scholarly journals Structural Strength Deterioration Characteristics and a Model of Undisturbed Loess under the Action of Wetting and Freeze-Thaw Cycles

2019 ◽  
Vol 2019 ◽  
pp. 1-23
Author(s):  
Haicheng She ◽  
Zaiqiang Hu ◽  
Zhan Qu ◽  
Hongru Li ◽  
Hu Guo ◽  
...  
Keyword(s):  
Compression Test ◽  
Structural Strength ◽  
Expansion Ratio ◽  
Damage Variable ◽  
Model Parameters ◽  
Confined Compression ◽  
Compression Curve ◽  
Freeze Thaw ◽  
Thaw Cycle ◽  
Statistical Damage

Undisturbed loess is affected by external environmental disturbances, such as wetting and freeze-thaw cycles, which cause microstructural changes that have an important impact on the structural strength of the loess. These changes in turn affect the stability of structures such as embankments, slopes, and guards. This article takes the Q3 undisturbed loess in Lintong District, Xi’an, as an example. The effects of wetting and freeze-thaw cycles on the loess expansion ratio and pore structure were studied by wetting tests, freeze-thaw cycle tests, and scanning electron microscopy (SEM). The changes in the compression index and compression modulus were studied by a confined compression test. The loess e-lgp compression curve was obtained according to the confined compression test, and the newly defined concepts of the loess structural strength, residual structural strength, and structural strength damage variable, in addition to the e-lgp compression curve, were combined with the experimental data to calculate the damage value generated by the disturbance during the sampling and preparation of loess. The deterioration of the structural strength and damage variable of loess was analyzed. Based on the microscopic statistical damage theory and Weibull distribution, the model used the volume expansion ratio as a variable to establish a statistical damage model under wetting and freeze-thaw cycles. Finally, on the basis of the test, the model parameters were determined. The models were verified by taking loess from a foundation pit in the northern suburbs of Xi’an and were in good agreement with the results of the test. Ultimately, the models have good practicability and can provide guidance for engineering design and construction.

scholarly journals Fractional Stress Relaxation Model of Rock Freeze-Thaw Damage

2021 ◽  
Vol 2021 ◽  
pp. 1-8
Author(s):  
Q. Liu ◽  
W. Chen ◽  
J. K. Guo ◽  
R. F. Li ◽  
D. Ke ◽  
...  
Keyword(s):  
Stress Relaxation ◽  
Fatigue Loading ◽  
Element Model ◽  
Model Parameters ◽  
Rock Stress ◽  
Relaxation Equation ◽  
Freeze Thaw ◽  
Thaw Cycle ◽  
Nonlinear Stress ◽  
Freeze Thaw Cycle

Freeze-thaw cycle is a type of fatigue loading, and rock stress relaxation under freeze-thaw cycles takes into account the influence of the freeze-thaw cycle damage and deterioration. Rock stress relaxation under freeze-thaw cycles is one of the paramount issues in tunnel and slope stability research. To accurately describe the mechanical behaviour of stress relaxation of rocks under freeze-thaw, the software element is constructed based on the theory of fractional calculus to replace the ideal viscous element in the traditional element model. The freeze-thaw damage degradation of viscosity coefficient is considered. A new three-element model is established to better reflect the nonlinear stress relaxation behavior of rocks under freeze-thaw. The freeze-thaw and stress relaxation of rock are simulated by ABAQUS, the relevant model parameters are determined, and the stress relaxation equation of rock under freeze-thaw cycle is obtained based on numerical simulation results. The research shows that the test results are consistent with the calculated results, indicating that the constitutive equation can better describe the stress relaxation characteristics of rocks under freeze-thaw and provide theoretical basis for surrounding rock support in cold region.

scholarly journals Equation for the Degradation of Uniaxial Compression Stress of Concrete due to Freeze-Thaw Damage

2019 ◽  
Vol 2019 ◽  
pp. 1-8 ◽  
Author(s):  
Xiaolin Yang ◽  
Genhui Wang ◽  
Shiwu Gao ◽  
Min Song ◽  
Anqi Wang
Keyword(s):  
Concrete Strength ◽  
Damage Variable ◽  
Curved Surface ◽  
Large Temperature ◽  
Test Results ◽  
Stress Strain ◽  
Cycle Number ◽  
Freeze Thaw ◽  
Thaw Cycle ◽  
Freeze Thaw Cycle

To study the freeze-thaw damage characteristics of concrete, the uniaxial compressive tests of concrete under different number of freeze-thaw cycles were conducted, and the damage variable of freeze-thaw was obtained. The test results showed that the stress was a function of strain and freeze-thaw damage variable, and it can describe the degradation of concrete strength. Meanwhile, the equation for the stress-strain curved surface about strain and freeze-thaw damage variable was also proposed in this paper. The derivative function of the stress-strain curved surface equation with respect to strain presented the change of elastic modulus with the increase of freeze-thaw cycle number. Equation proposed in this paper can be used for predicting the concrete lifetime effectively in cold and large temperature difference regions.

scholarly journals Investigation of Sandstone Mesostructure Damage Caused by Freeze-Thaw Cycles via CT Image Enhancement Technology

2020 ◽  
Vol 2020 ◽  
pp. 1-13
Author(s):  
Hui Liu ◽  
Gengshe Yang ◽  
Yehui Yun ◽  
Jianghao Lin ◽  
Wanjun Ye ◽  
...  
Keyword(s):  
Ct Images ◽  
Damage Variable ◽  
Damage Propagation ◽  
Freeze Thaw ◽  
Grain Sizes ◽  
Thaw Cycle ◽  
Defect Sites ◽  
Freeze Thaw Cycle ◽  
Color Enhancement ◽  
Pseudo Color

The mesostructures of rocks determine their macromechanical properties. These rock mesostructures may be altered by the freeze-thaw cycles in cold regions. In this regard, this paper proposes a quantitative evaluation method based on computed tomography (CT) scanning technology for investigating the mesostructure and damage characteristics of sandstone subjected to freeze-thaw conditions. CT scan images of two sandstones with different grain sizes were obtained after 0, 20, 40, 60, 80, and 100 freeze-thaw cycles, using a high-precision CT scanner. Based on the microphysical information contained in these CT images, pseudo-color-enhancement of the CT images of rocks subjected to freeze-thaw cycles was realized. The use of such a pseudo-color-enhancement technique can improve the resolution of CT images. Thus, particle detachment, crack initiation, crack propagation, and increased porosity due to the volumetric expansion of water inside the rock could be detected and clearly observed. Furthermore, a numerical expression for the mesostructure and damage information contained in the pseudo-color-enhanced images is presented herein; this serves as a convenient method for quantitative analyses of sandstone damage under freeze-thaw cycles. An analysis of the pseudo-color-enhanced images shows that, under freeze-thaw cycles, damage propagation in sandstone originates from existing damage or defect sites. After the stages of crack (pore) formation, penetration, and propagation, the freeze-thaw cycle-induced damage increases gradually, while the effective bearing area of the rock decreases continuously. Herein, a schematic of a conceptual model for the freeze-thaw cycle-induced deterioration in sandstone mesostructures is presented. Damage propagation models for sandstones with two different grain sizes subjected to freeze-thaw cycles were also developed. Based on the damage mechanics theory, a damage variable expressed in terms of the pore area was defined. Moreover, the relationship between this damage variable and the freeze-thaw cycles was established.

scholarly journals Whole cell-based catalyst for enzymatic production of the osmolyte 2-O-α-glucosylglycerol

2021 ◽  
Vol 20 (1) ◽  
Author(s):  
Katharina N. Schwaiger ◽  
Monika Cserjan-Puschmann ◽  
Gerald Striedner ◽  
Bernd Nidetzky
Keyword(s):  
Specific Activity ◽  
Dry Mass ◽  
High Yield ◽  
Sucrose Phosphorylase ◽  
Bifidobacterium Adolescentis ◽  
Enzymatic Production ◽  
Intracellular Enzyme ◽  
Whole Cell ◽  
Freeze Thaw ◽  
Thaw Cycle

Abstract Background Glucosylglycerol (2-O-α-d-glucosyl-sn-glycerol; GG) is a natural osmolyte from bacteria and plants. It has promising applications as cosmetic and food-and-feed ingredient. Due to its natural scarcity, GG must be prepared through dedicated synthesis, and an industrial bioprocess for GG production has been implemented. This process uses sucrose phosphorylase (SucP)-catalyzed glycosylation of glycerol from sucrose, applying the isolated enzyme in immobilized form. A whole cell-based enzyme formulation might constitute an advanced catalyst for GG production. Here, recombinant production in Escherichia coli BL21(DE3) was compared systematically for the SucPs from Leuconostoc mesenteroides (LmSucP) and Bifidobacterium adolescentis (BaSucP) with the purpose of whole cell catalyst development. Results Expression from pQE30 and pET21 plasmids in E. coli BL21(DE3) gave recombinant protein at 40–50% share of total intracellular protein, with the monomeric LmSucP mostly soluble (≥ 80%) and the homodimeric BaSucP more prominently insoluble (~ 40%). The cell lysate specific activity of LmSucP was 2.8-fold (pET21; 70 ± 24 U/mg; N = 5) and 1.4-fold (pQE30; 54 ± 9 U/mg, N = 5) higher than that of BaSucP. Synthesis reactions revealed LmSucP was more regio-selective for glycerol glycosylation (~ 88%; position O2 compared to O1) than BaSucP (~ 66%), thus identifying LmSucP as the enzyme of choice for GG production. Fed-batch bioreactor cultivations at controlled low specific growth rate (µ = 0.05 h−1; 28 °C) for LmSucP production (pET21) yielded ~ 40 g cell dry mass (CDM)/L with an activity of 2.0 × 104 U/g CDM, corresponding to 39 U/mg protein. The same production from the pQE30 plasmid gave a lower yield of 6.5 × 103 U/g CDM, equivalent to 13 U/mg. A single freeze–thaw cycle exposed ~ 70% of the intracellular enzyme activity for GG production (~ 65 g/L, ~ 90% yield from sucrose), without releasing it from the cells during the reaction. Conclusions Compared to BaSucP, LmSucP is preferred for regio-selective GG production. Expression from pET21 and pQE30 plasmids enables high-yield bioreactor production of the enzyme as a whole cell catalyst. The freeze–thaw treated cells represent a highly active, solid formulation of the LmSucP for practical synthesis.

scholarly journals Effect of freeze–thaw cycle on physical and mechanical properties and damage characteristics of sandstone

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Longxiao Chen ◽  
Kesheng Li ◽  
Guilei Song ◽  
Deng Zhang ◽  
Chuanxiao Liu
Keyword(s):  
Mechanical Properties ◽  
Shear Failure ◽  
Triaxial Compression ◽  
Physical And Mechanical Properties ◽  
Freeze Thaw ◽  
Damage Characteristics ◽  
Large Pore ◽  
Natural Rock ◽  
Thaw Cycle ◽  
Freeze Thaw Cycle

AbstractRock deterioration under freeze–thaw cycles is a concern for in-service tunnel in cold regions. Previous studies focused on the change of rock mechanical properties under unidirectional stress, but the natural rock mass is under three dimensional stresses. This paper investigates influences of the number of freeze–thaw cycle on sandstone under low confining pressure. Twelve sandstone samples were tested subjected to triaxial compression. Additionally, the damage characteristics of sandstone internal microstructure were obtained by using acoustic emission (AE) and mercury intrusion porosimetry. Results indicated that the mechanical properties of sandstone were significantly reduced by freeze–thaw effect. Sandstone’ peak strength and elastic modulus were 7.28–37.96% and 6.38–40.87% less than for the control, respectively. The proportion of super-large pore and large pore in sandstone increased by 19.53–81.19%. We attributed the reduced sandstone’ mechanical properties to the degenerated sandstone microstructure, which, in turn, was associated with increased sandstone macropores. The macroscopic failure pattern of sandstone changed from splitting failure to shear failure with an increasing of freeze–thaw cycles. Moreover, the activity of AE signal increased at each stage, and the cumulative ringing count also showed upward trend with the increase of freeze–thaw number.

Non-linear creep damage model of sandstone under freeze-thaw cycle

2021 ◽  
Vol 28 (3) ◽  
pp. 954-967
Author(s):  
Jie-lin Li ◽  
Long-yin Zhu ◽  
Ke-ping Zhou ◽  
Hui Chen ◽  
Le Gao ◽  
...  
Keyword(s):  
Damage Model ◽  
Creep Damage ◽  
Freeze Thaw ◽  
Thaw Cycle ◽  
Linear Creep ◽  
Non Linear ◽  
Freeze Thaw Cycle ◽  
Creep Damage Model

scholarly journals Strength and Microscopic Damage Mechanism of Yellow Sandstone with Holes under Freezing and Thawing

2020 ◽  
Vol 2020 ◽  
pp. 1-13 ◽  
Author(s):  
Huren Rong ◽  
Jingyu Gu ◽  
Miren Rong ◽  
Hong Liu ◽  
Jiayao Zhang ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Pore Size ◽  
Power Function ◽  
Damage Mechanism ◽  
Uniaxial Compression Test ◽  
Freezing And Thawing ◽  
Freeze Thaw ◽  
Thaw Cycle ◽  
Freeze Thaw Cycle ◽  
Microscopic Damage

In order to study the damage characteristics of the yellow sandstone containing pores under the freeze-thaw cycle, the uniaxial compression test of saturated water-stained yellow sandstones with different freeze-thaw cycles was carried out by rock servo press, the microstructure was qualitatively analyzed by Zeiss 508 stereo microscope, and the microdamage mechanism was quantitatively studied by using specific surface area and pore size analyzer. The mechanism of weakening mechanical properties of single-hole yellow sandstone was expounded from the perspective of microstructure. The results show the following. (1) The number of freeze-thaw cycles and single-pore diameter have significant effects on the strength and elastic modulus of the yellow sandstone; the more the freeze-thaw cycles and the larger the pore size, the lower the strength of the yellow sandstone. (2) The damage modes of the yellow sandstone containing pores under the freeze-thaw cycle are divided into five types, and the yellow sandstone with pores is divided into two areas: the periphery of the hole and the distance from the hole; as the number of freeze-thaw cycles increases, different regions show different microscopic damage patterns. (3) The damage degree of yellow sandstone is different with freeze-thaw cycle and pore size. Freeze-thaw not only affects the mechanical properties of yellow sandstone but also accelerates the damage process of pores. (4) The damage of the yellow sandstone by freeze-thaw is logarithmic function, and the damage of the yellow sandstone is a power function. The damage equation of the yellow sandstone with pores under the freezing and thawing is a log-power function nonlinear change law and presents a good correlation.

Studies the Properties of Polyaspartic Polyurea Coated Concrete under Coaction of Salt Fog and Freeze-Thaw

2012 ◽  
Vol 455-456 ◽  
pp. 781-785
Author(s):  
Ping Lu ◽  
Xin Mao Li ◽  
Xue Qiang Ma ◽  
Wei Bo Huang
Keyword(s):  
Elastic Modulus ◽  
Frost Resistance ◽  
Dynamic Elastic Modulus ◽  
Freeze Thaw ◽  
Decrease Rate ◽  
Thaw Cycle ◽  
Freeze Thaw Cycle ◽  
Salt Fog

. This paper mainly studied the properties of PAE polyurea coated concrete under coactions of salt fog and freeze-thaw. After exposed salt fog conditions for 200d, T3, B2, F2 and TM four coated concrete relative dynamic elastic modulus have small changes, but different coated concrete variation amplitude is different. T3 coated concrete after 100 times of freeze-thaw cycle the relative dynamic elastic modulus began to drop, 200 times freeze-thaw cycle ends, relative dynamic elastic modulus variation is the largest, decrease rate is 95%, TM concrete during 200 times freeze-thaw cycle, relative dynamic elastic modulus almost no change, B2 concrete and F2 concrete the extent of change between coating T3 and TM. After 300 times the freeze-thaw cycle coated concrete didn't appear freeze-thaw damage phenomenon. Four kinds of coating concrete relative dynamic elastic modulus variation by large to small order: T3 coated concrete > B2 coated concrete >F2 coated concrete > TM coated concrete, concrete with the same 200d rule. Frost resistance order, by contrast, TM coated concrete > B2 coated concrete > F2 coated concrete > T3 coated concrete.

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