Prabir Kumar Das

Micromechanical investigation of phase separation in bitumen by combining atomic force microscopy with differential scanning calorimetry results

The thermo-rheological behaviour of bitumen depends largely on its chemical structure and intermolecular microstructures. Bitumen is a complex mixture of organic molecules of different sizes and polarities for which the micro-structural knowledge is still rather incomplete. Knowledge at that level can have great implications for behaviour at a larger scale and will help to optimise the bitumen in its production stage. The present study is focused on understanding the fundamental mechanisms behind the micro-structural phase appearance and the speed or mobility at which they change. To do so, atomic force microscopy was utilised at different temperatures to investigate the phase separation behaviour for four different types of bitumen and co-relate it with the differential scanning calorimetry measurements. Based on the experimental evidences, it was found that the observed phase separation is mainly due to the wax/paraffin fraction presence in bitumen and that the investigated bitumen behaves quite differently. Recommendations are made to continue this research into qualitative information to be used on the asphalt mix design level.

Microscale investigation of thin film surface ageing of bitumen

This paper investigates the mechanism of bitumen surface ageing, which was validated utilizing the atomic force microscopy and the differential scanning calorimetry. To validate the surface ageing, three different types of bitumen with different natural wax content were conditioned in four different modes: both ultraviolet and air, only ultraviolet, only air and without any exposure, for 15 and 30 days. From the atomic force microscopy investigation after 15 and 30 days of conditioning period, it was found that regardless the bitumen type, the percentage of microstructure on the surface reduced with the degree of exposure and time. Comparing all the four different exposures, it was observed that ultraviolet radiation caused more surface ageing than the oxidation. It was also found that the combined effect was not simply a summation or multiplication of the individual effects. The differential scanning calorimetry investigation showed that the amount of crystalline fractions in bitumen remain constant even after the systematic conditioning. Interestingly, during the cooling cycle, crystallization of wax molecules started earlier for the exposed specimens than the without exposed one. The analysis of the obtained results indicated that the ageing created a thin film upon the exposed surface, which acts as a barrier and creates difficulty for the wax induced microstructures to float up at the surface. From the differential scanning calorimetry analysis, it can be concluded that the ageing product induced impurities in the bitumen matrix, which acts as a promoter in the crystallization process.

Evaluation of fracture and moisture damage performance of wax modified asphalt mixtures

In this study the fracture and moisture damage characteristics of wax modified asphalt mixtures were evaluated. Two types of commercial waxes (FT-paraffin and Asphaltan B) were added to bitumen of penetration grade 70/100. Using this wax modified and unmodified bitumen; total 48 specimens were produced from two sources of aggregates and two levels of gradation. Bitumen properties were determined by conventional test methods, Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR) testing. Thermal Stress Restrained Specimen Test (TSRST) was used to evaluate low temperature cracking resistance and cracking behavior of asphalt mixture was investigated at 0°C using Superpave Indirect Tensile Test (IDT). The influence of wax on the asphalt mixture resistance to cracking and moisture damage performance has been evaluated using Hot Mix Asphalt (HMA) fracture mechanics and Superpave IDT test results. The addition of FT-paraffin and Asphaltan B showed better cracking and moisture damage resistance of the asphalt mixture compared to unmodified mixture, but FT-paraffin showed the largest effect on cracking resistance while Asphaltan B showed highest resistance to moisture damage. In BBR test results, mixtures modified with FT-paraffin showed lower limit m value (LmT) which implies minor negative effect in stress relaxation. However, according to TSRST results, the mixtures with both waxes had nearly same fracture temperature as mixture with unmodified bitumen.

Challenges While Performing AFM on Bitumen

Using modern microscopic techniques such as atomic force microscopy (AFM) has added significant knowledge on the microstructure of bitumen. The advantages of AFM are that it requires relatively simple sample preparation and operates under ambient conditions. As the use of AFM is becoming more widespread and useful the RILEM technical committee (TC) on nano bituminous materials NBM 231 has conducted a round robin study on this method, the results with respect to reproducibility, repeatability or accuracy limits are presented elsewhere. However, the execution of good quality AFM experiments especially on bitumen is still a challenging task. Sample extraction and preparation are very crucial and attention should be paid to obtain homogenous samples with a sufficient thickness and no surface contamination. The preparation should include a high temperature treatment to provide a smooth homogenous surface. Annealing/resting of the sample has to be sufficiently long, at least 24 h under ambient temperatures to ensure the formation of a (meta)stable micro-structure. Imaging should be done using non-contact (Tapping) mode with stiff cantilevers (resonance frequency ~300 kHz) with a minimum amount of damping as possible.

Micro-mechanical investigation of low temperature fatigue cracking behaviour of bitumen

In an effort to understand the effect of low temperature fatigue cracking, atomic force microscopy (AFM) was used to characterize the morphology of bitumen. In addition, thermal analysis and chemic ...

Differential Scanning Calorimetry Applied to Bitumen: Results of the RILEM NBM TG1 Round Robin Test

The application of Differential Scanning Calorimetry (DSC) has been proven useful in characterizing bituminous binders, distillates and crude oils. In this paper, results of the round robin test, organized by the Rilem TC 231 Nanotechnology-based Bituminous Materials (NBM) TG1 group are reported. The purpose is to investigate the repeatability and reproducibility of standard DSC measurements when applied to bituminous binders. In the full test program of the Rilem NBM group, DSC measurements are further compared to observations made in atomic force microscopy (AFM), AFM measurements are reported in a separate paper. Seven laboratories have participated in this round robin test. Four bituminous binders were investigated, containing various amounts of natural or added wax. The test program consisted of a well-defined isothermal annealing procedure, followed by a first heating and cooling scan, and afterwards followed by a second heating scan. At this stage, the data, as they were reported by the different participants, were compared. For the glass transition (Tg), mid temperatures, can be defined with a reasonable reproducibility, which improves if natural wax is not present. Regarding melting and crystallization, the shape of the melting curve is highly dependent on the thermal history of the samples. Peak temperatures of melting and crystallization phenomena were reported with a good reproducibility, while the reproducibility of melting enthalpies (or surface area’s under the melting and crystallization signals) was not satisfactory. Different reasons for this and recommendations for improving the results are discussed in the paper.

Atomic Force Microscopy to Characterize the Healing Potential of Asphaltic Materials

Worldwide, asphalt concrete is the most commonly used material for the top layer of pavements. The asphalt mixture’s ability to provide the necessary stiffness and strength via its strong aggregate skeleton, while at the same time offering a damping and self-restoring ability via its visco-elastic bituminous binder, makes it a uniquely qualified material for increased driving comfort and flexible maintenance and repair actions. Unfortunately, bitumen supply is diminishing as crude sources are depleted and more asphalt refineries install cokers to convert heavy crude components into fuels. It is therefore becoming of imminent urgency to optimize the lifetime of the virgin bitumen from the remaining available crude sources. With 90% of the total European road network having an asphalt surface or incorporating recycled asphalt mixture in one of its base layers, the annual production of asphalt mixtures in Europe is well over 300 million tonnes. It is therefore fair to state that asphalt mixtures play a significant role in the economic viability and international position of the European pavement industry.

Numerical study on the effect of mixture morphology on long-term asphalt mixture ageing

Asphalt mixtures with similar percentages of air voids can have different morphologies and can age differently. Prediction of ageing behaviour without considering the influence of mixture morphology may thus lead to erroneous conclusions and non-optimal mix design. This article investigates the long-term field ageing of asphalt mixtures by incorporating mixture morphology. For this, a computational analysis on diffusion–reaction process has been conducted by implementing fundamental mechanism of ageing and conducting a parametric sweep of the morphology. To investigate the ageing gradient along the depth of asphalt mixture, diffusion controlled oxidative ageing on one dense and one open-graded field core was investigated. The proposed model based on the mixture morphology information was able to predict the aged viscosity better than the existing model. As mixture morphology is controllable, having insight into how the morphology parameter influences the mixtures ageing susceptibility can be of great value to its design.

Towards a Multi-scale Framework to Optimize Ageing Resistance of Asphaltic Materials

This paper describes an ongoing research project that is aiming at developing a comprehensive multi-scale approach to optimize the ageing resistance of asphaltic mixtures. In this, ageing has been focused on oxidative ageing, but allows future extension to other ageing mechanisms. The developed framework considers three different scales: the nano, micro and meso-scale which are defined as the bitumen phase, the mastic phase and the mixture phase, respectively. In nano-scale, atomic force microscopy and calorimetry are coupled to each other to give insight into how bitumen phase separation evolves and the mobility of microstructure changes with temperature and ageing. On the micro-scale, the energy dissipation as a function of ageing is measured and coupled to the phase behavior information from the nano-scale. On the meso-scale a morphology framework is defined, capable of identifying the dominant mixture morphology parameters that control mixture performance under ageing conditions. By coupling the three scales, the dominant parameters that control ageing of asphaltic mixtures can be defined, modeled and analyzed and as such a tool is created that has the potential of enhancing the sustainability of asphaltic mixtures.