The disposal of industrial by-product tailings has become an important issue in solving environmental pollution. In this study, 15%, 30%, 50%, and 70% iron tailings were used to replace the natural sand in concrete, and 1.5% steel fiber and 0–0.75% PVA fibers were added to the iron tailings concrete. The effects of the iron tailings replacement rate and the fiber content on the mechanical properties, carbonization depth, and concrete porosity were studied in a carbonization environment. The results demonstrated that the compressive and splitting tensile strengths of concrete first increased and subsequently decreased with an increase in the iron tailings replacement rate, while the carbonation depth and porosity initially decreased and subsequently increased. When the replacement rate of iron tailings was 30%, the compressive strength and split tensile strength were increased by 7.6% and 17.7%, respectively, and the porosity was reduced by 8.9%. The compressive strength, carbonation depth and porosity of single-doped steel-fiber concrete were superior to those of ordinary iron tailings concrete. However, compared with single-doped steel fiber, the performance of steel-PVA fiber was further improved. Based on the mechanical properties, the carbonation depth test results of the three aforementioned types of concrete, the mathematical expression of the uniaxial compression stress–strain curve of iron tailings concrete, and the prediction equation of the carbonation depth of mixed-fiber iron tailings concrete were proposed. This study provides a reference for the application and popularization of fiber-reinforced iron tailings concrete in carbonization environments. With the rapid development of the steel industry and other industries, China produced 870 million tons of iron ore in 2020 (up 3.7% from the previous year), generating a large amount of iron tailings as a by-product. However, the utilization rate of iron tailings is very low, and only approximately 7% of them are recycled . The proAil  replacedAs an engineering material commonly used in building structures, ordinary concrete exhibits a high compressive strength but low tensile strength, ductility, and toughness, which are easily affected by the environment. To solve this problem, fiber-reinforced concrete has been widely examined. By adding fibers to the concrete matrix, it is possible to effectively control and delay the generation and expansion of cracks and increase its strength, ductility, and durability . On the Exposing concrete structures to a natural or harsh environment for a long time strongly affects their safety characteristics and usability; therefore, the durability of building structures is a very important parameter. Concrete carbonation is a common form of concrete deterioration. Although carbonation produces no adverse effects on concrete strength and may even increase its value, it ultimately causes the shrinkage and cracking of the concrete matrix, reduces concrete ductility, and leads to the destruction of the concrete structure and a reduction in its service life ,17.The main purpose of this paper was to study the effects of different iron tailings and fiber replacement rates on the mechanical properties and carbonization properties of concrete. The compressive strength, splitting tensile strength, carbonation depth, and porosity of iron tailings concrete and fiber-reinforced iron tailings concrete were evaluated. In order to protect the ecological environment and waste utilization, an appropriate amount of iron tail sand was used to replace natural sand to prepare iron tail sand concrete, and an appropriate amount of mixed fiber was added to enhance the mechanical properties and durability of the iron tail sand concrete in a carbonization environment to provide support for the engineering application of this material. The obtained results can help elucidate the carbonation mechanisms of iron tailings concrete and mixed-fiber concrete and provide a theoretical basis for the application of iron tailings and fibers in concrete preparation.The cementitious materials used in this test were Qinling ordinary Portland cement and fly ash. The strength grade was PO42.5R. The chemical compositions and physical properties of the materials are presented in The aggregates consisted of coarse aggregates and fine aggregates. The coarse aggregates were obtained from the crushed stone produced in the Xi ‘an area, with a particle size range of 5–25 mm (continuous gradation). The fine aggregates were composed of Chanhe natural sand and iron tailings, and their main components were analyzed by  scanning electron microscopy and  X-ray diffraction. The fibers used in this study included steel and PVA fibers, whose appearance and physical properties are shown in According to the replacement rates of the iron tailings and the fiber content when divided into ordinary concrete, iron tailings concrete, steel-fiber concrete, and steel–polyvinyl alcohol (PVA) fiber concrete, the substitution rates of iron tailings were 0%, 15%, 30%, 50%, and 70%. Research shows ,19,20 thThis paper refers to the specification requirements of the General Concrete Mix Design Code (JGJ55-2011)  after thFor ordinary concrete and iron tailings concrete, the specific preparation process was as follows: First, the quantitative coarse aggregate and fine aggregate were poured into the mixer and mixed dryly for 1 min. At this time, the coarse aggregate and fine aggregate were roughly evenly mixed. Second, the quantitative cement was slowly poured into the mixer and stirred for 1 min. Then, an appropriate amount of water-reducing agent was poured into a water container, evenly mixed with water, poured into a mixer, and stirred at a low speed for 2 min.In the fiber-containing group of specimens, only when the fiber was uniformly dispersed in the concrete matrix could the bridging crack resistance of the fiber be fully exerted; the knotting and clumping of fibers were avoided as much as possible. Therefore, this test used a dry mixing after wet mixing method. The preparation process of the fiber-containing concrete consisted of the following steps: First, coarse aggregates, fine aggregates, and cement were poured into the mixer for 2 min of dry stirring. Afterwards, the manually rubbed and scattered fibers were slowly and evenly distributed in the resultant mix: first steel fibers (3 min of agitation) and then PVA fibers (2 min of mixing). Finally, water was mixed with the water-reducing agent and stirred for 2 min.The mechanical properties of the prepared concrete specimens were tested in accordance with the Standard Test Method for Mechanical Properties of Ordinary Concrete (GB/T50081-2019)  and the Rapid carbonation was performed using the carbonization test chamber depicted in 2 spectrum, porosity, and pore size distribution. NMR primarily monitors the signal produced by hydrogen atoms, which can be used for quantitative measurements of the water content. Before the test, the test block was immersed in water with a temperature of 20 ± 2 °C for 3 d to ensure the uniform distribution of water molecules in the concrete structure.A MacroMR12-150H-I  nuclear magnetic resonance (NMR) instrument was used to determine the pore structure of the test block, including its TSEM was performed using a field-emission scanning electron microscope , as shown in The working performance of concrete is an important prerequisite for its excellent mechanical properties and durability . The sluAs shown in There are several reasons for the increase in compressive strength after adding iron tailings: (1) The iron tailings particles are finer than those of natural sand. Appropriate iron tailings fill the pores inside the concrete, optimize the pore structure, and increase the concrete density. 2) The iron tailings material with a rough surface improves the bonding between the cement and aggregate interfaces, thus increasing the concrete strength. Meanwhile, the iron tailings particles with high iron concentrations also increase the concrete strength. However, with an increase in the replacement rate of iron tailings, the concrete strength decreases because iron tailings have a larger specific surface area than natural sand. In addition, the water demand of the concrete mixture increases, and its fluidity decreases, leading to the separation of the mortar and aggregates, which is not conducive to compact forming; as a result, the strength of the iron tailings concrete decreases  The iron.As shown in The observed strength improvement in the fiber-reinforced concrete was mainly due to the increase in its crack resistance and toughening. The effect of introduced fibers on crack resistance mainly depends on their distribution in the concrete matrix. When fibers are evenly distributed in the matrix, they are strongly bonded to the slurry. When a crack expands, it can be quickly blocked by the adjacent steel fibers, preventing crack development and thereby increasing the crack resistance of the concrete specimen. The increased concrete toughness is due to the fiber’s bridging–debonding–sliding failure mode, which promotes energy dissipation. Moreover, the hydroxyl groups attached to the molecular chain of PVA fibers form strong bonds between the fibers and the concrete matrix. A large number of small PVA fibers increase their contact area with the concrete matrix, which facilitates stress transfer through cracks. In addition, they form a complex three-dimensional fiber network with a disordered distribution of steel fibers. Steel fibers inhibit the formation of large cracks in concrete, and PVA fibers inhibit early plastic shrinkage, dry shrinkage, and microcrack generation . When thWith an increase in the carbonation age, the concrete compressive strength stably grows. As shown in 2 reacts with Ca(OH)2 to generate water-insoluble calcium carbonate, which effectively fills the pores and microcracks inside the concrete matrix and reduces its porosity. With an increase in the carbonation age, the compactness of the concrete surface increases owing to the formation of dense calcite microcrystals on the surface of the carbonation layer, which inhibits the CO2 diffusion into the concrete matrix, decreasing the diffusion rate. Therefore, the concrete compressive strength first increases with an increase in the carbonation age and then gradually stabilizes. Note that carbonation has little influence on the compressive strength of the fiber-reinforced concrete because the addition of fibers enhances its internal structure and increases the concrete density, suppressing CO2 penetration.After the carbonation of concrete, the chemical composition and pore structure of its matrix changes. COThe splitting tensile strength of concrete first increased and then decreased with an increase in the iron tailings replacement rate, as shown in The tensile strength of concrete is not only an important index for determining its crack resistance but also a key parameter for indirectly measuring the bond strength between concrete components. In this work, the optimal bond strength was achieved at an iron tailings replacement rate of 30%. This concrete had the densest filling structure, resulting in improved bonding between the aggregate and the cement mortar. With an increase in the iron tailings replacement rate, the bonding strength between the aggregates decreased, leading to a decrease in the tensile strength.As shown in The rough surface of iron tailings enhances the bonding of steel fibers; as result, the fibers are difficult to pull out or break. When the concrete is subjected to a large load, tensile stress is transferred to the steel fiber through the concrete due to a bridging effect between the steel fiber and concrete. During the concrete splitting tensile failure, steel fibers absorb a large amount of energy, which compensates for the deficiency of the concrete matrix tension. After adding PVA fibers, the tensile strength of the concrete further increases because of the lower fiber spacing in the concrete structure, which limits the development of primary cracks caused by segregation, shrinkage, and other processes. However, when the PVA fiber content exceeds 0.5%, the concrete tensile strength decreases. Similar to the trend observed for the compressive strength, the excessive addition of PVA fibers induces their agglomeration, leading to an uneven distribution of fibers in the concrete matrix. If the number of fibers at the splitting interface is too high or too low, stress concentration occurs, and the tensile strength is reduced.The tensile strength of concrete always maintains a steady growth trend with increasing carbonation ages. As shown in fsts and fcu are the splitting tensile and compressive strengths, respectively.Based on the literature ,32, the Based on the obtained experimental data, a statistical regression analysis was conducted to derive Equation (2):Different national standards stipulate different calculation relationships between the compressive and splitting tensile strengths of concrete. According to the American Concrete Association standards ACI-363R and ACI-318-11 and the European Concrete Commission (CEB), the corresponding relationships are expressed by Equations (3)–(5), respectively:As shown in Similar to the cube compressive strength and splitting tensile strength, the axial compressive strength of the prepared concrete specimens first increased and then decreased with an increase in the iron tailings replacement rate and PVA fiber content. According to The elastic modulus represents the deformation resistance of the material under stress, that is, the stiffness of the material, while the compressive strength of concrete is achieved by homogenizing the constituent materials and their strength. An appropriate amount of iron tailings increases the compressive strength of concrete; as a result, the ability to resist elastic deformation is enhanced, and the elastic modulus increases.The relationship between the axial compressive strength of concrete and its elastic modulus can be fitted by the least-squares method to obtain the coefficients a = 5521 and b = 0.5 . The valThe stress–strain curve of concrete under compression provides an experimental basis for structural design and nonlinear analysis. In this study, a constitutive relation of the iron tailings concrete was fitted by the standard stress–strain curve equation of ordinary concrete .c/σp, x = εc/εp, and σc and σp are the compressive stress and peak compressive stress, respectively. εc and εp are the compressive strain and peak compressive strain, respectively; a is the ascending stage parameter; and b is the descending stage parameter. The entire stress–strain curve of each group of concrete specimens was fitted with Equations (7) and (8), and the calculation formulas of a and b were obtained by the least-squares method:According to the GB50010–2010 Code for Design of Concrete Structures , the comTo verify the accuracy of the derived stress–strain curve equation and parameter calculation procedure, the fitting data were compared with the test results, as shown in The carbonation depths of the studied concrete specimens are shown in 2, the main hydration product of cement, results in the formation of calcium silicate hydrate and other compounds, thus improving the internal pore structure and concrete density. Furthermore, the water absorption of iron tailings is higher than that of natural sand; hence, iron tailings reduce the relative humidity inside the concrete, inhibiting the CO2 reaction and decreasing the carbonation depth [Because the size of iron tailings particles is small, they can effectively fill the internal pores of concrete and make the concrete more compact; therefore, they increase the concrete carbonation resistance. In addition, iron tailings may potentially exhibit volcanic ash activity. With an increase in the carbonation age, the secondary hydration reaction between iron tailings and Ca(OH)on depth ,35,36.The addition of fibers significantly reduced the concrete carbonation depth, as shown in 2 continues to diffuse, it may lead to the corrosion of steel fibers; the produced rust destroys the bonding between the fibers and mortar, promoting the infiltration and diffusion of CO2. The incorporation of PVA fibers into the concrete matrix further increases its carbonation resistance. Owing to the large density and coarse diameter of steel fibers, their volume fraction in the concrete structure is very large, which weakens the grid effect of these fibers and facilitates CO2 penetration. After adding PVA fibers, their volume decreases. However, PVA fibers are hydrophilic, which promotes the accumulation of a large amount of water on their surfaces, creates favorable conditions for cement hydration, inhibits the early formation of plastic shrinkage and dry shrinkage cracks, reduces the cracking temperature, and increases the crack resistance of the concrete [The carbonation resistance of the fiber-reinforced concrete is mainly affected by the filling of internal pores and a reduction in the number of cracks. After a certain amount of steel fiber is added to the concrete, a complex three-dimensional disordered support system is formed in the matrix, which inhibits aggregate sinking, makes the mortar distribution more uniform, improves the concrete compactness, and enhances the concrete carbonation performance. However, as COconcrete ,38.uf1: compressive strength of the iron tailings concrete before carbonation, MPa;uf2: compressive strength of the steel–PVA fiber iron tailings concrete before carbonation, MPa;t: carbonation time;C: CO2 concentration;β(ρ1): influence coefficient of iron tailings, which is a function of their replacement rate (ρ1);β(ρ2): influence coefficient of steel–PVA fiber iron tailings, which is a function of the PVA fiber content (ρ2).The steel–PVA mixed-fiber iron tailings concrete is a new concrete-based composite material, and its carbonation proceeds similarly to the carbonation of ordinary concrete. In this study, the carbonation depth model of ordinary concrete was modified by considering the iron tailings replacement rate and the volume fractions of steel and PVA fibers. The empirical model assumes that carbonation depth is proportional to the square root of the carbonization time. Smolczyk established a calculation formula for the concrete carbonization depth and compressive strength based on experimental data and assumed that the carbonation depth was proportional to the reciprocal of the square root of the compressive strength. Based on these results, carbonation depth models were derived for the iron tailings concrete and steel–PVA fiber iron tailings (Equations (11) and (12), respectively) ,40:(11)X2 concentration and subjected to an independent variable regression analysis, as shown in Using the laws of mathematical statistics, the carbonation depth was expressed as a function of carbonation time, compressive strength, and COHere, Equations (13) and (14) are substituted into Equations (11) and (12), which are the carbonation depth equations of the iron tailings concrete and steel–PVA fiber iron tailings concrete, respectively:By utilizing Equations (15) and (16), the carbonation depths of different iron tailings concrete and steel–PVA hybrid fiber concrete samples were calculated. The ratios of the calculated and experimental carbonation depth values are listed in The average value of μ = 0.96 and mean square deviation of σ = 0.099 were obtained for the ratio of the calculated carbonation depth of the iron tailings concrete to the experimental value. The average value of μ = 1.02 and mean square deviation of σ = 0.13 were obtained for the ratio of the calculated carbonation depth of the steel-PVA fiber-reinforced concrete to the experimental value, which indicate a good agreement of the developed carbonation model with the experimental data.2 spectrum is related to the pore size, and the signal intensity reflects the number of pores. The T2 spectra of the concrete specimens fabricated at different replacement rates of iron tailings are shown in 2 spectrum is composed of one main and two small peaks. Compared with the control group (NC), the T2 spectrum of the T30 concrete was shifted to the left, and the relaxation time corresponding to the peak value and signal amplitude were decreased. Meanwhile, the T2 spectra of the T50 and T70 concrete specimens were shifted to the right, and their relaxation times corresponding to the peak values and signal amplitude were greater than those of the control group. These results confirm that an appropriate iron tailings replacement rate can reduce the number of concrete pores and increase the concrete density.The relaxation time of a T2 spectral integral area reflects a change in the concrete pore volume. 2 spectral areas and relative fractions of the characteristic peaks obtained at different iron tailings substitution rates. According to A change in the TPorosity is the most commonly used parameter for evaluating the pore characteristics of porous materials. 2 spectrum shifted to the left during the carbonation process, and the signal intensity continuously decreased. After seven days of carbonation, each peak of the T2 curve sharpened and decreased significantly. After 14 d of carbonation, the peaks continued the trend observed after 7 d of carbonation, and the amplitude of each peak value gradually decreased. When the carbonation age reached 56 d, both the second and third peaks were still detected . However, the carbonation effect became stronger with an increase in the carbonation age, and the concrete pore structure was constantly optimized.As shown in 2 spectral areas and relative fractions of the characteristic peaks obtained at different carbonation ages. It shows that the area of the first peak in the T2 spectrum decreased and the fraction of the first peak in each group increased with increasing carbonization age. Furthermore, the areas and relative fractions of the second and third peaks decreased with increasing carbonization age. This indicates that the hydration products obtained during carbonation convert the large pores into small pores in the concrete structure and improve its properties.Concrete porosity typically reflects the carbonation process. As shown in Compressive strength is the most basic mechanical property index of concrete. The fitting equation and regression parameters of the porosity–compressive strength model listed in c is the compressive strength and P is the total porosity of concrete.Here, σTo study the effect of the pore size on the concrete compressive strength, the pores were divided into gel pores (<10 nm), mesopores (10–100 nm), capillary pores, and macropores (>100 nm) ,41. BaseThe theoretical and experimental compressive strengths are compared in The failure zone of concrete under loading is usually located at the interface between the aggregate and cement paste, which is called an interfacial transition zone. This zone is a weak link in the concrete specimen, and cracks are preferentially initiated in the transition zone. As shown in According to the established fiber bonding mechanism, the matrix and fibers in the fiber-reinforced concrete interact with each other through chemical bonding at the interface to jointly bear the applied force. For this concrete type, the mechanical properties of the fiber–cement slurry interfacial transition zone represent a weak link affecting its macroscopic properties. When the concrete is subjected to an external force, shear stress is formed at the interface between the fibers and the matrix owing to the strain difference. This stress is continuously transmitted and weakened along the fiber direction until both the fibers and matrix deform synchronously. Therefore, the bond strength plays a decisive role in the reinforcement and crack resistance of fibers, and the fiber–matrix bond performance mainly depends on the bonding between the fibers and the surrounding materials as well as on the strengths of the materials in the interfacial transition zone. As shown in After carbonation, the plate-like calcium hydroxide and needle-rod ettringite particles disappear, yielding calcium carbonate, as shown in (1)After adding iron tailings to the concrete matrix, its compressive strength, splitting tensile strength, axial compressive strength, and elastic modulus increased. The compressive strength, splitting tensile strength, and axial compressive strength of the T30 iron tailings concrete were 7.6%, 17.7%, and 14.7% higher than those of ordinary concrete, respectively.(2)The addition of fibers further improved the mechanical and carbonation properties of the iron tailings concrete, and the effect of the steel–PVA fiber mixture was stronger than that of the single-doped steel fibers. With an increase in the PVA fiber content, the mechanical properties of the concrete deteriorated, and its carbonation depth decreased. Based on the obtained test results, a mathematical expression was derived for the stress–strain curve of the iron tailings concrete under compression, and the calculated values were in good agreement with the experimental data.(3)With an increase in the carbonation age, the compressive strength, splitting tensile strength, and axial compressive strength of the concrete increased. From the results of the carbonation test and a carbonation depth prediction model previously developed for ordinary concrete, a carbonation depth prediction model for the fiber-reinforced iron tailings concrete was established.(4)2 NMR spectrum was obtained, and the T2 spectral area decreased by 8.9%. The lowest total concrete porosity was achieved at this replacement rate, and its value was negatively correlated with the compressive strength.The replacement rate of iron tailings produced a significant impact on the concrete pore structure. When the replacement rate of iron tailings was 30%, the smallest intensity of the main peak in the T(5)With an increase in the carbonation age, the porosity of various concrete specimens decreased by 7.6%, 10.9%, 13.4%, and 15.1%, and the pore size decreased as well. Considering the existing pore size–compressive strength model, a pore size–compressive strength model was established by conducting a regression analysis of the NMR data.(6)The main purpose of this experiment was to explore the influence of the iron tailings replacement rate and the fiber content on the mechanical properties and carbonation properties of concrete. The test focused on the material properties, and the components are still unclear. In addition, the microstructure of this experiment was only tested by nuclear magnetic resonance and scanning electron microscopy. More advanced and more accurate instruments should be used to comprehensively explore the microstructure of hybrid fiber iron tailings concrete from different levels and angles, such as energy spectrum tests, CT scan tests, thermal analysis tests, etc.In this study, the mechanical and carbonation properties and microstructures of concrete specimens prepared at different iron tailings replacement rates, single-steel fiber-doped concrete, and hybrid steel–PVA fiber reinforced concrete were examined. From the obtained results, the following conclusions have been drawn:


The adjusted odds ratio (OR) values of VA for hypercho lesterolemia, hypertriglyceridemia, mixed hyperlipidemia and low high-density lipoprotein cholesterolemia were 3.283, 3.239, 5.219 and 0.346, respectively (p < 0.01). Meanwhile, significant age interactions affected the relationship between VA and TC, as well as TG and LDL-C (p < 0.01). Conclusion: Serum VA was positively correlated with blood lipids, but these associations were influenced by age. VA was a risk factor for dyslipidemias, such as hypercholesterolemia, hypertriglyceridemia and mixed hyperlipidemia, but was a protective factor for low high-density lipoprotein cholesterolemia.Background: To study the relationship between serum vitamin A (VA) level and blood lipid profiles in children and adolescents aged 6–18 years, as well as the effect of VA on dyslipidemia. Methods: The project adopted a multistage stratified cluster sampling method. The Food Frequency Questionnaire (FFQ) was used to obtain dietary factors data. Blood samples of subjects were taken via venipuncture. Generalized linear models were used to explore the correlation be-tween VA and biochemical indicators, as well as stratified and inter-actions analysis to explore the influence of confounders on these relationships. Generalized linear models were constructed to explore the association between VA and blood lipids. Restricted cubic splines were used to characterize dose–response associations between serum VA and dyslipidemia based on logistic regression. Results: Serum VA was positively correlated with TC, TG and HDL-C ( As a kind of fat-soluble vitamin, vitamin A (VA) is one of the most essential nutrients for maintaining the normal growth and development of children and is closely related to children’s health . AccordiPrevious studies showed that nutritional factors have im-portant influences on dyslipidemia . VA is kGiven that VA plays a complex role in lipid metabolism, further research should be done on lipid metabolism in response to the VA level. At the same time, we need to explore the rela-tionship between children and adolescents of different ages in order to provide effective health intervention strategies to reduce the risk of dyslipidemia and improve cardiovascular health. This study explored the correlations between serum VA level, lipid profile and dyslipidemia among children and adolescents aged 6–18 years. This study also explored the role of age in these associations.This study was based on the survey of “China Nutrition and Health Surveillance (2016–2017)”, which was approved by the Chinese Center for Disease Control and Prevention (China CDC). The project adopted a multi-stage stratified cluster sampling method and selected 13 districts and counties in Shandong Province, namely, Yishui, Shibei, Lingcheng, Lijin, Shouguang, Penglai, Wucheng, Lanshan, Dong’e, Laizhou, Linzi, Dingtao, and Sishui. A total of 26 primary schools, 26 junior high schools, and 13 high schools were included in this survey. A total of 3551 adolescents were incorporated into the study. Inclusion and exclusion criteria: At least 280 children and adolescents aged 6–17 years needed to be surveyed at each monitoring site, including ten classes in primary school grades 1–6, junior one, junior two, senior one and senior two. There were 28 students in one class, half male and half female. Subjects with obvious diseases were excluded. During the investigation process, tablet and computer programs were used to investigate quality control, and subjects who were unable to participate were promptly replaced.This survey was approved by the Ethics Committee of the National Institute for Nutrition and Health and the Chinese Centre for Disease Control and Prevention (number: 201614), and the participants understood all aspects of the informed consent and attended this survey voluntarily.A questionnaire survey, body examination, dietary interview and laboratory test were included in this research. The working groups from the nation and provincial working groups were responsible for the quality control of the survey. The staff at the district and county levels, who were uniformly trained by the local Center for Disease Control and Prevention, were responsible for collecting fasting blood samples and undertaking the interviews and questionnaires.All subjects underwent a physical examination, including routine basic items, such as blood pressure, weight, waist circumference and height measurements. Blood samples of subjects were taken via venipuncture. Serum and plasma samples were separated using the standard rules of the working group and were stored at −86 °C, then sent to the China CDC for a unified quantitative analysis in the laboratory through the cryogenic cold chain.The Food Frequency Questionnaire (FFQ) , designeThe participants were classified into three groups based on WHO criteria ,3 and ChMeanwhile, according to the expert consensus on the prevention and treatment of dyslipidemia among Chinese children and adolescents, dyslipidemias was defined as hypercholesterolemia (TC ≥ 5.18 mmol/L), hypertriglyceridemia (TG ≥ 1.70 mmol/L), mixed hyperlipidemia (TC ≥ 5.18 mmol/L and TG ≥ 1.70 mmol/L) and low high-density lipoprotein cholesterolemia (HDL-C ≤ 1.04 mmol/L) .Statistical analysis was performed using one-way ANOVA and post hoc comparisons involved the SNK test for continuous variables to compare different VA level groups in terms of biochemical indicators and demographic characteristics and the chi-square test for categorical variables. Then, the generalized linear models were used to explore the correlation between VA and biochemical indicators, as well as employed stratified and interactions analysis to explore the influence of confounders on these relationships.p < 0.05 was considered statistically significant.Logistic regression was used to calculate 95% confidence intervals and odds ratios (ORs). Generalized linear models were constructed to explore the association between VA and blood lipids. Model 1 was adjusted for sex . Model 2 was further adjusted for hs-CRP intake (continuous), fat intake (continuous); systolic pressure (continuous), diastolic pressure (continuous), BMI (continuous), hemoglobin (continuous), blood glucose (continuous), blood uric acid (continuous), serum creatinine (continuous), ferritin (continuous), transferrin receptor (continuous), albumin (continuous), total protein (continuous), serum Zn (continuous) and vitamin D (continuous). Model 3 was adjusted for age (continuous) and sex . Model 4 was further adjusted for hs-CRP intake (continuous), fat intake (continuous), systolic pressure (continuous), diastolic pressure (continuous), BMI (continuous), hemoglobin (continuous), blood glucose (continuous), blood uric acid (continuous), serum creatinine (continuous), ferritin (continuous), transferrin receptor (continuous), albumin (continuous), total protein (continuous), serum Zn (continuous) and vitamin D (continuous). Logistic regression analysis was used to evaluate the relationship between dyslipidemias and risk factors. The dependent variable in this model was whether the participants had different kinds of dyslipidemia . Restricted cubic splines were used to characterize dose–response associations between serum VA and dyslipidemia based on logistic regression. Residual confounding was also minimized for confounders using restricted cubic splines (smooth curves). R version 3.6.3 was used to generate graphs. Any p > 0.05). Subjects with a lower level of VA deficiency status had a significantly younger age (p < 0.01).The average age of the participants was 11.4 years and 49.7% were males among the 3551 participants in the study. Meanwhile, 63.8% of the subjects were aged 6–12 years and 36.2% were aged 13–18 years. There were 1.6% of the subjects with serum VA deficiency status (<0.7 μmol/L), 22.2% with edge VA deficiency status (0.7–1.05 μmol/L) and 76.2% with normal VA status (>1.05 μmol/L). As shown in p < 0.01) but higher hs-CRP (p < 0.01) when compared to the subjects in the serum vitamin A normal group . In the group of 13–18-year-olds, subjects with a higher level of VA status had significantly higher TC, TG and HDL-C (p < 0.001) but not LDL-C .As depicted in p < 0.01). Furthermore, hypercholesterolemia was negatively correlated with age  and positively correlated with vitamin D , ferritin  and total protein . Hypertriglyceridemia was negatively associated with vitamin D  and serum creatinine , and positively correlated with BMI , blood uric acid , transferrin receptor , albumin  and serum zinc . Mixed hyperlipidemia was negatively correlated with serum creatinine  and positively correlated with BMI . Low HDL cholesterolemia was negatively correlated with albumin  and positively correlated with BMI , serum uric acid , ferritin  and transferrin receptor .The results of the logistic regression analysis are shown in p < 0.0001). Meanwhile, a significant age interaction affected the relationship between VA and TC, TG and LDL-C (p < 0.01).The generalized linear model  showed tThe results revealed that serum VA was positively correlated with TC, TG, LDL-C and HDL-C levels, and these associations were influenced by age. Serum VA levels were positively correlated with TC, TG, HDL-C and LDL-C in the 6–12-year-olds group, while in the 13–18-year-olds group, serum VA levels were significantly positively correlated with TC, TG and HDL-C. It was shown that the metabolism and storage of VA during some disease progression is related to cholesterol and triglyceride . The stuFurthermore, VA was a risk factor for dyslipidemias, namely, hypercholesterolemia, hypertriglyceridemia and mixed hyperlipidemia, but was a protective factor for low high-density lipoprotein cholesterolemia. Studies showed that the use of a retinoid could cause hyperlipidemia in the treatment of dermatology diseases, where the incidence of hyperlipidemia was more than 30% . This alIn general, the physiological effects of VA were mediated by its metabolite retinoic acid (RA). Various subtypes of RA control gene expression by regulating the activity of RARs and RXRs ,35,36,37The study results also showed that there were 1.6% of the subjects with VA deficiency (<0.7 μmol/L) and 22.2% with edge VA deficiency (0.7–1.05 μmol/L). The overall VA deficiency rate was 23.8%, i.e., almost a quarter of surveyed children present serum VA deficiency. On the basis of the results of nutrition and health monitoring of Chinese residents (2010–2012), the prevalence of VA deficiency in children in China was 7.69% in urban areas and 5.53% in rural areas ,45; thisIn the current study, correlation analyses provided evidence that higher inflammatory parameters, such as C-reactive protein, were significantly associated with reduced plasma VA levels. This finding supported available information showing that reduced plasma levels of VA were often associated with infection and acute inflammatory conditions ,12,53,54The study had some limitations. First, the methods and results data were from a cross-sectional health examination survey. This study can only reveal correlative relationships, not causality. Further studies are needed to combine the effects of VA on mRNA and protein expression of critical enzymes of fatty acid synthesis in vivo and in vitro to find the causal relationship between VA level and blood lipids or the effect of dyslipidemia on the plasma VA level. Second, the study only observed participants aged 6–18 years, which might not reflect the relationship between VA and lipids in the whole population. In view of the above limitations, further prospective, large-sample cohort studies are needed.Despite the above limitations, the present study had several notable strengths. The study found that there was a close correlation between VA level and blood lipids. Very little was found in the literature on the association between serum VA and blood lipids in a large sample population, especially in children and adolescents, and this study has clarified this issue.p < 0.05). VA was a risk factor for dyslipidemias, namely, hypercholesterolemia, hypertriglyceridemia and mixed hyperlipidemia, but was a protective factor for low high-density lipoprotein cholesterolemia. These results will provide a reference for the discovery of pathogenesis in pediatric hyperlipidemia.In conclusion, serum VA was positively correlated with TC, as well as TG and HDL-C, but these associations were influenced by age. Serum VA levels were significantly positively correlated with TC, TG, HDL-C and LDL-C in the 6–12-year-olds group, while in the 13–18-year-olds group, serum VA levels were significantly positively correlated with TC, TG and HDL-C (


Cram’s supramolecular capsule Octacid4 can irreversibly and noncovalently self-assemble with small-molecule guests at room temperature, but how they self-assemble and what accelerates their assembly remain poorly understood. This article reports 81 distinct Octacid4•guest self-assembly pathways captured in unrestricted, unbiased molecular dynamics simulations. These pathways reveal that the self-assembly was initiated by the guest interaction with the cavity portal exterior of Octacid4 to increase the portal collisions that led to the portal expansion for guest ingress, and completed by the portal contraction caused by the guest docking inside the cavity to impede guest egress. The pathways also reveal that the self-assembly was accelerated by engaging populated host and guest conformations for the exterior interaction to increase the portal collision frequency. These revelations may help explain why the presence of an exterior binding site at the rim of the enzyme active site is a fundamental feature of fast enzymes such as acetylcholinesterase and why small molecules adopt local minimum conformations when binding to proteins. Further, these revelations suggest that irreversible noncovalent complexes with fast assembly rates could be developed—by engaging populated host and guest conformations for the exterior interactions—for materials technology, data storage and processing, molecular sensing and tagging, and drug therapy. Neocarcerand Octacid4 can assemble with small molecule guests at room temperature to produce irreversible noncovalent bimolecular complexes, but the nature of the self-assembly pathway remains poorly understood. Here, the author uses unrestricted, unbiased molecular dynamics simulations to capture 81 distinct Octacid4•guest self-assembly pathways, revealing that interactions at the exterior of Octacid4’s cavity portal are key. These complexes generally refer to irreversible covalent complexes that are self-assembled by two molecules between which there is a covalent bond resulting from the self-assembly process. The desirability of irreversible covalent complexes is apparent from the drug action mechanisms of aspirin5, penicillin6, and sotorasib7, all of which involve an irreversible covalent complex. Notably, sotorasib was approved in May 2021 (https://www.fda.gov/news-events/press-announcements/fda-approves-first-targeted-therapy-lung-cancer-mutation-previously-considered-resistant-drug) as the first-in-class personalized treatment for a lung-cancer mutation previously considered resistant to drug therapy due to sotorasib’s unique capability to clinically block the function of KRASG12C  via irreversible complexation. However, the irreversible covalent complexes from in situ cysteine conjugation are limited by the infrequent presence of the noncatalytic cysteine in a protein cavity, and inhibiting KRASG12C offers treatment for only a subset of cancer patients. A paradigm shift is needed for the design of irreversible bimolecular complexes.Irreversible bimolecular complexes are highly desirable because (1) the residence of one molecule inside the cavity of another molecule is permanent until the breakdown of the cavity-containing molecule, (2) the complex formation is one-to-one stoichiometric, and (3) for drug therapy, the permanent residence and 1:1 stoichiometry confer the cavity-residing molecule with desired high metabolic stability, high potency-to-mass ratio, low-acting dose, and low off-target activity10, the irreversible complexes include irreversible noncovalent complexes that are self-assembled by two molecules between which there is no covalent bond. Here, the intrinsic binding is the complexation governed by intermolecular interactions between the two molecules and between the solvent and each of the two molecules, while the constrictive binding is the complexation controlled by the thermal energy required for the guest to overcome the steric hindrance from the host during the assembly or disassembly process, as exemplified below by Cram’s supramolecular capsules known as carcerand11, hemicarcerand12, and neocarcerand14.In terms of both intrinsic binding from the thermodynamic perspective and constrictive binding from the kinetic perspective11. When the barrier is moderately high, assembly or disassembly requires annealing. For example, a guest can enter or exit the cavity of a host known as hemicarcerand once the bimolecular system is heated to expand the cavity portal, and the guest remains inside the cavity once the system is cooled to contract the portal; the disassembly of the hemicarcerand complex is disallowed without heating15. When the barrier is slightly high, assembly or disassembly can occur spontaneously and slowly at an ambient temperature. For some host•guest complexes, the disassembly can be disabled at the ambient temperature by the complexation that subsequently heightens the barrier for disassembly. For example, a guest can enter the cavity of a host known as neocarcerand at room temperature and remain in the cavity at room temperature unless the portal is opened by heating because the docking of the guest at the cavity induces a host conformational change that consequently closes all cavity portals14.The hallmark of the constrictive binding is the heightened energy barrier for the assembly or disassembly of a host•guest complex. This barrier can make the dissembled or assembled molecules kinetically stable, namely, it takes a long time to assemble the two disassembled molecules or disassemble the two assembled molecules if the barrier for the conversion is heightened. When the barrier is extremely high, assembly or disassembly requires covalent-bond making or breaking, respectively. For example, a guest can be noncovalently trapped in the cavity of the two bowl-shaped fragments that are rim-to-rim tethered by linkers (of a host known as carcerand) when the guest is present in the reaction medium for the tethering reaction; the disassembly of the resulting carcerand complex is not allowed unless the host is broken by excessive heat16, the applications of neocarcerand complexes are also limited due to their slow complexation rates. This underscores the need to understand how two molecules self-assemble into an irreversible noncovalent complex and what accelerates their assembly as these high-level questions hold the key to designing irreversible noncovalent bimolecular complexes with fast complexation rates for broad applications.The applications of carcerand and hemicarcerand are however limited because their complexes cannot be formed in situ and adiabatically. Although neocarcerand can form its complexes in situ and adiabatically13—1,4-dioxane (dioxane), p-xylene (xylene), and naphthalene  according to NMR experiments13. These complexation times are many orders of magnitude longer than current MD simulation times that are on the order of milliseconds.In view of the current state of computational work on guest/ligand-binding pathwaysapo Octacid4 that was surrounded by eight neutralizing sodium cations, 10 xylenes to mimic the use of the guest in 10 equivalent excess in the NMR experiments13, 60 NaCl molecules to approximate the ionic strength of the experimental conditions13, and 2359 TIP3P water molecules21 to mimic the experimental aqueous solution13. Indeed, no autonomous complexation was observed in any set of 20 14,251.6-ns simulations at 298, 340, 363, or 370 K  were performed using the fully deprotonated,  K Table  because 13, the experimentally observed self-assembly of Octacid4 with dioxane, xylene, or naphthalene in a sodium borate buffer at pH 9 is mechanistically driven by the sodium cation as a phase-transfer catalyst22 that chelates with the carboxylates on the Octacid4 surface and consequently accumulates the guest on the host surface via the cation–π interaction23 or the sodium chelation. The guest accumulation on the host surface is similar to immersing the sodium-chelated Octacid4 in a neat guest solution. Simulating the latter can substantially accelerate the self-assembly according to the law of mass action, enabling determination of relative complexation times of different guests with Octacid4 for mechanistic insights into the self-assembly. Simulating the latter also allows the use of linear-regression analysis to examine the convergency and internal consistency of the MD simulations. A goodness of fit (r2) of <0.70 for the natural logarithm plot of the host population versus the simulation time indicates problematic simulations. This is because the first-order rate  is expected for simulating the self-assembly of Octacid4 with its guest in large excess.According to the reported NMR experimentsapo Octacid4 that was surrounded by eight neutralizing sodium cations and 150 xylenes. Here, the number of xylenes was arbitrarily chosen. Gratifyingly, all 40 14,251.6-ns simulations with the fully deprotonated, apo Octacid4 with 150 xylenes captured the self-assembly of Octacid4 with xylene at 298 K. Additional simulations were performed using each of the five variations: (1) replacing the fully deprotonated, apo Octacid4 with the octa-anionic Octacid4 possessing five water molecules inside the cavity, (2) increasing the number of xylenes to 250, (3) replacing 150 xylenes with 150 dioxanes, (4) replacing 150 xylenes with 150 naphthalenes, or (5) changing the Berendsen thermostat to the Langevin thermostat . Here the complexation time was defined as the first time instant at which xylene was inside the host cavity and had its phenyl plane parallel to the axial axis of the host. This xylene orientation was found in the most populated Octacid4•xylene conformationTo dissect the self-assembly kinetics, the duration of the first step is herein termed priming time, and the duration of the last two steps is termed ingression time. These names are used because during the first step, the host and guest conformations are primed, through self-selection and conformational rearrangements, for guest ingress, and because during the last two steps, the guest enters the host cavity and rotates ~90° to form a complex.The priming time was defined as a time period from the beginning of the MD simulation to the last time instant at which the distance between any guest hydrogen atoms and any host methylene hydrogen atoms was greater than 2.6 Å. The hydrogen–hydrogen distance cutoff (abbreviated as HH cutoff) was set at 2.6 Å for the following reasons. Visual inspection of all 40 Octacid4•xylene pathways revealed that xylene was outside of the Octacid4 cavity as long as the center-of-mass distance cutoff (abbreviated as COM cutoff) for Octacid4 and its guest was ≥7 Å. However, xylene could have its terminal methyl group contact the host cavity portal to slightly enter the host cavity portal in some of the 40 Octacid4•xylene pathways at the COM cutoff of 7 or 8 Å. Xylene could also be relatively away from the portal in some pathways at the COM cutoff of ≥8 Å. Therefore, rather than using the COM cutoff, the HH cutoff of 2.6 Å was used to avoid both the methyl group contacting the portal (which consequently shortens the ingression time) and the guest being away from the portal (which consequently lengthens the ingression time). The ingression time was defined as a time period from the last time instant at which the distance between any guest hydrogen atoms and any host methylene hydrogen atoms was greater than 2.6 Å to the first time instant at which the guest rotated ~90° inside the cavity. The complexation time is now a sum of the priming and ingression times.This dissection reveals that the ingression time , xylene (40 pathways), and naphthalene (1 pathway) at 298 K, the sodium cation that chelated with the Octacid4 carboxylate either coordinated with the guest oxygen atom or formed the cation–π interaction with the guest aromatic ring. However, the chelation with the oxygen atom of the guest that subsequently entered the host cavity was observed in one of the two pathways of dioxane Table ; the traThe conformational analysis of the 43 pathways also identified two small but interesting clusters of Octacid4 conformations that were derived a priori from the MD simulations Fig. –S18. In The 81 self-assembly pathways described above exhibited internal or external consistencies as follows.r2 of 0.98 for all 40 simulations that captured the Octacid4•xylene self-assembly, 0.85 for the first 20 of the 40 simulations, and 0.96 for the last 20 of the 40 simulations  The natural logarithm of the Octacid4 population versus the simulation time exhibited a linear relationship with 13 and with the phase-transfer catalysis undergirding those NMR experiments.(2) As described above, the simulations using the fully deprotonated Octacid4 under various conditions all captured the self-assembly of Octacid4 with dioxane, xylene, and naphthalene, but no autonomous complexation was observed under the same conditions if the fully deprotonated host was replaced by the fully protonated host. These results are consistent with the use of the Octacid4 solution containing the sodium borate buffer at pH 9 to detect the Octacid4•guest complexation in the NMR experiments27.(3) One key finding of the present work is that the guest ingress is so fast that complexation time is determined primarily by the priming time. This finding is consistent with the report that many dense-phase reactions can be considered as gated reactions in that the rate of a local reaction is governed largely by the initial formation of a permissive atomic arrangement  within which the local transformation can proceed relatively rapidly15 that are closely related to Octacid4.(4) Conformational analyses of all 43 pathways of xylene, dioxane, and naphthalene at 298 K showed that Octacid4 adopted exclusively a cluster of V-shaped conformations to gulp its guests. These V-shaped conformations have the mean C17a–C6b distance  For the self-assembly at 298 K, 12 of the 40 captured pathways for xylene and all captured pathways for dioxane and naphthalene had two nearly parallel linkers that channeled the guest into the cavity Table . These l25 of dioxane that showed the stronger intramolecular repulsion in the half-chair conformation than that in the chair conformation  Relative to the noncovalent interaction gradient isosurfacesion Fig. . These o13.(7) Most importantly, the capturing of the self-assembly of Octacid4 with dioxane, xylene, and naphthalene in the simulations at 298 K is consistent with the Octacid4 complexation with these guests detected in the NMR experiments at 298 KCollectively, the 81 pathways indicate that the irreversible noncovalent self-assembly process of Octacid4 with dioxane, xylene, and naphthalene was (1) initiated by the guest interaction with the cavity portal exterior of the host to increase the collisions of the portal that led to the portal expansion for guest ingress, (2) accelerated by engaging populated conformations of host and guest for the exterior intermolecular interaction to increase the portal collision frequency, and (3) completed by the portal contraction caused by the guest docking inside the cavity to impede guest egress. This type of self-assembly is a process of stepwise conformational rearrangements of the two molecules—kinetically starting from the pre-assembly state (at which the guest interacts with the exterior of the host) to the assembly and post-assembly states —to gradually increase the attraction between the two molecules along a repulsion–free path until the maximal attraction is reached. The kinetics of this process was governed primarily by the complementarity between the two molecules during the priming time (on the order of microseconds or longer) rather than the ingression time (on the order of picoseconds to nanoseconds).The 40 Octacid4•xylene self-assembly pathways derived from the converged MD simulations  but the mean complexation time  that offers insight into the delicate balance between the sodium cation’s roles in lengthening and shortening the complexation time of Octacid4•xylene.This heterogeneity is akin to the heterogeneity of protein-folding pathways27 and the finding of the present work that the ingression time is so short that the complexation time is governed largely by the priming time, it is worth noting the need to avoid overcomplication of the self-assembly process, such as simulating the Octacid4•guest self-assembly using the octa-anionic Octacid4 possessing five water molecules inside the cavity or a more complicated host with a variable number of water molecules inside the cavity, for at least two reasons. First, it has not been determined experimentally whether the egression of water molecules that was observed in the present work will actually occur under the NMR experiment conditions once the water-bound Octacid4 is surrounded by a layer of water-insoluble xylene or naphthalene. Second, the water molecule is much smaller than dioxane, xylene, and naphthalene. The ingression and egression times of water are hence much shorter than those of the three guests, and binding of water molecules to the host cavity is likely opportunistic rather than intrinsic or constrictive. The inclusion of water molecules inside the cavity will affect the Octacid4•guest self-assembly but not likely to the extent that the mean complexation time will be significantly altered.Given the known theoretical work on gated reactions36 and why it is advantageous for the proapoptotic multidomain BAK protein to have a noncanonical BH3-binding groove37 that abuts the canonical one for constrictive complexation of BAK—due to the steric hindrance from R88 and Y89 on the edge of the canonical groove38—with proapoptotic BH3-only proteins. The realization that the self-assembly acceleration by the adoption of populated conformations for the exterior intermolecular interaction is an advance and may help explain why small molecules reportedly prefer to adopt local minimum conformations when binding to proteins39. Further, these realizations suggest that irreversible noncovalent complexes with fast assembly rates could be designed by accounting for the complementarity between guest and host both of which adopt populated conformations for the exterior intermolecular interactions during the priming time. This design strategy may facilitate a paradigm shift from irreversible covalent complex design to irreversible noncovalent complex design for materials technology, data storage and processing, molecular sensing and tagging, and drug therapy—especially the personalized drug therapies for cancer patients with somatic mutations other than the KRASG12C mutation7.Despite the heterogeneity issue as described above, the realization that the self-assembly initiation by the exterior intermolecular interaction and the control of the complexation time largely by the priming time is an advance and may help explain why the presence of a cryptic exterior site for substrate or inhibitor binding at the rim of the enzyme-active site is a fundamental feature of fast enzymes such as acetylcholinesteraseapo Octacid4 neutralized with 8 sodium ions  and tLEaP of the AmberTools 16 package  and then energy-minimized for 100 cycles of steepest-descent minimization followed by 900 cycles of conjugate-gradient minimization to remove close van der Waals contacts using SANDER of the AMBER 11 package , FF12MClm40, and a cutoff of 8.0 Å for noncovalent interactions. The tLEaP input file for building the fully protonated Octacid4 and the Cartesian coordinates of the energy-minimized Octacid4•guest , FF12MClm40, and a periodic boundary condition at 1 atm and 298/340/363/370 K. All simulations used (i) a dielectric constant of 1.0, (ii) the Berendsen coupling algorithm41 for thermostat and barostat, (iii) the particle mesh Ewald method42 to calculate electrostatic interactions of two atoms at a separation of >8 Å, (iv) Δt = 1.00 fs of the standard-mass time43, (v) the SHAKE-bond-length constraint applied to all bonds involving hydrogen, (vi) a protocol to save the image closest to the middle of the “primary box” to the restart and trajectory files, (vii) a formatted restart file, (viii) the revised alkali-ion parameters44, (ix) a cutoff of 8.0 Å for noncovalent interactions, (x) a uniform 10-fold reduction in the atomic masses of the entire simulation system (both solute and solvent)43, (xi) NTWX = 100 steps for coordinates’ output, and (xii) default values of all other inputs of PMEMD.The fully deprotonated, 43, FF12MClm is a revised AMBER protein forcefield with no parameterization for any Octacid4•guest complexes40. This forcefield is able to (1) capture the experimentally observed exponential decay of the non-native state population of fast-folding proteins over simulation time with r2 > 0.90 and (2) fold these proteins with agreements between simulated and experimental folding times within factors of 0.6–1.445. FF12MClm was used in this study to investigate the noncovalent self-assembly of small-molecule guests with Octacid4 whose aromatic linkers can flip between left‐ and right‐handed configurations and usher the guest into the host cavity. This was because of the effectiveness of FF12MClm in simulating the experimentally observed flipping between left‐ and right‐handed configurations for C14–C38 of bovine pancreatic trypsin inhibitor in solution40 and because of the need to compress the simulation time  by a factor of 101/2 through 10-fold uniform reduction of the system mass. While the hydrogen mass repartitioning scheme can also speed up simulations, it was not used in this study because it would affect dynamic properties of the system46. The forcefield parameters for the fully deprotonated Octacid4, dioxane, xylene, and naphthalene are available in the Supplementary Information of Ref. 14. The forcefield parameters for the neutral Octacid4 were developed using a published procedure14 and provided in Data Available in the Supporting Information of Ref. All MD simulations were performed using a dedicated cluster of 100 12-core Apple Mac Pros with Intel Westmere (2.40/2.93 GHz) and computers at the University of Minnesota Supercomputing Institute and the Mayo Clinic high-performance computing facility at the University of Illinois Urbana-Champaign National Center for Supercomputing Applications.47.The mean complexation time and its 95% confidence interval for the Octacid4•xylene self-assembly at 298 K was obtained from the 40 individual complexation times of xylene listed in Table 48 with keyword RANGE  for Fig. All noncovalent interaction gradient isosurfaces were generated using the NCIPLOT program Version 4)48 with kAll energy minimization and frequency calculations were performed using Gaussian 16  and HF/6-31 G* or B3LYP/6-31 G* on computers from the Mayo Clinic high-performance computing facility at the University of Illinois Urbana-Champaign National Center for Supercomputing Applications. Each frequency calculation was done using %mem = 40 Gb and %nproc = 16. The Cartesian coordinates of the Octacid4 conformations (with one or two pairs of bidendate linkers) before and after the minimization are provided in Data https://www.adobe.com).The time series of the Cartesian coordinates of all heavy atoms of Octacid4 and the guest  was extracted from the output file of an MD simulation and saved to a concatenated PDB file using the CPPTRAJ of the AMBER 14/16/18 package. The guest involved in the complexation was identified through the analysis of eight intermolecular distances between a guest atom  and the eight diphenoxymethane carbon atoms of Octacid4. The eight distances for each of 150 guests were calculated from the output file using the CPPTRAJ, and the guest was considered  to be inside the Octacid4 cavity if all eight distances were <9.0 Å. The concatenated PDB file was then converted to a set of PNG files using the PyMOL V1.7.0.3 showing the top or side view of the bimolecular complex with the guest in the stick-and-ball model and the host in the stick model. Before the PDB-to-PNG conversion, each complex conformation was superimposed over the proceeding one using the PyMOL. The resulting PNG files were lastly converted to a video file using Adobe Photoshop 2021 (Peer Review FileDescription of Additional Supplementary FilesSupplemental materialVideo S1Video S2Video S3Video S4Video S5Video S6Video S7Video S8Video S9Video S10Video S11Video S12Video S13Video S14Video S15Video S16Video S17Video S18Data S1Data S2Data S3


Fiber-reinforced polymer (FRP) confined concrete is regarded as an innovative and economical approach for structural repairation. Two typical materials [carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP)] are selected in this study to investigate the concrete strengthen effect in a severe environment. The resist ability of FRP-confined concrete is discussed when subjected to coupled erosion between sulfate erosion and freeze–thaw cycles. Electron microscopy examines concrete's surface and interior degradation during coupled erosion. The corrosion degree and principle of sodium sulfate are analyzed using pH, SEM electron microscope, and EDS energy spectrum. The axial compressive strength test is used to evaluate the reinforcement of the FRP-confined concrete column, and the stress–strain relationship for various FRP-confined techniques in a coupled erosion environment is obtained. The error analysis is performed to calibrate the experimental test result using four existed prediction models. All observations indicate that the deterioration process of FRP-confined concrete is complicated and dynamic under coupled effect. Sodium sulfate initially increases the initial strength of concrete. However, subsequent freeze–thaw cycles may aggravate concrete fractures, while sodium sulfate further degrades the strength of concrete through the cracking development. A precise numerical model is presented to simulate the stress–strain relationship, which is critical for the design and life cycle assessment of FRP-confined concrete. It is becoming more common in engineering applications as costs fall, such as Glass fiber (GFRP), carbon fiber (CFRP), basalt fiber (BFRP), and aramid fiber (AFRP), which are the most often used FRPs for structural strengthening5. The proposed FRP-confined technique may increase concrete performance and avoid premature collapse. However, various external environments in engineering often affect the durability of FRP-confined concrete, resulting in its strength failure.As an innovative concrete strengthened method explored since the 1970s, FRP has the advantages of low weight, high strength, corrosion resistance, fatigue resistance, and ease of building6 found that the ultimate stress and strain positively related to the fiber fabric thickness grows. Wu et al.7 obtained stress–strain curves for FRP-confined concrete using various fiber types to predict ultimate strain and load. Lin et al.8 discovered that the FRP stress–strain models for circular, square, rectangular, and elliptical bars are also highly dissimilar and developed a novel, design-oriented stress–strain model, using width ratio and corner radius as parameters. Lam et al.9observed that the uneven lap joint of FRP and curvature contribute to FRP's fracture strain and stress being smaller than the plate tensile test. Furthermore, scholars have studied partial confinement and novel confined techniques based on the different requirements in practical projects. Wang et al.10conducted axial compression tests concerning three confined modes, including totally, partially, and unconfined concrete. A stress–strain model is developed and provides confinement effect coefficients for partially confined concrete. Wu et al.11 developed a method for predicting the stress–strain relationship of FRP-confined concrete that accounts for size impact. Moran et al.12 evaluated confined concrete's axial monotonic compressive performance with FRP helical strip and obtained its stress–strain curve. However, the above researches mainly study the difference between partial and entirely confined concrete. The action of the various parts of the FRP partially confined concrete has not been studied in detail.Some researchers have investigated the concrete stress–strain variation laws with various cross-sectional shapes and sizes. Yan et al.14 found that the repairability of FRP-confined concrete decreased with the increase of the damage degree through FRP repairing experiments on initial damaged concrete. Ma et al.15 studied the effect of FRP-confined concrete columns' initial damage and believed that the damage degree had a negligible effect on the ultimate strength and a significant effect on the lateral and longitudinal strain. However, Cao et al.16observed the stress–strain and envelope stress–strain curves of FRP-confined concrete under the influence of initial damage. In addition to investigating the concrete initial damage condition, some studies have also appeared on the durability of FRP-confined concrete under harsh environmental conditions. These scholars studied the degradation of FRP-confined concrete in harsh environments and used damage assessment methods to establish degradation models to predict service life. Xie et al.17placed FRP-confined concrete in a hydrothermal environment and found that hydrothermal conditions significantly affected the mechanical properties of FRP, resulting in a gradual decrease in its compressive strength. In the acid-base environment, the interface between CFRP and concrete will be affected by degradation. The fracture energy release rate of the CFRP layer decreased significantly with the increase in immersion time, which eventually led to the failure of the interface specimen20. In addition, some scholars have also studied the effect of freezing and thawing on FRP-confined concrete. Liu et al.21pointed out that CFRP reinforcement has good durability under freeze–thaw cycles based on relative dynamic modulus, compressive strength, and stress–strain relationship. Moreover, a model related to the degradation of concrete mechanical properties is proposed. However, Peng et al.22 calculated the service life of CFRP-concrete binders using temperature and freeze–thaw cycle data. Guan et al.23 conducted a rapid freeze–thaw test on concrete and proposed an evaluation method for freezing resistance based on the thickness of the damaged layer under freeze–thaw action. Yazdani et al.24 investigated the effect of FRP layers on the penetration of chloride ions into concrete. The results show that the FRP layer is resistant to chemical corrosion and can separate the internal concrete from the external chloride ions. Liu et al.25 simulated the pull-out test environment of sulfate eroded FRP concrete, established a bond-slip model and predicted the degradation of the FRP-concrete interface. Wang et al.26 established a stress–strain model of FRP-confined sulfate-eroded concrete through uniaxial compression tests. Zhou et al.27 studied the damage of unconfined concrete caused by the coupling of salt-freeze–thaw cycles and used the logistic function to describe its degradation mechanism for the first time. These studies have made considerable progress in evaluating the durability of FRP-confined concrete. However, most researchers have focused on simulating the environment with erosion under a single adverse condition. Concrete is commonly damaged due to coupling erosion caused by various environmental conditions. These coupled environmental conditions have a severe deterioration effect on the performance of FRP-confined concrete.In addition, studies have also evaluated the effectiveness of FRP-confined concrete under different conditions in terms of compressive strength, strain change, initial elastic modulus, and strain hardening modulus. Tijani et al.Sulfates and freeze–thaw cycles are two typical significant parameters to impact the durability of concrete. FRP-confined technology has the potential to enhance the performance of concrete. It is widely used in engineering and research but has its limitations at the current time. Few types of research focus on the endurance of FRP-confined concrete concerning sulfate corrosion in cold regions. The erosion process of fully-confined, semi-confined, and unconfined concrete under the coupled action of sodium sulfate and freeze–thaw deserves more exploration, especially the new semi-confined technique described in this paper. The strengthening effects on concrete columns are also investigated by exchanging the sequence of FRP confinement and erosion. The electron microscope, pH test, SEM electron microscope, EDS energy spectrum analysis, and uniaxial mechanical test are all conducted to demonstrate the micro and macro changes of specimens caused by linked erosion. Additionally, this study discusses the law governing the stress–strain relationship generated by uniaxial mechanical testing. The experimental test's ultimate stress and strain value are verified with error analysis, which uses four existing ultimate stress–strain models. The presented model can adequately predict the ultimate strain and strength of the material, which is helpful for future FRP reinforcement engineering practice. Finally, it also serves as the conceptual foundation for the idea of salt frost resistance in FRP concrete.This study evaluates the deterioration of FRP-confined concrete using the sulfate solution corrosion couped with freeze–thaw cycles. The micro and macro changes caused by concrete erosion are demonstrated using a scanning electron microscope, a pH test, an EDS energy spectrum analysis, and a uniaxial mechanical test. In addition, the mechanical properties and stress–strain variations of FRP-confined concrete subjected to coupled erosion are investigated using axial compression experiments.The FRP-confined concrete comprises originally plain concrete, an external FRP wrapping material, and an epoxy adhesive. Two types of external confined materials are selected: CFRP and GFRP, and the material property is shown in Table 4 solution with a mass fraction of 10%, then pouring the solution into the freeze–thaw cycle box.The concrete design strength is 30 MPa, which results in a slump of between 40 and 100 mm for newly mixed cement concrete. The concrete mix proportions are shown in Table 3 forced mixer, and the entire batch of concrete is used to place the required specimens. First, the concrete ingredients are batched in Table Concrete mixtures are prepared in a 0.5 mAfter curing for 28 days, the specimens are wrapped with FRP materials. Three techniques for reinforced concrete columns are discussed in this study, which included entirely confined, semi-confined, and unconfined. Two types of CFRP and GFRP are used for the confined material. FRP fully confined concrete is wrapped FRP with 20 cm height by 39 cm length. The top and bottom of the FRP-confined concrete are not encapsulated by epoxy. As a newly suggested confined technique, the semi-confined test processes are described as follows.(1) FRP is cut into strips with dimensions of 2 cm in height by 39 cm in length.(2) A ruler is used to mark the concrete cylindrical surface to determine the location of the FRP strips, which are 2.5 cm apart from each other. Then wrap the tape around the concrete areas that do not require FRP.(3) The concrete surface is sanded with sandpaper until smooth and wiped with alcohol cotton, and its surface is applied with an epoxy resin. Then stick the FRP strips on the concrete surface by hand and squeeze out the gap to make the FRP fully fit the concrete surface to avoid the generation of air bubbles. Finally, the FRP strips are glued to the concrete surface from top to bottom according to the marks made by the ruler.(4) Check whether the concrete and FRP are separated after half an hour. If the FRP appears to slip or bulge, it should be corrected immediately. The molded specimens need to cure for 7 days to guarantee the hardened strength.(5) After curing, the tape on the concrete surface is peeled off by a utility knife, and finally, the FRP semi-confined concrete column is obtained.The results under different confinements are shown in Fig. There are four main parameters, and it aims to examine the influence of FRP-confined and erosion sequences on the anti-erosion performance of circular columns. Table (1) Confined materials are classified as CFRP or GFRP. The impact of two fiber types on concrete reinforcing is compared.(2) There are three confined techniques for concrete columns: entirely confined, semi-confined, and unconfined. The erosion resistance of semi-confined concrete columns is compared with the other two varieties.(3) The erosion conditions are freeze–thaw cycles coupled with sulfate solution, and the freeze–thaw cycles are 0, 50, and 100 times, respectively. The effect of coupled erosion on FRP-confined concrete columns is investigated.(4) The specimens are divided into three groups. The first group is FRP wrapped and then eroding, the second group is eroding first and then wrapped, and the third group is eroding first and then wrapped and then eroding again.The experiments programs are conducted using a universal testing machine, a tensile testing machine, a freeze–thaw cycle box (model CDR-Z), an electron microscope, a pH tester, a strain gauge, a displacement device, an SEM electron microscope, and an EDS energy spectrum analyzer in this study. The specimen is a concrete column of 10 cm in height by 20 cm in diameter. After being placed and compacted, the concrete will be cured for 28 days, as shown in Fig. d). All experiments are performed on three sample replicates, and average values are used to discuss test results. The mass loss and strength loss formulas of the specimen are determined as follows:d is the weight loss of the specimen after every 25 freeze–thaw cycles (%), W0 is the average weight of the concrete specimen before the freeze–thaw cycle (kg), and Wd is the average weight of the concrete specimen after every 25 freeze–thaw cycles Weight (kg).The freeze–thaw experiment used a rapid freezing method, as shown in Fig. d, and the formula is as follows:d is the strength loss rate (%) of the sample in every 50 freeze–thaw cycles, f0 is the average strength of the concrete sample before the freeze–thaw cycle (MPa), and fd is the average strength of the concrete sample in every 50 freeze–thaw cycles (MPa).The strength degradation coefficient of the specimen is characterized by KFigure A sample of the freeze–thaw solution is removed and placed in a container every 25 freeze–thaw cycles. Figure Visually examining the specimen's surface is performed using an electron microscope, and 400 magnification factors are selected. The surface damage degree of FRP semi-confined and unconfined concrete is quite severe when subjected to a freeze–thaw cycle coupled with sulfate erosion, whereas the surface damage degree of fully-confined concrete is negligible. As illustrated in Fig. The second category is the corrosion appearance of CFRP and GFRP semi-confined concrete columns subjected to a freeze–thaw cycle coupled with sulfate erosion, as illustrated in Fig. The third category is the eroded appearance of CFRP and GFRP fully confined concrete subjected to a freeze–thaw cycle coupled with sulfate erosion, as illustrated in Fig. 28. Therefore, it can be considered that erosion mainly changes the enhancement effect of FRP by changing the properties of the adhesive layer through freeze–thaw cycles. The concrete surface of FRP semi-confined concrete has the same erosion phenomenon as the unconfined concrete surface. Its FRP layer is consistent with the FRP layer of fully confined concrete, and the damage is not apparent. However, extensive erosion cracks occur at the intersection of fiber strips and exposed concrete in FRP semi-confined concrete. The erosion of the unconfined concrete surface becomes more severe as the number of freeze–thaw cycles increases.Contrast the phenomena observed following the application of the three confinement techniques described above. The fiber fabric in FRP fully confined concrete remained stable as the freeze–thaw cycles increased. On the other hand, the adhesive ring layer is thinner on the surface. Epoxy resin mainly reacts with active hydrogen ions in sulfuric acid to open the ring and hardly reacts with sulfateThe interiors of FRP fully-confined, semi-confined, and unconfined concrete exhibit significant differences when eroded by freeze–thaw cycles coupled with sulfate solutions. Transverse cuts are made in the test pieces, and the cross-sections are observed using a 400× magnification electron microscope. Figure 27 have shown that when concrete is placed in a sodium sulfate solution, sodium sulfate will penetrate the concrete, part of it will precipitate in the form of sodium sulfate crystals, and part of it will react with the cement. Sodium sulfate crystals and reaction products appear as white granules.Internal erosion of the FRP fully confined concrete is depicted in Fig. 29, the coupling erosion process of unconfined and FRP semi-confined concrete is divided into two stages. The first stage of concrete cracks is due to freeze–thaw expansion and contraction. As the sulfate penetrates the concrete and becomes visible, the appropriate sulfate fills the cracks created by freeze–thaw and shrinkage from hydration reactions. Therefore, sulfate has a particularly protective effect on concrete in the initial stage and can improve the mechanical properties of concrete to a certain extent. The second stage of sulfate erosion continues, entering cracks or voids and reacting with cement to form alum. As a result, the fracture volume expands and causes damage. At this time, the expansion and contraction reactions related to freezing and thawing will aggravate the internal damage of the concrete, resulting in a decrease in the bearing capacity.The concrete under FRP full confinement produces cracks under coupled erosion, but the section is smooth and crystal-free. On the other hand, FRP semi-confined and unconfined concrete sections develop internal cracks and crystals under coupled erosion. According to the picture description and previous research− causes the pH of the saturated solution to increase. The pH changes are more significant in 0–25 freeze–thaw cycles, and the changes are not apparent in 25–100 freeze–thaw cycles30. However, it is found here that the pH continued to increase over 25–100 freeze–thaw cycles. It can be explained that sodium sulfate reacts chemically with the interior of the concrete, changing the pH of the solution. Chemical composition analysis shows that the following reactions occur between concrete and sodium sulfate.Figure -, so the pH increases. Furthermore, because the reaction is reversible, the pH increases at a particular time and changes slowly.According to formulas  and 4),,4), it cThe mass loss of FRP fully-confined, semi-confined, and unconfined concrete during freeze−thaw cycles in sulfate solution is shown in Fig. The change of mass loss of FRP semi-confined concrete first decreased and then increased. After 50 freeze–thaw erosions, the mass loss of FRP semi-confined concrete is about 1.3%. The mass loss after 100 cycles is 0.8%. Therefore, it can be concluded that sodium sulfate penetrates the concrete in unconfined concrete. In addition, observation of the surface of the specimens also found that the fiber strips can resist mortar spalling in unconfined areas, thereby reducing mass loss.The change in mass loss of FRP fully confined concrete is different from the former two. It does not lose mass but gains mass. After 50 freeze–thaw erosions, the mass increased by about 0.08%. After 100 times, its mass increased by about 0.428%. Because the concrete is completely encased, the mortar on the concrete surface will not fall off, resulting in virtually any loss of mass. On the other hand, the penetration of water and sulfate from the high content surface into the low content of interior concrete can also increase concrete mass.There are few previous studies on the relationship between pH and mass loss of FRP-confined concrete under erosive conditions. Most studies mainly discuss the relationship between mass loss, elastic modulus, and strength loss. Figure Figure The following conclusions are obtained from the analysis. The electron microscope images of the three specimens are all 1k ×, and the cracks and erosion products in the images are found and observed. Unconfined concrete has the widest cracks and contains many particles. FRP semi-confined is inferior to unconfined concrete in crack width and particle count. FRP fully-confined concrete crack width after freeze–thaw coupled erosion is the smallest and free of particles. All these indicate that the FRP fully confined concrete is least affected by the coupled erosion of freeze–thaw. The chemical processes inside the FRP semi-confined and unconfined concrete columns lead to the formation of alum and gypsum, and the infiltration of sulfates affects the pores. Although freeze–thaw cycles are the primary cause of concrete cracking, sulfate and its products fill some of the cracks and pores in the first place. However, as the number and time of erosion increased, the cracks continued to grow, and the volume of alum produced increased, squeezing the cracks. Ultimately, freeze–thaw and sulfate attack can reduce the strength of the column.Axial compressive strength tests investigated the mechanical properties of two confinement materials and technologies subjected to freeze–thaw cycle erosion in a sodium sulfate solution. Figure 22. However, the strength of the test results increased. It can be explained that the increase in strength is due to the addition of sodium sulfate to fill the pores in the concrete and cracks caused by freeze–thaw when considering the result of electron microscopy, mass loss, and pH change. After 100 freeze–thaw cycles, the strength of CFRP fully-confined concrete increased by 3.9 MPa, and the strength of semi-confined concrete decreased by 2 MPa. GFRP fully confined concrete increased by 8.2 MPa, while GFRP semi-confined concrete decreased by 5.9 MPa. This is because the freeze–thaw cycles cause the cracks to get progressively worse. The excessive volume of alum and gypsum generated by the reaction of sodium sulfate with cement will also squeeze cracks and aggravate internal damage. The strength of unconfined concrete increased by 5.7 MPa after 50 freeze–thaw cycles and decreased by 4.5 MPa after 100 freeze–thaw cycles. The variation law of strength is similar to that of FRP semi-confined, which increases first and then decreases.After 50 freeze–thaw cycles, the strength of CFRP fully confined concrete increases by 2.3 MPa, and the strength of semi-confined concrete increases by 3.5 MPa. The strength of GFRP fully confined 3.4 MPa increases concrete, and similarly, 6.5 MPa increases for GFRP semi-confined concrete. Studies have shown that cracks are generated inside FRP-confined concrete, and its strength will decrease under a single freeze–thaw conditionThe strength variation of CFRP and GFRP-confined concrete under freeze–thaw cycles can also be compared in Fig. 22. However, under the action of freeze–thaw cycles in sodium sulfate solution, the mass loss of concrete with different confined methods and materials is not consistent. Therefore, this paper analyzes the correlation between the mass loss and the sample's compressive strength and uses the Origin software to fit the curve in Fig. 2 of the unconfined concrete is small, and other prediction formulas have a high fitting degree.Mass loss is an essential indicator for evaluating the frost resistance of concrete in the freeze–thaw test. Unconfined concrete mass loss increases continuously under a single freeze–thaw conditionThis study aimed to determine the effect of confinement on the operational performance of concrete eroded by freeze–thaw cycles in a sulfate solution. In this context, the term ‘working performance' refers primarily to the eroded concrete's ability to regain its strength following confinement and its ability to resist further erosion following confinement. Erosion conditions are 50 freeze–thaw cycles in 10% sodium sulfate solution. Figure Figure Figure Stress–strain models for concrete with different confined materials and techniques are investigated under coupled erosion conditions. Firstly, the stress–strain law of different concretes under the coupled action of sulfate and freeze–thaw is analyzed. Secondly, the applicability of the previous ultimate stress–strain prediction models is discussed, and the ultimate stress–strain models of CFRP and GFRP materials suitable for the coupled erosion conditions of sulfate and freeze–thaw cycles are established.31 of concrete under uniaxial compression is cited as shown in Eq. (The stress–strain curves for unconfined concrete after 0, 50, and 100 freeze–thaw cycles in sulfate solution are depicted in Fig. According to Guo's model, the smaller value of parameter ‘a', the narrower curve, and the smaller integral area are observed. It can be explained that it demonstrates ‘a' decrease in ductility and plastic deformation capability. On the other hand, the larger the parameter ‘a', the more significant the ability. Thus, parameter ‘a' can be used to compare the mechanical properties of concrete. Figure Figure The stress–strain curves of various confined materials and technologies during freeze–thaw cycles in sodium sulfate solution are shown in Fig. The stress–strain curves of CFRP and GFRP fully confined concrete columns under freeze–thaw conditions are shown in Fig. Figure cc/ε′co is defined with the extension ratio, which can indicate the degree of improvement of the deformation capacity of FRP-confined concrete compared with unconfined concrete. Similarly, the f′cc/f′co is defined with the enhancement ratio, indicating the improvement of the axial compressive capacity of FRP-confined concrete compared with unconfined ordinary concrete. The observation table shows that CFRP fully confined concrete's confined ratio is the largest, followed by GFRP fully confined concrete, CFRP semi-confined concrete, and post-confined concrete. The extension of GFRP is more significant than that of CFRP, which is related to the elastic modulus of GFRP. The enhancement ratio of FRP semi-confined concrete is about half of that of fully confined concrete. Post-confined concrete has a slight increase in extension compared to un-eroded confined concrete.Table Numerous researchers have proposed an ultimate stress–strain model for FRP-confined concrete subjected to erosion. However, the model has not been studied in a freeze–thaw cycle environment with various confined materials and technologies in a sodium sulfate solution. The ultimate stress–strain calculation models for FRP-confined concrete developed by the four researchers are shown in Table cc/ε′co values in Table cc/f′co are calculated. It is compared with the actual f′cc/f′co value for the enhancement ratio error analysis in Fig. The actual ε′cc/f′co values in Table cc/ε′co are calculated. In Fig. The actual f′cc is the compressive strength of the FRP-confined concrete specimen; fco is the compressive strength of the unconfined concrete specimen; εcc is the limit point strain corresponding to the FRP-confined concrete; εco is the unconfined concrete. The limit point strain corresponding to the unconfined concrete. a, b, and c are the values to be fitted, and their values are shown in Fig. The previous limit stress–strain prediction model shows that the current research model can not meet the prediction when FRP-confined concrete is subjected to freeze–thaw cycles in a sodium sulfate solution. Therefore, a prediction model under this condition is established in this experiment based on the obtained data. This presented model has limited applicability and can only predict within 10% mass sodium sulfate solution and 100 freeze–thaw cycles. Exceeding this range requires more experiments for refinement. The test results are divided into two categories, namely CFRP-confined concrete and GFRP-confined concrete. The error rates of the prediction models are 0.075 and 0.089, respectively. All analysis results show that the model has a good prediction effect.Experiments are conducted to determine the effect of freeze–thaw cycles on FRP-confined concrete in a sulfate solution. External environmental conditions use sodium sulfate at 10% concentration and freeze–thaw cycles. The following conclusions are drawn following microscope observation, pH, mass loss, SEM and EDS analysis, axial compressive strength testing, and stress–strain curve analysis.1. The more freeze–thaw cycles, the more serious the surface damage, which gradually erodes from the surface to the inside. FRP semi-confined and unconfined surfaces are severely damaged, while FRP fully-confined surfaces are barely detectable. Semi-confined technology does not reduce the chemical attack of the environment on the concrete, but it can ensure the corresponding strength.2. The pH of unconfined concrete increases rapidly during erosion and gradually increases over time. EDS spectroscopy is used on the specimen to analyze further the white crystals observed in the SEM. Sodium sulfate reacts with calcium hydroxide to form calcium sulfate, and calcium sulfate reacts with calcium meta aluminate to form alum crystals. The predicted relationship between pH and mass loss under different confined techniques is established.3. The mechanical test confirms that FRP confined technology significantly improves the ultimate load of concrete. FRP semi-closed technology can achieve about 50% of the effect of fully closed technology. The mechanical effect of CFRP confinement is twice that of GFRP confinement. Under the action of coupled erosion, the strength of FRP semi-compressed and uncompressed concrete first increased and then decreased slightly. The strength of FRP fully restrained concrete shows a slow increase. Furthermore, concrete that has been eroded and then confined retains a high level of performance when eroded again. A predictive model of the relationship between mass loss and strength loss is built.4. The stress–strain curves of semi-confined and fully-confined concrete can be divided into three stages. The ultimate stress–strain model for FRP-confined concrete is validated by referring to previously published literature. A new model is presented to predict the ultimate strength under the coupled erosion of freeze–thaw and sulfate. It can predict the ultimate stress–strain of concrete confined by two materials, CFRP and GFRP, under the coupled conditions of freeze–thaw in a sulfate solution.