Evaluation of bio-binder modified asphalt’s adhesion behavior using sessile drop device and atomic force microscopy

Highlights

• Bio-binder would induce different influences on different asphalt’s adhesion properties.

• Adhesive forces obtained by using different AFM tips are different.

• Some connections are found between SFE data and AFM data.

• AFM can be regarded as a powerful method complementary to current contact angle measurement.

Abstract

Bio-binder is a renewable material which can be derived from natural resources. Applications of bio-binder in enhancing asphalt’s properties have gained profound progresses in past several years. However, limitation still exists in our knowledge on adhesion property of asphalt blended with bio-binder. This study aims to characterize adhesion behavior of bio-binder modified asphalt using sessile drop device (SDD) and atomic force microscopy (AFM). Relationship between the two test results is also investigated. It is shown that addition of bio-binder would introduce different impacts on surface properties for different base asphalts (Pen30 and Pen70 asphalts in this paper), which may be attributed to distinct compatibilities between bio-binder and different asphalts. And the work of adhesion data demonstrates that the bio-binder enhances the interaction between basalt and bio-binder modified Pen70 asphalt binders while depress the interaction between basalt and bio-binder modified Pen30 asphalt binders. It is also found that AFM adhesive force is tip-dependent which may be interpreted using different contact mechanics. In this paper, the high coefficient of variation in soft tip’s data may result from the capillary force and contamination during scanning. Interestingly, dispersive surface free energy and AFM results show some reasonable agreements for modified Pen70 asphalts, though there is still a gap between these two methods’ results. It is speculated that the variations between surface free energy method and AFM measurement can be attributed to the differences in probe substance, test scale and measured components. Findings in this paper not only promote the development of bio-binder modified asphalt, but also shed lights of the application of AFM in characterization of asphalt’s adhesion property.

Introduction

As a residue of petroleum industry, asphalt binder has long been used to bond aggregates in pavement engineering. However, petroleum is a non-renewable resource and may be exhausted in near future. In order to address this energy crisis, bio-binder has been developed to fully or partly replace traditional petroleum asphalt. In the past few years, efforts have been done to develop bio-binder from swine [1], wood waste [2], [3], waste cooking oil [4], [5] and other biomass [6]. It has been reported that incorporating bio-binder into asphalt not only lowers the construction cost, but also reduces the emission of greenhouse gas, which is both economical and environmental friendly [7]. Generally, bio-binder can be incorporated into asphalt in three ways: (1) as asphalt modifier (<10% asphalt replacement); (2) as asphalt extender (25–75% asphalt replacement) and (3) as direct alternative binder (100% asphalt replacement) [3]. Currently, research mainly focuses on using bio-binder as a modifier.

Previous research has provided thorough understanding on chemical and physical properties of bio-binder modified asphalt [1], [2], [3], [4], [5], [6]. It is found that bio-binder can be blended with asphalt homogeneously due to their similar elemental compositions. Compared with base asphalt, the performances of bio-binder modified asphalt have been significantly changed. The addition of bio-binder to conventional asphalt will reduce binder viscosity, which results in enhancing its workability [8]. Bio-binder also has the potential to improve the low-temperature properties of asphalt mixture, while the high-temperature properties will be deteriorated at the same time [9], [10], [11]. Besides, the addition of bio-binder will also significantly increase the asphalt mixture’s fatigue and cracking resistance, but slightly impact the tensile strength [11], [12]. Moreover, anti-aging performance of asphalt will also be improved using bio-binder modifier due to its functional groups [3]. Nevertheless, only a few researches have been conducted to investigate the adhesion behavior of bio-binder modified asphalt [13]. Surface free energy (SFE) method has been developed to characterize the adhesion work of asphalt binder-aggregate interface. And the atomic force microscopy is recommended as a feasible way to characterize adhesion because the contact between asphalt surface and tip can be used to simulate the interaction within asphalt-aggregate interface [14], [15]. Previous study also found that SFE results may have some correlations with AFM results [16]. However, no sufficient work has been conducted to look into this correlation. To this end, this paper prepared bio-binder modified asphalt samples using different dosages of additives. And contact angles between bio-binder modified asphalt samples and three probe liquids were measured using sessile drop device (SDD). Atomic force microscopy was also employed to acquire adhesive forces from each sample using silicon nitride tip to fabricate aggregate. Surface free energy and adhesive force data were then compared and the difference between these two methods was discussed herein.