Critical Degree Of Saturation Research Articles

Influence of relative compaction and degree of saturation on the deformation characteristics of bentonite under freeze-thaw cycles

Bentonite, consisting of clay minerals of the montmorillonite group, has been widely used as an adsorbent and backfill material in nuclear waste disposal and groundwater remediation. It is challenging to use bentonite as a filling material in cold regions since bentonite is highly sensitive to thermal environmental changes, during which its bulk volume and microstructure change significantly. In this study, a series of one-dimensional and three-dimensional freeze-thaw tests were carried out within a closed system to investigate the influencing factors of the deformation of bentonite under freeze-thaw cycles. Results show that the initial soil water content greatly impacts bentonite's deformation during freeze-thaw cycles. For an initial higher degree of saturation (Sr), the expansion caused by the formation of ice lenses has a greater impact than the shrinkage induced by dehydration, ice-cementation, and so on. Conversely, bentonite tends to shrink at a lower degree of saturation during freezing. And the critical degree of saturation that determines bentonite's behavior of frost heave or frost shrinkage seems to be roughly 0.8. As the number of freeze-thaw cycles rises, initially uncompacted bentonite clay becomes more compacted, and initially compacted bentonite clay remains unchanged.

Experimental and numerical evaluation of the critical degree of saturation and critical exposure time of acid generating filtered tailings

Tailings filtration has many advantages (e.g., water recirculation, progressive reclamation, reduction of geotechnical risks and of environmental footprint) and is increasingly used by mining companies to improve the geotechnical stability of tailings storage facilities (TSF). However, low water content also exposes filtered tailings to oxidation thus increasing the risk for acid mine drainage (AMD) generation. Tailings management techniques must therefore be adapted to prevent contamination during operations. More specifically, tailings degree of saturation and exposure time should be controlled so they do not remain exposed too long to oxygen before reclamation work starts. The main objective of this research was therefore to evaluate the combined influence of the mineralogy and the degree of saturation on the time before filtered tailings start generating AMD. This work included (i) physical, chemical, hydrogeological and mineralogical characterization of different tailings, (ii) laboratory kinetic tests, (iii) calibration and validation of reactive transport numerical simulations, and (iv) numerical extrapolations and analysis. Results showed that AMD started when carbonates were depleted and that the critical time (i.e., the time tailings could be left exposed before AMD starts) strongly depended on the mineralogy. Maintaining the degree of saturation above 90% could contribute to delay the generation of AMD, and the greater the degree of saturation, the longer the critical time. However, the critical time became independent of the degree of saturation and depended only on the mineralogy when Sr< 90%. The results of this study tend to show that it should be possible to plan the deposition of reactive filtered tailings to limit the generation of AMD, for example by mixing reactive tailings with other tailings containing a greater neutralization potential and/or by regularly disposing of a new layer of tailings on the surface, with the maximum delay between two layers being shorter than the critical time.

Evolution of the Unfrozen Water Content for Partially-Saturated Sandstones and the Critical Degree of Saturation

Evolution of the Unfrozen Water Content for Partially-Saturated Sandstones and the Critical Degree of Saturation

Study of the isothermal drying characteristics of normal concrete subjected to low air velocity convection

The drying of capillary moisture from structural concrete used in enclosed areas takes place by free convection characterized by low air velocity. The slow drying results in the persistence of an unsaturated state in concrete rendering it susceptible to deterioration. The study of critical process parameters, such as, mass loss flux, convective transfer coefficients, critical and equilibrium moisture contents and their dependence on the pertinent process variables is fundamental to design of concrete elements for durability. While these parameters have been widely researched for non-cementitious porous building materials, studies related to concrete are not comprehensive and far apart in literature. This study addresses the gap by investigating the drying characteristics of OPC-based normal concrete for varying levels of water-to-cement ratio, curing age and characteristic length of concrete specimens besides the dry bulb temperature and relative humidity of air. Interestingly, the constant rate mass flux is noted to depend on water-to-cement ratio thereby revealing the dependence of the convective transfer coefficients on surface characteristics. It is also observed that the Anomalous diffusion model satisfactorily describes the change in degree of saturation of a specimen with drying time and allows a reliable estimation of the critical degree of saturation. The latter, when plotted as a critical point curve, exhibits a non-uniform increase, as has been reported previously for a wide variety of other porous materials. The modified Oswin model is shown to provide a suitable description of the degree of saturation at equilibrium attained under steady-state temperature and relative humidity conditions.

Impact of Freeze-Thaw Cycles on the Long-Term Performance of Concrete Pavement and Related Improvement Measures: A Review.

Freeze–thaw damage is one of the most severe threats to the long-term performance of concrete pavement in cold regions. Currently, the freeze–thaw deterioration mechanism of concrete pavement has not been fully understood. This study summarizes the significant findings of concrete pavement freeze–thaw durability performance, identifies existing knowledge gaps, and proposes future research needs. The concrete material deterioration mechanism under freeze–thaw cycles is first critically reviewed. Current deterioration theories mainly include the hydrostatic pressure hypothesis, osmolarity, and salt crystallization pressure hypothesis. The critical saturation degree has been proposed to depict the influence of internal saturation on freeze–thaw damage development. Meanwhile, the influence of pore solution salinity on freeze–thaw damage level has not been widely investigated. Additionally, the deterioration mechanism of the typical D-cracking that occurs in concrete pavement has not been fully understood. Following this, we investigate the coupling effect between freeze–thaw and other loading or environmental factors. It is found that external loading can accelerate the development of freeze–thaw damage, and the acceleration becomes more evident under higher stress levels. Further, deicing salts can interact with concrete during freeze–thaw cycles, generating internal pores or leading to crystalline expansion pressure. Specifically, freeze–thaw development can be mitigated under relatively low ion concentration due to increased frozen points. The interactive mechanism between external loading, environmental ions, and freeze–thaw cycles has not been fully understood. Finally, the mitigation protocols to enhance frost resistance of concrete pavement are reviewed. Besides the widely used air-entraining process, the freeze–thaw durability of concrete can also be enhanced by using fiber reinforcement, pozzolanic materials, surface strengthening, Super Absorbent Polymers (SAPs), and Phase Change Materials. This study serves as a solid base of information to understand how to enhance the freeze–thaw durability of concrete pavement.

Influence of fines content and degree of saturation on the freezing deformation characteristics of unsaturated soils

In this study, a series of one-dimensional freezing deformation tests were conducted within a closed system to evaluate the influence of fines content and initial water content on the frost susceptibility of soils. Also, the pulsed nuclear magnetic resonance (P-NMR) method was adopted to investigate the pore water distribution and deformation mechanism of freezing soils. Results show that as soil temperature decreases, pore water freezes in two stages: the capillary (or free) water freezes first, followed by certain amounts of adsorbed water turned into pore ice. Soil specimens with higher fines content usually have higher unfrozen water content due to the more abundant adsorbed water from fine particles. The initial water content influences the soil-freezing characteristic curve significantly when the fines content is relatively high (e.g., FC = 50%), while this influence becomes negligible with lower fines content. In addition, the freezing deformation properties of soil within a closed system are affected by the initial water content. A higher degree of saturation (e.g., Sr > 0.5) results in frost heave, and a lower degree of saturation (e.g., Sr < 0.4) results in frost shrinkage. In fact, there is a critical degree of saturation (between 0.4 and 0.5) that separates soil's properties of frost heave and frost shrinkage. Frost heave or frost shrinkage of soil depends on which of the two, freezing of pore water or thinning of adsorbed water (i.e., dehydration), is predominant.

Application of traditional frost dilatometry methodology on half brick samples to identify the critical degree of saturation

An assessment of freeze–thaw deterioration risk of clay bricks should include frost dilatometry testing that predicts the moisture content at which frost decay occurs through laboratory analysis, commonly referred to as the critical degree of saturation (Scrit). It is typical to use brick ‘slices’ or ‘fingers’ while conducting frost dilatometry testing to obtain Scrit, however it is agreed that these sample sizes do not represent in-situ conditions. As such, this research involved performing frost dilatometry testing on eight half-brick samples to contribute towards the understanding of using larger samples in frost dilatometry testing. Microstrains along the x, y and z axes of the samples were measured, whereby the results show the trends of frost decay were non-uniform along certain axes. The half brick samples typically did not provide a confident Scrit, however the importance of developing this method further is highlighted. The study identified required modifications to the traditional frost dilatometry methodology to account for larger sized half-brick specimens and the likely barriers encountered as industry shifts towards testing whole-brick specimens to determine Scrit.

Undrained Responses of Partially Saturated Sand Induced by Biogas under Dynamic Cyclic Loading

Abstract Desaturation denotes a methodology used to mitigate sand liquefaction. Signifying a new form of material, biogas is deemed more feasible for the purpose of desaturation. The present study entailed the adoption of a microbial denitrification process for generating nitrogen gas to induce specimens that are partially saturated. These specimens were utilized for the consolidated undrained triaxial tests performed isotropically. Based on a simplified process concerning the evaluation of liquefaction, the study provided the crucial degree of saturation for determining its minimum degree for averting liquefaction under varying intensities of earthquake. According to test findings, the soils’ liquification resistance grows with an increase in the saturation degree. The number of loading cycles used for initializing liquefaction increases to 34 cycles from 18 cycles under undrained asymmetricity as the extent of saturation reduces to 95.9 % from 100 % when the similarity of stress ratio is observed. Furthermore, the ratio of liquification resistance goes up in a linear pattern initially as the degree of saturation decreases. Meanwhile the ratio declines as the relationship becomes increasingly nonlinear. As a result, there is a gradual reduction in liquefaction resistance as the magnitude of earthquake rises, with a decline in the critical saturation degree. Notably, the extent of this decline is predicated on the relative density.

A quantitative assessment of the parameters involved in the freeze–thaw damage of cement-based materials through numerical modelling

Degradation of concrete due to freeze–thaw cycles is a subject that is still debated. For many years, some predictive models have been developed by taking into account the physics, but the choice of parameters which drive the frost kinetics and the couplings are often arbitrarily chosen to fit experimental measurements. Moreover, recalibration becomes necessary each time the mix design or the environmental conditions vary. This article presents a quantitative assessment of the effect of the mechanisms induced by the freeze–thaw action on the internal damage of cementitious materials, using a descriptive numerical methodology at the microscale. The proposed thermo-mechanical model is based on thermodynamic considerations and physical processes related to freeze–thaw applied to virtual microstructures of cement pastes. A statistical study on the main parameters, which govern the frost action, has been performed to highlight their significance on the thermal strain. The numerical results show that the presence of supercooling and delayed nucleation of ice is by far the most harmful cause of degradation. Both the water-to-cement ratio and the minimum freezing temperature reached during freeze–thaw cycles contribute considerably to the deterioration of the material and a decrease in the Young's modulus. It is shown that the effect of de-icing salt ions present in the pore solution depends on the degree of saturation, and that the critical degree of saturation depends on the minimum temperature. The study also demonstrates the beneficial role of using entrained air to reduce damage caused to cement paste.

Determining the freeze-thaw performance of mortar samples using length change measurements during freezing

The Freeze-thaw (FT) performance of cementitious materials is dependent on the degree of saturation (DOS). It has been proposed that FT damage can occur during freezing when the DOS is greater than a critical DOS (DOSCR). In a previous study, a systematic methodology has been developed to determine the DOSCR of cement paste samples using high precision length change measurements during freezing and thawing in a thermomechanical analyzer (TMA). This paper explores the extension of the established methodology to mortar samples to determine their DOSCR. The influence of the drying temperature and the characteristics of FT cycle (temperature range, heating and cooling rate) were investigated to determine their impact on the FT performance of the mortar samples. The mortar samples tested in this study were obtained by making concrete and then wet sieving the concrete to obtain mortar. Mixtures with different air void content and air void quality were prepared in this study in order to determine the impact of air void content and air void quality on the FT performance of the samples using TMA testing. The results confirm that length change measurements can be used to determine DOSCR of mortar. A correlation considering the probability of failure was found to exist between the air void quality and the FT performance of mortar samples. The approach presented in this paper provides an alternative method that can complement other existing tests to determine the DOSCR for use in the sorption-based model.

The Effect of Gas Solubility on the Secondary Sorption in a Portland Cement Mortar Observed by X-ray CT

In this study, we investigate the secondary sorption in an air-entrained Portland cement mortar that is purged with different gases— $$\hbox {CO}_{2}, \hbox {CH}_{4}$$ , or $$\hbox {N}_{2}$$ . By altering the gas phase present in the void space, we are able to evaluate how gas solubility influences the secondary sorption. The rate of water sorption in the presence of different gases in the entrained voids was captured using successive 30 min X-ray micro-computed tomography scans for 24 h after water-specimen contact. The results show the higher the solubility of the gas, the faster the mortar reaches saturation. The air voids in the $$\hbox {CO}_{2}$$ -purged specimen begin to absorb water within 30 min and reach a critical degree of saturation within hours, while the air voids of $$\hbox {N}_{2}$$ and $$\hbox {CH}_{4}$$ -purged specimens show a significantly slower water absorption over 24 h. The high solubility of $$\hbox {CO}_{2}$$ was found to alter the X-ray mass attenuation value of the water as the water becomes $$\hbox {CO}_{2}$$ saturated. These tests reveal the gas type present in the void space significantly influences secondary sorption of Portland cement mortars. The findings of this study have implications on predicting mass transport in cement-based materials used in below ground carbon sequestration structures.

Saline water absorption behavior and critical saturation degree of recycled aggregate concrete during freeze-thaw cycles

For the economical and eco-environment friendly effect, the utilization of recycled aggregate concrete (RAC) has been widely promoted. Detailed exploration are required to better understand the durability performance of RAC. The saline solution and water absorption behavior, the variation of saturation degree, and their correlation with the freeze-thaw deterioration on RAC were investigated in this paper. The results indicated that the amount of liquid absorption and saturation degree increased with the time exposure to the liquid. The mass of liquid absorption decreased for RAC as the salt concentration increased in the solution. The experiment and study results revealed that the critical saturation degree for RAC was approximately 81%~83% in water. That was 72%~74% for RAC under 3% NaCl solution exposure. Once the critical saturation degree reached, serious internal damage and obvious strength loss initiated and developed inevitably during freeze-thaw cycles.

X-Ray computed tomography and traditional analysis of a capillary absorption test in cement pastes

Water absorption testing has the attraction of being simple, relatively quick and use water as a fluid which is the key substance in the possible concrete deterioration. However, in spite of the advantages its application has remained empirical, except for the testing of resistance to frost, through the identification of the critical degree of saturation. Rilem TC-116 studied this test, among others, for the characterization of concrete resistance against transport through the concrete pore network. Results indicated a low discriminating ability of the absorbency for qualifying concrete resistance and then, it seemed necessary to try to move forward by making the link between the absorption testing and the concrete pore microstructure. In the present work, thanks to the X-Ray computed tomography, it is described the evolution of the water front in three paste-specimen with different w/c ratios (different pore radius distributions). The observations allowed us to deduce that all the pores start to fill simultaneously, but the bigger ones fill quicker and rise less. The saturation degree is measured along the height of the sample, showing that each pore size has a different rate which becomes constant after the first hour. Washburn's equation was applied to the progressive advance of the water front, resulting in a pore radius much smaller than the one expected.

The role of ink-bottle pores in freeze-thaw damage of oolithic limestone

Many natural building stones are altered when subjected to freeze-thaw (FT) cycles. On the one hand, the sensitivity of a material to FT damage has been quantified in the past by the existence of a material-specific critical water saturation. On the other hand, it was noticed that natural stones with a high volume of ink-bottle pores, normally hold a relatively large FT resistance. The relationship between the pore structure, the saturation and the FT resistance is however not well understood. In this paper, the influence of water content and the porosity on FT behaviour is investigated macroscopically with temperature and strain measurements, established techniques in FT related research, and X-ray computed micro-tomography (µCT). Strain measurements performed on Savonnières limestone samples clearly show an increasing expansion with increasing water content, with the existence of a critical water saturation level between 70 and 80%. Differential X-ray imaging on differently saturated limestone samples in an unfrozen and frozen state then aided in explaining the origin of this critical saturation degree. By draining water from surrounding micro-pores through cryo-suction, ink-bottle ooid voids served as expansion reservoirs. When the majority of the ooid voids is water saturated prior to freezing, these voids lose their expansion reservoir ability and damaging pressures arise when water freezes inside undrained micro-pores. These findings not only help to understand the FT resistance of this limestone, but also give insight in the general FT behaviour of materials with bi- or multimodal pore-size distributions.

Using X-ray computed tomography to investigate mortar subjected to freeze-thaw cycles

This work uses X-ray computed micro tomography (XCT) to investigate the role of the critical degree of saturation (DOS) and air void system on the crack propagation of portland cement mortar subjected to freeze-thaw cycles. Three-dimensional imaging before and after freezing allow quantitative measurements on the location and volume of the formation of new cracks and the infilling of air voids. Cracks present before freeze-thaw cycles are observed to extend and new cracks initiate from the paste-aggregate interface. In addition, calcium-rich products were observed to fill >20% of the volume of the air voids with a diameter <50 μm. These observations provide insights into the freeze-thaw performance of mortar and are a step in understanding the damage in concrete.

Quantifying the freeze-thaw performance of air-entrained concrete using the time to reach critical saturation modelling approach

Many State Highway Agencies have been working to develop performance-based specifications for concrete pavements and concrete bridge decks in freeze-thaw environments. A time to reach critical saturation (TTRCS) model has been proposed to estimate the freeze-thaw performance of concrete. This study evaluates the TTRCS model for thirty different concrete mixtures with varying w/c, air volumes, and quality of air void (size and spacing). Simple quality control test procedures are used to determine the input parameters for the TTRCS model. The estimated time to reach critical saturation is compared with the measured durability factor using ASTM C 666–15. Results indicate that 86% of the mixtures with air volume above 4.5% and a Sequential Air Method (SAM) Number below 0.30 have a normalized time to reach critical saturation of greater than 20, and a durability factor that is greater than 75%. The mixtures with a high range water reducer require a higher volume fraction of entrained air to satisfy the recommended limit for the durability factor. This appears to be due to an interaction between the high range water reducer and air entraining admixture which results in greater air void spacing (i.e., larger air voids). However, the addition of high range water reducer was also found to increase the time to reach critical saturation in the mixtures with a low w/c due to a refined pore structure with a reduction in the connectivity of the matrix pores. Reducing the w/c improves the freeze-thaw performance due to a reduction in the pore volume, connectivity, and absorption rate of the concrete. A relationship is developed to estimate the time to reach critical saturation based on SAM Number and apparent formation factor. In addition, a relationship is proposed to estimate the critical degree of saturation based on air void content and quality.

Simulation of the volumetric deformation and changes in the pore structure of unsaturated cement-based materials subjected to freezing/thawing

This paper proposes a model to investigate the freezing behavior of cement-based materials of different water saturation degrees. Based on the thermodynamics of phase change of water (crystals-liquid-vapor) in pore and the initial pore size distribution of cement paste, changes in the pore structure and the freezing strains can be obtained by using a poro-elastic-plastic approach. To make it clear to calculate the volumetric change and frost damage, the water saturation degree in the model, covering all the probability of water content in reality, was divided into three categories: (a) smaller than critical saturation degree Scr, (b) equal to Scr and (c) higher than Scr. This model was validated from experimental results in literature, and was applied to investigate the influences of air content, water-to-cement ratio and curing period on cement paste after freeze-thaw cycles (F-T cycles). This framework may be a useful tool to study the frost resistance of unsaturated cement-based materials.

Toward the prediction of pore volumes and freeze-thaw performance of concrete using thermodynamic modelling

Many properties of concrete are related to its pore volume and pore structure. This paper describes an approach to predict gel and capillary pore volumes and other hardened properties of pastes using the pore partitioning model, and scaling these calculated properties to concrete by including entrained and entrapped air and aggregates. This paper focuses on illustrating how parameters that can be used for determining the freeze-thaw performance can be computed using a critical saturation model. Specifically, the degree of saturation and formation factor of the concrete are predicted when the matrix pores are saturated. Based on the model simulations, the secondary rate of sorption can be estimated using the formation factor to predict the potential for reaching a critical degree of saturation which is a measure of concrete's resistance to freeze-thaw damage. This approach allows for the evaluation of the performance of concrete mixtures with a wide variety of binder compositions and mixture proportions. The model predicts the saturated formation factor with a maximum error of 13% and the matrix saturation with a maximum error of 7%. The model can also be used to provide insight into the amount of entrained air needed for a given replacement and reactivity of binder.

Examining the Influence of the Degree of Saturation on Length Change and Freeze-Thaw Damage

Abstract Concrete can be susceptible to damage when it is exposed to freezing and thawing (FT). It has been shown that the potential for damage depends on the degree of saturation (DOS). When concrete has a low DOS, it does not show damage during an FT cycle. However, when the DOS exceeds a specific value (critical DOS), the concrete shows damage resulting from the FT cycle. The objective of this article is to measure the length change for cementitious specimens with varying DOS. It is hypothesized that these length change measurements can be used to quantify the critical DOS (DOSCR) for cementitious paste specimens exposed to FT. Specimens with a low DOS show a nearly linear length change response throughout the entire range of temperatures tested. Specimens with a high DOS (i.e., DOS &amp;gt; DOSCR) show a notable change in length at temperatures associated with the freezing and thawing points of water. The strain reduction associated with the melting of ice is calculated by normalizing the length reduction during the melting of ice with the initial length of the specimen. A DOSCR can be obtained by fitting a line between the strain reduction and the DOS. Another parameter known as residual strain can be used to quantify damage. Residual strain is defined as strain that exists in the specimen after an FT cycle. Only specimens with a high DOS (DOS &amp;gt; DOSCR) show residual strain. The approach presented in this paper provides an alternative method that can complement other existing tests to determine the DOSCR for use in the sorption-based model.

Quantitative analysis of the influence of saturation on rock strength reduction considering the distribution of water

Uniaxial compressive strength (UCS) of rocks usually decreases by the presence of water. It is widely observed that the strength of rock decreases rapidly with increasing water content at low saturation levels, but decreases insignificantly with increasing water content at high saturation levels. In this study, a parameter (critical degree of saturation) is established to separate the two stages. Another parameter (variation of saturation degree per unit length) is established to describe the distribution of water in cylindrical specimen. Based on the two parameters, an analytical solution of normalized UCS of rock with different degrees of saturation is proposed. Using published experimental data, a desired fitting result is obtained. A simplified and conservative form of the analytical solution is proposed for rock engineering. The analytical solution is expected to provide some new insights to understand the weakening effect of rock strength by the presence of water.