Computational Constitutive Model for Predicting Nonlinear Viscoelastic Damage and Fracture Failure of Asphalt Concrete Mixtures

A computational constitutive model was developed to predict damage and fracture failure of asphalt concrete mixtures. Complex heterogeneity and inelastic mechanical behavior are addressed by the model by using finite-element methods and elastic–viscoelastic constitutive relations. Damage evolution due to progressive cracking is represented by randomly oriented interface fracture, which is governed by a newly developed nonlinear viscoelastic cohesive zone model. Computational simulations demonstrate that damage evolution and failure of asphalt concrete mixtures is dependent on the mechanical properties of the mixture. This approach is suitable for the relative evaluation of asphalt concrete mixtures by simply employing material properties and fracture properties of mixture components rather than by performing expensive laboratory tests recursively, which are typically required for continuum damage mechanics modeling.     

Visualization and Simulation of Asphalt Concrete with Randomly Generated Three-Dimensional Models

The objective of this study is to visualize and simulate microscale properties of asphalt concrete with three-dimensional discrete element models under mechanical loading. The microstructure of the asphalt concrete sample was composed of three ingredients: coarse aggregates, sand mastic (a combination of fines, fine aggregates, and asphalt binder), and air voids. Coarse aggregates were represented with the irregular polyhedron particles which were randomly created with an algorithm developed for this study. The gaps among the irregular particles were filled with air voids and discrete elements of sand mastic. The mechanical behaviors of the three ingredients were simulated with specific constitutive models at different contacts of discrete elements. Based on the geometric and mechanical models, visualization and simulation of asphalt mixtures were conducted in this study.     

Application of the Viscoelastic Continuum Damage Model to the Indirect Tension Test at a Single Temperature

The viscoelastic continuum damage model, developed based on Schapery’s correspondence principle and the continuum damage mechanics, has received a great deal of attention because of its mathematical soundness and effectiveness in describing damage growth in viscoelastic media and has been used to make reliable estimations on the fatigue lives of asphalt mixtures. Its applications to field mixtures, however, have been limited because the model requires performing the uniaxial tension test. As an alternative, this study developed an analytical methodology for applying the model to the indirect tension test, which has been successfully used in testing both laboratory-made and field-cored mixtures. From the results of the indirect tension tests conducted on asphalt mixture at three different crosshead-controlled rates, it was found that the stress-pseudostrain curves could be superimposed onto one equality line in the linear viscoelastic range of the given mixture, and its rate dependency was successfully eliminated in the C1 and S1 plots. This indicates that the methodology developed for the indirect tension test has a capability of evaluating damage development in asphalt mixtures through the viscoelastic continuum damage model. It would be potentially of great benefit to pavement engineers who want to estimate the remaining lives of field mixtures.     

基于细观结构有限元模型的环氧沥青混合料间接拉伸试验研究

运用有限元方法建立环氧沥青混合料细观结构模型,对其间接拉伸试验(IDT)进行数值模拟.首先借助图像处理技术得到由集料和沥青砂浆组成的环氧沥青混合料二相细观结构,并通过蠕变试验获取沥青砂浆常温下的黏弹性材料参数,最后结合有限元手段建立包含集料、砂浆等在内的混合料细观结构有限元模型.数值模拟结果表明,有限元计算的混合料劲度模量与实际IDT试验结果吻合较好,通过改变加载方向、加载速率等参数,发现对混合料细观结构的劲度模量以及局部点位应力均造成一定影响,分析主要原因可能是由沥青混合料的内部结构分布不均匀性以及沥青砂浆的黏弹性特点所造成.研究成果可为微观有限元方法进一步推广应用于不同条件下沥青混合料微观力学响应仿真提供理论依据.     

Three-Dimensional Discrete Element Models for Asphalt Mixtures

The main objective of this paper is to develop three-dimensional (3D) microstructure-based discrete element models of asphalt mixtures to study the dynamic modulus from the stress-strain response under compressive loads. The 3D microstructure of the asphalt mixture was obtained from a number of two-dimensional (2D) images. In the 2D discrete element model, the aggregate and mastic were simulated with the captured aggregate and mastic images. The 3D models were reconstructed with a number of 2D models. This stress-strain response of the 3D model was computed under the loading cycles. The stress-strain response was used to predict the asphalt mixture’s stiffness (modulus) by using the aggregate and mastic stiffness. The moduli of the 3D models were compared with the experimental measurements. It was found that the 3D discrete element models were able to predict the mixture moduli across a range of temperatures and loading frequencies. The 3D model prediction was found to be better than that of the 2D model. In addition, the effects of different air void percentages and aggregate moduli to the mixture moduli were investigated and discussed.     

Viscoelastic Model for Discrete Element Simulation of Asphalt Mixtures

This paper presents a viscoelastic model of asphalt mixtures with the discrete element method, where the viscoelastic behaviors of asphalt mastics (fine aggregates, fines, and asphalt binder) are represented by a Burger’s model. Aggregates are simulated with irregular shape particles consisting of balls bonded together by elastic contact models, and the interplaces between aggregates are filled with balls bonded with viscoelastic Burger’s model to represent asphalt mastic. Digital samples were prepared with the image analysis technique. The micromechanical model was developed with four constitutive laws to represent the interactions at contacts of discrete elements (balls) within an aggregate, within mastic, between an aggregate and mastic, and between two adjacent aggregates. Each of these constitutive laws consists of three parts: a stiffness model, a slip model, and a bonding model in order to provide a relationship between the contact force and relative displacement and also in order to describe slipping and tensile strength at a particular contact. The relationship between the microscale model input and macroscale material properties was derived, and an iterative procedure was developed to fit the dynamic modulus test data of asphalt mastic with Burger’s model. The favorable agreement between the discrete element prediction and the lab results on dynamic moduli and phase angles indicates that the viscoelastic discrete element model developed in this study is very capable of simulating constitutive behavior of asphalt mixtures.     

Thermomechanical Framework for the Constitutive Modeling of Asphalt Concrete

This study is concerned with the constitutive modeling of asphalt concrete. Unlike most constitutive models for asphalt concrete that do not take into account the evolution of the microstructure of the material, this study incorporates the evolution of the microstructure by using a framework that recognizes that a body’s natural configurations can evolve as the microstructure changes. The general framework, on which this study is based, is cast within a full thermomechanical setting. In this paper, we develop models within the context of a mechanical framework that stems from the general framework for models based on the full thermodynamic framework and the resulting equations represent a nonlinear rate type viscoelastic model. The creep and stress relaxation experiments of Monismith and Secor are used for validating the efficacy of the model, and it is found that the predictions of the theory agree very well with the available experimental results. The advantages of using such a framework are many, especially when one wants to model the diverse mechanical and thermodynamic response characteristics of asphalt and asphalt concrete.     

On constrained sintering—III. Rigid inclusions

In Part II of this series we examined constitutive equations that have been used primarily to analyze constrained sintering problems, including sintering of a matrix phase with rigid inclusions. For the latter problem, two important effects have been identified: generation of internal stresses (which could lead to formation of crack-like defects) and deviation of the densification behavior from the rule of mixtures. Currently available analyses give very different results for the stresses, which we show to be due to the choice of the constitutive laws. The analyses that give large values of internal stresses and significant slowing down of densification use constitutive laws that underestimate the shear relaxation of the densifying body, which leads to a negative value for the Poisson's ratio. Since this has never been observed in sinter-forging experiments, it is concluded that either the internal stresses are small (as predicted by the constitutive laws given in Part I) or the basic assumptions of linearity and isotropy used in all of the analyses are incorrect. We discuss some phenomena that could be important in explaining the densification of composites.     

Percolation Threshold of Sand-Clay Binary Mixtures

Many poorly graded granular materials of engineering importance can be characterized as gap-graded binary mixtures. Such mixtures display a volume-change response at a threshold value of the coarse fraction that is reminiscent of systems described by percolation theory. An experimental investigation on a sand-clay mixture is presented that clearly displays threshold behavior and sheds light on the role that each soil fraction plays in transferring loads through the medium. There are two key effects. First, an analysis of void ratio of the interpore clay fraction for varying compaction energies reveals an abrupt reduction in clay density at the threshold fraction of sand, whereby it is virtually impossible to impart compaction on the clay fraction at sand contents exceeding this threshold. Second, although force chains cannot be observed directly, analysis of the sand in terms of its component void ratio, computed based on treating the clay as part of the void space, shows that the sand carries a majority of the load at component void ratios that are too high to form stable force chains. The traditional interrelationship between mean stress and void ratio based on critical state theory breaks down when the sand content nears its threshold fraction. When the sand content is near the threshold limit, increasing mean stress results in a greater dilative tendency. Results are compared with findings on consolidation of sand-bentonite mixtures, and so-called reverse behavior of sand-silt mixtures.     

Prediction of Creep Stiffness of Asphalt Mixture with Micromechanical Finite-Element and Discrete-Element Models

This study presents micromechanical finite-element (FE) and discrete-element (DE) models for the prediction of viscoelastic creep stiffness of asphalt mixture. Asphalt mixture is composed of graded aggregates bound with mastic (asphalt mixed with fines and fine aggregates) and air voids. The two-dimensional (2D) microstructure of asphalt mixture was obtained by optically scanning the smoothly sawn surface of superpave gyratory compacted asphalt mixture specimens. For the FE method, the micromechanical model of asphalt mixture uses an equivalent lattice network structure whereby interparticle load transfer is simulated through an effective asphalt mastic zone. The ABAQUS FE model integrates a user material subroutine that combines continuum elements with viscoelastic properties for the effective asphalt mastic and rigid body elements for each aggregate. An incremental FE algorithm was employed in an ABAQUS user material model for the asphalt mastic to predict global viscoelastic behavior of asphalt mixture. In regard to the DE model, the outlines of aggregates were converted into polygons based on a 2D scanned mixture microstructure. The polygons were then mapped onto a sheet of uniformly sized disks, and the intrinsic and interface properties of the aggregates and mastic were assigned for the simulation. An experimental program was developed to measure the properties of sand mastic for simulation inputs. The laboratory measurements of the mixture creep stiffness were compared with FE and DE model predictions over a reduced time. The results indicated both methods were applicable for mixture creep stiffness prediction.     

Use of waterless molding mixtures as a substitute for the traditional sandy-argillaceous mixtures

In the last years, there is carried out an intensive search for new molding mixtures capable of replacing sand-argillaceous mixtures whose properties have already been well studied. Unfortunately, the majority of them form a mold in the course of the hardening of a bonding agent, which requires creating a complex system for the regeneration of the used sand. In this connection, we investigated a waterless molding mixture with the use of organobentonite as the binder, which does not require any changes in the technological process of green-sand molding. The influence of the amount and composition of the solvent on the properties of waterless mixtures is estimated.     

Obtaining casts of aluminum alloys by foundry in loose molds fabricated on installations of three-dimensional printing

Three-dimensional printing is used more and more often at Russian enterprises in preparing the production of cast articles for fabricating models and casting molds. A ZPrinter 310 Plus and a ZCast molding sand are used at the Moscow Institute of Steel and Alloys to obtain molds by this method in the production of pilot casts of aluminum and magnesium alloys. Because of the absence of reference data on the strength and heat-transfer properties of these mixtures, it is difficult to optimize the thickness of the mold wall during their design. In connection with this, the physical and production properties of the ZCast mixture were determined as applied to different conditions of formation of the mold wall before it was poured with the aluminum alloy. The results showed that the method of rapid prototyping the models and molds can be effectively used in foundry for obtaining casts of aluminum alloys, especially at the stage of preparing the production of cast articles.     

How Reliable is the Plasticity Index for Estimating the Liquefaction Potential of Clayey Sands?

This paper examines the validity of the plasticity index (PI) as a criterion for estimating the liquefaction potential of clayey soils under cyclic loading. The results of undrained cyclic stress-controlled ring-shear tests on artificial mixtures of sand with different clays saturated with water indicated that an increase in PI decreased the soil potential to liquefy, and soil with PI>15 seemed to be nonliquefiable, a finding that is in agreement with the results of other researchers. However, in this study some deviations from this relation were found when a bentonite–sand mixture was treated with solutions of different ions, thus bringing into question the effectiveness of PI as a measure of the liquefaction potential of clayey soil having a certain pore water chemistry.     

Influence of Optimized Tire Shreds on Shear Strength Parameters of Sand

This paper presents the usefulness of optimizing the size of waste tire shreds on shear strength parameters of sand reinforced with shredded waste tires. A relatively, uniform sand has been mixed with randomly distributed waste tire shreds with rectangular shape and compacted at 2° of compaction. Waste tire shreds were prepared with a special cutter in three widths of 2, 3, and 4?cm and various lengths for each shred width. Three shred contents of 15, 30, and 50% by volume were chosen and mixed with the sand to obtain a uniformly distributed mixture. In order to compare the shear strength of different sand–tire shred samples, two compaction efforts in terms of sand matrix unit weights of 15.5 and 16.8?kN/m3 were considered. The results show that the influencing parameters on shear strength characteristics of sand–shred mixtures are normal stress, sand matrix unit weight, shred content, shred width, and aspect ratio of tire shreds. With the selected widths of shreds, compaction efforts, shred contents, and the variations of aspect ratios, it is possible to increase the initial friction angle ?1 up to 113.5%, that is ?1 = 67°. The average value for the influence of aspect ratio variations on increase in friction angle of the mixtures for all tests has been found to be about 25%. These average values for lower and higher compacted samples containing different widths and aspect rations were 37.6 and 17.2%, respectively. It has been investigated that for a given width of tire rectangular shreds, there is solely a certain length, which gives the greatest initial friction angle for sand–tire shred mixtures. This is the main contribution of this paper.     

Performance and Leaching Assessment of Flowable Slurry

This project was conducted to evaluate the performance and leaching of controlled low strength materials (CLSM) incorporating fly ash and foundry sand. Two different CLSM (or flowable slurry) reference mixtures (equivalent to available production CLSM mixtures) were proportioned for unconfined compressive strength levels in the range of 0.3–0.7 MPa (50–100 psi), at 28 days, using two sources of ASTM Class F fly ash. For each reference mixture, other mixtures were proportioned using two sources of foundry sand (molten metal-casting mold sand) as a replacement for fly ash in the range of 30–85%. The ingredients of the slurry mixtures—fly ash, clean foundry sand, and used foundry sand—were tested for their physical and chemical properties and their leachate characteristics. Portland cement used as the primary binder was also tested for its properties. All CLSM mixtures made with and without foundry sand were evaluated for settlement, setting and hardening characteristics, compressive strength, permeability, and leachate characteristics. The leachate results of these CLSM-making materials were below the enforcement standards (ES) of the Wisconsin Department of Natural Resources (WDNR) ground-water quality standards (GWQS). They also met practically all the parameters of the drinking water standards. A number of CLSM mixtures incorporating fly ash and foundry sand are recommended for construction applications.     

Accelerated Discrete-Element Modeling of Asphalt-Based Materials with the Frequency-Temperature Superposition Principle

This paper presents a methodology to reduce the computation time for discrete-element (DE) modeling of asphalt-based materials, based on the frequency-temperature superposition principle. Laboratory tests on the dynamic modulus of asphalt sand mastics and asphalt mixtures were conducted at temperatures of -5, 4, 13, and 21°C and frequencies of 1, 5, 10, and 25?Hz, respectively. The test results of the asphalt sand mastics were fitted with the Burger’s model to obtain the microparameters for DE models. To reduce the computation time of the DE modeling, the regular loading frequencies were amplified to virtual frequencies. Simultaneously, the Burger’s model parameters (microparameters in DE models) of asphalt sand mastic at regular frequencies were modified to those at virtual frequencies on the basis of the frequency-temperature superposition principle. Because the virtual frequencies were much larger than the regular frequencies, the computation time was significantly reduced by conducting the DE modeling with the virtual frequencies and the corresponding modified Burger’s model parameters. The modeling work, which typically takes several months or years with the traditional methods, only took a few hours or less in this study.     

Modeling Viscoelastic Crack Growth in Hot-Mix Asphalt Concrete Mixtures Using a Disk-Shaped Compact Tension Test

The growth of viscoelastic cracks in hot-mix asphalt (HMA) concrete mixtures was simulated by using a disk-shaped compaction test. Modeling techniques, including the interconversion and continuous spectrum methods, the harmony search algorithm, and the state-variable approach, substantially enhanced the computational efficiency of the generalized J-integral. The generalized J-integral was used for determining the viscoelastic crack growth parameter on the basis of the fracture mechanics under cyclic loading conditions. This paper shows that the parameter can be used for simulating the viscoelastic crack growth in HMA concrete mixtures even under conditions of different loading magnitudes and frequencies.     

CD45RC isoforms define two types of CD4 memory T cells, one of which depends on persisting antigen

A method to directly identify proteins contained in mixtures by microcolumn reversed-phase liquid chromatography electrospray ionization tandem mass spectrometry (LC/MS/MS) is studied. In this method, the mixture of proteins is digested with a proteolytic enzyme to produce a large collection of peptides. The complex peptide mixture is then separated on-line with a tandem mass spectrometer, acquiring large numbers of tandem mass spectra. The tandem mass spectra are then used to search a protein database to identify the proteins present. Results from standard protein mixtures show that proteins present in simple mixtures can be readily identified with a 30-fold difference in molar quantity, that the identifications are reproducible, and that proteins within the mixture can be identified at low femtomole levels. Based on these studies, methodology has been developed for direct LC/MS/MS analysis of proteins enriched by immunoaffinity precipitation, specific interaction with a protein-protein fusion product, and specific interaction with a macromolecular complex. The approach described in this article provides a rapid method for the direct identification of proteins in mixtures.     

Analytical Modeling of Three-Layered HMA Mixtures

This paper presents the analytical modeling of three-layered hot mix asphalt (HMA) mixtures. Conventional HMA mixtures can be regarded as a two-layered composite with asphalt-coated aggregate particles dispersed in an equivalent medium. The three-layered HMA mixtures can be constructed by introducing an intermediate layer of stiff binder coated over coarse aggregates prior to mixing them with hot asphalt cement. Based on the equivalent medium theorem reported by Eshelby in 1957 in “The Determination of the Elastic Field of an Ellipsoidal Inclusion, and Related Problems,” HMA mixtures were treated as particulate-filled composite materials. Theoretical formulations of the composite modulus were developed and finite element analysis of stress concentration in the asphalt binder was conducted. To validate the analytical results, a three-layered HMA utilizing natural asphalt (gilsonite) as the intermediate layer was prepared and dynamic modulus and indirect tensile strength tests were conducted. The lab-scale tests agreed with the theoretical results and further ascertained the benefits of utilizing the three-layered structure in HMA mixtures.     

Mixture risk assessment: a case study of Monsanto experiences

Monsanto employs several pragmatic approaches for evaluating the toxicity of mixtures. These approaches are similar to those recommended by many national and international agencies. When conducting hazard and risk assessments, priority is always given to using data collected directly on the mixture of concern. To provide an example of the first tier of evaluation, actual data on acute respiratory irritation studies on mixtures were evaluated to determine whether the principle of additivity was applicable to the mixture evaluated. If actual data on the mixture are unavailable, extrapolation across similar mixtures is considered. Because many formulations are quite similar in composition, the toxicity data from one mixture can be extended to a closely related mixture in a scientifically justifiable manner. An example of a family of products where such extrapolations have been made is presented to exemplify this second approach. Lastly, if data on similar mixtures are unavailable, data on component fractions are used to predict the toxicity of the mixture. In this third approach, process knowledge and scientific judgement are used to determine how the known toxicological properties of the individual fractions affect toxicity of the mixture. Three examples of plant effluents where toxicological data on fractions were used to predict the toxicity of the mixture are discussed. The results of the analysis are used to discuss the predictive value of each of the above mentioned toxicological approaches for evaluating chemical mixtures.