Determination of saturated surface-dry condition of clay–sand mixed soils for soil–cement concrete construction

Saturated surface-dry condition of soil needs to be known when soil–cement concrete mix is rationally designed. Based on the effective water concept, determination of the saturated surface-dry moisture content (hereafter referred to as w _ssd) of clay–sand mixed soil, necessary for determining unit water of soil–cement concrete construction, is dealt with in this study. The saturated surface-dry condition of soil, where soil water exhibits the same chemical potential as that of cement paste, can be determined both with drying rate method and pF method. Mixed soils were prepared in combination with six cohesive soils, standard sand and a recycled fine aggregate. Drying rate method and pF method were proven to be effective in determining w _ssd of any combinations of the soils. Relationship between volume fractions of cohesive soil and sand and w _ssd was found to be a linear equation.     

A particle packing model for sand–silt mixtures with the effect of dual-skeleton

The study of particle packing models for binary mixtures is important in the field of granular materials, from both theoretical and practical perspectives. A number of particle packing models have been developed for predicting packing density (or void ratio) of a binary mixture. However, the measured results and the predicted values do not always agree with each other, particularly in the range of fines content between 25 and 50%. It is postulated herein that the discrepancies between the measured results and the predicted values are primarily due to the incorrect assumptions used in the existing models. In the existing models, the packing density is determined from one of the following two assumed mechanisms of particle mixing: (1) the mixed packing has a dominant large-particle skeleton and the small particles fill the voids of the large-particle skeleton, or (2) the mixed packing has a dominant small-particle skeleton and the large particles are embedded in the small-particle skeleton. It is obvious that the first assumed mechanism is only applicable for mixtures with low fines content, whereas the second assumed mechanism is only applicable to mixtures with high fines content. Therefore, the predictions from existing models are unsuitable for mixtures with medium fines content, such as a mixture of fines content between 25 and 50%. In this study, a 3-D discrete element simulation is carried out to show that, for a mixture of medium fines content, the packing structure has a dual-skeleton, which is neither dominated by a large nor small-particle skeleton. Then, we postulate that, in the mixed packing, both mechanisms can take place: filling of small particles and embedment of large particles. The concepts of “dual-skeleton index” and “index size” are proposed to account for the interactive effects of filling and embedment. Based on this postulation, we develop an analytical method, which has the capability of predicting minimum void ratio for sand–silt mixtures with various fines contents. The developed model is then validated by the experimental results obtained from 16 types of sand–silt mixtures.     

Macrotexture influence on vibrational mechanisms of the tyre–road noise of an asphalt rubber pavement

The road surface is one of the most important factors that have influence on the current traffic noise. Usually, for dense surfaces, impacts of the tyre on the pavement generate vibrations which are the dominant mechanisms in the tyre–road noise. In this study, the effect of muffling these vibrations, by the incorporation of crumb rubber (CR) from wasted tyres into asphalt pavements, has been evaluated acoustically. Close proximity measurements have been carried out to register the sound emission generated in the contact zone between a reference tyre and an experimental asphalt pavement with CR. The analysis of the measurements indicates that the incorporation of CR as well as the air voids content has less influence than the macrotexture of the road surface on the acoustical behaviour of this experimental asphalt pavement.     

Resilient modulus–moisture content relationships for pavement engineering applications

In recent years, there has been an increased interest in determining the influence of moisture changes on the resilient modulus (M_R) of subgrade soils beneath pavement structures. Efforts have also been made to develop mathematical models that predict the change in M_R values with moisture. These models are expected to account for seasonal variations in subgrade moisture content. This study evaluates the variation of resilient modulus with post-compaction moisture content of soils in the State of Oklahoma and the State of Pennsylvania. A series of specimens was compacted at optimum moisture content and then tested for resilient modulus; other series of specimens were prepared at optimum moisture content and then either wetted or dried prior to M_R testing. Employed wetting and drying procedures are time-efficient in developing the M_R–moisture relationships. Results showed that M_R–moisture content relationships varied with soil types and M_R values varied inversely with changes of moisture content. In addition, an M_R–moisture model predicting the variation of resilient modulus with moisture contents is proposed. This model can be used to predict changes in the bearing capacity of pavements due to seasonal variations of moisture content.     

Multi-time-scale dynamics of road–vehicle systems

This paper investigates higher order simulation schemes and associated covariance equations, extended correspondingly. The methods are derived by means of multiply iterated integrals and applied to road–vehicle systems. To avoid numerical instabilities in case of high vehicle speeds, multi-time-scale dynamics are introduced by scaling the time and noise increments according to the main system frequencies. In the stationary case, the covariances are time-invariant so that Euler schemes can be applied with bigger time steps without systematic errors. Note that deterministic methods as the classical Runge–Kutta approach are consistently applicable to the drift term only, but not to the diffusion of stochastic differential equations.     

Discussion of the paper titled “Design of clay/cement mixtures for extruded building products by Khelifi et al.”

In this study, the paper of Khelifi et al. is discussed. The discussion is about the mathematical models.     

Mechanical properties and water uptake of epoxy–clay nanocomposites containing different clay loadings

Nanocomposites based on diglycidyl ether of bisphenol A (DGEBA) epoxy reinforced with 1–10 wt% I.30E nanoclay were fabricated using high shear mixing technique and characterized to determine the effects of clay loading on their mechanical, thermal, and water uptake properties. The XRD and TEM analyses revealed that the structures of the resultant nanocomposites were a combination of disordered intercalated and exfoliated morphologies. Tensile strength increased for nanocomposite containing 1 % clay loading and decreased for higher nanoclay loading. Unlike strength, the stiffness increased almost linearly with clay loading, showing 46 % improvement in modulus of elasticity for nanocomposites containing 5 % of nanoclay. Water uptake measurements indicated enhancement in the barrier properties of epoxy matrix as nanoclay loading increased from 1 up to 5 wt%.     

Preparation and characterization of polypropylene–wheat straw–clay composites

Preparation of polypropylene hybrid composite consisting of wheat straw and clay as reinforcement materials was investigated. The composite samples were prepared through melt blending method using a co-rotating twin-screw extruder. The composition of constituents of hybrid composite such as percentages of wheat straw, clay and maleic anhydride grafted polypropylene as a coupling agent was varied in order to investigate their influence on water absorption and flexural properties. The XRD analysis of composite samples containing clay showed shift in d₀₀₁ peak to lower 2θ indicating slight intercalation of polymer in clay sheets. The results of the study indicate that the increase in wheat straw and clay content in a composite increases the flexural modulus and reduces the resistance for water absorption. The increase in PP-MA coupling agent also increases the flexural modulus and resistance for water absorption. The morphological study by scanning electron microscope reveals that the addition of coupling agent increases the interfacial adhesion between the fibers and polymer matrix which is evidenced further from increased flexural modulus. Further, the particle size of wheat straw was analyzed before and after extrusion in order to investigate the effect of extrusion on wheat straw dimensions. The addition of clay as additional filler had no significant role on water absorption and flexural properties of the composite.     

Diagrams of entropy for ammonia–water mixtures: Applications to absorption refrigeration systems

The objective of this work is to calculate the entropy of ammonia–water mixture as a function of temperature, pressure, concentration, and other thermodynamic properties associated to absorption process, to support energy and exergy analysis of absorption refrigeration systems. This calculation is possible because a novel mathematical modelling was developed for this attempt. This determination will allow simulation and optimisation of absorption refrigeration systems, giving major importance in determining the values of thermodynamic properties of ammonia–water mixtures, such as enthalpy and entropy. A mathematical modelling for thermodynamics properties calculation at liquid and vapour phases of ammonia–water system is developed. The studies were based on the enthalpy vs. concentration diagram obtaining the enthalpy in the liquid phase corresponding at a temperature range from 80 °C to −40 °C. The mixtures enthalpy values were calculated for ammonia (h_1c) and water (h_2c) by using a non-linear regression program. The evaluation of thermodynamic properties in this work was discretised by formulating appropriate equations for each type of substance. However, thermodynamic properties of mixtures can be determined based on data from simple substances and mixing laws, or from an equation of state that considers the mixture concentration. The consistency of experimental data indicates the most suitable method to be used in entropy calculation.     

Validation of the time–temperature superposition principle for crack propagation in bituminous mixtures

The time–temperature superposition principle (TTSP) is known to be valid in the small strain domain where the behaviour of bituminous mixtures is linear viscoelastic (LVE). The behaviour is then called thermorheologically simple. In this work, an experimental campaign was performed at University of Lyon/ENTPE (France) to check the validity of the TTSP in the linear domain in the tridimensional case and also when cracks occur and propagate in bituminous mixture. A four-point bending test, which has been designed at University of Lyon/ENTPE, was used as crack propagation test. First, a complex modulus test is performed on cylindrical specimen in the LVE domain. Then, a series of crack propagation tests are carried out at different temperatures and different imposed displacement rates. The same shift factors obtained for master curve of complex modulus is also applied for the crack propagation tests analysis. The results allow obtaining a unique curve, for identical loadings when plotting as a function of reduced time. This result confirms that the TTSP is also valid for crack propagation in bituminous mixtures.     

Argon–oxygen mixtures for Joule–Thomson cryocooling at elevated altitude

At high altitude where the ambient pressure is low, argon as a coolant of a Joule–Thomson cryocooler solidifies and is not able to preserve the temperature of a cooled object with the same success as by liquefied and boiling argon. Instead of other strategies of thermal storage techniques, it is proposed to add oxygen to argon so to suppress the triple point of the mixture below that of argon. An experiment with 20% oxygen mixture demonstrated that indeed a cryocooler for argon operates well with the mixture while enabling solid free higher altitude run. An analysis of pure oxygen and its mixtures with argon shows that the performance of the mixed coolant is reasonably close to that of pure argon.     

Mechanical and dielectric properties of epoxy–clay nanocomposites

Epoxy–clay nanocomposites were prepared using two types of surface-treated montmorillonite (Closite 30B and Nanomer I28E). Wide angle X-ray scattering showed that all the nanocomposites had an intercalated structure. Improvements in tensile and fracture properties were found. The pure epoxy polymer was very brittle with a fracture energy, G _c, of 131 J m^?2. The addition of the nanoclays significantly increased the value of G _c, up to 240 J m^?2 for 5 wt% C30B. The toughening mechanisms acting in the nanocomposites were identified using scanning electron microscopy as crack deflection and plastic deformation of the epoxy matrix around the clay platelets following debonding. From electrical testing, the permittivity and loss angle of the nanocomposites decreased, and their breakdown strength increased as desired for insulation applications. The breakdown strength of the pure epoxy was found to be 11.7 kV mm^?1, while for a 2 wt% C30B nanocomposite, it increased to 14.7 kV mm^?1. It was concluded that the restriction of chain mobility inhibited electrical polarisation and thus decreased the permittivity and loss angle. The electrical damage zone was analysed using scanning electron microscopy. It was found that the higher resistance-to-surface degradation by partial discharges and the creation of a tortuous electrical path, which delayed the propagation of the electrical tree, were the main factors which improved the breakdown strengths of the nanocomposites.     

Mill processing and properties of rubber–clay nanocomposites

The dispersion of nanoscale composites in elastomers, which generally have higher molecular weight and viscosity as compared to plastics, is a challenge. Several techniques have been proposed for improvement of the dispersion of nanofillers in the polymers [1]. For example, the interaction of natural layered silicates can be improved by ion-exchange of hydrated cation with organic cations such as introducing bulky alkylammoniums to obtain larger interlayer spacing and provide the galleries for the polymer chain diffusion. The resultant swollen nanoclay was dried and dispersed in the polymer matrix by means of high shear mixers [2–3]. In this paper we describe the results from a new method of incorporating nanofillers into solid rubber by use of a conventional two-roll mill, which we call the modified mill method. The properties of the resultant material are compared with that of the material prepared by a latex method. We also test processability parameters, tensile behavior and crosslink density of carbon black composites prepared by the same two methods to provide a comparison between the nanocomposites. The rubber–clay nanocomposites prepared by the mill method are shown to have a fine dispersed phase structure and good reinforcement properties.     

Modeling of vapor–liquid–liquid equilibria in binary mixtures

Vapor compression and Joule–Thomson (JT) cycles provide cooling power at the boiling temperatures of the refrigerants. Maintaining a fixed pressure in the evaporator allows for a stable cooling temperature at the boiling point of a pure refrigerant. In these coolers enhanced cooling power can be achieved by using mixed refrigerants. However, gas mixtures usually do not change their phase at a constant temperature, therefore, the cooling temperature has to be actively controlled. An exception to this rule holds for binary mixtures that can form a vapor–liquid–liquid equilibrium (VLLE).Phase equilibria in binary mixtures are usually modeled based on experimental results only. In the present study only the vapor pressures of the pure mixture components are required. The calculated results of nitrogen–ethane, nitrogen–ethylene, and nitrogen–propane mixtures are compared with experimental data presented in literature showing deviations of less than 1%.     

Silver–clay nanohybrid structure for effective and diffusion-controlled antimicrobial activity

We describe here the synthesis and antimicrobial activity of silver–clay nanohybrid structure that was processed to exhibit a combination of accelerated and diffusion-controlled antimicrobial activity, with long term impact. The antimicrobial activity is assessed in terms of interaction with Escherichia coli, where the constituents of the nanohybrid structure play a synergistic role. Clay provides a physically stable surface for nucleation of silver nanoparticles. Additionally, the parallel- stacked layered structure of clay facilitates diffusion-controlled antimicrobial activity of in-situ precipitated silver. The antimicrobial activity is about four orders of magnitude greater than ex-situ precipitated bare silver particles. The study emphasizes the significance of controlling antimicrobial activity in nanostructured systems, which in the present case is enhanced and controlled antimicrobial activity with long term implication.     

Dispersion and reaction sintering of alumina–titania mixtures

Alumina–aluminium titanate (A–AT) composites are typically produced either by mixing the alumina matrix powder with already formed aluminium titanate or by reaction sintering of alumina and titania powders. Reaction-sintered materials usually exhibit limited final density and extensive microcracking. This paper describes the preparation of A–AT nanocomposites by slip casting and reaction sintering, using aqueous suspensions of submicrometre-sized alumina and nanometre-sized titania at a respective weight ratio of 87:13, which is typical for plasma-sprayed coatings. The colloidal stability of aqueous suspensions of the two individual powders and of their respective mixtures was determined first, measuring zeta potential and rheological behaviour as a function of deflocculant content and sonication time. The bimodal distribution yielded green relative densities of up to 70% of theoretical density. Dynamic and static sintering studies showed that aluminium titanate had already formed at 1400 °C.     

Preparation of polystyrene–clay nanocomposite by solution intercalation technique

Polymer–clay nanocomposites of commercial polystyrene (PS) and clay laponite were prepared via solution intercalation technique. Laponite was modified suitably with the well known cationic surfactant cetyltrimethyl ammonium bromide by ion-exchange reaction to render laponite miscible with hydrophobic PS. X-ray diffraction analysis in combination with scanning electron microscopy gives an idea of structural and morphological information of PS–laponite nanocomposite for different varying organo-laponite contents. Intercalation of PS chain occurs into the interlayer spacings of laponite for low organo-laponite concentration in the PS–O-laponite mixture. However, aggregation and agglomeration occur at higher clay concentration. The molecular bond vibrational profile of laponite as well as PS–laponite nanocomposite have been explored by Fourier transform infrared spectroscopy. Thermogravimetric analysis along with differential scanning calorimetry results reveal the enhancement of both thermal stability and glass transition temperature of PS due to the incorporation of clay platelets.     

Effects of nano-kaolinite clay on the freeze–thaw resistance of concrete

This paper investigates the effects of nano-kaolinite clay (NKC) on the freezing and thawing (F–T) behavior of concrete. In our experiments, we substituted NKC for 0%, 1%, 3%, and 5% of mixtures of ordinary Portland, cement, by weight. The blended concrete was prepared using w/c ratio as 0.5. A rapid freeze–thaw Cabinet was then used to measure the resistance of ordinary Portland cement concrete, as opposed to the concrete/NKC mixture, to examine deterioration caused by repeated F–T actions. We regularly measured the properties of the concrete specimens, including the pore structure, mass, electrical resistivity, chloride diffusion coefficient, compressive strength and dynamic modulus of elasticity. A computed tomography scan test evaluated the porosity characteristics of the concrete. This paper also applied scanning electron microscopy and X-ray diffraction tests in order to investigate the micro morphology and chemical element distributions inside of the concrete. The experimental results and visual comparisons revealed that the introduction of NKC improves the F–T resistivity values, as compared to the control concrete. The samples with 5% NKC exhibited the highest compressive strength, chloride diffusion resistivity, relative dynamic modulus of elasticity, and the most electrical resistivity after 125 F–T cycles. We designated the anti-freezing durability coefficient (DF) as the index to assess the F–T resistivity of concrete. The following research discusses the relationship between the concrete’s DF and the number of F–T cycles, compressive strength, chloride diffusion coefficient, and the electrical resistivity of the concrete samples.