Pseudomonas aeruginosa PAO1 bacterial pathogen were exposed to a phage 14/1, gentamycin or a combination of them both in a spatially structured environment. We found that even though antibiotic and phage–antibiotic treatments were equally effective at controlling bacteria in the beginning of the experiment, combination treatment rapidly lost its efficacy in both planktonic and biofilm populations. In a mechanistic manner, this was due to rapid resistance evolution: While both antibiotic and phage selected for increased resistance on their own, phage selection correlated positively with increase in antibiotic resistance, while biofilm growth, which provided generalized resistance mechanism, was favoured most in the combination treatment. Only relatively small cost of resistance and weak evidence for coevolutionary dynamics were observed. Together, these results suggest that spatial heterogeneity can promote rapid evolution of generalized resistance mechanisms without corresponding increase in phage infectivity, which could potentially limit the effectiveness of phage–antibiotic treatments in the evolutionary timescale.Antimicrobial resistance has been estimated to be responsible for over 700,000 deaths per year; therefore, new antimicrobial therapies are urgently needed. One way to increase the efficiency of antibiotics is to use them in combination with bacteria‐specific parasitic viruses, phages, which have been shown to exert additive or synergistic effects in controlling bacteria. However, it is still unclear to what extent these combinatory effects are limited by rapid evolution of resistance, especially when the pathogen grows as biofilm on surfaces typical for many persistent and chronic infections. To study this, we used a microcosm system, where genetically isogenic populations of  One altP. aeruginosa pathogen to both phage and sublethal concentration of gentamycin and monitored ecological and evolutionary dynamics in both homogenous (planktonic) and spatial (biofilm) dimensions of the microcosms. In particular, we explored how phage–antibiotic combination affects the evolution of resistance mechanisms and how this process depends on the spatial structure of the environment typical for clinical infections.Instead of replacing antibiotics, phage therapy could be used to complement antimicrobial therapeutic strategies  of the multidrug efflux systems MexAB and MexXY as a receptor‐binding site . In order to study changes in both planktonic and biofilm populations, all microcosms were seeded with 25 glass beads to allow destructive sampling and isolation of biofilm bacterial populations . In addition to measuring phage and bacterial population density dynamics, we quantified changes in pathogen resistance to antibiotics and phages, associated costs of resistance and phage–bacteria coevolutionary dynamics during the experiment.Here, we used an experimental evolution approach to study the effects of phage–antibiotic combinations on 22.1Pseudomonas aeruginosa PAO1 was used as a bacterial pathogen in this study. Isogenic starting stocks were cultured from a frozen ancestral stock in Lysogeny broth (LB) media and spread on LB agar plates. A single colony was chosen and used as a starting culture in the selection experiment. Phage 14/1  chloroform was added and cultures vortexed and centrifuged at 11,000× g for 5 min. Phage stocks were stored at 4°C.2.2P. aeruginosa evolving alone (control), in the presence of gentamycin antibiotic, in the presence of phage 14/1 or evolving in the presence of both phage and antibiotic (combination treatment) , and the addition of phage 14/1 additively reduced bacterial densities  was replaced with fresh media (LB or LB with 3 μg/ml gentamycin), minimizing disruption to the biofilm. It was experimentally confirmed that 3 μg/ml gentamycin in LB remained effective at reducing bacterial density after 72 hr at 37°C . Planktonic bacterial population densities were measured by diluting and plating subsets of collected aquatic samples at every sampling time point. To measure changes in biofilm bacterial population densities, biofilm colonies were isolated from glass beads from each replicate population using a previously developed method . Biofilm population densities were normalized similarly with each replicate population, and the Figure 2.4g for 5 min. The densities of phages retained in the supernatant were determined as plaque‐forming units (PFU/ml) using serial dilutions of phage suspension and the agar overlay method  chloroform, vortexing and centrifugation at 11,000 × d Adams, . In brie2.5600 nm, Tecan Infinite 200). Antibiotic resistance was determined as the bacterial growth in the presence of 3 μg/ml gentamycin relative to bacterial growth in the absence of gentamycin. Larger difference thus denotes for antibiotic resistance . All evolved colonies were grown for 24 hr at 37°C after all bacterial isolates were replicated into fresh 96‐well plates containing LB with 3 μg/ml gentamycin. Bacterial isolates were further incubated for 24 hr at 37°C after which the growth was determined as optical density  where higher growth of evolved bacteria in the presence of phage relative to density of bacteria grown without phage denotes the evolution of phage resistance  as described above and grown for 24 hr at 37°C. All bacterial isolates were then replicated into new 96‐well plates containing LB. Following incubation at 37°C, growth was determined as optical density at 600 nm at 24 and 48 hr postinoculation  and phage density (PFU/ml) data were log10‐transformed prior to analysis to fulfil ANOVA assumption of homogeneity of variance. When reporting results, the main effects and interactions of all factors are named before F‐statistics and p‐values.Data were analysed using linear mixed models and factorial ANOVA approach where populations were set as subjects, isolation time point as a repeated factor and origin of isolation (planktonic vs. biofilm) nested under replicate populations. Bacterial density or resistance was explained by the presence of phage, presence of antibiotic and colony origin (from biofilm or planktonic phase of microcosms). The bacterial  or phage‐antibiotic  treatments, which were equally effective at reducing bacterial densities . However, the dynamics of density reduction caused by phage‐alone and phage‐antibiotic treatments were very different: Phage‐antibiotic treatment was initially equally effective at reducing bacterial densities as the antibiotic‐alone treatment but lost its efficiency by the third sampling (Day 9) time point, and from then on, had comparable effect on bacterial densities as the phage‐alone treatment .Both phage and antibiotic reduced total bacterial densities alone and in combination . Also, the effect of phage and antibiotic was different for planktonic and biofilm populations . In planktonic populations, phage‐alone had the weakest, and phage–antibiotic intermediate and antibiotic‐alone treatment the strongest negative effect on bacterial populations . Similar to total bacterial density analysis, phage–antibiotic treatment was initially equally effective as the antibiotic‐alone treatment but lost its efficiency by the third sampling (Day 9) time point . In the case of biofilm populations, phage and antibiotic alone were equally effective at reducing bacterial densities on average, while phage–antibiotic combination was the least effective . All treatments became more effective at reducing the densities of biofilm populations towards the end of the experiment . However, this effect was less strong in phage treatments . Together, these results suggest that phage–antibiotic combination performed worse than antibiotic‐only treatment and this difference became more pronounced during the selection experiment.We found that bacterial densities were much higher in planktonic compared to biofilm populations on average , with combined antibiotic and phage treatment consistently having lower phage densities compared to phage‐only treatment. Phage densities increased slightly in the absence and decreased slightly in the presence of antibiotics during the experiment . Prior exposure to phage selection also had a significant positive effect on bacterial growth in the presence of antibiotic  indicative of potential cross‐resistance. A significant interaction was also found between antibiotic and phage treatments on antibiotic resistance evolution : Although both treatments had positive effects on the evolution of antibiotic resistance alone, the effect of phage selection was relatively stronger compared to antibiotic selection, and highest levels of resistance were observed in the phage–antibiotic combination treatment. No significant difference was found between the growth of biofilm and planktonic bacterial populations in the presence of antibiotics . Antibiotic selection alone did not affect the evolution of phage resistance . However, we found a significant interaction between spatial origin and phage treatment , with phage selection having a larger effect on the resistance evolution of biofilm compared to planktonic populations. Together, these results suggest that phage resistance evolved to higher levels in bacterial biofilms during the selection experiment even in the absence of phage selection.Previous exposure to a phage during the selection experiment increased bacterial growth in the presence of ancestral phage relative to the control treatment . However, distinct changes in mean phage resistance between different treatments and spatial origin were found. In contrast to endpoint measurement, phage resistance evolved to higher levels in planktonic compared to biofilm populations on average . Furthermore, antibiotic selection promoted the evolution of phage resistance more clearly in the planktonic bacterial populations . The rate and dynamics of phage resistance evolution also fluctuated in time depending on the treatment : While phage resistance increased over time in combination treatment, more fluctuations were observed in the phage‐only treatment , which suggest that reduction in maximum density was observed in the antibiotic‐only and phage‐only but not in combination treatment. While the spatial density had nonsignificant main effect, we found a significant interaction between the antibiotic treatment and the spatial dimension . This suggests that bacteria treated with antibiotics suffered a larger reduction in maximum density when they were isolated from the planktonic phase of the microcosms. Cost of resistance was also measured in terms of reduction in maximum growth rate relative to ancestral bacterium , while spatial dimension had no significant effect on maximum growth rates. Together, these results suggest that evolution of resistance incurred relatively small cost, which was mainly observed with planktonic bacteria that had evolved in the presence of either antibiotic or the phage (maximum density), and in general, the cost of adaptation was the lowest for bacteria that were exposed to combination treatment (both maximum density and growth rate).We found that prior evolutionary history with phage or antibiotic did not affect bacterial maximum density relative to control treatment (phage: 4Here, we conducted a long‐term evolutionary study to look phage–antibiotic synergy in spatially structured environment. Even though gentamycin and the phage exerted additive effects in the short‐term preliminary experiment, we did not observe evidence of combined effects in our long‐term selection experiment. In contrast, while phage–antibiotic combination was equally good at controlling bacterial growth as antibiotic‐alone treatment in the beginning, it became less effective from Day 9 due to rapid resistance evolution. Crucially, the level of resistance evolved the highest against both antibiotics and phages in the combination treatment indicative of generalized resistance evolution. This could be explained by biofilm‐mediated resistance against both phage and antibiotic, phage‐mediated correlated selection for increased antibiotic resistance, lack of cost of adaptation and weak phage infectivity evolution. Together, these results suggest that generalized resistance mechanisms could considerably limit the efficacy of phage–antibiotic treatments in the long term.Escherichia coli biofilms (Ryan, Alkawareek, Donnelly, & Gilmore, P. aeruginosa cultures (Torres‐Barceló et al., E. coli isolates (Allen, Pfrunder‐Cardozo, Meinel, Egli, & Hall, Even though we found evidence for additive phage–antibiotic effects in our preliminary experiment, these were quickly lost during the selection experiment. Instead, phage–antibiotic combination performed worse compared to antibiotic‐alone treatment on average and was as equally effective as the antibiotic‐alone treatment for just the first 6 days. This is in contrast with previous studies reporting beneficial effects of phage–antibiotic combinations on We found that bacteria evolved more resistant to both phage and antibiotic when exposed to selection by these agents individually, and in both cases, the resistance evolved higher within biofilm compared to planktonic populations. Crucially, resistance to both selective agents evolved the highest in the phage–antibiotic combination treatment where both planktonic and biofilm populations showed equally high levels of resistance. Relatively stronger resistance evolution in combination treatment could also explain why phage population densities were consistently lower in the presence of antibiotics even though the number of host bacteria remained approximately the same between phage and phage–antibiotic treatments. The evolution of generalized resistance mechanism could be explained in various ways.First, it is possible that selection by one selective agent led to cross‐resistance against the other agent. For example, selection by one phage or antibiotic has been shown to lead to resistance to other phages and antibiotics, respectively (Davies & Davies, E. coli against phage attack via two separate mechanisms: by inhibiting phage transport into the biofilm and by coating bacterial surface and binding phage particles, thereby preventing their attachment to the cell exterior (Vidakovic et al., P. aeruginosa.Second, biofilms could have provided effective resistance mechanism against both phage and antibiotics Abedon, , and henThird, while evolution of resistance is often expected to lead to fitness costs (Andersson & Hughes, One neglected aspect of phage–antibiotic combinations is the phage's ability to coevolve more infective during the treatments. In contrast to previous, relatively short‐term work with this system (Friman et al., In conclusion, here we show that evolution of generalized resistance mechanism can constrain the long‐term efficiency of phage–antibiotic combinations in a spatially structured environment. However, it should be noted that we used only a single phage and antibiotic combination in our experiment and it is thus somewhat unclear how far these results could be generalized. Unravelling the genetic basis of generalized resistance mechanism will be one of the next targets of research. While phage–antibiotic synergies could be driven by specific species–antibiotic combinations, it is also possible that mutation basis of adaptation plays important role. For example, in case of collateral sensitivity and cross‐resistance, it has been shown that the mutational basis of resistance to one antibiotic plays key role whether the subsequent antibiotic will have negative or neutral effect for the bacterial growth and that considerable variation can exists even between different replicate lines derived from the same selective environment (Barbosa et al., https://doi.org/10.5061/dryad.9f77pr0Data for this study are available at Dryad digital repository:  Click here for additional data file.


This paper presents a continuation of the evaluation of utilizing Nano Carbon Tubes (CNTs) in reinforced concrete (CNT-CRETE). The compressive, tensile and bond strengths of the samples with and without CNTs were investigated. Scanning Electron Microscope (SME) was utilized to study the microstructure of the prepared samples. In addition, the corrosion resistance of CNT-CRETE, was measured and compared to traditional concrete. Four mixes were prepared, with 0.01%, 0.02%, and 0.03%, CNTs by weight of cement, along with a control mix without CNTs. The results of the experimental work showed that adding CNTs led to an increase in the compressive, tensile and bond strengths of specimens compared to those of the control specimen. SEM examination for control and CNTs specimens showed that CNTs specimen were well structured compared with the control specimen and this affirms that CNTs act as bridges across micro cracks, which explains the improvement in mechanical properties. The diameter of steel bars played a significant role in failure mechanism for pull-out testing and corrosion resistance. In general, adding CNTs to the concrete mix increased the rate of corrosion for steel bars within the low risk limits. Benefits from using CNTs were limited to moderate. Mineral/chemical admixtures or fibers provide better improvements in the mechanical properties of concrete without the problems associated with dispersing CNTs and the health hazard of handling a Nano material. Several models were developed for bond strength, taking into consideration the effect of the bar diameter, embedded bar length in concrete, concrete strength, cover thickness and crack spacing are cited in literature8. ACI 408 committee9 explained that the bond between steel and concrete occurs by chemical adhesion between both materials. Frictional forces result from the interface between concrete & steel, and mechanical bearing of the steel ribs against the concrete surface are responsible of carrying the tensile forces in concrete section. The development length of steel bars, required to ensure adequate transfer of stresses, can be calculated using equations specified in various Building Codes11. Corrosion of reinforcement, as a result of exposure of reinforced concrete to aggressive environments, has a negative effect on the bond strength of steel reinforcement in structural concrete. With further exposure, the increased volume of corrosion products may cause the concrete cover to crack, and will definitely result in bond strength degradation14.Bond strength of steel reinforcement in concrete structures is one of the important factors in structural design15. The bond behaviour of steel reinforcement in reinforced concrete elements, including the ultimate bond strength, the load-slip relationships and the effect of different corrosion rates were studied in literature18. The bond behaviour between normal aggregate concrete or recycled aggregate concrete and corroded steel bars was also investigated19. The partly corroded steel bars exhibit different bond behaviour from that of the whole-surface corroded bars20. The effect of corrosion on stiffness and maximum strength of bond was expressed as a function of corrosion percentage21. A frictional model was developed and a damage model for bond strength assuming a scalar damage parameter to account for the bond strength loss in the full development of slip22 while a simple probabilistic bond strength model considering corrosion using a multivariable regression analysis and based on a comprehensive database was proposed23. An empirical model were developed for the ultimate bond strength by evaluating bond strength in different concrete mixes, different covers and different corrosion levels24.The bond strength loss for corroded reinforcing bar is much more critical than the cross-sectional area loss25. CNTs shall interact most intimately with CSH due to their Nano-scale characteristics. The interface or CNTs with C-S-H is achieved by the large number of atoms present at the nanotube surface. CNTs have high flexibility, and its microstructure shape as tubes is capable of bending in circles and forming bridges crossing the micro and Nano-cracks developed in the cement composites, which in turn, increases the strength of the cement composites26. CNTs can be widely distributed in the concrete paste, and as a Nano material, its interaction with the paste is more intense than that of the larger fibers. CNTs possess a high modulus of elasticity, tensile strength, and yield strain. Such properties make CNTs an attractive reinforcing material for concrete. The unique features offered by CNTs provide an excellent opportunity to develop CNTs-reinforced structural concrete27. Addition of CNTs to the concrete mix and repair mixtures was studied in literature29. CNTs improve cement composites in tension due to its ability to prevent the initiation of micro cracks in the cement matrix30.Concrete as a cementitious composite material has a complex network of binding particles known as calcium silicate hydrate (C-S-H). Using Carbon Nanotubes (CNTs) improves the mechanical properties of concrete, as reported in the literature29. Earlier published work was devoted to the effect of adding Nano silica materials in reinforced concrete mixes on different mechanical properties including bond and corrosion resistance31. In a recent publication32 the effect of CNTs on the bond strength of concrete was also studied. However, their mixes had compressive strengths in excess of 46.2 MPa, classifying these as high strength concrete. In addition, they added high percentages of CNTs, namely 0.05 and 0.1%. They only used one bar diameter (12 mm), to prepare the samples. In the current study, the maximum strength of the mixes was 35 MPa in the range of typical normal strength concrete and the content of CNTs was limited to a maximum of 0.03% by weight of cement. Therefore, the current investigation will add to the body of knowledge concerning the effect of a moderate percentage of CNTs on the bond strength of normal strength concrete, utilizing two steel bar diameters (12 and 16 mm) for the bond strength test. The authors did not cite any other publications presenting results of experimental work assessing the bond strength of CNT-CRETE.Members of this research group have been investigating the capabilities of CNT-CRETE33. On the other hand, Konsta-Gdoutos et al.34 reported that the inclusion of 0.1 wt% CNTs decrease the corrosion rate and significantly increased the resistance to corrosion by delaying the onset of the corrosion reaction. All previous studies on the effect of CNTs on corrosion rates were conducted on mortar samples. The authors cited no corrosion studies conducted on CNT-CRETE. In addition, the current study compared the benefits from using CNTs in concrete with those attained by using chemical/mineral admixtures or fibers to provide practical guidance to civil engineers.The results in the literature on the effect CNTs on the corrosion rate of steel embedded in mortar samples are contradictory. The addition of CNT caused higher corrosion intensities for cement mortars with up to 0.5% CNT subject to simulated sea water and accelerated carbonation3 was used as coarse aggregate; its sieve analysis is given in Fig. 35.Ordinary Portland Cement (OPC) according to ASTM C150 standard was used in all mixes of this study. Cement physical and chemical properties are shown in Table 3 for 2 minutes, then the mixture of the tap water, CNTs was added to the concrete mixer and mixing was continued for another two minutes.The representative transmission electron microscope micrograph and XRD of the CNTs are shown in Figs 36 and tensile strength of concrete after 28 days according to ASTM C 49637, respectively. Three specimens for control concrete mix and three specimens for each concrete mix contained CNTs at percentages of 0.01%, 0.02%, and 0.03% by weight of cement. The well-mixed concrete mixture, for each batch was poured into moulds to form three cubes of size 150 mm × 150 mm × 150 mm for the compressive strength testing and three cylinders 150Twelve concrete cubes of dimensions 150 mm × 150 mm × 150 mm and twelve cylinders 150Half of the length of the embedded part of the bar will be de-bonded using polyvinyl chloride tubing to ensure the bond slip failure dominates over other types of failure. The cube specimens are de-moulded after 24 hours and cured at room temperature in normal pure water until the day of testing.A 1000 KN universal testing machine was used for the compressive, tensile and bond strength testing of the RC and CNT-CRETE specimens. Scanning electron microscopy (SEM) made in a JEOL JSM6300 from Oxford Instruments. The secondary electron images were obtained in samples coated with Au and using a voltage of 20 kV.The specimens were immersed in a 3% NaCl solution by weight of water. The impressed current direction is to be adjusted such that the reinforcing steel served as the anode while a stainless steel rod positioned in the tank was acting as a cathode. In order to establish different levels of reinforcement corrosion, accelerated corrosion time was set over 1, 7 and 15 days. A schematic representation of the electrochemical test setup is shown in Fig. Figure 34 when up to 0.06% CNTs were included in concrete mixes. Hawreen and Bogas32, argued that the increase in compressive strength of CNT-CRETE compared to normal concretedue to the CNTs filler, nucleation and bridging effects. They also found that the increase ranges between 3 and 21% for their mixes containing different types of CNTs at 0.05 and 0.1% by weight of cement. They considered this to be a modest improvement that can be achieved by other means. Moreover, they attributed this modest improvement to the CNTs’ inability to significantly enhance the quality of the aggregate–paste bond. In another publication, when 0.13% CNTs were used in mortar, the improvement in compressive strength was 37% and 20% after 1 day and 28 days of curing, respectively38. So it appears that CNTs is more beneficial at the early ages when the mortar is weak.The compressive strength results at 7 and 28 days and the splitting tensile strength are reported in Table 39 found that adding 15% silica fume to a 38.3 MPa mix, lead to 23.5% increase in compressive strength. In another research40, it was found that 8% silica fume increased the compressive strength by 18% when added to a 44 MPa control concrete. Just adding 0.5% superplasticiser41, whilst keeping all other proportions constant increased the compressive strength of a 34 MPa mix by 40%.For comparison, Amudhavalli and MathewAdding of CNTs into concrete mix led to an increase in the splitting tensile strength of studied specimens compared to that of the control specimen. The tensile strength in the specimens including CNTs were higher than that of the control mix by a range from 10 and 20%, at CNTs dosages of 0.01% to 0.03%, respectively.42 found that the splitting tensile strength was increased by 3.1 to 37.5% when up 0.06% of CNTs were added to the samples. Their samples contained both long and short CNTs. They reported that SME images show breakage of CNTs along the fractured surfaces of the samples. No CNTs were pulled from the cement paste during the fracture. Hunashyal et al.43 found that the mortar samples with 0.5% CNTs exhibited an increase of 19% in direct tensile strength compared to plain mortar samples. They reported less strain at failure of CNTs samples. For comparison, the inclusion of 1% steel fibers (having a tensile strength of 2000 MPa) into a w/c = 0.55 concrete mix, lead to an increase in the splitting tensile strength by 36.3%44. The increase was higher for mixes with lower w/c, and the samples exhibited higher ductility due to fiber inclusion. In another investigation45, it was observed that addition of 0.5%, 50 mm copper coated crimped round steel fiber having aspect ratio 53.85 to concrete having a compressive strength of 25 MPa resulted in an increase in the splitting tensile strength by 61.1%. Shah et al.41 found that adding 1.5% superplasticiser to a 34 MPa mix, whilst keeping all other ingredients the same, increased its splitting tensile strength by more than 120%.Qissab and Abbas46. However, there is evidence47 from extensive SEM observations that despite the best efforts to maintain the dispersion of nano filaments within the aqueous solution, combining a well-dispersed aqueous mixture with cement and water resulted in a relatively poor dispersion within the hydrated cement paste.Figure 11. The increase of corrosion rate by increasing CNTs percent agrees with the findings of Konsta-Gdoutos et al.34 who reported that adding 0.5 wt% CNTs mortar mix exhibited higher corrosion rate values than those for 0.1 wt% CNTs samples. However, Konsta-Gdoutos et al.34 could not explain their results because when they added Nano filler material (CNFs) to the 0.1 wt% CNTs, the onset of the corrosion was delayed. In the authors view, the delay in the onset of corrosion is due to a marginal decrease in permeability leading to reduction in the ingress of aggressive agents. However, the increased tendency to corrosion needs further investigation.The test results in this study show that corrosion rates are affected by diameters of steel bars. For example, steel reinforcement of 16 mm diameter had a lower corrosion resistance than that of steel reinforcement of diameter 12 mm. It was observed that addition of CNTs increased the rate of corrosion for steel reinforcement especially for 16 mm diameter steel reinforcement as shown in Fig. 48 that adding multi walled CNTs, like those used in the current investigation, resulted in a significant reduction in the resistivity of cement paste. The reduction in resistivity was more for pastes with higher percentages of CNTs. This reduction in resistivity means an increase in electric current conductivity leading to higher rates of corrosion. In another publication49, it was observed that: “the addition of CNT to the cement matrix could imply the development of higher levels of corrosion in aggressive conditions, such as carbonation and contamination by chloride ions”. This was explained from a chemical perspective by stating that: “The union of two different conductive materials with different nobility implies that the less noble tends to develop higher corrosion rates than the same element without such electrical contact. On the other hand, the material with higher nobility develops lower corrosion rates. The former argument is consistent because the electrons of the less noble material (steel) will cause cathodic protection on the other one (CNT). For this reason, a higher content of CNTs implies higher levels of the Icorr values.” In simple words increased corrosion rates of steel in cement composites with CNTs, since steel is less noble than CNTs. This explains the results of the current and previous investigations on the corrosion in the presence of CNTs.Since, corrosion is an electrochemical process; the effect of CNTs on corrosion can be explained from both an electrical and chemical perspectives. It was found50 that the bond pull out force was 17 and 20 KN for 16 mm and 20 mm bars, respectively. However, the embedment length for those samples was only 65 mm, whereas in the current study it was 150 mm, which explains the higher forces reported in the current study. Bompa, and Elghazouli51 measured pullout forces of 118 and 189 KN for steel bars of 16 and 20 mm diameter. However, their samples had a compressive strength of 78.6 MPa, which explains why they recorded higher pullout forces.Pull-out or splitting is the common failure mode of steel-concrete bond test. The diameter of steel bars plays a significant role in failure mechanism. Failure of steel bars of 12 mm diameter was by pull out mode whereby the bar pulled out slowly as the load dropped steadily as shown in Fig. Figure 10. Figure It can be seen that all curves yielded initially a stiff response that varied up to around 13% of the maximum force and beyond which the bars free end started to slip noticeably. This was explained earlier in the literature that, in this region, the bond resistance consists mainly of chemical adhesion and friction at the interface between the bar and concrete matrix50, 0.5% hooked end steel fibers increased the pull out force in a 44 MPa mix by 17 and 42% for 16 and 20 mm bars. Other reports52, found that with higher hooked end fiber contents, up to 3% by volume, in a w/c = 0.32 concrete (designed to have a compressive strength of 40 MPa), the pull out force of a 16 mm bar was increased by 48%.In another investigationIt was found that adding CNTs to concrete led to some increase in both of the compressive and tensile strength of specimens compared to that of the regular concrete control specimens. Adding 0.01% to 0.03% CNTs led to an increase ranges approximately from 7 to 20% for compressive strength, and from 10 to 20%, for tensile strength.Results obtained from scanning electron microscopy analysis for control and CNTs specimens show that CNTs specimen is well structured and dispersed compared with the control specimen, and this affirms that CNTs act as bridges across micro cracks, which is helpful to improve the bond ability.It was found that the ultimate force and corresponding slip were directly dependent on the diameter of steel bar. The total force of pull out forces for steel bars of diameters 12 mm and 16 mm were 56 KN and 67 KN, respectively. This confirmed the dominating role of the lateral surface area of the steel bars in failure mechanism for pull-out testing.CNTs concrete has a better bond strength performance compared to a corresponding concrete specimen. In addition, adding CNTs improves the bond strength of steel reinforcement bars of 12 mm and 16 mm diameters by 36% and 21%, respectively.Steel reinforcement of 16 mm diameter has a lower corrosion resistance than that of steel reinforcement of diameter 12 mm. The maximum rate of corrosion in the assessed samples did not exceed 1.85% and this value is still considered as low risk according to ACI 318. In general, the CNT-CRETE has higher corrosion tendency compared to plain concrete. The increase of corrosion rate in the presence of CNTs was explained from both the electrical and chemical perspectives.The benefits of CNTs inclusion on compressive, tensile and bond strengths can be achieved, or in many cases exceeded, by adding chemical admixtures, such as superplasticisers, cement replacement materials, such as silica fume, or steel fibers.Given the effort needed for dispersion prior to mixing, the results of the current and previous investigations on the topic, and the health hazards associated with handling nano materials; and the increased tendency to corrosion of CNT-CRETE; the authors are not in favor of using CNTs in concrete to induce mechanical properties improvements.The experimental work carried out in this investigation assessed the effect of adding CNTs on the corrosion rates and pullout forces in concrete cubes. The main conclusions can be drawn as follows:


A total of seven NSCFRC mixtures incorporating 0%–0.35% of carbon fiber by volume of concrete and 0%–2% Nano SiO2 by weight of the binder were studied. The key mechanical properties such as compressive strength, tensile strength, and flexural strength of NSCFRC with 0.25% carbon fiber and 1% NS were 6.8%, 20.3%, and 11.7% higher than PC , respectively. Scanning Electron Microscopy (SEM) shows that Nano SiO2 reduced the internal porosity and increased the compactness of the concrete matrix. Furthermore, the experimental result demonstrates that NSCFRC can improve the mechanical properties of concrete after high-temperature and equations were obtained to describe the evolution of residual properties at elevated temperatures. Results suggested that the effect of carbon fibers on the residual properties of concrete after high-temperature is less than steel fiber and polypropylene fiber. It was also indicated that adding appropriate Nano SiO2 to concrete is an effective means to improve the residual performance after high-temperature.This study presents the key mechanical and residual properties after high-temperature of different Nano SiO Concrete is one of the most widely used building materials, but its tensile strength is relatively low compared to compressive strength. This defect limits the application of concrete in many ways. As a common reinforcement method, fiber reinforcement is currently being considered more widely ,2,3,4. CSimilar to other flexible fibers, carbon fiber generally has no significant influence on the improvements of compressive strength of concrete and may decrease the compressive strength when the high-volume fractions. Previous studies have shown that the addition of 0.25–1 vol.% carbon fiber reduces the compressive strength of concrete by 15%–50% ,14. Furt2 (NS) was added to CFRC to prepare NSCFRC. It is well documented that the mechanical and physical properties of concrete can be greatly improved by combining the pozzolanic nano materials such as SiO2 with concrete [2, the formation of C-S-H gel nanocrystals can be accelerated and most of the pores of traditional cement-based concrete can be filled [2 particles can significantly improve the compressive, flexural, tensile strength and toughness of concreteIn addition, NS can improve the mechanical properties of concrete after high-temperature. Yan et al. [2 can evidently improve normal-temperature and high-temperature compressive strengths of the concrete.In order to improve the mechanical properties of CFRC after high-temperature, Nano SiOconcrete ,16. Due e filled . Formerle filled  shown thn et al.  research2 on the mechanical properties at room temperature and high-temperature. This study presents the experimental results obtained on seven NSCFRC mixtures subjected to high temperatures. The main objective of this study was to investigate the effect of carbon fibers and Nano SiOThe study is realized in two stages. Firstly, this paper supplies results on the mechanical properties of seven mixtures at room temperature. The effect of carbon fiber content and NS content on the mechanical properties and microstructure of concrete was revealed. Secondly, seven mixtures heated to 375 °C, 575 °C, 775 °C, and natural cooling. The evolution of residual mechanical properties with temperature were investigated in order to explain the influence of carbon fiber and NS on the mechanical properties of concrete after high temperature.Normal Portland Cement , Crushed Limestone Coarse Aggregate (CA), Natural Sand Fine Aggregate (FA), Nano Silica (NS) , Water Reducer (WR) , Carbon Fibers (CFs) , Normal Tap Water (W), Dispersant (D)  and Defoaming Agent (DA)  were used in this study.2. The diameter and length of the selected carbon fibers were 7 μm and 7 mm, respectively. Hence, the aspect ratio was 1000. The selected Nano SiO2 is Evonik A380 with specific surface area and particle size of 380 m2/g and 7 nm, respectively. The carbon fiber surface is hydrophobic and difficult to disperse uniformly in the cement paste, which directly affects the mechanical and other properties of the CFRC. Hydroxyethyl Cellulose  was used as a dispersant in this study. Defoamer  was used to reduce pores generated during concrete mixing.2 by weight of the binder (C + NS) were studied. Nano SiO2 also improved the dispersion of carbon fibers in NSCFRC.2 were pre-dispersed. The carbon fiber was placed in a beaker and a certain amount of water was added. The SiO2 and the dispersing agent being added, ultrasonically shaken and stirred for 10 min and the defoamer being added in this process reduces the bubbles generated during the mixing process. All dry components (cement and aggregate) were first pre-mixed together in the mixing machine  for 2 min. The dispersed carbon fiber and Nano SiO2, the remaining water and WR were added into the forced mixer, mixed the concrete mixture was mixed further for 2 min. Finally, concrete was poured into the molds and kept for 24 h, when they were demolded and conserved in curing room (25 °C and Relative Humidity > 95%) until 28 days of age.The carbon fiber and Nano SiOFor each different concrete, 36 specimens were prepared and tested: 24 cubic specimens (150 mm × 150 mm × 150 mm) for the compression test and splitting tension test, and 12 prismatic specimens (100 mm × 100 mm × 400 mm) were made for the flexure test . The purpose of this work was to evaluate the mechanical properties of NSCFRC after high temperatures. Therefore, the following tests were carried out under 4 different conditions . For each different concrete 27 specimens included 18 cubic specimens and nine prismatic specimens were heated by box type resistance furnace  at 375, 575, and 775 °C. Once reached the target temperature, it was maintained for 120 min to obtain a homogeneous thermal state within the specimens. After that, turn off the oven and store the sample inside for natural cooling All samples were tested after cooling up to ambient temperature.The compressive strength, splitting tensile strength, and flexure strength of NSCFRC and control mixtures were determined according to GB/T50081-2016.The microstructure of the fractured concrete surface was observed by Scanning Electron Microscopy  t-test with 95% confidence, the population mean is not significantly. The experimental results were statistically acceptable. 2) provided the highest compressive strength of 47.1 MPa, 6.8% higher than PC (0% carbon fiber and 0% Nano SiO2). However, the compressive strength of other specimens generally declined. C3 (0.35% carbon fiber and 1% Nano SiO2) provided the lowest compressive strength of 39.9 MPa. Comparing with PC  and S0 , the compressive strength of carbon fibers decreased by 15.5%. Carbon fibers had a negative effect on the compressive strength of concrete. The compressive strength of NSCFRC was between 81.0 and 106.5% of PC in the presence of carbon fibers. A previous study reported that carbon fibers significantly decreased the compressive strength of CFRC, because it increased internal porosity and reduced the compactness of the concrete matrix [The compressive strength results and associated standard deviation (SD) for different NSCFRC as show in e matrix . Other se matrix .2) had the highest compressive strength, which proved that Nano SiO2 can effectively improve the compressive strength of concrete. Furthermore, the compressive strength of the specimens increased with the increase in Nano SiO2 content. The compressive strength for S0 (0.15% carbon fiber and 0% Nano SiO2) was 37.3 MPa and for S2 (0.15% carbon fiber and 1% Nano SiO2) was 41.2 MPa. The compressive strength increased by 10% for 1% Nano SiO2, since it reduced the internal porosity and increased the compactness of the concrete matrix. The study by Wang et al. [2 significantly increased the compressive strength of Lightweight aggregate concrete (LWAC). Cheng et al. [2 greatly improved the strength, and abrasion resistance of concrete. Zhang et al. [2 is limited to a certain range, the incorporation of concrete. Therefore, in this experiment, the optimal content of concrete to compressive strength was kept at 1 wt.% NS and 0.25 vol.% CFs.Appropriate amount of carbon fiber and NS can improve the compressive strength of concrete. C2  had the tensile strength of 3.32 MPa, the splitting tensile strength increased by 20% for 0.25% carbon fiber, 1% Nano SiO2. 2 on the splitting tensile strength of concrete. The tensile strength of NSCFRC increased first. Many articles reported a similar trend in the tensile strength results of CFRC, a higher load is obtained because the increase in carbon fiber content reduces the crack growth [The splitting tensile strength results and associated standard deviation (SD) for different NSCFRC as show in k growth ,20. When2 content increased. S0  had a tensile strength of 2.98 MPa while S3  had a tensile strength of 3.24 MPa, the splitting tensile strength increased by 8.7% for 2% Nano SiO2. The tensile strength of NSCFRC decreased first and then increased as the Nano SiO2 is less than 3%, the formation of C-S-H gel can be accelerated due to the increase of crystalline Ca (OH)2 content in the initial stage of hydration reaction, thereby improving the tensile strength of concrete specimens. When the SiO2 content is more than 3%, the reduction of the tensile strength is caused by the decrease of the crystalline Ca(OH)2 content required for the formation of the C-S-H gel. Other researchers have reported similar content, but the optimal dosage of NS was different [Nazari et al.  reportedifferent ,27. NS cThe flexural strength results and associated standard deviation (SD) for different NSCFRC as show in 2 content increased. The flexural strength ranged from 3.57 to 3.85 MPa. S2 (0.15% CFs and 1% NS) had the highest flexural strength of 3.85 MPa while S3 (0.15% CFs and 2% NS) had the lowest flexural strength of 3.57 MPa. The flexural strength of S2 was 6.4% higher than that of S0 . The results indicated that Nano SiO2 were able to improve the flexural strength of concrete due to the Pozzolanic and filling effects. A previous study reported that the increased SiO2 nanoparticles content (more than 4 wt. %) causes reduced flexural strength due to unsuitable dispersion of nanoparticles in the concrete matrix [e matrix . This wa2, which provided the highest mechanical properties due to compact matrix and evenly distributed carbon fiber. 2 = 0.98175:T; cf,25 is the compressive strength of concrete before exposure to high temperature; T is the elevated temperature experienced by the concrete (unit: °C).The experimental results were worked to design equations by the means of regression analyses. Equation (1) corresponds to NSCFRC with 0.25% CFS and 1% NS, with a coefficient of determination RIn 2 = 0.9999T; ctf,25 is the tensile strength of concrete before exposure to high temperature; T is the elevated temperature experienced by the concrete (unit: °C).The experimental results were worked to design equations by the means of regression analyses. Equation (2) corresponds to NSCFRC with 0.25% CFs and 1% NS, with a coefficient of determination R2) provided the highest flexural strength of 1.86 MPa, and was 40% higher than that of PC. The normalized residual flexural strength of C3 was 6% higher than that of PC. After 775 °C, C3  had the highest flexural strength while PC had the lowest flexural strength. The normalized residual flexural strength of C3 was 10% higher than PC.From the previous section, it can be seen that the NSCFRC has better residual mechanical properties compared to the concrete without fiber and polypropylene fiber reinforced concrete but lower than steel fiber reinforced concrete. Polypropylene fiber had a low melting point and melted at high temperatures, forming a large number of small pores, which caused the internal pore structure of the concrete to change. The pore connectivity was strengthened, providing a channel for the decomposition and evaporation of moisture inside the concrete, which greatly reduced the internal pressure; thereby, Polypropylene fiber reinforced concrete has much better resistance to thermal spalling compared to the concrete without fiber. Steel fiber had high thermal conductivity and can effectively transfer high temperature to the interior of concrete, reducing the temperature gradient of concrete . Carbon NS can effectively improve the mechanical properties of concrete after high-temperature. Comparing S0 to S3, after 775 °C, the normalized residual compressive strength, residual tensile strength, and residual flexural strength increased by 7.5, 3.5, and 6.3%, respectively. NS can react with C-H in cement to form C-S-H, and effectively fill concrete pores and increase the matrix compactness. It effectively alleviates the deterioration of concrete at high temperatures .NSCFRC provided relatively good mechanical properties than PC. The compressive strength, tensile strength and flexural strength of NSCFRC with 0.25% carbon fiber and 1% NS were 6.8%, 20.3% and 11.7% higher than that of PC, respectively.The existence of NS can comprehensively improve the mechanical properties of concrete. The compressive strength, tensile strength and flexural strength of S2 (0.15% CFs and 1% NS) were 10%, 6.7%, and 6.4% higher than that of S0 (0.15% CFs without NS), respectively, since it reduces the internal porosity and increases the compactness of the concrete matrix. NS can react with C-H in cement to form C-S-H, which reduces the internal porosity and increases the matrix compactness.Similar to previous reports, carbon fiber substantially decreased the compressive strength of CFRC because it increased the internal porosity and reduced the compactness of the concrete matrix. However, it can increase the tensile strength and flexural strength of concrete because it reduces the crack growth across the crack surface, thus resulting in failure at a higher load.NSCFRC can improve the mechanical properties of concrete after high-temperature. The effect of high temperature on the residual mechanical properties of NSCFRC was less than in PC, but greater than that of steel fiber-reinforced concrete. After 775 °C, the normalized residual compressive strength, residual tensile strength, and residual flexural strength of NSCFRC with 0.25% carbon fiber and 1% NS were 5.2%, 10.9%, and 8.9% higher than those of PC, respectively.NS can effectively improve the mechanical properties of concrete after high-temperature. NS can reduce the internal porosity and increase the matrix compactness after high-temperature. The synergistic effect of NS and carbon fiber was the main factor for the improvement of mechanical properties of NSCFRC after high-temperature.This study reports the experimental study on the mechanical properties of NSCFRC after room-temperature and high-temperature. Based on the test results and associated discussion, the following conclusions were drawn:


Over-prescription of antibiotics is prevalent all over the world, contributing to the development of antibiotic resistance. The importance of understanding how physicians prescribe antibiotics is increasingly highlighted for the purpose of promoting good practice. This study aimed to identify factors that shape the antibiotic prescribing behaviors of physicians in primary care based on the theory of planned behavior (TPB).Data were collected from 503 prescribers within 65 primary care facilities in Hubei, tapping into four behavioral aspects leading to antibiotic prescribing based on TPB, namely, attitudes (the degree to which a prescriber is in favor of the use of antibiotics), subjective norms , perceived control of behaviors  and intentions (the degree to which a prescriber is willing to prescribe antibiotics). A total of 440,268 prescriptions were audited to assess physician antibiotic prescribing practices. The four behavioral constructs were further linked with physician’s actual use of antibiotics using structural equation modelling (SEM) based on TPB.p < 0.001) and a lower intention to reduce antibiotic use . Those who perceived lower social pressure  and higher control over prescribing behaviors  reported a higher intention to reduce the use of antibiotics. However, such intention did not translate into prescribing practice (p > 0.05), although stronger perceived behavioral control was directly linked with lower antibiotic prescriptions . Weaker perceived behavioral control was evident in the participants who showed a less favorable attitude toward antibiotics .On average, 40.54% (SD = 20.82%) of the outpatient encounters resulted in a prescription for an antibiotic given by the participants and 9.81% (SD = 10.18%) of the patients were given two or more antibiotics. The participants showing a more favorable attitude toward antibiotics had a higher intention to prescribe antibiotics  contains supplementary material, which is available to authorized users. Antibiotic products, commonly used in modern medical practices, have saved hundreds of millions of lives over the past century. However, the emergence of antibiotic resistant “super-bugs” in the recent few decades has resulted in increasing concerns over the arrival of a “post-antibiotic era” . The lacThe irrational and over-use of antibiotics in human beings and animals is blamed for fueling antibiotic resistance –6. Over-Researchers have attempted to understand how prescribing decisions are made using various behavioral theories, such as the knowledge-attitudes-practices (KAP) model, the operant learning theory (OLT), the social cognitive theory (SCT), the theory of reasoned action (TRA) and the theory of planned behavior (TPB). Godin and colleagues concluded in a systematic review that the TPB is the most appropriate model to predict prescribing behaviors . SeveralThis study aimed to establish a TPB model for antibiotic prescribing practice in primary care in Hubei of China. Such a model can provide evidence to support the design of interventional measures.This study was conducted in Hubei, a province in central China with a population of 58.85 million. The social and economic development of Hubei ranks in the middle range of all the regions in China .This study focused on the primary care sector, which attracts about 60% of all outpatient visits in China . The stuAbout 60% of outpatient visits to primary care facilities in Hubei involved an antibiotic prescription , which iThe theoretical framework was adapted from the TPB model Fig. . The modThe TPB model assumed that antibiotic prescribing practice is influenced by behavioral intentions and the perceived behavioral control of the prescribers, with the former serving as a motivational factor while the latter reflecting the ability of the prescribers to fulfill their intentions. Attitudes, subjective norms and perceived behavioral controls are linked to each other and they can influence the behavioral intentions of the prescribers .Percentage of prescriptions containing antibiotics = Number of prescriptions containing antibiotics/Number of total prescriptions × 100%Percentage of prescriptions containing two or more antibiotics = Number of prescriptions containing two or more antibiotics/Total number of prescriptions × 100%Two indicators of prescribing practice were calculated by auditing the prescriptions in the participating facilities.Attitudes (5 items)A questionnaire was developed in line with the guidelines for TPB surveys . It cont2.Subjective norms (6 items)Attitudes were defined as the degree to which a prescriber is in favor of the use of antibiotics in outpatient encounters . Respond3.Perceived behavioral control (5 items)Subjective norms measure the perceived social pressure to which a prescriber is subject in relation to antibiotic prescriptions . Respond4.Behavioral intentions (6 items)Perceived behavioral control measures how easy a prescriber feels in making a rational decision on antibiotic prescriptions . It inclBehavioral intentions measure the degree to which a prescriber is willing to prescribe antibiotics . Two subn = 503). The confirmatory factor analysis (CFA) demonstrated an excellent fitness of data into the proposed model: root mean square error of approximation (RMSEA) = 0.039 (< 0.05); Tucker–Lewis index (TLI) = 0.993 (> 0.95); comparative fit index (CFI) = 0.991 (> 0.95) and weighted root mean square residual (WRMR) = 0.934 (< 1) [The questionnaire was piloted on 21 physicians from three primary care facilities. The participants were asked to complete the questionnaire and provide feedback on the clarity and difficulty of items, which resulted in some revisions to the items. The reliability and validity of the revised questionnaire were tested in the final survey (34 (< 1) , 32. Higbeing able to prescribe antibiotics independently;Having authorized ≥100 prescriptions over the past three months prior to the survey (to ensure a reliable estimation)All physicians from the sampled primary care facilities were invited to participate in the survey. Those who met the following criteria were included in this study:A multi-stage cluster sampling strategy was adopted. The first stage involved a random selection of one urban city and two rural counties in western, central and eastern Hubei, respectively.We estimated that a minimal sample size of 6 primary care facilities in each city/county involving 23 respondents was required according to the calculator developed by Dhand and Khatkar  . We incrIn the third stage, trained interviewers were paired and dispatched to the selected CHCs and THCs. All of the physicians on duty were approached and invited to participate in the survey. If less than 70 respondents were found in a city or county from the selected primary care facilities, an additional primary care facility (if available) was added to the sample. Eventually, 19 urban CHCs and 46 rural THCs were involved in this study.Prescription data of the participating primary care institutions from January 1st to March 31st, 2018 (three months prior to the survey of the prescribers) were extracted from the local governments and the participating institutions. We calculated the two prescribing indicators for each prescriber.The approached physicians who prescribed medicines were asked to complete the questionnaire independently. Written informed consent was obtained by the interviewers from each respondent prior to the survey. On average, the survey took around 10 min to complete. The returned questionnaires were examined by the interviewers to ensure completeness before a token gift (roughly $1.65) was given to the participant.In total, 712 questionnaires were distributed and 664 were returned. After matching with the prescription data, 503 met the inclusion criteria and were included for final analyses. This resulted in an effective response rate of 71%.We calculated the mean score and standard deviation for each measurement. A mean score of 3 represented a neutral response  analyses were performed based on the proposed TPB model Fig. . Means ap value < 0.05 was considered statistically significant.The statistical analyses were performed using STATA (version 12.0) and Mplus (version 6.0). A The respondents had a mean age of 43.40 years, with a standard deviation (SD) of 9.57. Most of the respondents were male (70.58%) and worked in THCs (78.53%). On average, the respondents had engaged in clinical practice for 16.28 (SD = 10.08) years. Only a small proportion (10.14%) of the respondents held a senior title . The majority (74.95%) had received some training in regard to the use of antibiotics in the year prior to the survey  prescriptions each. About 40.54% (SD = 20.82%) of the prescriptions contained an antibiotic and 9.81% (SD = 10.18%) contained two or more antibiotics . Their perceptions of social pressure (subjective norms) on antibiotic prescriptions were almost neutral . The majority (66.8%) felt positive in behavioral control .A low intention to prescribe antibiotics  and high intentions to reduce antibiotic prescriptions  were evident.A very good fitness of data into the TPB model was found: RMSEA = 0.029 (< 0.05); CFI = 0.994 (> 0.95); TLI = 0.992 (> 0.95) and WRMR = 0.927 (< 1).p < 0.001) and lower intentions to reduce antibiotic prescriptions . Greater social pressures (subjective norms) were linked to higher intentions to reduce antibiotic prescriptions . A greater sense of control over behaviors was associated with higher intentions to reduce antibiotic prescriptions  and a lower percentage of prescriptions containing antibiotics . However, intentions did not show significant associations with the two indicators for prescribing practice (p > 0.05).The TPB model Fig.  indicateThis study demonstrated that physicians over-prescribed antibiotics in primary care settings in Hubei of China, with 40% of prescriptions containing an antibiotic and 10% containing two and more antibiotics, respectively. These rates are much higher than the recommended levels proposed by the WHO . The comp > 0.05). The reduction of actual use of antibiotics is shaped by higher levels of behavioral control, a feeling of being able to translate intentions into practices .The TPB model fits well with the survey data: intentions to prescribe antibiotics are predicted by the attitudes, subjective norms and sense of behavioral control of the prescribers. However, such intentions are not directly linked to actual prescribing behaviors  incentives and consumer demands.Additional file 1:Table S1. Survery instruments (Translated version). (DOCX 29 kb)Additional file 2:Detailed information of physicians’ responses to items based on TPB. (DOCX 19 kb)


Concrete properties to a large extent depend on mix design and processing, currently leaving only limited options to actively modify concrete properties during or after casting. This paper gives a (p)review on a more advanced active control of properties of concrete, based on the application of external signals to trigger an intended response in the material, either in fresh or hardened state. Current practices in concrete industry that could be considered as active control are briefly summarized. More advanced active control mechanisms as studied in other fields, e.g. based on hydrogels and other functional polymers, are reviewed and some principles are listed. A specific focus is further given on potential methods for active rheology control. Based on the concepts developed in other fields, substantial progress could be made in order to achieve active control of fresh and hardened concrete properties. However, several challenges remain, like the stability and functioning of the responsive material in a cementitious environment, the applicability of the control signal in a cementitious material, and the economy, logistics and safety of a control system on a construction site or in precast industry. Finding solutions to these challenges will lead to marvelous opportunities in general, and for 3D and even 4D printing more particularly. The quality and performance of concrete as a construction material depend on mix design, processing and casting. The type, quality and dosage of constituent materials are key elements in view of obtaining the right concrete for the right job. Scientific and technological knowledge concerning the properties of concrete , supplemIn addition to the mix design, the material properties will be influenced by the mixing process and the casting operation. Mixing procedure and intensity can significantly influence the properties of the fresh material , 5. CastHowever, many variations, intentionally or not, play an additional role, sometimes making it a hard challenge to achieve intended concrete properties in fresh and hardened state. As the properties of the constituent materials vary in-between deliveries, maintaining constant concrete quality in spite of the changing constituent properties is not always straightforward. This is illustrated in  by studyQuality control consequently is an important aspect in concrete industry, involving quality control of the constituent materials, quality control of the mixing process, quality control during execution, and quality control (compliance testing) of the final properties. While the quality control actions can be very active in the sense that a lot of people spend a lot of energy in getting things done in the right way, we typically passively rely on the behaviour of the concrete. Once the concrete is mixed and flowing in the pumping pipes or formworks, we have no active methods to change the behaviour of the concrete, e.g. requiring it to flow faster or become stiffer. It is true that pumping pressures could be actively changed, or vibraton methods could be actively modified, but we nevertheless are stuck with a material with a pre-defined rheological response to the external changes. We have no means to modify the material behaviour while processing is ongoing, or when the material is in final position. In other words, we cannot actively change the constitutive behaviour of the concrete, either in fresh or hardened state. We can only change external parameters (e.g. pumping pressure) to invoke an effect (e.g. increase in discharge rate) obeying the constitutive material behaviour.In other fields, active control methods to change material properties “on demand” do exist. As will be further explained in this paper, special fluids are available responding to an external electric or magnetic trigger signal, changing the behaviour of the material from more fluid-like to more solid-like (or vice versa) on demand . These rAchieving active rheology control (ARC) and active stiffening control (ASC) is the final goal of the ERC (European Research Council) Advanced Grant Project “Smart casting of concrete structures by active control of rheology” (SmartCast) . Active In this paper, an overview is first given of current practices with active intervention in concrete industry, some of them very traditional, some of them already somewhat pushing the boundaries of current knowledge and understanding. Afterwards, active control mechanisms in other fields are briefly summarized, showing materials, signals and mechanisms. Further focus is then given to methods and applications related to the control of rheological behaviour. Finally, some specific challenges are discussed when transferring existing technologies from other fields to concrete industry in view of enabling active rheology and stiffening control of cementitious materials.In current concrete practice, active interventions are manifold, although mostly not in the sense of actively modifying material behaviour by triggering a response of mixed-in special components. Nevertheless, an overview of current active intervention methods  particles leads to high friction, obstructing the flow. In fresh state, an obvious active mechanical intervention is given by the vibration process, using poker vibration or other techniques. By means of a poker, a vibration with a certain frequency and amplitude is  imposed to the concrete. This “signal” has a certain range of action, triggering the (solid) particles in the cementitious material to respond (vibrate). As a consequence, the yield stress drops to zero within the affected zone , and theVibration of fresh concrete can thus be seen as the active application of an external signal , triggering a response from some constituents in the bulk of the concrete (in this case vibration of solid particles), changing the rheological properties of the fresh concrete (reducing the yield stress to zero). In this way, it could be said that active rheology control of fresh concrete is already performed in concrete industry since the 1930s due to the introduction of poker vibrators. However, active rheology control can go much further, as will be illustrated further on in this paper.An active chemical intervention is often applied in case of sprayed concrete, introducing accelerating admixtures in the spraying head. The added accelerating admixture triggers the cement particles, speeding up the hydration process in a chemical way, leading to faster setting (stiffening) and hardening of the sprayed concrete. Similar but even more complex chemical interventions can be applied when at the end of a working day the concrete within a long pumping pipe  is removed for pipe cleaning. The concrete flowing back in a truck mixer can be retarded for a long night by adding a chemical retarder. The next morning, while resuming pumping operations, a chemical accelerator can be added, counteracting the effect of the retarder. This kind of active combination of retarders and accelerators has e.g. been studied by Lesage .The active intervention in this case is giving a chemical signal to the cement particles, leading to a control of the hydration rate. After addition of the accelerator the triggered response of the cement particles is irreversible. However, the triggered response of the added retarder can be reversed by later addition of an accelerator. This kind of active intervention is limited to certain moments in the process, e.g. when the concrete is flowing through the spray head in case of sprayed concrete, or when the concrete is in the truck mixer in the case of re-use of concrete in a pumping pipe. The control is no longer possible at other timings, and it is also not possible to adjust the control levels once the admixtures have been added.Another example of active chemical intervention is the use of carbon dioxide for the accelerated curing of cementitious materials, often in combination with the application of a higher temperature. This active chemical intervention has already been reported by Klemm and Berger , but hasThe effect of temperature on the hydration process of cement is well-known, and can be calculated by means of the Arrhenius equation , with thAnother type of active control involving a thermal field is given by cooling measures to mitigate thermal cracking in hardening massive concrete elements . In embeFreshly cast concrete needs to be protected against premature drying, in view of mitigating shrinkage cracking and creating optimal conditions for the hydration process. For onsite casting, protection against premature drying during the first days is for many elements achieved in a passive way by the formworks. After striking of formworks, or for those elements not protected by formworks (e.g. slabs), curing can be performed in an active way by water spraying or other techniques. The active intervention providing sufficient moisture near the concrete surface is interacting with the pore system, based on capillary forces and diffusion processes. As a result, the concrete is maintaining sufficient moisture conditions, mitigating or reducing early age shrinkage cracking, and improving the hydration process .Modern concepts of internal curing by means of porous aggregate particles or super absorbent polymers (SAP)  cannot b2-content , in view of accelerated hardening of the concrete element [Concrete also includes air, showing a potential effect on the rheological properties depending on the size of air bubbles and the applied shear stresses . During  element . ExternaMagnetic fields can be relatively easily applied in industry, and can be an element of control during a production process. A magnetic field could be applied to magnetize water before its addition to cementitious materials. A magnetic field changes the physical properties of water, including e.g. specific heat and evaporation rate . Some reSoto-Bernal et al.  studied The application of magnetic fields to control rheology of cementitious materials containing magnetic particles in real time has been studied by Nair and Ferron . They coself-compactor elements for concrete.Abavisani et al.  also conThe same group of authors further studied the effect of alternating magnetic fields and alternating current on the behaviour of reinforced concrete columns , considereal-time compressive behaviour controlling of reinforced concrete elements, even enabling a control of failure mode. However, their claims are based on contradictory results, and are not fundamentally supported by a clear mechanism explaining the potential influence of the (electro-)magnetic field on the material and structural properties. While the principle of active control of concrete properties by magnetic signals has been set forward, it is not fully clear which mechanism could be the basis for the explanation of the reported effect of magnetization on hydration and microstructure development. This will need further research attention, with more rigorous test protocols in order to fully exclude potential influences of other parameters.Applying the magnetic field on the hardened concrete seems to lead to contradictory results, with sometimes an increase  and someOn the other hand, it seems clear that mixed-in magnetic particles or magnetizable materials like steel chips as well as preplaced steel rebars are responding to the magnetic signal, with effects on rheology, compactability and even mechanical performance. This opens a window for new developments and applications, possibly involving new types of magnetizable materials  or new fAn electric control signal (AC current) has been mentioned in previous paragraph, where it was combined with the research on the magnetic control of the mechanical performance of hardened reinforced concrete columns. Other literature results are available on the application of electric fields on fresh or hardening concrete. As an example, Kovtun et al.  apply diA more innovative application of an electrical field can be found in the papers of Goudjil et al. , 49. TheInspired by this electrically controlled formwork removal technique, Choi et al.  applied Another example of electric intervention is the technique of cathodic protection by means of applied current in view of protecting an existing structure from reinforcement corrosion . This teAs an extension of electric signals, microwaves can also be mentioned in this overview. Microwaves are electromagnetic waves with high frequencies (300 MHz to 300 GHz) and wavelengths ranging from 1 mm to 1 m. A dielectric material placed in a high frequency electric field will heat up due to internal friction, as the induced vibration of the dipolar molecules is hindered by molecular attraction forces. As already reported by Xuequan et al. , the proIn other fields, functional materials get a lot of research attention. For a multitude of potential applications, controllable materials are developed, responding to specific trigger signals, and resulting in an aimed-for effect. In this section, an (non-exhaustive) overview is given of some applications, type of materials, type of signals and type of responses. Some specific attention is given to different options to control rheology. The applications and control methods used in other fields can be inspiring in view of reaching active control of properties of concrete.drug delivery on demand [A major field of research studying stimuli-responsive materials is given by n demand –63. Compn demand .soft robotics and micro-actuators, when the functional polymer can transform chemical energy into a mechanical action like folding or bending [self-propelled nanomotors [Stimuli-responsive polymers are also used for  bending –69. Typinomotors .adhesion on demand [smart windows [information into triggered hydrogel by very precise control of the electric signal given to self-assembling molecules [In this non-exhaustive overview of applications of functional materials, reference can also be made to n demand , smart w windows  and evenolecules .hydrogel, generally defined as three-dimensional network structures consisting of polymeric chains joined by tie points or joints and swollen in water up to thermodynamic equilibrium [The majority of current applications relying on stimuli-responsive control is based on ilibrium . Hydrogeilibrium . It is ailibrium . The conilibrium .vitrimers, covalently crosslinked polymer networks. Vitrimers can be modified in different ways, and even be combined with carbon nanotubes, resulting in many different functionalities, in response to even six different stimuli [Although gels offer great possibilities, they have limitations concerning multi-responsivity and multi-functionality. An alternative approach is the use of  stimuli .core–shell type particles with monodisperse polymeric cores and uniform shells with independent control of properties of core and shell [A specific group of materials is given by nd shell . Core–shshape memory polymers (SMP). The mechanism can be based on different controllable transitions, such as reversible molecule cross-linking or anisotropic-isotropic transition. They can be reinforced by different materials, e.g. carbon nanotubes, carbon nanofibers, exfoliated nanoclay…, to improve the mechanical performance of the resulting material, called shape memory polymer composites (SMPC) [3O4-particles, or other magnetizable materials in order to make them magneto-responsive [Stimuli-responsive polymers intended for the (reversible) control of shape are also called s (SMPC) . They cosponsive , 80.hyperbranched polymers to make them magneto-responsive. Hyperbranched polymers show good possibilities to make them stimuli-responsive in general, offering good options to include a wide choice of terminal functional groups and branches [Magnetic nanoparticles (MNP) can also be incorporated into branches .The concept and design of stimuli-responsive polymers in general is getting a lot of research attention nowadays. New design strategies  and new Most common signals concern temperature and pH (or pH difference), or the application of an electric, magnetic, or electromagnetic field. Many other signals, however, have shown to be applicable in view of inducing a triggered response in functional materials, like e.g. pressure , redox-sMost stimuli-responsive polymers respond to one or more trigger signals. In case of dual or multi-responsive materials, the desired action depending on the given combination of trigger signals is an important point of attention. In case of two signals, as an example, it could be that the desired response is only activated in the presence of both stimuli, while another option could be the activation in case of any single stimulus. Different other logic combinations can be chosen, as illustrated in .The applicability of the trigger signal depends on the type of material, and further depends on practical considerations. While for some applications photonic signals are convenient and environmentally friendly, the responsive process  is typically not showing a high efficiency . For opaTriggering crosslinking resulting in gel formation, often by means of thermally induced sol–gel transition Cleavage of polymer molecules, resulting in collapse of a hydrogel or a hyperbranched polymer, or influencing micellization , 91, 94Controlling swelling/de-swelling and the release of an agent, due to changes in osmotic pressure by asymmetric distribution of ions in electric field , or by cChanging the permeability of a responsive membrane, by closing or opening “chemical gates”, e.g. by pH control Sorption control of water vapour of e.g. electro-responsive polypyrole film Control of solubility, often by changing temperature Changing the nature of a complex molecule from hydrophilic to hydrophobic, also enabling control over the micellization process 2 and redox-stimulated transformation of entangled worms to rodlike micelles [Changing the self-assembly, e.g. COmicelles Changing the orientation of the polymers, e.g. by the application of an electric and/or magnetic field , 101Controlling entanglement of polymer chains Controlling aggregation/dispersion of particles in a medium Control of trans–cis isomerization of polymers Shape-morphing Oscillatory shape change, e.g. oscillatory bending , 104Crystallization/melting transition Crystal anisotropic/isotropic transition …The listed responses can induce changes in numerous material properties, or can lead to specific actions (like e.g. drug release). Some of the listed responses could have good potential for rheology control. This is further discussed in the following paragraph.Roy et al.  give an The rheological behaviour of a material is governed by fundamental physical properties, characterized by specific parameters, further depending on the type of material . For solWhen particles are introduced into a liquid matrix, the behaviour can be different depending on the size and nature of the particles, and the resulting interparticle forces. In suspensions, depending on the density of the particles and of the suspending liquid, particles could segregate down- or upwards, as governed by the interplay between gravitational forces and buoyancy forces. However, particles also interact, depending on their concentration in the suspending liquid, counteracting settlement forces. As a result, the viscosity of the suspension will be different from the viscosity of the suspending medium. For low particle concentrations, up to 30% of the maximum concentration, the viscosity increase is limited. However, at larger concentrations, the viscosity can increase significantly, as described in the Krieger–Dougherty equation or similar models . FurtherIn case of small particles, in the range of µm and below, colloidal interactions become dominant, mainly Van der Waals forces combined with Brownian motion, and the particles tend to form clusters or aggregates. This aggregation phenomenon could be counteracted by electrostatic repulsion or by steric hindrance, as typically done by superplasticizers in cementitious materials . DependiG′ (representing the energy storage or elastic property of the fluid) and the loss modulus G″ (representing the viscous dissipation in the fluid) [When the particles in the liquid are polymers, droplets or air bubbles, the overall rheological behaviour of the resulting fluid can become even more complex, with additional governing parameters. Although advanced mathematical models are available , based oe fluid) . In caseConsidering the governing physics and the resulting characteristic parameters of complex fluids, changing or controlling the rheological behaviour can involve different mechanisms or approaches. A non-exhaustive overview is given hereafter.A well-known concept to change the rheological properties of a fluid is given by electrorheological (ER) fluids , 112 andτy) in ER fluids is depending on the electric field strength (E0) in the following way:a typically in the range of 1.5–2.0 [G″ typically shows an initial plateau as a function of shear stress, while the storage modulus G′ is higher than the loss modulus G″. In electro- or magneto-activated state, the suspension is dominantly showing an elastic behaviour. However, when the shear stress exceeds a critical value, the storage modulus G′ shows a sudden decrease, physically governed by the rupture of the internal structure and the resulting onset of material flow [In terms of rheological properties, it is reported that the yield stress , it is clear that new ways could be found to actively control concrete properties in general, and rheological properties of the fresh concrete more specifically. However, many difficulties have to be overcome when applying control mechanisms from other fields to the field of cementitious materials. The authors selected three concrete characteristics that are forming the main obstacles for now:The fascinating mechanism displayed by different types of hydrogels, as discussed in Sect. The application of control signals in cementitious materials also comes with some challenges. Some signals will not be applicable, like light signals, as they will not be able to travel easily through an opaque cementitious material. Electric control of hydrogels or other functional polymers is often based on an isolating dielectric medium suspending the functional polymers . In contAnother challenge is posed by the large bulk volume and its inert constituents. As an example, while casting a massive foundation slab or a high wall, will it be possible for the trigger signals to reach the bulk of the concrete? With the intended amplitude and/or frequency? In any location? These questions involve different aspects, including fundamental knowledge on the interaction between the physical fields and large volume of concrete in a real environment, involving e.g. pumping pipes and formwork. Furthermore, there are as well practical questions concerning the required installations to provoke the intended fields. Safety issues will become important when working with electromagnetic or other signals on a real construction site.In some cases, however, bulk control will not be required. As an example, the pumping behaviour to a very large extent depends on the properties of the lubrication layer near the pipe surface . In thatStability and functioning of the responsive material in a cementitious environment, typically showing higher pH values than in other fieldsApplicability of the control signal in a cementitious material, depending on its physical properties like resistivity, conductivity, opacity…Economy, logistics and safety of a control system on a construction site or in precast industryIn spite of the many challenges, a good opportunity might be given by digital fabrication methods, including 3D printing. Within the relatively limited volume of a printing head, it will be more practical and realistic to provide control signals to modify the rheology of the fresh cementitious material when moving from pumping line to printed layer. As the rheological requirements in a pumping pipe and for a printed layer are significantly different, an active control of the material properties in the printing head would be a very efficient approach. Active control features embedded in 3D printed concrete could open the door to so-called 4D printing in construction industry, as already envisioned by Momeni et al. [Summarizing the challenges, the following aspects will require due attention when developing active control features for fresh and hardened concrete:i et al. .This paper gives a (p)review on the active control of properties of concrete, based on the application of external signals to trigger an intended response in the material, either in fresh or hardened state. Current practices in concrete industry that could be considered as active control have been listed. More advanced active control mechanisms as studied in other fields, e.g. based on hydrogels and other functional polymers, are reviewed and briefly summarized. A specific focus is further given on potential methods for active rheology control. Based on the concepts developed in other fields, substantial progress could be made in order to achieve active control of fresh and hardened concrete properties. However, several challenges remain, like the stability and functioning of the responsive material in a cementitious environment, the applicability of the control signal in a cementitious material, and the economy, logistics and safety of a control system on a construction site or in precast industry. Finding solutions to these challenges will lead to marvelous opportunities in general, and for 3D and even 4D printing more particularly.