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.

Though the cement-based batteries and supercapacitors provide a new way to store the electricity, the related research and techniques are still at the infancy, and more issues need to be addressed in the future: 1) Effects of environment factors on the energy storage performance; 2) Balance between the cost and performance in terms of active components in cement-based electrodes and electrolytes; 3) Prediction of the lifespan of cement-based batteries and supercapacitors; 4) Recycle of some active components in cement-based electrodes, etc. The enhancements observed in the energy storage performance through the utilization of cement-based materials underscore the significance of incorporating conductive additives as a pivotal strategy for increasing energy density, power density, and current density. Nevertheless, several policy implications must be addressed to ensure the successful integration of active components in the development of intelligent, dependable, and sustainable cement-based batteries and supercapacitors. These include fostering effective collaborative partnerships among universities, research institutes, private enterprises, and state-owned entities; implementing monetary incentives such as government subsidies, tax exemptions, and specialized loans to incentivize the recycling of conductive waste in cement-based batteries and supercapacitors; facilitating the exchange of knowledge between scientists and industry stakeholders; integrating sustainable electro-tribological technologies into eco-labeling initiatives and the development of green conductive additives; and providing essential financial backing, technical support, and consultancy services to promote the practical application of cement-based batteries and supercapacitors across the energy sector.

Electromagnetic protection cementitious composites are widely applied in various civil-military building sectors, such as military command centers and research institutes and so on. The effects of filler components and structural design on the electromagnetic protection capability of cementitious composites were briefly summarized, providing a reference for the future development direction of functional cementitious composites. In order to minimize trial error costs, electromagnetic simulation analysis needs to be conducted to assess the contribution of various filler components in cementitious composites to electromagnetic protection effectiveness. The coupling effects among multiple factors should also be taken into account in this analysis. To elevate the feasibility of practical applications of metastructure in cementitious composites, the accuracy of the design of conductive resonance layers and long-term durability needs to be emphasized. Additionally, further research is required to study the impact of metastructure on the mechanical properties of cementitious composites. Furthermore, to support the carbon neutrality strategy, the development of electromagnetic protection cementitious composites based on waste materials including straw and iron tailings is a potential direction for reducing economic costs and addressing electromagnetic pollution issues.

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.

The wide range and strip-like distribution of high-speed rail crossings, as well as the exposed service characteristics, result in ballastless track concrete not only directly bearing train loads, but also facing freeze-thaw damage in severely cold or cold areas such as high-latitude cold regions and high-altitude mountainous areas. In the service process, the coupling beween environmental effects and train loads has significantly affected the ballastless track concrete at the micro- and macro-levels. However, current standards do not consider the effects of high-speed train fatigue loads and freeze-thaw cycles on the service performance of concrete. During the operation of high-speed trains, the flexural fatigue load frequency that causes damage to the concrete exceeds 20 Hz, which is a significant difference from the test system commonly used in current fatigue tests.

To reflect the actual service conditions of ballastless track more realistically in cold regions, a coupled test system of 'freeze-thaw cycles and 20 Hz fatigue loads' was designed. The effects of freeze-thaw cycles on the performance of concrete were analyzed from the perspectives of mechanical properties, fatigue life, and stiffness. The freeze-thaw cycles were divided into freezing and thawing stages, and their damage characteristics and energy transfer laws were analyzed. A life prediction model considering the coupling of freeze-thaw and fatigue was established from a probabilistic perspective to predict the service life of ballastless track manufactured sand concrete in cold regions.

The study used C60 manufactured sand concrete as the research object, with specimen dimensions of 100 mm (width) ×100 mm (height) × 400 mm (length). A SUNS-890 electro-hydraulic servo static and dynamic universal testing machine was used for fatigue tests. The stress level (maximum stress f_max/flexural strength f_f) was fixed at 0.6, stress ratio (minimum stress f_min/peak stress f_max) was 0.1, loading frequency was 20 Hz, and displacement data was simultaneously recorded during loading. After being saturated, the concrete underwent a series of freeze-thaw cycles, including 0, 150, 300, 450 cycles, and 600 cycles, before undergoing fatigue testing. To analyze the evolutionary of concrete performance during different stages, the freeze-thaw cycles were divided into two stages: freezing (-20 ℃) and thawing (0 ℃ and 20 ℃). After saturation, the specimens were placed at -20, 0 ℃, and 20 ℃ to conduct bending fatigue tests. Simultaneously, in order to analyze the temperature rise caused by fatigue loads at a constant temperature, a temperature measurement point was taken every 25 mm along the side of the concrete during the loading process, and a thermometer was used to measure its temperature.

After 300 freeze-thaw cycles, the ballastless track concrete exhibits accelerated brittle failure characteristics. Both flexural strength and fatigue performance show a trend of accelerated decline with freeze-thaw cycles exceeding 300 times, resulting in a 46.3% decrease in fatigue life and a 12.9% increase in stiffness degradation after 600 freeze-thaw cycles. The internal and surface defects caused by freeze-thaw cycles serve as initiation points for fatigue damage propagation, with the increased actual stress level making the concrete more prone to fatigue fracture. Defect connectivity induced by the cycles becomes the main factor contributing to the deterioration of concrete performance. From an energy perspective, the loading process involves both potential and internal energy, with concrete dissipating energy mainly through deformation energy generated by deformation. The decrease in maximum strain and increase in residual strain caused by freeze-thaw cycles result in reduced deformability of the concrete, thereby negatively affecting its fatigue performance. Ice within the concrete pores during the freezing stage has a reinforcing effect, leading to an improvement in fatigue performance. Additionally, ice helps alleviate internal friction and accelerates internal energy dissipation, thereby mitigating fatigue damage during the freezing stage. By introducing a fatigue damage factor, a life analysis model for concrete under freeze-thaw + fatigue service conditions are established. Predictions of concrete life under different environments are made based on freeze-thaw environmental criteria, revealing that C60 manufactured sand concrete can meet the freeze-thaw design requirements of cold region at level D1 and level D2, but considerations must be made for the intensified damage from fatigue loads.

A test regime of 'freeze-thaw cycle + high-frequency fatigue' was established to investigate the damage mechanism of concrete under the coupled action of freeze-thaw and fatigue from the perspectives of damage progression and energy transfer. It was found that high-speed train fatigue loads exacerbated the freeze-thaw damage of ballast-free track concrete, with defect connectivity caused by freeze-thaw cycles being the main reason for the decline in concrete fatigue performance. Freeze-thaw cycles lead to a decrease in maximum strain and an increase in residual strain, resulting in reduced deformability of the concrete. Compared to fatigue performance at room temperature, ice within the concrete pores during the freezing stage has a reinforcing effect, and ice in the frozen state can accelerate internal energy dissipation, thereby alleviating fatigue damage. A life analysis model for concrete under 'freeze-thaw + fatigue' service conditions, considering a fatigue damage factor, was successfully established to predict the service life of ballast-free track C60 manufactured sand concrete.

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.

The integration with ML is a major advancement in the field of cement-based materials. ML is able to process and to learn from huge datasets to predict a range of material properties. This approach not only solves the complex problem of nonlinear regression of materials, but also marks a new era in materials research. However, to realize the full potential of machine learning in this area, challenges such as imbalances in data quality and quantity, insufficient model interpretability and limited model commonality need to be addressed. The research prospect of cement-based materials by integrating with artificial intelligence is promising. And, with the continuous improvement of AI capability and computational power, we can foresee that more complex models combined with more advanced algorithms will be able to predict and design cement-based materials more accurately. This progress will likely lead to the development of new materials with enhanced properties, contributing significantly to the field of construction and material science. This research area remains a hotspot, promisingly exciting developments and breakthroughs in the near future.

As a heterogeneous multiphase material, cement-based material has a significant correlation between its macroscopic properties and microstructure, and exhibits significant scale dependent characteristics. Representative element is defined as the smallest element that can represent the macroscopic property of multi-scale material. It is usually very difficult to study materials at the full scale. Selecting representative element of appropriate size can to some extent represent the overall characteristics, providing possibilities for microscopic calculations. Therefore, the appropriate selection of representative elementary size (RES) is of great significance for further elaborating the structure-performance relationship in micro computational research. For cement mortar, there has been considerable research on the RES of mortar's microstructure of, but there is a lack of research on the RES of mortar's mesostructure. Only scholar such as Winslow reported a value of 10-30 mm in early research with relatively single dependent indicator. In view of this, combined with digital image processing and analysis techniques, the statistical characteristics of the spatial distribution of aggregates were calculated from two-dimensional and three-dimensional perspectives by using the average proportion function, two-point correlation function and radial distribution function in this paper. Finally, the representative elementary size of the mortar's mesostructure was explored.

Three types of cement mortars blended with cement, fly ash (FA), silica fume (SF) and sand were prepared in this research. The cement used was P·II52.5 in accordance with the relevant Chinese standard. The density of cement and FA are 3.11 g/cm³ and 2.27 g/cm³, respectively. The specific surface area of FA and SF are 420 m²/kg and 22 000 m²/kg, respectively. The fineness modulus of natural sands and manufactured sands are 1.66 and 2.25, respectively. Besides, one kind of polycarboxylic-type high performance water reducer was adopted as the superplasticizer and the water reducing ratio was not less than 40%.

The raw materials (cement, fly ash, silica fume, and sand) were mixed uniformly and then the mixed water and additives were added and stirred for 2-3 min until the mixture enters the viscous flow state. Then the uniform mixed slurry was poured at molds and vibrated to enhance compactness. The specimens were demolded after 1 d and then cured in the standard curing room ((20±2) ℃, room humidity >95%). After 28 d of standard curing, the specimens with size of 15 mm×15 mm×15 mm were cut and dried in vacuum at 60 ℃ for 3 d. Afterwards, computed micro-tomography (CT) testing was conducted and 1 000 CT slice images for each specimen were obtained with a resolution ratio of 10 μm.

After image processing, the influence of noise has been removed and the distribution information of aggregates has been obtained. It can be found that the presentation of natural sand (NS) in CT image is relatively clear and the particle size is relatively uniform, while the shape of manufactured sand (MS) is irregular and the particle size is uneven. Comparing the calculation results of the average volume fraction with the design volume fraction, it can be seen that the former is significantly lower than the latter. The reason is that the composition of the MS is uneven and uneven grayscale values inside may happen due to the crushing process. Therefore, some aggregate information will be lost during the image processing, resulting in a significant deviation between the final result and the design volume fraction. Although the grayscale range of the NS group is very close to that of the cement matrix, its composition is uniform and its shape is regular. After image processing, its distribution information can be retained to a high extent, so the deviation between its calculated volume fraction and the designed volume fraction is very small.

As the aggregate fraction increases, there is a significant fluctuation in the RES obtained by calculating the average area fraction on a single CT image, while the average volume fraction obtained by stacking multiple CT images on a three-dimensional structure is equivalent to the average value on a larger area, and the RES results tend to be consistent. The RES is not related to the volume fraction of sand, but is significantly affected by its type. For mortar with natural sand, RES is 2.75-2.90 mm, which is about 9 times of its average particle size. For mortar with manufactured sand, RES is 3.1-3.2 mm, which is about 7 times of its average particle size. Compared with the result mentioned by Winslow that the RES of mortar or concrete is within the range of 10-30 mm, the accuracy of the conclusion in this paper has been improved by one order of magnitude.

The RES of mortar prepared in this paper is not related to the volume fraction of sand, but is significantly affected by its type. For mortar with natural sand, the RES obtained through the average volume proportion function is 2.75 mm, and the RES is 3.1 mm for mortar with manufactured sand. For mortar with natural sand, the RES obtained through two-point probability function and radially distribution function are both 2.9 mm. For mortar with manufactured sand, the RES obtained through two-point probability function and radially distribution function are 3.2 mm and 3.1 mm, respectively.

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.

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.