Accumulating vast amounts of pollutants drives modern civilization toward sustainable development. Construction waste is one of the prominent issues impeding progress toward net-zero. Pollutants must be utilized in constructing civil engineering structures for a green ecosystem. On the other hand, large-scale production of industrial steel fibers (ISFs) causes significant damage to the goal of a sustainable environment. Recycled steel fibers (RSFs) from waste tires have been suggested to replace ISFs. This research critically examines RSF’s application in the mechanical properties’ improvement of concrete and mortar. A statistical analysis of dimensional parameters of RSFs, their properties, and their use in manufacturing various cement-based composites are given. Furthermore, comparative assessments are carried out among the improvements in compressive, split tensile, and flexural strengths of plain and RSF-incorporated concrete and mortar. In addition, the optimum contents of RSF for each strength property are also discussed. The influence of RSFs parameters on various strength properties of concrete and mortars is discussed. The possible applications of RSF for various civil engineering structures are reviewed. The limitations and errors noticed in previous review papers are also outlined. It is found that the maximum enhancement in compressive strength (CS), split tensile strength (STS), and flexure strength (FS) are 78%, 149%, and 157%, respectively, with the addition of RSF into concrete. RSF increased cement mortars’ CS, STS, and FS by 46%, 50.6%, and 69%, respectively. The current study encourages the building sector to use RSFs for sustainable concrete. Great attention on the global increase in tires is required, especially in the European Union ,2. More Concrete is extensively used in building materials because of its high compressive strength, durability, and environmental compatibility. Concrete is used in various architectural projects, including foundations, walls, bridges, roadways, dams, and reservoirs . However2 emissions of the concrete mixture with raw steel fibers recovered from waste tires (RSF) to a concrete recipe with industrial steel fibers (ISF). It was concluded that the concrete recipe with RSF uses 31.3 percent less energy and emits 30.8 percent less CO2. However, due to intricate and energy-intensive manufacturing procedures, industrially manufactured steel fibers are the second component (after cement) that substantially impacts the natural environment. Therefore, energy consumption and greenhouse gas emissions could be reduced by substituting waste fibers obtained from the recovery of rubber from used tires for industrially manufactured steel fibers. Pawelska-Mazur and Kaszynska  compared3 to 40 USD/m3. According to the findings, the contribution of employing RSF substitutes for ISF in terms of economy and environment is significant. Furthermore, RSF’s social role cannot be overlooked. Waste tires pose a significant environmental problem [Qin and Kaewunruen  demonstr problem . As an e problem . Further problem . In addi problem . Accordi problem ,27,28,29Various mistakes were noticed in the previous review papers related to the possible use of raw steel fiber recovered from waste tires (RSF) in concrete. In some review papers, the steel fibers from industrial waste and lath machines are combinedly presented with that of raw steel fiber recovered from waste tires. For example, Liew and Akbar  reported3 of RSF increased the compressive strength of the concrete. However, research by the authors [In some papers, the RSF content of mortars is referenced for concluding the optimized content of RSF for concrete. For example, Liew and Akbar  and Ahme authors  was rela authors  was rela authors  reportedSome review papers dealt with only limited studies of the considered spectrum for a conclusion, such as Awolusi et al. , who basThe Balea et al.  review pThe given mistakes did not nullify the usefulness of the papers, but helped the readers read the papers while considering the mentioned mistakes in the view. Moreover, including the limitations mentioned above, the other problem with the previous review papers is that the effect of RSF is taken for both mortar and concrete. The influence of different doses of RSF independently on concrete and mortar has not been discussed and analyzed. In the current paper, the significance of RSF on the mechanical properties of concrete and zero coarse aggregate composite (mortar) are demonstrated independently. Secondly, the optimized contents of RSFs are discussed and analyzed separately for each composite for corresponding strength properties. Finally, the mistakes noted in the previous review papers related to different strength values are reported correctly.This review primarily focuses on the mechanical properties of RSF-reinforced cement composites, i.e., concrete and mortar. Various search engines were used to find relevant literature. Papers were mainly collected using the bibliometric databases ScienceDirect, Google Scholar, Mendeley, and Web of Science, with three critical phrases used throughout searches: (1) recycled steel fiber; (2) waste tire steel fibers; and (3) waste steel fiber. As RSF-reinforced concrete is still a new topic, the review procedure used a cross-reference “snowballing” technique followed by Liew and Akbar . BibliogCollecting papers related to RSF-incorporated composites. Screening the most relevant papers using plain concrete or mortar as a reference.Arranging the material’s properties and strength properties of concrete and mortars separately.Determining the optimized content of RSF for each concrete and mortar and establishing the possible relationship between the strength properties of each of the composites with a dosage of RSF.Justifying the influence of RSF on a green and sustainable environment.Forty-three research studies were chosen for data analysis after seeking and gathering the most relevant publications. According to the compiled articles, the benefits of RSF in enhancing the mechanical characteristics of concrete and mortars may be accurately explained by analyzing data from various countries, authors, journals, and research institutes. In addition, its contribution to the performance of concrete and mortars was intended to be compared to that of plain concrete and plain mortar. Therefore, four steps were followed in the research order as given below:A variety of RSF fibers have been used in various research studies. Overall, the diameters, lengths, tensile strengths, and aspect ratios of the RSFs used varied significantly.Due to the extensive geometrical variation in recycled steel fibers, statistical analysis is required for frequency distribution and average fiber size for given numbers of representative fibers ,42,43,44The typical characteristics of the RSF used so far are reported in The frequency of the various ranges of diameter with a constant interval of 0.20 mm is shown in Similarly, the frequency of various ranges of the RSFs’ lengths with a constant interval of 15 mm other than the first interval, which is 8 mm, are shown in It is observed that a higher percentage (32.35%) of RSFs were used, whose length range was from 15 mm to 25 mm. The percentage of 23.57% of the lengths of the RSFs in the range of 40 mm to 60 mm is also quite considerable. However, tiny amounts (less than 5.88%) of fibers were used with an average diameter greater than 60 mm.The frequencies of the various ranges of the tensile strengths of the RSFs used in previous studies are demonstrated in The mechanical properties of the concrete incorporating raw steel fibers recovered from waste tires (RSFs) of previous research were demonstrated. Fundamentally, concrete is defined as a mixture of binder, fine aggregates, coarse aggregates, and water. Sometimes, additives or admixtures are added to obtain specific qualities. Hardened concrete’s three main mechanical properties are compressive strength, split tensile strength, and flexure strength. For percent comparison, the properties of the plain cement concrete of the concerned study were taken as a reference.3. For this purpose, the strength properties of each category were compared individually, and the effect of variation of fiber was reported. In addition, an optimized dose for RSF was demonstrated for each of the RSF’s contents: fiber by volume or weight of concrete or in kg/m3. In some cases, it is challenging to compare the strength properties of different research studies due to differences in the scales used by authors for adding the fibers to the concrete. This becomes very difficult when the complete set of parameters of the fibers is not provided for converting their weight to volume or for interconversion among different scales of adding the fibers. To deal with this issue, the strength properties are compared separately for concrete containing RSF’s content by volume fraction of concrete, RSF’s content added by weight fraction of the concrete, and RSF’s content added in kg/m3) than the industrial steel fibers (7200 kg/m3) [3, for inter-conversion of steel weight and volume [3, and the density of the fibers is not given. Therefore, the properties of the effect of the addition of RSF in kg/m3 are discussed in a separate section.Most previous research has assumed that the density of RSF is the same as that of industrial steel fibers (ISFs), presuming that the RSF is free of impurities/tire particles or that impurities/tire particles have been separated from the RSF before mixing with concrete. However, recycled steel fibers from waste tires (RSF) were utilized in some studies without separating impurities/tire particles . As a re0 kg/m3) . In addid volume . TherefoThe compressive strengths (CS) of the concrete reinforced with raw steel fibers recovered from waste tires (RSF) were collected. The percent comparison of the CS is demonstrated in The amount of fibers for specimens is given on the right axis of the graph, while an increase or decrease in CS to plain concrete is shown on the left y-axis of the graph. The read squares show the percentages of the fibers at which the CS was less than the PC. The rest of the different colors of the boxes represent a specific range of RSF doses used. The number of specimens is shown on the x-axis of the graph. Specimens 1 to 10 contain RSF from 0.13% to 0.30%. For specimen numbers 2, 3, 4, and 8, the CS decreased 18%, 11%, 15%, and 11%, respectively, from adding 0.15%, 0.19%, 0.19%, and 0.26% of RSF by volume fraction of concrete. The CS increased for specimens 1, 5, 6, 7, 9, and 10, by 3%, 13%, 20%, 23%, 12%, and 5% respectively. For RSF range from 0.46% to 0.50%, no significant decrease was reported except for specimens 12, 15, and 24, as shown in Moreover, contradictory results were reported for specimen numbers 12, 13, and 14, incorporating the same amount of 0.46% RSF. The possible reason is that for specimens 13 and 14, the authors used a planetary concrete mixer; thus, good fiber dispersion and better mixing were obtained ,66. On tAt 0.50% of RSF, all researchers found no improvement or slight increase in CS of RSF specimens 15 to 29 except a slight decrease of 3% and 4% for specimens 15 and 24, respectively. Only a few studies  tested the CS of the specimen incorporating RSF from 0.60% to 0.80% by volume fraction of concrete. Increases of 2% and 15% were reported for the incorporation of 0.75% RSF in two studies. At the same time, decreases of 25% and 13% were reported for specimens 30 and 33, respectively, at 0.60% RSF and 0.80% RSF, respectively. A rise in CS of the RSF specimens was reported at 1% RSF for specimens 34 to 48, except for specimens 34 and 39. Specimen 34 and 39 showed a decline of 8% and 1% in CS at 1% RSF. No decline in CS was reported by any studies for specimens 49 to 64 for concrete incorporating RSF from 1.25% to 5% RSF by volume fraction.The data given in Employing a planetary mixer for specimen preparation resulted in a considerable increase in the percentage of fibers supplied to the concrete mix and the dispersion of those fibers, thus significantly improving mix homogeneity. As a result, the blends were more workable for the same amount of water for specimens 6 and 7 ,66. On tThe RSF concrete specimens incorporating 0.40% to 0.50% of RSF were grouped. Significant decrease of 3%, 3%, and 4% were reported by Leone et al. , CentonzThe second-highest increase in CS was noted by Skarzynski and Suchorzewski  for 0.50The specimens incorporating the RSF content equal to or more than 1% are demonstrated in For maximum CS within the same range, both the diameter and length of the fibers need to be considered. In the previous studies, an average increase of 2.53% was noted in CS for 1% RSF content having an average diameter of 38.18 mm and 0.79 mm. There was no decrease in CS for specimens incorporated with 1.25% to 2% RSF content. The highest increase of 27% was noted in specimen 63, which incorporated RSF, 40 mm long and 0.89 mm thick, as reported by Köroğlu .The various lengths from 1.25% to 5% RSF were 7.30 mm, 20 mm, 30 mm, 40 mm, 45 mm, and 60 mm. The diameters of the RSF used were 0.22 mm and 0.245 mm for only specimens 49 and 61–64. While 0.89 mm thick RSFs were utilized in the rest of the specimens (50–60). The CS increased 9% at an average of 2.02% RSF, having an average length of 38 mm, and an average diameter of 1 mm. 2 = 0.12), but a reliable trend can be made possible in the near future with the comprehensive database of the compressive strength of concretes containing RSF by its volume fraction.The percent improvement in compressive strengths of concrete incorporating RSF by its volume fraction is shown in The compressive strengths (CS) of the concrete, including raw steel fibers recovered from waste tires (RSF) by weight fraction, are presented in Also shown is the % increase or decrease in the CS of RSF concrete. The right axis of the graph shows the number of fibers in specimens, while the left y-axis shows the increase or decrease in CS to plain concrete. The x-axis of the graph represents the number of specimens. The percentages of fibers where the CS is less than the PC are displayed in the read squares. The remaining colored boxes each indicate a certain range of RSF dosages. RSF is present in 0.10 percent to 0.75 percent of specimens one through five. For 0.50 percent and 0.75 percent of RSF, the CS declined 3% and 5%, respectively. The CS increased by 2%, 5%, and 4%, respectively, for specimens one, two, three, and five. For 1% to 2% RSF, a significant increase in CS was reported for specimens eight to twelve, except for specimen seven. The authors reported a 5% decrease in CS at 1% RSF. By considering the specimen including RSF from 2.5% to 3.5%, a decrease of 1%, 10% and, 5%, in CS was reported for specimens 15, 17, and 18, respectively, for 3%, 3.5%, and 3.5% of RSF, correspondingly. An increase of 34%, 37%, and 15%, in CS was reported for specimens 13, 14, and 16, respectively, reinforced with 2.5%, 2.5%, and 3% RSF, compatibly. For specimens 19 and 20, 17% and 7% decreased at 4% RSF. In contrast, an improvement of 78%, 67%, and 33% was reported for specimens 21, 22, and 23, respectively, for concrete incorporating RSF of 4%, 6%, and 6%, respectively.A decline of 3% and 5% was reported by Shah et al.  for 0.50Shah et al.  reportedTherefore, maximum strength can be attained for 2% to 3% RSF by keeping the diameters of the fibers below 7.62 mm for traditional concrete mixers. A decline of 17% and 7% were reported in CS for specimens 19 and 20 when diameters of RSF were 100.16 mm and 7.62 mm, respectively. Short RSF showed less decline in CS. Interesting results are reported when 4% and 6% RSF had reduced diameters and lengths added to concrete. It was investigated that the highest increase of 78% in CS was achieved using 29 mm long RSF, which had a diameter of 0.20 mm. When the same RSF (29 mm long and 0.20 thick) was increased from 4% by weight of concrete to 6%, the increase in CS reduced from 78% to 67% . An incr3, are demonstrated in 3. However, a 3.2 percent increase was reported for specimen seven incorporating 30 kg/m3 RSF. Specimens reinforced with RSF of 30 kg/m3 to 60 kg/m3 showed a considerable rise in CS. Specimens 9 and 11 showed a decrease of about 1% and 8% in CS at 40 kg/m3 and 60 kg/m3, respectively. The data given in The compressive strengths (CS) of the concrete, containing raw steel fibers recovered from waste tires (RSF) in kg/m3 of RSF, no significant increase (more than three percent) in CS of concrete was reported except for a decline in CS. A decline of 2% is noted in the CS of concrete for average RSF content of 15.71 kg/m3 (5 to 20 kg/m3). This is noticed for 50 mm to 52 mm-long fiber, whose average diameter ranges from 0.30 mm to 1.40 mm. Therefore, it can be suggested that the RSF content below 20 kg/m3 cannot help to improve the CS of the concrete.The given data are shown for a particular range of RSF content in 3, significant improvement was reported by the authors in CS, except for the 8.2% decrease reported by Sengul [3 of RSF. The possible reason for the decrease reported by [3 to 60 kg/m3 are 38.75 mm and 0.81 mm, respectively. Hence, it can be concluded that 60 kg/m3 of RSF can be a suitable dose for improving the CS within the limited diameter and length of the fibers. However, in-depth analysis and research are still needed to optimize the suitable diameter and length of the fibers for the different ranges of RSF dosage.On the other hand, for the RSF ranging from 20 to 40 kg/my Sengul  for utilorted by  is the fThe percent comparison of the split tensile strength (STS) of the concrete, including raw steel fibers recovered from waste tires (RSF) by its volume fraction, is presented in A certain decline was reported in STS when the content of RSF was lower than 1%. However, some specimens showed a substantial increase in STS even for RSF of less than 1%. The specimens 1, 5, 9, 10, and 11 incorporating RSF of 0.20%, 0.46%, 0.50%, 0.50%, and 0.80%, respectively, showed a decrease of 13%, 10%, 14%, 26%, and 10%, respectively, in the corresponding STS. On other hand, specimens 2, 3, 4, 6, 7, and 8, containing 0.23%, 0.30%, 0.40%, 0.46%, 0.50%, and 0.50%, respectively, showed a substantial improvement of 16%, 4%, 3%, 9%, 18%, and 43%, in respective STS. No improvement in STS was reported for specimen 11 at 0.60% RSF. A decline of 10% in STS was noted only for specimen 13 at 1% of RSF. For more than 0.60% of RSF, no decline was reported. The authors noted a significant improvement in STS for adding 1%, 1.50%, 2%, 3%, 4%, and 5% of RSF, as reflected in The data regarding STS of RSF-incorporated concrete and the properties of RSF used in each study are in In comparison, Aiello et al.  confirmeAn increase of more than 100% in STS was noted for specimens 18 and 19 using 1.5% and 2% of 30 mm long RSF. In comparison, a lower increase of 41% was reported in STS at 2% RSF . SimilarFurthermore, exploring the relation between the content of RSFs used in previous studies and percentage improvements in split tensile strengths in corresponding research is also required. Therefore, the relation between percent improvement in STS of RSF concrete concerning plain concrete and the content of the RSFs is presented in 2) is not very low (R2 = 0.36); however, an increased database of concrete STSs containing RSF by volume fraction can help obtain a clear image of the effect of more than 2% RSF. In addition, the various dimensional parameters of RSF must be considered to specify the trend of the increase in STS of the RSF concrete at a specific percentage of RSF.The available database’s coefficient of regression (RThe split tensile strength (STS) of the concretes containing raw steel fibers recovered from waste tires (RSF) by their weight fraction is shown in The percent comparison among the results was revealed and discussed. The STS of the plain concrete of the concerned study was taken as a reference  to assess the percent rise or decrease in the STS of RSF concrete. No decline was reported when including 0.25% to 6% of RSF in concrete by weight fraction. On the other hand, an increase of 9% to 96% was reported for RSF concrete compared to plain concrete. The STS of RSF-included concrete is summarized in Furthermore, when typical mixers were utilized, a considerable enhancement of 70.3 percent was seen in STS, even for long RSF (100 mm length) at 3.5 percent . Younis In contrast, a higher increase of 96% in split tensile strengths was noted for 4% and 6% RSF having small diameter and length compared to RSF of specimens 13 and 14. It showed that the diameter of the RSF can also have a detrimental effect on the concrete’s STS as the length of the RSF. Therefore, for optimizing RSF dose, the diameters of RSF need to be considered for the same length of RSF.3, is shown in 3 or less than 20 kg/m3 of RSF except for specimen five, which showed a 7% rise in STS at 20 kg/m3 RSF. Most of the research studied demonstrated a noticeable increase in STS when incorporating more than 20 kg/m3 RSF. Specimens seven to thirteen showed an increment of 18%, 34%, 7%, 43%, 7.5%, 4%, 27%, and 36%, respectively, in their STS as compared to plain concrete. It showed the potential of the RSF to increase the split tensile strength of the concrete when the ratio of RSF was 20 kg/m3 or more. The split tensile strength (STS) of the concrete, including raw steel fibers recovered from waste tires (RSF) in kg/m3, except for specimens five and six. The STS was improved when the content of the same RSF was increased for the same mix from 10 kg/m3 to 20 kg/m3 [The data in 20 kg/m3 . However3 RSF with an average length and diameter of 50.86 mm and 0.59 mm reduced STS. However, for all reported diameters and lengths, the RSF ranging from 20 to 60 kg/m3, the authors noted a significant increase in STS. The highest increase of 43% in STS was reported by Pawelska-Mazur and Kaszynska [3. For long fibers that were 50 mm long, the STS increased 34% at 40 kg/m3. When the content of the same-size fibers increases from 40 to 60 kg/m3, the STS enhancement lowers to 4% [3 content of RSF, the 50 mm long and 1.40 mm thick RSF showed a lesser improvement of 4% in STS, and small-sized RSF (30–35 mm long and 1 mm thick) showed a higher enhancement of 27% and 36% for 30 mm long and 35 mm long RSFs, respectively. This showed that at the same proportion of RSF, an increase in STS can differ for different sizes of RSFs.Overall, the average dose of 15.71 kg/maszynska  when RSFrs to 4% . At the The flexural strengths (FSs) of the concrete, in which the raw steel fibers recovered from waste tires (RSF) were added by its volume fraction, are demonstrated in The percent increase and decrease are reported with reference to plain concrete. All specimens demonstrated a significant rise in FS with RSF of 0.20 percent to 5 percent. The contents of the RSFs used were 0.20%, 0.40%, 0.50%, 0.60%, 0.80%, 1%, 1.5%, 2%, 3%, 4%, and 5% by volume fraction of concrete. The enhancement in FS to plain concrete was 2% to 457% for RSF concrete.The flexure strength of concrete specimens incorporating RSF by volume fraction and properties of RSF are provided in An increment of 30% was also reported for specimen 12 at 0.50% RSF with 26.17 mm length and 0.25 mm diameter. This confirms that RSF can help improve the FS when added in a small dose of 0.4% by volume of concrete. While the long fibers can perform well, the maximum optimized length for better performance of the concrete in flexure still needs to be explored in depth. For specimens 15 to 41, the flexure strength increased by adding content of RSF from 1% to 1.5% for the same-size RSF . SimilarLarge increases of 157%, 457%, and 429% in FS were reported for specimens 38 to 41 for incorporation of 3%, 4%, and 5% of RSF, respectively . All facInvestigating the relationship between the content of RSFs employed in prior experiments and % gains in flexure strengths in a subsequent study is necessary. 2) is good (R2 = 0.80); however, a more extensive database of split tensile testing results of concrete containing RSF by its volume fraction may assist in obtaining a clear picture of the effect for more than 2% RSF. Furthermore, the various RSF dimension aspects must be considered to specify the trend of the rise in STS of RSF concrete at a certain percentage of RSF.Most researchers examined the flexure behavior of RSF concrete using various sizes of RSF in amounts less than 1.5 percent, as can be seen. On the other hand, few studies have examined the effects of having more than 2% RSF with short fibers or an RSF mix (containing various RSFs of different sizes). Nevertheless, the trend line shows that the FS has the potential to improve at even more than 2 percent RSF. The graph’s coefficient of regression  were reported for specimen 11 at 2% RSF (29 mm long and 0.20 mm thick) . The pos3 are demonstrated in 3 RSF content for specimens two and five, respectively. For more than 20 kg/m3 of RSF, a decline in FS was not reported for any specimen. An increase of 32%, 29%, 2%, 21%, 38%, 68%, 20%, and 20% were reported for specimens 1, 4, 6, 7, 8, 9, and 10, respectively. The percentage comparisons among the flexure strengths (FS) of concrete, in which raw steel fibers recovered from discarded tires (RSF) were added in kg/m3 and 15 kg/m3 of 52 mm long and 0.30 mm thick RSFs for specimens one and four, respectively. The 9% decrease in FS noted for same-size RSF at 10 kg/m3 was not well-justified by the authors. Similarly, a decline of 4% in FS noted at 20 kg/m3 RSF (50 mm long and 0.60 mm thick) for specimen five was not logically justified [The results presented in ustified .3 and 20 kg/m3, respectively, for specimens three and six. It can be noted that at the lower percentage (5 kg/m3) for small-diameter (0.30 mm) RSF 32.1% increase in FS was noted, while the 4.5 times increase in RSF diameter (1.40 mm) showed little increase (21%) in FS at 20 kg/m3. This confirms that the diameter and length of raw steel fibers recovered from waste tires must be considered to optimize their flexure strength.For the same-size RSF (50 mm long and 0.60 mm thick), an increase of 2% and 21% was noticed in FS at 10 kg/mLike concrete, the hardened properties of the cement mortars incorporating raw steel fibers recovered from waste tires available in the previous papers were compared and discussed. Mortar is a mixture of binder, fine aggregates, and water. Sometimes, additives or admixtures are added to obtain unique characteristics. The effect of RSF on mortar is discussed separately because its composition and behavior differ from that of concrete. More cement paste is available for gripping raw steel fibers recovered from waste tires (RSF) than mortar concrete. Thus, the behavior and the effect of various contents of the RSF can be different for mortar than concrete. The three main mechanical properties are compressive strength, split tensile strength, and flexure strength. For percentage comparison, the properties of the plain cement mortars of the concerned study were taken as a reference.The compressive strengths (CS) of the mortars incorporating raw steel fibers recovered from waste tires (RSF) by their volume fraction are discussed in this section. The percentage comparison of the compressive strengths is demonstrated in The read squares show the percentages of fibers where the CS was less than the PC. The other colored boxes represent other RSF dose ranges. For percentage comparison, the percentage increase or decrease in CS RSF mortars is presented with reference to the plain mortar of the corresponding study. A decline of 0.7%, 4.5%, and 0.5%, in CS was reported for three specimens  incorporating 0.01%, 1.50%, and 3% of RSF, respectively. For other specimens, a significant increase in CS was reported. An increase of 10.3%, 9%, 11.7%, 35.1%, 17.4%, 9.8%, 25.2%, 16.6%, 46%, 23.3%, and 12.9% was noted in CS for specimens 2 to 9, and 11 to 13, respectively. Specimens 2 to 9 and 11 to 13, contained RSF of 0.02%, 0.35%, 0.57%, 0.57%, 0.70%, 2%, 2.05%, 1.50%, 1.50%, 2%, and 2.50%, respectively.The compressive strength of mortar specimens with RSF by percent volume and RSF features are shown in On the other hand, only a 3% increase in compressive strength was reported by using thirteen times more (0.13% by volume of concrete) of the lengthy RSF (26 mm long and 0.258 mm thick) in concrete with a high percentage (1.37%) of superplasticizer , while fA maximum increase of 35.1% in CS was noted for RSF, ranging from 0.10% to 1% for specimen five, including 21 mm long and 0.20 mm thick RSF . This validates the effect of RSF size on the CS for its same content. One percent to three percent of RSF, only for specimen 10, a decline of 4.5% was reported at 1.50% . The possible reason for this decline can be associated with the large thickness (0.92 mm) of short-length (31 mm) RSF, which might be unable to contribute to the CS of the mix in such a low volume. In the same study, CS was almost equal to plain mortar when RSF was increased from 1.50% to 3% for specimen 14. On the other hand, significant enhancement in CS was observed for other specimens. A maximum of 46% improvement was noted for specimen 11, including 1.5% RSF, 50 mm length, and 0.15 mm thickness . At the The percentage comparison of the split tensile strength (STS) of the mortars (zero coarse aggregate concrete) incorporating raw steel fibers recovered from waste tires (RSF) by volume fraction is demonstrated in Opposite to concrete, the inclusion of small-sized RSF (14.9 mm long and 0.32 mm thick) also significantly improved STS. However, only a 16% increase in split tensile strength was reported when using twenty times the amount (0.2% by volume of concrete) of 11 mm longer RSF (26 mm long and 0.258 mm thick) in concrete . The incThe percentage comparison of flexure strengths (FS) of the raw steel fibers recovered from waste tires (RSF) incorporated in mortar by volume fraction is demonstrated in Conversely, a slight increase of 4% for concrete was reported at 0.20% for 62 mm long and 0.80 mm thick RSF . It showSome standards for plain concrete specify the correlation between compressive strength and flexural or splitting strength. These include the relation between the compressive strength, split tensile strength, and flexure strength specified by the American Concrete Institute (ACI) and British Standards (BS). The equations are given below:fc′) ratio increases linearly by adding fiber to 2%, as seen in fc′) ratio also has a rising trend with an increase in the content of RSF. This showed that the ratio recommended for compressive strength and split tensile strength of plain concrete are not valid for RSF concrete and need to be modified accordingly.The potential relationship between flexural strength and split tensile strength is shown by Equations (1) and (2). The STS/√(2) to 2% RSF. However, the regression coefficient (R2) is lowered when the trend line for fiber content is drawn to 5% RSF as shown in Better compliance is supported by the moderate value (0.46) of the coefficient of regression (Rfc′) ratio can be made when more studies on FS of concrete incorporate more than 2% RSF. Therefore, the average value of the STS/√(fc′) ratio for 0.20% to 2% RSF is calculated using currently available data in Therefore, a more reliable comparison between the fiber content above 2% and the STS/√(fc′) ratio value is 0.67, which is more significant than the proposed ratio for plain concrete. Therefore, a more reliable ratio between split tensile strength can be obtained in the future by using a large amount of data related to the experimental results of the concrete’s compressive strength and split tensile strength.The average STS/√(fc′) ratio and RSF content of concrete specimens incorporating 0.20% to 2% RSF by volume fraction of concrete is shown in fc′) ratio rises with fiber, showing that the RSF helps improve flexural strength. Furthermore, the FS/√(fc′) ratio has rises in line with the growth in RSF content. This demonstrated that the plain concrete ratios for compressive and flexure strength are invalid for RSF concrete and should be changed accordingly. A linear trend of increase in the ratio of FS/√(fc′) is noted for an increase in the dose of the RSF content by volume fraction of concrete. The relation between the FS/√(fc′) and the content of the RSF due to a low regression coefficient (R2) of 0.15; however, it supported that the FS/√(fc′) ratio does not follow the same relation as the plain concrete. Therefore, more experimental data are required to formulate a unique FS/√(fc′) ratio for RSF concrete. It can be concluded that the existing experimental data are insufficient to conclude a reliable value for the FS/√(fc′) ratio. Therefore, more data are required to determine a reliable FS/√(fc′) ratio. However, an average value for the FS/√(fc′) ratio is premeditated for the currently available data for RSF concrete containing 0.20% to 2% RSF, as shown in The regression coefficient shows less correlation between the ratio of FS/√(fc′) ratio is 1.39, higher than the recommended plain concrete ratio. In the future, a more trustworthy ratio between flexure strength and compressive strength of concrete can be obtained by analyzing vast amounts of data linked to experimental results of the compressive and flexure strength of concrete.The average FS/√ have the potential to enhance the strength properties of concrete. It is expected that RSFs can replace industrial steel fibers (ISF) if uniformity is maintained in the RSFs by using more advanced procedures for their extraction and cutting. The possible applications of RSF concrete can be concluded based on the available data related to experimental testing of RSF concrete. Therefore, the applications of RSF concrete for various civil engineering structures and fields are discussed in comparison to other types of fibers in the section below.The controlling parameters for the design of rigid pavements, including street pavements and concrete roads, are flexure and compressive strength. According to the AASHTO (1993) equation , AASHTOWMoreover, the RSF can be an excellent replacement for costly artificial fibers like nylon and glass. Thus, RF-incorporated concrete has the potential to replace other types of costly and less durable natural fibers for achieving better pre-crack and post-crack properties of concrete for achieving long-term durability. Therefore, RFs are likely the best for concrete roads and street pavements. However, an in-depth analysis of the likelihood of RSF replacing the natural and other types of fibers still needs more research; the durability in particular needs to be tested. The cracking rate in hydraulic concrete structures, such as canal lining, may be influenced by various variables, including shrinkage, water absorption, permeability, differential settlement, and tensile strength . If the On the other hand, according to the available database, the possible increase in CS, STS, and FS of RSF-incorporated concrete is 78%, 149%, and 162%, respectively, reported by Younis , Abdul AThe beam-column joints of reinforced concrete (RC) structures are very important. Various authors suggest that the failure of the building starts from joints ,77. TherTo increase the beam-column joints’ performance, many materials have been tested to check their suitability for an increase in joint shear strength. Adibi et al.  employed3 RSF (17.50 mm long and 0.25 mm thick) by adding RSFs in kg/m3 to concrete.A maximum enhancement of 29% was reported in compressive strength (CS) at 0.40% RSF (62 mm long and 0.80 mm thick) added by the volume fraction of concrete. A maximum enhancement of 78% in CS at 4% RSF (29 mm long and 0.20 mm thick) by weight fraction of concrete was reported. The most significant improvement in CS was reported at 50 kg/m3 RSF (17.50 mm long and 0.25 mm thick) when adding RSFs in kg/m3 to concrete.Split tensile strength (STS) was increased maximally by up to 149% when two percent RSF (62 mm long) was added to concrete by volume fraction. The highest increase of 96% in STS at 6% RSF (29 mm in length and 0.20 mm in diameter) for RSF by weight fraction of concrete was reported. The most significant improvement of 43% in STS was reported at 50 kg/m3 RSFs (50 mm long and 0.60 mm thick) to concrete.The maximum increase in flexure strength (FS) was up to 157% when adding 3% RSF (45 mm long and 0.245 mm thick) into the concrete by volume fraction. An increase of 457% and 429% in FS was reported at 4% and 5% of RSF. However, such a high increase in FS needs to be explored and verified by additional research. At 6% RSF (29 mm in length and 0.20 mm in diameter) added to concrete by its weight proportion, the maximum increase in FS was 162 percent. The greatest improvement in FS was 68% when adding 40 kg/mThe size of RSFs have a significant impact on the concrete’s CS. A considerable rise in CS was observed by many researchers for the same proportion of lengthier RSFs.It was found that small RSFs work better than large RSFs for increasing the STS of concrete. In comparison, longer RSFs were more effective than smaller RSFs in improving the FS of concrete.The most significant increases of 46%, 50.6%, and 69%, were reported in CS, STS, and FS of cement mortars (zero coarse aggregate mixture), adding 1.50% RSF (50 mm long and 0.15 mm thick), 0.02% RSF (50 mm long and 0.15 mm thick), and 0.57% RSF (21 mm lengthy and 0.20 mm thick), respectively, by volume fraction of mortar.In mortar, the low proportion of lengthy RFs worked effectively in improving CS. It is noted that even at a low dose of RFs for mortar, a sizable increment in CS was observed. In contrast to RF mortar, the rise in CS for RF concrete was minimal. Compared to other RFs, medium-size (20 to 35 mm long) RFs performed better in improving their FS. Therefore, it is vital to introduce advanced techniques to improve the RSF mixes’ homogeneity and the dispersion of RSF within the mix of mortar and concrete.The increase or decrease of RSF on the concrete’s CS is mostly no more than 48%. However, opposite to the low volume of fiber concrete, if the fiber content was controlled at higher than moderate volume (1% to 2% by volume fraction of concrete), the improvement showed a slightly dropping trend with RSF. The impact of RSF on the compressive strength of various cement-based mixtures, especially mortar, is yet unknown; more research is needed to fully comprehend the fiber’s interaction with the matrix and its behavior under compressive loads. However, unlike mortar, experimental results reveal that maintaining the RSF fiber content at lower than 1% of the volume fraction of concrete slightly improves compressive strength capacity.For similar content of RSF, mechanical strengths can differ when using different lengths of RSF. It was observed that different authors noted variations in compressive, split tensile, and flexure strengths for the different lengths of RSF used in the same dose of RSFs. Therefore, for optimizing RSF’s content for any mix, dimensional parameters, especially the length of RSF, also need to be considered.A complete review of the mechanical properties of recycled steel fibers of waste tires (RSF), concrete, and mortars were presented in this paper. First, RSF’s contribution to hardened concrete and mortar qualities and long-term development were contrasted with plain ones. A vast number of papers have been studied in this respect. The effect of various doses of RSF on the compressive, tensile, and flexural strengths of the concrete and mortars were discussed. A critical review of previous papers was also given. The mistakes noted in the previous review papers were avoided here. An extensive database was used in this study. Detailed studies regarding the statistical analysis of RSFs used, properties of RSFs, and the percent increase or decrease in the strength properties of the RSF composite compared with plain concrete or mortar were reported. In addition, a detailed summary of the possible increase or decrease in compressive strengths, split tensile strength, and flexure strength reported in the previous specific dose of RSF were demonstrated. The effect of different parameters (length and diameter) of RSFs on various strength properties of concrete and mortars was also discussed. The following conclusions can be made from a thorough review of research data related to the strength properties of RSF concrete:An in-depth investigation of the hardened properties of concrete for various sizes of recycled steel fibers from discarded tires is still missing. Therefore, an extensive campaign of studies is required to evaluate different sizes of RFs at the same content levels. Nevertheless, it may help create a dataset that can assist in obtaining refined results for different applications. Evaluating the effect of more than 2% RSF (added by volume fraction of composites) on concrete and mortar strength properties is recommended. The existing database is insufficient for concluding the effect of using various sizes of RSF in more than 2% by volume fraction of concrete. The influence of using high doses (more than 2% by volume fraction of concrete) of small-sized RSFs needs to be explored.


The increasing volume of waste and the requirements of sustainable development are the reasons for the research on new waste management concepts. The research results presented in this paper show the effect of recycled aggregate on the selected properties of cement concrete. The aggregates obtained from three types of wastes are tested: recycled concrete paving, crushed ceramic bricks, and burnt sewage sludges. The recycled aggregates replaced 25% and 50% of the volume of the fine aggregate. The tested aggregates worsen the concrete mixes’ consistency and decrease, to some extent, the compressive strength of the concrete. However, the tensile splitting strength of the concrete with recycled aggregates is similar to that of the reference concrete. Using recycled aggregates worsens the tightness of the concrete, which manifests itself by increasing water penetration depth. The thermal properties of concrete are slightly affected by the type and content of the recycled aggregate. Considering the expected improvement in recycled aggregate processing, they can be an alternative to natural aggregates. Using recycled aggregates in cement concrete requires extensive studies to search for ways to increase their possible content without worsening concrete performance. The volume of produced industrial and vegetable wastes is continuously rising. The increasing pressure on activity in line with the sustainable development rules leads to research on using various industrial wastes in the construction and building materials industry . ConstruThe separate group of industrial wastes are the sludges, which are the materials remaining from the industrial sewage or their treatment process. The term sludge can also concern the precipitated suspension formed during conventional water purification and many other industrial processes . The wasThe other type of waste is the construction waste generated when constructing a new structure or renovating or demolishing an existing building. Most of the wastes mentioned above are such materials as timber, plasterboards, building ceramics, reinforcing steel, concrete, plastic, glass, and cardboard. Waste concrete is the basic raw material for producing recycled aggregate ,9.Agricultural wastes form another category. They are biodegradable with time, but they must first be stored in particular places . For utilization, they are often burnt, and the created powder can be used as a fine aggregate or filler for concrete and mortar ,11,12.All the above wastes can be utilized for manufacturing cement composites. However, only some are used on an industrial scale, particularly in cement production. The increasing volume of waste and the requirements of sustainable development are the reasons for the research on new waste management concepts. The conducted studies should make it possible to utilize the various wastes as raw materials in the building materials industry . ConcernIn Japan, the USA, and most EU countries, using aggregates made of construction wastes for cement concrete is standardized ,15,16. CThe recycled aggregates utilized as a substitute for the fine aggregate in concrete have high porosity, which means high water absorbability and weak interfacial transition zone between aggregate grains and cement matrix ,20,21. TThe aggregate from recycling the concrete paving;The aggregate from crushing the wastes created by ceramic brick production;The aggregate from burning the sewage sludges in the municipal water treatment plant.The subject of the studies presented in the paper was determining the relation between the type of recycled aggregate and selected properties of the concrete mix and hardened concrete. The aggregates obtained from three types of wastes with the established composition were tested:It was assumed that the recycled aggregates will replace 25% and 50% of the volume of the fine aggregate (river sand). The aggregate replacement was not accurate regarding grain size distribution, which differs from the previous studies reported in the literature discussed in this section. In the investigation described in this paper, the volume of the individual fractions of the recycled aggregates was selected to minimize the strength decrease. The sand with a grain size of 0–2 mm was substituted with the recycled aggregate with a grain size of 0.5–4 mm, which made it possible to obtain the concrete with high content of the recycled fine aggregate (up to 50%) without significant worsening of the mechanical performance.The presented research covered determining the chemical composition of the recycled aggregates regarding the possibility of their use in concrete and the influence of these aggregates on the performance of the concrete. Determining the technological parameters of the concrete mix, the mechanical performance, thermal properties, and tightness of the hardened concrete made it possible to thoroughly assess the modified concrete’s usability. The sewage sludges are produced in large volumes and are challenging to utilize. Therefore, the possibility of their use as a substitute for the fine aggregate in cement concrete is important and worth evaluating, yet there is little research on this topic. However, the most promising results were achieved for aggregate made of ceramic bricks wastes.3, 0–2 mm natural sand, 2–8 mm and 8–16 mm gravel, and tap water. The grain size distribution of sand and gravel is presented in Reference concrete specimens CR0 were prepared using Portland cement CEM I 42.5R with a specific density of 3.1 g/cm3, was used for this aim. The content of the superplasticizer was 1.5% of the cement mass.Because recycled aggregate usually causes a worsening of concrete mix workability , the CR03 and after compaction 0.93 g/cm3;Aggregate from concrete paving recycling; the paving was initially crushed, and the fractions larger than 2 mm were sifted on the construction sieves a. The co3 and after compaction 1.21 g/cm3;Aggregate from the wastes created during ceramic bricks production b. The fr3 and after compaction 0.64 g/cm3.Aggregate from Pomorzany Water Treatment Plant  in the form of slag (SS), created during thermal deactivation of the sewage sludges c. The slIn the subsequent concrete mixes, 25% and 50% of the sand volume was replaced by:The images of the used recycled aggregates are presented in The grain size distribution of the recycled aggregates is presented in Seven concrete mixes were designed. Designing the concrete mixes was based on the previous experience of the authors. It was assumed that the reference concrete mix should demonstrate high fluidity so that the worsening of its workability caused by the introduction of the recycled fine aggregate is as tiny as possible. The composition of the reference mix CR0 is presented in After mixing the components in the laboratory mixer, the concrete mixes were tested. Then, the specimens for hardened concrete testing were prepared. The strength and thermal conductivity were determined on the cubic specimens of 100 mm × 100 mm × 100 mm. The specimens, after preparation, were stored in the molds for 24 h. After demolding, the specimens were placed in the container filled with water at 20 ± 2 °C until testing.In the initial testing range, the recycled aggregates’ chemical analysis was performed using the Bruker Quantax EDS X-ray energy-dispersive spectrometer.λ, volumetric specific heat, cv, and thermal diffusivity coefficient, a, were determined.The consistency of the concrete mixes was determined by the slump method according to the European Standard EN 12350-2 [Compressive strength was determined after 7 and 28 days of curing. The tests were carried out according to the European Standard EN 12390-3 , using tTensile splitting strength was determined on the cubic specimens of 100 mm × 100 mm × 100 mm according to the European Standard EN 12390-5 [Additionally, the apparent density of concrete in the dry state and depth of water penetration under pressure according to the European Standard EN 12390-8  were tesThe EDS analysis was performed for the identification of the chemical elements in the recycled aggregates. No heavy metals nor chlorides, which would exclude the material as a substitute for the sand in concrete, were found in the aggregate from concrete recycling . The totThe chemical composition of the aggregate from ceramic bricks recycling is presented in The investigations of the ashes and slags from the thermal conversion of the sewage sludges are focused on their potential application in the production of building materials, such as glass ceramics ,43,44, bThe results of consistency testing are presented in The available studies results ,57 show Obtaining a desirable consistency of the concrete mix requires that the total water covers the water necessary for the cement hydration and the additional water absorbed by the aggregate. The water absorbed by the aggregate grains is then released. It can help cement hydration in the interfacial transition zone (ITZ) between the aggregate and hardened cement paste, thus improving the ITZ properties, including strength. However, the excess water, not used during hydration, evaporates and can leave the micropores weakening the composite’s structure. Therefore, designing the concrete mix with recycled aggregates always needs special experimental verification.3 and 2340 kg/m3 for CC25 and CC50, respectively; 2380 kg/m3 and 2350 kg/m3 for CB25 and CB50, respectively; 2375 kg/m3 and 2335 kg/m3 for CSS25 and CSS50, respectively.The average apparent densities of the concrete mixes were: 2370 kg/mThe compressive strength testing results of concrete after 7 and 28 days of curing are presented in Every concrete with recycled aggregate showed decreased compressive strength compared to the reference concrete RC0. Except for the concrete containing the aggregate from the crushed bricks; the concrete CB50, after 28 days of curing, demonstrated slightly higher compressive strength (rise by 3.6%). The concrete CB25 had slightly lower compressive strength after 7 days of curing (a decrease of 3.7%). Generally, the CB concrete’s compressive strength was similar to that of the reference concrete CR0 within the range of 5%. These results were caused by the grain size distribution of the crushed bricks. The recycled aggregate made of crushed bricks comprised 80% of the grains with a size of 1–4 mm. Such a grain size distribution made it possible to achieve compressive strength close to reference concrete despite lower density.The recycled aggregate made of sewage sludge had a grain distribution similar to that of the aggregate from the crushed bricks. However, due to its porous structure and low crushing resistance , concretIn most cases, the aggregate made by grinding and crushing the concrete from demolition or ceramic bricks and the aggregate from sewage sludge ash shows higher water absorption and lower apparent density ,28,29,57The studies on using sewage sludge (SS) in cement concrete are mainly focused on utilizing this waste as a partial substitute for cement ,47,50,60The tensile splitting strength testing results of concrete after 28 days of curing are presented in The highest tensile strength was observed, as in the case of the compressive strength, for the concrete CB50. The concrete CC50 and CSS50 demonstrated significantly lower tensile strength than the reference concrete CR0 . The tensile strength of the remaining composites was close to that of the concrete CR0. The good tensile strength is a consequence of the porous structure of the aggregate grains’ surface . The resThe results of testing water penetration depth (WPD) are presented in Using recycled aggregates obtained by crushing or grinding the wastes often worsens the concrete tightness. This worsening is caused by increased porosity of the aggregates after crushing. Such a situation has also been observed for the tested materials. The poorest tightness was observed for the composites containing the aggregate from sewage sludges, CSS25 and CSS50. Due to the WPD above 50 mm, the latter can only be used in the concrete structures exploited in the exposed class X0 or XC1. As described in Limiting the sewage sludge content in the fine aggregate to 25% is appropriate because of the compressive strength, consistency, and tightness. The water penetration depth under pressure exceeded 50 mm for the concrete with 50% of sewage sludge (CSS50). The poor tightness of the concrete containing 50% and more SS has also been confirmed in .λ. The recycled aggregates slightly decreased the thermal conductivity proportionally to their volumetric content. No evident influence of the aggregate type on λ values was found.vc observed .Even more minor was the influence of the recycled aggregate on the specific heat . Only ata, has been registered for the reference concrete CR0, where the average value of a was 1.19 × 10−6 m2/s, which is higher by 10% than in the case of the concrete CSS50, having the lowest a value  compressive strength compared to the reference concrete. The downfall of compressive strength is decreased with time, and after 56 or 90 days of curing, the compressive strength can be close to the reference concrete, as demonstrated in .The tensile splitting strength is the highest for concrete with crushed bricks as a recycled aggregate. Using the other tested aggregates led to a tensile strength similar to the reference concrete.Using the recycled aggregates worsens the tightness of the concrete, which manifests itself by increasing water penetration depth.The thermal properties of concrete are slightly affected by the type and content of the recycled aggregate. The thermal conductivity is slightly decreased, while the specific heat and thermal diffusivity are not significantly influenced.The obtained test results and the analysis of the available literature data show that considering the concrete mix’s and hardened concrete’s performance, the sewage sludge’s content in the fine aggregate should not exceed 25%.The presented research results show the effect of recycled aggregate on the selected properties of cement concrete. The used aggregates were obtained from recycling concrete and ceramic bricks, and from burnt sewage sludges. All these aggregates were introduced into the concrete mix as partial substitutes for the fine aggregate. The following conclusions can be formulated:The material modification of concrete requires, besides the control of the strength, the control of properties affecting the durability, such as tightness, frost resistance, and chemical resistance ,65. The Despite numerous research works, utilizing recycled aggregate for cement concrete production still needs extensive studies. The volume of industrial wastes is growing; therefore, further investigations should mainly concern the limited contents of various types of recycled aggregates and possible ways to increase these contents.


Concrete construction harms our environment, making it urgent to develop new methods for building with less materials. Structurally efficient shapes are, however, often expensive to produce, because they require non-standard formworks, thus, standard structures, which use more material than is often needed, remain cheaper. Digital fabrication has the potential to change this paradigm. One method is Digital Casting Systems (DCS), where the hydration of self-compacting concrete is controlled on the fly during production, shortening the required setting time and reducing hydrostatic pressure on the formwork to a minimum. This enables a productivity increase for standard concrete production. More importantly, though, it enables a rethinking of formworks, as the process requires only cheap thin formworks, thus, unlocking the possibility to produce optimised structural members with less bulk material and lower environmental cost. While DCS has already proven effective in building structural members, this process faces the challenge of moving into industry. This paper covers the next steps in doing so. First, we present the benchmark and expectations set by the industry. Second, we consider how we comply with these requirements and convert our fast-setting self-compacting mortar mix into a coarser one. Third, we present the next generation of our digital processing system, which moves closer to the industrial requirements in terms of size and the control system. Finally, two prototypes demonstrate how DSC: (a) increases standard bulk production by 50% and (b) can be cast into ultra-thin formworks. We discuss the results and the short-term industrial concerns for efficiency and robustness, which must be addressed for such a system to be fully implemented in industry. In the prefab industry, concrete is traditionally shaped by casting it into rigid and heavy-duty standard formworks. After 8 to 24 h, these formworks are removed and prepared for reuse. It is a highly efficient process that, nevertheless, still has margin for improvement. In particular, the setting and hardening rates of self-compacting concrete can be optimized to allow either for a faster demolding or for the use of alternative formworks at low cost that offer a larger range of geometrical freedom. In the first case, having a self-compacting concrete that sets and hardens more rapidly allows the production rate to be increased, which is particularly valuable in accommodating variations in market demand. While various accelerating admixtures can be used for this purpose, they are often associated with a rapid flow loss, which can be problematic in practice. Thus, concrete must be activated just before placing, while also ensuring enough open time for casting and, nevertheless, benefiting from a rapid strength gain shortly thereafter.Such systems may be referred to as setting on demand ,4,5,6,7 Using thin formworks reduces the price of elements of non-standard shapes that can often result from structural optimization, but they also present a solution for using less material in construction. Notably, the construction sector consumes 40–50% of the world’s natural resources and generates 30% of Europe’s waste ,11,12, sFirst, the paper presents a methodology, through which material developments at the laboratory scale may be streamlined to successful scale up from a concrete lab to comply with industrial requirements. Second, we present the next generation of our digital processing system, which both in terms of size and control system targets the processing of a coarser concrete mix, with aggregates up to 8 mm. Third, we demonstrate that our DCS can be used to remove standard formworks 50% faster compared to standard production processes in the concrete industry, and more importantly, that DCS enables the use of millimeter-thin formworks, which unlocks the possibility to produce structurally optimised shapes at low environmental cost. Finally, we discuss these results, as well as the short-term industrial concerns for efficiency and robustness that remain to be addressed for such a system to be fully implemented in industry.An important challenge when upscaling concrete mixes from laboratory to industry is to address the significant differences in not just the mixing energy (when using mixers with different volume capacity and configuration) but also in the mixing time, admixture addition, temperature changes, variation in water content caused by remains of water in the mixing equipment, humidity in the facilities and the storage condition of the material components, which is often outside. All of these factors can significantly impact both the rheological behavior and the hydration kinetics. Such differences in moving to large-scale production can be accentuated in the presence of chemical admixtures, raising issues for setting-on-demand systems. It is, thus, an objective of this paper to lay out a simple procedure, allowing for the efficient upscaling of such concrete formulations to an industrial context.The methodology presented includes various mix design optimization steps using cement pastes and mortars, which raises similar issues of mixing energy that must be accounted for. Setting-on-demand systems combine several admixtures and must achieve a very specific and challenging combination of rheological and hydration behavior. Thus, adequate protocols are required to minimize work at the industrial scale, while maximizing the amount of transferrable information that may be gained in the lab. This requires using adequate sample preparation protocols and accepting some degree of admixture adjustment as the mix is scaled up.The reactor presented in this study is built upon a previous system used for the robotic slip forming of façade mullions installed in the DFAB HOUSE, Empa Dübendorf (Switzerland) ,15 as weConsequently, to move towards becoming competitive with standard manufacturing processes used in the industry, the processing system has to be redesigned while also considering the changes in overall mix design required by the industry see . The majThe prefabrication of concrete elements may or may not involve heat curing. Whichever situation, having a concrete that can harden rapidly is interesting because it allows for a faster turn over, especially where heat curing is applied.The examples given in this paper were developed in collaboration with a Swiss prefabrication company. Specifically, the first case deals with the mass production of high-strength structural columns without heat curing for architecture and infrastructure. Their reference production conditions include steel or wooden formworks. Concrete-mix designs are pre-programmed, and a software controls the weighing and dosage of the different components, before delivering them to a pan mixer. Due to variable seasonal storage conditions, the water content of sand and aggregates may vary from one batch to the other, requiring constant monitoring of the added water to avoid detrimental changes in the final compressive strength.3 is mixed for 8 min, after which the concrete is poured into a hopper and transported to the casting hall. Via crane, the hopper is hovered above the formworks and the material is directly poured into traditional formworks (wood or steel) in one large batch. The casting time per column is 1–2 min, depending on the density of the reinforcement. After 8–10 h, the columns are demolded and vertically lifted by a crane into a non-insulated storage hall and positioned horizontally for further curing until being shipped to the customers. To avoid cracking during this step, a compressive strength of at least 15 MPa is required.Typically, 1 mThe well-established processing logistics and the short casting time make this process efficient. However, for large orders, lead time becomes long as the factory cannot house infinite amounts of formworks. Thus, the use of rapid-setting and hardening concrete presents an appealing solution. The project undertaken here shows that setting on demand would allow rheology to be closely controlled before placing, in order to increase the production rate by 50%, as well as to establish modified processing that would be applicable to the production of concrete elements of non-standard shape by digital casting into weak formworks.Final properties90 MPa after 28 days (strength class C75/90);Fire approved in accordance with standard European building codes;Up to 3 m tall columns with cross sections ranging from 30 × 30 cm to 50 × 50 cm;Good surface quality ;Similar price to the industrial reference.ProcessingCastable in less than 10 min.Material before accelerationGood flowability over a sufficient time .Material after accelerationReach 15 MPa within 4 h;For ordinary formworks: pumpable for at least 10 min after acceleration;For weak formwork: rapid setting of 2 kPa yield stress 1–2 min after acceleration.The prefab columns considered in this paper had to be developed in accordance with the following requirements, which were given by the industrial partner:Increasing the aggregate size from 4 mm to 8 mm (reducing the paste content);Including short polypropylene fibers (to obtain fire resistance);Changing the cement supplier (cement available at the company in question);2 footprint);Increasing cement replacement by limestone .To achieve this, the mix design was revised as follows:Our setting-on-demand system includes a set retarder in the concrete base mix, typically a sucrose solution, as well as superplasticizers. Since both these admixtures impact the setting time, their dosages must be carefully adapted to reach the desired setting times and rheological properties, including initial fluidity and flow retention. This optimization may require many iterations, so the best way to operate is typically to carry out preliminary tests in mortar rather than concrete. The same holds for other aspects of the mix design, such as the partial replacement of cement by limestone and/or other SCMs. The characterization of the used materials is presented in The mix design of the base-retarded concrete is presented in To overcome these limitations, we turned to using suspensions of calcium aluminate cement (CAC) that we had developed for 3D extrusion printing ,18. The The first step in adjusting the base mix design is to regulate the dosage of superplasticizer and retarder by also trying to keep the W/C ratio as low as possible, which improves the mechanical properties of the mix while making it sufficiently flowable. This is best done using a combination of spread flow measurements and calorimetry. The first is done using semi-empirical experiments based on slump flow tests according to standard procedures described in DIN EN 12350-5:2019-09 with an open cone 20 cm high with 20 cm diameter at the bottom and 13 cm at the top. The target slump flow was set between 50 and 60 cm over the duration of the open time to ensure enough flowability in the base mix. The sucrose dosage was adjusted to get the desired retardation of the onset of the acceleration period. Results reported in this paper were obtained on mortars using an isothermal calorimeter I-Cal 8000 HPC  at 23 °C.After the dosage of superplasticizer and retarder has been regulated, compressive strength tests may be performed on the mortars. Provided the same W/C ratio is used as what is planned for the concrete, there should be a good correlation between mortars and concretes. Correlations between heat release and compressive strength as reported by ,20,21 maThe second period refers to the demolding strength and is determined by a standard compressive strength measurement (BS EN 12390-3 with a force-controlled mode set at 0.96 kN/s) of 15 cm cubic samples. For this paper, a demolding strength of 15 MPa was targeted at 4 h.With regard to strength, there are three distinct time frames where target values must be reached, described as periods I, II, and III in The third period concerns the later strength after 7 and 28 days, which is determined with the same compressive strength measurement (BS EN 12390-3) on 15 cm cubic samples. After casting the cubic formworks, the samples are immediately stored at 20 °C/95%RH and demolded just before the test.Change from a discontinuous to a continuous pumping system, which means slow pumping rates can be controlled without starting and stopping the system. This required changing the minimum rotation of the motor of the PFT SWING L Pump from 454 to 46 rotations per minute (RPM).Change the mixing reactor from an open funnel to a closed non-pressurized system (both 1 and 2 were done to prevent backflow).Change the dimensions of the reactor from a single-pin to a double-pin mixing system to increase mixing energy and to have the possibility to mix aggregates up to 8 mm.Reduce the weight of the mixing unit.Increase flexibility in the mixing control (back and forth mixing) and individually control impellers.Record the resistance of the motors in voltage (V), which gives an indication of the viscosity of the mixture and can be used in a feedback loop to regulate the mixing rate on the fly during production. This measurement though reflects a high-shear behavior, while concrete placing is a low-shear process. Therefore, this sensor only offers limited insight into yield stress, which is the rheological property of greatest interest, but can nevertheless be used to monitor the consistency of the mixture.Change control software from an open-loop control system to a closed-loop control system.Change motors from asynchrony motor to server motors in order to:The goal of the new digital processing system was to meet industrial norms and assure that it can process the material robustly. Compared to the previous digital casting systems , the majc, the retarded mix is pumped into a reactor with double-mixing pins, Sm. This operates at a pre-set, but modifiable, rotation rate and includes a measurement of the power V consumed by the motor, as previously mentioned. The accelerating admixture is pumped from the CAC pump, fcac, while a peristaltic pump, fsp, doses a flow enhancer (superplasticizer) to adjust the fluidity on the fly in the reactor. The accelerated concrete exits the mixer at the top and flows into the formwork.In c) with the dosing speeds of the peristaltic pump (fsp) and the accelerating admixtures pump (fcac) to ensure constant dosage of the mix components using a customized software based on [After several developments on a prototypical level, we settled for the final design depicted in based on .The following section describes the results for the material development using different amounts of accelerator dosages, varying from 10 to 15 w%/cement of CAC, with two superplasticizer dosages in the accelerator paste, 6 and 10 w%/cement. Results also compare deviations based on the flow rate and mixing process. “Offline” refers to experiments conducted without the use of the “inline” digital process described in For period II, In the long term, strength increases more or less by an additional 30%, from 88 MPa at 28 days to values slightly above 110 MPa after four months . FurtherA central aspect of setting on demand is for the concrete and the accelerator to be stable over a long period of time before being combined. Isothermal calorimetry is a useful means to optimize the admixture dosages needed for that. As an example, For both accelerated samples, there is a much higher initial heat release that points to a much more active chemical system. Apart from the initial peak that reflects the reaction of the accelerator, the other peak is delayed by higher accelerator content. This corresponds to the hydration of tricalcium silicate and it can be seen to occur much sooner for the accelerator dosage of 10% than for 15%. A priori, this reflects a modification in the aluminate–silicate–sulfate balance, which is also influenced by a slight increase in superplasticizer dosage.The new mix design and processing unit were tested in two large-scale applications. The first was the high-strength reinforced concrete columns to be cast in a standard wooden formwork, as shown in For the standard columns, the advantage of digital casting over adding an accelerator or using a fast cement is that the open time can be better controlled. In particular, if more than one column is to be cast from a single batch, there is virtually no change in concrete workability between the first and last cast column.In our example, the formwork dimensions were 15 × 15 × 300 cm, which is representative of a prefabricated column on the Swiss market . The rebThe element could be demolded after 4 h (having reached 15 MPa).The formwork  Left witThe surface quality is very good  Right, aTwo columns were produced, the first one with 10% CAC with respect to the total cement in the mix and the second one with 15%. These accelerators were continuously added during the digital casting process using the processing and mixing reactor described in The fabrication of both columns was successful. They were cast in 20 min with a filling rate of 3 L/min. The first with 10% CAC was demolded after 4 h when 16 MPa had been reached in a room at 23 °C, while the second one, cast with 15% CAC, was demolded after only 3 h, having reached 18 MPa at approximately the same temperature.For the non-standard shape, polymer extrusion 3D printing was used to produce an ultra-thin formwork . In thisIn the present example, by using the DCS, we can capitalize on the above points and use a formwork with only 1.5 mm wall thickness. The formwork was designed as part of a separate study that had shown that different 3D-printed formworks cross sections have different breakage behavior . Thus, tAs in the previous example, we used the industry mix described in Overall, the digital casting of concrete into this thin formwork was successful and finalized after 66 min, with only a minor formwork breakage in the hexagonal part see . The craThrough this application, we were able to demonstrate that DCS is suitable for precisely adjusting the filling rates on the fly during casting, which, as exemplified in this case, may be particularly necessary to locally control the hydrostatic pressure in relation to varying cross sections throughout the formwork see .It is also important to note that this formwork was shredded and reused without problems for another print. This demonstrates that such formworks can be inscribed in a circular process, a process that has been tested in a separate study, but not yet reported.The examples presented in the previous two sub-sections demonstrate that the same concrete mix design and processing unit may be used for mass-produced standard columns, as well as for bespoke structures. This makes it much easier for industry to expand their offer beyond standard structures, also offering bespoke shapes, for which the formwork may be both easily 3D printed and also recycled.With regard to differences in the processing conditions and outcome, a summary is provided in This paper addressed some of the challenges for transferring advanced concrete manufacturing processes from the lab to industry. Specifically, it targeted delivering both a productivity increase for ordinary production and the facilitation of producing bespoke-shaped structures, both with the same process and material mix design.The mix design presented was developed with components used for daily production in industry. It has the characteristics of a pumpable mix, which can be accelerated using CAC pastes without causing sedimentation or cold joints during the casting process. It reached 15 MPa after 3–5 h, which enabled its demolding from standard formwork after the same period. Experiments not reported in this paper showed that achieving the same strength takes up to 6 h when temperatures are below 5 °C. This longer required time is not unexpected and remains reasonable. It suggests an energy of activations substantially lower than 50 kJ/mol, which has otherwise been reported for cement hydration ,28.We also underline that changes in rheology are an important and delicate issue. In our case, with the same materials both at ETH and at the industrial plant, we nevertheless had differences in initial fluidity and workability retention under the same conditions. This can result from small differences in the cement state  and/or mixing energy. In the case presented, dealing with this required increasing the cement content by 12%, with respect to the daily mix design used by the company. However, this was less cement than in the previous set-on-demand mixes used in digital casting . A key eIndependently of early strength and temperature variations, the selected mix design reaches compressive strengths up to about 90 MPa after 28 days, which complies with the requirements in the industry for high-strength columns. As previously mentioned, a downside of this mix is its higher cement and paste content. In addition to raising the carbon footprint, this also carries an economic cost. Overall, including the CAC cost, this material ends up being approximately 25% more expensive than the typical mix used in the industry.While the extra material cost can be, in part, compensated by the higher productivity, further changes in the mix design need to be considered. Among the various options, lowering the cement and/or paste content should be prioritized, while still considering that the material should remain pumpable when retarded and self-compacting after acceleration. Reducing the paste content will most certainly be facilitated by broadening the aggregate grading. However, this would call for additional iterations in the inline mixing unit . While tFire resistance is another important issue, but it could not be tested within the frame of this project. Such tests are cost intensive and, thus, are only done towards the end of a project. However, they are a requirement for certifying the material, so ultimately, they must be conducted to complete the transfer to industry. Nevertheless, as we included a similar number of polypropylene fibers as the reference certified concrete, our proposed mix should a priori be well-positioned to comply with fire-resistance requirements.In terms of processing, we presented the conversion of our semi-continuous digital processing system in a continuous and adaptive one, allowing us to react on the fly to changes in material properties, as well as to changes in cross-sectional geometry. As discussed in For the case of standard columns, the full-scale demonstrator enables production that is 50% faster than standard production. In practice, this is particularly valuable in periods where high throughput is needed. Companies not using heat curing will enjoy faster demolding, while companies using heat curing may resort to additional non-heated formworks to meet high demands without the high capital investment of additional heated formworks.Beyond this, we showed that it is possible to completely rethink what a formwork can look like. In particular, the option of 3D printing molds on demand in a material that can be easily recycled paves the way for new horizons. Importantly, and in contrast with extrusion printing, this allows steel reinforcement to be easily incorporated, making the process of structural certification much easier.Despite these achievements, the industry partner still did not adapt the system to their premises, as it is still considered too slow despite the fast demolding. One limiting factor is that, to make the best use of the faster-hardening concrete, it is necessary to establish faster and more flexible production methods. A first step in this direction is to extensively redesign the production pipeline from mixing to casting. Placing formworks on moving rails would also increase flexibility and require only one or a few digital casting systems. Indeed, as illustrated in The goal of the work presented here was to move our DCS system from an academic environment into an industrial pre-fab plant and to comply with the requirements set by the industry partner. In doing so, we aimed to demonstrate a potential for increased productivity for industry. Beyond this, we wished to stimulate a rethinking of formwork, showing that structurally efficient shapes can be produced using both less concrete and formwork material in the foreseeable future.In terms of mix design and processing adaptations, we demonstrated that a laboratory-developed process can be upscaled to meet key industry requirements, in terms of strength and productivity. We also presented one of the first processable digital concrete mix designs to include coarse aggregates. Importantly, we also showed that this mix design and processing unit make it possible to produce both standard elements in standard formwork and bespoke structures in ultra-thin formworks.2 footprint. Nevertheless, the method remains promising given the increased productivity and facilitated production of elements with structurally optimized shapes, so our future work lays in the inclusion of larger aggregates and use of low-carbon cement.In terms of cost and a reduction in cement, our mix still includes high amounts of cement and is costly , resultiIn regard to processing, we converted our system from a semi-continuous to a continuous system, which improved robustness and enabled formworks to be removed 50% faster than in normal practice. For ordinary production throughput, for which the current industry installations are optimized, the filling time (20 min) for standard formworks remains a limiting factor for the industry partner. Current research (not reported in this study) focuses on increasing flow rates by scaling up the mixing unit. That said, a change in the infrastructure and logistics within the plant could also be considered, as pointed out in Finally, the question of building structures with fewer materials remains open. Although this study showed that we can rethink formwork and thereby, use less bulk material, the main concern of the industry remains productivity. Still, as designers increasingly request non-standard optimized shapes, the industries that can offer that service will conquer this growing market segment.