Preparation and mechanical properties of asphalt mixtures reinforced by modified bamboo fibers

Highlights

• Green bamboo fibers (BFs) were used to reinforced asphalt mixture.

• A commercially practicable method was developed for modifying BFs.

• The modified BFs formed an superior interfacial adhesion with asphalt mixture.

• The modified BFs/asphalt mixture had increased tensile properties and stability.

Abstract

Cracking resistance and durability are the main concerns in asphalt mixture for pavement. The addition of fibers is an effective way for reinforcing asphalt mixture. Bamboo fibers (BFs) show unique characteristics such as high specific strength and modulus, renewability and low cost. Thus, BFs mechanically extracted from green bamboos were used to improve the mechanical properties of asphalt mixture and thus increase the service life of asphalt pavement. For improving the poor interfacial adhesion resulted from the hydrophobicity of asphalt and the hydrophilicity of BFs, a melamine–formaldehyde (MF) copolymer was synthesized and used to modify the BFs. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectronic spectroscopy (XPS), and nuclear magnetic resonance (NMR) analyses were conducted to reveal the surface chemistry of BFs. The results proved that the MF copolymer was grafted onto the surface of BFs via the formation of covalent bonding between hydroxyl groups of BFs and amino group of melamine. The mechanical properties of the asphalt mixture were evaluated by Marshall stability, direct tensile, and indirect tensile tests. Results showed that the addition of modified BFs increased the stability and the tensile strength of the asphalt mixture. SEM images of the fractured surface of asphalt mixture indicated that strong adhesion was formed between the modified BFs and asphalt matrices.

Introduction

Asphalt pavement is widely used in high-grade highways due to its comfortability for driving. However, asphalt pavement is prone to cracking due to its low tensile strength. Propagation of micro-cracks would reduce the service life of asphalt pavement [1], [2]. Incorporation of fibers into asphalt mixture is beneficial to the improvement of pavement strength and modulus. Steel fibers, glass fibers, basalt fibers, polymer fibers, and natural plant fibers have been investigated as reinforcements in asphalt pavements [1], [3], [4], [5].

Plant fibers have attracted much attention as asphalt mixture reinforcements. Research indicated that addition of cellulose fiber for hot-mix asphalt mixture (HMA) improved its penetration and softening point as well as the resistance to rutting of HMA [6]. The dynamic modulus and phase angle of asphalt mixture decreased due to the addition of cellulose fibers while the stiffness of fiber-modified asphalt mixtures could be reduced and the flexibility of mixtures was improved [7]. Aswathy et al. found that coconut shell charcoal could replace stone powder as a filler in stone mastic asphalt [8]. Oda et al. studied the application of natural fibers in stone matrix asphalt (SMA), and the results showed that the viscosity of asphalt mixed with natural fibers increased and the overflow of asphalt in the mixture was inhibited [9]. Aliha et al. concluded that the addition of plant fibers was beneficial to the improvement of low temperature performance of asphalt concrete [10]. Kar et al. determined that the optimal usage of sisal fiber in asphalt concrete was 0.3% for SMA mixture. And this mixture can be applied in flexible pavement [11]. The modification of plant fibers is usually carried out to improve their interfacial adhesion with asphalt matrix [12], [13]. The commonly used methods include physical treatment [14] and chemical modification [15]. Ghali et al. found that the mechanical properties and water resistance of the luffa fiber reinforced asphalt material were significantly improved after fiber treatment [16].

Green bamboo (Dendrocalamopsis oldhami Munro) is a fast-growing species and can be easily harvested for fibers extraction. BFs are potential reinforcements for asphalt mixture due to their intrinsic advantages such as low-cost, renewability, and high specific strength and stiffness [9], [17]. The properties of BFs-reinforced asphalt composites have been investigated [18], [19], [20]. However, the interfacial adhesion between BFs and asphalt matrices is poor due to the high hydrophilicity of BFs and the hydrophobicity of asphalt [21], [22]. This limits the reinforcing effects of BFs on asphalt mixture and application of BFs-asphalt composites in pavements.

Surface modification of BFs is an effective way to improve the interfacial adhesion between the fibers and asphalt. BF mainly consists of cellulose, hemicellulose, and lignin. The hydroxyl groups of BF cellulose can be grafted with a functional monomer or copolymer [23], thus a coupling agent is introduced between BFs and asphalt. In this study, a melamine–formaldehyde (MF) copolymer was synthesized and then used to modify BFs. The interaction between the MF copolymer and the BFs was characterized by FTIR, XPS, and NMR analyses. The mechanical properties of the BFs reinforced asphalt composites were evaluated. The interfacial adhesion between modified BFs and asphalt matrix was also studied.