Number Of Freeze-thaw Cycles Research Articles

Study on meso-deformation and failure mechanism of rock mass with micro-cracks under freeze-thaw loading

The long-term freeze-thaw effect leads to the development of rock fractures and strength degradation in cold region engineering construction, which poses a serious challenge to the stability of the project. In this paper, the microscopic model of sandstone freeze-thaw cycles was established using a particle flow code (PFC2D). Through numerical simulation, the variation law of mechanical properties of rock mass with micro-cracks is systematically studied from the aspects of displacement, crack development, strain and strength. The results show that: (i) The freeze-thaw loading displacement is concentrated on both sides of the initial micro-cracks, and the crack development is not controlled by it. When the number of freeze-thaw cycles is greater than 17, the displacement and crack development are significant. The crack increases with the increase of the crack distance ratio. (ii) When the number of freeze-thaw cycles is less than 15, the compressive crack develops at the end of the initial micro-crack, and the development is controlled by it. When the number of freeze-thaw cycles is greater than 21, the crack increases sharply, and the development is no longer controlled by the initial micro-cracks. When the number of freeze-thaw cycles is less than 15, the damage strain shows minimal variation but decreases sharply with the increase of freeze-thaw cycles. (iii) Under each crack distance ratio, the strength changes in three stages: when the number of freeze-thaw cycles is less than 15, the strength changes little, and the strength decreases sharply with the increase of freeze-thaw cycles. With the increase of the crack distance ratio, it increases first and then decreases. And it is more significant when the number of freeze-thaw cycles is greater than 17. The research results can provide theoretical support for the influence and evaluation of rock freeze-thaw action in cold regions.

Experimental Study of Shear Strength of Loess under Freeze-thaw Cycles

In order to study the shear strength characteristics of loess under the action of freeze-thaw cycle, through the freeze-thaw test, straight shear test, to analyze the change rule of water content and freeze-thaw cycle on loess cohesion, internal friction angle and shear strength. The test results show that: loess cohesion gradually decreases with the increase of water content and the number of freeze-thaw cycles, when the freeze-thaw 7 times, the change of cohesion gradually tends to stabilize; the angle of internal friction decreases gradually with the increase of water content and the number of freeze-thaw cycles, but the rate of decrease is smaller than the rate of cohesion; the change of shear strength is affected by the cohesion more than the angle of internal friction, and the shear strength at different times of freezing and thawing increased linearly with the vertical stress, and the relationship equations between the shear strength and the water content and the number of freezing and thawing cycles were established.

Experimental Study on the Microfabrication and Mechanical Properties of Freeze-Thaw Fractured Sandstone under Cyclic Loading and Unloading Effects.

A series of freeze-thaw cycling tests, as well as cyclic loading and unloading tests, have been conducted on nodular sandstones to investigate the effect of fatigue loading and freeze-thaw cycling on the damage evolution of fractured sandstones based on damage mechanics theory, the microstructure and sandstone pore fractal theory. The results show that the number of freeze-thaw cycles, the cyclic loading level, the pore distribution and the complex program are important factors affecting the damage evolution of rocks. As the number of freeze-thaw cycles rises, the peak strength, modulus of elasticity, modulus of deformation and damping ratio of the sandstone all declined. Additionally, the modulus of elasticity and deformation increase nonlinearly as the cyclic load level rises. With the rate of increase decreasing, while the dissipation energy due to hysteresis increases gradually and at an increasing rate, and the damping ratio as a whole shows a gradual decrease, with a tendency to increase at a later stage. The NRM (Nuclear Magnetic Resonance) demonstrated that the total porosity and micro-pores of the sandstone increased linearly with the number of freeze-thaw cycles and that the micro-porosity was more sensitive to freeze-thaw, gradually shifting towards meso-pores and macro-pores; simultaneously, the SEM (Scanning Electron Microscope) indicated that the more freeze-thaw cycles there are, the more micro-fractures and holes grow and penetrate each other and the more loose the structure is, with an overall nest-like appearance. To explore the mechanical behavior and mechanism of cracked rock in high-altitude and alpine areas, a damage model under the coupling of freeze-thaw-fatigue loading was established based on the loading and unloading response ratio theory and strain equivalence principle.

Experimental study and numerical simulation verification of the macro- and micromechanical properties of the sandstone–concrete interface under freeze–thaw cycles

In cold-region tunnel engineering, the bonding surface between concrete and surrounding rock is highly susceptible to engineering disasters such as the cracking of support structures under the influence of freeze-thaw cycles, which severely affects the stability of tunnel engineering. This study investigates the macroscopic and microscopic mechanical properties of the sandstone–concrete interface under freeze–thaw cycles and reveals the microdamage and fracture evolutionary laws during the freeze–thaw loading process via the coupled expansion of water–ice particle phase changes via particle flow numerical simulation methods, aiming to provide theoretical support for the design and construction of rock–soil and tunnel engineering in high-altitude frozen soil areas. The results indicate that the interface strength characteristics of the sandstone–concrete specimens exhibit a stable decreasing trend with an increase in the number of freeze-thaw cycles and a decrease in roughness. Taking the most unfavorable condition as an example, the exacerbation of freeze-thaw deterioration resulted in shear strength reductions of 16.53%, 27.01%, and 37.17% for specimens (JRC=3.727), while an increase in roughness led to shear strength increases of 11.00% and 15.14% for specimens (NT=60 cycles). The acoustic emission characteristics during the loading process of the sandstone–concrete interface specimens reflect the microcracking evolutionary activity of the specimens quite well, and setting 1.0 as the recommended value for the precursor determination of the b-value and CV(k) index is most reasonable. Under freeze-thaw cycles, cracks first initiate partial microcracks at the interface and on the outer side of the specimen, primarily dominated by tensile cracks and evolving with a "slow–fast" trend toward both sides of the interface. Under shear stress, particles at the interface first undergo slip dislocation due to their lower bonding strength, generating cracks, subsequently inducing significant displacement of particles on both sides of the interface, resulting in crack propagation toward both sides of the interface, ultimately penetrating and forming shear bands leading to macroscopic failure.

The Collapse Mechanism of Slope Rill Sidewall under Composite Erosion of Freeze-Thaw Cycles and Water

The composite erosion of freeze-thaw and water flow on slope rills is characterized by periodicity and spatial superposition. When revealing the collapse mechanism of slope rill sidewalls under the composite erosion of freeze-thaw and water flow, it is necessary to fully consider the effect of water migration and its impact on the stability of the rill sidewall. In this paper, we placed the self-developed collapse test system in an environmental chamber to carry out model tests on rill sidewall collapse on slopes under the composite erosion of freeze-thaw and water flow. We utilized three-dimensional reconstruction technology and the fixed grid coordinate method to reproduce the collapse process of the rill sidewall and precisely locate the top crack. We obtained the relationship between the water content of the specimen and mechanical indexes through the straight shear test. The main conclusions are as follows: The soil structure of the rill sidewall is significantly affected by the freeze-thaw cycle, which benefits capillary action in the soil. One freeze-thaw cycle has the most serious effect on the soil structure of the rill sidewall, and the change in the moisture field is more intense after the soil temperature drops below zero. The friction angle of the soil increases with the number of freeze-thaw cycles and tends to stabilize gradually. The effect of the freeze-thaw cycle on the rate of change of the water content of the soil at each position of the wall can be accurately described by a logarithmic function. The expression of the two-factor interaction effect on the rate of change of water content of soil at each position of the rill sidewall can be accurately fitted. We propose a calculation system for locating cracks at the top of the rill sidewall and determining the critical state of instability and collapse of the rill sidewall during the process of freeze-thaw and water flow composite erosion. The results of this research can help improve the accuracy of combined freeze-thaw and water flow erosion test equipment and the development of a prediction model for the collapse of the rill sidewall under compound erosion. This is of great significance for soil and water conservation and sustainability.

Bond performance of recycled coarse aggregate concrete with rebar under freeze–thaw environment: A review

Abstract In this article, a review focusing on the frost resistance of recycled coarse aggregate concrete (RAC) and the bond performance of RAC with rebar under freeze–thaw environment is carried out. The results show that there are still some controversies about the advantages and disadvantages of the frost resistance of RAC, and many influencing factors have not been considered. The mass of the RAC pullout specimens shows a trend of first increasing and then decreasing after freeze‒thaw cycles. The failure modes of the RAC pullout specimens after freeze‒thaw cycles include pullout failure, pullout-splitting failure, splitting failure, and rebar yielding failure. The bond performance of RAC with rebar shows a degradation trend after freeze‒thaw cycles, mainly in the decrease in bond strength and the increase in bond slip. The bond performance of RAC specimens is inferior to that of natural aggregate concrete specimens after freeze‒thaw cycles. The bond strength prediction models and the bond–slip constitutive relation prediction models of RAC with rebar after freeze‒thaw cycles have been summarized to ensure that engineers can better understand their applicability. The bond stress distribution between the RAC and rebar in the anchoring area is not uniform and constantly changes with the number of freeze‒thaw cycles. The degradation mechanism of the bond performance of RAC with rebar after freeze‒thaw cycles is analyzed.

Snow depth has greater influence on moss biocrusts' soil multifunctionality than the number of freeze-thaw cycles

Continued global warming and changes in precipitation patterns, especially regarding snow depth and freeze-thaw cycles (FTCs), have significantly affected the hydrothermal environment in winter at mid- and high-latitudes. In this study, we assessed the effects of changes in snow depth and FTCs, on the biogeochemical cycling of biological soil crusts. We collected moss crust soil samples from a temperate desert in northwest China, and measured the changes in the amount of snow (65 % snow removal, ambient snow, and 65 % snow increase) and FTCs (FT0, FT5, and FT15) under an artificial environment simulation, to determine the nutrient content and enzyme activities related to soil carbon, nitrogen, and phosphorus cycles. In terms of how snowfall and FTCs affect soil multifunctionality (SMF) of desert moss crusts, the research findings demonstrated that snow depth, FTCs, and their interplay have a significant influence on the carbon, nitrogen, phosphorus contents, enzyme activities, and SMF of moss crusts. Compared to snow removal, an increase in snow depth corresponded to a significant increase in the soil water content, and an improvement in soil carbon and nitrogen cycling and SMF. The number of FTCs increased, which led to enhancements in soil phosphorus cycling, soil total phosphorus, and available phosphorus content, together with a reduction in inorganic nitrogen content and the activities of certain enzymes involved in carbon and nitrogen cycling. The structural equation model indicated that snow depth had the greatest effect (total effect of 0.165) on SMF in moss crusts compared to FTCs (total effect of 0.048). Therefore, it is necessary to consider snow depth when examining the effects of climate change on nutrient cycling in the biological soil crusts of temperate deserts.

Influence of pre-seismic history on the cyclic response of seasonal frozen silty sand

The implementation of the Belt and Road strategy has increased construction activities in regions with seasonally frozen soil, such as the Qinghai–Tibet plateau. Numerous infrastructure systems in the region are built on seasonally frozen soil, rendering them susceptible to high-frequency seismic loads. Previous studies have demonstrated the significant influence of pre-seismic loads on the mechanical responses of soils. However, research on the influence of pre-seismic loads on seasonally frozen soil is limited. Therefore, two groups of cyclic triaxial tests are conducted in this study to investigate the impact of pre-seismic load and freeze-thaw history on the cyclic response of silty sand. Sample reconstitution and freeze-thaw procedures are described in detail. The test results indicate that the cyclic strength of silty sand decreases with the freeze-thaw history, although the influence diminishes with an increase in the number of freeze-thaw cycles. Additionally, a small pre-seismic load enhances the silty-sand strength, regardless of the freeze-thaw history it underwent. The findings of this study offer valuable insights for engineering construction in areas prone to seasonal freezing and earthquakes.

Effect of freeze-thaw cycles on aggregate breakdown of typical black soil during transportation

During the snowmelt period, the external erosive forces are dominated by freeze-thaw cycles and snowmelt runoff. These forces may affect soil structure and aggregate stability, thereby influencing snowmelt erosion. The process of snowmelt runoff can lead to the breakdown of aggregates during their transportation. However, few studies examined the effects of freeze-thaw cycles on the breakdown of aggregates during transportation. Focusing on 5-7 and 3-5 mm soil aggregates of typical black soil region in Northeast China, we analyzed the composition of water-stable aggregates, mean weight diameter (MWD), normalized mean weight diameter (NMWD), as well as breakdown rate of soil aggregates (BR) under different freeze-thaw cycles (0, 1, 5, 10, 15 and 20 times) and different transport distances (5, 10, 15, 20, 25 and 30 m). We further investigated the contribution (CT) of both freeze-thaw cycles and transport distances to BR. The results showed that: 1) After freeze-thaw cycles, the 5-7 and 3-5 mm aggregates were mainly composed of particles with a diameter of 0.5-1 mm. With increasing frequency of freeze-thaw cycles, the MWD generally showed a downward trend. Moreover, under the same number of freeze-thaw cycles, the NMWD of 3-5 mm aggregates was higher than that of 5-7 mm aggregates. 2) As the transport distance increased, the BR of 5-7and 3-5 mm aggregates gradually increased. Compared that under control group, the BR under one freeze-thaw cycle increased by 59.7%, 32.2%, 13.7%, 6.2%, 13.4%, 7.5%, and 60.0%, 39.0%, 18.4%, 13.0%, 6.3%, 6.1% at the condition of 5, 10, 15, 20, 25 and 30 m transport distances, respectively. However, with increasing frequency of freeze-thaw cycles, the BR increased slowly. 3) The breakdown of soil aggregates was mainly influenced by the transport distance (CT=54.6%) and freeze-thaw cycles (CT=26.2%). Freeze-thaw cycles primarily altered the stability of soil aggregates, which in turn affected the BR. Therefore, during the snowmelt period, freeze-thaw cycles reduced the stability of soil aggregates, leading to severe breakdown of soil aggregates during snowmelt runoff process. This made the soil more susceptible to migration with snowmelt runoff, which triggered soil erosion. Therefore, more attention should be paid on the prevention of soil erosion during snowmelt period.

Effect of freeze‒thaw cycles on root-Soil composite mechanical properties and slope stability.

Natural disasters such as landslides often occur on soil slopes in seasonally frozen areas that undergo freeze‒thaw cycling. Ecological slope protection is an effective way to prevent such disasters. To explore the change in the mechanical properties of soil under the influence of both root reinforcement and freeze‒thaw cycles and its influence on slope stability, the Baijiabao landslide in the Three Gorges Reservoir area was taken as an example. The mechanical properties of soil under different confining pressures, vegetation coverages (VCs) and numbers of freeze‒thaw cycles were studied via mechanical tests, such as triaxial compression tests, wave velocity tests and FLAC3D simulations. The results show that the shear strength of a root-soil composite increases with increasing confining pressure and VC and decreases with increasing number of freeze‒thaw cycles. Bermuda grass roots and confining pressure jointly improve the durability of soil under freeze‒thaw conditions. However, with an increase in the number of freeze‒thaw cycles, the resistance of root reinforcement to freeze‒thaw action gradually decreases. The observed effect of freeze‒thaw cycles on soil degradation was divided into three stages: a significant decrease in strength, a slight decrease in strength and strength stability. Freeze‒thaw cycles and VC mainly affect the cohesion of the soil and have little effect on the internal friction angle. Compared with that of a bare soil slope, the safety factor of a slope covered with plants is larger, the maximum displacement of a landslide is smaller, and it is less affected by freezing and thawing. These findings can provide a reference for research on ecological slope protection technology.

Experimental study on the frost resistance of molybdenum tailings concrete

The research aims to experimentally investigate the frost resistance of molybdenum tailings concrete (MoTC), involving C30 and C60 grade, with varying molybdenum tailings replacement ratios and freeze-thaw cycles. The results indicated that with an increase in the number of freeze-thaw cycles, both C30 and C60 grade MoTC showed an aggravated deterioration in strength and failure morphology. However, the deterioration of C60 grade MoTC was less severe than C30 grade. The more than 25% addition of molybdenum tailings significantly improved the frost resistance of C30 grade MoTC, but had minor effect on that of C60 grade MoTC. The mass loss ratio, cubic compressive strength, and dynamic elastic modulus of both C30 and C60 grade MoTC decreased with the increase in freeze-thaw cycles. The highest mass loss ratio for C30 grade MoTC was 100%, with the total loss of the dynamic elastic modulus and cubic compressive strength. For C60 grade MoTC, the highest mass loss ratio was 2.13%, with dynamic elastic modulus and cubic compressive strength reaching 12.8 and 23.79 MPa, respectively. Based on the test data, a predictive formula for the dynamic elastic modulus of C30 grade MoTC with considering the molybdenum tailings replacement ratios and freeze-thaw cycles was proposed, which shows good agreement between the results.

Damage characteristics of sandstone subjected to freeze–thaw cycles under different stress paths

The damage characteristics and failure modes of rock specimens subjected to different freeze–thaw (FT) cycles are vital to rock engineering in cold regions. Hence, in this study, the physical and mechanical parameters of sandstone specimens under different FT cycles were measured, and their deterioration laws were analyzed. The damage features of the rocks in different FT states under loading along various stress paths, including the main-frequency variation and crack propagation path evolution, were statistically analyzed. Two damage variables, one related to the acoustic emission (AE) and the other related to the rock strength, were combined to reveal the AE characteristics and damage evolution laws in the different damage states. The results showed that FT cycling led to a decrease in the peak stress, an increase in the number of microcracks, an increase in the AE activity, a shift in the main-frequency distribution toward the low-frequency range, and a shift in the damage mode from shear damage to a mixed damage dominated by tensile damage. The degree of strength deterioration in the rock specimens was affected by FT cycling under the different stress paths in the following influence order: conventional uniaxial loading > equal-amplitude cyclic loading> multistage cyclic loading; cyclic loading was more likely to induce tensile rupture than conventional uniaxial loading. The AE characteristic parameters were consistent with the stress–strain curve variations, which could reflect the damage evolution process of the rock specimens. The main frequencies showed a band-like evolution trend with the loading process and could be divided into three bands: a low-frequency interval (0–50 kHz), a medium-frequency interval (50–150 kHz), and a high-frequency interval (150–300 kHz); the main frequencies were concentrated in the low-frequency interval. With the increase in the number of FT cycles, the main frequency in the high-frequency interval and the number of corresponding microcracks gradually decreased.

Freeze-thaw effect on performance degradation of recycled glulam from structural members after cyclic loading

To achieve sustainable production with recycled glulam from after-service structural members, the physical and mechanical properties of the recycled glulam under different numbers of freeze-thaw cycles (60, 120, 180 times) were investigated. The results showed that the freeze-thaw cycle treatment had a serious damaging effect on the glue layer, rather than the laminates of the specimens, which showed the tackless phenomenon. The physical properties (color change, mass and dimensional variation) of the specimens changed most significantly after the first freeze-thaw cycles (60 cycles), and then showed slow growth. As the number of freeze-thaw cycles on the specimens increased, the strength, especially the shear strength of the specimens showed a significantly decreasing trend (45% for compressive strength and 100% for shear strength after 180 cycles), whilst the ductility shows an increasing trend. A strength prediction model of the recycled glulam under freeze-thaw cycles was established. Based on the shape characteristics of the tested stress-strain curves, the compression and shear performance models of recycled glulam influenced by the number of freeze-thaw cycles were proposed.

Effects of Freeze-Thaw Cycles on Interfacial Shear Transfer Characteristics in Section Steel Reinforced Concrete Composite Structures with Stud Connectors

This study examines how freeze-thaw cycles influence the shear transfer characteristics at the interface of section steel with stud connectors and concrete. Push out tests were performed on fourteen section steel reinforced concrete composite structures with stud connectors (SRCSC) and two without them. The analysis focused on examining the impact of variables such as the number of freeze-thaw cycles, the diameter of studs, and the arrangement of studs on the interfacial shear capacity of the SRCSCs. Experimental results showed that with an increase in the number of freeze-thaw cycles, the damage morphology of the SRCSCs transitions from stud shearing to concrete splitting. Concurrently, the interfacial shear capacity and shear stiffness of SRCSCs diminish. Large-diameter studs could effectively improve the interfacial shear properties of SRCSCs. Additionally, SRCSCs with studs positioned in the web demonstrate enhanced interfacial shear transfer capabilities compared to those with studs located in the flange. The proposed method for the residual interfacial shear capacity of the SRCSCs after freeze-thaw cycles was based on experimental data analysis. Results from this calculation method were consistent with the experimental observations.

Mucus-Mimicking Mucin-Based Hydrogels by Tandem Chemical and Physical Crosslinking.

Mucosal tissues represent a major interface between the body and the external environment and are covered by a highly hydrated mucins gel called mucus. Mucus lubricates, protects and modulates the moisture levels of the tissue and is capitalized in transmucosal drug delivery. Pharmaceutical researchers often use freshly excised animal mucosal membranes to assess mucoadhesion and muco-penetration of pharmaceutical formulations which may struggle with limited accessibility, reproducibility, and ethical questions. Aiming to develop a platform for the rationale study of the interaction of drugs and delivery systems with mucosal tissues, in this work mucus-mimicking mucin-based hydrogels are synthesized by the tandem chemical and physical crosslinking of mucin aqueous solutions. Chemical crosslinking is achieved with glutaraldehyde (0.3% and 0.75% w/v), while physical crosslinking by one or two freeze-thawing cycles. Hydrogels after one freeze-thawing cycle show water content of 97.6-98.1%, density of 0.0529-0.0648gcm⁻3, and storage and loss moduli of ≈40-60 and ≈3-5Pa, respectively, that resemble the properties of native gastrointestinal mucus. The mechanical stability of the hydrogels increases over the number of freeze-thawing cycles. Overall results highlight the potential of this simple, reproducible, and scalable method to produce artificial mucus-mimicking hydrogels for different applications in pharmaceutical research.

Machine learning prediction of concrete frost resistance and optimization design of mix proportions

This study explores nine machine learning (ML) methods, including linear, non-linear and ensemble learning models, using nine concrete parameters as characteristic variables. Including the dosage of cement (C), fly ash (FA), Ground granulated blast furnace slag (GGBS), coarse aggregate (G), fine aggregate (S), water reducing agent (WRA) and water (W), initial gas content (GC) and number of freeze-thaw cycles (NFTC), To predict relative dynamic elastic modulus (RDEM) and mass loss rate (MLR). Based on the linear correlation analysis and the evaluation of four performance indicators of R2, MSE, MAE and RMSE, it is found that the nonlinear model has better performance. In the prediction of RDEM, the integrated learning GBDT model has the best prediction ability. The evaluation indexes were R2 = 0.78, MSE = 0.0041, MAE = 0.0345, RMSE = 0.0157, SI = 0.0177, BIAS = 0.0294. In the prediction of MLR, ensemble learning Catboost algorithm model has the best prediction ability, and the evaluation indexes are R2 = 0.84, MSE = 0.0036, RMSE = 0.0597, MAE = 0.0312, SI = 5.5298, BIAS = 0.1772. Then, Monte Carlo fine-tuning method is used to optimize the concrete mix ratio, so as to obtain the best mix ratio.

Research on freeze-thaw damage and life prediction model of polyethylene fiber-reinforced cementitious composites based on reliability analysis

Engineered Cementitious Composite (ECC), is a fiber-reinforced cementitious composite with excellent ductility, toughness, impact resistance and multi-crack development characteristics. In addition to the excellent mechanical properties, the durability of ECC has also received equal attention. In order to investigate the frost resistance of polyethylene fiber-reinforced cementitious composites (PE-ECC), PE-ECC and normal concrete (NC) specimens were subjected to different numbers of freeze-thaw cycles, and the changes in compressive strength, mass loss rate and relative dynamic modulus of elasticity of the specimens before and after freezing and thawing were tested, so as to compare the freeze-resistant properties of PE-ECC and NC. A freeze-thaw damage model was established using the two-parameter Weibull distribution function, and the freeze-thaw reliability of NC and PE-ECC was analyzed based on the Wiener process analysis, based on which a life prediction model was established to predict the life of NC and PE-ECC under freeze-thaw environment. The results show that after 200 freeze-thaw cycles, the compressive strength loss rate, mass loss rate and the relative dynamic elastic modulus loss rate of NC and PE-ECC are 32.97%, 2.35%, 50.28% and 17.28%, 0.71%, 6.57%, respectively, and the damage indexes of PE-ECC are lower than that of NC, and the NC after 200 freeze-thaw cycles has reached the standard of freeze-thaw damage, while PE-ECC is still at a lower level of freeze-thaw damage; it shows that the frost resistance of PE-ECC is better than that of NC, and the addition of PE fibers can effectively improve the frost resistance of concrete; the results of the damage model and the reliability analysis coincide well with the experimental results, and the results of life prediction are given in some cold regions, which can provide a basis for the application of PE-ECC in practice.

Study on the durability deterioration law of marine concrete with nano-particles under the coupled effects of freeze-thaw cycles, flexural fatigue load and Cl− erosion

The marine concrete in cold region such as harbor, wharf and sea-bridge is inevitably subjected to the coupling effects of flexural fatigue load, freeze-thaw cycles and chloride ion (Cl−) erosion. This paper studies the durability of unreinforced concrete under the coupling effects of three factors. And in order to highlight the effects of freeze-thaw cycles, the two-factor coupling durability test of flexural fatigue load and Cl− erosion and the three-factor coupling durability test of freeze-thaw cycles, flexural fatigue load and Cl− erosion are designed. And the microstructures of various concretes are analyzed by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results show that under the coupling effects of three factors, on the one hand the icing of pore water can improve the deformation resistance of concrete; on the other hand, the expansion pressure can accelerate the deterioration of pores and promotes the development of fatigue cracks. The coupling effects of three factors significantly reduces the flexural fatigue life of concrete, and the more the number of freeze-thaw cycles, the more significant the adverse effects on the flexural fatigue life of concrete. The addition of nano-particles (The cement in concrete is replaced by the same content of nano-particles) can improve the adverse effects of freeze-thaw cycles on the flexural fatigue life of concrete, and enhance the flexural fatigue life of concrete. When the amount of nano-particles is optimal, the fatigue life of NC10 and NS20 is increased by 47.6% and 52.3% respectively. The Cl− erosion resistance of concrete increases firstly and then decreases with the increasing amount of nano-particles. The free Cl− content of the compression zone in NC10 and NS20 can be reduced by 22.4% and 24.2% respectively, and the free Cl− content of the tensile zone in NC10 and NS20 can be reduced by 16.6% and 20% respectively. Under the coupling effects of three factors, with the continues erosion of Cl−, the Ca/Si value and the atomic percentage of Ca in interface transition zone (ITZ) of concrete gradually are decreased, which makes the decreased extent of C–S–H gel and Ca(OH)2 in concrete. The addition of nano-particles can reduce the decrease extent of Ca atom percentage in ITZ, reduce the decomposition of C–S–H gel and Ca(OH)2, and improve the fatigue life and Cl− erosion resistance of concrete.

Strength and microscopic pore structure characterization of cement-fly ash stabilized organic soil under freeze-thaw cycles

In seasonal frost areas, an organic soil stratum is often encountered during engineering construction due to the widespread existence of organic soils. The soil stratum will experience frost heave in winter and thaw settlement in summer, resulting in a significant variation in its engineering behaviour, especially for organic soil stratum. In this study, with the help of cement, fly ash, and fulvic acid, cement-fly ash stabilized organic soil (CFOS) specimens were prepared and the unconfined compressive (UC) and mercury intrusion porosimetry (MIP) tests were carried out on CFOS specimens. The effects of fly ash content, number of freeze-thaw cycles (FT-N), and curing period on the strength, resilient modulus, and porosity were investigated. Test results revealed that the fly ash content increased from 0% to the optimum content of 5%, the unconfined compressive strength (UCS) and resilient modulus of CFOS with FT-12 increased by 50.30% and 118.92%, respectively. The pore size distribution (PSD) curve and fractal dimension (Dn) of specimens were obtained from the MIP test. The proportion of macropores was the main factor affecting the UCS of CFOS. With the increasing FT-N, the macropore proportion of CFOS with 5% fly ash content increased by 333.33%, and the UCS decreased by 28.25%. Based on a freeze-thaw damage model, the damage parameters of the CFOS specimens were extracted with the Dn as an independent variable. The microscopic pore characteristics and the relationship between the strength, Dn and damage parameter were analyzed. The microscopic pore structure of specimens with different fly ash contents experienced a change subjected to freeze-thaw cycles. The lower the strength, the lower the Dn of CFOS. The damage parameters quantitatively reflected the damage degree of specimens after freeze-thaw cycles on the micro scale. The damage parameter of CFOS with 5% fly ash content increased by 341.57% with the increase of FT-N. The introduction of fly ash was able to reduce freeze-thaw damage to the specimens. The damage parameter and CFOS strength exhibited a significant correlation. The relationships of the strength and microscopic pore structure of CFOS subjected to freeze-thaw cycles were conducive to a better understanding of the freeze-thaw damage mechanism of CFOS on the micro scale. These findings are beneficial for engineering construction design in seasonal frost areas.

Effect of repeated freeze-thaw cycles on mechanical properties of clay

To investigate the variation in the mechanical properties of clay under freeze-thaw cycles (FTCs), a series of experiments were conducted in the laboratory. Samples with different water contents and dry densities were subjected to FTCs ranging from 0 to 11 times. Then, cohesion, shear strength, internal friction angle and elastic modulus were obtained using triaxial test. The results show that with the increase in the number of FTCs, the shear strength, cohesion and elastic modulus decreased, while the internal friction angle increased slightly. However, the variation in the internal friction angle is not obvious, and the maximum increment is within 4°. The cohesion exhibited the most decrease after the first freeze-thaw action. Besides, under a same number of FTCs, four mechanical properties are significantly affected by water content and dry density. The shear strength, cohesion, elastic modulus and internal friction angle decrease with water content while increasing with dry density. Additionally, the elastic modulus is associated with confining pressure, which increases with confining pressure. This study provides evidence for the variation in mechanical properties of the soils subjected to FTCs and guides the design and construction of the cold regional engineering.