1. Introduction
The incorporation of biowaste enables the performance enhancement of asphalt at high and low temperatures, such as lignin waste, corn stover oil, and plant fibers, which have attracted widespread concerns for researchers [
1,
2,
3]. A vast amount of bagasse, a by-product of the sugar industry, is generated annually in the world. A majority of the bagasse is disposed to produce electricity by incineration or stockpiling along landfill sites, which ultimately threatens the atmospheric quality and occupies land resources [
4]. A growing aversion to pollution, together with concerns about the scarcity of resources, has motivated several countries to seek appropriate solutions regarding the utilization of sugarcane waste [
5,
6]. Except for biohydrogen production and ethanol production, bagasse has been mainly employed as a raw material for papermaking or prepared as fibers and then utilized in composite materials in recent years [
7,
8,
9,
10,
11]. In addition, bagasse has also been used in applications in the field of road engineering due to its low cost, high specific modulus, and acceptable mechanical properties [
12]. It has been verified that bagasse fibers and hemp fibers can both be used in asphalt mixtures, which can enhance the resilient properties of the asphalt mixtures and effectively improve the rutting resistance of pavement [
2,
13]. Nonetheless, bagasse fibers, as the binder retainer in polymer matrix composites, has faced some notable problems that need be solved urgently, such as high hygroscopicity, low thermal stability, and poor interfacial bonding with the matrix. Hence, it is necessary to further observe and improve the properties of bagasse fibers.
The interfacial adhesion between bagasse fibers and the polymer matrix plays a critical role in the properties of composites. Plant fibers are universally known to have strong hydrophilicity because of the quantities of hydroxyl groups on the surface, which leads to the incompatibility between the fibers and the hydrophobic polymer matrix [
14]. Moreover, pectin and waxy substances on the surface of fibers can prevent the interface reaction between fibers and the matrix [
15]. The incompatible interface affects the stress transfer of the polymer matrix to the fibers and reduces the performance of the composites [
16]. Morphologically, the main components of bagasse are composed of 30–50% cellulose, 19–54% hemicellulose, and 15–35% lignin, which means poor compatibility with the polymer matrix, similar to other plant fibers [
17]. Based on the above instances, it is vital to explore an appropriate treatment of bagasse fibers to fulfill its reinforcement effect in composites.
Currently, surface modification via chemical treatments is considered the most efficient strategy to promote the performance of plant fibers in composites. The popular methods involve alkalization, acetylation, benzoylation, silane treatment, coupling agent treatment, peroxide treatment, and isocyanate treatment, etc. [
15]. Conium maculatum fiber has been treated with alkali, silane, potassium dichromate, potassium permanganate, and silicone oil. The hydrogen-bond intensity (HBI) and the O/C atomic ratio of the fibers increased after treatment, and the highest increment was achieved by alkali treatment [
18]. Also, it was reported that fiber–matrix adhesion was promoted by fiber surface modifications, and better mechanical properties of hemp fiber-reinforced composites could be attained [
19]. Cornstalk fibers treated by sodium hydroxide solution can be a substitute for lignin fibers in asphalt pavement [
20]. Hence, combined with surface modification methods, plant fibers are widely used in composites owing to their advantages of being low-cost and environmentally friendly.
For demonstrating the modification mechanism and evaluating the effect of plant fibers in composite materials, several methods were extensively adopted to investigate the microstructure and chemical functional group of plant fibers as well as the performance of the composites [
17,
21,
22,
23]. In general, scanning electron microscopy (SEM) was conducted to observe the microstructure and the surface morphology of modified and un-modified plant fibers. Fourier transform infrared spectroscopy (FTIR) is widely used in cellulose research since it provides a relatively convenient approach to directly derive information about evolution during surface processing. In addition, an oil absorbing test was performed to examine the lipophilicity of fibers and evaluate the compatibility between fibers and the asphalt matrix [
24,
25,
26,
27,
28]. Furthermore, a variety of methods, including a cone penetration test, dynamic shear rheometer (DSR) test, and bending beam rheometer (BBR) test, were tentatively proposed to characterize the rheological properties of asphalt with and without fibers at different temperature ranges [
29,
30,
31,
32].
This work was initiated with the objective to study the physical and chemical properties of bagasse fibers before and after surface treatments and to analyze the effect of bagasse fibers on the properties of asphalt binders/mixtures. All bagasse fibers were prepared by a series of processing methods, including soaking, high-speed centrifuge machining, drying, and screening. The prepared bagasse fibers were modified by single, binary, and ternary methods with hydrochloric acid, sodium hydroxide, and sodium chlorite, respectively. Subsequently, the microstructure and properties of untreated and treated bagasse fibers were explored by SEM, FTIR, and an adsorption test. The rheological properties of asphalt binders with bagasse fibers or lignin fibers were analyzed by the DSR and BBR tests. Moreover, the reinforcement effects of bagasse fibers on asphalt mixtures were evaluated by a series of tests, including the wheel rutting test, bending test at a low temperature, and water stability test.
4. Conclusions and Future Trends
This paper observed the physical and chemical characteristics of treated and untreated bagasse fibers and analyzed the rheological properties of asphalt binders with and without various fibers. Subsequently, the performance of asphalt mixtures with fibers was evaluated. The following conclusions were obtained:
In accordance with the SEM images, it was verified that chemical modification could remove impurities on the surface of fibers, promote fibrillation, and make the fibers flexible. In addition, it was speculated that long-term heating may also damage the fiber structure and weaken the supporting function of the cell wall.
The result of FTIR revealed that sodium hydroxide had a significant impact on delignification, resulting in the presence of a cleaner cellulose phase with a high degree of polymerization, thus promoting the formation of a looser structure. Moreover, all the modification schemes could remove hydrophilic functional groups.
The result attained by the oil absorbing test showed that the five types of modified bagasse fibers exceed BF-1 regarding the value of the oil absorption ratio, which was caused by the loose structure of the fibers and partial removal of major components (i.e., cellulose, hemicellulose, and lignin). The increments were 15.0% for BF-2, 42.4% for BF-3, 31.6% for BF-4, 59.5% for BF-5, and 68.5% for BF-6.
In accordance with the DSR test, the asphalt binder with and without fibers, in terms of performance at a high temperature, ranked as follows: AB-BF-6 > AB-LF > AB-BF-5 > AB-BF-3 > AB-BF-4 > AB-BF-2 > AB-BF-1 > AB-0. The three-dimensional network comprised of fibers, and the SBS copolymer could hinder the fluidity of the asphalt. Meanwhile, the fibers could increase the stiffness of the asphalt binder by absorbing many asphalt components, enhancing the high-temperature deformation resistance of the asphalt binder, and the modification of fibers could also cause the promotion of comprehensive performance. The variation in the result of the BBR test was based on the same reason.
The road performance of the asphalt mixtures was significantly improved by bagasse fibers, confirmed by a series of comparative analyses. Modified bagasse fibers led to a much more significant enhancement in high-temperature performance compared to their impact on low-temperature performance. Furthermore, the ternary composite modification of bagasse fibers could effectively balance rutting resistance at a high temperature, cracking resistance at a low temperature, and the water stability of the asphalt mixtures. Overall, the performance of the asphalt mixtures with bagasse fibers by the ternary composite modification was close to that of lignin fibers.
In addition, the asphalt pavement was affected by complex factors, including loading, environment, and so on. The bonding mechanisms between the bagasse fibers and asphalt are a key issue that needs to be addressed for widespread applications of bagasse fibers. In the future, endeavors should be conducted to compare the performance of asphalt binders and asphalt mixtures with original bagasse fibers or modified bagasse fibers under hygrothermal environmental conditions, respectively. Furthermore, it is necessary to verify the pavement performance of asphalt mixtures with bagasse fibers in practical pavement construction and observe their long-term road performance.