Exploring fundamental aspects of bio-modified reclaimed asphalt binder

Abstract

Because of the increasing environmental awareness, reclaimed asphalt pavement (RAP) has become more and more important in road engineering as renewable source. Despite that, RAP cannot be used as it is because it is more brittle than virgin asphalt concrete thus it needs to be modified with additives known as rejuvenators. Modified reclaimed asphalt pavement is a novel composite material that has gained interest from some decades ago. This thesis explores the potential of using bio-based rejuvenators in RAP binder from different perspectives. Three different bio-based rejuvenators have been used: a sunflower seed oil which is known for its antioxidant properties, a cashew-nut shell based oil and a tall oil. The latter two are commercially available products commonly used in the asphalt industry as rejuvenators. In the first part of the thesis, I have evaluated the rheological as well the chemical properties of the bio-modified RAP binder before and after aging. By using SARA fractioning, I have highlighted how rejuvenators caused a change in RAP binder's polar/non-polar compositions. Nevertheless, from Fourier transform infrared spectroscopy I have detected how rejuvenators did not cause any changes at functional group level to the RAP binder. Analyses with the dynamic shear rheometer have shown how after the addition of rejuvenators an improvement in the mechanical performances of the RAP binder has been observed. Furthermore, I have shown how aging is an important parameter to be considered when evaluating the effectiveness of rejuvenators when studying the rheological performances. I have demonstrated how a holistic approach combing chemo-mechanical analysis is fundamental to understanding the behaviour of these bio-modified binders. In the second part of the thesis, I focused on the low temperature cracking performances of bio-modified RAP. High RAP mixtures are susceptible to cracking therefore; strong cracking resistance is desirable when producing such roads thus, it is fundamental for a bio-modified pavement to fulfil the required mechanical performances at low temperatures. By utilizing a so called fracture toughness test, I have observed how aging has a prominent effect on the fracture toughness. By combining rheological measurements and cracking experiments, the effectiveness of rejuvenators in restoring the properties of the RAP binder can be evaluated from different angles. In the third part of the thesis, I have analysed the microstructure of binders with atomic force microscopy (AFM). New features on the surface of the RAP binder and different mechanical properties have been observed when rejuvenators were added to the RAP binder. Quantitative nano-mechanical mapping (QNM) showed how aging caused a significant increase in terms of elastic moduli of all the binders. A qualitative correlation between bulk complex moduli (from rheological measurements) and phase images (from atomic force microscopy) has been shown. This correlation was found valid after the direct measurements of the elastic moduli at the samples’ surface with QNM. Furthermore, I have shown that the microstructures forming on the surface of the virgin binder due to wrinkling of the surface that has a higher measured modulus in comparison to the bulk. In the framework of this thesis, a multi-scale approach on the evaluation of rejuvenated binders has been proposed. It was demonstrated how a holistic approach is vital when solving questions addressing fundamental aspect of bio-modified reclaimed asphalt binder, thereby improving our understanding of challenges when producing high RAP mixtures and solutions to obtain the required performance of such mixtures.