The climatic features of plateau regions include low atmospheric pressure, large temperature variation, and intense radiation, presenting numerous challenges for concrete structures in these areas during the service life. The substantial diurnal temperature fluctuations expose concrete structures to the risk of cracking over prolonged temperature cycles, particularly notable in complex facade structures like bridge piers, where cracks often intersect and spread across the surface. In the plateau with significant temperature variations, the surface cracks that occur in bridge pier concrete do not directly impact the structural load-bearing capacity. However, if surface damage is allowed to progress, this deterioration accelerates the penetration of moisture and CO₂, among other corrosive agents, into the interior of the concrete, thereby causing the durability of the concrete structure to fail.

This study focuses on typical bridge pier concrete in plateau regions with significant temperature variations, employing finite element analysis and secondary development techniques to establish a temperature fatigue damage model for bridge pier concrete under such conditions. By incorporating cohesive elements to simulate the cracking behavior of bridge pier concrete under long-term temperature cycling, this research elucidates features such as the initial crack time and temperature gradient distribution in plateau regions with significant temperature variations. Furthermore, through an analysis of the fatigue stress evolution patterns and energy accumulation processes of bridge pier concrete under temperature cycling, this study aims to uncover its cracking mechanisms.

Based on the cracking mechanism of bridge piers in the high plateau environment with significant temperature variations, this study establishes a finite element model for a dual-lane circular bridge pier. The model dimensions are 7.4 m (length) × 3.0 m (width) × 9.0 m (height). The mesh consists of hexahedral elements, with element types C3D8R and cohesive elements embedded along the mesh boundaries. The concrete material properties for the bridge pier include an elastic modulus of 42 GPa, tensile strength of 2.5 MPa, poisson's ratio of 0.2, linear expansion coefficient of 1.5×10^-5, density of 2 420 kg/m³, and specific heat capacity of 1 100 J/(kg·℃). Diurnal temperature variations are set at 20-40, 20-60 ℃, and 20-80 ℃, respectively. Additionally, considering the difference between the sunny and shaded sides of the bridge pier concrete, a temperature variation of 10 ℃ is defined for the shaded side. The bottom of the bridge pier concrete is fully constrained, while the top is subjected to an upper load of 9 000 kN. Importantly, a threshold value is introduced to determine whether fatigue effects on concrete need to be considered during alternating diurnal temperature cycles.

Intense solar radiation and longer hours of sunshine in plateau regions lead to the appearance of randomly distributed cracks with widths less than 0.5 mm in bridge piers after 1-2 years of normal usage. The cracking mechanism of bridge pier concrete in plateau environments is as follows: the bridge pier concrete surface heats up to 50 ℃ due to thermal radiation and air convection. Heat is then transferred internally through a heat transfer process consistent with Fourier's law. The temperature gradients generated inside the bridge pier concrete, due to its low thermal conductivity, create temperature fatigue stresses, ultimately resulting in surface cracking under the influence of temperature fatigue stress.

Based on finite element analysis, this study demonstrates that in plateau environments with significant temperature variations, bridge pier concrete tends to initiate cracking from the bottom and gradually extend upwards. Additionally, it is shown that the alternation between day and night results in a more complex internal temperature field within the bridge pier concrete. It is noteworthy that as the diurnal temperature variation increases, the rate of damage accumulation on the surface of the bridge pier concrete gradually increases. Furthermore, the plastic dissipation energy generated in the bridge pier concrete over the same period also increases with the rise in diurnal temperature variation, thereby revealing the cracking mechanism of the bridge pier concrete.

This paper takes typical bridge piers in high plateau regions as an example and, through theoretical analysis, reveals the complete process of "thermal response-stress generation-fatigue damage accumulation" cracking mechanism of bridge piers in environments with significant temperature variations. Based on the cracking principles of bridge piers in such environments, a fatigue damage model under temperature fatigue is established using finite element simulation and secondary development techniques. Concurrently, cohesive elements are utilized to simulate the cracking behavior of bridge piers under prolonged temperature cycling. Subsequently, it is demonstrated that in environments with significant temperature variations, cracks in bridge piers extend upwards from the bottom according to a certain development pattern. Moreover, with the increase in temperature variation on the bridge pier surface, both the initial crack time and damage accumulation rate gradually accelerate.

Reinforced concrete structures are currently the most widely used structural form. However, a large number of concrete structures deteriorate with premature failure under the action of environmental factors. Numerous studies and engineering investigations have reported that cracking and expansion of microcracks in concrete structures under various conditions provide a channel for the diffusion of erosive media, which is the most important factor leading to the deterioration of concrete and thus reducing the durability of the structure. Shrinkage of concrete is an important causative factor for its cracking, and the rational use of mineral admixtures can effectively reduce the shrinkage, which is an important way to improve the crack resistance of concrete. Moreover, concrete is subjected to continuous tensile stress caused by contraction and temperature difference during the setting process. It has been proved that improving the early tensile creep of concrete can relax the tensile stress, and thus effectively inhibit the cracking of concrete. In this study, a design method of low shrinkage high creep cementitious materials was realized by adjusting the mineral admixture amount, mineral admixture fineness and water-cement ratio. The enhancement of cracking resistance was finnally obtained as expected.

P·II 42.5 cement, Class II fly ash, S95 slag, ultrafine fly ash and S115 slag were selected for this test. The mineral admixtures were used in a double mixing mode combining coarse and fine particles, i.e., double mixing of S95 slag with ultrafine fly ash, and double mixing of S115 slag with Class II fly ash, with the range of mineral admixtures to the total cementitious materials from 50% to 80%. The substitution rate of each component changed with a increment of 10%.

Three series of tests were included in this work: a full-age shrinkage test, an ultra-early tensile creep test, and a cracking test. The full-age shrinkage test apparatus was used to test the autogenous shrinkage from casting to 3 d and the drying shrinkage from 3 d to 28 d of cementitious materials. The ultra-early tensile device was used to test the tensile strain of cementitious materials from initial to final setting. This device used a polyurethane mold to hold the specimen and transfer the tensile load. The creep displacement was monitored by a laser displacement sensor. A low-elasticity circular restrained cracking mold was used to monitor the restraining stresses due to autogenous shrinkage from casting to demolding, as well as the restraining stresses caused by drying shrinkage after demolding. The mold was made of polyvinyl chloride resin plastic material. A pair of vertically placed strain gauges were pasted on the inner surface of the inner ring to detect the stresses.

Shrinkage test results show that at constant total replacement rate of mineral admixtures, the mixing group of ultrafine fly ash and S95 slag is smaller than the mixing group of Class II fly ash and S115 slag. The reason is that the activity of the slag is higher than that of the fly ash. Using of smaller fineness slag and larger fineness fly ash in combination is beneficial to decrease the difference in the reaction rate of these two materials, leading to a reduction in the porosity. When the total replacement rate of mineral admixture is 60%, the effect of improving the particle grade matching to reduce the shrinkage of the cement stone reaches maximum. In addition, the shrinkage of the cement stone increased with increasing water-cement ratio for all different mineral admixture substitution rates. Ultra-early tensile creep test results show that under relatively high total substitution rate (60%-80%), the ultra-early creep of cementite with different particle gradation shows obvious differences. Compared with the mixing group of Class II fly ash and S115 slag, the mixing group of ultra-fine fly ash and S95 slag exhibits larger ultra-early creep and later cracking. Contrary to the shrinkage law, under relatively high total substitution rate (60%-80%), increasing the substitution rate of ultrafine fly ash while simultaneously decreasing the substitution rate of S95 slag, the ultra-early creep is shown to increase first and then decrease. Increasing the water-cement ratio slightly increases the ultra-early specific creep and delays the cracking time. The use of low shrinkage high creep cement stone can effectively reduce the internal stress growth rate, resulting in shrinkage stress relax and cracking time delay.

Experimental studies are carried out on the early age shrinkage, creep and cracking properties of cementitious materials. The following conclusions are drawn. The cement stone using S95 slag with coarse particles and ultrafine fly ash with fine particles in combination exhibits characteristics of low shrinkage and high creep. When the total replacement rate of mineral admixture is in the range of 60%-80%, increasing the admixture of ultrafine fly ash while simultaneously decreasing the admixture of S95 slag, the shrinkage of specimens is observed to decrease first and then increase. While the ultra-early creep is characterized by increasing first and then decreasing. The use of low shrinkage high creep cement stone can effectively reduce the growth rate of the internal stress and delay the cracking time.

The interfacial bonding properties between steel fibers and cementitious matrix under harsh marine environment are crucial for the mechanical properties and durability of UHPC. However, studies on the deterioration mechanism of the steel fiber-cementitious matrix interface in marine environments are insufficient. There is a lack of effective approaches to improve the interfacial bonding properties. This study provides novel approach to strengthen the interfacial transition zone and enhance the interfacial bonding properties using silane coupling agent (SCA). Based on molecular dynamics simulation and steel fiber pull-out test, the variation rules of bonding performance and debonding strength of steel fiber-matrix interface in UHPC under harsh marine environment were clarified. The microstructural variation, bonding network evolution and bonding trend of interfacial chemical bonds for the pristine and SCA-modified interfaces in the harsh marine environment were investigated. Based on this, the interfacial bonding degradation mechanism of the pristine and modified interfaces under marine chloride environment was revealed. This study is expected to provide theoretical basis and methodological support for optimizing and regulating the durability of UHPC in harsh marine environment.

Based on molecular dynamics simulation, the variation of interfacial interaction energy and initial contact area before and after SCA modification in the harsh marine environment was calculated and the images of interfacial configurations in the marine environment were captured. Using interface debonding simulation and steel fiber pull-out experiments, the load-displacement curves and interfacial fracture energy pristine and SCA-modified interfaces were investigated during the debonding process in the marine environment. Microscopic morphology of interfacial transition zones in the marine environment was observed by scanning electron microscopy (SEM) and backscattered electron imaging (BSE). Moreover, molecular dynamics simulation was used to statistically characterize the evolution of the hydrogen bonding network in the interfacial region and to calculate the formation probability of chemical bonds during the interfacial debonding process.

The bonding property of the steel fiber-matrix interface under marine environment was severely damaged, where the initial contact area of the interface was significantly deteriorated. Compared to the dry environment, the initial contact areas of pristine and modified interfaces under the NaCl environment were reduced by 51.6% and 39.2%, respectively. Moreover, the interfacial transition zone around the SCA-modified steel fiber was dense. The interfacial bond strength and pull-out energy of the SCA-modified sample after seawater erosion were 23.6% and 21.3% higher than those of pristine sample, respectively. The SCA modification can effectively enhance the adsorption effect between interfacial molecules and improve the denseness of the interfacial transition zone, thus improving the interfacial debonding strength.

The hydrogen bonding network within the pristine interface consists only of O_CuO—H_CSH hydrogen bonds, while that within the SCA-modified interface consists of O_APS—H_CSH, O_CSH—H_APS and N_APS—H_CSH hydrogen bonds. The number of hydrogen bonds in the modified interface is over 2 times higher than that in the pristine interface. The high atomic compatibility between the cross-linked APS molecules and the C-S-H matrix leads to the formation of a variety of stable hydrogen-bonded connections in the interfacial region, which enhances interfacial bonding.

Under the marine environment, the bonding probability between the C-S-H matrix and water molecules in the pristine interface is higher than that between matrix and steel fibers. The bonding probability between the Cl^- and C-S-H matrix is much higher than that between the matrix and pristine steel fiber atoms in the dry environment. It is indicated that in marine environment, water molecules and aggressive ions are prone to intrude into the pristine interface, forming strong interactions with the interfacial materials and destroying the interfacial bonding between the steel fiber and matrix. Conversely, in the marine environment, the bonding probability between the modified steel fiber and the matrix is higher than that between the interfacial material and the water molecules. Specifically, in the NaCl environment, the bonding probability of silicon-oxygen covalent bonds between the cross-linked APS and the C-S-H matrix remains high. It is demonstrated that SCA modification facilitates the formation of a stable interfacial bonding network, enhancing the interfacial bonding properties and elevating the interfacial corrosion resistance to water molecules and aggressive ions.

The main conclusions of this paper are summarized as follows. 1) The marine chloride environment can seriously damage the integrity of the steel fiber-matrix interface and weaken the interaction energy between the steel fibers and matrix. The SCA modification can effectively enhance the adsorption effect for the interfacial molecules and improve the denseness of the interfacial transition zone, thus improving the interfacial debonding strength. 2) Strong interfacial bonding between the steel fiber and the matrix is achieved through the formation of hydrogen bonds. Water molecules in the erosive environment seize the initial hydrogen bonding bonding sites, generating O_water—H_CSH, O_CSH—H_water hydrogen bonds, breaking the bonding network connection between the steel fiber and the matrix. Moreover, Na⁺ and Cl^- are easy to accumulate on the surface of C-S-H matrix, cutting the covalent bond connection between steel fiber and matrix, expanding the invasion channel of water molecules, and eventually exacerbating the interfacial deterioration. 3) Due to the rich type of hydrogen bonding and large number of hydrogen bonds between SCA-modified steel fibers and the matrix, a strong hydrogen bonding network connection is formed within the interface, which improves its resistance to water molecules and aggressive ions. Furthermore, in the SCA-modified interface, the cross-linked APS molecules can develop stable silicon-oxygen covalent bonds with the matrix, and its good compatibility with the matrix promotes the crosslinking of silicate chains in the interfacial zone. This leads to enhanced interfacial bonding properties and ultimately improved interfacial corrosion resistance to the harsh marine environment.

On the Loess Plateau in western China, there exists a large number of saline soils, most of which contain salts, typically of sulfate and chloride salts. Since the sulfates in saline soils are sensitive to temperature changes, they are susceptible to salt crystallization expansion and frost heave at low temperatures, which can lead to destruction of concrete structures. Salty soils and freeze-thaw environments cause concrete to be subjected to the coupled effects of simultaneous sulfate attack and freezing and expansion, which further increases the deterioration degree of concrete and seriously threatens the durability of concrete in this area. Adding fibers to concrete can effectively inhibit the plastic shrinkage cracking and improve the durability of concrete such as wear resistance, impermeability and frost resistance. Basalt fiber (BF) has excellent properties such as high strength, light weight, high temperature resistance, high elastic modulus, fracture resistance and corrosion resistance, and it is easy to disperse in concrete and has excellent cooperative work performance with concrete. Therefore, in this paper, the durability of concrete is improved by incorporating an appropriate amount of BF into the concrete to reduce the initial defects and slow down the rate of corrosive ions into the interior of the concrete. Furthermore, the effect of different BF admixtures on the relative dynamic modulus of elasticity, mass loss, compressive strength, and splitting tensile strength of concrete during salt freezing were investigated. The effect of the included BF is also clarified.

The concrete was prepared by P·O 42.5 ordinary Portland cement, II grade fly ash, 5-20 mm continuous graded gravel and natural river sand fineness modulus of 2.83. The volume content of basalt fiber was 0%, 0.1%, 0.2% and 0.3%. The test was carried out with reference to the quick-freezing method in GB/T 50082—2009 'Test method for long-term performance and durability of ordinary concrete', using HC-HDK9/Y type fast freeze-thaw cycle tester. According to the content of corrosive ions in saline soil in western China, Na₂SO₄ salt solution with a concentration of 23 g/L was selected for freeze-thaw cycle test. The loss rates of mass, relative dynamic elastic modulus, the compressive strength and splitting tensile strength during the test were tested. At the same time, Numerical simulation tests were carried out using finite element software ABAQUS to analyze the changes of internal stress and strain fields of basalt fiber reinforced concrete (BFRC) during the tests.

The mass and relative dynamic elastic modulus of BFRC under the action of Na₂SO₄ solution-freeze-thaw cycle raised first and then decrease with growth of the number of freeze-thaw cycles. The compressive strength and split tensile strength decrease with the increase of the number of freeze-thaw cycles. The addition of BF slows down the formation of corrosion products and expansion products. Under the same number of freeze-thaw cycles, the addition of BF reduces the generation of microcracks in concrete and prevents crack propagation. The addition of BF improves the freeze-thaw resistance of concrete in Na₂SO₄ solution. And the freeze-thaw resistance performance gradually improves with the increase of the BF volumetric doping within the range of 0.1% to 0.3%. Moreover, the best performance of freeze resistance in Na₂SO₄ solution is achieved when BF doping is 0.3%.

The internal stress and strain fields of BFRC with different BF dosage during Na₂SO₄ solution-freeze-thaw cycle test decrease with increasing the BF dosage. At the same BF dosage, the stress and strain increase with increasing the number of freeze-thaw cycles, and the dynamic numerical simulation was in good agreement with the experimental results. Using the 'equivalent temperature load' method, the salt solution-freeze-thaw cycles test process can be effectively simulated by ABAQUS finite element software. The numerical simulation results also show that the stress field of concrete and BF as well as the strain field of BFRC have changed, and that BF plays the role of transferring and dispersing the stresses during the freeze-thaw cycling process, which leads to the phenomenon of stress redistribution inside the concrete, and improves the salt-freezing resistance of concrete.

In this work, the macroscopic properties and microscopic changes of BFRC with different amounts of BF were studied under the combined effect of Na₂SO₄ erosion and freeze-thaw. The main conclusions are that the addition of BF significantly reduces the loss rate of concrete mass, relative dynamic elastic modulus, compressive strength and splitting tensile strength during the freeze-thaw cycle of Na₂SO₄ solution. BF can transfer and disperse the stress, so that the stresses are redistributed inside the concrete during the freeze-thaw process, thus improving the salt-freeze resistance of concrete. In addition, when the BF content is 0.3%, BFRC has the best resistance to Na₂SO₄ salt erosion-freezing performance. Meanwhile, BF shows an important effect in the evolution of freeze-thaw damage evolution of concrete, which can be visualized effectively by finite element model.

With the implementation of national policies like the 'Belt and Road Initiative', major infrastructure projects are rapidly advancing into the western plateau regions. These regions, nestled deep inland, exhibit typical plateau environmental characteristics, including frequent temperature fluctuations, low precipitation, strong winds with high velocities, low atmospheric pressure, limited oxygen levels, and widespread saline soil/salt lakes, which pose serious challenges to the long-term serviceability of major infrastructure. Determining the service environment is a prerequisite for conducting durability analysis and design of concrete structures. However, compared to the increasingly mature research on extreme loads such as earthquakes and typhoons, research on the long-term environmental actions suffered by concrete is still in its infancy. Most existing studies analyzed environmental factors from the 'environmental' rather than the 'engineering' perspective, lacking suitable environmental analysis tools for actual engineering applications. Furthermore, although extensive experimental research has been conducted on concrete durability, the inconsistency between the actual service environment and laboratory accelerated tests can lead to significant discrepancies in concrete deterioration rates and damage mechanisms. Using performance indicators obtained under laboratory accelerated conditions to design the expected service life of concrete in actual engineering projects will result in significant errors. The absence of a long-term environmental action model for concrete is a key challenge to the quantitative design of concrete durability.

Based on 21 years of meteorological data from 687 weather stations and chemical composition data from 396 salt lakes, using ordinary kriging (OK) interpolation method, with the geographic coordinate system as World Geodetic System 1984 and the projected coordinate system as Albers, regional distribution maps of key environmental parameters were created. Python's distfit library was used to fit the best probability density function (PDF) of environmental data. The Kolmogorov-Smirnov (K-S) test, a non-parametric statistical method, was employed to determine if the sample data sets conformed to the theoretical PDF. Environmental data were categorized using the unsupervised learning method K-means clustering. An intelligent and efficient WeChat mini program was developed to provide technical support for durability analysis and longevity design of engineering structures under special environmental action in the Chinese plateau regions.

The action levels of temperature differences in China were divided into six zones, closely related to the terrain. The temperature differences in the western plateau regions were significantly greater than that in the eastern plain regions. In cities such as Xizang, Xinjiang, Sichuan, Qinghai, and Heilongjiang, the maximum cumulative monthly average temperature difference reached 23 ℃. The cumulative monthly average relative humidity gradually decreased from the southeast coast to the northwest inland, with most Chinese plateau regions having an average relative humidity of less than 40%. The frequency of freeze-thaw cycles differed significantly between the Chinese plateau and plain regions, with Xizang experiencing the highest annual average of up to 188 times. Taking freeze-thaw (F-T) temperature amplitude, the lowest freezing temperature and the annual freeze-thaw cycles as indicators, the F-T action levels in China were divided into six zones, nonfrozen zone, slight F-T zone, light F-T zone, moderate F-T zone, severe F-T zone, and extreme F-T zone, each showing significant differences in annual average freeze-thaw cycles. The variability of annual average atmospheric pressure was relatively small across regions, exhibiting a step-like pattern similar to China's terrain.

For regions with large temperature differences and strong aridity, the month with the most severe environmental action was considered the primary month, and representative values such as frequent occurrence, moderate occurrence, and rare occurrence were provided. For high-frequency freeze-thaw cycles, basic environmental parameters and key design parameters for different regions were provided. Given the relatively small variability of annual average atmospheric pressure across different regions, the mean value of each region was recommended. For hypersaline attack, representative values such as the 95th percentile, third quartile, median, and mean were provided, and it was suggested to set concentration gradients according to representative values.

Conclusions The main conclusions of this paper are summarized as follows. Special service environments affecting concrete durability, such as large temperature fluctuation, strong dryness, high-frequency freeze-thaw cycles, and low atmospheric pressure, have the most severe impact in the Qinghai-Xizang Plateau. There are significant differences in environmental actions across different regions of China, and even within the same province, it is not appropriate to summarize different cities in one group. Indicators characterizing the action level of freeze-thaw cycles include freeze-thaw temperature amplitude, lowest freezing temperature, and annual freeze-thaw cycles. There are distinct differences in freeze-thaw action levels in different regions of China, especially evident in the clear gradient differences in annual average freeze-thaw cycles. The formation and distribution of saline-alkali soils and salt lakes are closely related. A comprehensive analysis of aggressive ion concentrations in different salt lakes reveals strong variability, suggesting the setting of concentration gradients based on statistical results. A rapid query mini program for special environments has been developed on the WeChat platform, integrating meteorological data query, freeze-thaw action analysis, and environmental parameter zonation, offering technical support for improving the efficiency of obtaining real-time information on the special environment near the project location.

The bonding properties of Fiber Reinforced Plastics (FRP) -concrete composites is directly related to the load carrying capacity and durability of FRP reinforced concrete structures. In addition, the study of bond properties can help us better understand how FRP bars behave in concrete and how they react under specific environmental conditions (e.g., high temperature, humidity, corrosion, etc.). This is valuable for predicting and improving the long-term performance of concrete structures. However, in offshore engineering, structures are exposed to strong corrosive environments all year round. Under the erosion of internal FRP bars by corrosive ions and the highly alkaline environment of the concrete pore solution, the matrix on the surface of FRP bars firstly suffers from deterioration phenomena such as dissolution, plasticization, and microcrack formation. Consequently, the bonding properties of the FRP-concrete interface are seriously affected, leading to the deterioration or failure of the concrete structure. Therefore, improving the corrosion resistance of FRP reinforcement is an important measure to ensure the long-term service of FRP-reinforced concrete structures. In this paper, the resin matrix modification of GFRP bars was realized by self-synthesized organosilicones and surface hydrophobic modification of nanofillers to obtain highly corrosion-resistant GFRP bars. To investigate the degradation of the interfacial adhesion between highly corrosion-resistant GFRP reinforcement and seawater-seasand concrete (SSC) in simulated pore solution of SSC at 60 ℃, and to reveal the mechanism of GFRP reinforcement-concrete adhesion enhancement through interfacial microstructure analysis.

Organosilicon synthesis: Hydrogen-containing silicone oil (PMHS) was mixed with γ-(2,3-epoxypropyloxy) propyltrimethoxysilane (KH560) in a mass ratio of 1∶1, with ethanol as the solvent, and reacted under the catalytic effect of NaOH for 8 h at ambient temperature using a magnetic stirrer (200 r/min), and then kept for spare.

CNT surface modification: 0.03% (mass fraction) of CNTs was stirred and dispersed into an ethanol and KOH organic solvent, and the CNT modifier was subsequently obtained by adding hydrogen-containing silicone oil (PMHS) and ethyl silicate (TEOS) after ultrasonic dispersion.

Surface modification of EG: EG was dripped onto the surface of expanded graphite in the mass ratio of TEOS∶PMHS=3∶1 in a vacuum environment; subsequently, the samples were placed in a microwave oven at 800 W for a rapid expansion for 10 s. The modified expanded graphite was placed in the organic solvents of KOH and anhydrous ethanol with heating and stirring until the mixture was homogeneous.

Preparation of modified epoxy resin hybrids: A bisphenol A-type epoxy resin (EGEBA), methyl tetrahydrophthalic anhydride curing agent (MeTHPA), and an accelerator (DMP-30) were mixed in a mass ratio of 100∶85∶5. Subsequently, 5% of the self-synthesized organosilicon was mixed with CNT and EG surface-modified nanofillers with a total solid content of 0.03% into the epoxy emulsion, and the mixed solution was stirred homogeneously using ultrasonication combined with triple-roller shear dispersion to obtain the modified epoxy resin mixture.

Preparation and testing of pull-out specimens: modified GFRP bars were embedded in the upper surface of concrete to a depth of 5 mm, and the size of the concrete was set to 100 mm × 100 mm × 300 mm. Hydraulic jacks were used to carry out single-end pull-out tests on the bonded specimens, and a digital CCD camera was used to capture the displacement and deformation of the interface throughout the entire process of de-bonding.

Microstructural characterization: the scaled-down specimens were prepared, and the microstructural changes at the FRP-concrete interface of the scaled-down specimens after corrosion were observed using scanning electron microscopy and X-ray computed tomography.

From the pull-out damage pattern of the GFRP-concrete interface, it can be seen that after 120 d of corrosion, large areas of resin degradation on the surface of the N-GFRP reinforcement resulted in fiber exposure and carbon fiber rib detachment. Compared with N-GFRP, the corrosion degree of M-GFRP bar is less severe, and its surface only has speckled local yellowing and surface roughness phenomenon. Compared with the unmodified bars, the initial interfacial adhesion between corrosion-resistant GFRP bars and concrete was improved by 7.87%, and the interfacial adhesion was improved by about 15% in the 120 d-corrosion experiment. In addition, the maximum strain values at the interfaces of N-GFRP and M-GFRP with concrete decreased to 22.8×10^-3、28.7×10^-3, respectively in the 120 d-corrosion experiment. After the modification of the resin matrix of the FRP bars, the corrosion resistance of the bars is improved, which can better absorb and disperse the stress brought by the external load, making the stress distribution at the FRP-concrete interface more uniform and the deformation increased, thus slowing down the rate of loss of adhesive force.

Corrosion to 120 d, the interfacial spacing between N/M-GFRP and concrete reached 158.11 μm and 67.91 μm, respectively, which increased by about 100% and 60% compared to the uncorroded condition. The V_r value at the M-GFRP-concrete bonded interface (23.35%) was higher than that of N-GFRP (18.52%). The surface substrates at the N-GFRP reinforcement-concrete interface all were detached and a new layer was exposed. In contrast, the localized matrix spalling at the M-GFRP-concrete interface was less corrosive than that of the N-GFRP bars. Therefore, the debonding of GFRP-concrete mainly depends on the degradation degree of the matrix at the interface. The affinity between the resin matrix and the concrete was improved by matrix modification, and the introduction of hydrophobic network led to the improvement of the waterproofing performance of the bars, which effectively mitigated the degradation of the resin matrix of the bars, and then improved their bonding performance.

The main conclusions of this paper are summarized as following. Compared with the unmodified bars, the initial interfacial adhesion between corrosion-resistant GFRP bars and concrete was improved by 7.87%, and the interfacial adhesion was improved by about 15% in the 120 d-corrosion experiment. In addition, the maximum strain values at the interfaces of N-GFRP and M-GFRP with concrete decreased to 22.8×10^-3、28.7×10^-3, respectively in the 120 d-corrosion experiment. Compared with the two, the maximum strain at the interface increased by about 25.87% after modification. The damage modes of the debonding process were dominated by concrete cracking and GFRP bar pullout. The degradation rate of the interfacial matrix was mitigated by resin matrix modification, which improved the adhesive properties of the GFRP-concrete interface.

As one of the most prevalent safety risks faced by modern building structures, fire can lead to elevated spalling failure of UHPC, resulting in rapid loss of load-bearing capacity, posing significant challenges to the serviceability, lifespan, structural stability, and safety of structural engineering. Conducting relevant research holds practical application value in pushing the limits of UHPC materials and structures in terms of elevated resistance and fire prevention capabilities. The main conclusions of this paper are as follows: 1) At elevated temperatures, the strength of UHPC is primarily influenced by factors such as its strength grade, temperature, cooling method, raw material composition, moisture content, reinforcement or fiber type, and specimen dimensions. As the temperature increases, the compressive strength, flexural strength, tensile strength, and bond strength of UHPC decrease, with the loss of strength exhibiting distinct trends within different temperature ranges. 2) At elevated temperatures, UHPC undergoes phenomena such as evaporation and diffusion of pore water, deterioration of pore structure, decomposition of C-S-H gel, and failure of the interfacial transition zone, which initiate the formation of microcracks and accelerate their propagation. 3) Regarding the spalling behavior of concrete at elevated temperatures, scholars have proposed theories including the vapor pressure theory, thermal stress theory, and thermal cracking theory. While these theories can, to some extent, explain the degradation mechanisms of ordinary concrete in extreme temperature environments, they remain controversial, particularly when applied to UHPC.

Despite the extensive research conducted by scholars worldwide on UHPC in the context of building fires and elevated environments, as well as the significant achievements made in experimental and theoretical analyses of fire resistance for concrete materials or structures, the current literature reveals a lack of effective technical measures to enhance the elevated resistance of UHPC, necessitating further in-depth investigation. This field is plagued by numerous critical issues that urgently require research and resolution: 1) The elevated environment inevitably exerts negative effects on the performance of UHPC, and the enhancements achieved through existing research on improving the thermal resistance remain insufficient. It is thus imperative to develop novel intelligent elevated-resistant concrete that can efficiently prevent, resist, and insulate heat, while also integrating rapid fire warning response capabilities, through advancements in concrete material composition design, mix proportion optimization, and structural directional control. 2) At elevated temperatures, UHPC undergoes simultaneous processes of secondary hydration enhancement from unhydrated cement particles and elevated degradation of hydrated product phases. The key to deeply analyzing and comprehensively understanding the degradation mechanisms of UHPC at elevated temperatures lies in achieving in-situ microscopic characterization of hydrated product phases and accurately quantifying the contribution ratios and development patterns of these two processes to the strength of UHPC. 3) The spalling behavior of UHPC at elevated temperatures is complex and multidisciplinary. The current theories on elevated failure can only partially explain the elevated degradation of UHPC, making it difficult to predict the randomness and uncertainty of UHPC's elevated peeling and establish a correlation mechanism. It is crucial to comprehensively explore the elevated failure mechanisms of UHPC by integrating multiple disciplines such as material mechanics, thermodynamics, fracture mechanics, and blast dynamics.

White light emitting diode (LED) is widely used as a high-quality lighting source, and its performance depends critically on the selected light-emitting materials. Early white LEDs were mainly composed of a combination of blue light chips and yellow light emitting phosphors, but suffered from low color rendering index, high color temperature, and blue light hazards. To solve these problems, researchers have turned to improving the color rendering performance of white LEDs by using red phosphors that can be excited by blue light.

Professor Zhiguo Xia's group at the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, has proposed a new material design principle for Eu²⁺-activated oxide phosphors that are difficult to obtain red light emission: Eu²⁺ occupying a low coordination number of polyhedra can cause a large crystal field cleavage, which can in turn realize red light or even near-infrared emission. A novel Sr₃Ga₄O₉ phosphor matrix with multiple low-coordination cation lattice sites was designed and synthesized, and broadband red light emission under 450 nm blue light excitation was successfully achieved by doping Eu²⁺, with the emission peak located at 618 nm.

Sr₃Ga₄O₉∶Eu²⁺ and Sr₃Ga₄O₉∶Eu²⁺, 0.05Zn²⁺ phosphors were prepared by high-temperature solid-phase method. According to the proportional content of the different raw materials in the chemical formula, SrCO₃ (A.R.: 99.5% analytically pure), Ga₂O₃ (A.R.), H₃BO₃ (A.R.) and Eu₂O₃ (99.99%) were weighed and ground in an agate mortar for half an hour. The mixture was transferred to an alumina crucible and fired at 900 ℃ in an air atmosphere for 6 h, then cooled naturally to room temperature. The mixture was ground again to powder form and refilled into the crucible, which was placed in a tube furnace with a continuous flow of reducing gas ([V(N₂)∶V(H₂)] = 80%∶20%). The samples were burned at 1 200 ℃ for 10 h, cooled to room temperature naturally, and then ground carefully to obtain the desired phosphor samples.

A white light illumination device was fabricated using a blue LED chip stacked with green and red phosphors. The green light was partly provided by Lu₃Al₅O₁₂∶Ce³⁺ (LuAG∶Ce³⁺) commercial green phosphor, while the red light was contributed by the experimentally prepared Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ phosphor.

The Sr₃Ga₄O₉ cell structure belongs to the P-1 space group of the triclinic crystal system. The results of XRD characterization tests indicate that the prepared samples are in pure phase. A heterovalent substitution strategy was used to introduce Zn²⁺ into Sr₃Ga₄O₉, and the XRD results showed that this co-doping did not change the structure of the matrix material. Further analysis revealed that Zn²⁺ ions replaced Ga³⁺ sites, while Eu²⁺ may occupy two 6-coordinated Sr²⁺ lattice sites.

Under the excitation of 450 nm, the Sr₃Ga₄O₉∶xEu²⁺ phosphor emits red light at 618 nm, and the emission intensity reaches the maximum at x = 0.04. The low-temperature emission spectra at 78 K can be decomposed into two Gaussian peaks, which indicates that the Eu²⁺ occupy two different luminescent centers and emit the red light, respectively. The fluorescence lifetime decay curves of the Sr₃Ga₄O₉∶0.04Eu²⁺ sample at low temperature (78 K) can be well fitted by the double-exponential function, which again demonstrates that the Eu²⁺ ions occupy two different lattice sites.

After Zn doping, the luminescence intensity of Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ phosphor was enhanced and the thermal stability was also improved significantly. The enhancement of thermal stability is from the presence of defects in the crystal, which compensates for the heat loss. The average decay lifetime of the co-doped Zn²⁺ is significantly higher than that of the sample without Zn²⁺, and a clear bulge of new pyroelectric peaks at 210 ℃ and 325 ℃ is clearly seen. Thus the addition of Zn²⁺ can increase the concentration of traps and contribute to the formation of deeper defect energy levels.

For Sr₃Ga₄O₉∶0.04Eu²⁺, the thermal stability is poor due to its shallow trap depth, which causes the electrons to escape from the shallow traps immediately at room temperature. Whereas, Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ has deeper trap 2 and trap 3 in it, and thus maintains a good thermal stability even at high ambient temperatures.

The white LED device was fabricated to cover the entire visible region of the emission spectrum with a color rendering index of Ra = 87, a color temperature of CCT = 3 500 K, and CIE color coordinates of (0.410, 0.380).

Novel Sr₃Ga₄O₉∶Eu²⁺ red phosphors that can be excited by blue light were designed and prepared, and their crystal structures and spectral properties were investigated. It is found that the Sr₃Ga₄O₉∶Eu²⁺ red light emission comes from two low-coordination polyhedra Sr1O₆ and Sr3O₆ occupied by Eu²⁺ with 5d-4f leaps. Zn²⁺/Ga³⁺ heterovalent substitution occurs by the introduction of Zn²⁺ into Sr₃Ga₄O₉, which is proved to introduce a defective energy level through the fluorescence decay lifetime and thermoluminescence to improve the thermal stability of the phosphor. A white LED device prepared using a commercial blue LED chip, Sr₃Ga₄O₉∶Eu²⁺,0.05Zn²⁺ red phosphor, and a commercial green phosphor, LuAG∶Ce³⁺, has a high color-rendering index, Ra = 87, and a low correlated color temperature, CCT = 3 500 K, which suggests that it is suitable for warm-white-light lighting.

The easy dissolution of Fe under acidic conditions results in the decreased activity of traditional Fenton-like catalysts. It was reported that the encapsulated or framework stabilized Fe-based materials showed excellent stability. Zeolite is a crystalline silicate/aluminosilicate with a stable framework structure, high specific surface area, uniform microporous and mesoporous structure, good acid-base resistance property. These characteristics make it a promising carrier or main material for Fenton-like catalysts. Therefore, a Fe-doped nanosheet MFI (MFI-Fe) zeolite was synthesized in this study. The Fe sites was embedded in the framework of MFI-Fe zeolite to inhibit the dissolution of Fe under acidic conditions, which could improve the performance of MFI-Fe zeolite for activating H₂O₂ towards acidic red G (ARG) degradation in water.

With the assistance of diquaternary ammonium-type structural directing agents, the solid products were obtained through hydrothermal reactions (160 ℃, 5 d) using tetraethyl orthosilicate and ferric chloride as the main raw materials. Then, MFI-Fe zeolite was obtained by calcining the solid products in air at 550 ℃ for 3 h. X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR), N₂ adsorption-desorption and X-ray photoelectron spectroscopy (XPS) were applied to determine the physicochemical properties of MFI-Fe zeolite. The prepared MFI-Fe zeolite was applied to active H₂O₂ for ARG degradation. The effects of catalyst dosage, H₂O₂ dosage, solution pH value, reaction temperature, ARG concentration and coexisting ions on the degradation of ARG were studied. The concentration of ARG in the water was determined by an ultraviolet and visible (UV-Vis) spectrophotometer.

According to the XRD and FTIR results, the prepared MFI-Fe zeolite exhibited the characteristic peaks of MFI zeolite, and there were not impurity peaks of silicon oxide or iron oxide, indicating that both silicon and iron entered into the framework of MFI zeolite. The XPS spectra of MFI-Fe zeolite exhibited peaks of Fe 2p_3/2 and Fe 2p_1/2 at 711.8 eV and 725 eV, respectively. They were very close to the binding energies of the 2p_3/2 and 2p_1/2 peaks of Fe₂O₃. Moreover, there was no Fe²⁺ peak (approximately 708 eV) in MFI-Fe zeolite. These results indicated that Fe in MFI-Fe zeolite existed in the form of +3 valence and was directly connected to O. The SEM images showed that MFI-Fe zeolite was composed of many nanosheets. Si and Fe were uniformly distributed in the crystals. According to the N₂ adsorption and desorption results, the specific surface area and external surface area of MFI-Fe zeolite reached to 343.71 m²·g^-1 and 176.78 m²·g^-1, respectively. The average pore size of MFI-Fe zeolite reached 4.19 nm. These structural characteristics endowed MFI-Fe zeolite with excellent catalytic activity. The catalytic experiment results showed that ARG could only be rapidly removed when both H₂O₂ and MFI-Fe zeolite existed. The reaction mechanism results indicated that HO•was generated by the activating of H₂O₂ with MFI-Fe zeolite, leading to the degradation of ARG. In solution having pH values of 3-9, the removal rates of ARG gradually decreased with the increase of pH values. It can be explained that a lower pH value conduced to the activation of H₂O₂, thereby accelerating the degradation of ARG. Under the optimum conditions (initial pH=3, 25 ℃, MFI-Fe 0.40 g/L, H₂O₂ 20 mmol/L), the removal rate of ARG (20 mg/L) reached to 98.40% within 180 min. The coexistence of NO₃^-, Cl^-, and SO₄^2- in the solution had not effect on the removal of ARG. The degradation of ARG was slightly inhibited by humic acid (HA). The removal rate of ARG was significantly slowed down with the coexistence of HCO₃^-. It can be explained that the pH value of the solution would increase and partial HO• would be consumed when HCO₃^- is added into the reaction system. After reuse for 5 times, the concentrations of leached Fe were less than 0.42 mg/L, and the removal rates of ARG exceeded 87.64%.

A nanosheet MFI-Fe zeolite was synthesized and applied to active H₂O₂ for ARG degradation. When the pH value of the solution was between 3 and 9, the removal rates of ARG gradually decreased with the increase of the solution pH values. The reaction rate reached to the maximum (0.023 min^-1) at a pH value of 3. Under the optimal conditions (pH=3, 25 ℃, 0.40 g/L MFI-Fe and 20 mmol/L H₂O₂), the removal rate of ARG (20 mg/L) reached approximately 98.40% within 180 min. The coexistence of NO₃^-, Cl^-, and SO₄^2- ions in the solution had not effect on the removal of ARG, while the degradation of ARG was slightly inhibited by humic acid (HA). The removal rate of ARG decreased significantly with the coexistence of HCO₃^-. After reuse for 5 times, the concentrations of leached Fe were less than 0.42 mg/L, and the removal rates of ARG exceeded 87.64%.

Accompanied by industrial production, the issue of hexavalent Cr⁶⁺ contamination in water environments is becoming increasingly severe. The adsorption method for removing Cr⁶⁺ from water has attracted attention due to its operational simplicity and low cost. MOF as adsorbents possess structural advantages such as large surface area and high porosity. Hybridization of MOF can increase the adsorption sites, enhance reaction activity, and further improve the adsorption performance of MOF. Research on synthesizing MOFs using KHP as an organic ligand is scarce. In this study, KHP was utilized as the organic ligand to synthesize urea-doped N/MOF(Fe) via a solvothermal method. The optimal urea doping concentration of N/MOF(Fe) was characterized using SEM, TEM, BET, XRD, XPS, and other techniques. The effects of factors such as the initial concentration of Cr⁶⁺, the dosage of N/MOF(Fe), the initial pH of the solution, the presence of coexisting ions on the adsorption reaction, and the corresponding mechannisms, were analyzed. The adsorption kinetic equation was established, the adsorption isotherm model, and adsorption thermodynamic parameters, were calculated.

After determining the optimal preparation conditions for MOF(Fe) through orthogonal experiments, a solution was prepared by adding 0.306 g of KHP 1.212 g of Fe(NO₃)₃·9H₂O, 0.6 mL of acetic acid, and 0.028 mmol/L of urea into 24 mL of DMF. The solution was stirred until completely dissolved and then transferred to a reaction vessel lined with PTFE. The reaction was carried out at 180 ℃ for 20 h. Afterward, a reddish-brown precipitate was obtained by centrifugation. The precipitate was washed several times with deionized water and ethanol, followed by drying overnight at 80 ℃. The resulting material was named as N/MOF(Fe). A certain amount of the adsorbent was added to a 140 mL solution of Cr⁶⁺. At specific intervals, aliquots of the solution were taken out and the absorbance was measured to study the N/MOF(Fe) adsorption behavior.

Orthogonal experiments indicate that controlling the pH value is crucial for the adsorption of Cr⁶⁺ by MOF(Fe). The optimal preparation conditions for MOF(Fe) are a reaction temperature of 180 ℃, a reaction time of 20 hours, a metal-to-ligand molar ratio of 1.0∶0.7, and a pH value of 2.3. After urea doping, the material's adsorption capacity and adsorption rate were significantly enhanced. When the urea doping concentration was 0.028 mol/L, the adsorption rate of N/MOF(Fe) reached its maximum. N/MOF(Fe) exhibited a spherical structure with a lattice spacing of 0.12 nm. The diffraction peaks were broad, indicating low crystallinity and diffraction intensity. BET analysis showed that the adsorption of N/MOF(Fe) followed a type II isotherm with a hysteresis loop, indicating primarily mesoporous characteristics with pore size distribution mainly around 5 nm. With the increase in urea doping concentration, the pore volume and pore size of N/MOF(Fe) first increased and then decreased. The adsorption effect followed the same trend. Characterization by XRD and XPS revealed that the Fe³⁺ present in N/MOF(Fe) did not undergo a change in oxidation state during the adsorption process. There was a certain amount of Cr³⁺ in the solution.

The optimal experimental conditions were found to be an initial Cr⁶⁺ concentration of 5 mg/L, an adsorbent dosage of 0.8 g/L, and no adjustment of the initial pH of the solution. Electrostatic interactions enabled N/MOF(Fe) to exhibit superior adsorption performance in acidic environments. The SO₄^2- being divalent, exerted stronger electrostatic attraction compared to chromate ions in the solution, leading to a more significant competitive adsorption effect. Apart from SO₄^2-, other coexisting ions have little effect on the adsorption of N/MOF(Fe). BET analysis indicated that the material regenerated twice exhibited similar N₂ adsorption-desorption isotherms to the original material. The regeneration process had a slight effect on the pore size, pore volume, and specific surface area of N/MOF(Fe), but it did not significantly affect its adsorption performance. N/MOF(Fe) demonstrated good repeatability in reuse tests, with the adsorption rate remaining at around 70% even after multiple regenerations. The n in the Freundlich model is 8.13, indicating that the adsorption process is the main chemical adsorption and is easy to occur. In the intra-particle diffusion model, the intercepts of all three stages were non-zero, reflecting that surface diffusion is not the only limiting step. The a in the fitting result of Temkin model is 0.671, which proves that the adsorption of Cr⁶⁺ by N/MOF (Fe) is an exothermic process. By BQ, the mechanism of Cr⁶⁺ adsorption by N/MOF(Fe) was investigated, revealing the involvement of ·O₂^- in the reduction of Cr⁶⁺ during the adsorption process.

Urea was employed as a structure-directing agent to modify MOF(Fe), resulting in the preparation of N/MOF(Fe) via a solvothermal method. The adsorption reaction followed a second-order kinetic equation, and the adsorption model conformed to the Langmuir model. The adsorption process was characterized as a spontaneous exothermic reaction. N/MOF(Fe) demonstrated excellent regeneration performance, with the adsorption process involving physicochemical reactions. This process facilitated the in-situ adsorption and reduction of Cr⁶⁺ by N/MOF(Fe), rendering it more environmentally friendly compared to traditional adsorbents.