard deviation stress and strain relationship could also be plotted in the figure, which would lead to the relative length of the random field for concrete materials. That is also a key point deserving further research.For establishing the link between SFS vergence . For theP, and that from the nanoindentation by Q. Apparently, a distinct relative entropy P and Q. In other words, the constraint factor C meeting the minimum Assume that the random field in the stochastic damage model following a lognormal distribution is represented by  results ,24. Figus seeing , throughFrom the nanoscale to mesoscale, SFS could be generated based on the nanoindentation, which shows that the meso-fracture arises physically from the bulking of confined force chain system at the mesoscale. From the mesoscale to macroscale, SFS could be recognized based on the macro-testing, which provides the information that the damage of concrete is strongly dependent on the random fracture of mesoscale concrete element. In a word, the concrete damage under compression results from the random buckling of confined force chain system at the mesoscale.(1)The confined force chain buckling model proposed indicates the relationship between mesoscale strength and mesoscale hardness. The indentation hardness could be equivalent to the constrained compressive strength confined by the surrounding material around the indenter. The nanoindentation combined with the proposed model and random field theory provides direct access to the SPI of mesoscale fracture behavior of concrete. Meanwhile, the mesoscale fracture behavior of each constituent follows the homogeneous random field.(2)Nanoindentation combined with macro-testing under compression could lead to the constraint factor linking two scales from mesoscale to macroscale, by comparing the difference between distributions of mesoscale fracture behavior from nano- and macro-testing. This multiscale method provides an effective way to investigate concrete damage under compression, to offer the physical reality of concrete damage evolution, and to estimate the effect of concrete constituents on damage evolution. Up to now, it is interesting to see that the nature of mechanical properties, e.g., strength, creep and damage, could be anticipated based on the SPI by using nanoindentation ,18,19,20(3)At the mesoscale, the meso-fracture arises physically from the bulking of the confined force chain system. At the macroscale, the concrete damage is strongly dependent on the random fracture at the mesoscale. From mesoscale to macroscale, the accumulation of mesoscopic fracture results in the macroscopic damage. Notably, the mesoscale inhomogeneity and the mesoscale confined bulking are intrinsic to concrete, which may constitute the physical mechanism controlling concrete damage subjected to compression.(4)Our investigation provides the possibility to control damage and to strengthen cementitious materials. Additionally, evaluating the macroscopic properties based on the SPI at the lower scales could be a feasible option. However, in the present paper, only one concrete mix was studied. The concrete materials with higher and smaller water to cement ratios still deserve further investigation and thus the effect of concrete constituents, especially the aggregate on damage evolution could be systematically investigated in the future. Reasonable speculation could be given that the greater the concrete strength, the smaller difference in the second peak in In summary, this study sets a framework to investigate the damage of concrete under compression based on the SPI. Base on the multiscale research including experimental results, theoretical derivation and numerical analysis, the following conclusions could be drawn:


Slip-forming in concrete construction enables the continuous placement of concrete using a climbing form, the efficiency of which depends on appropriate slip-up timing. This implies the importance of knowing accurately the development of concrete strength over time, which has been assessed manually to date in construction fields. This paper presents a method for automating the slip-forming process by determining the optimal slip-up time using the in-situ strength of concrete. The strength of concrete is evaluated by a formula relating the strength to the surface wave velocity measured with ultrasonic sensors. Specifically, this study validates the applicability of the slip-form system with ultrasonic sensors for continuously monitoring the hardening of concrete through its application in several construction sites. To this end, a slip-form system with a pair of ultrasonic modules at the bottom of the panel was tested and the time variation of surface wave velocity in the concrete material was monitored during the slip-forming process. The results show that the proposed method can provide the optimal slip-up time of the form to automate the slip-forming process. This approach is expected to apply to other construction technologies that required the continuous monitoring of concrete strength for construction efficiency as well as quality maintenance. Slip-forming in concrete construction shortens the construction period significantly by enabling an automated form, moving at a rate of 2 to 4 m per day with the 24-h continuous placing of concrete . The forIn recognition of this need, various nondestructive evaluation methods, such as embedded piezoelectric transducers ,7,8 and Previous studies by the authors ,17 suggeThis chapter discusses the relationship between the compressive strength of concrete and Rayleigh wave velocity, especially in the short time after the casting of the concrete. Rayleigh waves, a type of elastic surface waves, travel along the shallow part of a solid medium. In a homogeneous and isotropic medium, the velocity of Rayleigh waves is known to be related to the dynamic modulus of elasticity  follows .(1)VR=0From Equation (1), the dynamic modulus of elasticity can be formally expressed by the Rayleigh wave velocity as: studies ,20 have CI318-14  prescribCI318-14  presentsEquations (4) and (5) imply that the compressive strength of concrete can be formally written as a function of gh waves ,24,25. Agh waves ,26,27. Igh waves  proposedA slip-form system for concrete construction consists of various components such as a hydraulic jack, steel rods, platforms, etc. For validating the applicability of the ultrasonic sensor system to the slip-forming of concrete, the system was applied to the construction of full-scale concrete towers using a slip-forming method. The test considered different temperature conditions to verify the system’s applicability to various seasons.The first application was on a 10 m high single concrete tower with a rectangular hollow section. The mix proportion and design strength of concrete are listed in In this study, it was determined that the slip-form would be lifted when the strength of the cast concrete reached the required range of 0.2 to 0.3 Mpa. The concrete casting for the building of the tower was done in early November when the temperature ranged between a minimum of 2.1 °C and a maximum of 16.3 °C. The average daily temperature was between 5.6 °C and 10.6 °C, as shown in The second application was on a 10 m high twin-shaft concrete tower with a rectangular hollow section, commonly adopted in cable-supporting bridges. Concrete casting for the twin tower was done in mid-November when the temperature ranged between a minimum of −1.8 °C and a maximum of 15.1 °C. The average daily temperature was between 4.7 and 10.6 °C as shown in This study investigated the applicability of a surface wave velocity method to the slip-forming of concrete structures. The wave velocity could be measured by a developed ultrasonic sensor module mounted on the slip-form. The slip-form system was applied to the construction of two full-scale concrete towers under different temperature conditions. The implementation showed that the surface wave velocity could provide a quantitative and objective indicator for the compressive strength of early-aged concrete inside the form. As a result, the surface wave velocity method with the ultrasonic module enabled the automation of slip-forming without relying on the subjective assessment of technicians on proper slip-up time. The ultrasonic module mounted on the slip-form did not provoke any leakage of concrete during casting and can be easily dismantled after the construction. With the timely slip-forming by the developed ultrasonic measurement system, the field implementation could yield an excellent quality to the concrete surface of the towers. Although the relationship between the compressive strength of concrete and the surface wave velocity was established for temperatures from 5 to 35 °C, it is applicable to temperatures outside of this range by an adjustment to the thermal coefficient. Further research is needed to investigate the applicability of the smart slip-forming method for a wider range of temperatures considering extreme weather conditions during construction.