A bio-based composite material to enhance sustainability in road infrastructure

QC 20260223

Abstract

This licentiate thesis investigates how forestry-derived biomaterials can reduce the fossil bitumen content of asphalt binders while maintaining functional performance. It focuses on lignin as a bio-based extender and on tall oil products as complementary softening bio-additives in a bio-composite binder. The work is motivated by two practical uncertainties in the literature: the ambiguous functional role of lignin in bitumen (often described as modifier-like or filler-like) and how this role affects the stiffness–flexibility trade-off.

The thesis addresses these questions through a combined approach: a systematic literature review and a targeted binder-scale experimental programme. The review confirms that lignin enhances high-temperature stiffness, rutting resistance, and ageing resistance, but it also identifies critical gaps: inconsistent mechanistic interpretation of lignin’s role, a lack of performance-balancing strategies, and insufficient comparative benchmarks.

Guided by these gaps, the experimental study evaluates a 70/100 paving-grade bitumen extended with 15 wt% kraft lignin (KL) or hydrolysis lignin (HL), using a limestone filler mastic (LSM) as an inert reference. Crude tall oil (CTO) and tall oil pitch (TOP PN) were assessed as secondary additives (5 and 10 wt%) in the KL-extended binder. Chemical and thermal analyses using Fourier-transform infrared spectroscopy and thermogravimetric analysis confirmed physical blending and thermal stability up to 190 °C for all binders. Rheological characterisation using dynamic shear rheometer and multiple stress creep and recovery testing revealed a clear functional distinction: KL behaved in a filler-like manner, showing a complex modulus and stress sensitivity very similar to the LSM mastic. In contrast, HL exhibited a modifier-like character, with significantly higher elastic recovery and lower non-recoverable creep compliance. Tall oil products acted as effective bio-fluxes; a 5 wt% dosage provided an optimal balance, improving workability and low-temperature flexibility while largely preserving the enhanced rutting resistance from KL. In contrast, a 10 wt% dosage, particularly of CTO, caused excessive softening, increased stress sensitivity, and a marked loss of high-temperature performance.

Overall, the thesis proposes a function-based framework for bio-composite binder design, where lignin type and tall oil dosage are selected according to their demonstrated role in the binder matrix, rather than treated as generic bitumen substitutes.

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