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.

Surface crack evolution patterns in freeze-thaw damage of fissured rock bodies

To explore the effects of freeze‒thaw cycles on the mechanical properties and crack evolution of fissured sandstone,biaxial compression experiments were carried out on sandstone subjected to freeze‒thaw cycles to characterize the changes in the physical and mechanical properties of fissured sandstone caused by freeze‒thaw cycles.The crack evolution and crack change process on the surface of the fissured sandstone were recorded and analysed in detail via digital image technology(DIC).Numerical simulation was used to reveal the expansion process and damage mode of fine-scale cracks under the action of freeze‒thaw cycles,and the simulation results were compared and analysed with the experimental data to verify the reliability of the numerical model.The results show that the mass loss,porosity,peak stress and elastic modulus all increase with increasing number of freeze‒thaw cycles.With an increase in the number of freeze‒thaw cycles,a substantial change in displacement occurs around the prefabricated cracks,and a stress concentration appears at the crack tip.As new cracks continue to sprout at the tips of the prefabricated cracks until the microcracks gradually penetrate into the main cracks,the displacement cloud becomes obviously discontinuous,and the contours of the displacement field in the crack fracture damage area simply intersect with the prefabricated cracks to form an obvious fracture.The damage patterns of the fractured sandstone after freeze‒thaw cycles clearly differ,forming a symmetrical"L"-shaped damage pattern at zero freeze‒thaw cycles,a symmetrical"V"-shaped damage pattern at 10 freeze‒thaw cycles,and a"V"-shaped damage pattern at 20 freeze‒thaw cycles.After 20 freeze‒thaw cycles,a"V"-shaped destruction pattern and"L"-shaped destruction pattern are formed;after 30 freeze‒thaw cycles,an"N"-shaped destruction pattern is formed.This shows that the failure mode of fractured sandstone gradually becomes more complicated with an increasing number of freeze‒thaw cycles.The effects of freeze‒thaw cycles on the direction and rate of crack propagation are revealed through a temperature‒load coupled model,which provides an important reference for an in-depth understanding of the freeze‒thaw failure mechanisms of fractured rock masses.

Quantification of the concrete freeze–thaw environment across the Qinghai–Tibet Plateau based on machine learning algorithms

The reasonable quantification of the concrete freezing environment on the Qinghai–Tibet Plateau(QTP) is the primary issue in frost resistant concrete design, which is one of the challenges that the QTP engineering managers should take into account. In this paper, we propose a more realistic method to calculate the number of concrete freeze–thaw cycles(NFTCs) on the QTP. The calculated results show that the NFTCs increase as the altitude of the meteorological station increases with the average NFTCs being 208.7. Four machine learning methods, i.e., the random forest(RF) model, generalized boosting method(GBM), generalized linear model(GLM), and generalized additive model(GAM), are used to fit the NFTCs. The root mean square error(RMSE) values of the RF, GBM, GLM, and GAM are 32.3, 4.3, 247.9, and 161.3, respectively. The R^(2) values of the RF, GBM, GLM, and GAM are 0.93, 0.99, 0.48, and 0.66, respectively. The GBM method performs the best compared to the other three methods, which was shown by the results of RMSE and R^(2) values. The quantitative results from the GBM method indicate that the lowest, medium, and highest NFTC values are distributed in the northern, central, and southern parts of the QTP, respectively. The annual NFTCs in the QTP region are mainly concentrated at 160 and above, and the average NFTCs is 200 across the QTP. Our results can provide scientific guidance and a theoretical basis for the freezing resistance design of concrete in various projects on the QTP.

Research Progress on Plant Anti-Freeze Proteins

Plant antifreeze proteins(AFPs)are special proteins that can protect plant cells from ice crystal damage in low-temperature environments,and they play a crucial role in the process of plants adapting to cold environ-ments.Proteins with these characteristics have been found infish living in cold regions,as well as many plants and insects.Although research on plant AFPs started relatively late,their application prospects are broad,leading to the attention of many researchers to the isolation,cloning,and genetic improvement of plant AFP genes.Studies have found that the distribution of AFPs in different species seems to be the result of independent evolu-tionary events.Unlike the AFPs found infish and insects,plant AFPs have multiple hydrophilic ice-binding domains,and their recrystallization inhibition activity is about 10–100 times that offish and insect AFPs.Although different plant AFPs have the characteristics of low TH and high RI,their DNA and amino acid sequences are completely different,with small homology.With in-depth research and analysis of the character-istics and mechanisms of plant AFPs,not only has our understanding of plant antifreeze mechanisms been enriched,but it can also be used to improve crop varieties and enhance their freezing tolerance,yield,and quality through genetic engineering.In addition,the study of plant AFPs also contributes to our understanding of freezing resistance mechanisms in other organisms and provides new research directions for thefield of biotech-nology.Therefore,based on the analysis of relevant literature,this article will delve into the concepts,character-istics,research methods,and mechanisms of plant AFPs,summarize the latest research progress and application prospects of AFPs in plant,and provide prospects for the future development of AFP gene research.

Thinning intensity aff ects carbon sequestration and release in seasonal freeze–thaw areas

To explore how to respond to seasonal freeze–thaw cycles on forest ecosystems in the context of climate change through thinning,we assessed the potential impact of thinning intensity on carbon cycle dynamics.By varying the number of temperature cycles,the eff ects of various thinning intensities in four seasons.The rate of mass,litter organic carbon,and soil organic carbon(SOC)loss in response to temperature variations was examined in two degrees of decomposition.The unfrozen season had the highest decomposition rate of litter,followed by the frozen season.Semi-decomposed litter had a higher decomposition rate than undecomposed litter.The decomposition rate of litter was the highest when the thinning intensity was 10%,while the litter and SOC were low.Forest litter had a good carbon sequestration impact in the unfrozen and freeze–thaw seasons,while the converse was confi rmed in the frozen and thaw seasons.The best carbon sequestration impact was identifi ed in litter,and soil layers under a 20–25%thinning intensity,and the infl uence of undecomposed litter on SOC was more noticeable than that of semi-decomposed litter.Both litter and soil can store carbon:however,carbon is transported from undecomposed litter to semi-decomposed litter and to the soil over time.In summary,the best thinning intensity being 20–25%.

The Antifreeze Critical Strength of Low-temperature Concrete Effected by Index

The antifreeze critical strength and the pre-curing time of low-temperature concrete were studied by means of guaranteed rate of compressive strength and antifreeze performance for the structural safety requirement of concrete engineering,suffering once freeze damage under air environment.It is shown that the antifreeze critical strength is 3.7-4.4MPa,pre-curing time is 18-32 h by guaranteed rate of compressive strength,and the antifreeze critical strength is 3.7-4.4MPa,pre-curing time is 18-32 h by guaranteed rate of antifreeze performance.It can be found that the method of guaranteed rate of compressive strength is sensitive to the defect which generated by freeze damage in the concrete interior.The method is fit to evaluate the antifreeze critical strength of low-temperature concrete.

智能冠状动脉运动追踪平台Snapshot Freeze在冠状动脉CT血管成像中的应用

目的探讨智能冠状动脉运动追踪平台Snapshot Freeze（SSF）在冠状动脉CT血管成像（CCTA）中的应用价值。方法连续选取61例患者,使用Discovery CT750 HD Freedom进行冠状动脉扫描,扫描后分别使用和不使用SSF重建,得到A、B两组图像;根据扫描实时心率将患者分为低心率组（心率≤70次/分,n=34）,中高心率组（心率71～80次/分,n=14）和高心率组（心率〉80次/分,n=13）。对比分析A、B组间和不同心率组间的图像质量差异。结果除左主干（S5）外,B组冠状动脉各节段图像质量评分均优于A组（P均〈0.05）;S5段剔除评分为5分者后,其余7例B组图像质量均优于A组（P=0.008）;未使用SSF重建时不可诊断的42个节段经用SSF重建后全部可用于诊断。未使用SSF重建的不同心率组间除S7、S9段外,其余各节段图像质量评分差异均有统计学意义（P均〈0.05）,图像质量随心率增加而下降;使用SSF重建后,不同心率组间各节段图像质量评分差异均无统计学意义（P均〉0.05）。未使用SSF重建的右冠状动脉中段（S2）在15段中评分最低（2.88±0.91）分,S3次之（3.65±1.32）分,且S2在低心率组亦有17例出现移动伪影影响其评估;使用SSF重建后,S2、S3移动伪影显著改善,评分分别提升至（4.32±0.59）分和（4.49±0.59）分,均满足诊断需求。结论 SSF能够有效地纠正高心率及心率波动导致的冠状动脉血管移动伪影,优化CCTA图像质量,尤其对右冠状动脉移动伪影有显著意义。

Experimental Study on the Degradation of Hydraulic Fully Graded Concrete under Freeze-thaw Cycles

In recent years,the winter temperature in southern China decreases year by year,and the phenomenon of freeze-thaw damage is also wide-spread in hydraulic buildings.In this paper,the freezing-thawing cycle tests of fully graded concrete specimens and corresponding wet sieving suitable for climate conditions in southern China were carried out,and the effects of freezing-thaw cycles on the appearance,mass loss,relative dynamic elastic modulus,internal temperature and strain of fully graded concrete and wet sieving concrete were studied.The splitting tensile strength test of concrete specimens with specified salt freezing cycles was carried out,and the relationship between the strength reduction of fully graded concrete and wet sieving concrete specimens and the number of freeze-thaw cycles was analyzed.The results show that with the increase of freeze-thaw cycles,the properties of fully graded concrete and wet sieving concrete degraded,and the degradation of compressive strength was the most significant,followed by the splitting tensile strength.In the initial stage of the freeze-thaw cycles,the degradation of each property was not obvious,and the deterioration rate of each property increased significantly after 100 freeze-thaw cycles.Besides,the relationship between the splitting tensile strength of fully graded concrete and wet sieving concrete and the internal wave velocity after freeze-thaw cycles was established.The test results can provide theoretical basis for the design,repair,maintenance and life prediction of dams and offshore platforms in southern China.

Freeze-thaw Damage Laws and Model of Hydraulic Fully Graded Concrete

On the basis of previous research results, the frost resistance of fully graded concrete and small wet sieving concrete specimens was compared and analyzed, and the characteristics and rules of freeze-thaw damage of hydraulic fully graded concrete after freeze-thaw cycles were analyzed through freeze-thaw test, impact-echo test, mechanical test and so on. Meanwhile, the damage characteristics of concrete in the salt and water freezing environment were compared. Through the mechanical test, it is found that the mechanical properties of both fully graded concrete and wet sieving concrete decreased to varying degrees after freeze-thaw cycles. The laws of uniaxial compression and tensile strength of concrete varying with the number of freeze-thaw cycles were obtained. The life of concrete after freeze-thaw cycles was predicted, and it is concluded that the life of concrete in the salt freezing environment was only 0.8 times that in the water freezing environment, proving once again that the damage of concrete in the salt freezing environment was more serious than that in the water freezing environment. Therefore, the grade of frost resistance should be improved appropriately for concrete engineering with salt solution. Finally, based on the damage theory and Lemaitre strain equivalence principle, the freeze-thaw damage evolution equation suitable for fully graded concrete was obtained.

Analysis of Meteorological Conditions of Freeze Damage to Citrus in Southern Shaanxi in the Winter of 2010 and Defensive Countermeasures

In order to take advantage of the climate resources more effectively ac- cording to the local circumstances and to plan and develop the citrus industry in Southern Shaanxi more reasonably. On the basis of the investigation of freeze dam- age to citrus occurring in Southern Shaanxi in the winter of 2010, the climatic back- ground for the formation of this freeze damage was analyzed. In combination with the freeze damage indicators during the overwintering period and the harmful accu- mulated cold during the cold wave, indexes for grading the freeze damage in southern Shaanxi were analyzed and verified, and the perspective of grading the freeze damage using the harmful accumulated cold during the cold wave was also presented. Through analyzing the extremely lowest temperature and the harmful ac- cumulated cold in the winter of 2010 and in history at 12 citrus growing counties （districts） in Ankang area and Hanzhong area, the reasons why the freeze damage to citrus during the overwintering period was severer in the west than in the east of Southern Shaanxi were discussed, and the results obtained were basically consistent with the actual situation observed from investigation. Finally, defensive countermea- sures against the freeze damage to citrus during the overwintering period were put forward from several aspects.

ULTRASTRUCTURE OF THE VEGETATIVE CELLS OF NOSTOC FLAGELLIFORME PREPARED WITH HIGH PRESSURE FREEZING AND FREEZE SUBSTITUTION TECHNIQUE

The ultrastructure of the vegetative cells of Nostoc flagelliforme Born. et Flah. was investigated with high pressure freezing and freeze substitution technique and compared with the results obtained by using conventional preparation methods. During the processes of chemical fixation, dehydration and embedding, the cell structures might be more artificially modified than that obtained from high pressure freezing and freeze substitution. With the present method, the sheath of N. flagelliforme could be well penetrated and no extra big space could exist between the cell and the sheath. The cell protoplasm rarely shrinked. Some fine structures of cell inclusions and unit membranes became visualized. Many bacteria were harbored in the sheath. In addition, the presence of big vacuoles in the cell of N. flagelliforme as well as the presence of bacteria in the sheath shown in the present preparation for cyanobacteria has not been described so far in the literature.

Effects of Facilities Condition on Occurrence of Freeze Injury and Fresh Pod Yield of Broad Bean

Using Tongxian No.2 as material, the effects of different film-covering time, different sowing time and different planting density on the occurrence of freeze injury and yield of fresh broad beans were investigated. The randomized block design was adopted. The results showed that with the delayed film covering, the incidence of mild freeze injury and number of headless seedlings were increased correspondingly, but the yield was increased; with the delayed sowing, the branch number per plant, effective branch number per plant, incidence of mild freeze injury and number of headless seedlings were all reduced, and the broad beans, sowed on September 30 th, obtained the highest yield; planting density showed on effect on the occurrence of freeze injury, and the yield was increased with the increase of planting density. Under the same film-covering time, the incidence of freeze injury was reduced with the delayed sowing time and it showed no changes when planting density was changed, but the yield was increased with the increase of planting density and it was highest when broad bean seeds were sowed on September 30th;under the same sowing time, the incidence of freeze injury was increased with the delayed film-covering time and it showed no changes when planting density was changed, and the yield was increased with the delayed film-covering time and increased planting density; under the same planting density, the incidence of freeze injury was increased with the delayed film-covering time but was reduced with the delayed sowing time, and the yield was increased with the delayed film-covering time and it was highest when the broad bean seeds were sowed on September30 th. Under same film-covering time and sowing time, the total branch number per plant and effective branch number per plant were reduced, but the yield was increased with the increase of planting density; under same film-covering time and planting density, the incidence of freeze injury was reduced with the delayed sowing time, and the yield was highest when broad bean seeds were sowed on September30th; under same sowing time and planting density, the incidence of freeze injury and the yield were all increased with the delayed film-covering time.

The mechanism underlying overwintering death in poplar:the cumulative effect of effective freeze-thaw damage

We analyzed the relationships linking overwintering death and frost cracking to temperature and sunlight as well as the effects of low temperatures and freeze–thaw cycles on bud-burst rates,relative electrical conductivity,and phloem and cambial ultrastructures of poplar.Overwintering death rates of poplar were not correlated with negative accumulated temperature or winter minimum temperature.Freeze–thaw cycles caused more bud damage than constant exposure to low temperatures.Resistance to freeze–thaw cycles differed among clones,and the budburst rate decreased with increasing exposure to freeze–thaw cycles.Cold-resistant clones had the lowest relative electrical conductivity.Chloroplasts exhibited the fastest and the most obvious reaction to freeze–thaw damage,whereas a single freeze–thaw cycle caused little damage to cambium ultrastructure.Several such cycles resulted in damage to plasma membranes,severe damage to organelles,dehydration of cells and cell death.We conclude that overwintering death of poplar is mainly attributed to the accumulation of effective freeze–thaw damage beyond the limits of freeze–thaw resistance.

多媒体教学中Freeze功能的应用

随着教育信息化的推进,多媒体在教学领域中的应用日益普及,然而如何应用和发挥好多媒体在教学中的功序优势还存在着很大的研究空间,本文通过对多媒体教学中一种常见的freeze功能应用研究,以引起广大教育工作者探索多媒体教学创新兴趣。

Deep Freeze软件在公共机房管理与维护方面的应用研究

机房管理与维护是保证学校正常教学活动的基础工作。科学的管理,灵活的技术,会使机房维护工作事半功倍。本文通过一个公共网络机房维护的实例,对Deep Freeze软件的特点、使用方法以及用此软件进行系统保护的具体操作过程进行了描述,提出了系统维护过程中应当注意的一系列技术性问题。

Monitoring Winter Wheat Freeze Injury Using Multi-Temporal MODIS Data

Freeze injury is an usual disaster for winter wheat in Shanxi Province, China, and monitoring freeze injury is of important economic significance. The aim of this article is to monitor and analyze the winter wheat freeze injury using remote sensing data, to monitor the occurrence and spatial distribution of winter wheat freeze in time, as well as the severity of the damage. The winter wheat freeze injury was monitored using multi-temporal moderate-resolution imaging spectroradiometer （MODIS） data, combined with ground meteorological data and field survey data, the change of normalized difference vegetation index （NDVI） before and after freeze injury was analyzed, as well as the effect of winter wheat growth recovery rate on yield. The results showed that the NDVI of winter wheat decreased dramatically after the suffering from freeze injury, which was the prominent feature for the winter wheat freeze injury monitoring. The degrees of winter wheat freeze injury were different in the three regions, of which, Yuncheng was the worst severity and the largest freeze injury area, the severity of freeze injury correlates with the breeding stage of the winter wheat. The yield of winter wheat showed positive correlation with its growth recovery rate （r=0.659^＊＊ which can be utilized to monitor the severity of winter wheat freeze injury as well as its impact on yield. It can effectively monitor the occurrence and severity of winter wheat freeze injury using horizontal and vertical profile distribution and growth wheat freeze injury in Shanxi Province. recovery rate, and provide a basis for monitoring the winter

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.

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.

Scheme of Weather-Based Indemnity Indices for Insuring Against Freeze Damage to Citrus Orchards in Zhejiang,China

We design a weather-based indemnity index for the insurance against freeze damage to citrus orchards so as to provide technological support for the development of policy-based agriculture. The indices are prepared by separating a relative meteorological yield from the yield that is dependent on tree age, high-yield and low-yield years, and environmental factors, and then using a risk assessment scheme to determine the percentage yield reduction due to the meteorological hazard. We thus develop a set of indices associated with cold temperature damage with which to construct more severe weather indices in conjunction with the yield percentage decrease. We then combine the insured regional citrus yield index with the insured meteorological counterpart to obtain a weather-based indemnity index for the varying degree of freeze damage to crops. When the freeze damage index （FDI） is greater than -7.0℃ for the coastal belt of Zhejiang Province, China, or greater than -9.0℃ for other regions of Zhejiang, weather-based indemnity index （WBII） is zero, meaning there is no compensation; when the FDI is from -7.0 to -7.9℃ for the coastal belt or from -9.0 to -9.9℃ for other regions, the WBII is 1 with 50% compensation; when the FDI is from -8.0 to -8.9℃ for the coastal belt or from -10.0 to -10.9℃ for other regions, the WBII is 2 with 70% compensation; and when the FDI is less than -9.0℃ for the coastal belt or less than -11.0℃ for other regions, the WBII is 3 with 90% compensation. The weather indemnity indices of insured orchards are developed in the interest of owners, thereby eliminating adverse selection and moral hazard issues and providing timely recompense from the insurer, and resolving the problem of high indemnity cost in agricultural insurance.

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.