A comparative study of the heterogeneous local mechanical response of two types of asphalt mixes

Two asphalt specimens featuring very different gradations, types of aggregates and binders are investigated in this study. A full-field measurement technique is used for this purpose: the grid method. Displacement and strain fields are captured during compression tests carried out on these specimens. The displacement and strain fields are analyzed and compared in light of the main characteristics of these materials. It is shown that a close relationship exists between gradation and ratio between local and global strain components. The strain recovery that follows the loading phase of the specimens is also analyzed and the difference between their mechanical response at the local level is also highlighted.     

The effect of crumb rubber modifier on the resistance of asphalt mixes to plastic deformation

This research analyzed the response of bituminous mixes manufactured with rubber to plastic deformation. For this purpose, a set of asphalt mixes containing different percentages of crumb rubber modifier (CRM) added by the dry process as well as the wet process were tested. It also compared the performance of a CRM mix to that of a mix made with high-performance polymer-modified bitumen. The mixes were assessed with the wheel-tracking test and the cyclic triaxial test. Their bearing capacity was also evaluated by determining their stiffness modulus at different temperatures. The results obtained showed that for the dosages and percentages of crumb rubber used, the addition of wet-process and dry-process CRM to asphalt mixes with conventional bitumen increased their resistance to plastic deformation. In fact, the performance of some CRM mixes was superior to that of the mix with high-performance modified bitumen. It also increased their stiffness modulus and creep modulus values and improved their resistance to plastic deformations caused by vehicle traffic loads.     

Investigating the effect of nanoparticles on the rutting behaviour of hot-mix asphalt

Rutting is considered as one of the major damages in asphalt mixtures. In this study, different types of nanoparticles such as TiO₂, Al₂O₃, Fe₂O₃ and ZnO in different percentages were added to the base asphalt binder in order to decrease the rutting potential of hot-mix asphalt (HMA). In the first step, asphalt binder tests for characteristics such as penetration grade, ductility, softening point and viscosity were performed on the asphalt binder modified by the nanoparticles. Then, after preparing HMA samples, the static creep test was done at two stress levels at a specific temperature. Results of this study showed that using the nanoparticles improved the behavioural properties of the asphalt binder and decreased rutting in asphalt mix samples. Furthermore, scanning electron microscope images taken from the asphalt binder samples modified by the nanoparticles demonstrated that these nanoparticles were properly distributed in the asphalt binder space and had a positive effect on the rutting performance of the asphalt mixes.     

Effect of aggregate nature on the fatigue-cracking behavior of asphalt mixes

Fatigue cracking in asphalt mixes is one of the most common road pavement distresses. When mixes are designed in the laboratory, it is very important to select the most suitable materials to ensure a good performance against this phenomenon. The types of binder and mortar strongly influence mix cohesion by providing tensile and shear strength. The mineral skeleton supplies it with compressive strength and bearing capacity. In this sense, the grading curve and shape of the aggregate affect crack growth (because of the internal friction), whereas the nature of the aggregate plays an important role in mix behavior (because of its adhesion to the bitumen and its resistance to fragmentation). This paper analyzes the impact of the nature of coarse aggregate on the fatigue-cracking behavior of asphalt mixes, an aspect that until now has received little attention. The UGR–FACT test was used to evaluate the cracking behavior of two mixes by varying the load amplitude, frequency, and test temperature. The results obtained showed that the nature of the coarse aggregate has an important effect in the fatigue-cracking behavior of asphalt mixes, and could be as important as bitumen type or mineral skeleton in mix design.     

Viscoplastic modeling of asphalt mixes with the effects of anisotropy, damage and aggregate characteristics

This paper presents an approach for constitutive modeling of the viscoplastic behavior of asphalt mixes. This approach utilizes an anisotropic non-associated flow rule based on the Drucker–Prager yield surface. The selection of this yield surface is motivated by the field stress paths and material properties associated with permanent deformation at high temperatures. The efficacy of the model is demonstrated by analyzing data from compressive triaxial tests conducted at different confining pressures and strain rates for three different mixes. The model parameters are related to the experimental measurements of aggregate shape characteristics, aggregate surface energy, inherent anisotropic distribution of aggregates, and microstructure damage measured using X-ray computed tomography and image analysis techniques. Establishing the relationship between the model parameters and material properties is important in order to optimize the mix properties, and achieve desirable mix performance.     

Influence of selected WMA additives on viscoelastic behaviour of asphalt mixes and pavements

Warm mix asphalt additives are effective in decreasing production, laying and compaction temperatures of asphalt mixes. However, there are still questions concerning influence of warm mix additives on properties of asphalt mixes and pavement performance. This paper presents results of the comprehensive research of viscoelastic behaviour of asphalt mixes and pavement structures with layers made with warm mix asphalt additives at high temperatures. Two additives of significantly different effects on mixes at higher temperatures were selected for analysis, namely aliphatic synthetic wax produced with the use of Fisher–Tropsch method and formulation of surfactant- based molecules (ionic and non-ionic). Viscoelastic properties of mixes with these two additives and, as a reference mix, with neat unmodified asphalt binder were determined in uniaxial compression with sinusoidal loading using Asphalt Mixture Performance Test. The viscoelastic analysis of pavement structures was performed with use of the VEROAD software and data from laboratory testing. Two different pavement structures were analysed, for light and heavy traffic. The temperature distribution in pavement structure during the hottest summer day in northern Poland in 2012 was taken into account. The model of pavement was loaded with moving wheel at different speeds. The analysis has shown that two tested warm mix additives had different effect on viscoelastic transient response at high temperatures. One of them (Fischer–Tropsch wax) evidently caused an increase in resistance of asphalt mix and pavement structure to loading at high temperature. The second additive (formulation of surfactant-based molecules) slightly reduced resistance of asphalt mix and pavement to loading at high temperatures as compared with the reference mix.     

Effect of waste plastic bottles on the stiffness and fatigue properties of modified asphalt mixes

Nowadays, the use of recycled waste materials as modifier additives in asphalt mixes could have several economic and environmental benefits. The main purpose of this research was to investigate the effect of waste plastic bottles (Polyethylene Terephthalate (PET)) on the stiffness and specially fatigue properties of asphalt mixes at two different temperatures of 5 and 20 °C. Likewise, the effect of PET was compared to styrene butadiene styrene (SBS) which is a conventional polymer additive which has been vastly used to modify asphalt mixes. Different PET contents (2–10% by weight of bitumen) were added directly to mixture as the method of dry process. Then the resilient modulus and fatigue tests were performed on cylindrical specimens with indirect tensile loading procedure. Overall, the mix stiffness reduced by increasing the PET content. Although stiffness of asphalt mix initially increased by adding lower amount of PET. Based on the results of resilient modulus test, the stiffness of PET modified mix was acceptable and warranted the proper deformation characteristics of these mixes at heavy loading conditions. At both temperatures, PET improved the fatigue behavior of studied mixes. PET modified mixes revealed comparable stiffness and fatigue behavior to SBS at 20 °C. However, at 5 °C the fatigue life of SBS modified mixes was to some extent higher than that of PET modified ones especially at higher strain levels of 200 microstrain.     

Predicting the tensile relaxation modulus of asphalt mixes based on the mix design and environmental factors

The main objective of this study was to predict the tensile relaxation modulus of asphalt mixes, without having to perform the common relaxation modulus tests, by developing a predictive model based on the mix characteristics, ageing condition, temperature and loading time. To this end, cylindrical asphalt mixture specimens containing crushed stone aggregates with 60/70 penetration asphalt binder were fabricated using two aggregate gradations, two binder contents, two air void levels and three ageing conditions with four replicates. Uniaxial tensile relaxation modulus tests were conducted on the specimens at four temperatures using the trapezoidal loading pattern at a low level of strain. Tensile relaxation modulus master curves of all the experimental combinations were constructed by the sigmoidal model. Statistical analysis of variance and regression analysis was performed on the test data and a predictive model was developed. Finally, the predictive model was verified using a group of measured values other than those used for the development of the model, and it was found that the predicted values correlated well with the measured ones.     

The effect of aggregate gradation on creep and moisture susceptibility performance of warm mix asphalt

It is clear that the purpose of mixture design is to select optimum asphalt content for a desired aggregate structure to meet the prescribed criteria. Aggregate makes up high proportion of volume and mass of mixtures; hence, it is considered as an important constituent of asphalt concrete. This study postulates that the gradation is an important characteristic of the aggregate in adoption of the optimum mixture. One aggregate source, three gradations and different percentages of Sasobit^® was used to manufacture hot mix asphalt and warm mix asphalt. The test results indicated that the aggregate gradation affects the rutting resistance and especially the moisture susceptibility of the introduced mixtures, differently. Rutting resistance was evaluated using the flow number parameter, and in order to determine the moisture sensitivity mechanism, a mechanical and visual inspection tests were carried out. At the end, it is concluded that the optimum aggregate gradation for these two types of mixtures is different.     

Influence of processed carbon black in the filler composition on the characteristics of baked carbon mixes

Several carbon products such as carbon brushes, special nuclear carbons, seal rings, etc. require carbon black in the filler composition. In the present study, the raw carbon black was mixed with a coal tar pitch and the resulting carbon mix was shaped, calcined and finally crushed into a fine powder for its subsequent use. The influence of this modified (processed) carbon black in the filler composition on the characteristics of the final calcined petroleum coke, processed carbon black and coal tar pitch-based carbon mixes has been investigated.     

Quantification of the inherent uncertainty in the relaxation modulus and creep compliance of asphalt mixes

Advanced material characterization of asphalt concrete is essential for realistic and accurate performance prediction of flexible pavements. However, such characterization requires rigorous testing regimes that involve mechanical testing of a large number of laboratory samples at various conditions and set-ups. Advanced measurement instrumentation in addition to meticulous and accurate data analysis and analytical representation are also of high importance. Such steps as well as the heterogeneous nature of asphalt concrete (AC) constitute major factors of inherent variability. Thus, it is imperative to model and quantify the variability of the needed asphalt material’s properties, mainly the linear viscoelastic response functions such as: relaxation modulus, \(E(t)\), and creep compliance, \(D(t)\). The objective of this paper is to characterize the inherent uncertainty of both \(E(t)\) and \(D(t)\) over the time domain of their master curves. This is achieved through a probabilistic framework using Monte Carlo simulations and First Order approximations, utilizing \(E^{*}\) data for six AC mixes with at least eight replicates per mix. The study shows that the inherent variability, presented by the coefficient of variation (COV), in \(E(t)\) and \(D(t)\) is low at small reduced times, and increases with the increase in reduced time. At small reduced times, the COV in \(E(t)\) and \(D(t)\) are similar in magnitude; however, differences become significant at large reduced times. Additionally, the probability distributions and COVs of \(E(t)\) and \(D(t)\) are mix dependent. Finally, a case study is considered in which the inherent uncertainty in \(D(t)\) is forward propagated to assess the effect of variability on the predicted number of cycles to fatigue failure of an asphalt mix.     

Using the surface free energy method to evaluate the effects of liquid antistrip additives on moisture sensitivity in hot mix asphalt

The objectives of this research are to evaluate the susceptibility of aggregates and asphalt binder with and without liquid antistrip (LAA) additives to moisture damage based on the properties that affect the adhesion bond between the aggregate and asphalt binder and the cohesion strength of the asphalt binder using the surface free energy (SFE) concept and laboratory testing. The percentage of the aggregate surface area that was exposed to water (P) due to each cycle was used as a screening parameter for evaluating the compatibility of the asphalt binder and aggregates in terms of the resistance to moisture damage. The results show that adding LAA causes the total SFE of the asphalt binder to increase, which results in a decrease in stripping between the aggregate and asphalt binder in the presence of water. Similar results were obtained from a dynamic modulus test. From the data obtained, we conclude that LAA caused a reduction of the magnitude of P that improves its resistance to moisture damage.     

Investigating the effect of some operating parameters on phosphate flotation kinetics by neural network

Artificial neural network (ANN) was used to simulate the effect of feed mean size, collector dosage, and impeller speed on flotation kinetic. Simulation were conducted using multi layer feed forward neural network assuming that flotation is following first order kinetic model.Simulation results showed that flotation rate constant (K) exponentially increased with increasing particle size up to a certain limit where it then decreased sharply. On the contrary, flotation rate constant (K) was inversely proportional with collector dosage but gradually increased with increasing impeller rotational speed.     

Estimating the moisture damage of asphalt mixture modified with nano zinc oxide

One of the main distresses of hot mix asphalt (HMA) is moisture damage. The most common method for decreasing this type of distress is using antistrip additives. In this study, the effect of nanoparticles was evaluated as an antistrip agent on the moisture damage of HMA. Two types of aggregates were evaluated in this study with different sensitivities against moisture damage (limestone and granite aggregate) and the asphalt binder with 60/70 penetration grade and nano zinc oxide (ZnO) in two different percentages by weight of the asphalt binder. The tests employed to evaluate the effects of modifying asphalt binder by nanomaterials on the moisture damage of asphalt mixture were surface free energy (SFE) and AASHTO T283. The results showed that the ratio of wet/dry values of indirect tensile strength for the mixtures containing nano ZnO for two types of aggregate were higher than the control mixtures. In addition, the results of the SFE method showed that adding nano ZnO increased the total SFE of the asphalt binder, which led to better coating of the aggregate with asphalt binder. Nano ZnO decreased the acid to base ratio of SFE of asphalt binder, while it led to improving adhesion between the asphalt binder and acidic aggregate that are prone to moisture damage.     

Aging and filler effects on the creep model parameters of thermoset composites

The effect of marble mineral filler on the creep response of polyester thermoset matrix before and after physical aging has been investigated. A number of composites with different filler content (0.29, 0.38, 0.45, 0.55) have been studied in order to understand the role of the filler for the creep response. A mathematical model has been chosen for interpreting the experimental results, its parameters having a definite physical sense and being related with the single components of deformation under creep conditions. This model represents the generalized equation of Maxwell for relaxing isotropic medium in the form presented by G.I. Gourevich (Maxwell–Gourevich equation). The influence of the filler and physical aging on the parameter values of the model has been discussed, respectively on the elastic and rubberlike elastic (inelastic) deformation. The possibilities of the proposed model to describe the creep response of the matrix and the composites before and after physical aging are shown.     

Study on the optimization design of mixing moisture content in foamed asphalt mix

This research discussed the rational mixing moisture content (MMC) in foamed asphalt (FA) mix design. Sieve analysis was firstly used to study the bitumen dispersion and bitumen-aggregate coating and bonding action in FA mixes with three levels of MMC. Then, through variance analysis, the impacts of MMC, bitumen content and their interaction effect on the mechanical properties of FA mixes were studied. Indirect tensile strength test and dry density determination were carried out on FA mixes consisting of different gradings and material types to explore the rational MMC. On base of that, monotonic triaxial test was performed on two types FA mixes with various filler contents to further investigate the rational MMC for FA mix. The research results indicated that improper MMC led to bitumen clots and affected cohesion of FA mix. MMC and bitumen content had important impacts on mechanical properties of FA mix, while their interaction effect could be ignored in mix design. Optimal MMC increased to a certain extent with the increase of fine aggregates content, especially filler content in the grading. 70?C80% of the optimum moisture content (OMC) was recommended as rational range of MMC for FA mix with 5?C15% filler content and 75?C85% of OMC for mix with 15?C20% filler content, which offered a reference for easy and simplified MMC design for FA mix.     

Effect of aggregate types on the performance of neat and EVA-modified asphalt mixtures

The objective of this study is to determine the effect of bitumen modification with varying percentage of ethylene vinyl acetate (EVA) for preparing the hot-mix asphalt containing various aggregates (marble, granite and quartzite) and compare the results with mixes prepared with neat viscosity grade 30. The selection of aggregates has been done based on their acidic and basic nature. The physical and mechanical properties of EVA-modified bitumen and neat bitumen aggregate mixes were evaluated. Moisture susceptibility tests and wheel tracking test were carried out on the samples. Brookfield viscometer and dynamic shear rheometer were used to determine the mixing and compacting temperatures of neat and modified bitumen. The results indicate that mixes prepared with aggregates that are basic in nature, i.e. with higher calciumcontent, show better bonding with both neat bitumen and EVA-modified bitumen compared with acidic aggregates, i.e. aggregates with higher silica content.     

Investigating the effect of chirality on structural parameters of chiral single-walled carbon nanotubes by molecular dynamics simulation

The structural parameters of thin single-walled carbon nanotubes (SWCNTs) vs. chiral angle were investigated using molecular dynamics (MD) simulation. A comparison was made between nanotube radius obtained from MD simulation and that obtained from ideal rolling graphene model. Brenner empirical bond order potential was used to describe the interaction between carbon atoms. SWCNTs (n, m) with n + m = 6, 8, 10 and 12 were considered. It was observed that chiral nanotubes have three unequal bond lengths and three unequal bond angles, while for armchair and zigzag SWCNTs there are two unequal parameters.