Identification of Microstructural Components of Bitumen by Means of Atomic Force Microscopy (AFM)

Identification of Microstructural Components of Bitumen by Means ofAtomic Force Microscopy (AFM)

Andreas Jager 1, Roman Lackner 1, Christoph Eisenmenger-Sittner 2 and Ronald Blab 3

1 Christian-Doppler Laboratory for “Performance-Based Optimization of Flexible Road Pavements”; Institute for Strength ofMaterials, Vienna University of Technology, Karlsplatz 13/202, A-1040 Vienna, Austria; [email protected]

2 Institute for Solid State Physics, Vienna University of Technology, Wiedner Hauptstraße 8-10, A-1040 Vienna, Austria3 Christian-Doppler Laboratory for “Performance-Based Optimization of Flexible Road Pavements”; Institute for Road Con-

struction and Maintenance, Vienna University of Technology, Gußhausstraße 28/233, A-1040 Vienna, Austria

Accounting for the large variation of asphalt mixes, resulting from variations of constituents and composition, and fromthe allowance of additives, a multiscale model for asphalt is currently developed at the Christian Doppler Laboratory for“Performance-based optimization of flexible road pavements”. The multiscale concept allows to relate macroscopic materialproperties of asphalt to phenomena and material properties of finer scales of observation. Starting with the characterizationof the finest scale, i.e., the bitumen-scale, Atomic Force Microscopy (AFM) is employed. Depending on the mode of mea-surement (tapping versus pulsed-force mode), the AFM provides insight into the surface topography or stiffness and adhesionproperties of bitumen. The obtained results will serve as input for upscaling in the context of the multiscale model in order toobtain the homogenized material behavior of bitumen at the next-higher scale, i.e., the mastic-scale.

1 Introduction

The AFM, introduced by [Binnig et al., 1986], provides insight into the surface topography (nm resolution) and mechanicalproperties of bitumen. During AFM measurements, the specimen is scanned by a silica cantilever with a small tip placed nearits free end. The deflection of the cantilever, ws, describing the interaction between the AFM tip and the sample surface, ismonitored by an optical lever method. Based on the deflection ws and the stiffness k of the cantilever, the force acting at thetip is obtained as F = k ws. Depending on the mode of measurement (non-contact mode (NCM) versus pulsed-force mode(PFM)), either the surface topography or mechanical properties of bitumen can be identified. For the conduction of the AFMexperiments, the Topometrix Explorer [TopoMetrix, 1996] was used.

2 Presentation of results

Figures 1(a) and (b) show AFM plots from NCM measurements of B50/70 and B160/220 at room temperature. Similar toexperimental results presented in [Loeber et al., 1996], randomly distributed “bee-shaped” structures are observed for bothtypes of bitumen. The impression of “bees” stems from alternating higher and lower parts in the surface topography ofbitumen. The matrix phase, surrounding the “bee-shaped” structures, can also be divided into two subdomains, characterizedby a difference in the topography of 5 nm.

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In addition to the images given in Figures 1(a) and (b), the AFM gives access to characteristic dimensions, i.e., the maximumtopographical change ∆H and the distance between the higher parts within one “bee”, d (see Figure 1(c)). Whereas nocorrelation is found between the type of bitumen and the topographical change ∆H , the distance d between the higher partsin the “bee-shaped” structure is approximately the same for all considered types of bitumen, namely 550 nm.

Figures 2(a) and (b) show plots of the output quantities Vs and Vad obtained from PFM measurements for B70/100 atT = 10◦C, and Figure 2(c) shows Vs for B50/70 at T = 15◦C. According to these results, the four subdomains identifiedfrom the surface topography (Figure 1) exhibit different mechanical properties. Hereby, the parts of the surface with largerstiffness (see light areas in Figure 2(a)) show a lower adhesive behavior, and vice versa. Figure 2(d) shows PFM results

PAMM · Proc. Appl. Math. Mech. 4, 400–401 (2004) / DOI 10.1002/pamm.200410181

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


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for B50/70 using two different penetration depths. By increasing the penetration depth, the rather local hard-matrix phasesurrounding the “bees” becomes interconnected.

In order to gain insight into the stiffness properties of the four subdomains observed for both bitumens shown in Figures2(a) and (c), the relative stiffness, associated with the mean value of Vs over selected parts of the PFM plots, are given inFigures 3(a) and (b). The values corresponding to the soft part of the matrix and to the soft part of the “bees” are almost thesame. However, they were obtained from different areas of the PFM plots, separated by the hard-matrix phase surroundingthe “bee-shaped” structures. Figure 3(c) shows the mean values of the relative stiffness [%] obtained from averaging over thewhole scanning domain (50× 50µm). The order of the stiffness of non-modified bitumen (i.e. B90/10, B50/70, B70/100, andB160/220) obtained from PFM testing corresponds well to the penetration depth determined by conventional test methods.

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Fig. 3 Mean values of relative stiffness [%] of the four phases for (a) B50/70 and (b) B70/100; (c) average relative stiffness [%] forconsidered types of bitumen

3 Discussion and outlook

In order to establish the chemomechanical interaction between the (four-phase) structure observed at the bitumen surface andthe constituents of bitumen, the mechanical properties of the different constituents must be taken into account. According to[Shell-Bitumen-U.K., 1990], asphaltenes and resins behave as solid-like material, resulting in an increased stiffness. Hence,these larger-size molecules seem to form the stiffer parts observed during PFM testing (Figures 2(a) to (d)). The soft matrix,on the other hand, may be composed of the smaller-size saturates and aromatics. As regards the observed “bee-shaped”structures at the bitumen surface, PFM results characterized by an increased penetration of the AFM tip into the bitumen,indicate that the stiffer parts of the matrix are interconnected. This suggests that the “bees” observed during NCM testingbelong to string-like structures, emerging and immersing at the bitumen surface. Having identified the “bees” as part of stringsemerging and immersing from the bitumen surface, the fact of alternating topography within the “bees” is still unanswered. Apossible explanation is given by the arrangement of micelles, whereby the size of the “bees” is significantly larger than typicaldimensions of micelles given in the literature.

Finally, the mechanical properties obtained from PFM testing as well as the identified microstructure will serve as inputfor upscaling within the multiscale model for asphalt, providing insight into the effect of the bitumen microstructure on themacroscopic properties of asphalt.

Literature

[Binnig et al., 1986] Binnig, G., Quate, C., and Gerber, C. (1986). Atomic force microscope. Physical Review Letters, 56(9):930–933.[TopoMetrix, 1996] TopoMetrix (1996). Explorer instrument operation manual. TopoMetrix Corporation, Santa Clara.[Loeber et al., 1996] Loeber, L., Sutton, O., Morel, J., Valleton, J.-M., and Muller, G. (1996). New direct observations of asphalt and asphalt

binders by scanning electron microscopy and atomic force microscopy. Journal of Microscopy, 182:32–39.[Shell-Bitumen-U.K., 1990] Shell-Bitumen-U.K. (1990). The Shell Bitumen Handbook, volume 1. Shell Bitumen UK, Chertsey.[Rozeveld et al., 1997] Rozeveld, S., Shin, E., Bhurke, A., France, L., and Drzal, L. (1997). Network morphology of straight and polymer

modified asphalt cements. Microscopy Research and Technique, 38:529–543.


Section 9 401

Identification of Microstructural Components of Bitumen by Means of Atomic Force Microscopy (AFM)

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