Modeling Crack Propagation in Bituminous Binders under a Rotational Shear Fatigue Load using Pseudo J-Integral Paris’ Law

Fatigue resistance of bituminous binders plays a critical role in determining the fatigue performance of asphalt pavements. It is reported in the literature that, under a rotational shear fatigue load like a dynamic shear rheometer (DSR) test, the crack grows in the cylindrical bitumen sample as a circumferential crack that is initiated at the periphery of the sample and propagates toward the center of the sample. This study aims to model this crack propagation in bituminous binders under rotational shear fatigue load by time sweep (TS) fatigue test using the DSR. The crack length in the TS test is determined using a damage mechanics-based DSR-C model which is a function of the shear moduli and phase angles under undamaged and damaged conditions. The crack evolution is modeled by a pseudo J-integral based Paris’ law. Samples of virgin bitumen 40/60 and polymer-modified bitumen X-70 under unaged and aged conditions are tested by the TS tests at different temperatures, frequencies, and strain levels. Results show that the pseudo J-integral Paris’ law is able to predict accurately the crack propagation in bituminous binders under the rotational shear fatigue load. The crack grows faster in aged bitumen or at lower temperatures. The Paris’ law model parameters (A and n) are independent of loading frequency or load amplitude. They are fundamental material properties and can be determined at one loading frequency and amplitude, then can be implemented to predict the growth of cracks in bituminous binders at different loading frequencies or amplitudes

Multiscale Modelling of Bonding Performance of Bituminous Materials

Bonding performance of bituminous materials fundamentally determines the resistance of asphalt to fatigue cracking which is one of the major distresses in asphalt pavements. Running laboratory fatigue tests of bitumen or asphalt is costly and time consuming, resulting in empirical-based fatigue life prediction models. To reveal the debonding mechanism of fatigue damage in bitumen, a hierarchical multiscale modelling framework was developed in this thesis to understand and model the bonding performance of bitumen in order to accurately and reliably predict fatigue crack initiation and propagation in bitumen. At the nanoscale, the bonding performance of bitumen was characterised by bond energy using molecular dynamics (MD) simulations, which was validated by contact angle measurements. The MD modelling and simulations were performed to understand the effects of minerals and oxidative ageing on the bonding performance of bitumen at the molecular level. At the microscale, cohesive debonding performance of bitumen under a rotational shear fatigue load was first quantified by a DSR-C model to predict fatigue crack length. Then, based on the quantification of cohesive debonding behaviour at the nanoscale, an energy-based fatigue crack initiation criterion was developed for bitumen using a viscoelastic Griffith’s theory. At the macroscale, the cohesive debonding behaviour in bitumen was modelled for fatigue crack propagation by a pseudo J-integral based Paris’ law. Dynamic shear rheometer (DSR) tests were conducted to validate the developed models at different environmental and loading conditions. Results indicated that the cohesive bond energy predicted at the nanoscale can be used as a fundamental material property and scale-independent input for the cohesive debonding modelling for fatigue cracking of bitumen at the microscale. Adhesive bonding performance of bitumen with minerals is attributed to non-bond energy including van der Waals and electrostatic interactions. Bitumen oxidative ageing trends to strengthen interfacial electrostatic interaction since the introduction of oxygen atoms makes bitumen polarity stronger. The developed DSR-C model is capable of accurately predicting the fatigue crack length in bitumen. The energy-based crack initiation criterion along with the DSR fatigue tests can act as a substitute for surface energy tests. The crack propagation model based on the pseudo J-integral Paris’ law is able to characterise the fatigue crack evolution in bitumen. The Paris’ law coefficients A and n are temperature dependent fundamental material property, which are demonstrated to be independent of loading frequency or loading amplitude

Molecular dynamics investigation of interfacial adhesion between oxidised bitumen and mineral surfaces

The interfacial adhesion between oxidised bitumen and mineral surfaces at dry and wet conditions was investigated using molecular dynamics (MD) simulations. Molecular models were built for virgin and oxidised bitumen components including saturate, aromatic, resin and asphaltenes. The bitumen models and four representative mineral substrates (namely quartz, calcite, albite and microcline) were employed to construct bitumen-mineral interface systems. These models were validated by the experimental results and MD simulations reported in the literature. The hardening mechanism of the aged bitumen was analysed by comparing the density, cohesive energy density and fraction of free volume between the virgin and oxidised bitumen. Work of adhesion was computed to quantify the adhesive bonding property of the bitumen-mineral interface systems for the virgin, lightly oxidised and heavily oxidised bitumen models under dry and wet conditions. Results show that the oxidised products (carbonyl and sulfoxide) strengthen the intermolecular bonding, resulting in molecular aggregation and physical hardening of the aged bitumen. When bitumen becomes oxidised at the dry condition, the interfacial adhesion of bitumen-acidic minerals (quartz) is dominated by van der Waals interaction which decreases due to the increased bitumen-quartz intermolecular distance caused by the aggregated bitumen molecules during aging. In comparison, the interfacial adhesion of bitumen-strong alkali minerals (albite and microcline) is dominated by electrostatic energy which increases due to higher polarity introduced by the oxidised products. For the bitumen-weak alkali mineral (calcite), the interfacial adhesion is attributed to both electrostatic energy and van der Waals energy, where compared to the virgin bitumen, the electrostatic energy becomes lower for the lightly-oxidised bitumen due to the increased bitumen-mineral distance but becomes higher for the heavily-oxidised bitumen due to higher polarity. At wet condition, water is the dominating factor that affects (weakens) the interfacial adhesion between the bitumen and the acidic minerals (quartz), and the oxidative aging of bitumen is the major factor that affects (strengthens) the interfacial adhesion between the bitumen and the strongly alkaline minerals (albite and microcline). For the weak alkali minerals such as calcite, both water and bitumen aging can significantly affect the interfacial adhesion

Impact of minerals and water on bitumen-mineral adhesion and debonding behaviours using molecular dynamics simulations

This study aims to evaluate the effects of mineral types and water on the adhesion properties and debonding behaviours of bitumen-mineral interface systems. A molecular dynamics modelling approach was employed to simulate the interactions between minerals and bitumen with and without the presence of water. Four representative minerals (quartz, calcite, albite and microcline) were selected to build the mineral-bitumen interface systems and the mineral-water-bitumen interface systems in the molecular dynamics models. The adhesion property between minerals and bitumen was quantified by work of adhesion, defined as the energy required to separate a unit area of the bitumen-mineral interface. The debonding behaviour between minerals and bitumen is characterised by work of debonding, defined as the energy required to displace bitumen by water at the mineral-bitumen interface. The simulation results were validated by available experimental results reported in the literature. It was found that the work of adhesion and the work of debonding for the four bitumen-minerals interface systems are ranked microcline > albite > calcite > quartz at both dry and wet conditions. Moisture can reduce the adhesion between minerals and bitumen by 82%, 84%, 18% and 1% for the quartz, calcite, albite and microcline, respectively. The adhesion between minerals and bitumen is attributed to the non-bond interaction energy, in which the major component is van der Waals interaction for neutral minerals (e.g., quartz) and the electrostatic interaction for the alkali minerals (e.g., calcite, albite and microcline). The bitumen-mineral debonding is a thermodynamically favourable process with reduced total potential energy of the system. It is concluded that the bitumen-mineral adhesion and debonding behaviours strongly depends on the chemistry and mineralogical properties of the minerals. This work provides a fundamental understanding of the adhesion and debonding behaviours of the bitumen-mineral interface at the atomistic scale

Fast and Accurate Recognition of Chinese Clinical Named Entities with Residual Dilated Convolutions

Clinical Named Entity Recognition (CNER) aims to identify and classify clinical terms such as diseases, symptoms, treatments, exams, and body parts in electronic health records, which is a fundamental and crucial task for clinical and translation research. In recent years, deep learning methods have achieved significant success in CNER tasks. However, these methods depend greatly on Recurrent Neural Networks (RNNs), which maintain a vector of hidden activations that are propagated through time, thus causing too much time to train models. In this paper, we propose a Residual Dilated Convolutional Neural Network with Conditional Random Field (RD-CNN-CRF) to solve it. Specifically, Chinese characters and dictionary features are first projected into dense vector representations, then they are fed into the residual dilated convolutional neural network to capture contextual features. Finally, a conditional random field is employed to capture dependencies between neighboring tags. Computational results on the CCKS-2017 Task 2 benchmark dataset show that our proposed RD-CNN-CRF method competes favorably with state-of-the-art RNN-based methods both in terms of computational performance and training time.Comment: 8 pages, 3 figures. Accepted as regular paper by 2018 IEEE International Conference on Bioinformatics and Biomedicine. arXiv admin note: text overlap with arXiv:1804.0501

Modelling crack initiation in bituminous binders under a rotational shear fatigue load

This study aims to model fatigue crack initiation in bituminous binders. An energy-based crack initiation criterion is developed for bitumen under a rotational shear fatigue load. Based on a damage mechanics analysis of fatigue cracking process, the crack initiation is defined and local energy redistribution around crack tips due to ‘factory-roof’ cracking is quantified. A quantitative energy criterion is proposed for the fatigue crack initiation in the bitumen using viscoelastic Griffith’s theory. The crack initiation criterion is validated through comparing the predicted and measured surface energy of the bitumen. The results show that bitumen fatigue cracking under the rotational shear fatigue load can be divided into two stages: the edge flow damage and the ‘factory-roof’ cracking. The crack initiation is dependent of the shear modulus and surface energy of bituminous binders, critical crack size, and loading amplitude. The energy-based crack initiation criterion along with the DSR fatigue tests can be potentially used to determine the material surface energy

Crack Length Based Healing Characterisation of Bitumen at Different Levels of Cracking Damage

This study aims to characterise the healing properties of asphalt binders at different damage levels. The healing and the damage levels were quantified by crack length (CL) in binder samples generated by rotational shear fatigue loads in strain-controlled dynamic shear rheometer (DSR) tests. For comparison, the healing was also characterised by the commonly used material parameters including pseudo shear stiffness (S) and dissipated pseudo strain energy (DPSE). A normalized healing index was formulated using the above three parameters, respectively. A healing test of polymer-modified bitumen was designed based on DSR including a strain-controlled time sweep test plus a rest duration and followed by another strain-controlled time sweep test. The healing tests were performed at different rest start times (5min, 10min and 20min), rest durations (5s, 10s, 0.5min, 1min, 2min, 5min, 10min, 20min, and 40min), and amplitudes of the shear strain (5%, 7%, and 10%) at 20°C and 10Hz. Results show that the CL-based healing index is a fundamental and accurate parameter to evaluate the healing rate and healing potential of the bitumen. DPSE-based healing index is applicable only when the energy is mainly dissipated to generate cracks. Healing is underestimated when characterised using S or DPSE-based healing indices. Healing index increases due to the advance of rest duration or the decrease of the damage level, and the healing rate can be essentially modelled by Ramberg-Osgood model. Higher damage levels (introduced by higher load levels or longer loading time) can effectively decrease the binders’ healing potentials when the binders are at the relatively low damaged levels. The healing potential becomes low when the material is at severe damage state, thus will remain at that low levels even though the damage level increases