Volume Change Characteristics Research Articles

Numerical investigation in effects of refrigerants’ thermodynamic properties on performance of supersonic ejector

ABSTRACT Using zeotropic mixtures with unique temperature glide characteristics as the working medium in heat-driven ejector refrigeration system will potentially reduce the irreversible losses in heat exchange process. However, the intrinsic link between the pure/mixture refrigerants’ thermophysical properties and the entrainment performance of supersonic ejector has not yet been elucidated. In this study, a novel ejector CFD model is established by coupling the real-gas thermodynamic equation and species transport equation, then the relationships between thermophysical properties of pure/mixture refrigerants and ejector’s dimensionless operational parameters were thoroughly investigated. The results indicated that the ejector expansion ratio rises as primary flow inlet temperature or refrigerant’s normal boiling point increases, while the dimensionless parameters of primary nozzle are relatively insensitive to the variation of primary flow inlet temperature and refrigerant’s boiling point, resulted in changes of supersonic jet expansion behaviour. Moreover, the specific volume and adiabatic index change characteristics resulted in minor fluctuations of primary nozzle’s dimensionless parameters. When the bubble point temperature is fixed at 30℃ in the condenser, the saturated vapor/liquid phase pressure glide characteristics of R600a/R1234ze mixture may lead to an increasement of ejector’s compression ratio.

Constitutive model of MWCNTs/PDMS composites considering volume change and electromechanical response characteristics under impact load

Multi-walled carbon nanotubes/polydimethylsiloxane (MWCNTs/PDMS) composites have excellent electromechanical conversion ability which can be applied in the field of intelligent sensing. However, the volume changes and electromechanical response mechanism of MWCNTs/PDMS under dynamic load remain unclear. In this paper, the electromechanical response of MWCNTs/PDMS with different mass ratio of MWCNTs are studied by quasi-static compression, SHPB experiment and finite element numerical simulation. The quasi-static compression results show that volume ratios of MWCNTs/PDMS are decreased with compression strain (0.845 when strain is 0.5), then, the Yeoh hyperelastic constitutive model is established considering volume change. When the mass ratio of MWCNTs is 1 wt%, MWCNTs/PDMS shows the best mechanical properties. The stationary phases of strain rate of MWCNTs/PDMS decrease with increasing of impact velocity, showing obvious viscoelastic mechanical properties. The impact induced voltages of MWCNTs/PDMS are generally larger than PDMS (10∼15 m/s). The flexoelectric effect of MWCNTs/PDMS under impact is further studied based on numerical simulations. The results show that the strain gradients are within 10−2∼10−1 m−1, the related flexoelectric coefficients are within 10−10–10−9 C/m. In summary, this study can further expand the application of MWCNTs/PDMS in the field of intelligent sensing.

Compaction, strength, and volume change characteristics of excavated clayey soil stabilized with composite admixture of cement and autoclaved aerated concrete powder

Low-strength and highly compressible excavated clayey soils are common in geotechnical engineering, which cannot serve as a bearing stratum and typically end up being disposed of in landfills. Autoclaved aerated concrete powder (AACP) is a lightweight and porous waste material, with its stockpiles rapidly accumulating worldwide. To promote sustainable development in geotechnical engineering, a type of composite admixture consisting of cement and AACP was developed to modify clayey soils in this study. The physical and mechanical properties of untreated and the composite admixture-treated soil samples were investigated via Atterberg limits, compaction, bender element, constraint compression, free swell, and unconfined compressive strength (UCS) tests. The physicochemical and microstructural observations, including soil pH, scanning electron microscopy, and mercury intrusion porosimetry analyses, were conducted to interpret the macroscopic mechanical behaviors. Test results showed that the incorporation of AACP improved the workability of clayey soils, while cement further enhanced their mechanical properties. Hydration compounds primarily filled the voids with a diameter ranging from 0.1 to 1 µm. From the perspective of volume change behavior, 8% cement content was recommended. Shear wave velocity showed a strong correlation with the UCS, demonstrating that the bender element technique was an effective non-destructive tool for assessing the strength of compacted samples.

Experimental Investigation and Numerical Model for Chloride Diffusivity of Long-Age Fly Ash Cement Slurry

Fly ash is a by-product of coal-fired thermal power plants and offers great potential for the use of resources. To effectively improve the durability of reinforced concrete structures in marine environment and achieve waste to treasure, fly ash is widely used as a pozzolanic material due to its long-hydration characteristics and effects of micro-aggregate, micro-filling and secondary hydration. In this study, both the experimental investigation and numerical simulation are carried out to study the chloride transport characteristics of fly ash cement paste. The variation in chloride diffusivity with fly ash content, water-to-binder ratio and curing age up to 360 days is studied via accelerated conductivity measurement, and it is found that the above three experimental variables have a significant impact on the chloride diffusivity. For the influence of the dosage of fly ash, the optimum dosage is 30%. By introducing specific rules for the particle distribution, the fresh fly ash cement paste is first made. Based on the volume change characteristics of fly ash and cement particles after hydration, the vector hydration model of fly ash cement paste is established by considering the water shortage effect caused by hydration layer interference. After the accuracy of this hydration model is verified by the results from third-party experiments, the random walk algorithm is proposed to calculate the diffusion coefficient of the reconstructed mineral admixture cement paste. By comprehensive comparison with the experimental results from the third-party and self-conducted experiments, the numerical model for predicting the chloride diffusivity of fly ash cement paste is verified.

Hydrogel Micropillar Array for Temperature Sensing in Fluid.

Localized temperature sensing and control on a micron-scale have diverse applications in biological systems. We present a micron-sized hydrogel pillar array as potential temperature probes and actuators by exploiting sensitive temperature dependence of their volume change. Soft lithography-based molding processes were presented to fabricate poly N-isopropyl acrylamide (p-NIPAAm)-based hydrogel pillar array on a glass substrate. Au nanorods as light-induced heating elements were embedded within the hydrogel pillars, and near-infrared (NIR) light was used to modulate temperature in a local area. First, static responses of the micro-pillar array were characterized as a function of its temperature. It was shown that the hydrogel had a sensitive volume transition near its low critical solution temperature (LCST). Furthermore, we showed that LCST could be readily adjusted by utilizing copolymerizing with acrylamide (AAM). To demonstrate the feasibility of spatiotemporal temperature mapping and modulation using the presented pillar array, pulsed NIR light was illuminated on a local area of the hydrogel pillar array, and its responses were recorded. Dynamic temperature change in water was mapped based on the abrupt volume change characteristics of the hydrogel pillar, and its potential actuation using NIR light was successfully demonstrated. Considering that the structure can be arrayed in a two-dimensional pixel format with high spatial resolution and high sensitive temperature characteristics, the presented method and the device structure can have diverse applications to change and sense local temperatures in liquid. This is particularly useful in biological systems, where their physiological temperature can be modulated and mapped with high spatial resolution.

Influence of the mix parameters on shrinkage properties of environment-friendly mortar

ABSTRACT Cracks in concrete structures are generally initiated due to the shrinkage i.e. the volume change characteristics of the concrete structures. In this experimental study, effects of the mix parameters related to alkaline liquid (AL) and recycled fine aggregate (RFA) on the shrinkage behaviour of environment-friendly mortar mixes produced with fly ash (FA)-based geopolymer binder and RFA were investigated and reported. To find out the effects of AL, concentration of liquid sodium hydroxide (LSH) was varied from 6M to 16M, ratio of liquid sodium silicate (LSS) to LSH in AL was varied from 1.0 to 2.5 and AL/FA ratio was considered as 0.4 and 0.6. Different fly ash-based geopolymer mortar mix were produced depending on above-said combinations of mix parameters along with the RFA content (by weight) of 10%, 20%, 30%, 40% and 50% in lieu of natural fine aggregate. Prismatic specimens (25 mm × 25 mm × 285 mm) were cast and cured at ambient air temperature to determine the shrinkage behaviour. Higher RFA content in mix, higher LSS/LSH ratio in AL and higher AL/FA ratio resulted in higher shrinkage value. But, lesser shrinkage value was noticed for those specimens of mortar mix with the consideration of higher concentration of LSH in AL with varying RFA content.

Application of eco-friendly and smart materials in geotechnical engineering for subgrade stabilization: a review

The performance of road pavement mainly depends upon the properties of subgrade soil. The failures occur in case of road pavement is due to poor strength characteristics and high volume change behaviour of subgrade soil. Thus, considering the performance criteria, subgrade soil needs to be treated so as to make it suitable for the construction of road pavement. It was observed that, soil stabilization using various admixtures is more effective and commonly used practice for enhancing the properties of subgrade soil. The previous research shows that subgrade soil stabilization can be achieved with the application of various environmental friendly (Bio – Enzymes) and smart materials (Agricultural and Industrial waste) in required quantity to enhance the physical, strength and volume change characteristics of natural soil. It was also observed that, the combined use of such materials makes the subgrade soil more suitable for the construction of road pavement. From the experimental investigations of various available research works on subgrade stabilization, it was observed that strength and volume change properties such as California Bearing Ratio (CBR), Unconfined Compressive Strength (UCS) and Swell Pressure of subgrade soil can be effectively changed with the help of use of such materials. It was also observed that use of conventional methods has negative impact on environment. Thus, subgrade can be stabilized with eco – friendly and cost effective materials to achieve both economic and serviceability criteria.

Effect of Freeze-Thaw Cycles on Shear Strength Properties of Loess Reinforced with Lignin Fiber

Freeze-thaw cycles caused by climate change can change the structure and strength of the soil. In seasonally frozen soil areas, the use of improved loess as a filling material must consider the effects of freeze-thaw cycles. With the increasingly severe global environmental problems, the search for suitable new environmental protection improvement materials has become one of the hotspots in soil performance improvement research. The purpose of this paper is to use lignin fiber to improve the engineering performance and freeze-thaw resistance of loess and to reduce the negative impact of engineering construction on the environment of the loess area. Based on a series of triaxial shear tests, the effects of freeze-thaw cycles on the stress-strain relationship, shear strength, and Mohr-Coulomb’s strength parameters of loess reinforced with lignin fiber were analyzed. Combined with the volume change and microstructure characteristics of fiber-reinforced loess before and after the freeze-thaw cycles, the reasons for the effects of the freeze-thaw cycles on the shear strength characteristics of fiber-reinforced loess are discussed. The research results showed that after 15 freeze-thaw cycles, the shear strength of loess reinforced with 1% fiber increased by 0.15%, 2.05%, and 1.35% at 80, 140, and 200 kPa, respectively. The shear strength of the reinforced loess with other fiber contents decreases to different degrees, and the maximum reduction ratio can reach 9.54%. Freeze-thaw cycles changed the variation of shear strength and strength parameters with fiber content. When the fiber content is less than 1%, the shear strength, cohesion, and friction angle of fiber-reinforced loess increase the fastest after freeze-thaw cycles. When the fiber content is 1%, the overall destruction effect of freeze-thaw cycles on fiber-reinforced loess is inhibited, and the soil has the best freeze-thaw resistance.

The Influence of Mechanical Granulation Process and Granular Bentonite Plasticity on Self-Sealing and Volume Change Behavior

Geosynthetic clay liners (GCLs) are widely used as hydraulic barriers in landfills and mine tailings facilities due to their low hydraulic conductivity. The average grain size of the granular bentonites (GBs) used in GCLs varies depending on the manufacturer and also between batches from the same manufacturer. The influence of GB grain size and plasticity on the sealing ability and volume-change characteristics under extreme chemical loads is important for long-term stability. Furthermore, GCLs are subjected to mechanical loading from landfill waste. The influence of different grain-size distributions in GB samples was assessed in terms of their self-sealing ability, hydraulic infiltration, and volume-change behavior under extreme chemical loadings and a 50-kPa mechanical load. The performance of fine-grained GBs was found to be satisfactory, in terms of hydraulic infiltration and volume change, under the test conditions. The plasticity of the GB also influenced its sealing and swelling ability under chemomechanical loading. The mechanism underpinning the influence of plasticity and grain size on the self-sealing behavior and hydraulic conductivity equilibrium is explained. We recommend fine-grained GBs for use in GCLs for containment applications.

Mechanism of Polyurethane Binder Curing Reaction and Evaluation of Polyurethane Mixture Properties

This study focuses on analyzing the curing reaction mechanism of polyurethane (PU) binders and comprehensively evaluating the PU mixture’s properties. The former was investigated by conducting a Fourier transform infrared spectroscopy (FTIR) test on PU binders with different curing times. The volume change characteristics, construction operation time, and strength formation law were clarified through the splitting tensile test of PU mixtures under different environmental conditions. The optimal PU mixture stacking time and curing time under different environmental conditions were determined. The properties of the PU mixture and asphalt mixture were evaluated and compared through a rutting test, low-temperature bending test, freeze–thaw splitting test, and four-point bending fatigue test. The results show that the physical and chemical curing of the PU binder occurred within the first 24 h of curing, and the reaction speed gradually accelerated to form a polyurea structure 24 h later. It is recommended to stack the PU mixture for 4 h before compaction and to cure it for 2 days before opening under the conditions of 50% humidity and 15–40 °C surrounding temperature. The PU mixture shows better temperature stability and fatigue resistance than the asphalt mixture, and the splitting tensile strength of the PU mixture before and after the freeze–thaw splitting test is also higher. It is clear that the PU mixture is a green road building material with good performance.

Laboratory Assessment of the Mechanical Properties of an Unsaturated Mid-Atlantic Silty Sand

Natural soil formation processes involve variable particle size distributions that vary spatially within a deposit, giving rise to an array of soils that transition from one general soil type to another within essentially similar deposits. Characterization of such transition soils in the active zone is something past research has largely overlooked. An evaluation of the unsaturated strength behavior of a transition soil was carried out on a native mid-Atlantic silty sand. A suite of unsaturated consolidated drained axisymmetric triaxial shear tests was performed, and the stress–strain and volume change characteristics of the transition silty sand under different values of matric suction, confining pressure, strain rate, and fines content were investigated. It is found that the triaxial shear strength increases with increasing matric suction, confining pressure, strain rate, and fines content. The volumetric strain increases with increase in matric suction and confining pressure, and it decreases as fines content and strain rate are increased. The experimental results are used to further validate a state-dependent constitutive model for unsaturated soils that was recently proposed based on the theory of bounding surface plasticity and the framework of hyperelasticity. It is shown that the model simulations are in good agreement with the experimental data.

Drained Triaxial Compressive Shear Response of Cemented Paste Backfill (CPB)

Cemented paste backfill (CPB) has been widely used as local and regional underground support to reduce host rock wall closure in mined-out areas and also to reduce rockfall and rockburst incidents. However, analyzing the rock mass—CPB interactions in the first month after backfill placement must account for the CPB’s time-dependent strength, stiffness, and volume change characteristics during binder hydration. This article presents the first such comprehensive study made for CPB from the Williams mine in Ontario, Canada. Monotonic isotopically consolidated drained compressive triaxial tests were conducted on cured CPB specimens using the lubricated-ends test technique. The specimens had 3.0–7.5% Cement Content (CC); Curing Times (CTs) were from 3 to 28 days; and the confining stress ranged from 25 to 350 kPa. During shearing, all tests exhibited an initial contractive phase leading to a Characteristic State (CHS or point of volumetric strain reversal) followed by dilation with a maximum dilation rate corresponding to peak stress at the Failure State (FS). Both CHS and FS are adequately described by the Mohr–Coulomb criterion and a framework formulation was proposed to predict the evolution of CHS and FS based on CC and CT. Furthermore, the volumetric strains at CHS and FS can be defined as linear functions of the respective axial strains at the CHS and FS. Quantification of the observed behaviors through these functional relationships can help develop future constitutive models that better represent CPB’s transient response to triaxial stress loading while curing, which is essential to understanding as-placed backfill properties and its interaction with surrounding rock mass.

Importance of volumetric shrinkage curve (VSC) for determination of soil–water retention curve (SWRC) for low plastic natural soils

The study of unsaturated behavior of soils pivots around the relationship between soil suction and the amount of water content present in the soil mass, which is termed as soil–water retention curve (SWRC). The drying SWRC is generally determined by neglecting the volume change behavior of the soil for low plastic soils. Such a procedure is bound by an inherent assumption that the soil exhibit zero or negligible volume change and can lead to approximations in important SWRC parameters, including saturated volumetric water content, residual water content, and fitting parameters, which are essential for the estimation of unsaturated hydraulic characteristics of the soil. This study deals with the determination of SWRC parameters for a range of soils starting from non-plastic to high plastic soils, with and without considering volume change characteristics. For this purpose, the SWRC of six different natural soils were experimentally measured using a miniature tensiometer (T5) and a dew point potentiometer (WP4C), while the VSC of the used soils were measured using the balloon method. Efforts were made to critically understand the influence of no volume change assumption on the SWRC parameters, unsaturated hydraulic conductivity function, and flow of water through 1D soil column. The experimental results indicated that the no volume change assumption underpredicted the water retention curves of low plastic soils, and this error in SWRC increases with the soil plasticity. The measured VSC of the used soils further revealed that there could be a range of saturated volumetric water content (VWC) depending on the initial state of the soil sample, which also influences the sensitivity of the SWRC parameters. Therefore, the present study proposed a new methodology to determine unambiguous SWRC (in terms of VWC-suction) for a particular soil type combining both the measured VSC data and SWRC data. Based on the obtained results, it was recommended that the details of VSC must be incorporated along with the measured SWRC data for soils having plasticity and clay content higher than 15 and 11%, respectively.

Optimization, quantification of Strength and Design Benefits of Flexible Pavements Resting on Soft Soil Subgrades Treated with Coir Waste and Lime

ABSTRACT Soft soil as a pavement subgrade leads to early pavement failure or increased pavement thickness. Ground improvement techniques have been used as a solution to this. The published studies on coir waste treated soft soil depict that the adverse volume change characteristics of clayey soils are mitigated by the resistance offered by coir waste. However, on analyzing, it can be seen that the engineering properties of coir waste treated soil are on a lower side. Hence, an additional stabilizer lime was used in combination with coir waste to investigate the enhancement in strength and reduction in deformation behavior. This paper presents the experimental studies carried out to investigate how the addition of lime to soil treated with coir waste affects various aspects like compression behavior, undrained shear strength, and deformation behavior. The degree of expansion and effect of the curing period was also analyzed and an optimized design model was arrived to quantify the strength in terms of lime content added to the coir waste treated soil and its curing period. Design charts for pavement thickness over treated subgrade soil were also developed.

Elastoplastic Behavior of Compacted Kaolin under Consolidated Drained and Shearing Infiltration Conditions

Abstract This study presents a comprehensive laboratory study to characterize the elastoplastic behavior of compacted kaolin under consolidated drained (CD) and shearing-infiltration (SI) triaxial conditions. The laboratory tests include a soil-water characteristic curve (SWCC), isotropic consolidation (IC), CD, and SI tests under different net confining stresses to consider in situ stress state and matric suctions that describe the volume change characteristics of unsaturated soil with respect to two stress state variables (i.e., net normal stress and matric suction). Consistently prepared specimens of statically compacted kaolin were used in this study. The results of the SWCC tests demonstrated that the air-entry value and yield suction (so) increased with an increase in net confining stress. The IC tests indicated the strong influence of matric suction on compressibility and stiffness of the compacted kaolin. The results of SI tests indicated that water infiltration reduced the matric suction of the soil and was accompanied by a degradation in deviator stress. It was also found from the CD and SI tests that the failure envelope of compacted kaolin was unique. For practical purposes of transient analyses, therefore, the CD and SI tests, as well as the SWCC and IC tests, might be performed to obtain more rigorously elastoplastic behavior of unsaturated soil under CD and SI conditions.

On the dilatancy of root-permeated soils under partially saturated conditions

Mechanical contribution of roots to the shear strength of saturated soils is not the only function of vegetation in favour of slope stability. Plants also cause an increase in the strength due to plant-induced suction, resulting from partial saturation. Although understanding, quantifying and modelling the shearing behaviour of soils is directly related to the volume change characteristics of soil, dilative behaviour of rooted soils has not been of much interest. Large-scale suction-monitored direct shear tests were performed on specimens planted with a mixture of species from different plant functional groups. Increase in matric suction, which was linked to root biomass and root : shoot ratio, resulted in higher maximum dilatancy angles. It was also found that maximum dilatancy is controlled by net normal stress, but not effective stress.

Strength and Volume Change Characteristics of Clayey Soils: Performance Evaluation of Enzymes

This study was conducted to evaluate the strength and volume change characteristics of a sedimentary residual soil mixed with bentonite (S1) when treated with three different enzymes. In addition, three reference clays including bentonite, illite, and kaolinite were also treated with enzymes to study the effect on their strength characteristics. Soil samples prepared at the optimum moisture content (OMC) were sealed and cured for four months. After curing, reference clays were tested for unconfined compressive strength (UCS). For swell tests, the S1 soil samples were placed on porous stones, which were immersed in water to allow capillary soaking of the samples. S1 samples were allowed to dry at ambient temperature for shrinkage test until the rate of reduction in volume became negligible. On completion of swell tests, the samples were tested for UCS to determine the decrease in strength due to saturation. No increase in strength and decrease in volume change were observed for any of the enzymes and dosages. Field Emission Scanning Electron Microscope (FESEM) showed some dense packing of particles for treated samples, whereas X-ray diffraction (XRD) did not reveal any change; in fact, the pattern for untreated and treated soil samples were indistinguishable.

Volume change behavior of root-permeated soils under partially saturated conditions

Vegetation, particularly roots, serves different functions in relation to increased shear strength under saturated and partially saturated conditions. Quantification of mechanical contribution of roots due to their tensile strength, and relationships of various vegetation parameters and plant-induced suction, as well as shear strength, have been widely studied. Although shear strength is directly related to the volume change characteristics of soil, dilative or contractive behaviour of root-permeated soils has not been of significant interest so far. This study investigates how volume change during shearing is related to the hydrological and mechanical characteristics of vegetated soils relevant to slope stability and shear strength of root-permeated soils under partially saturated conditions. Direct shear tests, on specimens planted with a mixture of species from different plant functional groups, were performed with an Inclinable Large-scale Direct Shear Apparatus (ILDSA). Matric suctions were monitored throughout the test with tensiometers. Vertical and horizontal displacement graphs were plotted to investigate the volume change behaviour. Maximum dilatancy angle was found to be positively correlated with plant-induced suction and net normalised stress, both of which were linked to root biomass and the root:shoot ratio. It was found that maximum dilatancy is controlled by matric suction and net normal stress.

Hydraulic and volume change characteristics of compacted clay liner blended with exfoliated vermiculite

Clays are used for the construction of waste containment barriers, because of its low hydraulic conductivity. Due to the difficulty in obtaining the natural clay satisfying the essential requirement as a liner material; commercially available bentonites are being used for the construction of clay liners. Bentonite undergoes large volume changes upon saturation. Various physical and chemical stabilisation techniques are in practice to stabilise the bentonite. Previous studies reported that contaminants with high concentrations attack clay minerals leading to increase in their permeability. Exfoliated vermiculite, finds several applications in construction industry, also found to be good absorbent material. Hence, this study has been carried out to find the optimal percentage of vermiculite that can be added to the bentonite so that it satisfies the essential requirements of clay liners and absorb the heavy metals from contaminants. Several index properties and engineering properties tests on bentonite-vermiculite mixtures were performed. From the studies, it was found that 30% of vermiculite can be added to bentonite, so that the essential requirements of liner can be maintained.

Hydraulic and volume change characteristics of compacted clay liner blended with exfoliated vermiculite

Hydraulic and volume change characteristics of compacted clay liner blended with exfoliated vermiculite