Coupling Effect of Cryogenic Freeze-Thaw Cycles and Chloride Ion Erosion Effect in Pre-Cracked Reinforced Concrete

Chloride (Cl−) ion erosion effects can seriously impact the safety and service life of marine liquefied natural gas(LNG) storage tanks and other polar offshore structures. This study investigates the impact of different low-temperaturecycles (20°C, –80°C, and −160°C) and concrete specimen crack widths (0, 0.3, and 0.6 mm) on the Cl−ion diffusion performance through rapid erosion tests conducted on pre-cracked concrete. The results show thatthe minimum temperature and crack width of freeze-thaw cycles enhance the erosive effect of chloride ions. TheCl− ion concentration and growth rate increased with the increasing crack width. Based on the experimental modeland in accordance with Fick’s second law of diffusion, the Cl− ion diffusion equation was modified by introducingcorrection factors in consideration of the freeze-thaw temperature, crack width, and their coupling effect.The experimental and fitting results obtained from this model can provide excellent reference for practical engineeringapplications.

Effects of dry-wet cycles on the mechanical properties of sandstone with unloading-induced damage

Sandstone is the fundamental material in various engineering and construction projects.However,the mechanical integrity of sandstone can be compromised by initial unloading damage resulting from activities such as engineering excavations.Furthermore,this degradation is further exacerbated under periodic dry-wet environmental conditions.This study investigated the effects of dry-wet cycles and unloading on the mechanical properties of jointed fine sandstone using uniaxial and triaxial compression tests.These tests were performed on rock samples subjected to varying unloading degrees and different numbers of dry-wet cycles.The results demonstrate that with an increase in the unloading degree from 0%to 70%,there is a corresponding decrease in peak stress ranging from 10%to 33%.Additionally,the cohesion exhibits a reduction of approximately 20%to 25%,while the internal friction angle experiences a decline of about 3.5%to 6%.These findings emphasize a significant unloading effect.Moreover,the degree of peak stress degradation in unloading jointed fine sandstone diminishes with an increase in confining pressure,suggesting that confining pressure mitigates the deterioration caused by dry-wet cycles.Additionally,as the number of dry-wet cycles increases,there is a notable decline in the mechanical properties of the sandstone,evidencing significant dry-wet degradation.Utilizing the Drucker Prager criterion,this study establishes a strength criterion and fracture criterion,denoted as σ_(1)(m,n) and K_(T)^(Ⅱ)(m, n), to quantify the combined impacts of dry-wet cycles and unloading on jointed fine sandstone,which provides a comprehensive understanding of its mechanical behavior under such conditions.

Effect of dry-wet cycles on dynamic properties and microstructures of sandstone:Experiments and modelling

Underground pumped storage power plant(UPSP)is an innovative concept for space recycling of abandoned mines.Its realization requires better understanding of the dynamic performance and durability of reservoir rock.This paper conducted ultrasonic detection,split Hopkinson pressure bar(SHPB)impact,mercury intrusion porosimetry(MIP),and backscatter electron observation(BSE)tests to investigate the dynamical behaviour and microstructure of sandstone with cyclical dry-wet damage.A coupling FEM-DEM model was constructed for reappearing mesoscopic structure damage.The results show that dry-wet cycles decrease the dynamic compressive strength(DCS)with a maximum reduction of 39.40%,the elastic limit strength is reduced from 41.75 to 25.62 MPa.The sieved fragments obtain the highest crack growth rate during the 23rd dry-wet cycle with a predictable life of 25 cycles for each rock particle.The pore fractal features of the macropores and micro-meso pores show great differences between the early and late cycles,which verifies the computational statistics analysis of particle deterioration.The numerical results show that the failure patterns are governed by the strain in pre-peak stage and the shear cracks are dominant.The dry-wet cycles reduce the energy transfer efficiency and lead to the discretization of force chain and crack fields.

Effect of Freeze-Thaw Cycles on Chloride Transportation in Concrete: Prediction Model and Experiment

This research aims to investigate the effect of frost damage on chloride transportation mechanism in ordinary andfiber concrete with both theoretical and experimental methods.The proposed theoretical model takes into account the varying damage levels caused by concrete cover depth and freeze-thaw cycles,which are the two primary parameters affecting the expression of the chloride diffusion coefficient.In the experiment,three types of concrete were prepared:ordinary Portland concrete(OPC),polypropylenefiber concrete(PFC),and steelfiber concrete(SFC).These were then immersed in NaCl solution for 120 days after undergoing 10,25,and 50 freeze-thaw cycles.The damage coefficient of the tested concrete was determined by measuring the dynamic elas-tic modulus.The results indicated that the relative dynamic elasticity modulus of the specimens decreased with each freeze-thaw cycle,and the chloride diffusion coefficient of the specimens increased as the degree of frost degradation increased.Samples containing steel and polypropylenefibers exhibited greater resistance to cyclic water freezing compared to the controlled concrete withoutfibers.A model has been also developed that takes into account the damage caused by freezing-thawing cycles and the depth of the concrete,which can predict variations in free chloride concentration at different depths.The calculated values were in good agreement with the test results for depths between 10 to 30 mm.This new damage-induced diffusion model can helpfill the gap in research on the effects of freeze-thaw cycles on chloride diffusion.

Performance of interface between TRC and existing concrete under a chloride dry-wet cycle environment

Textile-reinforced concrete(TRC)is suitable to repair and reinforce concrete structures in harsh environments.The performance of the interface between TRC and existing concrete is an important factor in determining the strengthening effect of TRC.In this paper,a double-sided shear test was performed to investigate the effects of the chloride dry-wet cycles on the average shear strength and slip at the interface between the TRC and existing concrete,also considering the existing concrete strength,bond length,textile layer and short-cut fiber arrangements.In addition,X-ray diffraction(XRD)technology was used to analyze the microscopic matter at the interface in the corrosive environment.The experimental results indicate that the interface performance between TRC and existing concrete would decrease with continued chloride dry-wet cycles.Compared with the specimen with a single layer of textile reinforcement,the specimens with two layers of textile with added PVA or AR-glass short-cut fibers could further improve the properties of the interface between the TRC layer and existing concrete.For the TRC with a single layer of textile,the average shear strength tended to decrease with increasing bond length.In addition,the strength grade of the existing concrete had a minor effect on the interface properties.

Effect of Dry-Wet Cycles on the Transport and Mechanical Properties of Cement Mortar Subjected to Sulfate Attack

This study deals with the analysis of the detrimental effects of a“sulfate attack”on cement mortar for different dry-wet cycles.The mass loss,tensile strength,and gas permeability coefficient were determined and analyzed under different exposure conditions.At the same time,nitrogen adsorption(NAD),scanning electron microscopy(SEM),and X-ray diffraction(XRD)techniques were used to analyze the corresponding variations in the microstructure and the corrosion products.The results show that certain properties of the cement mortar evolve differently according to the durations of the dry-wet cycles and that some damage is caused to the mortars in aqueous solution.The pores fill with corrosion products,increasing the mortar specimen mass and tensile strength while reducing the permeability coefficient and pore size distribution.As corrosion proceeds,the crystallization pressure of the corrosion products increases,resulting in a 16%reduction in tensile strength from the initial value and a 2.6-factor increase in the permeability coefficient,indicating sensitivity to sulfate attack damage.Furthermore,the main corrosion products generated in the experiment are gypsum and ettringite.Application of osmotic pressure and extension of the immersion time can accelerate the erosion process.

Stability analysis of gravity anchor foundation of layered argillaceous sandstone under dry-wet cycles

To investigate the stability of gravity anchors of suspension bridges,in-situ tests of the vertical bearing capacity of the bedrock,shear resistance of the anchor-rock interface,shear resistance of the bedrock were conducted in a suspension bridge project.Under dry-wet cycles,the deterioration law of the mechanical properties of argillaceous sandstone was identified in laboratory tests:the elastic modulus,cohesion and friction of the argillaceous sandstone deteriorated significantly at first few dry-wet cycles and then declined slowly after 10 cycles,ultimately these three mechanical parameters were reduced to about 1/3,1/3,2/3 of the initial value respectively.Moreover,numerical simulation was used to restore in-situ shear tests and a good agreement was obtained.Base on the results of in-situ and laboratory tests,the stability of the gravity anchor foundation under natural conditions and drywet cycles was calculated and its failure modes were analyzed.The results demonstrated that the dry-wet cycles caused uneven settlement of the anchor foundation,resulting in more serious stress concentration in the substrate.The dry-wet cycles remarkably reduced the stability coefficient of the anchor foundation,whose failure mode shifted from overturning failure under natural conditions to sliding failure.When there was weak interlayer in the rock layer,the anti-sliding stability of the anchor foundation was affected drastically.

The law of strength damage and deterioration of jointed sandstone after dry-wet cycles

Under the periodic rise and fall of the water level in the Three Gorges Reservoir in China,the rock mass in the ebb and flow zone of the slope is always in a state of a dry-wet cycle.In order to explore the influence of dry-wet cycle on mechanical properties of jointed sandstone,the triaxial and uniaxial compression tests of dry-wet cycle of jointed sandstone were carried out.For the experiment,four groups of samples with different numbers of joints were set up,and the jointed rock samples were subjected to 20 dry-wet cycles.Using both the triaxial compression test and the Mohr-Coulomb(M-C)rock fracture criterion,the strength envelope of the sandstone samples was fitted,and their strength degradation was further analyzed and studied.The results show that:(1)The peak intensity and elastic modulus of the sandstone samples decrease with increased number of dry-wet cycles.(2)The total deterioration of mechanical properties of intact rock samples is bigger than that of jointed sandstone samples as the number of dry-wet cycles increases.(3)With the increase of confining pressure,the peak intensity of intact sandstone samples increases much more than that of jointed sandstone samples,which indicates that joints and their numbers have obvious influence.(4)Joints and their numbers play an important role in guiding the damage effects of sandstone samples,which weaken the damage caused by dry-wet cycles.Therefore,the envelope of the M-C strength criterion of intact sandstone samples moves more than that of jointed sandstone samples.

Intracellular chloride regulates the G_1/S cell cycle progression in gastric cancer cells

Recent studies show that ion channels/transporters play important roles in fundamental cellular functions. Several reports indicating the important roles of Cl- channels/transporters on cell proliferation suggest that the intracellular chloride concentration ([Cl-]i) regulated by them would be one of critical messengers. We investigated whether the [Cl-]i controls cell proliferation and cell cycle progression in human gastric cancer cells. Our studies indicated that furosemide, a blocker of Na+ /K+ /2Cl- cotransporter (NKCC), diminished cell growth by delaying the G1-S phase progression in gastric cancer cells with high expression and activity of NKCC. Furthermore, we found that the culture in the low Cl- medium (replacement of Cl- by NO3-) decreased the [Cl-]i and inhibited cell growth of gastric cancer cells and that this inhibition of cell growth was due to cell cycle arrest at the G0/G1 phase caused by diminutionof CDK2 and phosphorylated Rb. The culture of cells in the low Cl- medium significantly increased expressions of p21 mRNA and protein. In addition, the low Cl- medium induced phosphorylation of mitogen activated protein kinases (MAPKs). Treatment with an inhibitor of p38 or JNK significantly suppressed p21 upregulation caused by culture in a low Cl- medium and rescued gastric cancer cells from the low Cl- -induced G1 cell cycle arrest. These findings revealed that the [Cl-]i affects the cell proliferation via activation of MAPKs through upregulation of p21 in gastric cancer cells. Our results suggest that the [Cl-]i regulates important cellular functions in gastric cancer cells, leading to the development of novel therapeutic strategies.

Chloride Ion Transmission Model under the Drying-wetting Cycles and Its Solution

The chloride ion transmission model considering diffusion and convection was established respectively for different zones in concrete by analyzing chloride ion transmission mechanism under the dryingwetting cycles. The finite difference method was adopted to solve the model. The equation of chloride ion transmission model in the convection and diffusion zone of concrete was discreted by the group explicit scheme with right single point （GER method） and the equation in diffusion zone was discreted by FTCS difference scheme. According to relative humidity characteristics in concrete under drying-wetting cycles, the seepage velocity equation was formulated based on Kelvin Equation and Darcy＇s Law. The time-variant equations of chloride ion concentration of concrete surface and the boundary surface of the convection and diffusion zone were established. Based on the software MATLAB the numerical calculation was carried out by using the model and basic material parameters from the experiments. The calculation of chloride ion concentration distribution in concrete is in good agreement with the drying-wetting cycles experiments. It can be shown that the chloride ion transmission model and the seepage velocity equation are reasonable and practical. Studies have shown that the chloride ion transmission in concrete considering convection and diffusion under the drying-wetting cycles is the better correlation with the actual situation than that only considering the diffusion.

Effect of Carbonation and Drying-Wetting Cycles on Chloride Diffusion Behavior of Coral Aggregate Seawater Concrete

Based on seawater immersion,drying-wetting cycles,carbonation and drying-wetting cycles for coral aggregate sea-water concrete(CASC)with different strength grades,the effect of carbonation and drying-wetting cycles on chloride diffusion be-havior of CASC is studied.The results show that the free surface chloride concentration(Cs),free chloride diffusion coefficient(Df)and time-dependent index(m)of CASC in the drying-wetting cycles is obviously higher than that in seawater immersion.The Df and m of CASC of carbonation and drying-wetting cycles is higher than that in the drying-wetting cycles.Carbonation increases the Df and m of CASC,which is against CASC to resist chloride corrosion.The corrosion possibility of CASC structures in different ex-posed areas is as follows:splash zone(carbonation and drying-wetting cycles)>tidal zone(drying-wetting cycles)>underwater zone(seawater immersion).Besides,the chloride diffusion rate of C65-CASC is 17.8%-63.4%higher than that of C65-ordinary aggre-gate concrete(OAC)in seawater immersion(underwater zone).Therefore,anti-corrosion measures should be adopted to improve the service life of CASC structure in the oceanic environment.

Effects of dry-wet cycles on three-dimensional pore structure and permeability characteristics of granite residual soil using X-ray micro computed tomography

Due to seasonal climate alterations,the microstructure and permeability of granite residual soil are easily affected by multiple dry-wet cycles.The X-ray micro computed tomography(micro-CT)acted as a nondestructive tool for characterizing the microstructure of soil samples exposed to a range of damage levels induced by dry-wet cycles.Subsequently,the variations of pore distribution and permeability due to drywet cycling effects were revealed based on three-dimensional(3D)pore distribution analysis and seepage simulations.According to the results,granite residual soils could be separated into four different components,namely,pores,clay,quartz,and hematite,from micro-CT images.The reconstructed 3D pore models dynamically demonstrated the expanding and connecting patterns of pore structures during drywet cycles.The values of porosity and connectivity are positively correlated with the number of dry-wet cycles,which were expressed by exponential and linear functions,respectively.The pore volume probability distribution curves of granite residual soil coincide with the χ^(2)distribution curve,which verifies the effectiveness of the assumption of χ^(2)distribution probability.The pore volume distribution curves suggest that the pores in soils were divided into four types based on their volumes,i.e.micropores,mesopores,macropores,and cracks.From a quantitative and visual perspective,considerable small pores are gradually transformed into cracks with a large volume and a high connectivity.Under the action of dry-wet cycles,the number of seepage flow streamlines which contribute to water permeation in seepage simulation increases distinctly,as well as the permeability and hydraulic conductivity.The calculated hydraulic conductivity is comparable with measured ones with an acceptable error margin in general,verifying the accuracy of seepage simulations based on micro-CT results.

Shot noise analysis on corrosion behavior of zinc alloy (ZnAl4Cul) under dry-wet cycles

The corrosion behaviors of zinc alloy （ZnAl4Cul） in 3.5% （mass fraction） NaCl, 7.3% （mass fraction） Na2SO4 and simulated acid rain solutions were investigated using electrochemical measurements. The potential noise during dry-wet cycle was monitored and analyzed by fast Fourier transform （FFT）, fast wavelet transform （FWT）, shot noise theory and stochastic theory. Cumulative probability curves of event frequency fn indicate that the corrosion events in the dry cycles are greater than those in the wet cycles. Uniform corrosion was observed in the NaCl solution compared with more localized corrosion in the Na2SO4 solution, which is evidenced by FWT and SEM. Conditional events generation rate r（t） for diffusion controlled reactions decreases with increasing the time. r（t） values for uniform corrosion and diffusion controlled process are the largest in the wet cycle in 3.5% NaCl solution. The values of r（t） for pitting corrosion in Na2SO4 solution are observed to become large during spraying periods, and r（t） for pitting corrosion has the largest value in the Na2SO4 solution. The intergranular corrosion of zinc is serious in simulated acid rain solution.

Law of Water Content Change in Subgrade Soil Under Action of Dry-Wet Cycle

Due to the influence of the groundwater level,the internal humidity of the subgrade changes and the stability of the subgrade is affected. The main purpose of this paper is to obtain a reliable model of subgrade soil water content variation under the action of dry-wet cycle through sensor readings. Thus,an indoor soil column model test system is designed,and the readings of the sensors are used to determine the changing law of moisture field in the subgrade soil. The sensor readings indicate that the water content gradually decreases along the height of the soil column,and the water in the upper part of the soil column continuously loses,while the water in the lower part migrates upward to supplement. With the increase of dry-wet cycle index,the water holding capacity of soil decreases,and the soil surface gradually cracks and tends to rupture.

The Quantification of Micro-structural Damage of Weak Muddy Intercalation in Dry-wet Cycles Combining in-situ SEM and DIP

In order to reflect truly the damage evolution mechanism of weak muddy intercalation in dry-wet cycles, two typical weak muddy intercalations were selected for dry-wet cycles. The mineral changes of specimens were analyzed via X-ray diffraction after dry-wet cycles. By combining in-situ SEM and digital image processing(DIP), the damage evolution process and damage characteristic parameters of each stage were obtained. The experimental results indicate that the hydration and dissolution of minerals can not be a determinant factor in structure damage. The micro-structural damage is due to disintegration of mineral aggregates, leading to changes in the number and size of cracks and pores. The damage degree of specimens is related to its initial structure, and the micro-structural damage intensifies and finally tends to stabilize with cycle times increased.

Investigation of Sulfate Attack Resistance of Shotcrete under Dry-wet Cycles

In order to research the sulfate attack resistance of shotcrete, the sulfate attack of shotcrete in the presence and absence of steel fiber was experimentally studied by using dry-wet cycle method. Meanwhile, compared with ordinary concrete by the same mixture, the difference of sulfate attack resistance of shotcrete was studied. The experimental results showed that, with dry-wet cycles increasing, the changes of loss rate of relative dynamic elastic modulus and mass loss rate of specimens included three stages： initial descent stage, stable stage, and rapid descent stage, respectively. However, the changes of mechanical properties first increased and then decreased. Furthermore, the corrosion products of shotcrete after sulfate attack were observed by using the method of XRD, thermal analysis, and SEM, respectively, and the failure mode of shotcrete turned from ettringite destruction to ettringite-gypsum comprehensive failure. Meanwhile, the contents of ettringite and gypsum increased with increasing dry-wet cycle. Simultaneously, the stratified powders drilled from shotcrete under 150＇s dry-wet cycle were analyzed for the mineral phase composition and thermal analysis. With the drywet cycle increasing, the content of ettringite first increased and then decreased and tended to stable. However, the determination of gypsum decreased gradually and even to 0 when the depth was more than 12 mm.

Degradation of bond between steel bar and freeze-thaw concrete after electrochemical chloride extraction

The effect of electrochemical chloride extraction （ECE） on bond strength between steel bar and freeze-thaw concrete contaminated by chloride was experimentally investigated for beam specimens with dimensions of 100 mm × 100 mm × 400 ram. During the experiment, 3% NaC1 （vs mass of cement, mass fraction） was mixed into concrete to simulate chloride contamination, and the specimens experienced 0, 25, 50, 75 freeze-thaw cycles before ECE. In the process of ECE, different current densities and durations were adopted. It is indicated that the bond strength between reinforcement and concrete decreases with the increase of freeze-thaw cycles; the more the current and the electric quantity of ECE are, the more the loss of bond strength is; and the largest loss is up to 58.7%. So, it is important to choose proper parameters of ECE for the reinforced concrete structures contaminated by chloride and subjected to freeze-thaw cycles.

Chloride diffusivity in flexural cracked Portland cement concrete and fly ash concrete beams

In order to examine the effect of load-induced transverse cracks on the chloride penetration in flexural concrete beams, two different concretes, Portland cement concrete(PCC) and fly ash concrete(FAC), were tested with various crack widths. Total 14 reinforced concrete(RC) beams, ten of which were self-anchored in a three-point bending mode, were immersed into a 5% NaCl solution with the condition of dry-wet cycles. Then, the free chloride ion contents were determined by rapid chloride testing(RCT) method. Based on the proposed analytical models of chloride penetration in sound and cracked concrete subjected to dry-wet cycles, the apparent chloride diffusion coefficient and chloride diffusivity of concrete were discussed. It can be found that the performance of chloride diffusivity in both concretes will be improved with the increase of crack width, and that the influence of convection action will also be augmented. Based on the two samples obtained in sound concrete after 15 and 30 cycles, the time-exponent, m, for chloride diffusion coefficient was determined to be 0.58, 0.42, 0.62 and 0.77 for PCC1, PCC2, FAC1 and FAC2 specimens, respectively. Finally, two influencing factors of fly ash content and crack width on chloride diffusivity were obtained by regression analysis of test data, and it can be seen that factors kf and kw can be expressed with quadratic polynomial functions of fly ash content, f, and crack width, w, respectively.

Chloride Ion Critical Content in Reinforced Concrete

Chloride ion critical content was studied under soaking and cycle of dry and wet conditions, with three electrochemical nondestructive measuring techniques, i e, half-cell potential, A C impedance, and time potential. The experimental results show that chloride ion critical content is primarily determined by the water cement ratio, while for the same concrete mixture the chloride ion critical content in soaking conditions is larger than that in a cycle of dry and wet conditions.

Exploring the mechanical behavior and microstructure of compacted loess subjected to dry-wet cycles and chemical contamination

Due to climatic factors and rapid urbanization,the soil in the Loess Plateau,China,experiences the coupled effects of dry-wet cycles and chemical contamination.Understanding the mechanical behavior and corresponding microstructural evolution of contaminated loess subjected to dry-wet cycles is essential to elucidate the soil degradation mechanism.Therefore,direct shear and consolidation tests were performed to investigate the variations in mechanical properties of compacted loess contaminated with acetic acid,sodium hydroxide,and sodium sulfate during dry-wet cycles.The mechanical response mechanisms were investigated using zeta potential,mineral chemical composition,and scanning electron microscopy(SEM)tests.The results indicate that the mechanical deterioration of sodium hydroxidecontaminated loess during dry-wet cycles decreases with increasing contaminant concentration,which is mainly attributed to the thickening of the electrical double layer(EDL)by Nat and the precipitation of calcite,as well as the formation of colloidal flocs induced by OH,thus inhibiting the development of large pores during the dry-wet process.In contrast,the attenuation of mechanical properties of both acetic acid-and sodium sulfate-contaminated loess becomes more severe with increasing contaminant concentration,with the latter being more particularly significant.This is primarily due to the reduction of the EDL thickness and the erosion of cement in the acidic environment,which facilitates the connectivity of pores during dry-wet cycles.Furthermore,the salt expansion generated by the drying process of saline loess further intensifies the structural disturbance.Consequently,the mechanical performance of compacted loess is sensitive to both pollutant type and concentration,exhibiting different response patterns in the dry-wet cycling condition.