Comparison of microscopic techniques to study the diversity of the bitumen microstructure.

Bitumen characterisation and differentiation usually involve a combination of mechanical and chemical analyses. However, these methods provide limited information on the diversity caused by the binders' origin or processing method. Thus, the question arises whether the bitumen microstructure can be used to identify these issues. In this study, microscopic methods, including brightfield, darkfield and fluorescence optical inverse microscopy (OIM), as well as confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM), were used to investigate the bitumen surface. Five different binders varying in their origin and production method were selected. The results show that CLSM, AFM and OIM darkfield can adequately capture a specific surface microstructure known as the bee structure, whereas brightfield in the OIM and optical CLSM show the surrounding peri phase, which exhibits a strong fluorescence. All visbroken binders show bee structures surrounded by a pronounced peri phase. On the other hand, one of the straight distilled binders does not show any microstructure, while the second straight distilled binder displays smaller bee structures surrounded by a small peri phase. Results from the image processing evaluation reveal that the area covered by bee structures is in the range of 2.4 - 4.3% for those binders that developed a surface microstructure. These results indicate a good accordance between the three microscopic techniques selected. However, a clear differentiation between the binders is difficult to obtain. Nonetheless, this work shows how these techniques can be used to their maximum capabilities regarding the obtained microstructural information and may help solve future questions regarding ageing, modification or rejuvenation.

Identification of Corrosion Minerals Using Shortwave Infrared Hyperspectral Imaging.

In this study, we propose a new method to identify corrosion minerals in carbon steel using hyperspectral imaging (HSI) in the shortwave infrared range (900-1700 nm). Seven samples were artificially corroded using a neutral salt spray test and examined using a hyperspectral camera. A normalized cross-correlation algorithm is used to identify four different corrosion minerals (goethite, magnetite, lepidocrocite and hematite), using reference spectra. A Fourier Transform Infrared spectrometer (FTIR) analysis of the scraped corrosion powders was used as a ground truth to validate the results obtained by the hyperspectral camera. This comparison shows that the HSI technique effectively detects the dominant mineral present in the samples. In addition, HSI can also accurately predict the changes in mineral composition that occur over time.

Coupling AFM and CLSM to investigate the effect of ageing on the bee structures of bitumen.

Bitumen's surface microstructure is in general well-documented and most recent theories postulate the effect of crystalline wax on the so-called bee structures. Although it is widely accepted that ageing has an effect on this unique microstructure yet conflicting literature exists on clear trends. This paper explores the effect of ageing on the bee structures of bitumen, employing two advanced microscopic techniques: an Atomic Force Microscopy (AFM) and a Confocal Laser Scanning Microscopy (CLSM). Four waxy and two wax-free bituminous binders are investigated before and after sequential laboratory short- and long-term ageing. The study demonstrates that the number of bees per µm2 and bee area percentage decrease with ageing, whereas their size is increasing. A systematic analysis of the waveform characteristics is also provided for the peaks and valleys as well as the shape probabilistic values of the bee structures. In conclusion, the results of the two techniques are in good agreement, reporting similar trends upon ageing for the relative bee density. Differences are mainly identified in the waveform calculations. It is believed that this systematic investigation may assist in enhancing the understanding of the effect of ageing on the surface microstructure, the wax distribution as well as on the wax crystallisation.

Application of Atomic Force (AFM), Environmental Scanning Electron (ESEM) and Confocal Laser Scanning Microscopy (CLSM) in bitumen: A review of the ageing effect.

Undoubtedly bitumen's viscoelastic performance has received much attention in the literature. Especially, the oxidative ageing phenomenon of bitumen has been studied by several scholars from different physicochemical and mechanical perspectives due to its direct impact on asphalt performance. The microstructural patterns observed with ageing utilising different microscopic techniques have not remained unexplored, and an increasing interest has been expressed to understand the bitumen's architecture by coupling it with different theories. This review aims to provide a useful guide for the road engineer by collecting all the existing microstructural trends that have been reported upon ageing by utilising some of the most promising microscopic techniques. The study demonstrates the changes being observed for the size of the so-called bee structures via Atomic Force Microscopy (AFM). The apparent fibril microstructure captured with Environmental Scanning Electron Microscopy (ESEM) consistently reported in the literature to become denser and coarser with ageing. The existing findings of Confocal Laser Scanning Microscopy (CLSM) revealed the conflicting observations that exist for the fluorescent centres of bitumen upon oxidation, concerning their size and number. Finally, this paper provides a comparative analysis of the three techniques for bitumen applications and recommends a systematic sample preparation protocol to move towards more consistent observations between the different research groups.

Fatigue Resistance of Bituminous Mixtures and Mortars Containing High Reclaimed Asphalt Content.

With the increased use of reclaimed asphalt (RA), the ability of bituminous materials to resist fatigue cracking may face a decline mainly due to the aged reclaimed asphalt binder (RAB), especially when RA is used at higher rates and not sufficiently treated. In this study, the bulk scale (asphalt) and its subscale (mortar) were employed to evaluate the effect on fatigue resistance when a RAB is added, by considering three replacement rates: 0%, 40%, and 70% RAB. The fatigue testing of asphalt mixtures was carried out using a four-point bending (4PB) setup, while the mortars were tested using a new column-like geometry utilising a dynamic shear rheometer (DSR). The fatigue properties were further analysed using dissipated energy concepts. The aim of this study was, first, to assess whether the inclusion of a RAB can provide at least similar fatigue properties compared to an all-virgin mix, and second, to evaluate whether the proposed treatment is beneficial for the mixtures with a RAB. The asphalt tests revealed that the inclusion of a 40% RAB led to increased fatigue resistance, whereas the mortar tests showed that the inclusion of RAB has an inverse effect on fatigue life.

Physicochemical and Rheological Properties of a Transparent Asphalt Binder Modified with Nano-TiO2.

Transparent binder is used to substitute conventional black asphalt binder and to provide light-colored pavements, whereas nano-TiO2 has the potential to promote photocatalytic and self-cleaning properties. Together, these materials provide multifunction effects and benefits when the pavement is submitted to high solar irradiation. This paper analyzes the physicochemical and rheological properties of a transparent binder modified with 0.5%, 3.0%, 6.0%, and 10.0% nano-TiO2 and compares it to the transparent base binder and conventional and polymer modified binders (PMB) without nano-TiO2. Their penetration, softening point, dynamic viscosity, master curve, black diagram, Linear Amplitude Sweep (LAS), Multiple Stress Creep Recovery (MSCR), and Fourier Transform Infrared Spectroscopy (FTIR) were obtained. The transparent binders (base and modified) seem to be workable considering their viscosity, and exhibited values between the conventional binder and PMB with respect to rutting resistance, penetration, and softening point. They showed similar behavior to the PMB, demonstrating signs of polymer modification. The addition of TiO2 seemed to reduce fatigue life, except for the 0.5% content. Nevertheless, its addition in high contents increased the rutting resistance. The TiO2 modification seems to have little effect on the chemical functional indices. The best percentage of TiO2 was 0.5%, with respect to fatigue, and 10.0% with respect to permanent deformation.