Jiqing Zhu

This report presents the results of a project aimed at evaluating the performance of asphalt pavements incorporating bio‑extended bituminous binders, with a focus on long‑term structural behaviour and surface characteristics. The project focused on bio-extended penetration-graded bitumen containing Tall Oil Pitch (TOP) and a polymer-modified bitumen (PMB) with a biogenic component as alternatives to conventional bituminous binders. Both penetration-graded bitumen and PMB were investigated. The experimental program consisted of full‑scale accelerated pavement testing (APT) using a Heavy Vehicle Simulator (HVS) at the indoor testing facility of the Swedish National Road and Transport Research Institute (VTI), complemented by surface performance monitoring of a walking and cycling path constructed in Sundbyberg, Stockholm. Four full‑scale pavement structures, penetration-graded reference and its bio‑extended variant, as well as PMB reference and PMB with biogenic component, were constructed using asphalt mixtures produced in asphalt plants. The pavement structures were instrumented with asphalt strain gauges (ASGs) and temperature sensors to measure temperatures and tensile strains related to fatigue cracking. Rutting performance was evaluated through repeated loading at test temperatures of 10 °C and 20 °C. The APT results showed that the structures incorporating bio-based binders exhibited similar or improved resistance to permanent deformation compared with conventional reference structures, particularly at the elevated temperature of 20 °C. Measured tensile strains at the bottom of the asphalt layers were generally lower or comparable for bio-based variants, indicating favourable fatigue‑related condition. No operational challenges were observed during asphalt production, transportation, or paving, and the biobased mixtures demonstrated good workability and compaction characteristics. The walking and cycling path was monitored over a three‑year period to assess surface texture and roughness using Mean Profile Depth (MPD) and International Roughness Index (IRI). The measured values remained within the range of reference data for newly paved paths in Sweden throughout the monitoring period. Overall, the results demonstrate the technical feasibility of bio-based asphalt mixtures as a sustainable alternative for asphalt pavements without compromising performance.

The article investigates the chemical and rheological properties of bituminous binders extended with tall oil pitch (TOP), a bio-oil by-product from the pulp and paper industry. Bio-extended binders were formulated to incorporate different percentages of TOP, reaching the target penetration grades. Comprehensive chemical analyses, including Fourier-transform infrared FTIR spectroscopy and SARA (saturates, aromatics, resins, and asphaltenes) analysis by thin-layer chromatography with flame ionisation detection (TLC-FID), were conducted to investigate the compositional changes induced by TOP addition. Laboratory ageing of the binders was carried out. Rheological characterisation was performed using dynamic shear rheometer (DSR) to assess the impact on viscoelastic behaviour. The results indicate that TOP-extended bituminous binders exhibit distinct chemical signatures from bitumen, with notable differences in functional groups and SARA fractions. Furthermore, the incorporation of TOP changes the viscoelastic behaviour of binder. The specific impact and its extent seem to depend on the temperature and frequency range as well as the content of TOP in the binder. These findings are expected to contribute to the development of more sustainable asphalt pavements, highlighting the potential of TOP as a viable extender for bituminous binders.

This paper investigates the use of a plant-based bio-oil in bituminous binders as a partial replacement of petroleum-based bitumen for asphalt mixtures. Its effects on the ageing behaviour of bituminous binders and asphalt mixtures are studied. A total of six bituminous binders and their asphalt mixtures were prepared and analysed in laboratory, including three different binder formulations with varying percentages of bio-oil and their respective reference binders. Both the bituminous binders and asphalt mixtures were subjected to ageing protocols in laboratory. Softening point test, rheological and dynamic mechanical analyses were conducted to evaluate the changes in properties of the materials before and after ageing. The results indicate that the mechanical properties of aged binders and mixtures show very similar relationships as between the fresh materials, but the relationships after ageing are at changed levels due to the laboratory conditioning. This supports further studies to verify their functional performance in asphalt pavements.

The carbon footprint of bio-extended bituminous binders aiming for pavement decarbonisation was quantified using a “cradle-to-pavement” life cycle assessment. The binders were extended with a plant-based bio-oil. By biogenic carbon accounting, the impact of biogenic carbon flow was analysed through different approaches. The results indicated that using the bio-oil as a bitumen extender slightly increases the fossil global warming potential (GWP) of asphalt pavements, by about 1%. Both the absolute GWP levels and the effect of polymer modification on average annual GWP are variable, depending on the input data. When the “−1/+1” approach is adopted, two of the three analysed cases showed a lower average annual GWP of asphalt pavements with polymer-modified bitumen while one analysed case showed almost unchanged average annual GWP. All analysed cases confirmed that the biogenic carbon accounting by the “−1/+ X” approach reduces the total and average annual GWP significantly for the cases with biogenic components.

A two-level three-factor factorial design experiment was conducted to study the influences of three critical specimen preparation parameters on the measurement results of bitumen by a dynamic shear rheometer (DSR). The investigated factors were (1) the pre-heating temperature (HT) for manufacturing the specimen, (2) the bonding temperature (BT) onto the rheometer, and (3) the trimming (Trim) operation for preparing the specimen after bonding. The analysed data were the measured shear modulus |G*|, phase angle δ, and the characteristic temperatures of bitumen’s specific stiffness TX with corresponding phase angle δ_TX according to the European standard EN 14770:2023. Five types of bitumen were tested, including three penetration grades and two modified bitumen specimens (with polymer and wax additives). In addition, a repeatability evaluation of the test results was conducted. We found that the trimming operation for preparing the specimen has a noticeable impact when using smaller plates (PP08) for the DSR measurement. At higher test temperatures when using larger plates (PP25), the trimming operation does not significantly impact the measured parameters, in contrast to the HT and BT. Except for bitumen type 70/100, modified binders are more susceptible to variation in the analysed parameters than unmodified ones. The three-way interaction Trim:BT:HT tends to cause relatively little variation in measured data. Interactions between two factors Trim:BT, Trim:HT, and BT:HT contribute more to the fluctuation in δ value than in TX and |G*|. The variation employed in this study affects the test repeatability of wax-modified bitumen significantly; however, for unmodified binders the repeatability of TX and δ_TX are within 0.4–2.1 °C and 0.3–3.1°, respectively.

The Multiple Stress Creep and Recovery test is recognised to better characterise the high-temperature property of bituminous binders than the traditional methods, especially for polymer modified bitumen (PmB). However, some experimental parameters may affect the test results, such as the preloading, measurement location, stress level. In this study, the effects of these parameters were evaluated through an interlaboratory experiment. Four bituminous binders were short-term aged and analysed by five different laboratories. The variability in most results met the requirements of the European standard, while some failed to pass the American precision criteria. The preloading at 0.1 kPa with ten cycles was found to have a recordable impact on PmB samples, but a less significant effect was observed for the unmodified ones. With the measurement locations, the influence is limited compared to other parameters. As for stress sensitivity, higher stresses lead to remarkable differences.