[Objective] Current micromechanical models pay limited attention to parameter uncertainty and interactions, which makes it difficult for their response results to reflect the dispersed nature of the properties of cement-based materials. Therefore, it is necessary to explore an analysis method that can simultaneously capture the effects of multiple parameters and their interactions on the responses of micromechanical models of cement-based materials. [Methods] To address the discrete distribution of response results in existing micromechanical models and to identify and control the influencing factors causing this phenomenon, a multi-scale micromechanical model of cement-based materials was constructed in this study. Cement-based materials were divided into four scales: calcium silicate hydrate gel, cement paste, cement mortar, and concrete. Considering the mineral composition of cement phases, aggregates, and the ITZ, a multi-scale micromechanical model capable of accounting for the randomness of input parameters was proposed. Meanwhile, probabilistic methods were applied to the constructed micromechanical model, and global sensitivity analysis was employed to quantify the effects of input parameter uncertainty on the elastic modulus of cement-based materials. [Results] The results showed that the proposed model exhibited good applicability in simulating the relationship between elastic modulus and hydration degree of cement-based materials across multiple scales and showed good agreement with experimental results. The discreteness of the model response results mainly originated from the cross-scale propagation of input parameter uncertainty, indicating that uncertainty at the concrete scale incorporated the uncertainties of input parameters at the mortar and cement paste scales. The total-order sensitivity indices, ranked from largest to smallest, were the elastic modulus of sand and coarse aggregates, the volume fraction of sand and coarse aggregates, the elastic modulus of hydration products, the volume fraction of cement clinker, and the elastic modulus of cement clinker. To identify the dominant sources of uncertainty within the model framework, particular attention should be paid to the elastic modulus of sand and coarse aggregates, whereas the volume fraction and elastic modulus of cement clinker can be regarded as insensitive factors. [Conclusion] Screening the number of input parameters has important practical significance for reducing computational complexity and improving the efficiency of model response analysis.

[Objective] This study aims to reveal the exothermic properties and hydration mechanism of low-heat Portland cement (LHC) (hereafter referred to as low-heat cement) mixed with fly ash, the composition of pore solution during hydration, and the evolution mechanism of hydration products. [Methods] Long-term exothermic tests were conducted on the cementitious materials, and a quantitative relationship between heat release and hydration progress was established, from which the hydration state was obtained. Based on the theory of Gibbs free energy minimization using GEMS software, thermodynamic calculations were performed to construct a cement hydration model, whose validity was verified through comparative analysis between simulated and measured Ca(OH)₂ contents in neat paste. [Results] (1) The exotherm from low-heat cement hydration was mainly concentrated within 28 days. Fly ash significantly reduced the hydration heat of low-heat cement: a 20%-50% replacement ratio resulting in a reduction of 14.6%-32.7% in total exotherm after 720 days of hydration. Predictive models for exotherm and degree of hydration of the cementitious system were established. At hydration stabilization, the degree of hydration of low-heat cement reached 86.0%, and the reaction degree of fly ash was 53.3%. (2) The Ca(OH)₂ content calculated by the cement hydration model showed small deviation from measured values, indicating that the developed model adequately characterized the hydration process of low-heat cement. The cement was dominated by dicalcium silicate (C₂S). Compared with ordinary and medium-heat Portland cements, it produced more calcium silicate hydrate (C-S-H) gel and less Ca(OH)₂, which explained its higher later-age strength. (3) Different types of C-S-H were found to exhibit distinct saturation indices, ranked from highest to lowest as C_1.5S_0.67H_2.5, C_0.83S_0.67H_1.83, C_1.33SH_2.17, and C_0.67SH_1.5 (the last being unstable). All types showed linear positive correlations with the saturation index of Ca(OH)_2, as well as with OH^- and silicon ion concentrations, and a negative correlation with calcium ion concentration, with the relevant relationships established. (4) After fly ash incorporation (0-25%), the ettringite (AFt) content in the cementitious system gradually decreased to zero, while the monosulfate (AFm) content continuously increased. (5) At low fly ash dosages (0-20%), Si phases in fly ash first reacted with Ca(OH)₂ to form C-S-H with a higher Ca/Si ratio, increasing the average from 1.61 to 1.63. At higher dosages (20%-50%), C-S-H content decreased by 16.5%, and both Ca/Si ratio and pH declined markedly, primarily due to exhaustion of CH, reduced Ca phase, and increased Si and Al phases. At very high dosages (65%-80%), severe deficiency of Ca phase along with excess Si, Al, and Fe phases and lower pH caused extensive dissolution of C-S-H and a reduction in its Ca/Si ratio. (6) A relationship among pH, solution composition, and product saturation indices was established. At pH=12.86, the saturation indices of the above products were highly similar. When pH≤12.86, significant changes in the stable states of the products occurred. [Conclusion] Incorporation of fly ash further reduces the exotherm of low-heat cement, and heat release is significantly positively correlated with hydration progress. At low dosages, both the content and properties of C-S-H gel are improved, which benefits macroscopic mechanical performance and durability. pH 12.86 appears to be the pH inflection point for the stable state of hydration products, which should be considered when designing low-heat cement concrete with high fly ash dosages.

[Objective] This study aims to optimize the coupling conditions for CO₂ reinforcement of recycled coarse aggregate (RCA) by investigating the interactions of three key factors—CO₂ concentration, carbonation temperature, and relative humidity—using response surface methodology (RSM). The innovation lies in using a systematic RSM-based approach to model and optimize the carbonation process, overcoming the limitations of traditional methods by capturing complex inter-factor interactions. This provides a more efficient and reliable framework for enhancing RCA performance in sustainable construction applications. [Methods] A Box-Behnken design using Design-Expert software was applied, encompassing 17 sets of carbonation tests to evaluate the effects of CO₂ concentration (20%-60%), carbonation temperature (20-60℃), and relative humidity (35%-65%). RCA was derived from waste concrete blocks in Baotou, China, and characterized in accordance with GB/T 25177-2010, with particle sizes between 4.75 and 31.5 mm. The measured responses included crush value (indicator of mechanical strength), water absorption (indicator of porosity), and apparent density (indicator of compactness). Carbonation experiments were performed in a controlled environment, and the obtained data were utilized to develop quadratic regression models using RSM. Analysis of variance (ANOVA) was conducted to assess the significance, reliability, and interactions of the models, using evaluation criteria including F-statistic, p-value, coefficient of determination (R²), adjusted R², predicted R², coefficient of variation (C_V), and signal-to-noise ratio (Adeq Precision). Optimization was performed using the numerical module of Design-Expert to determine the optimal carbonation conditions, which were validated experimentally to confirm model accuracy. [Results] The interaction between carbonation temperature and relative humidity had the strongest effect (p>0.05 for BC interaction), followed by the CO₂ concentration-temperature (AB) and CO₂ concentration-relative humidity (AC) interactions. The CO₂ concentration-temperature (AB) interaction was the most significant, resulting in a parabolic response. Water absorption initially decreased with increasing CO₂ concentration and temperature, but increased under extreme conditions due to reduced CO₂ diffusion and calcium ion dissolution. The CO₂ concentration-relative humidity (AC) interaction was the most significant, making apparent density peak under moderate conditions (e.g., 42% CO₂ concentration and 44 ℃) and decline at extremes due to moisture-induced calcium loss or CO₂ saturation. The optimization process determined the optimal carbonation conditions as 38% CO₂ concentration, 41 ℃ carbonation temperature, and 49% relative humidity. Under these conditions, the predicted values were 14.3% for crush value, 3.80% for water absorption, and 2 700 kg/m³ for apparent density. Experimental validation produced measured values of 14.6% (crush value), 3.85% (water absorption), and 2 702 kg/m³ (apparent density), with relative errors of 2.1%, 1.3%, and 0.1%, respectively. All relative errors were below 5%, confirming model accuracy. Compared with untreated RCA, the optimized carbonation treatment reduced crush value by 18.0%, decreased water absorption by 20.5%, and increased apparent density by 0.9%, demonstrating practical effectiveness. Response surface diagrams and contour plots illustrated these interactions. For example, the temperature-relative humidity interaction for crush value showed a steep elliptical contour, while the CO₂ concentration-relative humidity interaction for apparent density presented a flat parabolic surface. These results highlighted the innovation of applying RSM to decipher complex multi-factor couplings, which previous studies did not fully address. [Conclusion] This study successfully develops and validates RSM-based regression models for optimizing the CO₂ reinforcement of RCA, with high reliability and precision confirmed by statistical indicators and experimental validation. The optimal conditions effectively improve RCA properties and provide a sustainable solution for waste concrete recycling and carbon emission reduction. The developed model offers a reliable reference for industrial applications, facilitating the adoption of CO₂ -modified RCA in concrete production. Future research can apply this approach to other aggregate types or larger-scale scenarios, further advancing a circular economy in the construction industry.

[Objective] This study aims to optimize the two-dimensional adaptive analysis strategy of the independent cover-based manifold method, focusing on addressing its deficiencies in error control and mesh distribution, thereby significantly enhancing computational efficiency and engineering practicality. [Methods] Based on the arbitrarily shaped and connected cover meshes of the independent cover-based manifold method, a “split-one-into-two” mesh splitting algorithm was employed for arbitrary refinement, and the degree of continuity of physical field derivatives was adopted as the error control indicator, forming an adaptive analysis strategy. An optimization scheme was proposed. 1) Adopting an absolute error indicator to replace the relative error indicator: the original relative error indicator tended to cause over-refinement in regions of minor stress and was overly sensitive in concave corner singularity regions. Using the absolute error indicator not only simplified the error judgment logic but also permitted larger error thresholds to be set near singular points such as concave corners, thereby effectively avoiding over-refinement. 2) Introducing a local mesh pre-partitioning and short strip elimination strategy: to address the issue of excessively high mesh density and irregular distribution in concave corner regions, a local pre-partitioning strategy was proposed, which pre-set the initial mesh in these regions by inwardly offsetting and reversely extending the edges of the concave corner. Simultaneously, an adjacent point merging algorithm was introduced during the mesh splitting process, which avoided the generation of extremely short connection strips and improved the conditioning of the system equations. [Results] Verification through two typical hydraulic structure examples, the square-hole and the gravity-dam model, demonstrated that the optimized scheme achieved a breakthrough improvement in computational efficiency. For the square-hole example, the original adaptive strategy generated 310 covers, corresponding to 6 520 degrees of freedom (DOFs). Under the same accuracy objective, the optimized scheme required only 59 covers and 933 DOFs. This represented a reduction of approximately 81% in the number of meshes and approximately 86% in DOFs. For the gravity-dam example, the original strategy generated 228 covers and 4 810 DOFs, whereas the optimized scheme required only 106 covers and 2 354 DOFs, achieving significant results of over 53% reduction in the number of meshes and 51% reduction in DOFs. The most notable achievement of the optimized scheme was in the effective suppression of mesh over-refinement near concave corner singularity regions. The calculation results demonstrated that the new strategy could generate more reasonable meshes, while ensuring computational accuracy, it substantially reduced the computational scale, and greatly enhanced the computational efficiency. [Conclusion] The proposed optimization strategy significantly enhances the efficiency of adaptive analysis while maintaining high accuracy. Through absolute error control and local mesh pre-partitioning, it effectively solves the problems of mesh over-refinement and unreasonable distribution near concave corner singularities, laying a foundation for subsequent three-dimensional adaptive analysis and engineering applications. Future research includes: criteria for selecting error thresholds and the highest order of cover series; further automating the local mesh pre-partitioning process to enable it to handle more complex geometries, ultimately achieving the goal of efficient and fully automatic simulation analysis for hydraulic structures.

[Objective] A systematic investigation is conducted on the detrimental effects of high geothermal temperatures (up to 60 ℃) encountered during Sichuan-Tibet Railway tunnel construction on the performance of shotcrete mortar. The primary objectives are to elucidate the impacts of elevated temperature on hydration kinetics, mechanical strength development, and microstructural evolution, with a focus on understanding the paradoxical phenomenon of early-age strength enhancement versus long-term performance degradation. The research aims to identify critical temperature thresholds and underlying mechanisms responsible for material deterioration, providing insights essential for developing durable shotcrete formulations in geothermal environments. [Methods] Cement mortar specimens that simulated shotcrete (Ordinary Portland Cement, water-cement ratio of 0.5, sand-cement ratio of 1.5) incorporating a commercial alkali-free accelerator (8% dosage) were prepared and cured under controlled humidity at 20 ℃ (as a reference), 40 ℃, and 60 ℃. A multi-technique experimental approach was employed. Isothermal calorimetry was used to track hydration heat flow. The compressive strength was measured at 1, 3, 7, and 28 days. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) were employed to identify phase composition and transformation. Low-field nuclear magnetic resonance (LF-NMR) was utilized to quantify pore structure parameters. Scanning electron microscopy/backscattered electron (SEM/BSE) imaging was applied to characterize the microstructural morphology and interfacial transition zones between cement paste and aggregate. [Results] Elevated temperatures significantly accelerated early hydration. The 60 ℃ specimens achieved 1-day compressive strength that accounted for 64% of their 28-day reference strength, which was more than double the ratio observed at 20 ℃. Calorimetry revealed intensified and earlier heat release peaks at higher temperatures, indicating rapid C₃A dissolution and AFt formation. However, temperatures ≥60 ℃ triggered severe microstructural degradation: XRD/FT-IR confirmed the destabilization of ettringite (AFt) and its conversion to weaker monosulfate (AFm), with AFm content increasing by 300% at 80 ℃. LF-NMR analysis demonstrated pore structure coarsening, with harmful pores (≥50 nm) increasing by 45% at 60 ℃ due to disordered hydration product precipitation and moisture loss. SEM/BSE imaging revealed thermal stress-induced microcracks (10-50 μm wide) at paste-aggregate interfaces and excessive CH crystallization. These synergistic effects caused pronounced strength regression—specimens at 60 ℃ peaked at 1-day strength (21.8 MPa) but declined to 34.9 MPa by 28 days, retaining only 78% of the strength exhibited by 20 ℃ controls. [Conclusion] This study identifies 60 ℃ as the critical threshold for irreversible shotcrete degradation in geothermal environments. Its principal innovation lies in decoupling the dualistic temperature response: while temperatures ≤40 ℃ enhance early strength through accelerated hydration, ≥60 ℃ induces multi-scale deterioration via AFt→AFm conversion, moisture loss-induced porosity, thermal microcracking, and pore coarsening. The quantified phase transformations and pore structure evolution provide a mechanistic explanation for long-term strength regression. These findings underscore the necessity of developing thermal-stable accelerators, optimizing binders with SCMs to suppress AFm formation, and implementing active cooling strategies for tunnels exceeding 60 ℃. The study establishes a scientific basis for designing high-performance shotcrete capable of withstanding extreme geothermal conditions in major infrastructure projects like the Sichuan-Tibet Railway.

[Objective] The Code for Design of Hydraulic Concrete Structures (NB/T 11011—2022) is the main reference for the design of reinforced concrete structures in hydropower projects. However, certain provisions in the standard, such as those concerning the design reference period and partial factors, remain difficult to apply or insufficiently clear, which may cause deviations in engineering practice. [Method] Drawing on many years of design experience with hydraulic structures of large and medium-sized hydropower stations, we analyze and discuss the issues related to the design reference period, service life, target reliability index, variable action values, partial factor setting, and seismic combinations. We also conduct cross-industry comparisons with building construction, highway, railway, and port codes, parameter back-analysis, and engineering practice verification. [Result] The structural target reliability index β_t in the hydropower sector was set at a relatively low level compared with that of other domestic industries, and that the statistical data on which it was based were outdated, making it difficult to reflect the current reliability level of hydraulic structures. Moreover, relevant provisions on the design reference period of statistical variable actions were missing, while the prescribed live load values for main and auxiliary powerhouse buildings and platforms of hydropower stations were found to match more appropriately with large and medium-sized plants designed for a 100-year reference period. Although the categories of partial factors differ from those in other sectors, the variability of the basic variables and their interactions that these factors account for is essentially consistent, and inclusions and overlaps exist in their conceptual definitions. The current values of partial factors for persistent design situations cannot always ensure that the safety level of hydraulic concrete structures is higher than that of building structures. In addition, treating seismic action solely as an accidental effect is inappropriate, as it ignores the fact that small- and medium-scale earthquakes are now statistically analyzable, which may lead to insufficient reliability in seismic combination limit states. [Conclusions] In response to these problems, we suggest that 1) future revisions of the standard improve the setting of the target reliability index β, differentiate the values of β according to the design reference period, and strengthen reliability research as well as statistical studies for different structure types such as dams, tunnels, powerhouses, and frame structures, so as to gradually achieve differentiated β settings across structure types; 2) specify a single design reference period for hydraulic concrete structures, with 100 years set as the benchmark in line with the service requirements of most major structures and a corresponding β value to match current engineering practice; 3) limit the design reference period, remove the specification of controllable variable actions, and adjust partial factor values to improve the safety level for persistent design situations. Finally, the paper outlines the equivalence relationship between partial factors in hydropower projects and those in other industries. The conclusions are intended to serve as a reference for engineering design practice and for future code revisions.

[Objective] This study aims to address the cracking and leakage in the foundation galleries of core rockfill dams situated on deep overburden layers by proposing a novel HECC (Hydraulic Engineered Cementitious Composite) dam foundation gallery structure. The structure involves a plastic hinge section made of the HECC material at large stress positions to adapt to the gallery deformation. [Methods] First, three-dimensional finite element numerical simulation method was adopted to analyze the deformation properties of the gallery, reveal the cracking mechanism of the gallery, identify the prone-to-crack parts, and determine the position of the HECC material. Then, by comparing the stress distribution of the gallery before and after installing the HECC plastic hinge section, the anti-cracking effect of the composite gallery structure was verified. [Results] (1) Under hydrostatic loads and self-weight, the gallery exhibited complex flexural deformation. Overall, the maximum displacement occurred at the center of the riverbed, with deformation gradually decreasing towards the banks, showing distinct reverse bending zones at both ends. (2) For conventional concrete gallery, significant tensile stress zones were primarily located at the ends. Within 30 m of the right end and 35 m of the left end, the tensile stress exceeded the tensile strength of the concrete. This was particularly pronounced in the 10 m to 20 m range from each end, where high levels of tensile stress were observed at the gallery’s crown and sidewalls. (3) In the proposed foundation gallery structure, 20m-long HECC plastic hinge sections were installed at the ends on both the left and right banks. These HECC sections fully entered plastic state, with the maximum equivalent plastic strain remaining within the material’s allowable limits, ensuring that the HECC segments would not leak. (4) As the HECC sections underwent plastic deformation, they accommodated most of the deformation from the main gallery body. Through stress redistribution, the overall stress level in the conventional concrete portion of the gallery was significantly reduced. The area where tensile stress exceeded the design strength in the typical cross-sections of the high-stress zones at the ends was markedly decreased, with a maximum reduction of approximately 70%, thus significantly lowering the risk of cracking. [Conclusions] (1) The regions within 30m of the ends of the foundation gallery in asphalt core rockfill dam are areas of intense stress variation and are prone to cracking. (2) The introduction of HECC plastic hinge sections in these high-stress zones at both ends allows the HECC sections to enter a plastic state first. Through internal stress redistribution, the stress in the conventional gallery sections is substantially reduced. The areas where tensile stress exceeds the design strength in typical cross-sections, especially in the high-stress end zones, are significantly reduced, thereby reducing the risk of cracking. (3) The HECC plastic hinge sections leverages the material’s inherent strain-hardening, ultra-high toughness, crack dispersion, and self-healing properties. This not only ensures the sections themselves remain impermeable but also effectively reduces or prevents cracking and leakage issues in the concrete gallery.

[Objective] Asphalt concrete is widely favored as an impermeable panel in pumped-storage power station design due to its excellent impermeability, deformation adaptability, self-healing ability, and ease of construction. At present, research on the application of neutral aggregates in asphalt concrete panels is limited, especially regarding experimental results on their key performance. This paper aims to promote the application of neutral aggregates in asphalt concrete panels in severe cold regions of western China. [Methods] Laboratory mix design and performance tests of asphalt concrete panels were carried out using neutral aggregate diabase. Thirteen groups of specimens were prepared to analyze porosity, 48 h Marshall flow value, splitting tensile strength, and splitting failure strain of asphalt concrete with different mix proportions. The optimal mix proportion was proposed as: gradation index 0.4, asphalt content 7%, and filler content 12%. To verify the adhesion between diabase coarse aggregate particles and asphalt, the boiling test was conducted. [Results] After 5 minutes of boiling, the asphalt film coating on the surface of diabase coarse aggregate particles was completely preserved, with the percentage of stripped area approaching zero and an adhesion level of Ⅴ. To compare the performance differences between neutral and alkaline aggregate asphalt concrete, comparative performance tests were conducted on asphalt concrete with diabase and limestone aggregates. The results showed that asphalt concrete with diabase aggregate exhibited better slope flow value and maximum flexural strain than that with limestone aggregate, while other mechanical and deformation properties showed no significant difference. [Conclusions] Under the recommended mix proportions, the permeability coefficient, water stability, slope flow value, splitting test, small beam bending test, direct tensile test, and bonding strength with cement concrete of the diabase aggregate asphalt concrete impermeable layer all meet the relevant technical requirements for asphalt concrete face dams. The freezing temperature of the recommended asphalt concrete mix is lower than -37 ℃, enabling it to withstand the local minimum temperature of -31.9 ℃ and satisfy design specifications. Based on the results of the static triaxial test, the Duncan-Chang model parameters of the asphalt concrete impermeable layer material with the recommended mix proportion are obtained, which can be applied to numerical calculations of dam stress-strain.

[Objective] This study aims to investigate the effects of different temperature and humidity curing conditions on the freeze-thaw resistance of concrete, and to conduct freeze-thaw cycle tests on cured specimens for quantitative analysis of mechanical properties and internal micropore structure. [Methods] Nuclear magnetic resonance technology (NMR) was used to measure internal pore parameters of concrete under freeze-thaw cycles and to study the evolution of pore distribution with the increasing number of cycles. Based on the NMR test results, the proportions of different pore categories and the changes in porosity during the freeze-thaw cycles were analysed dynamically. [Results] Compared with concrete cured at 20 ℃-95 % RH, whose relative dynamic modulus of elasticity remained generally stable after 300 cycles, concretes cured at 3 ℃-50% RH and 10 ℃-70% RH deteriorated sharply after 200 and 150 cycles, respectively, indicating that insufficient curing temperature and humidity led to complete hydration reactions, thereby severely affecting concrete durability. After different numbers of freeze-thaw cycles, the T₂ relaxation time distributions of concrete exhibited good continuity, with the overall pattern consisting of three parts: small pores, medium pores, and large pores (micro-cracks). However, for concrete cured under insufficient conditions, the third peak became markedly pronounced because the reduced temperature and humidity altered the distribution and connectivity of internal pores and micro-cracks, and the freeze-thaw cycles further increased the number of large pores and micro-cracks. According to porosity measurements and pore distribution characteristics, insufficient curing temperature and humidity resulted in inadequate hydration products filling the original pores, resulting in increased porosity. As the freeze-thaw cycles progressed, when subject to osmotic and frost-heave pressures, different pore types suffered damage and interconnection, and new micro-cracks even formed in the interfacial transition zone, causing porosity to increase continuously. Based on the freeze-thaw damage test results, the cumulative damage after different freeze-thaw periods was obtained using the relative dynamic modulus of elasticity as the damage variable. Comparison between test results and the established damage model yielded correlation coefficients all greater than 0.98, and a concrete freeze-thaw damage model was developed using the Boltzmann equation. [Conclusions] Analysis of the relationship between freeze-thaw cycles and cumulative damage shows that as the number of freeze-thaw cycles increases, internal cracks propagate and permeability intensifies, resulting in continuous accumulation of macroscopic damage. Investigating the freeze-thaw damage process of concrete under different curing conditions not only helps improve macroscopic mechanical properties and durability performance by optimising pore structure but also reveals the meso-scale damage evolution mechanism of hydraulic concrete under freeze-thaw action.

[Objective] With the implementation of China’s dual-carbon strategy, pumped storage has become increasingly important in the new-type power system dominated by renewable energy resources. As the operating intensity of pumped storage units continues to increase, vibration problems of pumped storage powerhouses have become increasingly common. It is necessary to summarize solutions to vibration problems in operating pumped storage power stations and units to guide the design of future stations. [Methods] This study presented the identification of vibration sources and solutions to vibration issues of pumped storage power stations in Zhanghewan, Heimifeng, and Guangzhou. Two widely used structural design schemes for pumped storage powerhouses—thick-plate continuous wall structure and plate-girder frame structure—were presented, with a discussion of their advantages and disadvantages. Based on the actual cases, the layout of vibration measurement points for both the powerhouse and the unit was provided. Test methods and evaluation indicators were established for powerhouse vibration. [Results] Three typical methods for alleviating vibration in the powerhouse and pumped storage unit were proposed: controlling the energy generated by hydraulic excitation sources, staggering the frequencies between stationary parts and hydraulic excitation sources, improving the local or overall stiffness of structures. For structure stiffness and vibration resistance, the thick-plate continuous wall structure and plate-girder frame structure showed no significant difference. For the measurement and calculation of powerhouse vibration, more attentions should be paid to individual structural components to prevent local resonance with hydraulic excitation sources. [Conclusion] Both the thick-plate continuous wall structure and plate-girder frame structure can be widely used for pumped storage power stations, depending on specific engineering requirements. The natural frequencies of overall and local powerhouse structures should maintain a frequency deviation of 20% from hydraulic excitation source frequencies. If the vibration velocity is used as an evaluation indicator, the maximum vibration values should be less than 10 mm/s on the powerhouse floor and 5 mm/s in the pit. This study provides important guidance for improving the structural design of pumped storage power stations in China.

[Objective] This study focuses on the issue of cracking in the support structures of high-temperature water diversion tunnels during the operation period caused by excessive tensile stress. An active thermal control strategy involving the application of thermal insulation coatings with specific thicknesses before water flow in the tunnel is proposed and systematically quantified. This study evaluates the regulatory effect of this strategy on the temperature and stress fields throughout the life cycle of the tunnel (from construction to operation) and its role in improving crack resistance safety. [Methods] Based on a three-dimensional thermo-mechanical coupled finite element method, a typical high-temperature water diversion tunnel was used as the engineering background. The temperature evolution and stress response of the structure under different thermal insulation coating thicknesses were precisely simulated. [Results] The thermal insulation coating significantly improved the temperature gradient of the secondary lining. As the coating thickness increased, the temperature difference between the inner and outer sides notably decreased. The application of thermal insulation coating before water flow in the tunnel effectively suppressed the temperature difference between the inner and outer sides of the secondary lining and the resulting tensile stresses. The coating thickness was positively correlated with the reduction in tensile stress, leading to a corresponding decrease in the area of zones that did not meet crack resistance safety criteria. In particular, when the coating thickness was 2 mm, the peak tensile stresses at all key locations of the secondary lining were below the ultimate tensile strength of the material. Except for localized high-stress zones, the crack resistance safety factor in the majority of the zones remained stable above 1.6, significantly outperforming the no-coating or thin-coating schemes. [Conclusion] Pre-applying a thermal insulation coating of appropriate thickness (such as 2 mm) before water flow in the tunnel is a highly efficient and innovative thermal control and cracking prevention strategy. This source-intervention approach significantly reduces the tensile stresses induced by temperature loads, fundamentally enhancing the structural safety and durability of high-temperature tunnels during long-term operation. The research findings provide direct quantitative design guidance and key technical support for similar engineering projects.

[Objective] The Three Gorges Hydropower Station is the only mega hydropower station in the world that employs three embedding methods of spiral cases. The operational conditions and differences among the units with these different embedding methods have attracted widespread attention. As the supporting system of hydroelectric generators, both the steel spiral cases and surrounding reinforced concrete structures have significant differences in construction processes and structural bearing characteristics due to different embedding methods. This study aims to comprehensively evaluate structural safety conditions, providing a basis for the safe operation of the power station and offering references for structural design and embedding method selection at other power stations. [Methods] Based on the monitoring data from the real-machine performance tests conducted during the experimental impoundment period of the Three Gorges Project, units with three types of spiral case embedding methods in the right-bank power plant were selected as the research subjects. The three-dimensional finite element method was used to conduct feedback comparative analysis of relevant monitoring parameters to summarize the structural bearing patterns and differences among different embedding methods of spiral cases. [Results] The finite element calculation results were consistent with the patterns of the monitored data, with close numerical values, validating the credibility of the monitoring results and the rationality of the design research and calculation methods. Under water pressure, the tensile stresses generated in both concrete and steel spiral cases were significantly greater in the circumferential direction than in the flow direction. The concrete bearing ratio of the direct embedding method was significantly higher than that of the other two schemes, while the stresses in concrete and steel spiral cases were generally at the same level in the pressure-maintaining and cushion methods. The concrete stress distribution in the direct embedding and pressure-maintaining methods was more uniform than in the cushion method. The stresses in the steel flow passage components, reinforcement bars, and the relative uplift displacement of lower frame foundation all remained within the design limits. The non-uniform uplift displacement of the lower frame foundation in the direct embedding method was notably greater than in other methods. Under thermal loading, concrete and steel spiral cases exhibited identical stress variation patterns. Tensile stresses developed during cooling and compressive stresses developed during heating. The stress in the flow direction was significantly greater than in the circumferential direction, and the magnitude of the stress increments was minimally affected by the spiral case embedding method. [Conclusions] Comprehensive analysis indicates that during the operation period, the structural concrete and steel components under all three embedding methods remain in the elastic stage of the materials, and the units are operating under the normal conditions anticipated in the design.

[Objective] This study aims to establish an efficient and reliable mechanical model to accurately evaluate internal forces and stresses within the force transmission system of fully balanced rack-and-pinion vertical shiplifts, providing reliable theoretical foundations for engineering design, structural optimization, and safety assessment. [Methods] Based on a comprehensive analysis of the structural configuration, material properties, and loading conditions of the nut-column force transmission system, a novel semi-infinite double elastic coupling foundation beam model is proposed. In this approach, the nut column and adjustment beam were modeled as interacting elastic bodies while incorporating their mutual deformation and force transmission mechanisms. To describe axial load transmission at the interface with complex contact mechanics, exponential distribution functions were introduced. The model integrated elastic foundation beam theory with fundamental mechanics of materials, enabling the derivation of closed-form expressions for deflection, internal forces (bending moments and shear forces), and stresses (normal and shear) in both the nut column and adjustment beam. These expressions accurately characterized the distinct mechanical behavior of the force transmission system under different loading scenarios. [Results] Theoretical predictions were validated through physical model testing, yielding several key findings. (1) Model validation: At x=L=4 950 mm, both deflections and internal forces of the nut column and adjustment beam approached zero, demonstrating the validity of the semi-infinite beam assumption. This simplified boundary condition treatment and provided a novel analytical approach for similar structural systems. (2) Parameter sensitivity insights: By comparing results under two interface constraint assumptions with α=π and α=2π, the model revealed the significant impact of α on structural response. While the α=2π assumption yielded more conservative results, α=π aligned better with experimental data. (3) New understanding of stress distribution: Contrary to conventional assumptions, the maximum bending moment in the nut column occurred not at the end but near it, indicating that the traditional use of end-moment M0 as the design load was inadequate. Moreover, except for axial force, all other internal forces in the adjustment beam exceeded those in the nut column, providing critical insights for structural optimization. (4) Stress comparison: Calculated stress values, though slightly exceeding experimental results, remained within allowable stress limits, demonstrating the rationality of the current design. The results obtained with α=π exhibited better agreement with experimental data, further confirming the model’s reliability. [Conclusion] The nut column force transmission system of the Three Gorges Ship Lift is structurally robust. The proposed semi-infinite elastic foundation beam model proves effective and applicable for similar designs. Incorporating the Pasternak elastic foundation beam model in future studies can further enhance analytical accuracy, providing a promising direction for both theoretical advancements and engineering applications.

[Objectives] When constructing concrete hydraulic projects in cold and high-altitude regions, or when promoting interconnection of water networks through the construction of concrete-based backbone projects such as sluices, ship locks, and pumping stations, long intervals in concrete pouring are inevitable, and the temperature difference between the upper and lower layers of concrete becomes a key concern. Although the upper-lower temperature difference, the allowable foundation temperature difference, and the internal-external temperature difference are three important temperature control indicators in the thermal control and crack prevention of mass concrete, the vague or overly theoretical definition of the upper-lower temperature difference makes its calculation inconvenient and monitoring difficult to implement effectively, making it hard to apply this indicator in actual construction. Therefore, effectively addressing the calculation and monitoring of the temperature difference between the upper and lower layers of concrete is of urgent engineering significance. [Methods] Based on existing definitions of the temperature difference between the upper and lower layers and considering the operability of monitoring, this study proposed four calculation methods following the principle of matching “calculation method-monitoring index”. These methods are based respectively on the “temperature within the geometric vertical centerline range” and the “temperature within the overall influence range” of the upper and lower pouring segments. Through simulating the temperature field and creep stress field of mass concrete with different interval durations, samples of maximum tensile stress in concrete and samples of upper-lower temperature differences under different interval conditions were obtained. Subsequently, statistical tests were conducted on the above samples to derive the corresponding probability density distribution functions of the maximum tensile stress and temperature difference between upper and lower layers. Then, the failure probability of the maximum tensile stress was determined based on the allowable tensile stress of concrete. Finally, assuming that the failure probabilities of the maximum tensile stress and temperature difference between upper and lower layers were equal, the allowable temperature differences between the upper and lower layers of concrete corresponding to the four calculation methods were proposed. [Results] Based on the concrete pouring of the bottom slab-guide angle section of a large ship lock, simulation calculations of the temperature field and creep stress field of mass concrete were carried out for different seasons (spring, summer, autumn, and winter) and various interval durations (30 to 180 days). According to the proposed method for determining the allowable temperature difference between upper and lower layers, four allowable temperature differences were derived: 29.14 ℃, 19.95 ℃, 20.29 ℃, and 18.02 ℃, respectively. These values showed some deviations from the currently recommended range of 15-20 ℃ in existing standards. [Conclusions] Because the calculation methods correspond to their respective monitoring indicators, different approaches to calculating the temperature difference between the upper and lower layers result in significant differences in the allowable temperature differences. Considering the operability of on-site monitoring, it is recommended to use the calculation method based on the “temperature within the geometric vertical centerline range of the upper and lower pouring segments” and its corresponding monitoring indicator to monitor the temperature difference between upper and lower layers of mass concrete on site.

[Objective] To promote the application of large-aggregate asphalt concrete in water conservancy projects, this study investigates the stress-strain and dilatancy characteristics of large-aggregate asphalt concrete under the same mix ratio but under varying influencing factors. [Methods] Under large shear deformation conditions (ε_a=30% ), static triaxial tests were carried out on asphalt concrete with D_max=26.5, 31.5, and 37.5 mm. The dilatancy characteristics were elucidated from the perspectives of confining pressure and different maximum aggregate sizes. The relationship between the phase transformation stress ratio (M_pt) of asphalt concrete and confining pressure as well as different maximum aggregate sizes was comparatively analyzed, and an expression for determining whether dilatancy occurred in the specimen based on initial parameters was established. To further demonstrate the applicability of large-aggregate asphalt concrete, the D_max=19 mm asphalt concrete in the core wall was replaced with D_max=37.5 mm asphalt concrete. Based on a finite element model that ignored the contact and dilatancy between the core wall and the rockfill body, stress-deformation calculations were performed on the asphalt concrete core wall of a typical project in Xinjiang to simulate the behavior of the core wall with large-aggregate asphalt concrete and analyze the influence of maximum aggregate size on the calculation parameters. [Results] (1) With increasing aggregate size, the stress-strain curve of asphalt concrete changed from the softening type to the hardening type. (2) Under the same confining pressure conditions, the tangent modulus E_t of large-aggregate asphalt concrete was lower than that of D_max=19 mm asphalt concrete. As the confining pressure increased, both the maximum deviatoric stress and the maximum volumetric strain of D_max=37.5 mm asphalt concrete decreased compared to D_max=19 mm asphalt concrete, indicating that appropriately increasing the maximum aggregate size could weaken the shear dilatancy. (3) An empirical expression for calculating the phase transformation stress ratio M_pt based on initial physical parameters (confining pressure, different maximum aggregate sizes) was proposed, which could serve as a criterion for the transformation between shear contraction and dilatancy in asphalt concrete. A larger M_pt value indicated stronger shear dilatancy. (4) Furthermore, the finite element analysis results showed that there were almost no differences in settlement rate, maximum minor principal stress, and maximum major principal stress of the core walls. The dilatancy characteristics of large-aggregate asphalt concrete met the requirements of high-stress and deep overburden conditions for high dam projects. [Conclusion] Under the conditions of this study, increasing the maximum aggregate size in the asphalt concrete core wall has almost no effect on its stress condition. The experimental results provide a theoretical basis for the promotion and application of large-aggregate asphalt concrete in high dam projects under high-stress and deep overburden conditions.

[Objectives] This study aims to investigate the lateral bearing characteristics of bucket foundation breakwaters, focusing on their failure modes, ultimate bearing capacity, the relationship between rotation and displacement, soil-structure interaction, and cyclic bearing behaviors. The goal is to improve computational theories of bucket foundation breakwaters and support their engineering design and applications. [Methods] Based on the actual structural configurations, a 1∶20 large-scale test model was designed, along with a foundation bed model simulating the sandy soil conditions of port areas, to simulate the service performance of bucket foundation breakwaters. The loading process covered the entire phase from post-installation to failure. While validating the test conditions using the finite element analysis, supplementary finite element analysis was conducted for different structural configurations and soil conditions, enabling a systematic study of the bearing characteristics and load response of bucket foundation breakwaters. [Results] (1) In the early stage of installation, the sidewall friction increased linearly, with the average friction measured in the test being 41.23 kPa and the average friction coefficient 0.405. (2) The failure mode was general shear failure. The displacement pattern at the limit state obtained from numerical analysis was consistent with the test results. When the displacement reached 150.3 mm, the maximum displacement of the soil on the rear side was 54.2 mm. (3) The ultimate bearing capacity obtained from the model test was 14.26 kN, while the finite element analysis calculated 15.07 kN, with a relative error of 5.68%. The deformation process of bucket foundation breakwaters could be divided into three stages: quasi-elastic stage, plastic stage, and failure stage. In the quasi-elastic stage, the displacement increased linearly, with an elastic limit displacement of about 1.0%L and a plastic limit displacement of about 3.0%L. (4) Before failure, earth pressure in the passive zone increased with displacement, reaching a maximum increment of 74.2 kPa. Earth pressure in the active zone was significantly smaller, with a maximum increment of 10.2 kPa. The variation trends of earth pressure on the connecting wall and the cylinder wall were consistent, and the deformation coordination between the foundation and the internal soil was relatively good. (5) The bearing capacity of bucket foundation breakwaters in sandy soil was better than in silty clay, which was better than in silt. Under the same displacement, rotation was more pronounced in sandy soil, while under identical loading conditions, the most significant rotation occurred in silt. (6) The ultimate bearing capacity of the bucket foundation breakwaters was negatively correlated with both the length-to-height ratio and width-to-height ratio, but positively correlated with the length-to-height ratio. (7) Under lateral cyclic loading, the bucket foundation breakwaters showed cyclic hardening during positive rotation and cyclic degradation during negative rotation. The stiffness reduction was more pronounced during positive rotation. [Conclusions] The deformation of bucket foundation breakwaters has three stages: quasi-elastic, plastic, and failure stages. The overall failure mode is general shear failure, and there is a high degree of deformation coordination between the foundation and the soil. Soil type significantly affects bearing capacity, with sandy soil performing the best and silt the worst. In addition, the geometry layout greatly influences performance. Under cyclic loading conditions, the bucket foundation breakwaters exhibit enhanced plastic deformation capacity, good energy dissipation, and excellent seismic performance.

[Objectives] This study conducts a systematic investigation into the influence of mud content on the mechanical properties and microstructure of Cemented Sand and Gravel (CSG), focusing on the low mud content range (<5%) that has not been fully addressed in previous research. The objectives include: identifying key factors affecting CSG strength through orthogonal experimental design; determining the optimal mix proportion balancing technical performance and economy; and revealing the micro-mechanism by which mud content affects CSG properties. [Methods] A four-factor (mud content, cement content, fly ash content, water-binder ratio) and four-level orthogonal experimental design (L₁₆(4⁴)) was used. Compressive strength, splitting tensile strength, and elastic modulus of CSG specimens were tested for 16 mix proportions at 7 days, 28 days, and 90 days. By graded washing of natural aggregates, the mud content was controlled at 0.39%, 1.28%, 2.05%, and 6.97%. Techniques such as X-ray diffraction (XRD), scanning electron microscope with energy dispersive spectrometer (SEM-EDS), and back scattered electron-image analysis (BSE-IA) were used to analyze hydration products, pore structure, and interface bonding characteristics. [Results] 1. Mechanical properties: Mud content was the most influential factor on compressive and splitting tensile strengths, with a significance ranking of: mud content > fly ash > cement > water-binder ratio. The optimal mix proportion—cement 60 kg/m³, fly ash 60 kg/m³, water-binder ratio 1.1, and mud content 2.05%—achieved a 28-day compressive strength of 7.68 MPa and an elastic modulus of 20.3 GPa. When the mud content increased to 6.97%, the elastic modulus decreased by 46.3% compared to the optimal group. Strength was age-dependent: compressive strength increased continuously (with an increase of >20% in each stage), while the growth rate of splitting tensile strength slowed after 28 days, stabilizing at 8%-11% of the compressive strength. 2. Microstructural Mechanism: In the low mud content (2.05%) group, the hydration process proceeded smoothly, promoting the formation of calcium silicate hydrate (C-S-H) gel, which effectively filled pores and cemented aggregates to form a dense structure. In contrast, high mud content (6.97%) caused unreacted mud powder to accumulate, which interfered with hydration and created interfacial cracks and large pores. XRD and EDS analyses further showed that excessive mud powder adsorbed free water, inhibited the secondary hydration of fly ash, and retained flaky calcium hydroxide (CH) crystals, ultimately reducing the overall integrity of the material. [Conclusions] This study innovatively fills the research gap on the influence of low mud content (<5%) on CSG performance. The proposed optimal mix proportion offers both economic and performance advantages, providing a practical solution for the direct use of natural aggregates with mud content in engineering (thus avoiding excessive washing). Microstructural evidence shows that appropriate mud content can improve material density through hydration products, while excessive mud content disrupts the hydration process and interfacial bonding between CSG components.

[Objectives] Traditional concrete is prone to brittle cracking under complex thermal-mechanical coupling conditions, which significantly increases the risk of gas leakage from underground gas storage reservoirs in Compressed Air Energy Storage (CAES) power stations. High-toughness cementitious composites (HTCC), due to their excellent toughness and impermeability, are considered as a potential structural lining material for CAES underground gas reservoirs. This study systematically investigates the gas permeability property and the evolution mechanism of the micro-pore structure of HTCC under thermal-mechanical coupling from an experimental perspective. A quantitative relationship between operational parameters (e.g., temperature and pressure) and gas permeability property is established, providing references for material selection in energy storage infrastructure. [Methods] Five groups of HTCC test specimens with different mix proportions were prepared. Their basic mechanical properties were evaluated through uniaxial tensile tests, and the mix with the best mechanical performance was selected to prepare ten groups of test specimens. Based on typical CAES operational conditions, nine test schemes were designed under a pressure of 10 MPa and temperature of 150 ℃. A self-developed temperature and pressure synchronized cyclic loading tester was used to simulate these operational conditions, and the ten groups of HTCC test specimens were subjected to ten cycles of loading. After the cycles, high-pressure gas permeability tests and mercury intrusion porosimetry tests were conducted to evaluate the effects of thermal-mechanical coupling on the gas permeability property and pore structure of HTCC. [Results] (1) The tensile-compressive strength ratio of HTCC reached 0.16, with a peak tensile strain exceeding 0.7% and an average crack width between 41-49 μm. HTCC demonstrated excellent tensile toughness and crack control capability, making it highly suitable for use in concrete lining structures of CAES reservoirs, and with optimized mix design, may also be applicable to the sealing layer. (2) The average gas permeability of the HTCC control group was 4.09×10^-18 m², and significant increases in permeability were observed after temperature and pressure synchronized cyclic loading. Under three pressure combinations (0-5 MPa, 0-7.5 MPa, and 0-10 MPa), when temperature increased from 25-50 ℃ to 25-150 ℃, three groups of test specimens showed maximum gas permeability increases of 112.7%, 183.6%, and 508.8%, respectively, compared to the control group. Moreover, temperature and pressure had distinct effects on permeability, with permeability being more sensitive to pressure than to temperature. (3) The gas permeability gradually decreased with increasing inlet pressure but tended to stabilize when the inlet pressure exceeded 3 MPa. (4) When the reservoir pressure was within 0-7.5 MPa, and the internal temperature reached 100 ℃, although the pore structure of HTCC changed, the critical pore diameter remained stable, and the permeability stayed within the order of 10^-18 m², which generally met the impermeability requirements of CAES reservoirs. However, when the operating pressure reached 10 MPa, the critical pore diameter increased, pore coarsening occurred, and new cracks formed, resulting in rapid degradation of impermeability. Therefore, if HTCC was to be used as the lining or sealing layer under 10 MPa pressure, it was recommended that its design compressive strength should exceed 40 MPa. [Conclusions] With excellent tensile toughness and crack control capability, HTCC can be applied to concrete lining structures of underground gas reservoirs in CAES power stations. When the operating pressure reaches 10 MPa, the impermeability of HTCC deteriorates rapidly. If HTCC is used as the sealing layer, its mix design should be optimized accordingly.

To explore the mechanical properties of ultra-high performance concrete (UHPC) with different steel fibers under uniaxial tension, we designed ten sets of uniaxial tensile tests on dog-bone-shaped UHPC specimens. We investigated how the aspect ratio (37, 50, 64, 65), dosage (2%, 3%, 4%), shape (end-hooked and curved), and forming methods(mixing and slurry) of steel fibers affect the tensile strength, stress-strain curves, tensile toughness, and failure process of UHPC. Results reveal that as the aspect ratio of hooked steel fiber rises, the tensile strength of UHPC increases by 6.54%-9.37%, and the residual strength ratio in the strain-softening segment grows by 5.00%-38.30%. When the volumetric dosage of end-hooked steel fiber increases from 2% to 3% and 4%, the tensile strength, peak strain, and residual strength ratio in the strain-softening segment of UHPC increase, along with an enhancement in tensile toughness. Compared with specimens formed by the mixing method, those formed by the slurry method with end-hooked steel fibers show no significant change in strength, but the peak strain increases by 53.61%-91.96%. The stress-strain curve of UHPC with curved steel fibers demonstrates strain-hardening characteristics, and its failure process involves the propagation of multiple cracks. In comparison to specimens with end-hooked steel fibers of the same aspect ratio, UHPC with curved steel fibers exhibits a 19.41-19.96-fold increase in peak tensile strain and an 18.00%-70.03% increase in the residual strength ratio in strain-softening segment. This indicates that the toughening effect of curved steel fibers is superior to that of end-hooked steel fibers. By using the hardening index and the residual strength ratio in the strain-softening segment, we can comprehensively evaluate the strain-hardening characteristics before peak axial-tensile strain and the axial-tensile toughness after peak in UHPC.

To tackle the high carbon emissions issue resulting from doubling the cement dosage in ultra-high performance concrete (UHPC), a low-carbon ultra-high performance concrete (LC-UHPC) was developed by replacing a large proportion of Portland cement with granulated blast furnace slag, fly ash, and silica fume. Eleven groups in a total of 154 specimens were fabricated with three factors, namely, cement replace ratio, steel fiber volume content, and water-binder ratio taken into account. Through cube compression tests at different ages, flexural tests, and uniaxial compression tests, the mechanical properties of LC-UHPC, including failure patterns, basic strength, and deformation capacity, were analyzed. Based on the test results, a mathematical equation for the stress-strain curve under uniaxial compression was derived. Results indicated that the LC-UHPC displays shear failure mode under uniaxial compression. Moreover, the addition of steel fibers significantly enhances the mechanical properties of LC-UHPC. Compared with conventional UHPC, up to 70% of the cement in LC-UHPC can be replaced, and its 28-day compressive strength can reach 149.09 MPa. The established axial stress-strain equation can accurately predict the mechanical responses of LC-UHPC under uniaxial compression. This equation provides valuable insights for studying the mechanical properties of LC-UHPC and the design of related structural components.