Multiphysics Modelling and Mitigation of Ageing in Asphalt Pavements

Bitumen binders oxidise with air and cause ageing deterioration of asphalt pavements in the form of hardening, cracking and an overall decline in the mechanical performance. The degree of oxidative ageing is governed by many coupled internal and external physics and variables, making it difficult to predict. Mathematical models were established to represent the multiple physics that contribute to oxidative ageing of asphalt pavements. By coupling them together, these models can simultaneously simulate heat transfer, oxygen diffusion and oxidation kinetics to predict the oxidative ageing of asphalt pavements. The challenge lies in the non-linearity and circular dependency of these physics, making them difficult to solve and converge in numerical applications.The current study establishes a partial differential equation (PDE) based Finite Element (FE) modelling framework to solve these multiple physics using a weak form method. The framework is validated using field measurements of within-pavement temperatures and oxidative ageing products. A laboratory-based experimental investigation is conducted to select anti-ageing compounds (AACs)to mitigate oxidative ageing of bitumen binders effectively. Thin-film oven ageing and Fourier transform infrared spectroscopy (FTIR) tests are used primarily to select potential AACs. Detailed rheological and chemical tests are followed to detect any effects of these AACs on the long-term ageing performance of bitumen binders. Results indicate that the weak-form PDE-based model can effectively address the circular dependency among ageing-related Multiphysics. The model can reliably predict hourly profiles of temperature, oxygen pressure and oxidation products growth along the pavement depth, in different climate zones and for extended periods of field ageing.The air void content and distribution play a vital role in limiting oxidative ageing. Dense asphalt pavements with a low air voids content ( 9%) will have a full access to oxygen from the atmosphere, thus the average carbonyl content is high and uniform across asphalt pavement depth with no clear ageing gradient. A normalised carbonyl index (NCI) was proposed to quantify the oxidative ageing of bitumen modified by AACs. The activation energy of oxidation is found to be a suitable parameter to evaluate and assort the effectiveness of different AACs. Bitumen samples modified with 12% (1 furfural: 5 Irganox 1076), 15% Irganox 1076 and 3.5% (3 DLTDP: 4 furfural) demonstrated the best anti-ageing behaviour by retarding carbonyl content growth and decreasing the fatigue damage among selected AACs without sacrificing the stiffness of binder. The study represents a step forward in the understanding of long-term behaviour of asphalt pavements

An Equation-Based Multiphysics Modelling Framework for Oxidative Ageing of Asphalt Pavements

Long-term oxidative ageing occurs in asphalt pavements when they are exposed to the ambient environment for extended periods. This ageing phenomenon is dependent on multiple physical fields, including heat transfer, oxygen diffusion from air into interconnected air voids of asphalt pavement, oxygen diffusion from air void channels to asphalt mastic inside, and growth of oxidation products in bitumen. Most existing oxidative ageing models were established via coupling of limited physical fields. However, to accurately determine the oxidative ageing effect on pavement performance, there is a need to develop a multiphysics model that integrates all ageing-related physical fields comprehensively. The challenge lies in that the ageing-related physics are circularly dependent, time-dependent and highly nonlinear. This study developed a multiphysics and time-dependent finite element model that successfully addressed the issues of high nonlinearity and circular dependency of oxidative ageing in the asphalt pavements. Specifically, a differential equation-based approach was employed to efficiently couple the multiple physical fields into one integrated model. The multiphysics framework included a pavement temperature prediction model and an integrated ageing model. The model involved a variety of inputs such as site-specific hourly climate data, parameters for oxidation kinetics of bituminous binder, volumetric properties of asphalt mixture, thermal and diffusive properties of pavement materials, and pavement structure. The pavement temperature model was validated using the pavement temperature profiles for different climate regions in the Long-Term Pavement Performance (LTPP) database. The integrated ageing model was validated using the Fourier-transform infrared spectroscopy (FTIR) data of field-aged asphalt cores in the literature. Results showed that the model can accurately predict the change in pavement temperature profile on an hourly basis and reliably predict the degree of oxidative ageing across pavement depth for different climate zones

Intermediate pyrolysis of organic fraction of municipal solid waste and rheological study of the pyrolysis oil for potential use as bio-bitumen

This work presents a study on intermediate pyrolysis of the organic fraction of municipal solid waste (OFMSW) and characterisation of organic liquid product (pyrolysis oils) with particular focus on aging and rheological characteristics. The feedstock was a real municipal waste sample received from a local waste treatment plant. Shredded into small particles, it contained a high amount of moisture (51.2%) and ash (17.4%). A pilot-scale intermediate pyrolysis system was used to process the material. The process mass balance showed that the yield pyrolysis oil was 10.6%. GC-MS and FTIR experiments showed that the accelerated aging (80 °C for 24 h) did not cause an obvious change in the liquid chemical composition, but led to a significant reduction in the solids and moisture contents. The dynamic viscosity tests demonstrated that the intermediate pyrolysis oil derived from OFMSW is a non-Newtonian fluid. The dynamic viscosity of the pyrolysis oil reduced with the increase of temperature or shear rate, which can be modelled by WLF function and the Carreau model, respectively. A shear rate-temperature superposition method was proposed to construct the viscosity master curve at a wide range of shear rate, where WLF function was employed to model the shear rate-temperature shift factor. The accelerated aging caused an obvious reduction in dynamic viscosity, resulting from the decomposition of the semisolid organic agglomerates in the solids content during the aging of the OFMSW intermediate pyrolysis oil. The relatively high viscosity and reduced viscosity after aging of the OFMSW pyrolysis oil has indicated its potential for application as a substitute of the light fraction in the bitumen for road construction

Deriving and Characterising Alternative Bitumen from Waste Plastics

This study presents research on laboratory production and experimental characterisation of an alternative bitumen using municipal waste plastics. Six different waste plastics (A1 - A6) produced by a local waste recycling manufacturers were selected and characterised to investigate their feasibility in modifying the bitumen binders. Thermal characteristics were firstly obtained using Differential Scanning Calorimetry (DSC) device and the chemical functional groups were identified by Fourier Transform Infrared Spectroscopy (FT-IR) test to determine the plastic types existing in the recycled plastics. Then the rheological properties of the bitumen modified with two nominated plastic waste (A1 and A2) were examined using the Dynamic Shear Rheometer (DSR) device by conducting frequency sweep tests. Additionally, the engineering performance of waste plastics-derived bitumen was also obtained and compared against the control bitumen, including fatigue, rutting and healing performance using Time Sweep (TS) test, Multiple Stress Creep and Recovery (MSCR) test and Healing test, respectively. Results show that A1and A2 consist of low-density polyethene (LDPE) and polypropylene (PP), respectively. The recycled waste plastic A5 and A6 (both classified under the same category but collected from different plants and batches) are mainly consisting of LDPE. Whereas, other recycled plastics (A3 and A4) consist of a variety of materials and impurities. Thus, A1 and A2 were chosen as bitumen binder extenders. A1-modified bitumen exhibited more elastic and less viscous behaviour than the control bitumen, showed by increased shear modulus and reduced phase angle. Whereas, A2 (consisting of PP) caused a significant drop in the shear modulus. Both recycled LDPE and PP-modified bitumen had a substantially improved resistance to rutting and fatigue cracking compared to the control bitumen. Furthermore, waste LDPE-modified bitumen sustained increased healing potential compared to waste PP-modified bitumen, where the latter did not show noticeable improvement to the healing performance

Rheological and fatigue characterisation of bitumen modified by anti-ageing compounds

When exposed to the ambient environment for an extended period, bitumen ages and causes the failure of asphalt pavement, the addition of anti-ageing compounds (AACs) to bitumen binders can prolong the service life of the pavement. This study investigates the effects of anti-ageing compounds on the fatigue performance of bitumen when subjected to different ageing conditions. The AAC-modified bitumen binders were tested by dynamic shear rheometer (DSR) and Fourier transform infrared spectroscopy test (FTIR) at different ageing conditions including unaged, short-term ageing by thin film oven test (TFOT) and long-term ageing by pressure ageing vessel (PAV). The fatigue performance of the AAC-modified bitumen was characterised by the dissipated energy ratio (DER), SHRP fatigue parameter and the DSR-cracking (DSR-C) approach developed by the authors. Linear amplitude sweep (LAS) tests were firstly run at 20 °C, 10 Hz and 0.1–15% controlled shear strain conditions, to obtain the phase angles and shear moduli at the undamaged conditions. Time sweep (TS) tests were then conducted at 5% shear strain, also at 20 °C and 10 Hz, and up to 24,000 loading cycles to obtain phase angles, shear moduli and DER at damaged conditions. The crack lengths in the TS tests were calculated by DSR-C model and then validated by the image analysis method. The results suggest that, compared to the DER or SHRP fatigue parameter, DSR-C predicted crack lengths show a more consistent and reliable agreement with laboratory measurements. DSR-C test can differentiate fatigue cracking performance among binders modified with AACs, and the normalised carbonyl index may be a reliable parameter to reflect the impact of ageing products on the fatigue resistance of AAC-modified binders. Bitumen samples modified with 12% (1 furfural: 5 Irganox 1076), 15% Irganox 1076 and 3.5% (3 DLTDP: 4 furfural) demonstrated the best anti-ageing behaviour by retarding carbonyl content growth and decreasing the fatigue damage among selected AACs without sacrificing the stiffness of binder. However, rutting susceptibility at earlier stages of service life needs further investigations

Recovering the properties of aged bitumen using bio-rejuvenators derived from municipal wastes

Ageing of bitumen leads to significant performance deterioration of asphalt pavements and leads to material properties that are not conducive to recycling. Aiming to maximise the reusability of bitumen, this study investigated the feasibility of rejuvenating aged bitumen using bio-based rejuvenators synthesised from municipal wastes. Two bio-rejuvenators were used in this study, namely Rej-A which was a crude polymer with bio-waste pyrolysis dense fractions and Rej-B which was a filtered pyrolysis wax further derived from Rej-A. The bio-rejuvenators, virgin, aged, and rejuvenated bitumen were characterised using a comprehensive testing programme of gas chromatography and mass spectrometry (GC-MS), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), multiple stress creep and recovery (MSCR), linear amplitude sweep (LAS), and frequency sweep tests. It was observed that both bio-rejuvenators produced in this study can effectively recover the rheological properties of aged bitumen, improving its fatigue performance, e.g. the fatigue lives (at 15 % strain level) of Rej-A and Rej-B rejuvenated bitumen were 5.4 times and 3.0 times of that for aged bitumen when the dosage was 14 %. The rejuvenated bitumen was more sensitive to strain while less sensitive to temperature compared with virgin bitumen. Overall, Rej-A outperformed Rej-B in recovering the properties of aged bitumen. However, Rej-A was thermally unstable, undergoing 15.6 % mass loss when heated to 160 °C