Endometriosis is defined as the presence of endometrial foci, localized beyond their primary site, i.e., the uterine cavity. The etiology of this disease is rather complex. Its development is supported by hormonal, immunological, and environmental factors. During recent years, particular attention has been focused on the genetic mechanisms that may be of particular significance for the increased incidence rates of endometriosis. According to most recent studies, ESR2 and CYP19A1 genes may account for the potential risk factors of infertility associated with endometriosis. The paper presents a thorough review of the latest reports and data concerning the genetic background of the risk for endometriosis development. The inherited susceptibility to contract endometriosis is then justified by the fact of the growing interest in the identification of genes and genetic polymorphisms, predisposing  women to the group at risk for endometriosis development.ESR2 gene and of the CYP19A1 gene for the susceptibility to and the incidence of endometriosis [Literature data indicate the particular role of estrogen receptor genes, especially of the etriosis ,62,63,64ESR2 gene, is a dominating isoform in patients with endometriosis [CYP19A1 is a gene which encodes aromatase—an enzyme participating in the biosynthesis of estrogens. The background of the observed changes is still far from being known and understood.The estrogen receptors, which act as transcriptive factors, play a significant role in the growth and differentiation of endometrial cells, as well as in numerous biological functions in both, eutopic and ectopic endometrium. The ERβ receptor, encoded by the etriosis . A diffeESR2 and CYP19A1, respectively.During the recent years, particular attention has been focused on the genetic mechanisms that may be of particular significance for the increased incidence rates of endometriosis. According to literature data, numerous studies are underway, seeking the markers of endometriosis. One of the ways is an evaluation of the expression levels of estrogen receptor genes and of the enzyme genes responsible for metabolism of drugs, including Endometriosis is an estrogen-dependent disease. This confirms the fact of its occurrence in women of child-bearing age and of the remission of its symptoms after menopause or ovariectomy. This disease entity presents a specific ability to receive estrogen stimuli that stimulate its development. An enhanced local production of estrogens takes place within the endometrial tissue in result of an increased expression level of P450 cytochrome aromatase. Aromatase impacts the rate of estradiol synthesis . In addiCYP1A1, CYP19, and GSTM1), predisposing to endometriosis [Genetic polymorphism occurs within the endometrial tissue, concerning estrogen and progesterone receptors ,67, as wetriosis ,69,70,71etriosis . Literatetriosis .CYP1A1, CYP19, and GSTM1 polymorphic genes, which increase the level of P450 aromatase and estrogen production [The activation of estrogen receptors in endometriosis can be direct or indirect. The indirect activation is supported by oduction .Estrone, 17β-estradiol (E2), and estriol belong to the estrogens produced in the human body. These are steroid hormones, produced from cholesterol in result of androgen aromatization under the influence of the enzymes from the P450 cytochrome group, localized in the internal mitochondrial membrane. The last, irreversible stage in estrogen production is demethylation, which occurs under the effect of p450 aromatase ,76.—to stimulate the development of female genital organs; —to form secondary and tertiary sexual characteristics; —to stimulate proliferation of the stromal cells in endometrium and the growth of the uterine muscle mass, as well as the peristalsis of the uterus and of the ovarian ducts; and —to stimulate the development of the breasts and of the external genital organs, in preparation of the breasts for lactation .The function of estrogens in the genital system is: Estrogens reveal a relationship with the development of breast, prostate, uterine, ovarian, and endometrial cancer ,78.CYP19A1 and ESR2. The level of estrogens in the body largely determines the activities of the enzymes that take part in their synthesis and catabolism, encoded, among others, by polymorphic genes. Therefore, studies are underway to reveal correlations between polymorphisms and estrogen-dependent diseases, the latter including breast cancer, endometrial cancer, osteoporosis, and, also, endometriosis ,80,81. TCYP19A1 gene is localized on 15q21.2 chromosome, contains 10 exons, and encodes the aromatase of P450 cytochrome, the main component of the aromatase enzyme, which plays the most direct role in estrogen production [The oduction .CYP19A1 gene is associated with differences in the activity of aromatase. There are reports about possible correlations between this phenomenon and CYP19A1 polymorphisms [The CYP19 aromatase converts testosterone and androstenedione into estradiol and estrone . The focorphisms ,85,86.CYP19A1 gene is highly polymorphic. In its area (within intron 4), there is a polymorphism consisting in the occurrence of 7- to 12-fold tetranucleid repetitions of TTTA sequence with an additional, the shortest allele with seven repetitions of the sequence, together with the deletion of three TCT nucleotides above the microsatellite region [The e region ,88.In the case of endometriosis, it was studied together with 3pz I/D (insertion/deletion) polymorphism in intron 4. No correlation was found between TTTA repetitions and the incidence of endometriosis. In the case of 3pz I/D polymorphism, a weak but significant correlation was found with the incidence of the disease. Patients with endometriosis were most often the carriers of D/D genotype .CYP19A1 gene, besides the microsatellite polymorphism, there is also the SNP polymorphism in the noncoding region 3′, which is the transition of thymine into cytosine. It causes a decrease of the transcript stability [In the tability ,88.FSHR , HSD17B3 (rs2066479), and CYP19 (rs700519) genes may modulate the risk of endometriosis in Taiwanese Chinese women [FSHR, rs6166  of FSHR, rs2066479  of HSD17B3, and rs700519  of CYP19) were strongly associated with decreased risks of endometriosis [Wang et al. reported that single nucleotide polymorphisms of se women . The stuetriosis . CYP19A1 gene in women with endometriosis. This polymorphism may then play some role in the increase of the risk for endometriosis [In the studies of Vietri et al. a considerable prevalence was demonstrated of homozygotes A of Val89 polymorphism of the etriosis .CYP19A1 gene with endometriosis [The latest research indicates a correlation of rs2899470 and 1531G  >  A of the etriosis ,93.CYP19A1 gene with endometriosis. The studies of Yi et al., while being a broad metanalysis, did not demonstrate any correlations between rs10046 polymorphism of the CYP19A1 gene and the disease [CYP19A1 gene were subject to a thorough analysis. Of all the studied genetic variants, only the rs700518AA genotype was associated with infertility in patients with endometriosis [However, not all reports confirm the correlation of SNP polymorphisms of the  disease . In anotetriosis .CYP19A1 gene encodes aromatase, the activity of which was identified, besides normal tissues, in pathologically changed tissues, such as endometriosis, uterine fibromas, breast cancer, and endometrial cancer [CYP19A1 gene demonstrates a tissue-specific expression and its regulation makes use of alternative promoters. The highly polymorphic l cancer ,96,97. TESR1 gene, and the estrogen receptor β (ERβ), encoded by the ESR2 gene. The activity of estrogens on target cells is possible via estrogen receptors. Two types of receptors can be distinguished in the human body: the estrogen receptor α (ER α), encoded by the ERβ is the main estrogen receptor in patients with endometriosis. Its presence is a significant and useful predictive factor. The modulation of gene transcription by estrogens is called the genomic activity of estrogens. The nongenomic mechanism of activity is characterized by a rapid response after an exposure to the hormone. The nongenomic system leads to post-translative modifications of signaling proteins. It is the ER receptor, localized in the cellular membrane, that is responsible for the nongenomic activity. Regarding this activity, neither the receptor binding to DNA nor the synthesis of gene mRNAs is needed.The nongenomic activity of ERβ plays a dominating role in the progression of endometriosis .The mechanism of the nongenomic activity of estrogens is very complex. Despite continued studies, it remains not fully exploited. The works, which are now conducted to learn the mechanism, turn toward studies on the integration of cellular signaling pathways, both on the surface and inside the mitochondria, which decide about cell apoptosis or survival. ERβ belongs to the family of nuclear receptors, the activation of which leads to receptor connection with a specific DNA sequence of the gene promoter, which may either activate or inhibit the transcription process. The genomic mechanism of estrogen activity involves the connection of estrogens to the receptor. Then, the complex is transferred to the cell nucleus. The receptor undergoes dimerization and binds to the estrogen response element (ERE). ERE is localized in the promoter of specific genes. In addition, the hormone binding induces conformational changes of the receptor in the ligand binding domain, which enables the connection of coactivator proteins .The ERE sequence does not have more than one-third of human genes, the control of which takes place via the ER. The mechanism, by which estrogens control the transcription of these genes, remains unknown. It is known that estrogens regulate the expression of genes without binding to DNA, making use of the modulation of the functions of other transcriptive factor classes via the protein–protein interaction. The interaction of ER with AP-1, a known transcriptive factor, may be a good example. AP-1 (the activator protein 1) is a protein complex, built of protein dimers of Fos and Jun families.A certain number of genes, which are controlled by estrogens and deprived of ERE, contain binding sites for the orphan nuclear hormone receptor SF-1 (SFRE) SF-1 response element, which bears the function of a direct binding site of the estrogen receptor (ERα) ,100.ESR2 gene, localized on 14q23.2 chromosome [ERβ is encoded by the romosome ,102,103.ESR2 gene transcription and in mRNA levels. The gene variability may then affect the extent of the estrogen receptor biosynthesis [The changes in ERβ biosynthesis usually precede the changes in the ynthesis .ESR2, a correlation was found between the occurrence of menstrual disorders in women of the Chinese population with rs1256049 and rs4986938 polymorphisms [ESR2 gene and endometriosis [In the case of orphisms . A correetriosis .The rs4986938 polymorphism of the ESR2 gene was also associated with advanced endometriosis observed in the Japanese population. No such correlation was confirmed in Greek, Italian, or Korean populations ,108.ESR2 gene, is associated with the higher incidence of endometriosis in stage IV [A team of Japanese researchers demonstrated that the specific polymorphism, which is observed in stage IV .No specific marker for endometriosis has yet been identified. Groups of compounds are now searched, which, in a specific combination, would ensure a maximum sensitivity and specificity of non-invasive diagnostics of the disease, especially in its early stages ,111. TheIt has been proven that changes in the expression of genes and proteins, typical for ectopic foci of the disease, may also be found in the endometrium in the uterine cavity ,113. ManESR2 gene expression level in endometriosis are rather scarce. Such studies were undertaken by Osinski et al. [HSD3B2), hydroxysteroid 17-beta dehydrogenase of type 1 (HSD17B1) and of type 2 (HSD17B2), and the estrogen receptors of type 1 (ESR1) and of type 2 (ESR2), as well as of the androgen receptor (AR). The abovementioned genes are involved in metabolism and steroid activity in eutopic endometrium. The objective of that study was to examine whether infertility, associated with endometriosis, may result from disturbed expression of HSD3B2, HSD17B1, HSD17B2, and ESR1/2 genes. The expression of the abovementioned genes was assayed in follicular phase and in luteal phase in women with preserved fertility and in patients suffering from endometriosis. Increased expression levels of HSD3B2 gene and of ESR1 gene were demonstrated in endometrium of infertile women with endometriosis. Those researchers drew a conclusion that it could have been associated with an abnormal activity of E2 estradiol on endometrium in that group of women.The literature data on i et al. , targetiESR1, ESR2, beta hormone luteinizing gene, FOXP subunit, complement component 3 gene, and the Fc receptor-like 3 (FCRL3) gene [CYP17, VDR, MUC17, COX-2, WNT4, E-cadherin, CYP19, CYP17, NFKB1, and TYK2 genes, as well as the variants of MUC2 gene, may also contribute to the risk of infertility associated with endometriosis [According to the most recent studies, the following genes may account for the potential risk factors of infertility associated with endometriosis: L3) gene ,118,119.etriosis ,124,125.ESR2 gene expression, observed in endometriosis as related to the types and sites of lesions associated with the disease, are confirmed by world literature reports.Disorders in the level of ESR1, ESR2, and PGR genes in the cases of ovarian endometriosis and eutopic endometrium [ESR1 and PGR expression levels were substantially lower, while the level of ESR2 gene expression was significantly higher in endometriosis than in normal endometrium. DNA methylation levels were also studied. DNA methylation in ESR1 gene was significantly higher in endometriosis than in eutopic endometrium. No major differences were observed in DNA methylation degrees for either ESR2 or PGR. The researchers made a suggestion that the observed abnormal DNA methylation had been associated with a low level of ESR1 expression, whereas the degree of methylation did not affect the expression changes of ESR2 or PGR genes in endometriosis.The team of Maekawa et al. employed the RT-PCR technique  to assay the expression levels of ometrium . The ESRESR1 to ESR2 expression levels, resulting from an excessive increase in the expression of the latter gene. The cells also demonstrated a deficit of the progesterone receptor, leading to resistance to progesterone and a defective retinoid synthesis. The expression of nuclear receptors, including the low levels of ESR1 and PGR and a high level of ESR2, corresponded to the expression of the factors, which was found in the stem cells of uterine fibroid. It is suggested that the endometrial stromal cells may present certain features of the stem cells, present in other uterine tissues. In the study by Colon-Caraballo et al., [ESR1, ESR2 estrogen, and PGR progesterone receptors were assayed in various types of endometrial changes and in eutopic endometrium of women with endometriosis and in a control group. Endometrial changes in the ovaries demonstrated low levels of ESR1 and PGR expression and high levels of ESR2 expression, while changes in the ovarian ducts were characterized by high expression levels of all the three receptors. Differences, observed among endometrial specimens, regarded lower ESR1 expression and higher ESR2 expression in the proliferative endometrium from the study group. A tendency toward the lack of nuclear activity of PGR was identified in the proliferative endometrium from the patients. The highest ESR2:ESR1 expression ratio was observed in endometrial changes of the ovaries and in the excretory endometrium. The results, obtained by Colon-Caraballo et al., are valuable, as they may extend our knowledge regarding a potential, possible anticipation of individual reactions to hormonal therapies and provide a basis to develop personalized therapeutic methods for women with endometriosis.The latest studies of the team of Yilmaz et al.  demonstr et al.,  the exprESR2 gene expression level was characterized by exceptionally high differences between the cells of endometriotic lesions and the cells of normal endometrium, while the mechanisms of the difference remained unknown, the authors assumed a hypothesis that the change in DNA methylation process was the mechanism responsible for the elevated ESR2 mRNA levels in the endometriotic cells. An area of CpG sites was localized at the promoter region (position –197/+ 359) of ESR2 gene. A sequencing process in that region revealed a significantly higher degree of methylation in the primary cells of endometrium vs. that in the endometriotic cells. The applied demethylating agent (5-aza-2′-deoxycytidine) significantly increased ESR2 mRNA levels in endometrial cells. It was demonstrated that the (−197/+ 372) region, which induced activity in the promoter, had also a CpG site and the ESR2 promoter activity was strongly inactivated by methylation in vitro. Summing up, methylation of the CpG site at the ESR2 promoter region was the main mechanism responsible for the differentiated expression of ESR2 gene, both in endometriosis and in normal endometrium. The studies of Xue et al.  demonstrESR2 gene mRNA levels. A team of Yu et al. [Changes in the biosynthesis of the ERβ receptor were the consequence of the fluctuations in u et al.  assayed The study by Matsuzaka et al.  indicateIt was confirmed in the study by Xue et al., who suggested that differences in the ERα to ERβ ratio were more visible in various endometrial lesions. In particular, the ERα to ERβ expression ratio was much higher in active (red) endometriosis than in black endometriosis or in the case of endometrial cysts of the ovary . Literature data demonstrate that the decreased ERα to ERβ expression ratio in endometrial changes may inhibit the expression of the progesterone receptor and contribute to resistance to hormonal therapy . NR1H4 gene) inhibited the estrogen signaling pathway in breast cancer and testis tumors. A team of Wu et al. [ESR2 promoter, which caused a decrease in ERβ level. Despite the carried-out studies, the significance of FXR for therapeutic purposes in endometriosis is, however, still poorly recognized and, thus, requires further research.The latest data suggest that the farnesoid x receptor (FXR) may be a potential target for a future therapy in endometriosis . It was u et al.  conducteCYP19A1 is a gene that encodes P450 cytochrome aromatase—an enzyme, participating in the biosynthesis of estrogens. A different aromatase expression has been proven in endometriosis foci [CYP19A1 gene expression and the incidence of endometriosis. sis foci . The basCYP genes’ expression in the cell are based on several mechanisms: —The control of transcription of the p450 isoenzyme encoding genes, —The mRNA stabilization, and—Post-translative modifications.It is known that the control processes of the In result of the transcription process induction, the transcriptive factors bind with DNA at the regulatory site, which is appropriate for them. The process is controlled by endo- or exogenous low-molecular ligands. By attaching to nuclear receptors, the ligands may, together with other specific proteins, lead to expression changes of the genes that are responsible for the metabolism catalyzed by P450 cytochromes .CYP1A1, CYP19, and GSTM1 enzymes, which are responsible for the metabolism of medicines, predisposes to endometriosis.It is known that the genetic polymorphism of CYP1A1, CYP19, and GSTM1 polymorphic enzymes, which increase the level of P450 aromatase and estrogen production [The activation of estrogen receptors in endometriosis can also be implemented indirectly via an abnormal activity of oduction .CYP1A1, CYP19, and GSTM1 genetic polymorphisms, the latest research concentrates on identifying the significance of the expression level of these genes in endometriosis.Besides the analysis of CYP1A1 and CYP1B1 gene mRNA was higher in endometriosis-related changes than in normal, eutopic endometrium [Piccinato et al. showed that the expression of ometrium . The expCYP19A1 expression in endometriosis may result from epigenetic modifications in the regulatory regions of the gene as, for example, DNA methylation or from the modification of histones [A reduced histones .CYP19A1 gene, were confirmed in endometriosis by the team of Izawa et al. [Changes in the methylation status of CpG islands, contained in a et al. .CYP19A1 gene. It is hypomethylated in endometriosis . However, its hypermethylation was found in eutopic endometrium , in the differentiation of trophoblasts (miR-19b and miR-106a), and in endometrial cancer (miR98) . There aThe latest studies indicate that the overexpression of miR-370-3p inhibits cell proliferation and induces apoptosis in endometrial cells . The resIt has been shown that a decreased level of miR-23a and b in endometrium of women with endometriosis releases an elevated expression level of SF-1, as well as of the steroidogenic acute regulatory protein (StAR) and of CYP19. It suggests some role of miR-23a and b in the acquisition of steroidogenic activity by endometrial tissues .CYP19A1 expression might not be correlated with endometriosis. The researchers conducted their studies to see whether the expression of aromatase in eutopic endometrium correlated with the presence of endometriosis and its clinical staging in patients with infertility and/or dysmenorrhea. The patients were submitted to laparoscopy and hysteroscopy. The study involved 106 women in the reproductive age and with symptoms of dysmenorrhea and infertility. A control group consisted of 16 patients with asymptomatic endometriosis. The results of the studies demonstrated that aromatase expression in the endometrium was connected with the occurrence of dysmenorrhea and infertility, regardless of whether endometriosis was present or not. A team of Maia et al.  obtainedESR1, ESR2, PGR, and CYP19A1 genes, unveil the existence of certain correlations of their expression with endometriosis. A significant increase of ESR2 and CYP19A1 gene expression was observed in the studied groups of women with endometriosis, which may indicate a certain role of this factor in the pathogenesis of endometriosis. The results, obtained in experimental study, contribute to better knowledge of and information on the molecular mechanisms that support the development of endometriosis. In consideration of the rather small number of studies on the expression and CYP19A1 and ESR2 gene in patients with endometriosis, the assumed impact of experimental studies may bring a real added value, extending our knowledge of the effects of genetic factors on the development of endometriosis. Learning and understanding the relationships between ESR2 and CYP19A1 gene expression and endometriosis may help design and develop new therapeutic concepts and strategies in the context of the disease. Studies on gene expression aspects are apt to constitute a prima facie target of major concern, regarding personalized therapy.The results of experimental study, concerning the analysis of While reviewing the studies on endometriosis from recent years, it is becoming apparent that the currently available therapeutic measures paint a rather bleak picture, not translating into a bright future for the patients. Regrettably, the available treatment protocols of this disease remain still ineffective. Nevertheless, the continuous progress of knowledge, especially at the genetic level, which has been made over recent years, allows the identification of certain molecular targets for new therapeutic methods. Great hopes are associated with new research trends, as, for example, the application of miRNA or lncRNA, which controls the key cell pathways in the development of the disease, as molecular markers in endometriosis . However


Technological developments in construction have led to an increase in the use of 3D modelling using CAD environments. The popularity of this approach has increased in tandem with developments in industry branches which use 3D printers to print concrete based printing materials in construction, as these allow freedom in shaping the dimensions of supporting elements. One of the biggest challenges for researchers working on this highly innovative technology is that of cement material shrinkage. This article presents the findings of research on an original method of measuring deformations caused by shrinkage in 3D-printed concrete elements. It also discusses the results of tests on base mixes, as well as comparisons between the influence of internal and external curing methods on the development of deformations and their final outcomes. Furthermore, the article discusses differences between deformations formed after seven days of hardening without curing, with those which occur when two common, traditional concrete curing methods are used: foil insulation and shrinkage reducing admixtures. In addition, the article examines the effects of internal curing on the 1, 7, 14, 21 and 28 day mechanical properties of concrete, in accordance with EN 196-1 and EN 12390-2. Studies have shown that the optimal amount of shrinkage reducing admixtures is 4% (in relation to the mass of cement), resulting in a reduction in total shrinkage of 23%. The use of a shrinkage reducing admixture in 3D-printed concrete does not affect their strength after 28 days, but slows the strength development during the first 7 days. The last few years have witnessed considerable advancements in the construction field, which has widely begun to adopt automated and computerised industrial processes . CAD env3D printing utilising concrete mixes is currently one of the most rapidly developing fields in construction; the best evidence of this is the number of new research projects and private endeavours focusing on this technology worldwide ,8,9,10.However, despite the intensive research being carried out at many research institutes, the introduction of this technology on an industrial scale is still associated with numerous unsolved material questions and construction-related problems. Much research has focused on the influence of the level of adhesion between layers on the durability of the entire element, as well as on the mechanical behaviour of mixes during the 3D printing process. . The difMix composition and the immediate onset of the phenomenon of drying bring about a number of problems related to the dynamic development of shrinkage deformations in the printed material. High binder contents, the lack of internal limitations from coarse aggregates and the erection of elements without formwork, exposes printed materials to a lot of quickly occurring shrinkage caused by drying and self-drying . The dryThis article presents the results of tests and analyses related to the influence of both active and passive traditional concrete curing methods on the development of 3D-printed concrete shrinkage as well as on its properties regrading durability. Two types of curing were adopted: external curing based on isolating samples with foil, as well as internal curing using anti-shrinkage admixture (SRA) in the amounts of 2% and 4% per weight of cement.The concrete mixes under investigation were prepared with CEM I 52,5R Portland cement (European Standard EN 197-1:2011 ), fly asThree 1350-mm-long samples of concrete were printed from each mix for shrinkage tests. Furthermore, typically casted 40 mm × 40 mm × 160 mm prism specimens were also made for the purposes of compressive strength testing in accordance with EN 196-1  and flexBoth external and internal curing were utilised in the experiment: the former method consisted of isolating the printed specimens with foil, while the latter method involved adding shrinkage reducing agents (SRA) in the amounts of 2% and 4% per mass of cement. Water volume was reduced when hydrated SRA was added to the mixes, thus maintaining both a constant w/c coefficient and consistency.A custom 3D printer, with the kinematics of a cartesian coordinate robot , was useThe length of the printed sample—limited by the printer’s maximum working area length—was 1350 mm, while the extruded layer was approximately 35 mm in width and 15 mm in height . Only onA novelty of the presented approach is to test the total shrinkage of the mixtures used for 3D printing starting just after the addition of water to mixture. The proposed method enables the measurement of volumetric changes taking place in the initial stages of material maturation—especially during the first 24 h. There is a lack of research that analyzes this phenomenon. Standardized measurement methods based on Graf-Kaufman, Amsler tests or similar tests proposed in ASTM C157  make it Throughout the entire testing procedure, the specimens produced for the purposes of shrinkage testing were placed on a layer of foil, which was situated on the working platform. At the end of the printing procedure, excessive material was removed from the extruded samples so as to adhere to the desired sample length of 1350 mm, after which the reference points were installed. Next, the specimens which were to harden underneath foil were hermetically sealed in it, with care taken so as not to cause any deformations. All the specimens were immediately transferred to the environmental chamber, at which point—after installation of the reference points—the process of deformation measurement began.To determine the influence of the SRA admixture on the mechanical properties of the concrete, typically casted 40 mm × 40 mm × 160 mm specimens were produced, according to EN 196-1. For each type of concrete, three specimens were produced for each testing time. After 1, 7, 14, 21 and 28 days, the specimens were tested for their flexural and compressive strengths. A standard testing machine, in accordance with EN 196-1, was used for measurement. The specimens produced for strength testing were formed and stored in accordance with EN 12390-2.The measurement of shrinkage deformations in the base mix, which did not contain any SRA, was carried out for 28 days. This involved measurement of the total shrinkage, which was a sum of both autogenous shrinkage and drying shrinkage ,20,21,22The cured specimens were measured for seven days. Further measurements were not carried out because deformations ceased to appear after the first week of maturation. The maximum average deformations, in the specimens where drying was minimised via foil isolation, measured −1031 µm/m. The results of the measurements of shrinkage deformations in mixes containing a 2% addition of SRA have been presented in Compressive and flexural strength tests were carried out on normalised samples for all of the mixes, in accordance with EN 196-1, after 1, 7, 14, 21 and 28 days. The samples were kept in an environmental chamber at a temperature of 20 ± 2 °C and a relative humidity of RH = 65 ± 5%. According to the obtained results, it can be seen that the increase in deformations in specimens subject to drying stabilised after three days. For all samples subject to drying, the largest increase in deformation occurred in the first three hours after printing. This amounted to around 66% of all the deformations in mix C580/SF83/FA166, and 77% and 61% of deformations in the mixes with 2% and 4% of SRA, respectively. The mixes with 2% SRA showed more shrinkage, and at a higher rate of increase, than those with 4% SRA. A decrease in shrinkage-reducing admixture from 4% to 2% resulted in an increase in the scale and rate of shrinkage in the first six hours. Adding SRA in the amount of 4% per mass of cement reduced both the total amount of deformations, as well as their rate of increase, in the first nine hours. In the case of the samples isolated by foil, the growth of deformations was slower and more uniform, which is characteristic of autogenous deformations over time . The samA comparison of the shrinkage deformations in all the concrete samples tested, measured at 6 h, 12 h, 24 h and 7 days, can be found in The manner in which the deformations formed was very similar across all of the samples subjected to drying, irrespective of the internal curing method applied. Both the base mix and the mix with 2% SRA had similar gradients after the first six hours, indicating similar deformation growth rates. A shifting of the slope in the graph for cement C580/SF83/FA166/SRA2 towards the vertical axis, was a result of the difference in the time which lapsed between sample production and the beginning of deformation measurement between these samples and the base samples. The gradient was much smaller for the concrete with an SRA content of 4%, with the progression of concrete deformation being much smoother. A stabilisation in the increase in deformations occurred for all mixes in the next six hours. From this point on, shrinkage in the base concrete was higher than in the concretes containing SRA.The deformation growth rate in the base concrete, as well as in that with added SRA, was clearly suppressed in the first 24 h of measurement. This is a point in time at which the hardening of a mixture has finished and when the cement matrix is so compacted that there is no more moisture loss. A very small increase in deformations, resulting from autogenous shrinkage in the material, can be observed between the first and seventh days of measurement. The gradients of all of the graphs are convergent, which confirms that the cause of deformation was autogenous shrinkage .In the seven day period analysed, in comparison to the base concrete, the 2% and 4% additions of SRA caused 7% and 23% reductions in shrinkage, respectively. The most significant reduction in shrinkage occurred in the concrete cured externally with foil covering; this had only 20% of the deformations of the base mix. Accordingly, external curing was the most effective method for limiting shrinkage in the concrete under analysis, with no fractures or cracks observed for any of the samples measured.The use of shrinkage-reducing agents did not significantly affect the compressive strengths of the mixes analysed in this study, with the differences after 28 days being around 4%. However, it should be noted that an increasing SRA dosage resulted in a progressively slower increase in strength, in the first seven days. This relationship was not observed in the case of bending tensile strength, but both mixes with added SRA achieved higher strengths after 28 days, which might have been a result of the minimisation of microcracks in the cement matrix and its more compacted structure.On the basis of the tests undertaken in this study, it is clear that measurement must start immediately after sample printing, as a result of the way in which shrinkage progresses. In many articles about high-performance concrete (HPC), which has similar mechanical properties to 3D-printed concrete, authors focus on autogenous shrinkage. For concrete with a low water-to-cement ratio (w/c = 0.2–0.4) deformation caused by autogenous shrinkage varies from 330 to 850 µm/m ,41,42,43While anti-shrinkage admixtures do not have a significant effect on compressive strength after 28 days, they nonetheless suppresses the rate of its increase. This should be kept in mind when producing elements via 3DCP, as these are usually placed under load-bearing soon after their erection.In analysing the results presented in this paper, as well as the most recent literature in the field of 3D concrete printing, it would be prudent to consider future possible research avenues regarding deformations in production materials, as well as to analyse methods of predicting shrinkage in 3DCP ,53. The No. PL231299: Method for measuring the self-generated shrinkage of poured out composite materials and the workstand for measuring the self-generated shrinkage of poured out composite materials


The methyl substituent tends to accelerate the reactions. However, an aromatic ring bonded to N and O makes the reaction more difficult, whereas its linkage to P and S favour it. On the whole, ligands with O and S heteroatoms seem to disfavour these processes more than ligands with N and P heteroatoms, respectively. Phosphido and thiolato ligands tend to yield a coupling product with the bipy ligand, which is not the general case for hydroxo, alcoxo or amido ligands. When the Y ligand has an O/N and an H atom the most likely product is the one containing a coupling with the carbonyl ligand, which is not always obtained when Y contains P/S. Only for OMe and OPh, the product resulting from formal insertion into the Re-Y bond is the preferred.A comparative theoretical study on the reactivity of the complexes [ReY(CO) The first step is common and consists in a nucleophilic attack from the heteroatom  of the Y ligand onto an acetylene carbon atom, generating a zwitteronic species, which evolves to afford different types of products   and HMAD uncovered that the reaction rate is, on the whole, controlled by the first stage of the reaction mechanism, that is, the nucleophilic attack of the Y ligand on the non-substituted alkyne carbon [pTol and PPh2 cases is not reliable to rationalize either the effect of an aryl group or the effect of the donor atom directly linked to Re. The situation is even worse if we wanted to extract chemical trends when comparing the four cases mentioned above at the same time. The results obtained for the reactivity of the [Re(PPh2)(CO)3(bipy)] complex are not comparable with those found for [ReY(CO)3(bipy)]  because there are different substituents and donor atoms belonging to different groups of the Periodic System.Our theoretical investigations on the reaction between the complexes  . The theoretical results obtained on these hypothetical reactions along with those previously found for the reactivity between the complexes [ReY(CO)3(bipy)]  and HMAD may be beneficial in gaining a broader understanding of the effect of Y ligand focusing on the alkyl and aryl substituents and the heteroatom directly bonded to Re.Therefore, to get more general conclusions about these processes, ultimately aiming at tuning the Re complexes in order to obtain more interesting synthetic, industrial or biochemical applications ,60, a mo3(bipy)]  and HMAD will be done as follows. First, we will report a general description of the types of potential energy surfaces found taking into consideration our DLPNO-CCSD(T) data. Second, we will analyze the effect of alkyl and aryl substituents at the nucleophilic ligand Y on the energetics of the reactive processes investigated. The same objective will be pursued when analyzing the influence of replacing, within Groups 15 and 16, the heteroatom of the second period by its counterpart of the third period . Finally, we will provide a general discussion by considering all the results mentioned in the previous subsections.The presentation and analysis of the results obtained for the reaction between the 15 complexes [ReY(CO)3(N-N)]  towards activated alkynes (HMAD and/or DMAD) have shown the formation of three types of products  through the TS TS1 to give a zwitterionic intermediate (I1), in which a new HO/Ph2P-C1 bond is formed and, consequently, the original triple bond between C1 and the substituted acetylenic carbon of the HMAD fragment (C2) presents now a double bond character  wherein C2 is readily oriented to yield any of the products. I1′ is the common precursor species of ccoP/ccohP, ccbP, and insP via the TS ccoTS2, ccbTS2, and insTS2, respectively , respectively  instead of bI1 and/or bI1′  reacts with HMAD. In the presence of an aromatic ring, its available conjugation with the N atom, which does not occur with the P atom, overcomes the destabilization observed in the two previous cases (see below).We now focus on the replacement of the heteroatom of the Y ligand when going from the second period to the third one along Groups 15 and 16 of the periodic table. The substitution of the nitrogen atom of NH/mol see . The rat PH2 see . From NHized see . Besidesases see , all thePHMe see . The Gibduct see . By contvely see . Even TSvidences , while P2 by PH2  ligands. ccoP and ccbP compete with each other to be the preferred product for SMe while insP is the most stable one. In contrast, when replacing OPh by SPh, all the species stabilize between 0.1 and 9.8 kcal/mol. TS1, ccbTS2, and insTS2 (ccoTS2 was not located for OPh) become 2.7, 7.8, and 1.7 kcal/mol more stable when going from OPh to SPh, with insTS2 again the highest in energy ]  towards HMAD displayed a variety of PES. However, all of them can be grouped into four different types on the basis of the number and the first splitting species as collected in 1R2 family always present the type-I PES the three ligands of the OR family  show three different types of PES named as I, II, and IV, respectively. The remaining Y ligands , which present two splitting species, have been grouped within the type-III PES.The investigation on the reaction mechanism of the reactivity of the 15 Re complexes   complexes towards HMAD [3(N-N)]  towards activated alkynes  complexes  with HMDA gave rise to several potential energy surfaces, all of them with an initial large energy TS for the nucleophilic attack of Y to HMAD followed by diverse reaction channels towards three kinds of products, ccoP/ccohP, ccbP, and insP. Ligands with N and S show an alternative approach of the reactants leading to ccbP. The particular features of each ligand cause slight differences that determine the resulting product. Thus, ligands NH2, OH, PH2, and SH are poorer nucleophiles for the reaction with HMAD, whereas the presence of a moderate electron-donating substituent like a methyl group replacing one of the H atoms in these ligands favours the processes. The energy destabilization of the HOMO of the reactant complexes seems to be responsible for the diminution of the Gibbs energy barrier corresponding to the nucleophilic attack step. The presence of an aryl substituent replacing one of the H atoms in these ligands increases even more the energy destabilization of the HOMO of the reactant complexes, but this does not always imply an easier process as factors other than the HOMO energy may be predominant. Particularly, an aromatic ring bonded to N and O makes the reaction more difficult, whereas its linkage to P and S favours it. Since the π electrons in the aromatic ring conjugate with the lone pair at N atom in NHpTol, this pair becomes less available for the nucleophilic attack on the HMAD molecule. The larger size of P prevents this conjugation, which explains the easier reactivity of PHPh compared to that of NHpTol. It is also interesting to note that a more contracted donor atom at the Y ligand tends to disfavour the reactivity of these Re complexes towards HMAD. On the whole, ligands with N and P atoms show lower rate-determining Gibbs energy barriers than ligands with O and S atoms, respectively. This trend is ascribed to the stability of the HOMO of the reactant complexes and the availability of the attacking electron lone pair of the donor atom of the Y ligand.The systematic study of the reaction of [ReY(CO)insP is very stable) due to the strong bond formed between these large atoms and Re, which is weaker in the case of N and O atoms. P and S ligands tend to yield ccbP as the main product, which is not the general case for O and N ones. When the Y ligand has an O/N and an H atom the most likely product is ccohP, which is not always obtained when Y contains P/S. For OMe and OPh, the weakness of the Re-O bonds together with the instability of ccoP and the absence of an O-bonded hydrogen atom explain why insP is the preferred product only for these two situations.Concerning the reaction products, the insertion route is greatly penalized for P and S ligands (although


Recently, 3D concrete printing has progressed rapidly in the construction industry. However, this technique still contains several factors that influence the buildability and mechanical properties of the printed concrete. Therefore, this study investigated the effects of the nozzle speed, the interlayer interval time, the rotations per minute (RPMs) of the screw in the 3D printing device, and the presence of lateral supports on the buildability of 3D concrete printing. In addition, this paper presents the results of the mechanical properties, including the compressive, splitting tensile, and flexural tensile strengths of 3D printed concrete. The buildability of 3D printed structures was improved with an extended interlayer interval time of up to 300 s. The printing processes were interrupted because of tearing of concrete filaments, which was related to excessive RPMs of the mixing screw. The test results also showed that a lateral support with a wide contact surface could improve the resistance to buckling failure for 3D printed structures. The test results of the mechanical properties of the 3D printed concrete specimens indicated that the compressive, splitting tensile, and flexural tensile strengths significantly depended on the bonding behavior at the interlayers of the printed specimens. In addition, although metal laths were expected to improve the tensile strength of the printed specimens, they adversely affected the tensile performance due to weak bonding between the reinforcements and concrete filaments. The 3D printing technique has become promising for prefabricated structures because of its outstanding flexibility in both architectural and structural designs . WithoutAlthough the 3D printing method is considered a promising technique, some factors still influence the printing performance. For example, the printing performance is strongly dependent on the fresh properties of concrete, such as its rheology and green strength ,14,15, aPanda et al.  investigThe effect of the interlayer interval time (delay time) during 3D concrete printing also has a significant influence on the buildability as well as the interlayer bonding strength of filaments. According to Le et al.  and SanjMoreover, the printed structure may collapse suddenly by buckling during printing; the buckling behavior depends on the geometry and support conditions of the structure. Thus, lateral support or reinforcement is needed ,24. For In addition, the anisotropic strength of 3D printed concrete is a concern because the printing direction significantly affects the material properties of 3D printed concrete in the hardened state ,26,27,28Based on previous studies, the aim of this study was to investigate the buildability of 3D printed concrete during printing and the mechanical properties of the 3D printed concrete in a hardened state. The effects of the nozzle speed, interlayer interval time, and presence of lateral support on the buildability of the 3D concrete printing were investigated. To investigate the mechanical properties of 3D printed concrete, compressive, splitting tensile, and flexural tensile tests in three different loading directions were carried out. Finally, the mechanical properties of the monolithic specimens and printed specimens were compared.3, as shown in Currently, several printing techniques have been applied in building construction, such as contour crafting , Apis-CoTo investigate the buildability and mechanical properties of 3D printed concrete, ten concrete structures were printed, as listed in Three structures, namely, S1, S2, and S3 in Group 1, were printed to investigate the effect of the interlayer interval time and rotations per minute (RPMs) of the mixing screw on the buildability of the structures. Accordingly, the three structures in Group 1 had the same shapes and dimensions but different interlayer interval times or RPMs of the mixing screw.Three structures, namely, S4, S5, and S6 in Group 2, were printed to investigate the effect of lateral supports inside walls on the buildability of the structures. Accordingly, wall members of the three structures were supported laterally by members with zigzag shapes inside walls. Structures S4 and S5 had the same rectangular wall dimensions, but the details of the connected parts between the wall and lateral supports were different from each other. For structure S4, the wall and lateral supports were connected to the wall by point contact, as shown in Four structures, namely, S7 through S10 in Group 3, were printed to investigate the effect of the interlayer interval time on the buildability of the structures and mechanical properties in the hardened state of the 3D printed concrete. Among these four structures, structures S7 and S8 were duplicates, and structures S9 and S10 were also duplicates except for the interlayer reinforcement. The four structures were printed with the same printing process, as shown in 3 was used for the mixture. Additionally, two types of cementitious materials, namely, silica fume  and class C fly ash , were added to the mixture. To improve the extrusion and bonding properties, SF with a SiO2 content of 91.3% was used as a supplementary cementitious material. Class C FA used in the mixture had a density of 2.26 g/cm3 and a loss on ignition of 2.8%. Sand with a size range from 0.16–0.2 mm was selected to ensure good extrusion during the printing process. To increase the concrete filament extrusion under the low water-binder ratio of 0.29, a high-performance water-reducing agent  was added to the mixture. Finally, the mixture included a viscosity agent to prevent segregation of the materials and enhance the viscosity of the mixture.The mixture used for the 3D printing process is given in The repeatability of the process is a critical requirement for 3D printing technology. The width, height, and material characteristics of the printed structures should remain almost the same for each printing process. To achieve repeatable printing in this study, the nozzle height and speed, concrete pumping rate, and RPMs of the mixing screw were controlled by an accurate printing device. Moreover, the test structures were printed on a stable platform. A nozzle height of 10 mm from the existing printed layer was maintained during the printing process. In addition, to control the quality of the mixture, the properties of the materials in the fresh state were measured.The rheological properties of printing concrete primarily affect extrusion. Therefore, the fluidity and extrusion rate of concrete were measured before printing the structures. The fluidity of the concrete filament was estimated by a flow table test according to ASTM C 230 . The extThe measurement of early-age mechanical strength is one of the possible ways to find the stiffness of printing materials. Green strength measurement by a uniaxial unconfined compression test on cylindrical samples has been found to be useful in concrete printing applications. Voigt et al.  found thFor green strength measurement, the specimen preparation and testing process should be designed to minimize unintentional breakdown of the thixotropic behavior of the concrete . MoreoveThe critical criterion for fabricating concrete structures via 3D printing is the vertical deposition of filament layers. The buildability is a significant parameter to evaluate the printable performance of concrete filaments. The buildability refers to the resistance of the deposited fresh concrete to deformation during construction and the ability of the concrete to retain its extruded shape . The 3D The buildability of 3D printed concrete structures was investigated based on the layer deposition process. Among the six S1 through S6 structures, S1, S2, S4, and S6 collapsed suddenly, as shown in To investigate the influence of the interlayer interval time on the buildability of the 3D printed concrete, three different nozzle speeds of 50, 80, and 100 s and different structural shapes were applied to print the concrete structures, as shown in First, structures S1, S2, and S3 were printed with the same dimensions of 1500 mm × 300 mm along the centerline of the wall but with different nozzle speeds. Due to the different nozzle speeds, structure S1 was printed with an interlayer interval time of 36 s, while structures S2 and S3 were printed with an interlayer interval time of 45 s. The maximum number of printed layers in structures S1, S2, and S3 were 19, 49, and 29, respectively. The maximum number of printed layers upon the collapse of structure S1 was much lower than those of structures S2 and S3. Therefore, the test results showed that the buildability of the 3D concrete printed structures increased as the interlayer interval time increased. According to previous studies ,37,38,39Structures S2 and S3 were printed with the same interlayer interval time but with different RPM values of the screw in the storage bin during printing of the structures. The printing of structure S3 was stopped when tearing of the filaments in the structure in the top layer was observed, as shown in The buildability of structures S5 and S7, which had similar geometries, was also compared to investigate the buildability of 3D printed concrete at different interlayer interval times. Structure S7 with an interlayer interval time of 300 s was printed up to 52 layers, whereas structure S5 with an interval time of 57 s was printed up to 28 layers. The comparison also showed that an increased interlayer interval time favorably affected the buildability of the printed concrete.The different interlayer interval times in structures S6 and S9 were compared because the two structures had the same long wall length of 500 mm between lateral supports. Structure S6 was printed with an interval time of 30 s, while the interlayer interval time of structure S9 was extended to ten times that of structure S6. Structure S6 was stacked up to 33 layers and then collapsed by buckling along the long wall. Moreover, structure S9 was printed up to 52 layers, and then printing of the structure was stopped because it was to be used for investigating the mechanical properties of the concrete in the hardened state.Therefore, a comparison of printed structures with interlayer interval times ranging from 36 to 300 s showed that an increased interlayer interval time favorably affected the buildability of the concrete. The interlayer interval time of the printing process influenced the green strength of the 3D printed concrete. If the interlayer interval time was too short, the green strength of the 3D printed concrete was not high enough to support the upper layers, which might lead to early collapse of the structure, and vice versa.The 3D printed concrete structures in Group 1 in this study failed by buckling, so it was important for the 3D printed concrete structures to resist buckling. The lateral supports printed inside the structure could enhance the resistance to buckling failure along the long wall of the structures. Two types of zigzag lateral supports were printed inside structures S4 and S5, as presented in Structure S4 contained lateral supports that were printed inside the structure and connected to the long wall by a contacting point. Structure S5 contained lateral supports inside the structure, the arrangement of which was similar to that in structure S4, but they were connected to the long wall by a contact with a width of 100 mm.The test results showed that structure S4 was printed up to 22 layers and then collapsed by buckling failure along the long wall. Moreover, 28 layers were stacked in structure S5, and then the printing process was interrupted because of the tearing of the concrete in the top layer, as shown in Similarly, structure S6 was printed with lateral supports that were connected to the long wall by contact points with a width of 100 mm. Structure S6 was stacked up to 33 layers prior to collapse, although it was printed with a shorter interlayer interval time of 30 s than that of 55 s for structure S4. The number of printed layers in structure S6 was higher than that in structure S4.Structure S6 failed due to buckling because this structure was printed with a short interlayer interval time.Therefore, the zigzag-shaped lateral supports printed inside the structure, especially with long connecting widths, could increase the resistance to buckling failure in 3D printed concrete structures. The test results in this study are in good agreement with those of a previous study , where 3A numerical model was developed to predict different failure modes based on the material properties of the concrete in the fresh state and on the failure criterion by Wolfs et al. . AccordiThis study originally focused on the experimental program and discussion of the test results in terms of buildability. Prediction of the failure mode or numerical resistance for the structures printed in this study could not be performed because the heterogeneous strength and stiffness characteristics of the concrete used in this study, which are necessary to analyze the numerical resistance to failure, could not be estimated from the green strength test. Therefore, numerical predictions of failure modes based on the developed model should be investigated in a future study.In this study, after the printed structures were hardened, the walls of the 3D printed structure in the hardened state from Group 3, as shown in A typical representation of specimens cut from a 3D printed concrete wall is shown in In addition, the monolithic specimens were cast in dimensions of 50 mm × 50 mm × 50 mm for the cubic specimens and 40 mm × 40 mm × 160 mm for the prismatic specimens, as proposed in ASTM C109/C 109M-07 and C348-18 ,42. The Unlike the monolithic specimens, the printed specimens had anisotropic mechanical properties that depended on the different loading directions; this dependency was based on the layer stacking directions of the 3D printed concrete. Accordingly, the printed specimens were tested in three mutually perpendicular loading directions, as shown in l is the length (mm), b is the width of the specimen (mm), and P is the maximum load (N).The compressive strength test was performed on the cubic specimens. Before the compressive strength testing of the monolithic and printed specimens was performed, the lengths of all sides of the specimens were measured. The compressive strengths of the printed specimens were measured in three loading directions, namely, I, II, and III. However, loading directions II and III were actually the same, and thus, the compressive tests were performed in loading directions I and II. For each loading direction, five monolithic and printed specimens were tested for their compressive strengths in accordance with ASTM C109/C 109M-07 . The coml is the length of line contact of the load (mm), b is the nominal cross-sectional dimension (mm), and P is the maximum load (N).The splitting tensile strength test was also determined for the cubic specimens. For the printed specimens, splitting tensile tests were conducted in three loading directions, namely, I, II, and III. For each loading direction, five monolithic and printed cubic specimens were tested for their splitting tensile strengths in accordance with ASTM C 496/C 496M-04 . The spll is the distance between the supports (mm), b is the length of the squared section of the prism specimen (mm), and P is the maximum load (N).For the flexural tensile test, the test in loading direction III could not be performed because of problems encountered while cutting the specimens out of the 3D printed structures. Five prismatic specimens were used for the flexural tensile test in each loading direction (I and II) in accordance with ASTM C348-18 . The fleAfter five specimens were tested for each strength, the means and standard deviations of the test results were calculated to estimate the strength properties of the monolithic and printed specimens.Compressive strength tests were performed for both the monolithic and printed cube specimens. The printed specimens were categorized into those without metal lath reinforcement and those reinforced by metal lath reinforcement. The compressive test results are given in The compressive strength of the monolithic specimen was 72.8 MPa, while those of the printed specimens without metal lath reinforcement in loading directions I and II were 23.5 and 31.0 MPa, respectively. For the printed specimens reinforced by the metal lath, the compressive strengths in loading directions I and II were 24.6 and 24.0 MPa, respectively. The compressive strengths of the printed cubic specimens ranged from approximately 32.3 to 42.6% lower than those of the monolithic specimens.For loading direction I, the compressive strengths of the printed specimens with and without metal lath reinforcement showed similar values. The failure patterns were accompanied by the propagation of vertical cracks over the height of the specimens, as shown in For loading direction II (or III), the failure of the printed cubic specimens resulted from interface failures between the layers. The failure pattern also indicated that the compressive strengths were highly dependent on the bonding capacity at the interlayers of the printed specimens because the specimens in this loading direction were subjected to lateral deformation under vertical compressive loading.The compressive strength of the printed specimens without a metal lath was greater than that of the specimens reinforced by a metal lath. This implied that the bonding performance of the printed specimens without a metal lath at the interlayers was higher than that of the printed specimens reinforced by a metal lath at the interlayers. Metal lath reinforcements at the interlayers were expected to improve the bonding and tensile performance of the 3D printed concrete. In contrast, the metal lath reinforcement adversely affected the interlayer adhesion performance due to weak bonding between the reinforcement and concrete filaments. The failure of the printed specimen reinforced by the metal lath shown in Splitting tensile testing was performed on both the monolithic and printed specimens in different loading directions. A comparison of the splitting tensile strengths for the monolithic and printed specimens is shown in The splitting tensile strengths in loading directions I and II of the printed specimens with and without metal laths were not significantly different. Therefore, the interlayer reinforcement hardly affected the splitting tensile strength, although the metal lath reinforcement at the interlayers was expected to improve the splitting tensile strength of the printed specimens. The failure pattern of the printed specimens in loading direction II is shown in The effect of the imperfect adhesion between the concrete filaments and reinforcements on the tensile strength was also observed in the splitting tensile strength in loading direction III. The splitting tensile strength of the specimens reinforced by the metal laths was lower than that of the specimens without the metal laths. The test results were different from the expectation that the metal lath reinforcement at the interlayer would increase the splitting tensile strength of the printed specimens in this loading direction. The failure pattern of the printed specimens in loading direction III is shown in Finally, the test results in this study implied that the bonding conditions between the concrete filaments and reinforcements at the interlayers played a significant role in the splitting tensile behavior. Therefore, the adhesion between the concrete filaments and reinforcements should be confirmed to improve the tensile behavior of 3D printed concrete structures.The printed prismatic specimens with and without metal laths were tested in two different loading directions, namely, I and II. The flexural strength testing of specimens in loading direction III was not performed because of difficulties in preparing those specimens. A comparison of the flexural tensile strength results for the monolithic and printed specimens is shown in The flexural tensile strength of the monolithic specimen was 11.9 MPa, while those of the printed specimens without metal lath reinforcement were 6.5 and 6.1 MPa in loading directions I and II, respectively. The test results show that the flexural tensile strengths of the printed specimens without reinforcement were approximately half that of the monolithic cast specimens.In addition, the flexural tensile strengths of the printed specimens reinforced by metal laths were 18.5 and 11.0 MPa for loading directions I and II, respectively. The flexural tensile strength of the printed specimens without metal lath reinforcement was lower than that of printed specimens reinforced by metal laths. This was because metal lath reinforcement in the tensile zone acted as a tensile reinforcement. Accordingly, the metal lath reinforcement increased the flexural tensile behavior of the 3D printed specimens in both loading directions I and II.However, the flexural tensile behavior in loading direction II, as shown in The interlayer interval time significantly influenced the buildability of 3D printed concrete. The test results showed that an extended interlayer interval time of up to 300 s contributed to the green strength of the 3D printed concrete and thus increased the buildability of the 3D printed concrete.The 3D printed concrete structures with lateral supports could increase the resistance to collapse due to buckling failure. In particular, a wide connecting width between the lateral support and the structure wall improved the capacity of layer decomposition in the 3D printed concrete.The compressive strengths of the 3D printed cubic specimens were 32.3~42.6% lower than those of the monolithic specimens. In addition, the weak bonding performance between the reinforcement and concrete filaments caused the low compressive strength of the printed specimens in direction II.The splitting tensile strength of the 3D printed concrete specimens was 11.1~39.7% lower than that of the monolithic specimens, depending on the loading directions. The splitting tensile failure was accompanied by debonding between the printed layers, which implied that the bonding performance between the concrete filaments and reinforcements should be confirmed when interlayer reinforcements are included.The flexural tensile strengths of the 3D printed specimens without reinforcements at the interlayers were approximately half that of the monolithic cast specimens. The flexural tensile strength of the printed specimens was improved by the metal lath reinforcements. The failure pattern also revealed that the bonding between the interlayers and reinforcements might influence the flexural strength.The buildability and mechanical properties of 3D printed concrete were investigated in this study. Based on the extensive experimental results, the following conclusions can be drawn:


A significant increase of the use of high-performance fiber-reinforced concrete (HPFRC) to strengthen reinforced concrete structures (RC) has been noted for the past few years, thereby achieving composite RC-HPFRC elements. Such a technique tries to take advantage of the superior material properties of HPFRC in the ultimate and service load regimes. Many of the existing works on RC-HPFRC elements have focused on the strength increase at the ultimate load state and much less effort has been devoted to the serviceability response. The in-service performance of RC structures is governed by the behavior of the tension chord, which determines the crack pattern  and deformations. The presence of HPFRC is supposed to improve serviceability due to its strain-hardening and tension-softening capacities. In this paper, the experimental analysis of composite RC-HPFRC tension members is dealt with. Specimens consisting of a RC tie strengthened with two 35 mm thick HPFRC layers have been subjected to loads in the service range so that the deformational and cracking response can be analyzed. The HPFRC has been a cement-based mortar with 3% volumetric amount of short straight steel fibers with a compressive and tensile strength of 144 MPa and 8.5 MPa, respectively. The experiments have shown that RC-HPFRC has higher stiffness, first cracking strength and reduced crack widths and deformations compared to companion unstrengthened RC. To understand the observed behavioral stages, the experimental results are compared with an analytical tension chord model, which is a simplified version of a previous general model by the authors consisting of 4 key points. In addition, the influence of time-dependent shrinkage has been included in the presented approach. In the last years, the use of high-performance fiber-reinforced concrete (HPFRC) as strengthening material for existing concrete structures has increased significantly ,2,3. TheThe composite action of reinforced concrete (RC) members strengthened with HPFRC layers (referred to as RC-HPFRC elements hereafter) is a complex mechanism due to the different mechanical properties of both layers and their interaction. In tension, conventional concrete has a low tensile strength and negligible softening capacity, while HPFRC typically has a three-stage tensile response ,15,16,17Most existing works on RC-HPFRC have focused on the contribution of the HPFRC layer to the ultimate capacity of composite members, which is not only due to the utilization of the tensile capacity of HPFRC for bending-critical elements, but also includes strength gains of up to 2–3 times the resistance of unstrengthened shear-critical members due to the additional mechanisms mobilized by the RC-HPFRC interaction after interlayer detachment ,19,20. IThe present paper represents a step forward in the authors’ research towards a better understanding of the composite behavior of RC-HPFRC members. Previous research ,25 has fThe main objective of the present paper is to evaluate the benefits on stiffness, cracking strength, and ability to reduce crack widths and deformations by strengthening conventional RC with HPFRC layers. Specific tests have been carried out on composite RC-HPFRC tension members under service load levels, as well as on a companion conventional RC specimen. Crack widths and deformations are analyzed to understand the improved serviceability and the potential durability of RC-HPFRC with respect to RC with the help of a novel simplified theoretical approach able to define the behavioral phases of RC-HPFRC tension members with 4 key points. A significant novelty of the model with respect to previous approaches  is the i3 of cement CEM I 42.5R, a maximum aggregate size of 12 mm and water/cement ratio of 0.5. The average compressive strength was 33 MPa at 28 days and 37 MPa at testing age (from 3 cylinders of 150 mm × 300 mm). The indirect tensile strength at testing age was 2.7 MPa (from 3 cylinders of 150 mm × 300 mm).The experimental research has studied composite RC-HPFRC tension members. The RC part has consisted of a conventional concrete prism centrically reinforced with a 16 mm ribbed steel bar (refer to The HPFRC has been manufactured from a Compact Reinforced Concrete (CRC) dry-mortar premix kindly supplied by Hi-Con A/S . The mortar is a portland cement/micro silica-based binder including super plasticizer and sand aggregates of 4 mm maximum size. The mortar was mixed with the water with a water/premix weight ratio of 0.08 and a 3% volumetric amount of steel fibers in a vertical pan mixer following the instructions of the supplier. The fibers were straight, with a length of 12.5 mm, diameter of 0.3 mm, and yield strength of 2950 MPa. The average compressive strength on 15 cm cubes was 132 MPa at 28 days and 144 MPa at the age of testing of RC-HPFRC members (from 3 cubes at each age).The tensile properties of HPFRC have been determined by inverse analysis  from 5 fThe experimental campaign included four tests on tension members. A reference test (T0) consisted of a RC specimen, while the other tests  were composite RC-HPFRC tension members. Therefore, test T0 can serve as a reference to be compared with the response of strengthened elements T1–T3. The dimensions of specimens and set-up configuration are represented in The HPFRC strengthening layers were applied when the age of the RC was 60 days. The concrete surfaces were prepared by bush-hammering to achieve adequate roughness and facilitate composite action of HPFRC and conventional concrete . The appThe specimens were stored in horizontal position, with the HPFRC layers at the bottom and top sides, in laboratory environment until the testing date. The tension tests were carried out 88 days after application of HPFRC, i.e., the total age of the conventional concrete was 148 days.The specimens were tested in tension with displacement control at a rate of 0.02 mm/s applied from the upper end with a hydraulic actuator , until a total deformation sufficient to cover the in-service range of concrete structures (strains of 1.5–2.0‰). The transducers used for the instrumentation were the following refer to : 3 LVDTsAll specimens were manufactured and tested at the Laboratory of Structures of the Technical University of Madrid, Spain (UPM).Digital image correlation (DIC) is a powerful photogrammetric technique which shows the deformation of a 2-D plain surface by taking a sequence of photographs of a speckle pattern . In the case of the RC-HPFRC specimens studied in this paper, one of the lateral sides with both the RC and HPFRC layers visible was painted with a speckle pattern consisting of randomly placed black points of 1–3 mm on a white background refer to b,c. The The global behavior of tested specimens can be represented by the load-deformation diagrams of The initial stiffness of the specimens, given by the slope of the first elastic stage, was 528 MN for T1–T3 (on average) and 318 MN for T0, which means a 66% stiffness increase provided by the contribution of the HPFRC layers. In this first stage, the composite behavior was monolithic because there were no cracks. Therefore, the stiffness increase can be attributed to the elastic stiffness of the HPFRC layers refer to .The average value of the first cracking load has been 30.0 kN for specimens T1–T3 and 19.3 kN for specimen T0, which means a significant 55% additional cracking strength achieved by the addition of the HPFRC layers. After the first crack formation, the global stiffness of the second stage of all specimens logically decreased. The average stiffness of the second stage, calculated as the slope of a secant line of the cracked part of the tests, was 24.2 MN for specimens T1–T3 (on average) and 27.9 MN for T0. The smaller secant stiffness of strengthened specimens is due to the fact that the second part departs from a higher cracking strength, but at the end of the service range their behavior tries to approximate the response of RC member, as the bridging capacity of the HPFRC through cracks and the tension stiffening contribution diminishes. Due to the testing procedure with displacement control, subsequent crack formation in specimens is accompanied by moderate load releases and subsequent reloading, which can be observed through some load oscillations in the diagrams of The number of cracks formed in the specimens has been detected with the help of the DIC technique. Two methods can be used. On the one hand, the contours of the longitudinal strains derived from the DIC analysis can be obtained, as plotted in c (c being the concrete cover). It is noted that other codes have reduced such contribution to 2c [c [The analysis of strain contours and longitudinal displacements along sections 1 to 3 has shown the following number of cracks of each specimen: 6 (T0), 7 (T1), 8 (T2), and 6 (T3). Moreover, the average crack spacing is: 11.8 cm (T0), 12.7 cm (T1), 9.6 cm (T2) and 15.2 cm (T3). Therefore, an average value of 12.5 cm has been obtained for composite members T1–T3, which is 6% higher than the crack spacing of T0. It seems that such a slight increase has been due to the larger concrete cover of composite specimens on the strengthened sides. The crack spacing calculated with the rules of Eurocode 2  would beon to 2c  and 1c [to 2c [c .It is also interesting that the strain contours and representation of longitudinal displacements b have inIn addition to the detection of crack positions, the DIC analysis offers the possibility to calculate the width of detected cracks and verify the benefits given by HPFRC strengthening. This can be done with the use of virtual extensometers bridging the two mouths of the cracks. Such a technique automatically provides the values of the displacement jumps sketched in pc = 2.5‰ in The small crack width at the HPFRC layers is indicative of the strain-hardening capacity of the material. According to the strain hardening limit derived from material characterization . In the present paper, the analytical model is limited to the strain hardening stage of the HPFRC due to the fact that its experimentally determined upper bound  until first crack formation. At this stage there is perfect bond at the interfaces (concrete-steel and HPFRC-concrete). Therefore, there are no relative slips, and the three materials have the same strain at any cross-section. Point A is characterized by the first cracking load, which takes place when the strain reaches the tensile capacity of the weakest material , i.e., εPhase 2: the concrete is cracked, while steel and HPFRC are elastic. At the cracks, the externally applied load is carried by the steel and the HPFRC. Due to concrete cracking, there is relative slip at the interfaces. For compatibility, the strain of steel and HPFRC are the same at the cracks. The first point of phase 2 (point B) corresponds to the same load as point A, but with a sudden strain increase due to the stress release at the concrete upon cracking:The horizontal transition between points A and B is a simplified approximation of the crack formation stage analogous to the one done in RC models ,42,43.ccε, refer to The end of phase 2 takes place when the elastic limit of the HPFRC is reached (pc) is reached and a macrocrack opens. The load and strain of point D can be calculated by:Phase 3: the concrete is cracked, the steel is elastic, and HPFRC is hardening. As in phase 2, there is relative slip at the interfaces, and the load is carried by steel and HPFRC at the cracks, where they have the same strain. The first point of phase 3 is point C. The end of phase 3 occurs when the strain-hardening limit of HPFRC  produces shortening of RC tension members and formation of self-equilibrated internal stresses (compression at the steel and tension at the concrete). Note that t is the time from end of concrete curing to the time of loading. According to Bischoff [cr,redF in Equation (10).c and Δσs are the internal stresses developed in the concrete and the steel. It has been extensively reported that the time-dependent shrinkage of concrete affects the behavior of tension members ,45,46. IBischoff , the loaIn case of composite RC-HPFRC members, the shrinkage effect is more complicated due to the construction sequence and the presence of two materials shrinking from different times . Such time-dependent effects have not been considered in the previous general model  and it iPhase 1: Before the instant of application of the HPFRC strengthening layers (denoted as Ut hereafter), the shrinkage of the conventional concrete produces the following shortening of the tension member and internal stresses:Ut) to the time of loading (t), both conventional concrete and HPFRC shrink. The free shrinkage strains of conventional concrete and HPFRC are εcs and εUs in absolute values, respectively. The composite tension chord is then monolithically shortened by the following strain:From the installation of the HPFRC layers  is the creep coefficient from Ut to t. The total modification of the elastic uncracked stage at the time of loading (t) can be included in the tension chord model by modifying the behavior of Phase 1 with Equations (11) and (13) for the origin offset and Equations (12) and (14) for the first cracking strength reduction Ec(tU):Phases 2 and 3: After the conventional concrete cracks, the load is carried by the HPFRC and the steel reinforcement. The behavior of Phases 2 and 3 must be modified by an origin offset (Δε’) and by the reduction of the load carrying capacity of the HPFRC  in order to easily focus on the serviceability range (F/yF < 1.0). On the other hand, the crack width has been measured at two points of the flexural crack formed at the midspan section: at the level of the longitudinal steel reinforcement and at the centroid of the HPFRC strengthening layer.In order to further evaluate the potential benefits of the application of HPFRC strengthening layers with appreciable strain hardening capacity as those investigated in the present research, the results of previous experiments on beams tested by the authors  are assepc = 1.2‰) can be noted from the tensile properties listed in The HPFRC consisted of a CRC with 2% of steel fibers. The main difference with respect to the HPFRC used in the tension members T1–T3 was the smaller number of fibers, which was responsible for the poorer tensile properties. The small hardening capacity of the HPFRC (εUh = 35 mm (the same HPFRC thickness as the one used in tension members T1–T3), the crack width at the steel reinforcement level and at the HPFRC was approximately the same from the first crack formation, without regard of the load level. Such a result is a more deficient response than the one shown in Uh = 55 mm . Nevertheless, the increase of the consumption of high-performance material to achieve this is a significant 57% (55/35-1).The relevance of the tensile properties can be discussed with the load-crack width diagrams plotted in  = 55 mm  shows thTherefore, from the comparison of the results of tension members T1–T3 with those of bending tests of , it can The in-service behavior of RC-HPFRC tension members is characterized by larger stiffness, higher first cracking strength and smaller crack widths than comparative unstrengthened RC members. According to the test results presented in this paper, the increase of initial stiffness and first cracking strength have been 66% and 55%, respectively. The widest crack widths measured at the axis of the specimens was 0.3 mm and 0.2 mm for RC and RC-HPFRC specimens, respectively. The experimental results showed that the tension stiffening contribution of RC-HPFRC members is significant and should not be neglected for serviceability verifications.The width along each single crack is smaller in the HPFRC layers than in the conventional concrete layer. According to the experimental results, crack widths were not larger than 0.05 mm at the HPFRC layers, while 0.2 mm was measured at the RC layer. Moreover, the study has demonstrated that the HPFRC can stay in the microcracking phase within the service-load range if a HPFRC mix with strain-hardening capacity is used (as in the present paper), which is rather favorable to achieve increased permeability and durability of the strengthened member.While in RC tension members the load is fully carried by the steel reinforcement at the cracks in the service load range, a non-negligible contribution of the HPFRC can also be expected in cracked sections of RC-HPFRC elements due to the strain hardening and softening capacities.From the analysis carried out in the present paper, it is fundamental to take into account the time-dependent shrinkage effect in order to fully understand the different behavioral stages of composite RC-HPFRC tension members. A simple mechanical model consisting of four key points has been proposed. An important ability of the model is the consideration of the shrinkage of the two concretes .With respect to previous research by the authors , this paA simplified model for service load domains was presented consisting of 4 hey points, which includes the complex phenomena related with the shrinkage of the two concretes .From the study carried out in the present paper, the following conclusions can be drawn:


According to the basic principle of dense packing of particles, and considering the interaction between particles, a dense packing model of granular materials in concrete was proposed. During the establishment of this model, binary particle packing tests of crushed stone and sand were carried out. The fitting analysis of the test results determines the relationship between the particle size ratio and the remaining volume fraction of the particle packing, and then the actual void fraction of the particle packing was obtained, based on which the water–binder ratio was combined to determine the amount of various materials in the concrete. The proposed concrete mix design method was used to prepare concrete, and its compressive strength and elastic modulus were tested experimentally. The test results show that the aggregate volume fraction of the prepared concrete increased, and the workability of the concrete mixture with the appropriate amount of water reducing agent meets the design requirements. When the water–binder ratio was 0.42, 0.47, or 0.52, the compressive strength of the concrete increased compared with the control concrete, and the degree of improvement in compressive strength increased with the decrease in water–binder ratio; when the water-binder ratio was 0.42, 0.47, or 0.52, the static elastic modulus of the concrete increased compared with the control concrete, and the degree of improvement in elastic modulus also increased with the decrease in water–binder ratio. The elastic modulus and compressive strength of the prepared concrete have a positive correlation. Findings show that the concrete mix design method proposed by this research is feasible and advanced in a sense. The concrete mix ratio is the proportional relationship between the amount of each constituent material in concrete, and the performance of concrete is closely related to its mix ratio. There are different methods for designing mix ratios in different countries. The American ACI method is simple and easy, and the concrete mix ratio can be determined by looking up the relevant forms . The BriAmong the constituent materials of concrete, coarse aggregate, fine aggregate, and cementitious materials are all granular materials, and it can be said that concrete is a dense solid formed by filling these granular materials with each other . AccordiThis research proposes a new method for designing thew concrete mix ratio. The method is based on the binary particle packing test of crushed stone and sand, and the relationship between the particle size ratio and the remaining volume fraction of particle packing is determined through the analysis of the test results. Furthermore, the actual voidage of the particle packing is obtained, and then the water–binder ratio is combined to determine the amount of various materials in the concrete. Then, the proposed concrete mix design method is used to prepare concrete, and its compressive strength and elastic modulus, the two most basic parameters in the design of concrete structures, are tested.Coarse aggregates, fine aggregates, and cementitious materials are granular materials in concrete. Considering the mutual filling of particles with different particle sizes, the particles are packed with the goal of achieving maximum compactness or minimum void ratio after the particles are mixed. The basic model expression for particle packing is as follows:Coarse aggregate volume fraction:Fine aggregate volume fraction:Volume fraction of cementitious material:3.Total volume fraction for particles packing:This packing model is proposed based on the theoretical dense packing between particles with different particle sizes; that is, fine aggregates are filled in the gaps of coarse aggregates, and cementitious materials are filled in the gaps of coarse and fine aggregates.When particles of different sizes are blended, they have a spatial influence on each other, described by the wedging effect . When coThe wedging effect makes the actual voidage of the particle pack larger than the theoretical voidage of the particle pack, and this interaction between the particles is related to the particle size ratio between the particles .Standard for Technical Requirements and Test Method of Sand and Crushed Stone (or Gravel) for Ordinary Concrete .Particles with sizes of 22.75, 17.5, 12.75, 7.125, 3.555, 1.77, 0.89, 0.45, and 0.225 mm were selected, and each size was the arithmetic mean of the adjacent sieve sizes of the standard sieve for concrete sand and gravel specified in 3;3; 3.The selected particles were combined in pairs and arranged into 20 groups according to the ratio of the particle size from small to large, and then the 20 groups of particles were subjected to particle packing tests. According to the basic model of particle packing (Equations (1) and (2)), the amount of coarse and fine aggregates is determined, and due to the wedging effect, there will be a residual number of particles in the actual packing process. The residual amount can be measured in the packing test, and the residual particle volume fraction is calculated asThe particle binary packing test and its results are shown in ix had a similar effect on the residual particle volume fractions ix increased, both ix exceeded 5, the effect of particles with a diameter of The relationship curve between the particle size ratio and the residual particle volume fraction is fitted from the test results, as shown in In order to simplify the model, the following assumptions were made: (1) the concrete packing system was composed of different materials, and each material took its volume average particle size as its characteristic particle size; (2) there was a large difference in particle size between the concrete composition materials, such as coarse aggregate, fine aggregate, and cementitious material. We ignored the influence between non-adjacent size particles; that is, we only considered the effects between coarse aggregate and fine aggregate and the effects between fine aggregate and cementitious material, while the effects between coarse aggregate and cementitious material were ignored.On the basis of the results of the particle packing test and the above assumptions, the basic model (Equations (1)–(4)) was modified, and the modified model expression is as follows:Actual volume fraction of coarse aggregate:Actual volume fraction of fine aggregate:Actual volume fraction of cementitious material:Actual total volume fraction for particles packing:Substituting the residual particle volume fractions into Equations (8)–(10), we get the following:Actual volume fraction of coarse aggregate:Actual volume fraction of fine aggregate:Actual volume fraction of cementitious material:Solved by Equations (12)–(14):Coarse aggregate actual voidage:Fine aggregate actual voidage:Actual voidage of cementitious material:(1) Cement: ordinary Portland cement with strength grade 42.5; the main components are shown in (2) Fine aggregate: river sand in Zone II; the main technical properties are shown in (3) Coarse aggregate: crushed stone, 5–25 mm continuous gradation; the main technical properties are shown in 3 and pH of 7.0–8.0.(4) Water reducing agent: highly efficient polycarboxylic acid water reducing agent, light yellow liquid with density of 1.1 g/cm(5) Water: drinking water.On the basis of the main technical parameters of the concrete constituent materials, the actual voidages of the granular material in the concrete were calculated from Equations (15)–(17), as shown in It can be seen from Three water–binder ratios of 0.42, 0.47, and 0.52 were selected, and the design slump of the concrete mixture was 70–90 mm.3 concrete is as follows:3; V—concrete volume, taken as 1 m3; ξ—the volume shrinkage coefficient is expressed as follows:yw is the volume fraction of water, expressed as follows:W/B is the water–binder ratio.The calculation process of the amount of crushed stone, sand, cement, and water in 1 m3 concrete, see C-1, C-2, and C-3 in Put Equations (8)–(10), (22), and (23) and various material parameters into Equations (18)–(21), and you can get the amount of crushed stone, sand, cement, and water in 1 mSpecification for Mix Proportion Design of Ordinary Concrete . The amount of the water reducing agent given in In Standard for Test Method of Mechanical Properties on Ordinary Concrete . In this method, there are three test pieces with a size of 100 mm × 100 mm × 100 mm in each group, and the test result is the average of the compressive strength test values of the three test pieces. The test results are shown in The test was performed in accordance with the concrete compressive strength test method specified in It can be seen from Standard for Test Method of Mechanical Properties on Ordinary Concrete . In the method, each group had six specimens with the size of 150 mm × 150 mm × 300 mm, which were cured for 28 d before testing. Among them, three specimens were used to test the axial compressive strength of concrete, and the other three specimens were used to test the elastic modulus of concrete. The test results are taken as the average value of the tested elastic modulus of the three specimens, as shown in The test was carried out according to the test method of elastic modulus of concrete under static compression specified in As can be seen from The relationship between stress and strain before the ultimate stress of the concrete under axial compression is shown in The concrete mix design method proposed in this study was based on dense particle packing. Since the aggregate was a granular material with the largest proportion in concrete, the volume fraction of aggregate in the concrete prepared by this method increased to different degrees. The volume fraction of aggregate is shown in Sections of the concrete specimen were randomly taken. The coarse aggregate information was extracted on the section using the IPP image processing software, and grayscale processing was performed, as shown in Concrete is a multiphase composite material composed of mortar, coarse aggregate, and an interface transition zone between the two. Since aggregate has a large volume fraction in concrete, especially coarse aggregate, it has a great influence on mechanical parameters such as compressive strength and elastic modulus under the conditions of a certain cementitious material and water–cement ratio .In concrete of group C, the higher aggregate volume fraction makes the aggregate more closely distributed in the concrete The design method of concrete mix proportion based on interparticle voidage proposed in the research is a kind of design method for concrete mix proportion according to the principle of dense particle packing.(2)The functional relationship between the particle size ratio and the remaining volume fraction of the particles is the basis for determining the actual voidage of the granular material in the concrete.(3)The coarse aggregates in the concrete prepared by the method proposed in this research are closely distributed, and even overlap each other, forming a relatively dense concrete core skeleton. Micro-cracks and defects in the interface transition zone of the concrete are also reduced to a certain extent.(4)In the concrete, there is an increase in aggregate volume fraction and a corresponding decrease in the amount of cement, and when an appropriate amount of water reducing agent is added, the workability of the concrete mixture meets the design requirements.(5)When the water–binder ratio was 0.42, 0.47, or 0.52, the compressive strength of concrete with ages of 3, 7, 28, or 90 d all increased compared with that of the control concrete at the same age, and the degree of increase in compressive strength increased as the water–binder ratio decreased.(6)When the water–binder ratio was 0.42, 0.47, or 0.52, the static elastic modulus of concrete increased compared with that of the control concrete, and the degree of increase in static elastic modulus increased as the water–binder ratio decreased, showing that the elastic modulus is positively correlated with the compressive strength.This research proposed a concrete mix design method based on the principle of dense packing of particles, and the method was used to prepare concrete. Compressive strength and elastic modulus of the concrete were experimentally studied. The coarse aggregate distribution and interface transition zone in the concrete were observed, and the following conclusions were drawn: