Performance Analysis of Bio-Based Asphalt Mixtures Containing Lignin

Abstract

Given the need to promote the circular economy and sustainability, one of the main current trends in road materials construction is to employ industrial residues and by-products deriving from renewable sources as extender, replacement or modifier of bitumen, obtaining the so-called “bio-binders”. As regards, lignin can represent a potentially attractive solution, because it is the most abundant natural biopolymer, available in large quantity and characterized by certain chemical similarity with bitumen. In this context, this study focuses on the evaluation of two dense-grade asphalt mixtures for binder layer made with bio-binders containing two different lignins, as partial replacement of bitumen. A preliminary phase allowed to optimize lignin content (30% by bio-binder weight) based on empirical test (i.e. dynamic viscosity, penetration and softening point tests) with the aim of maximizing the bitumen replacement and at the same time obtaining two bio-binders having a consistency similar to a reference plain bitumen. In the second phase, two bio-based mixtures were produced by using the before-optimized bio-binders in different attempt contents. After mixing, specimens were produced by means of a gyratory compactor at set gyrations. Then, the two lignin-based asphalt mixtures were compared with the reference mixture in terms of workability, Indirect Tensile Strength (ITS) and water sensitivity. Despite the bio-based mixtures revealed a slightly penalized workability, overall results showed that they are characterized by fully comparable performances to the reference one, allowing a reduction of the effective bitumen content.

Investigation of unaged and long-term aged bio-based asphalt mixtures containing lignin according to the VECD theory

“…The results, summarized in Table 3, confirm the lower aging susceptibility of the mixture containing lignin, as demonstrated by the percentage increase of E 1 after aging (D = 7.3% for M50/70 vs. 1.8% for M70/100_S30). This preliminary finding could be attributable to the recognised antioxidant properties of lignin [18,59,60], to the lower content of effective bitumen in the presence of lignin [37] or to a combination of both effects.…”

“…The bio-binder was properly designed in order to have a similar consistency to that of the plain 50/70 pen grade bitumen commonly employed at Italian latitudes and used to produce the reference mixture M50/70. The characteristics of the reference bitumen and the lignin-based binder are listed in Table 1 [37].…”

“…The two mixtures differed for the binder content, which was optimised based on volumetric considerations, as described by Gaudenzi et al [37]. As a result of such optimisation, a bitumen content equal to 5.4% was adopted for the reference mixture M50/70, whereas a bio-binder content of 6.4% (corresponding to 4.6% of total bitumen plus 1.8% of lignin) was used to produce the lignin-based asphalt mixture M70/ 100_S30, with a consequent bitumen saving of 15%.…”

“…Within this context, this study focuses on the application of the S-VECD approach to compare the fatigue performance of an asphalt mixture containing lignin and a conventional asphalt mixture (selected as reference), both in unaged and long-term aged conditions. Binder phase analysis [36] and a conventional mechanical characterization [26,37] have been previously conducted on the same materials. Thus, a correlation between the S-VECD findings and the previous results gathered from the conventional approach was also sought.…”