Rheological properties of SEBS, EVA and EBA polymer modified bitumens

The rheological properties of various polymer modified bitumens were studied. Three bitumens from two different sources were mixed with styrene-ethylenbutylene-styrene (SEBS), ethylene vinyl acetate (EVA) and ethylene butyl acrylate (EBA) copolymers at different polymer contents. The rheological properties of the modified binders were investigated by means of dynamic mechanical analysis and creep test (bending beam rheometer). The results indicated that polymer modification increased binder elastic responses and dynamic moduli at intermediate and high temperatures, and reduced binder complex and stiffness moduli at low temperatures. Polymer modification also reduced temperature susceptibility, glass transition temperature as well as limiting stiffness temperature. The degree of the improvement generally increased with polymer content, but varied with bitumen source/grade and polymer type.     

Effect of ageing on the morphology and creep and recovery of polymer-modified bitumens

Polymer additives are used to improve the properties of road bitumens including their oxidative resistance. However, their usage as anti-oxidative materials remains relatively unclear. This study aims to investigate the changes in the morphology and the rheological response of polymer modified bitumens used in road pavement construction caused by ageing. An elastomer (radial styrene butadiene styrene, SBS) and a plastomer (ethyl vinyl acetate, EVA) polymer were mixed with one base bitumen at three polymer concentrations. The bitumens were RTFO and PAV aged. The morphology of the bitumens was captured by fluorescence microscopy while the rheological properties were measured by means of the multiple stress creep and recovery (MSCR) test. The results show that the morphology of the SBS modified bitumen degrades with ageing as a function of polymer concentration and dispersion, with higher dispersion being more resistant. The morphology of the EVA modified bitumen has a low ageing susceptibility irrespective of polymer concentration. The MSCR response of EVA modified bitumens does not differ from that found for unmodified bitumen, where the hardening produces a decrease in the non-recoverable compliance. In the case of SBS modified bitumen, the degradation of the polymer backbone affects the bitumen hardening as much as the polymer phase dispersed and networked in the bitumen phase. Furthermore, in the case of the elastomer, the average percent recovery is in agreement with the variation of the morphology with ageing. Therefore, the use of the average percent recovery as a valuable rheological index of the integrity of the polymer network can be advocated.     

Influence of polymer modification on low temperature behaviour of bituminous binders and mixtures

Effects of polymer modification on low-temperature properties of bituminous binders and mixtures were studied. Three bitumens were blended with 6% SBS, SEBS, EVA or EBA. Dense graded asphalt mixtures were prepared using a gyratory compactor. The low-temperature properties of the binders were characterised using dynamic shear rheometer and bending beam rheometer, and the low-temperature cracking of the mixtures evaluated by tensile stress restrained specimen test. The results indicated that low-temperature parameters were greatly dependent on the base bitumen, and in most cases, polymer modification did not show significant benefits as compared to the corresponding base bitumen. The mixture cracking temperature was found to correlate with the limiting temperatures (in bending beam rheometer) of the binders, weakly with Fraass breaking point, but not with parameters obtained using dynamic shear rheometer. Upon isothermal storage at low temperatures, the bitumens displayed physical hardening, and effect of polymer modification was small. However, physical hardening as measured by TSRST (tensile stress restrained specimen test) was not observed for the mixtures studied.     

Effect of ageing on different modified bituminous binders: comparison between RTFOT and radiation ageing

Standard laboratory ageing methods of bitumen only take into account the effect of thermo-oxidation during the service life of a pavement but the effect of high energy cosmic radiation on site is not simulated in these procedures. The aim of the present work is to compare the laboratory simulated short term bitumen ageing (rolling thin film oven test) with ageing produced by short exposures of bitumen samples to Ultra Violet and gamma radiation. The influence of ageing agents on the thermal properties and rheological performance of the pristine and modified bitumen binders has been evaluated in this study. The thermal behavior of various aged bitumens is characterized by both isothermal as well as non-isothermal thermogravimetric analysis. The thermoanalytic investigations on bituminous samples are carried out to evaluate the thermal stabilities and activation energies of the binders and the life time prediction of the materials is made with the help of the kinetic information. It is found that modified bituminous binders are more resistant to heat and radiation. Different rheological tests are conducted by dynamic shear rheometer to examine the effect of ageing in terms of bitumen oxidation and polymer phase degradation which has a major consequence on high temperature rutting or low temperature cracking. Type of modifier is found to be of decisive importance. Creep and recovery tests show that the structure-time dependency of pristine aged bitumen is influenced much by stress and temperature than in the case of modified aged bitumens. The study has revealed that the elastomeric modifier protects the bituminous binder more than plastic modifier or nano filler. Finally, a fair correlation has been made between standard RTFO ageing and radiation aging.     

Styrene butadiene styrene polymer modification of road bitumens

This paper describes the polymer modification of road bitumens with SBS. Six polymer modified bitumens (PMBs) were produced by mixing bitumen from two crude oil sources with an SBS copolymer at three polymer contents. The rheological characteristics of the SBS PMBs were analysed by means of dynamic mechanical analysis using a dynamic shear rheometer (DSR). The results of the investigation indicate that the degree of SBS modification is a function of bitumen source, bitumen-polymer compatibility and polymer concentration. When the polymer concentration and bitumen-polymer compatibility allow a continuous polymer network to be established, modification is provided by a highly elastic network which increases the viscosity, stiffness and elastic response of the PMB, particularly at high service temperatures. However, ageing of the SBS PMBs tends to result in a reduction of the molecular size of the SBS copolymer with a decrease in the elastic response of the modified road bitumen.     

Determination of polymer content in modified bitumen

Polymer-modified bitumen (PMB) has been increasingly used to enhance pavement performance. Two styrenebutadiene-styrene (SBS) copolymers were mixed with two bitumens by weight of the blend. This paper aims at developing the procedure to determine the proper polymer content to be mixed with bitumen. Tests including storage stability test, dynamic shear rheometer and scanning electron microscopy (SEM) were conducted to investigate the viscoelastic properties and microstructures of PMB. The addition of polymers increased the viscosity, softening point, toughness and complex modulus of bitumens. SEM results indicated that, as the polymer content increased, SBS gradually became the dominant phase that resulted in an increase in PMB's mechanic properties. Good compatibility produced an elastic network into the PMB up to 6% polymer concentration. The optimum polymer content was determined based on the rheological properties and the formation of the critical network. Adding higher polymer contents could lead to the separation of polymer and bitumen. The softening point temperature difference between top and bottom samples should be controlled within 2°C to monitor PMB's stability.     

Compatibility and storage stability of styrene-butadiene-styrene copolymer modified bitumens

The compatibility and storage stability of styrene-butadiene-styrene copolymer (SBS) modified bitumens were studied using fluorescence microscopy and dynamic mechanical analysis. Chemical characteristics of base bitumens and SBS polymers were determined by means of thin layer and gel permeation chromatography, respectively. The results showed that the morphology and phase separation of the modified binders varied with the characteristics of bitumens and polymers and were influenced by polymer content. At a given polymer content, the modified binders produced from bitumens with a higher content of aromatics exhibited better compatibility and higher storage stability. An increase in asphaltenes was observed to adversely affect storage stability. When mixing a small amount of polymer with bitumen, the modified binder showed dispersed polymer particles. At relatively high polymer content, depending on the base bitumens used, a continuous SBS phase was observed. On the other hand, the storage stability of modified binders decreased with increasing SBS content. Compared with branched SBS polymer, linear SBS displayed a finer dispersion in modified binder, and consequently, a lower phase separation was observed during hot storage. The degree of SBS dispersion in bitumen influenced storage stability and the rheological properties of modified binders; however, no definite relationships among them could be established.

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Résumé La compatibilité et la stabilité au stockage des bitumes modifiés par des copolymères styrène-butadiène-styrène (SBS) ont été étudiées au moyen de la microscopie à fluorescence et de l'analyse rhéologique dynamique. Les caractéristiques chimiques des bitumes de base ont été déterminées par chromatographie en couche mince alors que celles des polymères SBS l'ont été par chromatographie à pennéation de gel. Les résultats ont montré que la morphologie et la séparation de phases des liants modifiés variaient sclon les caractéristiques des bitumes et des polymères et qu'elles étaient influencées par le contenu en polymères. Pour une teneur en polymères donnée, les liants modifiés à base de bitumes ayant un fort taux d'aromatiques ont montré une meilleure compatibilité et une plus grande stabilité au stockage. Il a été observé qu'une augmentation de la teneur en asphaltènes dégradait la stabilité au stockage. Pour des teneurs en polymère faibles, l'observation des liants modifiés montre des particules de polym``ere dispersées dans la phase bitume. à des teneurs relativement importantes en polymère, mais variables selon le bitume de base utilisé, on observe une phase polymère continue. Par contre, la stabilité au stockage des liants modifiés diminue lorsque la teneur en SBS augmente. En comparaison avec un polymère SBS de type radial (ramifié), le polymère SBS de type linéaire donne des dispersions plus fines qui se traduisent par une moindre tendance à la séparation de phases lors du stockage à chaud. Le degré de dispersion du polymère SBS dans le bitume influence à la fois la stabilité au stockage et les propriétés rhéologiques des liants modifiés. On n'a cependant pas pu établir de relations précises entre ces propriétés.

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Editorial note Mr. Ulf Isacsson is a RILEM Senior Member. He is a member of TC 152-PBM on Performance of Bituminous Materials.     

Time dependent viscoelastic rheological response of pure,modified and synthetic bituminous binders

Bitumen is a viscoelastic material that exhibits both elastic and viscous components of response and displays both a temperature and time dependent relationship between applied stresses and resultant strains. In addition, as bitumen is responsible for the viscoelastic behaviour of all bituminous materials, it plays a dominant role in defining many of the aspects of asphalt road performance, such as strength and stiffness, permanent deformation and cracking. Although conventional bituminous materials perform satisfactorily in most highway pavement applications, there are situations that require the modification of the binder to enhance the properties of existing asphalt material. The best known form of modification is by means of polymer modification, traditionally used to improve the temperature and time susceptibility of bitumen. Tyre rubber modification is another form using recycled crumb tyre rubber to alter the properties of conventional bitumen. In addition, alternative binders (synthetic polymeric binders as well as renewable, environmental-friendly bio-binders) have entered the bitumen market over the last few years due to concerns over the continued availability of bitumen from current crudes and refinery processes. This paper provides a detailed rheological assessment, under both temperature and time regimes, of a range of conventional, modified and alternative binders in terms of the materials dynamic (oscillatory) viscoelastic response. The rheological results show the improved viscoelastic properties of polymer- and rubber-modified binders in terms of increased complex shear modulus and elastic response, particularly at high temperatures and low frequencies. The synthetic binders were found to demonstrate complex rheological behaviour relative to that seen for conventional bituminous binders.     

Polymer modified bitumen: Optimization and selection

Polymer modification of bitumen has been commonly performed since the 1980s in order to decrease bitumen (and pavement) susceptibility to high and low temperatures, allowing reduction in common failure mechanisms as rutting and cracking. Bitumen modification has been commonly performed by addition of thermoplastic or elastomeric polymers. However, there are just a few studies on bitumen blends using multiple materials, seeking for specific advantages provided by addition of these modifiers. This work describes the results obtained after the preparation of multicomponent polymer-bitumen blends (MC) based on an 80/100 penetration grade bitumen with varying amounts of (i) Polyethylene wax (PW); (ii) Styrene–Butadiene–Styrene copolymer (SBS); and (iii) crumb rubber (CR). Ideal blends depending on the amount of polymer modifiers added were found by using an experimental design procedure. It was possible to propose charts allowing optimizing and selecting appropriate polymer modified bitumens (PMB) depending on target properties for a given application by following Ashby’s materials selection methodology.     

Evaluation and optimization of the engineering properties of polymer-modified asphalt

Polymers are increasingly being used to modify asphalt and enhance highway pavement performance. This paper reports the development of a procedure to evaluate and optimize a polymer-modified asphalt (PMA). Two asphalt cements and two styrene-butadiene-styrene (SBS) copolymers were mixed at ten concentration levels. The engineering properties and morphologies of the binders were investigated using a dynamic shear rheometer, scanning electron microscopy (SEM), and other rheological techniques. The morphology of the PMA was characterized by the SBS concentration and the microstructure of the copolymer. Polymer modification increased the elastic responses and dynamic moduli of asphalt binders. As the SBS concentration increased, the copolymer gradually became the dominant phase, accompanied by a change in engineering properties. Results from SEM demonstrated that, up to 6% concentration, good compatibility exists between SBS and asphalt binder. The modified binders show either a continuous asphalt phase with dispersed SBS particles or a continuous polymer phase with dispersed asphalt globules, or two interlocked continuous phases. The optimum SBS content was determined based on the formation of a critical network between asphalt and polymer.     

From virgin to recycled bitumen: A microstructural view

In the present work, soft and hard bitumens recovered from unaged, aged and recycled asphalt concrete (AC) mixtures, which in laboratory tests performed mechanically as well as an AC mixture produced with virgin materials, were investigated regarding rheological, thermal and surface microstructural aspects. For comparison purposes, bitumen containing 50 wt% of virgin bitumen and 50 wt% of bitumen recovered from reclaimed asphalt pavement (RAP) was studied. Some properties of the bitumens remained unchanged throughout the preparation of the AC mixture, aging and recycling: Soft and hard bitumens retained their general rheological properties significantly, and their thermal and surface microstructural properties partially. Soft bitumens presented larger “bee” structures and, therefore, higher surface roughness, while hard bitumens presented smaller “bee” structures and, thus, lower surface roughness. Furthermore, soft bitumens seemed to contain higher crystalline-like content than hard bitumens. For the soft cases, the unaged recovered bitumen did not show the same characteristics (rheological and surface microstructure) as the virgin bitumen. Similarly the recovered recycled bitumen did not show the same characteristics (surface microstructure) as the bitumen prepared from the mixture of virgin bitumen and RAP bitumen. Aging of the AC mixture changed the rheological properties of the soft bitumen by increasing the complex modulus and decreasing the phase angle. Similarly, recycling changed the rheological properties by increasing the complex modulus and decreasing the phase angle. Compositional changes occurred during AC mixture preparation (possibly also aging and recycling) for both soft and hard bitumens. Consequently, more “phases” were observed on the surface microstructure for the recovered bitumens as compared with the virgin bitumens. However, no significant trend was found for the surface microstructure characteristics between the unaged, aged and recycled recovered bitumens. Moreover, the nature of the virgin bitumen influenced the properties of the recycled recovered bitumen, e.g. the glass transition temperature.     

Cracking of asphalt at low temperature as related to bitumen rheology

A laboratory investigation of the influence of bitumen rheology on low temperature behaviour of asphalt mixtures is described. Five bitumens from four sources and three different mixture types were studied. Rheological characteristics of the binders were measured using conventional methods (penetration, softening point and viscosity) as well as dynamic mechanical analysis (DMA). Low temperature properties of asphalt characterized by the fracture temperature were measured using thermal stress restrained specimen test (TSRST). Statistically significant relations between rheological characteristics of bitumens and TSRST fracture temperatures of asphalt specimens were established. © 1998 Chapman & Hall     

Dynamic mechanical characterization of asphalt concrete mixes with modified asphalt binders

The primary objective of this work is to characterize and compare the dynamic mechanical behavior of asphalt concrete mixes with styrene butadiene styrene (SBS) polymer and crumb rubber modified asphalt binders with the behavior of mixes with unmodified viscosity grade asphalt binders. Asphalt binders are characterized for their physical and rheological properties. Simple performance tests like dynamic modulus, dynamic and static creep tests are carried out at varying temperatures and time. Dynamic modulus master curves constructed using numerical optimization technique is used to explain the time and temperature dependency of modified and unmodified asphalt binder mixes. Creep parameters estimated through regression analysis explained the permanent deformation characteristics of asphalt concrete mixes. From the dynamic mechanical characterization studies, it is found that asphalt concrete mixes with SBS polymer modified asphalt binder showed significantly higher values of dynamic modulus and reduced rate of deformation at higher temperatures when compared to asphalt concrete mixes with crumb rubber and unmodified asphalt binders. From the concept of energy dissipation, it is found that SBS polymer modification substantially reduces the energy loss at higher temperatures. Multi-factorial analysis of variance carried out using generalized liner model showed that temperature, frequency and asphalt binder type significant influences the mechanical response of asphalt concrete mixes. The mechanical response of SBS polymer modified asphalt binders are significantly correlated with the rutting resistance of asphalt concrete mixes.     

Low-temperature cracking of polymer-modified asphalt

A laboratory investigation was conducted to study different types of polymer-modified asphalt with regard to low temperature behaviour. The thermal stress restrained specimen test (TSRST) was used to assess the disposition of asphalt mixtures to cracking at low temperature. Five polymer modified bitumens and three mixture types (dense graded, stone mastic and porous asphalt) were investigated. In addition, three different “gussasphalt” mixtures were analysed. Based on the results obtained, it was concluded, among other things, that (1) the polymer type, mixture type and degree of ageing influence the low temperature properties of asphalt mixtures; (2) the increase in fracture temperature during ageing is dependent on mixture and polymer type; (3) the use of modified binders may improve the low temperature properties of ‘gussasphalt” mixtures.     

Study of rheological properties of pure and polymer-modified Brazilian asphalt binders

In recent years, many authors have suggested techniques to study asphalt binders and establish structure-property correlations for these materials. Several works have shown that rheological analysis provides reliable information about the stability, elasticity, thermal susceptibility, and also that rheological behavior depends on chemical composition and structure of pure and modified binders. The objective of this work was to study the effect of the asphalt binder chemical composition and modifier polymer type on the linear viscoelastic properties and to correlate these properties to the PMB's thermal susceptibility. A set of polymer-modified asphalt binders (PMB) was prepared using two Brazilian asphalts. Both pure binders and PMB were analyzed by classical and dynamic rotational rheology tests. The rheological analysis results were also compared to storage stability data and PMB's morphology. The fitting capability of the Christensen-Anderson and Christensen-Anderson-Marasteanu rheological models was analyzed for the two pure asphalt binders. Both models presented lack of fitness in the regions of lowest and highest frequencies. When using elastomeric modifiers, it was possible to state the existence and interrelation between the width of the phase angle master curve plateau and the PMB's thermal susceptibility and stability. This interrelation was supported by PMB's storage stability data and microscopy analysis.     

Evolution of bituminous mix behaviour submitted to UV rays in laboratory compared to field exposure

Laboratory methods to simulate the short and long term ageing occurring during the service life of pure and polymer modified bitumens in a pavement are standardized but none of them takes into account the influence of ultraviolet (UV) radiations. Recently, attentions have been paid to the laboratory ageing tests with UV radiation applied to the bituminous binders. Even if this effect of UV radiation on binder’s films have been largely demonstrated and studied, however there are few studies dealing with the photo-oxidation of binders in a bituminous mix in relation with voids, film thickness and permeability. So, the aim of our study was to investigate whether the influence of UV light on bitumen ageing might be assessed in the bituminous mixes using an experimental apparatus dedicated to the ageing of paint. The results show that the influence of UV radiation on the ageing of bituminous mixes containing an elastomer modified bitumen can not be totally ignored: compared with thermal aging, the UV impact can be distinguished and found to be dominant for the production of carbonyl functions, the disappearance of C=C double bond of SBS and the increase of binder’s hardening. So, this study has highlighted, on the one hand, that inside the bituminous mix, the UV radiations do increase the rate of oxidation and, on the other hand, that the evolution’s kinetics due to a pure thermal oxidation or a photo-oxidation processes are different: the evolution due to 44 months of on site ageing is better assessed by photo-oxidation process than by pure thermal oxidation. Consequently, the UV exposure may affect the bitumen’s properties of pavement upper layers more strongly than the PAV simulation in laboratory (without UV action) does.     

A model combining structure and properties of a 160/220 bituminous binder modified with polymer/clay nanocomposites. A rheological and morphological study

The present contribution focuses on the modification of a 160/220 bituminous binder with clay and polymer/clay nanocomposites. Bitumen/polymer/clay ternary blends were prepared using styrene–butadiene–styrene, ethylene vinyl acetate and ethylene methylacrylate copolymers mixed with an organomodified montmorillonite. Dynamic mechanical analyses were performed in the extended domain of stress, temperature and frequency to analyse the thermorheological behaviour of the blends. The time–temperature superposition principle was applied to shift the experimental data recorded at different temperatures and generate master curves of the linear viscoelastic functions. For all blends, the mechanical response of the system was found to be strongly and intimately influenced by the nanocomposite modification. In some cases, a solid-like behaviour appears and delays the Newtonian transition. Morphological analyses performed with fluorescence microscopy allowed to associate the binder properties with the presence of clay silicates, which alter the colloidal equilibrium of the bitumen and enhances the compatibility between bitumen and polymers. Based on the morphological and rheological results, a structural model of the prepared blends is proposed.     

Ageing in modified bitumen using FTIR spectroscopy

Modified binders exhibit complex rheological behaviour due to the interaction of the modifiers with the base binder and the manner in which they age during field applications. Quantification of the interaction and the ageing of such modified binders is currently a necessity. In this investigation, three modified binders (elastomer, plastomer, and crumb rubber) and the base bitumen were subjected to different ageing conditions and the evolution of the chemical functionalities during ageing were tracked using FTIR spectroscopy. Analysis of the spectra of modified binders immediately after the production process showed that while the elastomer modified binders exhibited physical interaction, the plastomer and crumb rubber modified binders exhibited physical and chemical interactions. During ageing, there was no subsequent evolution of the interactions and one could only see chemical functionalities related to oxidation. Analysis of the spectra showed that the carbonyl and sulphoxide exhibit identical trends whereas aliphaticity and aromaticity deviate drastically.     

Wax morphology in bitumen

Wax crystallisation and melting in bitumen is usually considered detrimental to bitumen quality and asphalt performance. The objectives of this paper are to study wax morphology in bitumen and to investigate effects of time, temperature, and thermal cycling on wax crystallisation. Various samples were selected, including eight waxy bitumens of different sources and three laboratory blends prepared by adding a slack wax and two isolated bitumen waxes to the non-waxy bitumen. Test methods used were differential scanning calorimetry (DSC), polarised light microscopy (PLM), confocal laser scanning microscopy (CLSM), and freeze etching (fracture) in combination with transmission electron microscopy (FF-TEM). The DSC results indicated that the selected bitumen samples differ widely in wax content and wax crystallisation starting and melting out temperatures. It was found that non-waxy bitumen displayed no structure or crystals neither in PLM, CLSM or FF-TEM, while waxy bitumens from different crude origins showed a large variation of structures. The morphology of wax crystals was highly dependent on crystallisation temperature as well as temperature history. The wax which has been isolated from waxy bitumen and mixed into non-waxy bitumen displayed similar morphology as the wax in the original bitumen. It was also found that bitumen wax usually melted at temperatures lower than 60°C although in one case a temperature of 80°C was needed until complete melting of the wax.