Effects of freeze-thaw cycles on sandstone in sunny and shady slopes

A growing rock engineering activity in cold regions is facing the threat of freeze-thaw(FT)weathering,especially in high mountains where the sunny-shady slope effects strongly control the difference in weathering behavior of rocks.In this paper,an investigation of the degradation of petrophysical characteristics of sandstone specimens subjected to FT cycle tests to simulate the sunny-shady slope effects is presented.To this aim,non-destructive and repeatable testing techniques including weight,ultrasonic waves,and nuclear magnetic resonance methods on standard specimens were performed.For the sunny slope specimens,accompanied by the enlargement of small pores,100 FT cycles caused a significant decrease in P-wave velocity with an average of 23%,but a consistent rise of 0.18%in mass loss,34%in porosity,67%in pore geometrical mean radius,and a remarkable 14.5-fold increase in permeability.However,slight changes with some abnormal trends in physical parameters of the shady slope specimens were observed during FT cycling,which can be attributed to superficial granular disaggregation and pore throat obstruction.Thermal shocks enhance rock weathering on sunny slopes during FT cycles,while FT weathering on shady slopes is restricted to the small pores and the superficial cover.These two factors are primarily responsible for the differences in FT weathering intensity between sunny and shady slopes.The conclusions derived from the interpretation of the experimental results may provide theoretical guidance for the design of slope-failure prevention measures and the selection of transportation routes in cold mountainous regions.

Effect of freeze-thaw cycles on uniaxial mechanical properties of cohesive coarse-grained soils

Freeze-thaw cycles are closely related to the slope instability in high-altitude mountain regions. In this study, cohesive coarse-grained soils were collected from a high-altitude slope in the Qinghai–Tibet Plateau to study the effect of cyclic freeze-thaw on their uniaxial mechanical properties. The soil specimens were remolded with three dry densities and three moisture contents. Then, after performing a series of freeze-thaw tests in a closed system without water supply, the soil specimens were subjected to a uniaxial compression test. The results showed that the stress-strain curves of the tested soils mainly performed as strain-softening. The softening feature intensified with the increasing dry density but weakened with an increase in freeze-thaw cycles and moisture content. The uniaxial compressive strength, resilient modulus, residual strength and softening modulus decreased considerably with the increase of freeze-thaw cycles. After more than nine freeze-thaw cycles, these four parameters tended to be stable. These parameters increased with the increase of dry density and decreased with the increasing moisture content, except for the residual strength which did not exhibit any clear variation with an increase in moisture content. The residual strength, however, generally increased with an increase in dry density. The soil structural damage caused by frozen water expansion during the freeze-thaw is the major cause for the changes in mechanical behaviors of cohesive coarse-grained soils. With results in this study, the deterioration effect of freeze-thaw cycles on the mechanical properties of soils should be considered during the slope stability analysis in high-altitude mountain regions.

Experimental study of dynamic resilient modulus of subgrade soils under coupling of freeze–thaw cycles and dynamic load

Although the dynamic properties of subgrade soils in seasonally frozen areas have already been studied, few researchers have considered the influence of shallow groundwater during the freeze–thaw(F–T) cycles. So a multifunctional F–T cycle system was developed to imitate the groundwater recharge in the subgrade during the freezing process and a large number of dynamic triaxial experiments were conducted after the F–T cycles. Some significant factors including the F–T cycle number, compaction degree, confining pressure, cyclic deviator stress, loading frequency, and water content were investigated for the resilient modulus of soils. The experimental results indicated that the dynamic resilient modulus of the subgrade was negatively correlated with the cyclic deviator stress, F–T cycle number, and initial water content, whereas the degree of compaction, confining pressure, and loading frequency could enhance the resilient modulus. Furthermore, a modified model considering the F–T cycle number and stress state was established to predict the dynamic resilient modulus. The calculated results of this modified model were very close to the experimental results. Consequently, calculation of the resilient modulus for F–T cycles considering the dynamic load was appropriate. This study provides reference for research focusing on F–T cycles with groundwater supply and the dynamic resilient moduli of subgrade soils in seasonally frozen areas.

Effects of Freeze/Thaw Cycles and Gas Purging Method on Polymer Electrolyte Membrane Fuel Cells

At subzero temperature, the startup capability and performance of polymer electrolyte membrane fuel cell (PEMFC) deteriorates markedly. The object of this work is to study the degradation mechanism of key components of PEMFC-membrane-electrode assembly (MEA) and seek feasible measures to avoid degradation. The effect of freeze/thaw cycles on the structure of MEA is investigated based on porosity and SEM measurement. The performance of a single cell was also tested before and after repetitious freeze/thaw cycles. The experimental results indicated that the performance of a PEMFC decreased along with the total operating time as well as the pore size distribution shifting and micro configuration changing. However, when the redundant water had been removed by gas purging, the performance of the PEMFC stack was almost resumed when it experienced again the same subzero temperature test. These results show that it is necessary to remove the water in PEMFCs to maintain stable performance under subzero temperature and gas purging is proved to be the effective operation.

Effects of Freeze–thaw Cycles on Soil Mechanical and Physical Properties in the Qinghai–Tibet Plateau

Extreme freeze-thaw action occurs on the Qinghai-Tibet Plateau due to its unique climate resulting from high elevation and cold temperature.This action causes damage to the surface soil structure, as soil erosion in the Qinghai-Tibet Plateau is dominated by freeze-thaw erosion.In this research,freezing–thawing process of the soil samples collected from the Qinghai–Tibet Plateau was carried out by laboratory experiments to determinate the volume variation of soil as well as physical and mechanical properties, such as porosity, granularity and uniaxial compressive strength, after the soil experiences various freeze–thaw cycles.Results show that cohesion and uniaxial compressive strength decreased as the volume and porosity of the soil increased after experiencing various freeze–thaw cycles, especially in the first six freeze–thaw cycles.Consequently, the physical and mechanical properties of the soil were altered.However, granularity and internal friction angle did not vary significantly with an increase in the freeze–thaw cycle.The structural damage among soil particles due to frozen water expansion was the major cause of changes in soil mechanical behavior in the Qinghai–Tibet Plateau.

Cracking in an expansive soil under freeze–thaw cycles

Expansive soils located in cold regions can easily endure the action of frost heaving and cyclic freezing–thawing. Cracking can also occur in expansive clayey soils under freeze–thaw cycles, of which little attention has been paid on this issue.In this study, laboratory experiment and cracking analysis were performed on an expansive soil. Crack patterns were quantitatively analyzed using the fractal concept. The relationships among crack pattern, water loss, number of freeze–thaw cycles, and fractal dimension were discussed. It was found that crack patterns on the surface exhibit a hierarchical network structure that is fractal at a statistical level. Cracks induced by freeze–thaw cycles are shorter, more irregularly oriented,and slowly evolves from an irregularly rectilinear pattern towards a polygonal or quasi–hexagonal one; water loss, closely related to specimen thickness, plays a significant role in the process of soil cracking; crack development under freeze-thaw cycles are not only attributed to capillary effect, but also to expansion and absorption effects.

Typical Soft–Sediment Deformation Structures Induced by Freeze/Thaw Cycles: A Case Study of Quaternary Alluvial Deposits in the Northern Qiangtang Basin, Tibetan Plateau

With the objective of establishing a distinction between deformation structures caused by freeze/thaw cycles and those resulting from seismic activity, we studied three well–exposed alluvial deposits in a section at Dogai Coring, northern Qiangtang Basin, Tibetan Plateau. Deformation is present in the form of plastic structures(diapirs, folds and clastic dykes), brittle structures(micro–faults) and cryogenic wedges. These soft–sediment deformation features(except the micro–faults) are mainly characterized by meter–scale, non–interlayered, low–speed and low–pressure displacements within soft sediments, most commonly in the form of plastic deformation. Taking into account the geographic setting, lithology and deformation features, we interpret these soft–sediment deformation features as the products of freeze/thaw cycles, rather than of earthquake–induced shock waves, thus reflecting regional temperature changes and fluctuations of hydrothermal conditions in the uppermost sediments. The micro–faults(close to linear hot springs) are ascribed to regional fault activity;however, we were unable to identify the nature of the micro–faults, perhaps due to disturbance by subsequent freeze/thaw cycles. This study may serve as a guide to recognizing the differences between deformation structures attributed to freeze/thaw cycles and seismic processes.

Deformation monitoring and analysis at two frost mounds during freeze–thaw cycles along the Qinghai–Tibet Engineering Corridor

This paper presents various deformation-monitoring technologies employed to monitor the frost heave and thaw settlement of two mounds along the Qinghai–Tibet Engineering Corridor(QTEC), China. The QTEC is known as a critical infrastructure and passage connecting inland China and the Qinghai–Tibet Plateau(QTP). Three technologies—global navigation satellite system(GNSS), terrestrial laser scanning(TLS), and unmanned aerial vehicle(UAV)—were used to estimate the freeze/thaw–induced 3D surface deformation of two frost mounds. Our results showed that (1) the two frost mounds exhibited mainly thaw settlement in thawing periods and frost heave in the freezing period, but frost heave dominated after repeated freeze–thaw cycles;(2) different zones of the mounds showed different deformation characteristics;(3) active-layer thickness(ALT) and elevation changes were highly correlated during thaw periods;(4) integrated 3D-measurement technologies can achieve a better understanding and assessment of hazards in the permafrost area.

Deterioration Mechanism of Sulfate Attack on Concrete under Freeze-thaw Cycles

The experiments of concrete attacked by sulfate solution under freeze-thaw cycles were investigated. The sulfate solution includes two types of 5% Na2SO4 and 5% MgSO4. Through the experiment, microstructural analyses such as SEM, XRD and TGA measurements were performed on the selected samples after freeze-thaw cycles. The corrosion products of the concrete were distinguished and quantitatively compared by the thermal analysis. Besides, the damage mechanism considering the dynamic modulus of elastically of concrete under the coupling effect was also investigated. The experimental results show that, under the action of freeze-thaw cycles and sulfate attack, the main attack products in concrete are ettringite and gypsum. The corrosion products exposed to MgSO4 solution are more than those to Na2SO4 solution. Furthermore, the content of gypsum in concrete is less than that of ettringite in test, and some of gypsum can be observed only after a certain corrosion extent. It is also shown that MgSO4 solution has a promoting effect to the damage of concrete under freeze-thaw cycles. Whereas for Na：SO4 solution, the damage of concrete has restrained before 300 freeze-thaw cycles, but the sulfate attack accelerates the deterioration process in its further test period.

Effect of Freeze-thaw Cycles on Bond Strength between Steel Bars and Concrete

The effect of freezing and thawing cycles on mechanical properties of concrete （compressive, splitting tensile strength） was experimentally investigated. According to the pullout test data of three kinds of deformed steel bars, the bond stress-slip curves after freezing and thawing were obtained. The empirical equations of peak bond strength were proposed that the damage accounted for effects of freezing and thawing cycle. Meanwhile, the mechanism of bond deterioration between steel bars and concrete after freezing and thawing cycles was discussed. All these conclusions will be useful to the durability design and reliability calculation of RC structures in cold region.

The influence of freeze-thaw cycles on the granulometric composition of Moscow morainic clay

The freeze-thaw cycling process considerably changes the composition, structure, and properties of soils. Since the grain size is the most important factor in determining soil characteristics, our current research primarily aims to investigate dynamic changes of the soil fraction when exposed to freeze-thaw conditions. We observed two series of Moscow morainic clayey specimens （gQⅡm）： （Ⅰ） the original series, and （Ⅱ） the remolded series. We subjected each series of soil specimens to different frequencies of freeze-thaw cycles （3, 6, 20, and 40 cycles）, and we used granulometric tests to analyze both series before and after exposure to freeze-thaw conditions. As a result of our experiments, the granulometric compositions tended to be distributed evenly after 40 freeze-thaw processes （i.e., content of fraction for 0.1-0.05 mm was increased after 40 freeze-thaw cycles） because the division of coarse grains and the aggregation of fine grains were synchronized during the freeze-thaw process. The soil grains in both series changed bi-directionally. In the original series, changes of the sand grains were conjugated with the clay grains, and in the remolded series, changes of the sand grains were conjugated with the silt grains, because potential energy difference caused the division and aggregation processes to relate to the counteraction process. The even distribution of soil grain size indicated the state of equilibrium or balance. The granulometric compositions were altered the most during the sixth freeze-thaw cycle, because the coefficient of the intensity variation of the grain fineness （Kvar） had its maximum value at that time.

Shear behavior of ultrafine magnetite tailings subjected to freeze-thaw cycles

To study the shear behavior of the ultrafine magnetite tailings subjected to freeze-thaw cycles,unconsolidated-undrained shear tests were conducted on ultrafine-grained tailings that were subjected to 1-11 cycles of freeze-thaw and defined as a type of clayey silt under confining pressures of 100,200,and 300 kPa.Taking the number of freeze-thaw cycles,cooling temperature,initial dry density,and moisture content as the four main influencing factors of shear behavior of the tailings samples,the shear stress-strain curve,compression modulus,failure strength,cohesion,and internal friction angle were measured.The results show that the freeze-thaw cycle has an obvious weakening effect on the shear behavior of the tailings material,and the shear mechanical parameters are affected by a combination of confining pressure,freeze-thaw cycle condition,and initial physical-mechanical properties of the tailings samples.Through the microstructural analysis of the tailings samples subjected to freeze-thaw cycles,it shows that the freeze-thaw cycle mainly affects the porosity,bound water,and arrangement of the tailings particles.Subsequently,the macroscopic changes in shear strength indexes emerge,and then the stability of the tailings dam will decrease.

Review of the influence of freeze-thaw cycles on the physical and mechanical properties of soil

Seasonally frozen soil is a four-phase material and its physical-mechanical properties are more complex compared to the unfrozen soil. Its physical properties changes during the freeze-thaw process; repeated fieeze-thaw cycles change the characteristics of soil, which can render the soil from an unstable state to a new dynamic equilibrium state. The freezing process changes the structttre coupled between the soil particle arrangements, which will change the mechanical properties of the soil. The method of significance and interaction between different fac tors should be considered to measure the influence on the propties of soil under freeze-thaw cycles.

BEHAVIOR OF AIR-ENTRAINED CONCRETE AFTER FREEZE-THAW CYCLES

The experimental study of air-entrained concrete specimens subjected to different cycles of freeze-thaw was completed. The dynamic modulus of elasticity, weight loss, the cubic compressive strength, compressive strength, tensile strength and cleavage strength of air-entrained concrete were measured after 0, 100, 200, 300, 400 cycles of freeze-thaw. The experimental results showed that the dynamic modulus of elasticity and strength decreased as the freeze-thaw was repeated. The influences of freeze-thaw cycles on the mechanical properties, the dynamic modulus of elasticity and weight loss were analyzed according to the experimental results. It can serve as a reference for the maintenance, design and the life prediction of dams, hydraulic structures, offshore structures, concrete roads and bridges in northern cold regions.

Effects of freeze–thaw cycles on soil N_2O concentration and flux in the permafrost regions of the Qinghai–Tibetan Plateau

Nitrous oxide(N_2 O) is one of the most important greenhouse gases in the atmosphere; freeze–thaw cycles(FTCs) might strongly influence the emission of soil N_2 O on the Qinghai–Tibetan Plateau(QTP). However, there is a lack of in situ research on the characteristics of soil N_2 O concentration and flux in response to variations in soil properties caused by FTCs.Here, we report the effect of FTC-induced changes in soil properties on the soil N_2 O concentration and flux in the permafrost region of the higher reaches of the Shule River Basin on the northeastern margin of the QTP. We measured chemical properties of the topsoil, activities of soil microorganisms, and air temperature(AT), as well as soil N_2 O concentration and flux, over an annual cycle from July 31, 2011, to July 30, 2012. The results showed that soil N_2 O concentration was significantly affected by soil temperature(ST), soil moisture(SM), soil salinity(SS), soil polyphenol oxidase(SPO), soil alkaline phosphatase(SAP), and soil culturable actinomycetes(SCA), ranked as SM>SS>ST>SPO>SAP>SCA, whereas ST significantly increased soil N_2 O flux, compared with SS. Overall, our study indicated that the soil N_2 O concentration and flux in permafrost zone FTCs were strongly affected by soil properties, especially soil moisture, soil salinity, and soil temperature.

Evolution and Characterization of Damage of Concrete under Freeze-thaw Cycles

To study the internal damage of concrete under freeze-thaw cycles, concrete strains were measured using embedded strain gauges. Residual strain and coefficients of freezing expansion （CFE） derived from strain-temperature curves were used to quantify the damage degree. The experimental results show that irreversible residual strain increases with the number of freeze-thaw cycles. After 50 cycles, residual strains of C20 and C35 concretes are 320με and 100με in water, and 120με and 60 με in saline solution, respectively. In lower temperature range （- 10 ℃ to - 25 ℃） CFE of C20 and C35 concretes decrease by 9.82 × 10-6/K and 8.44×10-6/K in water, and 9.38×10-6/K and 5.47×10-6/K in saline solution, respectively. Both residual strains and CFEs indicate that during the first 50 freeze-thaw cycles, the internal damage of concrete in saline solution is less than that of concrete in water. Thus residual strain and CFE can be used to measure the frost damage of concrete.

Acoustic experimental study of two types of rock from the Tibetan Plateau under the condition of freeze-thaw cycles

Under the condition of freeze-thaw cycles, two types of rocks （granite and andesite）, used as slope protection for the Qinghai-Tibet Railway, were tested according to the special climatic conditions in the Tibetan Plateau, and their various damage processes in ap- pearance were carefully observed. Observation results show that damage of andesite was more serious than that of granite. Using an acoustic instrument, ultrasonic velocity was tested. The changing trends of velocity with the number of freeze-thaw cycles were analyzed, and the freeze-thaw cycle damaging the physical and mechanical properties of rocks can be seen. According to the changing trends of ultrasonic velocity with the number of freeze-thaw cycles, mechanical parameters of rocks, such as dynamic elasticity modulus, Poisson＇s ratio, and dynamic bulk modulus were analyzed. It is found that they all have declining trends as the number of fi＇eeze-thaw cycles increases, and in particular, when the cycle number reaches a certain extent, the Poisson＇s ratio of rocks begins to become negative.

Calculation of salt-frost heave of sulfate saline soil due to long-term freeze−thaw cycles

Based on salt-frost heave tests of sulfate saline soil under repeated freeze−thaw cycles,this paper discusses the mechanism of the salt-frost heave under long-term freeze−thaw cycles.The results show that the salt-frost heave can be restricted considerably by loads,and there is a critical load for the salt-frost heave cumulative effect.Under this load,peak values of salt-frost heave approach a constant,and the residual values become 0.There is no longer structure heave or cumulative effect of saline soil exposed to freeze−thaw cycles under the critical load.Taking cumulative effect into account in calculations of salt-frost heave,a salt-frost heave model under freeze−thaw cycles is developed.

Behavior of High Water-cement Ratio Concrete under Biaxial Compression after Freeze-thaw Cycles

The high water-cement ratio concrete specimens under biaxial compression that completed in a triaxial testing machine were experimentally studied. Strength and deformations of plain concrete specimens in two loading direction under biaxial compression with stress ratio of a=0, 0.25, 0.5, 0.75, 1.0 were obtained after 0, 25, 50 cycles of freeze-thaw. Influences of freeze-thaw cycles and stress ratio on the peak stress and deformation of this point were analyzed according to the experimental results, Based on the test data, the failure criterion expressed in terms of principal stress after different cycles of freeze-thaw, and the failure criterion with consideration of the influence of freeze-thaw cycle and stress ratio were proposed respectively.

Compressive strength and frost heave resistance of different types of semi-rigid base materials after freeze-thaw cycles

Freeze-thaw damage is the most common disease of semi-rigid bases in cold regions, which may greatly affect the dura- bility of roadways. In this study, the compressive strength and frost resistance of four different types of semi-rigid bases （lime-fly ash-stabilized sand, cement-stabilized sand, lime-fly ash-stabilized gravel, and cement-stabilized gravel） are assessed by varying the materials content. Based on freeze-thaw and compressive strength tests, this paper presents the performance of the different materials, each having different physical properties, and the optimal amounts of materials contents are proposed.