The impact of aircraft on airport pavements is varied and closely related to their operational durability. The article presents the impact of the annealing process related to the forced impact of airplanes on airport pavements. The composition of cement concrete with ceramic dust, which is characterized by increased thermal resistance, has been proposed. Two research cycles were programmed, differentiated by the annealing scheme and the way in which the temperature influences the annealing time. Samples stored at a temperature of 20 ± 2 °C were subjected to testing. The tests were carried out for two diagrams: A and B. The first—diagram A—included the continuous impact of the flue gas stream on the samples for a period of 350 min with a test step every 25 min. For the second—diagram B—the samples were alternately heated (1 min) and cooled (15 min). The influence of the proposed pavement mix on changes in the internal structure of cement concrete and the increase in its resistance to high temperatures was determined. In the microstructure of the CC-1 concrete matrix, it was found that there were plate-granular portlandite crystals up to 10 µm in size and ettringite crystals with a length of 8 µm. In the CC-2 concrete, the ettringite crystals were less numerous and had a length of up to 5 µm, there were also continuous contact zones between the aggregate grains and the cement matrix (diagrams A). The alternating annealing/cooling (diagram B) resulted in the ettringite crystals in the CC-1 matrix being up to 10 µm long, and in the CC-2 concrete up to 7 µm long. The contact zone between the aggregate grain and the matrix in CC-2 concrete was continuous, and the microcracks in CC-1 concrete were up to 8 nm. Regardless of the heating diagram, in the surface zone, there were larger microcracks in the CC-1 concrete than in the CC-2 concrete. For diagram A they were 14 µm and 4 µm and for diagram B they were 35 µm and 5 µm, respectively. It was found that concrete with ceramic dust is characterized by a lower and more stable temperature increase. In scheme A, the average temperature increase on the heated surface ranged from 46 °C to 79.5 °C for CC-1 concrete, and from 33.3 °C to 61.3 °C for CC-2 concrete. However, in scheme B, the temperature after 350 heating cycles for CC-1 concrete increased to 129.8 °C, and for CC-2 concrete to 116.6 °C. After the cooling period, the temperature of CC-1 and CC-2 concrete was comparable and amounted to 76.4 C and 76.3 °C, respectively. CC-2 concrete heats to lower values, and favorable changes in internal structure translate into higher strength and durability . Airfield pavements have a specific system of structural layers designed for the traffic of aircraft and wheeled vehicles of airport service . During The work  specifieAccording to , the gasTo an increased extent, the destructive effects of aircraft are recorded at military airports. In  the authAn important environmental aspect is a possibility of reusing materials in the composition of concrete mixtures. In , the autThe specific nature of pavement operation, the necessity of using the structure in diversified environmental conditions, and providing high criteria of traffic safety are determined by the necessity of providing relevant concrete quality of the pavement. Particular attention should be paid to the opportunity of using modern technological solutions. This paper presents the extended scope of experimental tests on the effect of the heat flux generated on the engine dynamometer. In the comparative analysis, the influence of the pavement mix used on the thermal resistance of concrete and its internal structure was determined.The durability of airfield pavements is understood as the opportunity of failure-free operation within the assumed operation period, and, taking into consideration the influence of diversified environmental conditions, is specified as several coexisting and interdependent factors. The issue of concrete structure’s durability, in terms of the multi-criteria aspects, have been discussed, among other issues, in the following works ,53,54,55The applicable standard  includes2O). Cement content in the concrete mix is diversified and closely dependent on the parameters of the designed concrete. Airfield pavement concrete should be resistant to environmental impacts, such as frost corrosion (XF4), corrosion caused by carbonatation (XC4), corrosion caused by chlorides (XD3), the abrasion impact (XM3), and class (XA2) [General requirements for airport pavements are defined in the paper . Accordiss (XA2) ,28. The The concrete mix includes water in the amount dependent on the assumed w/c ratio. The maximum value of w/c ratio according to standard requirements  is 0.4. -in the case of coarse aggregate their geometrical characteristics  which indicate the range of good grain size distribution, in accordance with . The folCompressive strength in accordance with the guidelines . The tesBending strength in accordance with the guidelines of . The tesTensile strength during splitting in accordance with the guidelines of . The tesAbsorbability of concrete in water and de-icers according to . The tesFrost resistance of concrete according to , which i2;Resistance to surface flaking of concrete, according to , should No phenomenon of permeability of water through concrete is specified according to .In the scope of experimental works that enable the assessment of concrete parameters, it is required to mark the possibility of using the designed concrete for specific construction purposes. The assessment of the suitability of concrete for the construction of airport pavements means, among others, the need to determine selected mechanical and physical parameters of concrete. These parameters, in accordance with , should Obtaining durable concrete composite, apart from the aforementioned factors, is referred to the technological process concerning proper concrete incorporation, compaction, and curing, and the maintenance process being conducted on time during the operation. The discussed determining factors of concrete durability presented in the following works ,78,79,803, absorbability (W) according to [DE) according to [D10, and crushing resistance (LA) according to [35.The reactive and physical and mechanical properties of the aggregate used has a significant impact on the amount of rheological deformation of concrete. The tests performed and the results presented in  showed trding to —0.6%, abrding to —MD10, anrding to —LA35.3S, belite -C2S, C4AF, C3A, traces of CaO and MgO), and setting time regulators  and carbonation products . The cement used in the composition of concrete mixes met the criteria of reduced alkali content and the specific surface area see . The ideCeramic dust included crystalline components: quartz, mullite, cristobalite, corundum, very small amounts of clay minerals, and anorthite . The chaBased on the results obtained from the thermal analysis of the ceramic dust used , it was The analysis of the research results to date, especially the parameters of the proposed ceramic dust , indicates the possibility of applying an additive to the composition of airport concrete. Particularly beneficial is the effect of ceramic dust, as noted in ,81, on c-assumed exposure classes (according to ) reflect-assumed concrete strength class (according to provisions of )—C40/50;-W/C mix coefficient—indicating the quality of slurry, each time this value did not exceed the limit value of 0,4, which was assumed according to standard requirements of , as the -minimum cement content in the concrete mix, as derived from the assumed concrete exposure class, which should not be lower than that presented in -3;total cement and fine aggregate content at a maximum level up to 450 kg/m-required air content in the concrete mix—each time the air content w defined in accordance with  and was -grain size distribution curve of concrete mix—each of the designed curves in case of concrete mixes was within good grain size distribution, accepted according to the requirements of  dependinAt the mix design stage, preliminary assumptions were accepted, which included defining the following:According to our own research, the aggregate composition of concrete mix was diverse in individual components content and complied with aggregate composition tightness requirements. In the case of each mix type, the coarse aggregate was considered the basis. Fine aggregate in the form of washed sand was also taken into consideration in aggregate composition. Considering the analyzed research problem, there is a quantitative diversification of individual mix components. From the designing perspective of cement concrete intended for airfield pavements, the significant factor was the selection of coarse aggregate used in the case of aggregate composition. The designing process of specific mixes was planned according to analytical and experimental methods taking into consideration the following, which defines the necessary amount of individual components based on three equation methods according to ; verificThe designed aggregate mixtures , which wConcrete samples of CC-1 and CC-2 series, after 28 days of curing in accordance with the requirements of the  standardin is the number of events in the group and ix is the successive values of the variable.The arithmetic mean was used as a position measure  accordinStandard deviation and a relative measure of dispersion (coefficient of variation to compare the measurements of the variation of the two characteristics or traits in different groups) were determined from Equations (2) and (3) according to .(2)SD=1t-test (Equation (4) according to [in is the size of the i-th group. i-th group, iSD is thestandard deviation of the i-th group.The analyzes were performed with the Student’s rding to ) for unrF) referred to sample cross-sectional area (cA), to which the destructive force is applied, the compressive strength was defined (cf), with the value approximately equal to 0.5 MPa—Equation (5). In the case of each concrete series, basic statistical characteristics were defined which allowed concrete verification in terms of its compliance with the standard requirements of [Determining the compressive strength was conducted in cubic samples of the following dimensions 15 × 15 × 15 cm, according to the requirements of . Every sments of .(5)fc=FThe qualitative phase composition tests were carried out in a TUR-M62 diffractometer . The quantification of selected components was performed on a thermal analysis device  of the SDT Q600 type .Selected samples in the case of each concrete series after the assumed curing periods were intended for microscope observation in the scanning electron microscope and computed tomography. Microscope analysis required earlier preparation of concrete samples according to procedures and methodology of individual microscope techniques.2. Preparation of samples and interpretation of the obtained results were in compliance with those described in literature works [Microscope analysis was conducted by means of scanning electron microscope Zeiss-SUPRA type and FEI Quanta 250 FEG. Recent fractures or micro sections were prepared each time from concrete samples. In order to eliminate the negative influence of electron beam on the image and therefore improve the quality, each sample was sprayed with alayer of platinum 10 nm thick in the device Baltec SCD 005 attachment CEA 035. The surface of preparations was subject to observations by means of SEM and it was smaller than 1.0 cmre works ,87. The re works  Using thExamination of the analyzed concrete samples was conducted by means of computer tomography V/Tome made by GE, as described in the work . The exaThe average values of the recorded temperatures are shown in The temperature increase recorded for each successive 25 min of heating is summarized in It was found that the temperature change of CC-1 concrete was more heterogeneous than that of CC-2 concrete . The recStatistical analysis showed that for individual test series the values obtained by CC-1 and CC-2 concrete are clearly different see . For exaCC-1 and CC-2 concrete, exposed to alternating cycles of heating and cooling, changed their thermal parameters . The aveIn the case of alternating heating cycles, a significant differentiation of the temperature gradient in individual periods was found for CC-1 and CC-2 concrete. For CC-1 concrete , the higAdditionally, for CC-2 concrete , the higIt was found that at the annealing stage, the increase in the average concrete temperature for CC-1 and CC-2 concrete varied, as shown in In the case of CC-2 concrete, after 20 s of heating, the average values ranged from 76.7 °C after 25 thermal cycles to 103.4 °C after 350 cycles. After 40 s of soaking, the mean values ranged from 84.4 °C to 111.3 °C after 350 cycles, respectively. On the other hand, after 60 s of heating, the mean temperature values ranged from 92.2 °C to 116.6 °C. Temperature changes were always characterized by a linear course.The nature of sample cooling after the annealing process is different for CC-1 and CC-2 concrete . In the riods ss . On the The analysis of the change in the mean value of compressive strength was carried out for the concretes in diagram A and diagram B.The analysis of changes in the average compressive strength of concrete CC-1 and CC-2 (diagram A) showed a significant differentiation . In the The analysis of changes in the average compressive strength of concrete CC-1 and CC-2 (diagram B) showed a significant differentiation . In the Exemplary microphotographs obtained during SEM observations are presented in The results observed in  allowed It was found that the CC-1 and CC-2 concrete samples, which were subjected to thermal cycles according to scheme A, were clearly differentiated in terms of internal structure . In the It was found that the CC-1 and CC-2 concrete samples, which were subjected to thermal cycles according to scheme B, were clearly differentiated in terms of internal structure . In the Exemplary data obtained during the TEM observations are presented in The concrete compositions analyzed in the study differed in the material composition and the share of hydration products. This was the cause of heterogeneous stresses and the appearance of a distorted microstructure during linear changes during thermal cycles. It was observed that the ettringite in the heated CC-1 concrete was also present in the cracks, which proves the destruction of the microstructure. In each of the cases (Scheme A and Scheme B), the edge of the expanding cement slurry is a representation of the grain outline of the aggregate. This is due to the use of an aggregate that (as shown in the paper) does not expand. The changes in the volume of CC-1 concrete were caused by the expansion of the grout, which caused the microcracks to open in the contact zone and were the site of the development of ettringite crystals. In addition, ettringite crystals were developed. In the case of CC-2 concrete (after 28 days of maturation), there were no microcracks in the contact zone, which limited the propagation of cracks after thermal cycles and the aging process. The aging process in the case of concretes subjected to thermal diagrams consisted of the intensification of ettringite crystallization, especially in the existing microcracks and in the spaces of the contact zone between the aggregate grain and the cement matrix. The change in the length and geometry of the cracks is caused by the stresses appearing as a result of heating and cooling and is more intensively carried out according to the scheme in the B process. The CC-1 ettringite appearing in the microcracks in the concrete causes increased expansion and enlargement of the crack width. The appearance of microcracks in CC-1 concrete already after 28 days of maturation intensifies the range of crack opening after thermal cycles and, consequently, increases the expansion. The confirmation of the obtained results is consistent with the results presented in , which cNon-reactive aggregate and cement, which contains a reduced alkali content and ceramic dust in the concrete mix were used, produced concrete with increased resistance to the diversified influence of thermal cycles.The presented research was carried out to assess the influence of thermal cycles on the change of selected physical and mechanical parameters and the structure of the internal microstructure of concrete intended for airport pavements. The laboratory results allowed for the formulation of the following conclusions:The increase in concrete resistance was registered regardless of the heating pattern. In the case of scheme A, after the impact of 350 thermal cycles, a higher increase in the average temperature of CC-1 concrete was observed than that of CC-2 concrete, in the range from 27.7 °C to 33.5 °C and in the range from 25 °C to 28 °C, respectively. The analysis of the temperature gradient in the 25-min interval showed that CC-2 concrete is characterized by lower values than CC-1 concrete, in the range from 1.2 °C to 3.2 °C and in the range from 0.7 °C to 6.6 °C, respectively. A change in the temperature gradient in terms of the sample cross-section was also observed. In this case, higher values were recorded for CC-1 concrete than for CC-2 concrete. The temperature gradient for CC-1 concrete in the top-bottom section of the sample was from 5.9 °C to 12.7 °C, in the top-center zone from 2.0 °C to 4.2 °C, and in the middle-bottom zone from 3.3 °C to 8.8 °C. In the case of CC-2 concrete, the temperature gradient in the top-bottom section of the sample was from 3.5 °C to 6.5 °C, in the top-center zone from 0.7 °C to 1.6 °C, and in the middle-bottom zone from 2.0 °C to 5.1 °C.In the case of cyclic heating and cooling of concretes according to scheme B, it was found that CC-2 concrete heats up to lower temperatures than CC-1 concrete. The average temperature of CC-2 concrete was over 8 lower than that of CC-1 concrete. The temperature difference of CC-1 concrete analyzed after 60 s soaking and 900 s cooling was in the range from 40.3 °C to 56.3 °C, while for CC-2 concrete it was in the range from 38.7 °C to 43.5 °C, respectively. The temperature difference of CC-1 concrete analyzed after 60 s of heating and 150 s of cooling was in the range from 24.3 °C to 34.4 °C, while for CC-2 concrete it was in the range from 10.6 °C to 20.5 °C, respectively. The temperature difference of CC-1 concrete analyzed after 60 s heating and 20 s cooling down ranged from 5.5 °C to 23.1 °C, while for CC-2 concrete it was in the range from 11.7 °C to 18.8 °C, respectively.2.Changes in the internal structure were observed, in particular concerning the porosity characteristics, the structure of the contact zone between the aggregate grain and the matrix, and the structure of the cement matrix.In the case of annealing according to scheme A, it was observed that in the CC-2 concrete, less extensive ettringite crystallization occurred. It concerned the cement matrix and the contact zones between the matrix and aggregate grains. In the cement matrix, the length of individual crystals did not exceed 5 μm in CC-2 concrete, while in the CC-1 concrete it reached the value of 8 μm. There was also a noticeable difference in the contact zone between the cement matrix and aggregate grains, for CC-2 concrete the crystal length did not exceed 1 nm, and for CC-1 concrete it was 5 μm. The extensive crystallization of ettringite in CC-1 concrete increased the width of the microcracks. On the other hand, in CC-2 concrete, the contact zones between the aggregate grains and the cement matrix remained continuous. This change translated into the achievement of higher values of compressive strength of concrete subjected to annealing cycles.In the case of annealing according to scheme B, it was observed that in the CC-2 concrete, less extensive ettringite crystallization occurred, in favor of extensive crystallization of portlandite. In the cement matrix of CC-1 concrete, the length of individual ettringite crystals was up to 10 μm. In the contact zone between the cement matrix and aggregate grains for CC-1 concrete, the length of the crystal reached 8 μm. On the other hand, in the CC-2 concrete, the contact zones between the aggregate grains and the cement matrix remained continuous, and the crystallization of hydrated calcium silicates with a size of up to 7 μm prevailed.3.A significant increase in the compressive strength of concrete with the proposed ceramic dust was found, regardless of the course of the heating process.In the case of CC-2 concrete, an increase in compressive strength was recorded in the range of up to 350 annealing cycles, regardless of the adopted scheme. In scheme A, the strength increased from 49.2 MPa to 83.9 MPa, and in scheme B, from 58.5 MPa to 75.0 MPa. However, in the case of concrete without the addition of the proposed dust (CC-1), the compressive strength in scheme A increased in the range from 42.5 MPa to 85.7 MPa after 200 cycles, and after subsequent cycles it decreased, reaching the average value of 67.1 MPa after 350 cycles. In scheme B, the tendency was similar, CC-1 concrete was characterized by an increase in strength up to 100 cycles (in the range from 48.3 MPa to 71.9 MPa), and then a decrease in strength by subsequent cycles to the value of 55.4 MPa after 350 cycles was observed.


Freeze–thaw cycles (FTCs) and steel bar corrosion (SBC) are the most common service conditions of hydraulic concrete and have significant impacts on its durability. Using pullout and microscopic tests of different FTC and SBC rates, we selected the mass loss rate, ultrasonic velocity, bond strength and bond slip in order to describe the changes in the macro-properties, and also selected the porosity and pore size distribution as micro-parameters in order to explore the influence of FTCs and SBC on the mechanical properties of hydraulic concrete. The results showed that the bond strength decreased as the FTCs increased due to the microstructure damage caused by FTC and SBC, which affects the mechanical properties. A corrosion rate of ≤3% offset the damage caused by 50 FTCs. FTCs and SBC resulted in superimposed damage effects on the concrete. In addition, we established a bond strength damage model based on the joint FTCs and SBC and quantitatively described the degradation law of the macro-mechanical properties. The analysis shows that the influence of FTCs on the bond strength was greater than that of the SBC. These research results can provide a reference and experimental support for the frost-resistant design and durability prediction of hydraulic concrete structures in cold environments. Concrete structures in northern China encounter complex service environments during their service life . The facSufficient bonding force is the basis for the capacity of the two materials, steel bar and concrete, to work together and coordinate deformation . In receThe abovementioned studies mainly focused on how the FTC influences the bond performance between the reinforcement and concrete (bond strength and bond slip) from a macro perspective, with less attention given to the influence of the joint actions of FTCs and SBC on the bond performance. A lower FTC, in the current research, may not be sufficient to explain the effects of greater TFC damage on the mechanical properties of reinforced concrete. In addition, because of the limited sample points selected for the microscopic experiments, many studies do not clearly describe the relationship between the macro-properties and microstructure, and there are few studies presenting bond strength damage models based on the number of FTCs and rates of SBC. In conclusion, further study of the influences of FTC and SBC on the macro-performance, on the microlevel, is necessary. An understanding of the damage mechanism of reinforced concrete structures under the combined actions of FTCs and SBC is also of great significance for the prediction of the durability of concrete and alterations to the corrosion resistance of reinforced concrete in cold environments. In this paper, the effects of FTCs and SBC on reinforced concrete and the relationship between the evolutions of the microstructure and mechanical properties are studied using the central pullout test, combined with the evolution of damage indices . Through the observation and study of concrete that is close to the central point of the steel bar, using a scanning electron microscope, the relationship between porosity, the most probable pore diameter and bond strength is established. In addition, we also offer a model for the bond strength damage under the joint actions of FTCs and SBC, and the influences of FTCs and SBC on the bond strength between the reinforcement and concrete are quantitatively analyzed. The test results provide supporting data and a reference for the evaluation of the durability and structural safety of concrete structures in cold environments.The experimental program aimed to test the bond strength between the steel bars and concrete with different degrees of corrosion following FTCs and to explore how the FTCs and SBC influenced the bond strength. A series of orthogonal tests with different FTCs and different SBC rates were designed. Firstly, the samples were placed under different FTCs: F0 (0 FTC), F50 (50 FTC), F100 (100 FTC), F150 (150 FTC) and F200 (200 FTC). Different electrochemical-accelerated corrosion processes were then conducted for X0 (uncorroded), X1 , X2  and X3 . The experimental procedure included the FTC process, accelerated corrosion process and central pullout process. The macroscopic parameters tested included the corrosion quality, ultrasonic wave velocity, bond strength and bond slip. The microscopic parameters tested included the porosity and pore size distribution (PSD). The general test procedure is shown in The cement used was ordinary Portland cement P. O 42.5, produced by the Hubei Cement Plant, Yichang, China. Sand with a fineness modulus of 2.78 was used as the fine aggregate. The coarse aggregate was crushed stone with diameters ranging from 5 to 40 mm. Yichang Building Materials Co., Ltd., Yichang, China. provided the steel bars and class I fly ash for this experiment. The high-efficiency water-reducing agent and air-entraining agent were provided by Shanxi building materials Co., Ltd., Weinan, China. Tap water was used in all the mixtures. Deformed steel bars with a diameter, yield strength and elastic modulus of 20 mm, 568 MPa and 200 GPa, respectively, were used to prepare the pullout specimens. The materials used in this experiment were all from China. The mix proportions and performance of the concrete are shown in Pullout tests were performed using a cuboid concrete specimen with a side length of 100 mm × 100 mm × 200 mm and a concentric steel bar placement to investigate the bond behavior between the concrete and the deformed steel bars after the FTC. The number of FTCs and SBC rate were used as variables in the experiment. Three equal specimens were drawn for each condition, giving a total of 60 specimens. The length of the bond anchorage zone was 7 d (140 mm), and the bond zone was established in the middle of the specimen. Simultaneously, the unbonded section was set up to eliminate the local extrusion effect at the loading end. The steel bar had an unbonded section at the loading and free ends. To ensure that the bond stress distribution was more uniform and to avoid local damage at the loading end, the steel bar in the unbonded section was covered with PVC plastic pipe. In addition, after installing the steel bar, the casing was filled using the melted paraffin method to prevent the cement paste from entering the casing when the concrete was poured and to ensure the length of the bond between the steel bar and concrete. The free end of the PVC pipe was filled with wax, and the specimen was soaked in water before freezing and thawing to avoid contact between the steel bar and water. The exposed steel bar was coated with glycerol to prevent corrosion, and the specimen was placed in the standard maintenance room after the molds had been removed. The Chinese standards, namely, the Design Code for Concrete-Face Rockfill Dams  and “SteAfter casting, the specimens were removed from the molds after 24 h and then moved into the standard maintenance room and kept at a temperature of 20 ± 3 °C and 95% relative humidity for 24 days. The specimens were immersed in water for 4 days before the freeze–thaw cycle tests were conducted in order to ensure that the concrete was water-saturated. The FTC tests were performed using a freeze-thaw machine (type HDK-9) following the Chinese code GB/T 50082-2009 . In eachThe accelerated corrosion testing of the steel bars began after the end of the FTC tests. According to the grouping, each group of FTC tests included four corrosion states, and 60 specimens were subjected to electrochemical migration acceleration tests with different degrees of corrosion. The full immersion method was used in this experiment to eliminate the effect of humidity on the concrete resistance and the chloride ion transfer rate . In geneTo investigate the effects of the FTCs and SBC on the bond performance between the steel bar and concrete, the bond strength and bond slip were measured using the central pullout test, in accordance with the Chinese Code of Standard for Test Methods for Concrete Structures GB/T 50152-2012 . All theThe ultrasonic wave velocity test is a non-destructive testing method used to obtain the strength and defects of concrete by measuring the change in the ultrasonic wave velocity in the concrete. It can accurately reflect changes in the internal damage of the concrete after the freezing and thawing cycle. Our test used the Pundit Lab ultrasonic detector  to measure the ultrasonic wave velocity of the reinforced concrete specimens. Before the instrument was used, a special coupling agent was applied to the end of the transducer, and it was calibrated with a calibration rod. Then, the sensor frequency, test path, width and calibration coefficient were set according to the specification requirements. Before the experiment, the couplant was also applied to the measuring position on the surface of the test piece, and the ultrasonic wave velocity of the test piece was measured using the counter measurement method.The macroscopic mechanical properties of concrete, such as the bond strength and bond slip between the steel bar and concrete, are determined by its internal microstructure. The internal pore structure of concrete, in particular the porosity and pore size distribution, affects the macroscopic mechanical properties of concrete. After the central pullout test, representative samples with a thickness, length and width of 1 mm, 3 mm and 3 mm, respectively, were collected from the bonding area between the reinforcement and the concrete using a diamond saw. A JEOL JSM 7500F scanning electron microscope (SEM) was used to analyze the microstructural arrangements of the C-S-H and C–H gels and other hydrated products in concrete under the joint actions of the FTCs and SBC. Before the SEM test, the concrete samples were sprayed with high-pressure gold to obtain high-quality images. The images were processed and analyzed based on different grey level values using Image-Pro Plus V6.0  ,31 to deUnder the designed FTC load, loading tests of the pullout specimens with different rates of SBC were conducted, and the entire bond failure process was studied. Two bond failure modes, i.e., the splitting and pullout failures, were noted. The pullout test results showed that, before 100 FTCs, the specimens suffered total split failure, the splitting failure shown in During the FTC process, the concrete surface gradually transformed from compact to porous, which induced a decrease in the mass of the specimens. The mass loss can be used to quantify the spalling of concrete after FTCs. The corrosion products caused by SBC also led to changes in the quality of the specimens. To study the effects of different FTC and SBC rates on the mass loss rate, the effects of the FTCs on the mass loss rate under the same corrosion degree were compared, and the relationships between the material quality and corrosion rate with the same number of FTCs were also compared.In For the uncorroded experimental group, the mass loss rate was negative before 50 FTCs. This is because, at the beginning of the FTCs, the microcracks caused by the freeze-thaw damage do not cause the concrete to peel off but allow water to enter the concrete and increase the total mass. After 100 FTCs, the mass loss rates of F100, F150 and F200 concrete specimens were 3.32%, 5.21% and 5.66%, respectively. It can be seen that the mass loss rate of the specimens increased as the FTCs increased. It can also be concluded from τ can be calculated as follows:τ is the average bond strength  and P is the pullout force of the steel bar . In Newtons, d is the diameter of the steel bar (in millimeters) and P, bond strength τ and bond slip s corresponding to bond strength were calculated. Bond stress τ is defined as the average shear stress along the buried length between the steel bar and the surrounding concrete. Based on the pullout test results, the bond stress of the steel bars in the specimens was constant with the anchorage length. Hence, an average bond strength A comparison of the 0 FTC specimens showed that the bond strength loss of all the specimens could be obtained. The bond slip behavior is the main evaluative indicator of the structural safety of reinforced concrete. The bond slip behavior of the concrete specimens under the joint actions of the FTCs and SBC is shown in According to the abovementioned macroscopic test results, FTCs and SBC have significant effects on the bond properties of reinforced concrete. Nine experimental groups with FTCs  and SBC rates  were selected for the comparison of the scanning electron microscope images before and after corrosion in order to further explore the influences of FTCs and SBC on the bond performance between steel bars and concrete from a microscopic point of view. The pore structures of the specimens with the three corrosion rates subjected to the actions of the FTCs are shown in 2O3 products. Because of the large volume of Fe2O3 and the expansion force acting on the surrounding cement materials, microcracks were observed in the structure, with an increased porosity. Generally speaking, the microscopic properties of concrete changed with increasing freeze–thaw cycles, and the mechanical properties of the concrete matrix decreased, leading to a poor bonding performance. With a similar corrosion rate, the structure became continuously looser as the FTCs increased, indicating that the damage to the microstructure caused by FTCs provides a channel for the transport of water and ions and accelerates the formation of corrosion products. In summary, under the combined actions of the FTCs and SBC, the deterioration process of the concrete involved the expansion of a large number of macropores and microcracks. With the development and expansion of microcracks caused by the joint actions of the FTCs and SBC, the internal structure of the concrete specimens was no longer compact. Therefore, the periodic FTCs and excessive corrosion rates produced a large number of harmful pores in the concrete, which greatly reduced its durability. Additionally, the ultrasonic wave velocity and mechanical properties of the concrete gradually decreased.The 0, 100 and 200 FTC samples were selected for the analysis of the change law of the porosity close to the steel bar at different SBC rates combined with FTC in order to further quantify the relationship between the microstructure and macroscopic properties. In addition, the influences of the FTC and SBC rates on the bond strength was explained from a microscopic point of view. The relationship between the corrosion rate and porosity after the FTCs is shown in The pores in concrete are formed by cement hydration, and they are distributed in the cement mortar. The porosity, PSD and pore connectivity affect the transport properties of water and chloride ions in concrete, as well as its mechanical properties. Of these, PSD is the best method for evaluating the comprehensive properties of hydrated cement paste. Under the actions of 0, 100 and 200 FTCs, the specimens with the three corrosion rates of 0%, 3% and 12% were selected as samples for the PSD analysis.Based on a fitting analysis of the meso- and macroscopic parameters of the concrete specimens with different reinforcement corrosion rates under freeze-thaw conditions, the degradation mechanism of the bond strength under the combined actions of the FTCs and SBC was considered. The relationship between the pore structure and bond strength was evaluated, and the influenced of the FTCs and SBC on the bond strength of the concrete is discussed below.Chloride penetration is the major cause of steel corrosion in reinforced concrete. From the abovementioned micromorphology analyses combined with the macroscopic strength degradation law, on the one hand, we can observe that FTCs change the internal structure of concrete and the chemical adsorption between the cement gel and steel bar surface, and may weaken the friction between the concrete and steel bar interface, which reduces the bond strength between the concrete and steel bar. On the other hand, the increase in the number of macropores and microcracks caused by FTCs provides more channels for chloride ion penetration and accelerates the steel bar corrosion. With increasing volume, the corrosion products produce a squeezing pressure on the concrete around the steel bar, which causes the cracks to expand further. The FTCs not only increase the most probable pore diameter, but also increases the transport performance of the water and chloride ions in the concrete, making it easier for external oxygen to come into contact with the steel bar. Under the combined actions of FTCs and SBC, the bond strength between the steel bar and concrete decreases, and the bond performance deteriorates.At the microlevel, freeze-thaw damage changes the number of connected holes and water transport performance in concrete. These pores affect the microstructure distribution of the concrete and accelerate the migration of the pore water. In addition, more water becomes ice, and volume expansion produces tensile stress, resulting in new microcracks. Thus, the bond strength is reduced. The generation of new cracks creates favorable conditions for corrosion and produces more corrosion products, while the corrosion products of the volume expansion also produce tensile stress on the concrete, and the damage of the FTCs and SBC on the bond strength of the concrete therefore occurs as a cumulative effect.The relationship between the bond strength and pore structure of the specimens with different corrosion rates under the action of the FTCs was analyzed in order to study the effect of the FTCs on the bond strength between the steel bars and concrete. The relationship between the bond strength and porosity of concrete specimens with different corrosion rates under the action of the FTCs is shown in Considering that the most probable pore diameter represents the greatest degree of total porosity, n is the number of FTCs divided by 100, P is the porosity of concrete after the FTCs and C is the SBC rate. Equation (2) shows that the porosity decreases as the FTC and SBC rates increase. Substituting Equation (2) into the model of the relationship between the bond strength and porosity in τ is the average bond strength . Substituting the data measured in the laboratory into Equation (3), the results are consistent, and the error is small, which shows that the model can effectively predict the damage rate of the bond strength according to the number of FTCs and SBC rates. The bond strength damage model also shows that the effect of the FTCs on the bond strength is slightly greater than that of the SBC rate.According to the above analysis, the porosity of concrete is related to the number of FTCs and the SBC rate. Based on multiple regression analysis, the model of the relationship between the porosity and the number of FTCs and the SBC rate was established, and it can be expressed as follows:r is Pearson′s coefficient of correlation, r” lies between +1 and −1. Positive (+) values of “r” mean that there is a “direct correlation,” and negative (−) values of “r” mean that there is an “inverse correlation”. The Pearson model was used to analyze the bond strength, ultrasonic velocity and porosity data under different FTC and SBC rates in order to judge their compactness and study the relationship between the number of FTCs and SBC on macro-strength and micro-index. The results of the correlation analysis are shown in Correlation refers to the degree and direction in which two variables are related. Pearson′s relationship coefficient is the covariance of two factors separated by the result of their standard deviations , and it The results in (1)The effect of FTCs on corrosion is a process varying from micro-quantitative alterations to macro-qualitative alterations. The micro-damage of the cement paste caused by FTCs is cumulative and reduces the bond strength directly. Moreover, the micro-damage can also increase the transport process of ions and, then, accelerate the corrosion.(2)The corrosion products, at a lower corrosion rate (≤3%), fill a portion of the pores of the concrete and increase the friction between the steel bar and concrete. Then, it can alleviate the damage caused by 50 FTCs to an extent. The bond slip decreases as the corrosion rate increases but is almost unchanged when the corrosion rate exceeds 8%.(3)The relationship between the macro-mechanical properties and micro-pore structure shows that freeze-thaw damage increases the porosity and the most probable pore diameter of concrete, leads to a decrease in the concrete strength and reduces the bond strength. Freeze-thaw can also accelerate the corrosion rate, reducing the bond.(4)The damage model of the bond strength shows that the bond strength decreases with the increasing number of FTCs and SBC rates, and the effect of the FTCs on the bond strength is slightly greater than that of the SBC rate. The results of the correlation analysis also confirm that the effect of FTCs on the bond strength is greater than that of SBC rate.Through the accelerated corrosion testing of different steel bar corrosion (SBC) rates under freeze–thaw cycle (FTCs) conditions, the effects of FTCs and SBC on the bond strength between the steel bar and concrete were studied, and the evolution rules of the effects of the FTCs and SBC on the macroscopic strength and concrete microstructure were obtained. The main conclusions are summarized as follows:


Fundamental knowledge of the processes of cement gel formation for new generation concretes is a scientific deficit. Studies aimed at the formation of a cement gel for standard vibrated concrete research, and especially for centrifugally compacted concrete, are of interest because the structure of this concrete differs significantly from the structure of standard vibrated concrete. This article aims to study the fundamental dependencies of the theoretical and practical values that occur during compaction using vibration, as well as the centrifugal force of new emerging concrete structures. New theoretical findings about the processes of cement gel formation for three technologies were developed: vibrating, centrifuging, and vibrocentrifuging of concrete; the fundamental difference in gel formation has been determined, the main physical and chemical processes were described, and a significant effect of technology on the gel formation process was established. The influence of indirect characteristics based on the processes of cement gel formation, rheological properties of concrete mixtures, water squeezing processes, and the ratio between the liquid and solid phases in the mixture was evaluated. The process of formation of cement gel for centrifugally compacted cement systems was studied and graphical dependences were constructed, giving answers to the mechanism of interaction according to the principle “composition-rheological characteristics-structure-properties of concrete”. The quantitative aspect of the achieved result is expressed in the increase in the indicators demonstrated by centrifuged and especially vibrocentrifuged samples compared to vibrated ones. Additionally, in terms of strength indicators, vibrocentrifuged samples demonstrated an increase from 22% to 32%, depending on the type of strength, and the rheological characteristics of concrete mixes differed by 80% and 300% in terms of delamination. Fundamental knowledge about the processes of cement gel formation for new generation concretes is a scientific deficit. These ideas are not sufficiently developed in the existing scientific literature and are mainly based on theories of the hardening of binder cement systems and other components of the concrete mix, developed in the late 19th–early 20th century. Given the fact that the current construction industry receives more and more high-strength, structured concrete reaching classes above B100, a significant revision is required not only of engineering and technical parameters that are laid down when creating new knowledge about building materials, but also a revision of fundamental processes occurring not only at the macro but also at the micro level ,4,5,6.The influence of various factors on the formation of cement gel and the microstructure of the hardened cement paste are presented in Microstructural studies and modeling methods are especially important in studying the processes of cement gel formation and allow them to be controlled to improve and predict the characteristics of cement composites ,24,25,26As is known, in the initial period when cement is mixed with water, calcium hydroxide is released during the hydrolysis of tricalcium silicate, forming a supersaturated solution. This solution contains sulfate ions, alkali hydroxide, and a part of silica, alumina, and iron. A high concentration of calcium ions and sulfate ions is observed for a short time after mixing the cement with water since within a few minutes the first neoplasms—calcium hydroxide and ettringite—begin to precipitate from the solution.Approximately an hour later, the second stage of hydration occurs, which is characterized by the formation of very fine calcium hydrosilicates. Since only the surface layers of cement grains take part in the reaction, the newly formed hydrated phases, called cement gel, are characterized by very fine granulometry and the cement grain size decreases slightly. Neoplasms first “appear on the surface of cement grains. With an increase in the number of new formations and their packing density, the boundary layer becomes less permeable to water for approximately 2–6 h” ,38.The second stage of delayed hydration is commonly referred to as the “latent or induction period” of cement hydration. During the latent period, the cement paste is a dense suspension, stabilized by the action of flocculation forces. However, the attractive forces between cement particles in water are relatively weak, which can be explained as follows: gel-coated cement grains form a solvate layer around themselves and have a positive E-potential. The combined action of the solvated layer and electric charge prevents direct contact between the grains in contact. At the same time, these grains experience interparticle attraction, at least in some boundary regions. The forces of repulsion and attraction are balanced at some distance from the interface, where the potential energy of the particles is minimal. Cement dough under the influence of these forces acquires coherence and mobility. During the latent period, there is a gradual absorption of water by the surface shells of cement grains; the thickness of the water layers between the grains decreases, and the mobility of the dough and concrete mixture gradually decreases. Osmotic pressure appears in the gel shells. The inner layers of cement grains, reacting with water, tend to expand. As a result, the destruction of the gel shells occurs, the access of water deep into the cement grains is facilitated, and the process of cement hydration is accelerated ,38.This is the third stage of the hydration process. It is characterized by the beginning of the crystallization of calcium hydroxide from a solution. This process is very intense. Since at this stage the number of hydrated phases is relatively small, in the space between the cement particles there is a free growth of thin plates of calcium hydroxide, calcium hydrosilicates, and ettringite in the form of long fibers that are formed simultaneously. The fibers of neoplasms pass through the pores, divide them into smaller ones and create a spatial bond, enhancing the adhesion between the hydrated phases and cement grains. With an increase in the content of hydrated phases, direct contacts arise between them, the number of which increases; the cement paste seizes and a hardened cement paste is formed. The resulting rigid structure is at first very loose, but gradually it becomes denser. New hydrate phases continuously appear in the water-filled pores of this structure. The volume of pores and their sizes decrease, the number of contacts between neoplasms increases, and the gel shells on the cement grains thicken and compact, coalescing into a continuous cement gel with inclusions of unreacted centers of cement grains. Schematically, the process of transformations occurring in the cement–water system during cement hydration is shown in Vibrated, centrifuged, and vibrocentrifuged concretes, including those in structures, differ in their structure, composition, and also in their characteristics ,44,45,46The most traditional way of compacting concrete in the formwork of a reinforced concrete product is by vibrating. When vibrating for a sufficiently long time, the larger and denser aggregate grains sink under the influence of vibration, while the smaller and lighter ones rise, thereby creating differences in the properties and characteristics of concrete along the section height, that is, leading to variatropy. Variatropy in vibrated concrete has the character of segregation, i.e., the separation of the concrete mixture in the process of vibrating and migration of heavy coarse aggregate and other heavy fractions to the lower parts of concrete structures. This segregation, which is the actual variability of concrete over the section, leads to a change in the properties and characteristics of concrete along the height of the section of a linear reinforced concrete element ,38.In centrifuged concrete, variatropy has a different character. Namely, it is expressed in the difference in the characteristics of concrete along the annular section. Variatropia here arises due to the centrifugal compaction of the concrete mixture. Heavy fractions of aggregate and other concrete components migrate to the outer part of the element, and lighter fractions to the inner part of the element. However, such variatropy is more positive than variatropy during vibration as the outer layer over the entire section of the element acquires the strongest characteristics of concrete, which carries the main load ,43,44,45The most interesting, from our point of view, are vibrocentrifuged concretes in which variatropy is the result of the combined action of vibrating and centrifuging and is most pronounced. In such structures, with the strongest and most durable outer layer, the middle layer also has sufficient strength and can bear part of the load. The inner layer here also remains the weakest; however, the enhanced variatropy and the joint operation of the two bearing layers—the outer and middle ones—compensate for the weakness of the inner layer and, in total, lead to an even greater positive effect in the calculation, design, and operation of such structures ,43,45,46The process of forming a cement gel is fundamental to the hardening of concrete, and directly affects the formation of the structure and properties of such concrete. Of course, one of the key roles in this process is played by the composition of concrete. However, our study is not devoted to the influence of the composition, but rather to the concrete mixture compaction technology. This is because technological parameters are no less important at the stage of creating new concrete structures, and these technological parameters in turn depend on fundamental physical and chemical processes that can be controlled when forming new types of material structures. In this regard, it will be interesting to study the formation of a cement gel for standard vibrated concretes, especially for centrifugally compacted concretes, since the structure of such concretes will differ significantly from the structure of standard vibrated concretes. First, this is due to the influence of fundamental centrifugal forces, which create conditions for a fundamentally different process of hydration and the formation of a cement gel by squeezing out the liquid phase and a significant separation between the liquid and solid components in concretes ,40. Of cThus, the main goal of our study is to study the fundamental dependencies of theoretical and practical quantities that arise during compaction with the help of vibration, as well as the centrifugal force of new emerging concrete structures.The objectives of the study will include a detailed literary analysis of existing ideas that are already quite outdated, their development on the example of more advanced concrete structures with higher rates, and finally, the application of the obtained fundamental dependencies in the practical construction industry to establish the relationship between the fundamental processes of cement formation gel using various technologies, and an applied aspect expressed as an increase in the performance of hardened concretes used in modern construction.The experimental research plan is presented in the form of a diagram in For the manufacture of concrete using the technologies of vibration, centrifugation, and vibrocentrifugation; the compositions  were selAccording to the results of studies of the rheological characteristics of concrete mixtures, it was found that the density of a centrifugally compacted concrete mixture is up to 7% higher, the solution separation is 80% less, and the water separation is three times less than that of a vibrated mixture.The results of experimental studies of the physical and mechanical characteristics of vibrated concrete are presented in The results of the experimental studies on the integral and differential characteristics of the layers of variotropic sections of centrifuged and vibrocentrifuged concrete samples are presented in From the results presented in Comparing In addition, According to the results of experimental studies on the mechanical characteristics of centrifuged and vibrocentrifuged concrete, it can be concluded that the integral strength characteristics of vibrocentrifuged concrete are higher than those of centrifuged (by 7–18%) and vibrated concrete (by 22–32%), depending on the type of strength. At the same time, centrifuged concrete has better strength indicators than vibrated concrete. The same trend is typical for density.If we compare the differences in the values of the strength characteristics of layers of centrifuged and vibrocentrifuged concrete, then between the middle and inner layers, the difference is greater for vibrocentrifuged (70–81%) than for centrifuged (48–65%). While the difference between the outer and middle layers of vibrocentrifuged concrete is less (29–36%) than that of centrifuged concrete (45–55%), the differences in strength characteristics between the outer and inner layers of vibrocentrifuged and centrifuged concrete are approximately the same and differs only by a few percent. This indicates the strengthening of the middle layer of vibrocentrifuged concrete in comparison with centrifuged concrete.The differences in the values of the integral strength characteristics of vibrated, centrifuged and vibrocentrifuged concretes and the differential characteristics of centrifuged and vibrocentrifuged concretes are summarized in The calculation results presented in The analysis of the microstructure of the samples is in good agreement with the results of physical and mechanical tests of concrete, confirming the better properties and structure of vibrocentrifuged concrete compared to centrifuged concrete, including in terms of cement gel formation. An analysis of the microstructure also allows us to speak about the improvement of the properties that characterize the durability of vibrocentrifuged concrete in comparison with centrifuged and especially vibrated concrete, due to a decrease in the number and size of filtration channels, pores, and voids, leading to compaction and strengthening of the structure, and a decrease in concrete permeability .To highlight the essence of centrifugal molding and compaction it is necessary to understand when a mold with a uniformly distributed concrete mixture in it rotates around a fixed axis at a certain speed, a compressing centrifugal pressure arises, under the influence of which a liquid with finely dispersed fractions is squeezed out of the cement gel and larger particles approach each other of the solid phase. As a result of this process, the cohesion of the cement gel increases, and the compaction of the concrete mixture is achieved. The “ability of the concrete mixture to be evenly distributed over the inner surface of the rotating mold is determined by the viscoplastic properties of the cement gel. If you place a viscoplastic or short-term vibrationally liquefied concrete mixture into a rotating mold, then under the action of centrifugal radially directed pressure, it will begin to deform - spread in all directions. This will continue until there is an equilibrium between the forces that promote the distribution of the mixture in the mold and the structural compressive strength of the cement gel. With an increase in the rotation speed of the mold, the liquid phase will begin to be squeezed out of the cement gel and this will continue until the compacted concrete mixture acquires the properties of a pseudo-solid body: the deformation of which can manifest itself mainly in the form of an elastic aftereffect” ,40.The “centrifugal pressure is directly perceived by the liquid phase, in connection with which the excess hydrostatic pressure increases, under the influence of which the liquid is filtered and at the same time the concrete mixture is compacted”. In addition, the compressibility of the “cement gel decreases and an equilibrium state” is established, characterized by an internal redistribution of pressure and termination of the filtration process due to significant resistance to the movement of fluid through the pore channels. Pressing pressure, like hydrostatic pressure, varies unevenly across the wall thickness of the product: from a minimum on the inner to a maximum on the outer surface of the product, so the liquid phase cannot be simultaneously squeezed out of the entire thickness of the concrete. First, the liquid is squeezed out more completely from the outer layers of concrete, where the greatest pressing pressures act, and as it approaches the inner surface of the element less and less liquid phase is displaced. From the inner layer, where the pressing pressure is close to zero, the liquid does not separate at all since this is also prevented by the layer of previously squeezed-out liquid. It follows that the mechanism of water separation from the concrete mixture is a hydrodynamic process of fluid filtration from one layer of concrete to another, continuing until all excess fluid is squeezed out of the product and a new balance between internal and external forces is restored. Therefore, different layers of concrete will contain different amounts of the liquid phase. Significant uneven density of centrifugation of the cement gel over the thickness of the product is exacerbated by the presence of directed radial filtration channels, the cross-section and number of which increases from the outer to the inner surface of the element.The cross-section and the number of filtration channels depend on the initial water content in the cement gel: the less free liquid in it, the less it will be squeezed out during centrifugation and the filtration channels will be more dispersed. They may not occur at all if the liquid phase is not squeezed out. This is possible, for example, when vibration and centrifugation are combined in time. With layer-by-layer centrifugation, the number and cross-section of filtration channels are significantly reduced, along with their outlets in each compacted layer of cement gel overlap. If the squeezed liquid is removed after compaction of the first layer, then during compaction of the second layer it will exfoliate mainly only from it. At the same time, additional compaction of the first layer will occur (under the influence of increased pressing pressure from the mass of the two layers) and due to the deformation of the filtration channels, a certain amount of liquid will be displaced from them. Since before the distribution of the second layer, most of the liquid was removed from the first layer, during the compaction of the second layer, a less developed system of filtration channels is formed in it than when both layers are centrifuged simultaneously. The third, fourth, and other layers of cement gel will be compacted similarly. As a result of layer-by-layer compaction, the difference in values (W/C) of the individual layers of cement gel will decrease. With layer-by-layer centrifugation, a less porous (by 10–12%) structure of the hardened cement paste is formed than with single-layer centrifugation. In addition, with a decrease in porosity the pore structure changes, which significantly increases the water resistance of centrifuged concrete ,40.The most important physical parameter determining the duration of the involvement of elementary layers of cement gel in the compaction process and the end of the liquid phase exfoliation at a given pressing pressure is the time of centrifugation of the concrete mixture. As the centrifugation time increases the water separation process continues, but the strength of concrete, as a rule, decreases.The pressing pressure has a similar effect: the greater it is, the faster the liquid phase separates in the initial period; therefore, with an increase in the time of centrifugation beyond the optimal duration, the strength of concrete decreases. The natural decline in the strength of concrete at high values of pressing pressure after some optimal compaction time of the mixture is caused by a change in the properties of the structure of the cement gel during its severe dehydration. With severe dehydration of cement gel in compacted concrete, tensile stresses may occur, leading to the opening of radial cracks in the wall of the product. The crack opening is significantly affected by the elastic aftereffect of the steel split mold and partly by the uneven distribution of centrifugal pressure on the concrete mixture ,40.The uneven density of centrifuged concrete is expressed not only in the structural heterogeneity of the hardened cement paste but also in the nature of the distribution of filler grains along the wall thickness of the product. The larger grains are pressed against the outer surface of the product, and the smaller ones are concentrated closer to its inner surface, so the centrifuged concrete differs from the vibrated one by the heterogeneous distribution of filler grains along the thickness of the product. With a properly selected composition of concrete, a 0.5–2 mm thick sludge crust forms on the inner surface of the element wall, followed by a layer of hardened cement paste, then fine-grained concrete and concrete of ordinary texture. Following this, the distribution of cement along the section of the wall of the product also changes. Such heterogeneity reduces the strength of the centrifuged concrete compared to what it should have been with a uniform distribution of filler grains over the entire section of the element. In concrete, the cement gel fills only the intergranular space, occupying about 1/3 of its volume, so the filtration ducts formed in the cement gel are dispersed and change their direction when they meet the filler grains. In this regard, the length and type of filtration ducts change. Skirting the contact surface between the cement gel and the filler grains, the filtration ducts merge with the solvate layer on the surface of the filler grains and then the ducts form again, breaking through the cement gel layers. This is repeated until the pressed liquid comes out on the inner surface of the product. Consequently, during the centrifugation of the concrete mixture, the liquid is also squeezed out through filtration channels in a cement gel.The main disadvantage of the centrifugal method is the separation of the liquid phase (sludge) and the uneven distribution of components in concrete. The formed filtration channels during single-layer centrifugation significantly increase the permeability of concrete. An additional measure to reduce cement consumption and sludge yield is the use of vibration centrifugation since in this case very little liquid phase is squeezed out and less centrifugal pressure can be used to seal the concrete mixture. If centrifugation is combined with horizontally directed high-frequency vibration, then the initial water content in the cement gel can be reduced and compaction will occur without sludge detachment due to the redistribution of the liquid phase and other known factors ,40.(1)The processes of cement gel formation for three technologies were studied: vibrating, centrifuging, and vibrocentrifuging of concrete. The fundamental difference in gel formation was determined, the main physical and chemical processes were described, and a significant effect of technology on the gel formation process was established.(2)The influence of indirect characteristics based on the processes of cement gel formation, the rheological properties of concrete mixtures, the processes of squeezing water, and the ratio between the liquid and solid phases in the mixture was evaluated.(3)The formation of cement gel for centrifugally compacted cement systems has been studied, and graphical dependencies have been constructed, giving answers to the mechanism of interaction according to the principle “composition-rheological characteristics-structure-properties of concrete”.(4)The quantitative aspect of the result achieved is expressed in the increase in performance demonstrated by centrifuged and especially vibrocentrifuged samples compared to vibrating ones. The rheological characteristics of concrete mixes differed by 80% and 300% in terms of delamination. In terms of strength indicators, vibrocentrifuged samples showed an increase from 22% to 32% compared to those of vibrating and from 7% to 18% compared to those centrifuged, depending on the type of strength. The difference between the middle and inner layers is greater for vibrocentrifuged concrete (70–81%) than for centrifuged concrete (48–65%). The difference between the outer and middle layers of vibrocentrifuged concrete is less (29–36%) than that of centrifuged concrete (45–55%). This indicates the strengthening of the middle layer of vibrocentrifuged concrete in comparison with centrifuged concrete.(5)Analysis of the microstructure showed that the vibrocentrifuged hardened cement paste has a more solid structure with a smaller number and size of voids, pores, and filtration channels than centrifuged.Prospects for the development of the study are seen in the continuation and deepening of the study of the processes of cement gel formation for other compaction technologies, particularly vibrohydraulic pressing, which make it possible to obtain fundamentally different rheological characteristics of the concrete mixture, the processes occurring at the “liquid-solid” phase boundary, and the final impact of the ongoing fundamental processes on applied properties of the resulting concretes with the creation of an empirical and theoretical basis for the practical industry.The main characteristics of Portland cement CEM I 52.5  are presented in 3; true density—2650 kg/m3”.The used quartz sand produced by Arkhipovsky Quarry OJSC  had the following characteristics: “fineness modulus—1.66; the content of dust and clay particles is 1.2%; bulk density—1438 kg/m3; true density—2620 kg/m3; crushability—11.4% by weight; the content of lamellar (flaky) and acicular grains is 8.1% by mass; voidness—43%”.The used granite crushed stone produced by Pavlovsknerud JSC  had the following characteristics: “fraction size—5–20 mm; bulk density—1503 kg/mappointment of a nominal composition based on the calculation, experimental verification, and adjustment of the initial compositions;appointment of the working composition of concrete;adjustment of the working composition of concrete upon receipt of batches of materials of the same types, grades, and classes that were taken in the selection of the nominal composition, but taking into account their actual quality;checking in laboratory and production conditions of working compositions after their correction .Selection of the composition of the concrete mixture for vibrated (V) concrete in accordance with the requirements of GOST 27006 “Concretes. Rules for mix proposing”  include:The selection of the composition of the concrete mixture for the manufacture of centrifuged (C) and vibrocentrifuged (VC) concretes was carried out in accordance with the recommendations of VSN-1-90 “Fabrication Requirements for Centrifugal-Cast Support Pillars for Contact Lines, Communications Lines, and Lock-Out Systems”.W is the water content  in 1 m3 of concrete, l; C is cement consumption per 1 m3 of concrete, kg; The approximate value of the required residual water–cement ratio in the calculation and selection of the composition of centrifuged concrete was determined by the formula3 of concrete is calculated by the formula:3 depending on the type of plasticizer used.The required cement content per 1 mA and kg, respectively, were determined by the formulasThe absolute volume of the cement paste r is the mass ratio between sand and gravel.Here CS, kg, sand, S, and kg was determined by the formulasThe weight consumption of crushed stone The workability of concrete mixtures for the manufacture of concrete by various technologies was evaluated by the draft of the cone. The cone was installed on a smooth sheet and filled with concrete mixture through a funnel in three layers of the same height. Each layer was sealed by bayoneting with a metal rod 25 times. Then the cone was smoothly removed from the molded concrete mixture in a strictly vertical direction and installed next to it. Cone slump was determined by placing a smooth rod on top of the cone and measuring the distance from the lower surface of the rod to the surface of the GOST 10181 “Concrete mixtures. Methods of testing” .The delamination of the concrete mixture was evaluated by indicators of solution separation and water separation. The mortar “separation of a concrete mixture with coarse aggregate, which characterizes its delamination under dynamic action, was determined by comparing the content of the mortar component in the lower and upper parts of the concrete mixture, compacted in a measuring vessel or form”. Determination of the solution separation of the concrete mixture was carried out as follows. The concrete mixture was poured into a measuring vessel, compacted, and vibrated for 25 s; “After additional vibrating, the top layer of the concrete mixture with a height of about half the height of the measuring vessel was taken onto a pre-weighed baking sheet, and the mixture remaining in the lower part of the measuring vessel (mould) was vibrated until the surface of the mixture was leveled”. Then, “with an error of up to 5 mm, the height of the layer of the mixture remaining in the lower part of the measuring vessel (mould) was measured, and the height of the selected layer of the mixture was calculated”. The mixture remaining in the mold was spread on a second weighed baking sheet. The concrete mixture “divided into two portions from the upper and lower parts of the measuring vessel (mould) was weighed and subjected to wet sieving on a sieve with holes 5 mm in diameter”. Additionally, “During wet sieving, each weighed portion of the mixture, laid out on a sieve, was washed with a stream of clean water until the cement-sand mortar was completely removed from the surface of coarse aggregate grains” .mP, %, is determined by the formula:The solution separation of the concrete mix H is the “height of the measuring vessel”, mm; and The water separation of the concrete mixture “was determined after settling it in a measuring vessel for a certain period of time. The concrete mixture was placed in a measuring vessel and compacted on a vibrating platform for 25 s, then the vessel was covered with a glass sheet and left alone for 2 h. Then, the separated water was taken every 15 min with a pipette, collecting it in a beaker with a lid, and weighing after the end of the test” .3”; 3.The water separation of the concrete mixture 3 was calculated by the formula:m is the mass of the measuring vessel with the concrete mixture, g; V is the capacity of the measuring vessel in cm3.The average density of the concrete mixture Compressive strength, tensile strength in bending, and axial tensile strength were evaluated according to .For the manufacture of centrifuged and vibrocentrifuged concretes, special laboratory facilities were used for the manufacture of samples of this type; schemes of these facilities are presented in ,51. The The technique for manufacturing “prototypes for determining the strength integral and differential characteristics of concrete” is presented in ,42. The Technological and testing equipment and measuring instruments were used the same as in ,53,54,55


Infection after maxillofacial trauma remains an important complication, with a significant socio-economic impact. While consensus exists that systemic antibiotic prophylaxis reduces the risk of infection in the management of maxillofacial fractures, the type, and duration remain controversial. Therefore, the purpose of this scoping review was to provide an overview of the current evidence that supports the use of prophylactic antibiotics in the treatment of maxillofacial fractures. A comprehensive literature search on 1 January 2022, in PubMed, Web of Science, Embase, and Cochrane, revealed 16 articles. Most studies focused on the duration of systemic antibiotic prophylaxis and compared a one-day to a five-day regimen. Included studies showed considerable variability in design and research aims, which rendered them difficult to compare. Furthermore, a variety of antibiotic regimens were used, and most studies had a short follow-up period and unclear outcome parameters. This scoping review demonstrates the lack of well-constructed studies investigating the type and duration of systemic antibiotic prophylaxis in the treatment of maxillofacial trauma. Based on the included articles, prolonging antibiotic prophylaxis over 24 h for surgically treated fractures does not appear to be beneficial. Furthermore, there is no evidence for its use in conservatively treated fractures. These results should be interpreted with caution since all included studies had limitations. Infection after maxillofacial trauma remains an important complication , leadingSystemic antibiotic prophylaxis is an accepted strategy to prevent infection in daily clinical practice . HoweverContemporary guidelines state that antibiotic prophylaxis should be provided for no longer than 24 h . HoweverThe purpose of this scoping review was to investigate the association between the type and duration of systemic antibiotic prophylaxis and infection in the treatment of maxillofacial fractures. Furthermore, this study provides an overview of the currently available evidence with respect to this topic. Moreover, the aim is to identify the bottlenecks that prevent the maxillofacial community from adhering to guidelines and illustrate the implications of antibiotic overuse.All aspects of the five-steps methodology proposed by the Joanna Briggs Institute (JBI) guidance were followed and the study was written according to the Preferred Reporting Items for Systematic Reviews and the Meta-Analyses extension for Scoping Reviews (PRISMA-Scr) guidelines .The search strategy was peer-reviewed by a certified librarian. A comprehensive search was performed on 1 January 2022, in the PubMed, Web of Science, Embase, and Cochrane databases. The main search concepts were maxillofacial trauma and antibiotic prophylaxis. No date restrictions were applied. Search strings for each database are provided as To be included, published studies were required to present original data with the primary aim to investigate the association between systemic antibiotic prophylaxis  and infection, in both surgically and non-surgically treated patients with maxillofacial fractures. There were no restrictions related to the type or timing of the surgical intervention or the type or class of antibiotics. Furthermore, studies were only included when a comparison group was provided and when the antibiotic prophylaxis duration was clearly described. Moreover, studies needed to clearly describe which fractures were treated surgically and which were treated non-surgically. Exclusion criteria were articles that focused on basilar skull fractures and non-traumatic fractures . In addition, case reports, published abstracts, conference posters, letters, articles in languages other than English, and articles of which the full text was not available, were excluded.References were collected in EndNote and duplicates were removed. Two authors (FG and MV) independently screened the titles and abstracts of references identified in the search. In case of disagreement, a third reviewer (RC) was available for an additional discussion. After the selection of the title and abstract, potentially relevant articles were included for full-text screening. A PRISMA flow diagram provides an overview of the selection process and the number of papers retrieved and excluded .Two authors (FG and MV) independently extracted data from the selected studies and recorded the data for each included study in the ‘characteristics of included studies’ tables . An asseOverall, 7180 references were collected in EndNote. After the exclusion of duplicates, we retrieved 5480 articles. Based on the title and abstract, 5253 articles were excluded. Of the remaining 227 articles, 16 were included in this scoping review , 13 of wUpper and lower facial fractures were discussed separately because significantly more infections are seen in fractures of the tooth-bearing area of the mandible, which is frequently argued to be due to the proximity to the oral cavity . Table 1In total, we included ten randomized controlled trials (RCT) ,34,35,36Included studies showed a considerable variability in their design and research aims, which rendered them difficult to compare. Chole et al. compared antibiotic prophylaxis of one hour preoperatively and eight hours postoperatively, to no antibiotic prophylaxis at all . Campos Infection rates varied from 0% to 43.9% for all included studies in this review ,36,37,38Infection rates for mandibular fractures varied from 3.3% to 43.9% . None ofInfection rates for fractures of the midface and upper facial third varied from 0% to 12.5% . None ofAll included studies administered β-lactam antibiotics. Nine studies used cephalosporins, including first , second (cefotetan), third , and fourth (cefepime) generation cephalosporins ,30,33,37Furthermore, four retrospective cohort studies used a wide variety of antibiotics or combinations, without specifying the reasoning for administering different antibiotic types to different patients ,29,33,37Since none of the included studies compared the different types of antibiotics, and data on pathogens identified by culture at the time of the diagnosis of infection were lacking across studies, no statement could be made about the most suitable antibiotic type used for antibiotic prophylaxis in maxillofacial fracture treatment.Infection was the primary outcome measure in all included studies. Lovato et al. and Reiss et al. failed to describe the applied criteria for infection ,33. For Follow-up periods varied widely and ranged from less than one week to six months, with a median follow-up of six weeks ,36,37,38This scoping review included ten RCTs and six RCS investigating the effect of systemic antibiotic prophylaxis duration on the infection rate after treatment of maxillofacial fractures ,36,37,38Based on the included studies, prolonged antibiotic prophylaxis does not appear to be beneficial in the prevention of infection. None of the studies comparing an antibiotic prophylaxis duration of up to 24 h with longer durations found a significant difference in infection rates ,36,37,38The results of this scoping review are consistent with the most recent recommendations, which support discontinuing antibiotic prophylaxis after 24 h for all fracture types . For manFor conservatively treated fractures of the middle and upper facial third, there is no evidence for the use of systemic antibiotic prophylaxis at all .Our initial literature search revealed 29 studies on the type and duration of systemic antibiotic prophylaxis with infection as the primary outcome measure ,49,50,51Eleven of the studies in this scoping review included patients under the age of 18 years ,34,35,36Although all studies administered β-lactam antibiotics, the antibiotic agents that were prescribed varied widely. In retrospective studies, a wide variety of antibiotics or combinations was often administered without specifying the reasoning for administering the different antibiotic types to different patients ,29,33,37Furthermore, the duration of antibiotic prophylaxis varied and none of the studies had the same research question. Of the included prospective studies, one administered antibiotic prophylaxis one hour preoperatively and eight hours postoperatively , one evaStaphylococcal species) [Streptococcal species; oropharyngeal anaerobes ), broad-spectrum antibiotics may be indicated  [The selection of prophylactic antibiotics for infection prevention should consider the susceptibility of potential pathogens to these antibiotics . Therefospecies) . In maxiidazole) . Data onThe gold standard for a diagnosis of infection remains the deep tissue cultures, obtained from intraoperative samples . Data onStreptococcal species—which could be a possible cause of their insignificant study results [Included studies showed considerable variability in their antibiotic type, with varying specificity against the presumed bacterial flora. This is a significant limitation, as the use of an effective antibiotic agent is essential to validate study outcomes. Abubaker et al. administered penicillin G and penicillin VK—narrow-spectrum antibiotics that only cover  results . Further results ,29,33,37Future trials should thoroughly describe the causal pathogens related to infection after maxillofacial trauma, identified by culture. High quality, uncontaminated, deep tissue and implant samples are essential to validate culture outcomes . FurtherFive included studies on mandibular fractures failed to report whether they included open or closed fractures ,31,32,34Fracture stability is of the utmost importance in the prevention of infection. Instability leads to ongoing soft-tissue trauma, interruption of neo-vascularity and osteolysis of bone, which creates a supportive environment for bacterial proliferation . HoweverSurgical treatment of mandibular fractures is performed using ORIF or CR and maxillomandibular fixation (MMF). Standard MMF methods are either tooth-supported , or bone-supported devices such as intermaxillary fixation (IMF) screws . Out of Only one study on non-surgically treated maxillofacial fractures met our inclusion criteria . The stuThe lack of a uniform definition for infection after maxillofacial trauma also contributed to the scarcity of comparable data. Using inadequate outcome parameters risks underestimating or overestimating the actual number of complications, resulting in misleading study conclusions . The absTo date, the term FRI has not been used to describe infection following maxillofacial trauma. Although general treatment principles may differ, and significant differences exist with respect to the blood supply at the fracture site and the bacterial flora present at the site of injury, the basic diagnostic principles are similar. The definition of FRI is based on clinical, laboratory and radiological features that confirm or exclude the presence of infection. The described confirmatory and suggestive criteria could also apply to maxillofacial trauma and possibly be utilized to diagnose (and define) infection . The terIn this review, follow-up periods were mostly short with only four studies reaching six months ,34,35,36To the best of our knowledge, 11 systematic reviews on systemic antibiotic prophylaxis in maxillofacial fracture treatment were published over the past 15 years ,15,16,17Recent surveys have confirmed that surgeons do not always adhere to these recommendations, and a high percentage still prolong the use of antibiotic prophylaxis ,17. A suThe question arises as to why the above-mentioned recommendations have not yet been implemented in our daily clinical practice. Existing guidelines are mainly based on only a handful of clinical studies which still leads to uncertainty regarding the optimal type and duration of systemic antibiotic prophylaxis. Therefore, worldwide heterogeneity in prevention protocols still exists. Because of the microbiome present in the oral cavity and sinuses, maxillofacial surgeons fear contamination of the fracture site with commensal flora leading to infection  and intuClostridioides difficile infection, nausea, vomiting and abdominal pain, malaise, and fatigue [Antibiotics are a worldwide leading cause of adverse drug reactions and emergency department visits . Prolong fatigue ,73.Furthermore, prolonged antibiotic prophylaxis could possibly contribute to delayed infectious complications. Zosa et al. described a significantly increased infection risk for patients receiving prolonged antibiotic prophylaxis over seven days , and MilFinally, the overuse and improper use of antibiotics are considered important drivers for the emergence and spread of AMR . AMR occThe main limitation of this scoping review is the limited quality and heterogeneity of evidence available in the literature. In our opinion, none of the current studies provides sufficient evidence to serve as a starting point for future RCTs. Variability in study design, antibiotic type and duration, treatment, fracture type and location, renders existing studies difficult to evaluate and compare. Outcome measurements were mostly determined using unclear criteria and inadequate follow-up periods. Therefore, this review only provides limited evidence, and the results should be interpreted with caution.This scoping review demonstrates the lack of well-constructed studies investigating the association between the type and duration of systemic antibiotic prophylaxis and infection in the treatment of maxillofacial fractures. All current available studies with respect to this topic showed a significant risk of bias. The considerable variability in antibiotic duration seen throughout the literature highlights the need for further research and guidelines for the use of antibiotic prophylaxis in maxillofacial trauma. Based on currently available evidence, shortening the duration of antibiotic prophylaxis to one day or less for all operatively treated facial fractures should be sufficient. Furthermore, there is no evidence for the use of systemic antibiotic prophylaxis in conservatively treated fractures. In fact, overprescribing practices may contribute to increased long-term infectious complications and AMR. Furthermore, although multiple guidelines have been developed to prevent infections after maxillofacial trauma over recent years, worldwide heterogeneity still exists with respect to the implementation of these guidelines in our daily clinical practice. High-quality studies are needed to clarify the optimal duration of antibiotic prophylaxis in the management of maxillofacial fractures.