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.     

Attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectroscopy of oxidized polymer-modified bitumens

Oxidative age hardening of bitumen results in increasing fatigue susceptibility of bituminous mixtures, thus reducing the service life of asphalt pavements. Polymer additives to bitumen have been shown to improve its viscoelastic properties and, in some cases, reduce the level of bitumen hardening. Fourier transform infrared (FT-IR) spectroscopy enables evaluation of oxidation levels in bitumen by measuring the concentration of oxygen-containing chemical functionalities. This paper summarizes the results of the investigation of oxidative age hardening of polymer-modified bitumens (PMB) caused by accelerated aging in laboratory conditions. The PMB samples are prepared with different concentrations of styrene-butadiene-based co-polymers. Next, the PMB samples are aged using standard procedures that employ air blowing at 163 °C for 85 min followed by conditioning the samples at 100 °C and 2.1 MPa pressure for 20 to 48 hours. The resultant changes in their chemical composition are evaluated by portable attenuated total reflection (ATR) spectrometer. Measurements of ketone, sulfoxide, and hydroxyl content in PMB samples indicated similar oxidation pathways to those of non-modified bitumens. In addition, no evidence of polymer degradation due to accelerated aging of PMB was found in this study.     

Engineering characterization of recycled asphalt concrete and aged bitumen mixed recycling agent

A recycling agent is commonly used to restore the aged bitumen to a condition that resembles that of the virgin bitumen. Three reclaimed asphalt pavement (RAP) stockpiles were sampled, and the aged binders recovered from RAP binders were mixed with recycling agents at ten levels to produce bitumen blends. The blends using virgin bitumen as the softening agent exhibited a significantly different rheological behavior from ones using the rejuvenating agent. The addition of a recycling agent could shift up or down the master curve of the blend vertically, depending on the engineering properties of the recycling agent. A normalized viscosity ratio (NVR) model was used to characterize the rheological properties of aged bitumen mixed with softening and rejuvenating agents. An interaction parameter was introduced into the model to consider the physico-chemical reaction between aged bitumen and recycling agent. This mixing rule was compared to the method specified in the blending chart by the Asphalt Institute (AI). The blending chart was shown to be applicable to determine the amount of the softening agent required to meet the target viscosity. The NVR model appeared to be a better tool for the rejuvenating agent to predict the viscosity of a recovered bitumen blend than the AI chart.     

Numerical and experimental evaluation of the influence of the filler–bitumen interface in mastics

The successful use of additives in modified asphalt mixtures, such as warm mix asphalt, depends largely on the effect such modification has on the mastic. Previous research indicated that such modifiers do not simply change the bitumen properties, but can also change the interaction between the filler and the bitumen matrix. Understanding the effect of the properties of the fillers, the bitumen and their interaction is thus important for future asphalt mix design. In order to investigate this and to define the dominant relationships, this paper combines a numerical and experimental approach. In the experiments, the viscosities of modified and unmodified mastics with different filler concentrations and types were systematically investigated utilizing a novel testing protocol. In the numerical analyses, the Finite Element Method was utilized for a micro-mechanical analysis, in which the shape and size of the filler particles were varied in the bitumen matrix. Combining the experimental and numerical results allowed for a detailed investigation of the effect of the interface properties, with and without modifiers. The research further indicated that the effect of the shape and size of the fillers varied, depending on the interface properties. From the research relationships were established between the overall mastic viscosity and the influence of the filler–bitumen interface, considering shape and size. The conclusion of this paper can thus be useful for the effective development of modified asphalt mixtures and gives strong indications towards future research directions.     

环氧沥青混凝土桥面铺装材料研究与应用进展

随着桥梁设计理念、结构分析、施工技术等不断更新发展与完善,目前桥梁正逐渐朝着大跨度、高强度、长寿命、高耐久等方向发展,同时,未来桥梁也将面临更复杂的建设环境、更多的功能需求,因此必须加强研发与新型桥梁设计体系相匹配的特种桥面铺装材料。环氧沥青混凝土桥面铺装材料以高强度、耐高温、抗疲劳、抗老化等优异路用性能脱颖而出,逐渐受到关注。然而,环氧沥青混凝土制备工艺复杂、施工条件严苛、耐久性不足等问题日益凸显,这些缺点使得其在桥面铺装领域的推广受到一定阻碍。为此,研究者们针对如何提升环氧沥青混凝土使用品质及耐久性进行了深入研究并取得了一定成果。这一系列成果先后在大量桥面铺装实体工程中得以应用,良好的使用效果也为环氧沥青混凝土的进一步推广奠定了基础。环氧沥青混凝土在桥面铺装领域的研究成果可以概括为三个方面:铺装结构组合优化、制备工艺优化、混凝土原材料优化。其中铺装结构组合从早期的单质单层结构逐渐过渡到单质双层、异质双层结构,趋于合理的铺装结构组合使环氧沥青混凝土材料的性能得以充分发挥。制备工艺从热拌法发展到温拌甚至冷拌,在保证环氧沥青混凝土使用性能的同时减少了对环境的污染,并且在一定程度上降低了施工难度。而在原材料应用方面,研究者们不断对双组分环氧沥青与三组分环氧沥青的性能进行对比分析;同时,环氧沥青的改性方式趋于多样化,从单独使用改性沥青发展到同步使用改性环氧树脂,改性剂从纤维发展到高分子聚合物、超支化聚合物等。这些措施不仅改善了沥青与环氧树脂的相容性,也增强了环氧沥青混凝土的相关性能。此外,为使环氧沥青混凝土具有更好的稳定性,在级配优化方面也进行了深入研究。然而目前环氧沥青混凝土的一系列研究成果较为散乱,缺乏对其系统的总结与梳理,且环氧沥青混凝土的性能评价指标及要求仍需深入研究与完善。为进一步确定环氧沥青混凝土桥面铺装材料科学合理的性能评价指标及要求,本文全面梳理了国内外环氧沥青混凝土相关规范,系统调查了大量实体工程及研究动态,对比分析了不同主要原材料的环氧沥青混凝土对其路用性能的影响,最终推荐了环氧沥青混凝土桥面铺装结构组合、原材料类型、级配范围和性能评价指标及要求,为环氧沥青混凝土桥面铺装材料规范完善与质量控制奠定了基础。     

Rheological characterisation of bitumen diffusion

Diffusion is considered important in the process of mixing old and new binders during asphalt recycling. The degree of mixing is presumed to greatly influence the final properties of recycled asphalt concrete. Previously, studies have been undertaken to investigate diffusion using FTIR-ATR (Fourier Transform Infrared Spectroscopy using Attenuated Total Reflectance). A need was identified to verify, if the rates of diffusion detected using FTIR-ATR were accompanied by changes in rheological properties. In this paper, a dynamic shear rheometer (DSR) with parallel plates is used for monitoring diffusion. Diffusion coefficients obtained at 60, 80 and 100 °C from tests of a soft bitumen (rejuvenator) diffusing into a stiff one are presented. The diffusion coefficients determined are compared with the corresponding diffusion coefficients obtained using FTIR-ATR. The comparison shows that the rates of diffusion detected by the DSR are of the same magnitude, but somewhat higher than the ones detected by FTIR-ATR.     

老化SBS改性沥青二次改性再生工艺及机理研究

为研究老化SBS改性沥青的二次改性再生效果,比较不同工艺对再生效果的影响,首先通过常规试验、SHRP试验选择恰当的SBS改性沥青老化模拟方式,然后对添加SBS改性剂、新沥青进行再生的3种工艺展开研究,并借助红外光谱试验分析不同掺加顺序对老化SBS改性沥青再生效果的影响,确定了改性剂及新、旧沥青的最佳比例和最佳工艺。结果表明:采用RTFOT试验进行老化模拟更加方便、科学,RTFOT试验5h获得的改性沥青指标与服务年龄为7年的SBS改性沥青一致;SBS改性剂、新沥青添加顺序与老化沥青再生效果关系密切,先将SBS改性剂和新沥青混合制备改性沥青,再将改性沥青与老化沥青混合的工艺可使再生沥青性能最佳,其中SBS最佳掺量为4%,新、旧沥青质量比为2。     

Effect of crumb rubber gradation on a rubberized cold recycled mixture for road pavements

Cold recycling technique represents the most environmental friendly solution for pavement rehabilitation nowadays. In fact, this technique allows the use of the highest percentage of reclaimed asphalt avoiding the energy consumption related to aggregates heating required by the traditional hot mix asphalt design. The mix design represents a key phase of the cold mix production. The study of workability and compactability properties combined with a deep laboratory investigation is required. The idea of introducing crumb rubber in the cold mixtures was developed based on the concept of maximizing the valorization of recycled materials together with the goal of achieving high performance. In the present research project, two different gradations of crumb rubber, processed with the traditional grading method, have been adopted for the production of a cold recycled mixture stabilized with bitumen emulsion and cement. The spring-back effects of the rubber particles, which occur after compaction, together with the Indirect Tensile Strength and the Indirect Tensile Stiffness Modulus have been studied. The results show that the gradation of the adopted crumb rubber sensibly affects the compaction and mechanical properties of the cold recycled mixture.     

Effect of warm mix asphalt chemical additives on the mechanical performance of asphalt binders

Warm mix asphalt (WMA) has become very popular in asphalt pavement construction because it allows reducing both energy consumptions and carbon emissions. WMA can be obtained by using different types of additives and can be produced, applied, and compacted at temperatures 20–40 °C lower than hot mix asphalt. WMA additives allow reducing the working temperatures without compromising the final performance of the asphalt concrete. Many WMA additives are available on the worldwide market and some of them reduce the viscosity of asphalts binder (organic additives or foam) whereas others do not act on this sense (chemical additives). This study focuses on the effect of chemical additives on the performance of asphalt binders for WMA production. To this purpose, a neat bitumen, a polymer modified bitumen (PMB) and two different chemical additives were selected. All the binders were characterized through conventional tests, DSR, MSCR, FTIR and microscopic analysis. The result clearly showed that the influence of the chemical additives on the neat bitumen is negligible or non-existent. On the contrary, significant changes were observed in the modified bitumen properties. Specifically, chemical additives reduce the viscosity temperature susceptibility of PMBs in the temperature range between 80 and 140 °C, increase the rutting resistance potential and the elastic response of PMBs at high temperatures. Moreover, a morphological inspection supported the modifications observed in the rheological properties of PMBs.     

Influence of cement content and environmental humidity on asphalt emulsion and cement composites performance

Asphalt and cement concrete are the most popular materials used in the construction of roads, highways, bridge deck surface layers and pavements in airports and other areas with heavy wheel roads. Whereas asphalt possesses, compared to concrete, the advantages of a short curing period, high skid resistance and easy maintenance, it also shows lower fatigue durability, ravelling and rutting due to repeated concentrated loads and susceptibility to temperature changes and moisture. On the other hand, concrete pavements are initially more expensive, have lower driving comfort and are susceptible to cracking due to volume changes and to salt damage. A material with low-environmental impact and with advantages of both asphalt and concrete may be obtained by combining bitumen emulsions and a cementitious material. In this paper, cold asphalt mixtures with different amounts of cement were tested with Marshall stability tests. Selected mixtures were also cured at different environmental relative humidity (35, 70 and 90 % RH). By monitoring the mass of the specimens and estimating the water bound by the cement, the total water remaining in the mixtures was calculated. Details of the microstructure in the mixtures were examined with X-ray microtomography. According to the results of the present study, cement contributes to the hardening of cold asphalt mixtures both by creating cement paste bridges between the aggregates and by removing water from the mixtures through cement hydration. Asphalt and cement composites appear to be promising materials for implementation in real pavements, although their rate of hardening needs to be improved further.     

Analysis of styrene-butadiene-styrene polymer modified bitumen using fluorescent microscopy and conventional test methods

This paper presents a laboratory study of modified bitumen containing styrene-butadiene-styrene (SBS) copolymer. Polymer modified bitumen (PMB) samples have been produced by mixing a 50/70 penetration grade unmodified (base) bitumen with SBS Kraton D1101 copolymer at five different polymer contents. The fundamental characteristics of the SBS PMB samples have been determined using conventional methods. The morphology of the samples as well as the percent area (%) distribution of SBS polymers throughout the base bitumen have been characterized and determined by means of fluorescence microscopy and Qwin Plus image analysis program, respectively. The mechanical properties of the hot-mix asphalt (HMA) containing SBS PMBs have also been analyzed and compared with HMA incorporating base bitumen. The effect of polymer addition on the short and long term aging characteristics of HMA have been evaluated by indirect tensile strength (ITS) test. The results indicated that polymer modification improved the conventional properties (penetration, softening point, etc.) and the mechanical properties (Marshall, ITS, etc.) of the base bitumen. It was also concluded that at low polymer contents, the samples revealed the existence of dispersed polymer particles in a continuous bitumen phase, whereas at high polymer contents a continuous polymer phase has been observed. Moreover, it was found out that the polymer addition minimizes the short and long term aging of HMA.     

A parametric study on the influence of steel wool fibers in dense asphalt concrete

Environmental conditions combined with traffic loads contribute to premature deterioration of asphalt concrete pavements, reducing their strength and durability over time. To improve it, fibers can be incorporated in the mixture. Additionally, electrically conductive fibers can be used for self-healing purposes. In this context, this paper evaluates the influence of flexible steel fibers (steel wool) on the mechanical and physical properties of dense asphalt concrete. With these purposes, 25 different mixtures, with the same aggregate gradation and amount of bitumen, but with two different fibers lengths, four different percentages, and four different diameters of steel wool have been considered. Additionally, the influence of fibers on test specimens with three different types of damage: water damage, salt water damage and ageing have been evaluated through particle loss tests. Moreover, the influence of different temperatures on the flexural strength of dense asphalt concrete with steel wool fibers has been studied. It was found that steel wool fibers do not significantly improve the mechanical properties and damage resistance of dense asphalt concrete. On the other hand, steel wool fibers can change the air void distribution of a mixture, and therefore even reduce its particle loss resistance. As a recommendation, it is indicated that, for induction heating purposes, short fibers, with big diameters should be used, since they do not seem to alter the original properties of dense asphalt concrete.     

Preparation of coated iron powders by chemical plating

Small iron particles can be produced by reduction of γ-Fe2O3and α-FeOOH with hydrogen; they show outstanding magnetic properties and can be used for recording media. The main difficulty concerns the tendency of these particles to oxidize. We have studied a preparation process in two steps: a) reduction of iron oxides or oxy-hydroxides; b) chemical plating in the same furnace. The magnetic properties of the iron powders chiefly depend on the reduction step. The water content in the gas leaving the furnace has been shown to be the main parameter determining the obtainment of powders with high magnetic properties. The iron powders, which were pyrophoric, have been quenched in chemical plating bath of suitable formulation and coated with cobalt or copper. Typical magnetic properties are: saturation magnetization σsat= 130-165 emu/g, intrinsic coercivityjHc= 350-450 Oe; best magnetic properties: σsat=155 emu/g,jHc=700 Oe. The coated powders have been submitted to heat treatment in air saturated with water, showing a good resistance to oxidation.     

Influence of organo-montmorillonites on fatigue properties of bitumen and mortar

This study investigated the influence of organo-montmorillonites on fatigue properties of bitumen and mortar. Two organo-montmorillonites, Mt1 and Mt2 with octadecyl trimethyl ammonium and with benzyl dimethyl hexadecyl ammonium surfactants were used to modify one base bitumen with a content of 4 wt.%, respectively. Mt2 was also used to prepare the mortar which consisted of bitumen, filler and sand at a weight ratio of 0.34:0.30:0.36. The dispersion state of montmorillonite in bitumen was determined by the X-ray diffraction. A cone and plate device was designed to perform the fatigue test on bitumen. A column specimen before and after a short time aging was prepared for the fatigue test of mortar. An intercalated morphology for both montmorillonites was observed in bitumen. Results indicated that the addition of organo-montmorillonites can change the fatigue properties of bitumen under a stress control mode. This change was influenced by the type of the surfactant which determined the interfacial interaction between the bitumen and the montmorillonite. The mortar with modified bitumen showed better fatigue resistance and aging resistance than one with base bitumen. Therefore, the organo-montmorillonite would be an alternative to modifiers used in the bitumen to improve the lifetime of asphalt pavements.     

Effect of ultraviolet aging on rheology, chemistry and morphology of ultraviolet absorber modified bitumen

Seeking a new method to solve the ultraviolet (UV) aging of bitumen is of great importance for road applications since the UV aging is considered to be one of the main causes leading to the performance deterioration of bitumen. In this paper, two UV absorbers (octabenzone and bumetrizole) were applied to modify the bitumen by melt blending. Effect of UV aging on rheology, chemistry and morphology of the UV absorber modified bitumens was investigated, by means of dynamic shear rheometer, thin-layer chromatography with flame ionization detection, Fourier transform infrared spectroscopy and atomic force microscopy, to reveal the mechanisms of action for bitumen and UV absorbers. Results show that the two UV absorbers show opposite influences on the UV aging performance of bitumen depending on the type of UV absorbers. The bumetrizole improves the UV aging resistance of bitumen remarkably since the bumetrizole can inhibit reactions of aromatization and oxidation of bitumen molecules to a certain extent during the UV aging, and thus slows down the rate at which the bitumen transforms from sol to gel. The bumetrizole modified bitumen has the potential to be used in pavement for improvement of the UV aging resistance during service life.     

透水沥青混合料的热物特性与热阻功能

为研究透水沥青混合料的热物性,基于热力学理论计算比较了透水沥青混合料和密级配沥青混合料的热导率、比热容、热扩散率等指标,并对透水沥青混合料的热物性指标与空隙率、含水率的关系展开分析;通过设计室内光照试验,测得相同的传热条件下两种材料表面和底部的温度变化。理论计算结果表明同密级配沥青混合料相比,透水沥青混合料的热导率降低约20%,热扩散率降低约10%;室内温度测试结果显示,透水沥青混合料试件表面较密级配沥青混合料试件温度低2～2.5℃,底部温度低3～3.5℃,说明透水沥青混合料具有热阻功能,对气温荷载变化的抵抗能力较强,验证了理论计算结果。     

Characterization of stripping properties of stone material in asphalt

Aggregates and bitumen together form a composite called asphalt concrete pavement. Moisture damage to asphalt concrete pavement can occur as stripping, and is a common problem that can lead to costly repairs. There is therefore a need to understand which stone aggregates adhere best to bituminous binder and result in a minimum of stripping. Lifshitz used the refractive index to estimate the dispersive non-polar van der Waal’s interaction component of adhesion, the predominant component in adhesion between minerals and bituminous binder. The impact of an intervening thin medium such as air or water on the adhesion can be estimated using Hamaker’s coefficient, which in turn can be related to stripping potential. Aggregates consist of minerals and minerals consist of different elements. The objective of this study was to investigate variation in the dispersive component of minerals via their refractive indices using data from mineral data sheets. The influence of the position of elements in the periodic table and chemical composition on refractive index of minerals was examined in order to classify mineral aggregates for asphalt road building with regard to dispersive adhesive properties and expected resistance to stripping. It is clear from this study that the elemental composition of a mineral will affect its refractive index and hence its dispersive adhesion to bitumen. Aggregates and minerals have been classified according to degree of stripping in the literature. In this study it was shown that aggregates and minerals that have a refractive index higher than approximately 1.6 are expected to be less susceptible to stripping. Also, minerals containing alkali metals are sensitive to stripping since they are partially soluble in water.     

Controlled Oxidation of an Fe + C + NaCl + H2O Mixture: Phase Composition of Reaction Products and Dynamics of Heat Generation

The Fe-containing products of the reaction in an Fe + C + NaCl + H₂O mixture were identified by Mössbauer spectroscopy at different stages of oxidation interrupted by storage in an inert atmosphere. The results indicate that the storage of the mixture away from atmospheric oxygen leads to partial reduction of iron oxides. According to thermal analysis data, the energy stored in the heat-generating mixture studied can be used intermittently: heat release rapidly ceases after the mixture is isolated from air and resumes upon further oxidation. The reduction of iron(III) oxides during storage in an inert atmosphere stabilizes the heat-generation parameters at a nearly constant level during multiple oxidation–storage cycles.     

Solid-state transformations during diffusion bonding of copper to iron

Solid-state bonding between dissimilar metals, produced at elevated temperatures with the application of a bonding pressure, causes structural changes in the microstructure of the zones nearest to the bond interface. These metallurgical transformations, produced by interdiffusion in the vicinity of the bond, decide the final properties of the joint. In the present paper, such diffusional transformations have been investigated for diffusion-bonded joints of Armco iron and copper with different oxygen contents (ETPC and OFLPC). The formation of iron oxide (wustite) has been observed in the ETPC-Armco iron joints. This oxide did not appear in OFLPC-Armco iron diffusion-bonded joints. This suggests that iron oxide forms by reaction of iron with oxygen dissolved in the ETPC base metal. The formation of copper particles in the iron base matrix, near the bond interface, has been observed. This may be due to two different processes: the solid-state precipitation of copper into iron and the eutectoid reaction (γ →ε +α) at bonding temperatures above 900° C.     

Computations of permeability tensor coefficients and anisotropy of asphalt concrete based on microstructure simulation of fluid flow

Asphalt concrete is the most widely used material for building the surface layer of pavements. It is a porous material that consists of a non-uniform arrangement of asphalt binder, aggregate particles and air voids. One of the primary factors controlling pavement performance is the fluid flow characteristics within the surface asphalt concrete layer.

This paper focuses on the numerical simulation of fluid flow in the three-dimensional (3-D) microstructure of asphalt concrete, and the calculation of permeability from the flow field. The asphalt concrete microstructure was captured using the non-destructive X-ray computed tomography (CT) technique. X-ray CT images were processed in order to identify and retain interconnected air voids and eliminate isolated voids. This image processing enhanced the efficiency of the model as it does not have to solve for flow in isolated voids that do not contribute to fluid flow. The X-ray CT images were analyzed and the results were used to determine the relationship between air void distribution and permeability directional distribution or anisotropy.

The computed permeability values were found to have good correlation with the experimental measurements. The major and minor principal directions of the permeability tensor were found to correspond to the horizontal and vertical directions, respectively. The results indicated that the non-uniform spatial distribution of air voids created more open flow paths in the horizontal directional than the vertical direction, and hence was the much higher permeability in the horizontal directions.