Xiaohu Lu

EFFECT OF AGEING ON BITUMEN CHEMISTRY AND RHEOLOGY

Abstract Effect of ageing on bitumen chemistry and rheology was studied. Seven bitumens were aged according to the thin film oven test (TFOT) and the rolling thin film oven test (RTFOT). The binders were characterised using infrared spectroscopy, chromatography and dynamic mechanical analysis. Statistical correlation between different chemical parameters, as well as between chemical and rheological parameters, was examined. The relationship between TFOT and RTFOT was also investigated. It was observed that ageing influenced bitumen chemistry and rheology significantly. However, chemical and rheological changes were generally not consistent, and consequently, ageing susceptibility of bitumens may be ranked differently when different evaluation methods are used. Regardless of the type of the parameters measured, a strong correlation was observed between TFOT and RTFOT, and the two ageing procedures show similar severity.

Testing and appraisal of polymer modified road bitumens—state of the art

Polymer of road applications and their effects of functional properties (permanent deformation, fatigue cracking, low temperature cracking, stripping, wear resistance and ageing) of asphalt pavements are described. The state of the art in test methods for polymer modified bitumens is presented. The methods are classified as standardized or non-standardized. A general description of each non-standardized test procedure is given. Current or proposed specification models for polymer modified bitumens are summarized. The present situation regarding test methods and specifications of polymer modified bitumens and the need for further development are discussed.

Chemical and Rheological Evaluation of Ageing Properties of SBS Polymer Modified Bitumens

Abstract The ageing properties of Styrene-Butadiene-Styrene (SBS) polymer modified bitumens were evaluated using Dynamic Mechanical Analysis (DMA), Gel Permeation Chromatography (GPC) and Fourier Transform InfraRed (FTIR) spectroscopy. The binders were aged by means of the Thin Film Oven Test (TFOT) and the Rolling Thin Film Oven Test (RTFOT), respectively. It was observed that ageing resulted in degradation of the SBS polymer (decrease in the polymer molecular weight) and oxidation of the bitumen (increase in the content of polar oxygen-containing molecules and increase in bitumen molecular weight). The polymer was observed to resist formation of sulphoxides. Changes in the rheological properties of aged-modified binders were dependent on a combined effect of bitumen oxidation and polymer degradation, which varied with bitumen source/grade and polymer type/content. In all cases, the aged modified binders showed better rheological properties than aged base bitumens. The study also indicated that the ageing index obtained using DMA was largely influenced by temperature and frequency. This parameter may be applied for evaluating the base bitumens and modified binders with a low polymer content (3% in this study), but not for modified binders with a high (6%) polymer content.

Modification of road bitumens with thermoplastic polymers

Polymer modified bitumens were prepared by blending bitumens with different thermoplastic polymers (SBS, SEBS, EVA and EBA). The fundamental properties (e.g. morphology, rheology and ageing) of the modified binders were studied using fluorescence microscopy, dynamic mechanical analysis, creep test (bending beam rheometer), and gel permeation chromatography. The results indicated that the morphology and rheological properties of the modified binders were influenced by characteristics and content of the polymer and nature of the bitumen. When a continuous polymer phase was formed at a sufficiently high polymer content (about 6%), the rheological properties of the binders were improved significantly. At a given polymer content, the modified binders containing SBS and SEBS differ widely from those containing EVA and EBA in their rheological behavior. The ageing properties of the binders were also strongly dependent on types of polymer. In most cases, the rheological changes during ageing were related to oxidation of bitumen and/or degradation of polymer. In addition, the effect of ageing was testing condition (e.g. temperature) dependent.

Rheological characterization of styrene-butadiene-styrene copolymer modified bitumens

Abstract The rheological properties of styrene-butadiene-styrene copolymer (SBS) modified bitumens were studied. The modified binders were prepared using a laboratory mixer. Five types of bitumen from four sources were mixed with two SBS polymers at different polymer content levels. The rheological properties of the modified binders were characterized using dynamic mechanical analysis over wide ranges of temperatures and frequencies. It was found that the addition of the SBS polymers increased the binder elasticity at high temperatures and improved the binder flexibility at low temperatures. The temperature susceptibility of bitumens was also reduced by SBS modification; however, this property cannot be evaluated with a single-valued parameter. The degree of modification with respect to the binder rheology varied with temperature and frequency, and was dependent on the bitumen source/grade and the polymer concentration and structure.

Characterization of bitumens modified with SEBS, EVA and EBA polymers

A laboratory evaluation of the modified bitumens containing styrene-ethylene-butylene-styrene (SEBS), ethylene vinyl acetate (EVA) and ethylene butyl acrylate (EBA) copolymers is presented. The morphology, storage stability and rheological and ageing properties of the modified binders were studied using fluorescence microscopy, dynamic mechanical analysis, creep test (bending beam rheometer) and conventional methods. The results indicated that the morphology and storage stability of the modified binders were largely dependent on the polymer content and were influenced by the characteristics of the base bitumens and the polymers. At a low polymer content (3% by weight), the modified binders showed dispersed polymer particles in a continuous bitumen matrix. At a sufficiently high polymer content (6% by weight), a continuous polymer phase was observed. Regardless of the nature of the two phases, the storage stability of the modified binders decreased as polymer content increased. Polymer modification improved bitumen rheological properties such as increased elastic responses at high temperatures and reduced creep stiffness at low temperatures. The degree of improvement generally increased with polymer content, but varied with bitumen source/grade and polymer type. Polymer modification also influenced bitumen ageing properties. Evaluation of ageing effect was dependent on testing conditions (e.g. temperature and frequency).

Artificial aging of polymer modified bitumens

This paper presents an investigation of artificial aging of polymer modified binders, prepared from three base bitumens and six polymers. Aging of the binders was performed using the Thin Film Oven Test (TFOT), the Rolling Thin Film Oven Test (RTFOT), and modified RTFOT (MRTFOT). The binders were characterized by means of infrared spectroscopy, different types of chromatography, and dynamic mechanical analysis. It was found that the effect of aging on the chemistry and rheology of the modified binders was influenced by the nature of the base bitumens and was strongly dependent on the characteristics of the polymers. For styrene–butadiene–styrene (SBS) and styrene–ethylene–butylene–styrene (SEBS) modified binders, aging decreased the complex modulus and increased the phase angle. Aging also increased the temperature susceptibility of these modified binders. The rheological changes of SBS modified bitumens were attributed to polymer degradation and bitumen oxidation. However, for SEBS modified bitumens, the mechanisms of aging are unclear. In the case of ethylene vinyl acetate (EVA) and ethylene butyl acrylate (EBA) modified binders, the process of aging increased the complex modulus and elastic response (decreased phase angle), and reduced temperature susceptibility. These changes were mainly due to the oxidative hardening of the base bitumens. The study also showed statistically significant correlation between TFOT, RTFOT, and MRTFOT. However, no definite conclusions could be drawn regarding the difference in severity of aging between these methods.

Influence of styrene-butadiene-styrene polymer modification on bitumen viscosity

Effects of polymer content/structure and bitumen type on viscosity characteristics of styrene-butadiene-styrene (SBS) polymer modified bitumens were investigated. The study indicated that SBS polymers were not inert additives and increases in kinematic and dynamic viscosities of the modified bitumens were not directly proportional to polymer content; a marked viscosity increase was observed when the polymer content increased from 3 to 6% by weight of the blend. Modification with a sufficiently high polymer content also increased the degree of non-Newtonian behaviour of the bitumens. The base bitumens and modified bitumens containing 3% SBS were observed to be essentially shear rate independent, while those containing 6 or 9% SBS displayed shear-thinning behaviour. Compared to the modified bitumens with linear SBS, the modified bitumens with branched SBS demonstrated a higher degree of shear-thinning behaviour. These effects were dependent on ranges of shear rate and temperature. The increased degree of non-Newtonian behaviour was observed to influence the correlations between kinematic and dynamic viscosities.

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.RésuméLes effets de l’ajout de polymères sur les propriétés à basses températures ont été étudiés sur des liants bitumineux tel quel ou en enrobés. Trois bitumes ont été mélangés avec 6% SBS, SEBS, EVA ou EBA. Les enrobés denses ont été préparés avec une presse à cisaillement giratoire. Les propriétés à basses températures ont été caractérisées sur les liants à l’aide d’un rhéomètre dynamique à cisaillement et de l’essai de flexion sur barreau (BBR) et sur les enrobés à l’aide d’un essai de retrait empêché (TSRST). Les résultats indiquent que les propriétés à basses températures dépendent essentiellement du bitume de base et que, dans la plupart des cas, l’ajout de polymère n’améliore pas de façon significative ses propriétés. La température de fissuration des enrobés a pu être corrélée avec les températures limites BBR du liant, faiblement avec le point de fragilité Fraass mais pas avec les paramètres obtenus en utilisant le rhéomètre dynamique à cisaillement. Lors d’un stockage à une température basse donnée, les bitumes présentent un durchissement physique qui est peu influencé par les polymères. Néanmoins, ce durcissement physique n’a pas été observé sur les enrobés lors des essais de TSRST.

Laboratory Study on the Low Temperature Physical Hardening of Conventional and Polymer Modified Bitumens

Abstract Physical hardening is a reversible process that may influence the long-term performance of a material. In this paper, physical hardening of five unmodified and 35 polymer modified bitumens was studied using a bending beam rheometer (BBR). The modified binders were prepared by mixing 3, 6 or 9% styrene–butadiene–styrene (SBS), styrene–ethylene–butylene–styrene (SEBS), ethylene vinyl acetate (EVA) or ethylene butyl acrylate (EBA) polymers with different base bitumens. The binders were isothermally stored at low temperatures (−15, −25 and −35°C) for different times ranging from 0.5 to 32 h. The results indicated that physical hardening significantly influenced the creep response of the binders, and the hardening index and horizontal shift factor were strong functions of isothermal storage temperature. Unlike the shift factor, the hardening index did not always increase with decreasing storage temperature. At a given storage temperature, a statistical correlation was observed between the two parameters. It was also shown that degree and kinetics of physical hardening were dependent on the base bitumens, and in most cases, the effect of polymer modification was insignificant.