Interfacial interactions and mass transfer at the interfacial region of bituminous hydrocarbon mixtures

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Interfacial interactions and mass transfer at

the interfacial region of bituminous

hydrocarbon mixtures.

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Interfacial interactions and mass transfer at the

interfacial region of bituminous

hydrocarbon mixtures

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 6 januari 2014 om 10.00 uur

door

Diederik Quirinus VAN LENT civiel ingenieur geboren te Rotterdam

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Prof. dr. S.J. Picken

Copromotor:

Ir. M.F.C. van de Ven

Samenstelling promotiecommissie:

Rector Magnificus, Technische Universiteit Delft, voorzitter Prof. dr. ir. A.A.A. Molenaar, Technische Universiteit Delft, promotor Prof. dr. S.J. Picken, Technische Universiteit Delft, promotor Ir. M.F.C. van de Ven, Technische Universiteit Delft, copromotor Prof. R.L. Lytton, Ph.D., P.E., Texas A&M University

Prof. dr. A. Scarpas, Technische Universiteit Delft Prof. dr. ir. S.M.J.G. Erkens, Technische Universiteit Delft Dr. ing. G.J.M. Koper, Technische Universiteit Delft

Prof. dr. ir. H.E.J.G. Schlangen, Technische Universiteit Delft, reservelid

Published and distributed by: Diederik van Lent

E-mail: d.q.vanlent@tudelft.nl, dqvanlent@gmail.com

ISBN 978-94-6203-509-6

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written consent from the publisher.

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Acknowledgements

During the experiments for my master thesis my supervisor asked me whether I would be interested in a follow-up research in the form of a PhD research. My first answer was that I had never thought about doing a PhD research. At the end of the research for my master thesis my supervisor asked me again to consider doing a PhD research. The result of this PhD research is presented in this dissertation. I realize that I am very fortunate because of this PhD research. I learned a lot and it was a very pleasant time. I would like to thank prof. dr. ir. Molenaar for the opportunity he has given me.

Furthermore I would like to thank prof. dr. ir. Molenaar and my other promotor prof. dr. Picken and my co-promotor ir. van de Ven for their support, advise and their time for guiding me through this PhD research.

This PhD research is partly financed by Kuwait Petroleum Research and Technology bv (KPRT). I am very grateful to them for this support. I would like to thank drs. Besamusca of KPRT for his always sincere interest in my research project and the vivid meetings. I would like to thank all employees of KPRT for their pleasant welcome and their help during the many months I spent performing experiments at their laboratory facility in Europoort.

The measurements of the surface energy of the Dutch road aggregates were performed as part of a project for BAM Wegen bv. They covered the expenses of my stay and the experimental program at Texas A&M University. I would like to express my appreciation to dr. ir. Jacobs and ir. Sluer (former BAM) also for their enthusiasm, vision and for offering me the opportunity to work at such a top level institution in a foreign country for three months.

Many experiments of this PhD research have been performed at other sections of Delft University of Technology. I would like to thank dr. van de Krol and ir. Didden from the section of Materials for Energy Conversion and Storage for their support and for allowing me to use their ellipsometer for the extensive experimental program. Thanks to dr. Poulis and dr. de Ruijter for allowing me to use their contact angle meter. I would like to thank ing. van der Linden for his help with the infrared measurements in chapter eight. I would like to thank ing. Lindenburg for allowing me to perform the contact angle measurements at their aircraft laboratory. I am very thankful that people from such a different scientific

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I would like to thank ing. van Vliet from research institute TNO for determining the molecular weight distribution of the bituminous samples by means of gel permeation chromatography.

I would like to thank all my present and former colleagues of the section of Road and Railway Engineering and of the section of Nanostructured Materials that helped me with the experiments or gave me such a pleasant time at Delft University of Technology.

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Summary

The adhesion between bitumen and aggregate is a complex process with numerous of variables. To improve the understanding of the bond between bitumen and aggregates in road applications, this research focuses on preferential adsorption, which is one aspect of bitumen-aggregate adhesion. The main objective is to find out whether there is preferential adsorption or segregation at the aggregate and the air interface. The investigation of the preferential adsorption was done on the bitumen as it is used in road applications. So in this research the bitumens were not diluted and the bitumens were not characterized by selected molecules in the bitumen.

To determine whether preferential adsorption can occur, the surface energy of common road construction aggregates was measured by means of sorption experiments. The measurements of the aggregates showed that the dominant polar component is significantly large enough. So if bitumen would consist of non-polar and polar components, then the potential interaction energy of the polar components will be larger than the non-polar components with the measured aggregates. The difference in interaction energy could lead to preferential adsorption of specific bitumen particles on the aggregates.

The next step was to determine the surface energy of bitumens and bitumen components. This was done by means of contact angle measurements. It was found that the components have a difference in surface energy relatively to each other. This means that preferential adsorption can occur and that, if preferential adsorption really takes place, the interaction energy at the bitumen surface could be different at different interfaces.

The adsorption of bitumen at an aggregate was investigated by means of refractometric measurements. Comparison of the calculated index of refraction of the not adsorbed reference bitumen with the measured index of refraction of adsorbed reference bitumen suggested that preferential adsorption took place on the crystal, but that it was certainly not clear. The index of refraction of bitumen at the crystal interface, at both 20°C and 90°C, has a high correlation with the penetration grade.

Ellipsometric measurements were used to investigate the bitumen-air interface. In general the bitumens showed a larger difference in results between the ellipsometric measurements and the refractometric measurements compared to

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bitumen-air interface in comparison with the bitumen-aggregate interface. It was found that the results of the ellipsometric measurements for bitumens were depending on the cooling rate of the bitumen samples during preparation and on time. The preparation temperature also showed to have effect on the results for some bitumens.

The difference found between the bitumen at the crystal interface and the bitumen at the air interface was investigated by means of Fourier transform infrared spectroscopy. A chemical characterization for the difference between the bitumen at the crystal interface and the bitumen at the air interface could not be made. The measurement inaccuracies made the chemical characterization of the reference bitumen at the air interface unreliable. Also no preferential adsorption of bitumen onto the crystal surface was observed with the infrared measurements

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Samenvatting

De adhesie tussen bitumen en toeslagmateriaal is een complex proces met vele verschillende invloeden. Om het begrip over de hechting tussen bitumen en toeslagmateriaal te vergroten, richt dit onderzoek zich op preferentiële adsorptie, een van de aspecten van de hechting tussen bitumen en toeslagmaterialen. Het hoofddoel is om te onderzoeken of preferentiële adsorptie of segregatie optreedt aan het steen- en het luchtoppervlak. Het onderzoek werd uitgevoerd op bitumen zoals ze ook gebruikt worden in wegenbouwtoepassingen. De bitumen zijn dus niet verdund en niet gekarakteriseerd door geselecteerde moleculen voorkomend in bitumen.

Om vast te stellen of preferentiële adsorptie plaatsvindt, is de oppervlakte-energie van gebruikelijke wegenbouwtoeslagmaterialen gemeten door middel van sorptie-experimenten. De metingen toonden dat de polaire component van de toeslagmaterialen groot genoeg is, zodat als bitumen uit niet polaire en polaire componenten bestaat, de potentiële interactie-energie van de polaire componenten groter zal zijn dan van de niet polaire componenten met de gemeten toeslagmaterialen. Het verschil in interactie-energie kan leiden tot preferentiële adsorptie van specifieke bitumendeeltjes op de toeslagmaterialen. De volgende stap was om de oppervlakte-energie van bitumen en bitumencomponenten te bepalen. Hiervoor werd gebruik gemaakt van contacthoekmetingen. Er werd gevonden dat de componenten onderling een verschil hebben in oppervlakte-energie. Dit betekent dat het mogelijk is dat preferentiële adsorptie plaatsvindt en dat, als preferentiële adsorptie werkelijk plaatsvindt, dat dan de interactie-energie aan het bitumenoppervlak verschillend kan zijn voor verschillende grensvlakken.

De adsorptie van bitumen op toeslagmateriaal is onderzocht door middel van refractometrie. Vergelijking van de berekende brekingsindex van niet geadsorbeerd referentiebitumen met de gemeten brekingsindex van geadsorbeerd referentiebitumen suggereert dat preferentiële adsorptie plaatsvindt, maar dat dit zeker niet overduidelijk is. De brekingsindex van bitumen aan het kristalopppervlak, op zowel 20°C als 90°C, heeft een hoge correlatie met de penetratie van de bitumen.

Ellipsometrische metingen zijn gebruikt om het bitumen-luchtgrensvlak te onderzoeken. In het algemeen hebben bitumen een groter verschil in de

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aan de grensvlakken. De meeste bitumen hebben aan het luchteroppervlak over het algemeen meer moleculen die een lagere brekingsindex geven, vergeleken met aan het kristaloppervlak. Er is gevonden dat de resultaten van de ellipsometrische metingen van bitumen afhankelijk zijn van de afkoelsnelheid van de bitumenmonsters gedurende de preparatie en van de tijd. De preparatietemperatuur had ook een invloed op de resultaten van sommige bitumen.

Het verschil dat gevonden is tussen bitumen aan het kristalgrensvlak en bitumen aan het luchtgrensvlak is onderzocht door middel van Fourier transform infrarood spectroscopie. Een chemische karakterisering voor het verschil tussen bitumen aan het kristalgrensvlak en bitumen aan het luchtgrensvlak kon niet worden gemaakt. De meetonnauwkeurigheid maakt de chemische karakterisering van bitumen aan het luchtoppervlak onbetrouwbaar. Met infrarood spectroscopie is er ook geen preferentiële adsorptie waargenomen van bitumen aan het kristaloppervlak.

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Chapter 1 Introduction

sphalt mixtures are composed of bituminous binder, sand, coarse aggregates and filler (mineral powder with a diameter smaller than 63 μm). The bitumen is the binder that holds all the other components together and ensures the cohesion of the mixture. Decomposition forces of asphalt mixtures in pavements originate from for instance traffic and weather conditions. With the increasing loads on roads and runways the binder becomes more and more important, especially when special structures and compositions of asphalt mixtures are used.

1.1 Porous asphalt mixtures

An asphalt mixture with a special composition with more than 20% of voids is widely used at Dutch highways. The main reasons for surfacing the primary road-network in Netherlands with this special type of porous asphalt concrete are traffic noise reduction and the significant reduction of splash and spray in wet weather conditions. However the service life of porous asphalt concrete is lower than the service life of dense asphalt concrete. The surface life of porous asphalt concrete is limited by the development of raveling at the surface. Raveling is defined as the loss of stones from the surface. It increases the roughness, may lead to dangerous driving conditions and increases the noise production. The bond of the individual aggregates to the rest of the pavement can break cohesively and adhesively. Cohesive failure occurs within the

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bituminous binder and adhesive failure is described as the loss of the bond at the interface of the bituminous binder and the aggregates (Molenaar 2009).

More strict requirements for the service life of top layers of asphalt roads cause larger risks for contractors. Therefore there is an increasing interest for development of road paving materials. In order to be able to develop road materials with enhanced characteristics, it is necessary to get a better insight into the processes controlling adhesion between bitumen and aggregates.

To understand the mechanisms leading to raveling, it is important that all the factors affecting the cohesive and adhesive performance characteristics of the materials are known. Furthermore the effects of traffic, environmental loadings, mixture composition and the physicochemical properties of the asphalt mixture on the performance of asphalt mixtures should be qualified and quantified (Woldekidan 2011).

1.1.1 Adhesive bond between bituminous binder and aggregate

The adhesive bond between bituminous binder and aggregates has been investigated extensively to reach to improvement of the bond between the bituminous binder and aggregates and to develop selection methods to reach to the best combinations of aggregates and bitumen types. The main challenge is that a bituminous binder is generally a non-polar material and the mineral aggregates are polar. The consequence is that the intermolecular interaction between water and aggregates is larger than the intermolecular interaction between bituminous binders and aggregates (Hefer and Little 2005). A chemical system will move towards a situation with the lowest possible potential energy. An interface of bituminous binder and an aggregate in contact with a water medium behaves the same. This means that the bituminous binder will be pushed away and replaced by water molecules at the mineral aggregate, deteriorating the bitumen-aggregate bond. This moisture damage phenomenon is called stripping (Lytton et al. 2005).

It is often reported that the resistance against stripping of bitumen-aggregate systems can be improved by adding anti-stripping agents (ASA) to asphalt mixtures. ASA have a special molecular structure as shown in figure 1.1. The polar head of the molecule gives a strong adhesion with the polar mineral surface and the non-polar tail has to ensure a strong bond with the non-polar bulk of the bituminous binder (Castaño et al. 2004).

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Figure 1.1: General molecular structure of anti-striping agents [Castaño et al. 2004]

Many anti-stripping agents contain a polar head with an amine group (NH2)

(Asphalt Institute 2002). These amine groups form a chemical bond with Si-OH groups at the surface of the mineral aggregate (Bagampadde et al. 2003). Some molecules in bituminous binders also contain such polar functional groups. Therefore it has been hypothesized that these functional molecules are preferentially adsorbed on the mineral surface. It has been shown that competition for a surface among different molecules has a large effect on the eventual result of the adsorption (Curtis et al. 1993). The combination of the anti-stripping agents and the bitumen functional molecules may negatively or positively influence the effective adhesive bond. However significant preferential adsorption has never been directly established in-situ in a bitumen-aggregate system. Preferential adsorption is expected to have a significant effect on the physical properties of the adhesive layer (Lee et al. 1990).

1.1.2. Mechanical modeling of asphalt mixtures

Knowledge of adsorption is of importance for mechanical modeling of asphalt pavements as well. Mechanical models for predicting raveling resistance of surface layers require advanced and accurate physicochemical properties of the adhesive zone between bituminous binder and mineral aggregates among many other parameters as shown in figure 1.2 (Woldekidan 2011).

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Temperature

Temperature Moisture

Moisture Cohesive

Physicochemistry Physicochemistry

Adhesive Mechanical loads

UV radiation UV radiation Mechanical loads

Ravelling Rutting

Bitumen content UV radiation Surface cracking Drainage Environmental loads

Compaction Temp. fluctuations Traffic loading Voids Moisture

Pavement deflection Aggregate percentage Boundary effects Geometrical effects

Response (Stress /Strain) Material Behaviour

Loads

Figure 1.2: Parameters for mechanistic modeling [Woldekidan 2011]

Recently at Delft University of Technology a meso mechanics based porous asphalt mixture performance design tool has been developed for predicting, among others, the raveling resistance. In this approach different geometrical models were developed, also for instance an idealized model with perfectly round and rigid spheres bound together via mortar bridges (figure 1.3). An adhesive zone was described with a thickness of 0.01 mm (Huurman 2008).

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Knowledge of the properties of the adhesion between bituminous binder and aggregates can help to improve to address moisture related damage in porous asphalt mixtures in a physicochemical way and it can also be used as input in meso mechanics based approaches in which the predicted performance of the porous asphalt surface is related to the performance of the constituent materials, mineral aggregates, bituminous binder and its adhesive zones.

1.2 Objectives of the research

The main objective of this research is the investigation of the adhesive zone between bituminous binder and mineral aggregates and specifically whether preferential adsorption takes place. The research will explore new physical methods for investigating adsorption. The measured property of the adhesive bond should give an insight in the physicochemical behavior of the adhesive region so that this adhesive zone at the mineral surface can be compared to for instance the bulk of bituminous binder and to the bituminous surface at the air interface. This will lead to an indication whether preferential adsorption could occur and also really occurs at the surface of the mineral aggregates in asphalt mixtures. This will lead to more knowledge of this one particular aspect of the complex process of the adhesion on bitumen and aggregate, which results to more understanding for improvement of the durability of the bond between bitumen and aggregates in road applications.

1.3 Organization of the dissertation

In this first chapter a general introduction is given on one of the major damage types of surfaces of porous asphalt pavements, raveling. It is shown that the adhesion between the bituminous binder and the mineral surface is important. In this introduction the objectives of this research are described.

In chapter 2 a literature review is given on the adhesion between bituminous binders and aggregates. The processes and mechanisms that result in or from adhesion are mainly related to the complex chemical structure of bituminous binders and the properties of mineral aggregates that affect bitumen-aggregate adhesion. The literature review is concluded with the description of important research done on the topic of adsorption of bitumen on aggregate surfaces. From this all, conclusions are drawn which resulted in the objectives and scope for the research conducted in this project.

In chapter 3 the research methodology based on the conclusions drawn from the literature review in the previous chapter is presented. The starting point, the limitations and the boundaries of the research are addressed. The bituminous

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binders that are used in the experimental program are presented together with a characterization of these materials.

Chapter 4 gives a literature review on surface energy. With the results of this literature review the experiments and methods are determined for the mineral aggregates, to investigate whether the difference in interaction energy of the aggregates could lead to preferential adsorption of certain components from the bituminous binder onto the surface of the aggregates.

In chapter 5 the results of surface energy measurements on a bituminous binder and its components are presented for evaluating the potential of preferential adsorption of certain bituminous components from a surface energy approach. Some nuances are given for the results which are based on a literature review and additional experiments.

Chapter 6 presents the experimental results of the refractometric measurements of the bituminous binder at the aggregate interface. The results are evaluated to establish whether preferential adsorption within a bituminous binder on an aggregate particle is shown.

In chapter 7 the results of ellipsometric measurements performed on bituminous binders at the air interface are given for establishing possible mass transfer at the bitumen-air interface. The results are compared to the results found for the bituminous binders at the aggregate interface from the previous chapter.

Chapter 8 presents a chemical characterization of the bituminous binder at the aggregate interface and at the air interface. These chemical characterizations are compared with the chemical characterization made for the bulk of the bituminous binder.

In the last chapter conclusions are drawn covering the whole research program. The possible differences between the refractometric measurements and the ellipsometric measurements are compared with the possible differences found for the chemical characterizations of bituminous binder at the aggregate interface and at the air interface. Finally some recommendations are formulated for further research.

References

Asphalt Institute (2002); Chemistry of asphalt aggregate interaction; Asphalt Institute Spring meeting; SL April 14, 2002

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Bagampadde, U., Isacsson, U. and Kiggundu, B.M. (2003); Fundamentals of stripping in bituminous pavements; State-of-the-Art; Research Report ISSN 1650-867X; Division of Highway Engineering, Royal Institute of Technology, Stockholm, Sweden.

Castaño, N., Ferré, P., Fossas, F. and Puñet, A. (2004); A real heat stable bitumen antistripping agent; Proceedings of the 8th Conference on Asphalt Pavements for Southern Africa (CAPSA), 2004

Curtis, C.W., Ensley, K. and Epps, J. (1993); Fundamental properties of asphalt-aggregate interactions including adhesion and absorption; SHRP-A-341; Strategic Highway Research Program, National Research Council, Washington, D.C., 1993 Hefer, A. and Little, D. (2005); Adhesion in bitumen-aggregate systems and quantification of the effects of water on the adhesive bond; report ICAR/505-1; Texas Transportation Institute; Texas A&M University System, College Station, Texas, USA

Huurman, M., (2008); ‘Lifetime Optimisation Tool’, LOT, Main report; report 7-07-170-1; Delft University of Technology, Delft, 2007

Lee, D.-Y., Guinn, J.A., Khandhal, P.S. and Dunning, R.L. (1990); Absorption of asphalt into porous aggregates; SHRP-A/UIR-90-009; Strategic Highway Research Program, National Research Council, Washington, D.C., 1990

Lytton, R.L., Masad, E.A., Zollinger, C., Bulut, R. and Little, D. (2005); Measurements of surface energy and its relationship to moisture damage; report FHWA/TX-05/0-4524-2; Texas Transportation Institute, Texas A&M University System, College Station, Texas, USA.

Molenaar, A.A.A. (2009); Structural Design of Pavements, part III, design of flexible pavements; Lecture notes CT4860; Delft University of Technology, Delft, 2009

Woldekidan, M.F. (2011); Response modelling of bitumen, bituminous mastic and mortar; Ph.D. dissertation; Delft University of Technology, Delft, 2011

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Chapter 2 Literature review on

adhesion between

bitumen and aggregates

n this chapter a literature review is given on the adsorption of bitumen and the adhesion between bitumen and aggregates. The literature review is started with a description of the complex composition of bitumen. This is followed by a treatise on the processes and mechanisms that result in or from adhesion. More attention is given to the properties of bitumen and aggregates that affect bitumen-aggregate adhesion. The literature survey is concluded with a description of important research done on the topic of adsorption of bitumen on aggregate surfaces. From this, conclusions are drawn which result in the objectives and scope for the research described in this thesis.

2.1 Bituminous hydrocarbon mixtures

Bitumen is a residue of fractional distillation of crude oil. It is considered to be a mixture of especially hydrocarbons. In general a small amount of functional groups containing sulfur, nitrogen and oxygen atoms can be found. The composition of a specific bitumen is dependent on the source of the crude oil and its refinery process. The elementary composition of bitumen in general terms can be given in ranges of 82-88% of carbon, 8-11% of hydrogen, 0-6% of sulfur, 0-1.5% of oxygen and 0-1% of nitrogen. Trace quantities of metals such as vanadium, nickel, iron, magnesium and calcium can also be found in bitumen (Read and Whiteoak 2003).

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The functional groups are the dipole parts of molecules in bitumen or the unsaturated molecules, e.g. double π-π bonding between oxygen and carbon atoms. Examples of these functional groups in bitumen are anhydrides, carboxylic acids, ketones, phenolics, polynuclear aromatics, pyridinics, pyrrolics, 2-quinolones, sulfides and sulfoxides (figure 2.1). Sulfoxides, anhydrides and ketons are rare in fresh bitumen. These three functional groups are mainly formed by oxidation during the lifetime of bitumen. On the other hand carboxylic acid for instance is found in both fresh and aged bitumen. The functional groups influence the adhesion between bitumen and aggregate, because the polar parts will aim and migrate towards the interface with the aggregate. In this way a layer is formed. The resulting bond strength is depending on this phenomenon (Jeon and Curtis 1990).

Figure 2.1: Examples of the important chemical functional groups present in bitumen molecules, (1) naturally occurring and (2) formed on oxidative ageing

[Jeon and Curtis 1990]

Long ago it was found that when quantities of alkanes were mixed with crude oils then precipitation occurs of black, friable solids. The molecules of these black solids have relatively a higher content of nitrogen, oxygen, sulfur and metals. It is also stated that these solids are more aromatic than the crude oil they precipitated from. These solids are called asphaltenes (Petersen et al. 1994).

The establishment of the precipitation of asphaltenes from the crude oil led to the assumption that the residue of crude oils has a colloidal system. The structure of bitumen as a colloidal system is extensively described by Read and Whiteoak

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(2003). In this colloidal structure micelles consist of high molecular weight asphaltenes. The micelles are surrounded and there by stabilized by high molecular weight aromatic resins (figure 2.2). The high molecular weight aromatic resins themselves are surrounded by less polar aromatic resins. These micelles are dispersed in a lower molecular weight oily medium. In bitumen with enough polar resins the micelles are well dispersed and have a good mobility. These bitumens are called sol type bitumen. In bitumen where the polar resins are not able to give the high molecular weight micelles enough mobility the micelles will not be dispersed. The asphaltenes are able to aggregate more resulting in an irregular structure of linked micelles. Bitumens with this behavior are called gel type bitumens. In practice most bitumens have an intermediate character between sol type and gel type (Read and Whiteoak 2003).

Figure 2.2: Colloidal system of a sol type bitumen (top) and a gel type bitumen (below) [Read and Whiteoak 2003]

The colloidal model for describing the structure of bitumen is also criticized (Petersen et al. 1994). The description of asphaltenes being highly polar

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molecules is not supported by everyone. The only heteroatoms that can give polarity to asphaltenes are oxygen and nitrogen. Average chemical compositions of asphaltenes only show about 5% of oxygen and nitrogen. From a chemical viewpoint the asphaltenes may not be considered polar, because they only carry a small polarity and only have small amounts of polarizable functional groups, although asphaltenes are surely more polar than n-heptane. The consideration of asphaltenes being non-polar molecules makes the colloidal structure of bitumen unlikely (Redelius 2009). By calculating the solubility parameters contributed by dispersion, polar and hydrogen bonding interactions of bitumen and asphaltenes, the solubility of the asphaltenes within bitumen was investigated (table 2.1). The calculated results show that there is not a large difference in the solubility behavior within the bitumen (figure 2.3). It was concluded that comparison of the solubility parameters for bitumen, asphaltenes, and maltenes confirms that asphaltenes are soluble within bitumen and that therefore bitumen should not be considered as a colloidal dispersion (Redelius 2007). A continuous model in which all bitumen molecules are soluble in each other and randomly distributed seems more appropriate. Asphaltenes do not form micelles or particles in bitumen. Bitumen is relatively homogeneous (Petersen et al. 1994, Redelius 2009).

Table 2.1: Calculated Hansen solubility parameters [Redelius 2009]

Sample δD MPa0.5 δP MPa0.5 δH MPa0.5 radius

Bitumen 18.4 3.9 3.6 5.8

Asphaltenes 19.6 3.4 4.4 5.3

Maltenes 17.7 5.8 2.5 6.7

n-Heptane 15.3 0 0

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Figure 2.3: Calculated Hansen Solubility parameters of asphaltenes, maltenes and n-heptane [Redelius 2009]

The nature of bitumens is affecting the viscosity and therefore also the wetting and adhesion properties of bitumens. The colloidal model surely allows possible aggregation on surfaces. However the continuous model does not exclude the possibility of preferential adsorption of certain molecules on surfaces. Although for instance asphaltenes are not polar in a chemical sense, they are surely more polar than n-heptane. Specific asphaltenes may also behave as polar molecules when they are in contact with polar molecules, like a polar solvent or polar surface (Redelius 2009).

Although asphaltenes are not always considered to be polar molecules, the surface tension of possible hydrocarbons in bitumen have a broad range. In general dispersion forces between spherical molecules are lower than between straight molecules. Larger hydrocarbon molecules have generally larger dispersive intermolecular forces than smaller hydrocarbon molecules. This is due to the fact that larger molecules consist of more electrons. This means that the polarizability of larger molecules is generally larger (Pieren et al. 1997). In a study the surface tension of bitumens was measured for instance by means of atomic force microscopy (figure 2.4). It was found that the surface tension of the

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bitumen samples was lower than certain types of molecules which are possibly also present in bitumen (Pauli 2008).

Figure 2.4: Surface tension of hydrocarbons in relation to the number of carbon atoms in the molecules [Pauli 2008]

By means of atomic force microscopy different molecules and phases were observed on the surface of bitumens (Masson et al. 2006, Jäger et al. 2004). With AFM the researchers found “bee” structures on the surface of the bitumen as shown in figure 2.5. The “bee” structures are symmetrically aligned with typical alternate dark and light lines. The strips were stated to be about 100-200 nm thick. The “bee” structures were only found in gel type bitumens (Loeber et al. 1996). The structures and morphology of the surface of the bitumen cannot be directly related to the composition yet (Masson et al. 2006).

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Figure 2.5: Bitumen surface observed with AFM in tapping mode [Loeber et al. 1996]

Even the surfaces of less complex hydrocarbon mixtures are not straightforward. It is determined that aliphatic hydrocarbon molecules in the liquid phase cannot lie on and also not parallel to the surface at the liquid-vapor interface. It is also believed that these hydrocarbon molecules cannot be randomly positioned at this surface. Hydrocarbon molecules preferentially expose their CH3 groups towards

the vapor phase and the degree of orientation depends on the length of the hydrocarbon chain. CH3 groups contribute differently to the overall value of the

surface tension of hydrocarbons than CH2 groups (Kloubek 1990). In general it

can be stated that molecules at the surfaces of liquids seem to be oriented in such a way that the least active or least polar groups are oriented toward the vapor phase. The free energy at the surface is reduced to a minimum (Harkins et al. 1917).

Bearsley et al. (2004) examined bitumens by means of confocal laser-scanning microscopy (CLSM). They observed fluorescing particles in a continuous matrix as shown in figure 2.6. The researchers believed that the fluorescing particles are asphaltene particles that are a dispersed phase in the continuous maltenes matrix. The size of the fluorescing particles was determined to be around 2-7 μm, similar to sizes of asphaltene particles reported by other researchers (Bearsley at al. 2004). However the particles are now believed to be wax crystals (Liu 2011).

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Figure 2.6: CLSM image of Safaniya 180/200 (left) and 80/100 bitumen (right) [Bearsley at al. 2004]

2.2 Saturates, aromatics, resins and asphaltenes

The complex structure of all the different molecules within bitumens brings us to the bitumen components. In order to bring structure within all the different molecules of bitumens a classification was made in four different bitumen components, saturates, aromatics, resins and asphaltenes (SARA).

An example to separate bitumen in these four fractions is given in figure 2.7. The asphaltenes are separated by adding a specific volume of n-heptane. The asphaltenes are per definition insoluble in n-heptane. This results in precipitation of the asphaltenes in the n-heptane mixture. The asphaltenes are now separated from the rest of the bituminous binder. The bituminous binder without asphaltenes is called the maltenes. The maltenes can be further separated in a column using a static chromatography medium, like silica gel or alumina powder. Elution with different liquids results in the separation of the maltenes. A first elution with n-heptane gives the saturates. After separation of the saturates, elution with toluene results in the separation of the aromatics. The column hold-up can be collected with elution with a mixture of toluene and methanol. The last collected bitumen components are the resins (Read and Whiteoak 2003).

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Figure 2.7: Separation of the different bitumen components [Read and Whiteoak 2003]

The saturates component of bitumen is described as aliphatic hydrocarbons (figure 2.8). This component is a straw or white colored non-polar viscous liquid. The aromatics component is a dark brown viscous liquid and it consists of non-polar carbon chains with a lot of unsaturated ring systems (figure 2.9). The resins component is also dark brown, but is more a solid or semi-solid material. They are described as polar and therefore strongly adhesive. The asphaltenes component (figure 2.10) is described as a black or brown amorphous solid and they are generally considered highly polar (Read and Whiteoak 2003).

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Figure 2.9: Molecular models of aromatics in bitumen [Read and Whiteoak 2003]

Figure 2.10: Molecular model of asphaltene in bitumen [Read and Whiteoak 2003]

The SARA classification of the molecules in bitumen is also criticized by other researchers. It is believed that the SARA classification leads to the idea that the four components are four different types of materials. Some researchers state explicitly that the SARA fractions are not four types of materials. They are only four fractions from bitumen from a continuous range of molecules. Because the four fractions are not a material, one should also not speculate about one particular molecular structure of, for instance, asphaltenes (Redelius 2009).

2.3 Adhesion between bitumen and aggregate

The maximum range of influential effects of intermolecular forces is in the order of 30 Å. Roughness on the finest mechanically treated surfaces still exhibits peaks and valleys of about 250 Å. In practice the roughness results therefore in only partial contact between two solid surfaces as presented in figure 2.11. Adhesion between the solid surfaces therefore requires binder material that can penetrate into the surface texture irregularities (Schliekelmann 1970). The binder can be pulled into pores. Absorption refers to the phenomena in which molecules of one phase (for instance the binder) dissolute with or into another phase. On the other

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hand adsorption is the accumulation of molecules of one phase onto the surface of another phase. For this reason adhesion of fluids should not be viewed as a static situation but as dynamic. Also during adhesive contact molecules are in random motion because of Brownian movement. In fluids the molecules wander around at high speeds in all directions and in solids the motion is limited, but the localized vibrations at the surface are still important (Kendall 2001).

Figure 2.11: Apparent non-adhesion because of incomplete contact between two different solid surfaces [Kendall 2001]

The adhesion between bitumen and aggregates is influenced by the viscosity and the bitumen properties mentioned in the previous paragraphs, surface energy and chemical composition. The aggregates in the asphalt mixture also have a large influence on the adhesion in an asphalt mixture. In this paragraph important properties of aggregates are described that have an influence on bitumen-aggregate adhesion (van Lent 2008). In the next section four general theories of adhesion are described.

2.3.1 Adhesion theories

Several theories of adhesion exist. Some theories combine elements from other theories and others use the same elements, but define it differently as a whole. In this paragraph four major theories for bitumen-aggregate adhesion are described, first the chemical reaction theory, followed by the molecular orientation theory, the thermodynamic approach and the mechanical theory (Kanitpong and Bahia 2003). This paragraph concludes with some observations concerning the bitumen-aggregate adhesion theories.

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2.3.1.1 Chemical reaction theory

This theory is based on the supposition that the adhesion between bitumen and aggregate is caused by chemical reactions (Kanitpong and Bahia 2003). After coating the aggregate with bitumen, adsorption of the bitumen chemical active components on the surface of the aggregate takes place. Especially during the high temperature of mixing the polar molecules in bitumen could displace the weaker adsorbed non-polar molecules of the bitumen on the aggregate surface (Hefer and Little 2005). The chemical active components from both the bitumen and the aggregate interact, resulting in chemical bonds between the bitumen and the aggregate. The bond caused by the chemical reactions cannot be broken without damaging the bulk of one or both materials (Bagampadde et al. 2003).

2.3.1.2 Molecular orientation theory

In this theory the functional groups from both the bitumen and aggregate cause an adhesive bond. The bitumen functional groups aim and migrate towards the aggregate surface, resulting from the electric field caused by the dipole charges of the functional groups on the aggregate surface. The bitumen functional groups align in accordance with the electrical field around the aggregate (Bagampadde et al. 2003). The level of adsorption and desorption of bitumen functional groups is influenced by the distance from the particle and the magnitude of the dipole charges. At close distances of the aggregate surface the bitumen functional groups with the highest potential will form a layer. The potential of the aggregate surface decreases with increasing distance from this surface, causing lower charged bitumen functional groups to form a less firmly attached layer around the first layer of bitumen functional groups. The layer with the stronger bond is called the Stern layer and the secondary layer is called the Gouy-Chapman layer (Nguyěn 2007). The adhesive bond caused by electrostatic and hydrogen bonding is lowered when water is present at the interaction layer. Water is more polar than most of the bitumen functional groups and molecules. Therefore a preference of the aggregate surface for water above bitumen exists. This provides a possible explanation for stripping (Bagampadde et al. 2003).

2.3.1.3 Thermodynamic approach

This approach for the adhesive bond strength between bitumen and aggregate is based on the change in interfacial energy of the bitumen after coating the aggregate (Bagampadde et al. 2003). This approach combines elements from the chemical reaction and the molecular orientation theory. After wetting the aggregate with bitumen, intermolecular forces at the aggregate surface and the bitumen interact, resulting in a released bonding energy. To break the bond

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between the aggregate surface and the bitumen energy is needed. This needed work to be applied equals the change in energy before and after coating the aggregate with bitumen. This work needed to break the bond is a measure for adhesion of the bitumen on the aggregate surface (Hefer and Little 2005).

2.3.1.4 Mechanical theory

The main assumption for this theory is that, during coating the aggregate, bitumen enters present pores, holes, cracks and unevenness in the aggregate surface texture (Nguyěn 2007). After hardening of the bitumen, the adhesive bond is caused by the surface friction between bitumen and aggregate surface. The theory may provide an explanation for the stronger adhesive bonds of rougher aggregate surface textures. First of all, more irregularities provide more interlock for the bitumen (Bagampadde et al. 2003). Secondly, a rougher aggregate surface has a larger physical area of contact and so provides a larger surface friction between bitumen and aggregate surface. Due to the roughness of the aggregate surface the stresses in the bitumen coating are redistributed as well. This results in less peak stress concentrations and so a decrease in the chance of rupture. However, the theory also states that with an increasing roughness of the aggregate surface, the wetting with bitumen becomes more difficult and the wetting could become incomplete. This may result in a less strong adhesive bond (Hefer and Little 2005).

Until now, it is not possible to state that one of the theories is flawed or that one theory describes the entire process of bitumen-aggregate adhesion. It is generally thought that the phenomena of adhesion are a combination of the four described bitumen-aggregate adhesion theories. Therefore adhesion should be explained by (all) elements mentioned in the four described bitumen-aggregate adhesion theories (van Lent 2008).

2.3.2 Parameters affecting adhesion in asphalt concrete

Asphalt concrete consists of two major components, bitumen and mineral particles (coarse aggregate, sand and filler). In the beginning of this literature review the most important influences of the bitumen affecting adhesion are given. Now attention is paid to important properties of the mineral particles.

2.3.2.1 Mineralogical composition

The aggregate mineralogy affects the adhesion in asphalt mixtures. The mineralogy of the aggregate affects the relative affinity between water and bitumen at this surface. Therefore the mineralogical composition is one of the major factors affecting stripping. Most natural aggregates are composed of a

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combination of different minerals. The most important minerals found in aggregates are silica minerals, feldspars, ferromagnesian minerals, carbonate minerals and clay minerals (table 2.2) (Bagampadde et al. 2003).

Table 2.2: Examples of the important minerals present in aggregate and examples of the major groups of rocks [Roberts et al. 1991]

Minerals Igneous Rocks Sedimentary rocks Metamorphic

rocks Silica Quartz Opal Chalcedony Tridymite Cristobalite Silicates Feldspars Ferromagnesians Hornblende Augite Clay Illites Kaolins Chlorites Montmorillonites Mica Zeolite Carbonate Calcite Dolomite Sulfate Gypsum Anhydrite Iron sulfide Pyrite Marcasite Pyrrhotite Iron Oxide Magnetite Hematite Goethite Ilmenite Limonite Granite Syenite Diorite Gabbro Peridotite Pegmatite Volcanic glass Obsidian Pumice Tuff Scoria Perlite Pitchstone Felsite Basalt Conglomerate Sandstone Quartzite Graywacke Subgraywacke Arkose Claystone, siltstone argillite, and shale Carbonates Limestone Dolomite Marl Chalk Chert Marble Metaquartzite Slate Phyllite Schist Amphibolite Hornfels Gneiss Serpentine

The mineralogical composition affects the surface texture of the aggregate. Many minerals show different kinds of texture when fractured. Quartz has a conchoidal fracture. This fracture has the appearance of series or arcs. Native metals such as copper have a hackly fracture. Other types of fracture are even and uneven fracture (Pearl 1955).

The mineralogical composition of aggregates depends on the origin of the rocks. On earth, rocks can be divided in three major groups: igneous, sedimentary and metamorphic rocks (Roberts et al. 1991). Igneous rocks have their primary origin in molten rock within the earth’s crust, called magma. Igneous rocks are intrusive when the magma was intruded or was forced into other rocks underneath earth’s surface. The term extrusive is used when the molten rock is forced to the surface resulting in lava flows or volcanic fragment blow out. Sedimentary rocks are formed by accumulation of erosion and weathering products of crumbled and decomposed rocks. The last group, metamorphic rock, contains igneous and sedimentary rocks, which have been drastically conversed, so that the nature and original state of the rocks are lost. Heat and pressure in the

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earth’s crust are contributors for creating metamorphic rocks and the formation of entirely new minerals (Pearl 1955).

2.3.2.2 Chemical composition

The chemical composition gives a detailed view of adhesion of bitumen onto the aggregate surface. After coating the aggregate with bitumen, the bitumen will be chemically bonded with the aggregate surface as a result from chemical reactions at the interface. It is argued that the level of polarity of the aggregates has a strong influence on these chemical reactions (Read and Whiteoak 2003).

Arrhenius acidic aggregates are considered to be hydrophilic. Hydrophilic, water loving, aggregates have a greater affinity for water than for a bitumen coating. So if such an aggregate, coated with bitumen, is immersed in water the aggregate tends to have a stronger attraction to the water molecules than to its bitumen layer. Through cracks water might contact the interface layer of the bitumen and the aggregate. The bitumen might be ‘stripped’ from the surface of the aggregate and replaced by the water in time. Hydrophobic, oil loving, and basic aggregate appear to have a better adhesion with the bitumen coating, and have a little more resistance to stripping. In most studies this has been confirmed, but not in all (Nguyěn 2007, Tarrer 1991).

It is found that the presence of certain metal components on the aggregate surface could be beneficial to adhesion and the presence of other metal components could be detrimental. Iron, calcium, magnesium and to some extend aluminum on the aggregate surface is sometimes beneficial. Alkali metals like potassium and sodium are considered to be detrimental. Because no aggregate surface is totally acidic or basic, the aggregate surface is always to some extend acidic and to some extend basic. Acidic parts of the surface could become negatively charged in water and basic parts positively charged. Minerals at the surface containing silica could become negatively charged, minerals containing the metals iron, calcium, magnesium and aluminum positively charged. The theory is that with the presence of both positively and negatively charged minerals present on the surface, a possibility exists to form salts with the bitumen functional groups. Insoluble salts in water give a better adhesive bonding. Soluble salts present on the aggregate surface weaken the adhesive bond, because the salts are easily dissolved. Soluble salts are the result of a reaction of acidic functional groups in the bitumen with potassium and sodium (Bagampadde et al. 2003).

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As mentioned before, aggregates are never totally acidic or basic. The aggregate surface is always to some extend acidic and to some extend basic. The silica content of some aggregates is shown in figure 2.12 (Mertens and Wright 1959).

Figure 2.12: Classification of aggregates by surface charges [Mertens and Wright 1959]

2.3.2.3 Surface energy

Directly related to the chemical and mineralogical composition is the surface energy of the aggregates. The intermolecular forces at the surface of the aggregates are able to interact with the molecules from the bitumen resulting in an adhesive bond. Different chemicals have different types of bonding mechanism for interaction with the molecules in the bitumen. For instance the polar molecules on the surface of the aggregates are more able to interact with the functional groups in the bitumen. Non-polar molecules at the surface are more able to interact with non-polar molecules in the bitumen. Sandstone has a silica (SiO2) content between 60 and 100 %. In literature it is found that silica

shows a mono-polar Lewis base character. This is of influence for the selected bitumen type and their resulting work of adhesion (Little and Bashin 2006). In the next chapter a literature review on surface energy is presented for a more detailed description.

2.3.2.4 Weathering

Weathering depends on the chemical and mineralogical composition. Some research has already been done on relating weathering to adsorption of bitumen and surface energy of the aggregates, but no clear results were found (Little and Bashin 2006). It is thought that freshly sawed aggregate surfaces have more free radicals to interact with the bitumen functional groups (Tarrer 1991). Weathering occurs to different extents. When aggregates are stored outside under influence of rain, frost and sunlight disintegration may take place (Roberts et al. 1991).

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When aggregates are protected from direct sunlight, frost and rainfall this weathering process will almost not occur. Only a quick ageing of the freshly sawed surface of the aggregate under influence of the air occurs. Some aggregate ageing effects, such as atmosphere contaminants accumulation and oxidation of organic debris, occur within few seconds after sawing (Little and Bashin 2006). In practice weathered aggregates are considered to be superior to freshly cleaved aggregates in pavements (Tarrer 1991).

2.3.2.5 Surface texture

During mixing of the asphalt mixture, surface texture influences proper coating of the aggregate with the bitumen binder. At first a smooth surface texture may be easier to coat with a proper binder film, but the mechanical bond is less in comparison to a rougher surface texture. A rougher surface has a larger surface area per unit mass, viz. specific surface area, resulting in stronger adhesive bonding. However when the aggregate is too rough it is possible that not the whole surface of the aggregate is coated. Then the contact area between the bitumen and aggregate is decreased resulting in a weaker adhesive bond (Roberts et al. 1991). In general one can say that roughness is always acting against you, whether making or breaking a bond (Kendall 2001).

2.3.2.6 Size

Aggregate particles for road construction in asphalt mixtures are normally smaller than 22 mm. For reasons of processability and aggregate shape, the maximum size of aggregates is limited. The size of crushed stone is limited to 16 mm for surface layers and to 22 mm for intermediate and lower layers. The finest aggregate particles, the filler, may approach sizes smaller than 2 μm. For larger aggregate sizes the surface texture must increase to ensure such a surface area per unit mass that a good adhesive bond is obtained (Nguyěn 2007).

2.3.2.7 Shape and angularity

Like surface texture, the shape of the aggregates affects the coating process of the bitumen binder onto the aggregates in the asphalt mixture during mixing. Rounder aggregate particles, such as most natural gravels and sands, are coated easier with a bitumen binder than angular shaped aggregates (Roberts et al. 1991). During service life angularity may offer good points of anchoring for the bitumen binder to improve adhesion. It is known however that an increased angularity also increases the probability of puncturing the bitumen film. In this way water can easier penetrate the layer of adhesive bonding and possibly cause stripping (Masad et al. 2006).

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2.3.2.8 Porosity and pore size distribution

A pore size distribution in which many pores are located at the surface of the aggregate, results in a rougher surface texture and so a larger surface area per unit mass to ensure stronger adhesive bonding. A high porosity at the surface also affects absorption of the bitumen binder. Absorbed bitumen is forced into the pores and is locked in, causing an even stronger adhesive bond. A disadvantage is that the amount of absorbed bitumen in the pores of the aggregate is not available for proper coating of the exterior of the aggregate. This amount could be compensated with an extra amount of bitumen added during mixing (Bagampadde et al. 2003). A too high porosity of the stone can however cause another disadvantage. With a too high porosity of the stone not all pores might be able to adsorb the bitumen. Air and water can be trapped inside the pores, causing serious detachment problems during service life (Tarrer 1991, Bagampadde et al. 2003).

2.3.2.9 Aggregate surface impurities

Deleterious materials on the aggregate surface affect to some extent the adhesion between bitumen and aggregate. These surface impurities include clay, dust (e.g. from aggregate crushing), coal, shale, free mica, salts and vegetation (Roberts et al. 1991, Bagampadde et al. 2003). These impurities can inhibit direct bonding between the aggregate and bitumen binder. In this way no proper adhesive bond is ensured (Bagampadde et al. 2003).

Another property of deleterious material that affects adhesion in asphalt mixtures is the tendency to attract water. By attracting more water the probability of stripping increases. Deleterious materials may also result in channels and trapped air on the aggregate surface causing water to penetrate more easily (Tarrer 1991).

2.4 Bitumen-aggregate adhesion experiments

Different approaches have been followed for the investigation of the adhesion between bitumen and aggregates. Mechanical tests can be performed on mixtures to investigate the susceptibility to water. The wetting behavior of bitumen on aggregate surfaces can also be evaluated in a physical way under an atmosphere of air and water. Another approach is to look at the adhesion and disbonding of bitumen and aggregates from a chemical viewpoint. In this approach the physical or chemical characteristics of the components and their effect on their mutual interactions and adsorption processes are investigated for evaluation of the adhesion (Scott 1978).

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Table 2.3: Summary of selected adhesion experiments related to water damage [Bagampadde et al. 2003]

Test type Examples

Dynamic immersion tests Nicholson test

Dow and Tyler wash test Rolling bottle test

Static immersion tests ASTM D-1664, AASHTO T182

Lee test

Holmes water displacement Oberbach test

German U-37 test

Boiling test ASTM D-3625

Riedel and Weber test Chemical immersion tests Riedel and Weber test

Abrasion tests Cold water abrasion tests

Abrasion-displacement tests Surface water abrasion tests Simulated traffic tests English trafficking tests

Test tracks

Quantitative coating evaluation tests Dyne adsorption test

Mechanical integration method Radioactive isotope tracer technique Tracer-salt tests

Light-reflection method Net adsorption test

Non-destructive test Sonic tests

Resilient modulus tests Immersion-mechanical tests Marshall immersion test

Moisture vapor susceptibility test Water susceptibility test

Lottman tests

Immersion-compression tests Freeze-thaw pedestal tests

Environmental conditioning system Tunnicliff and Root test

Miscellaneous tests Detachment tests

Briquette soaking test Swell tests

Stripping coefficient measurement Peeling test

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Bagampadde et al. (2003) list numerous experiments, mainly focusing on the adhesion and its susceptibility to moisture and water of separate asphalt components and of compacted asphalt mixture samples (table 2.3). Most experiments from this enumeration can be classified in the mechanical and the physical evaluation of bitumen-aggregate adhesion. This current research is more focused on contributing to understanding the phenomena involved in bitumen-aggregate adhesion. In the following paragraphs detailed descriptions are given of some important, more fundamental, investigations on bitumen-aggregate interactions.

2.4.1 Absorption of bitumen into aggregates

One of the possible processes occurring at the bitumen-aggregate interface is absorption. Absorbed binder into the aggregates is not available anymore for binding the aggregates in an asphalt mixture. Previous research was especially focused for this reason. Absorption of bitumen into the aggregates is especially influenced by the porosity and size of the aggregates and the viscosity of the bituminous binder. The main driving force for absorption is capillary pressure (Lee et al. 1990). For circular cylindrical pores absorption can be described by (Washburn 1921): 2 cos R p γ θ ⋅ ⋅ = (2.1)

where R = pore radius

γ = surface tension θ = contact angle

p = absolute exerted pressure

Many different test methods have been developed for the characterization of absorption of bituminous binder into aggregates. In these tests the absorption of bituminous binder is evaluated through correlations with absorption using other liquids, like water and kerosene (Kandhal and Khatri 1991). To study the absorption of bituminous binder into aggregates Hveem (1942) developed the Centrifuge Kerosene Equivalent test. In this method 100 g of dry aggregates passing through sieve size #4 (4.75 mm) are saturated with kerosene. The saturated aggregates are then centrifuged at a force of 400 times gravity to ensure that excess kerosene drains off the samples. After centrifuging for two minutes the aggregates are weighed to determine the amount of kerosene retained as per cent of dry aggregates. This value is the Centrifuge Kerosene Equivalent (CKE). If the specific gravity of these aggregates passing through sieve #4 is significantly different from 2.65, then a correction to the CKE has to be applied. This

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correction is a multiplication with the specific gravity of the fines defined by 2.65. With the corrected CKE and per cent aggregate passing through sieve #4, the oil ratio can be determined using figure 2.13. The required oil ratio can also be determined by the relation (Hveem 1942):

0.85 2.5 % # 4

100 100

mix

CKE pass

Oil Ratio = + (2.2)

where Oil Ratiomix = required oil ratio

CKE = corrected centrifuge kerosene equivalent %pass#4 = amount passing sieve size #4

Figure 2.13: Graph for determining the required oil ratio for asphalt mixtures [Hveem 1942]

This oil ratio is the estimated optimum bitumen content for a specific mixture (Asphalt Institute 1989). It includes the absorbed amount of binder into the aggregates and the still available amount of binder on the surface of the aggregates necessary for binding the aggregates in the mixture. The absorption of the coarser aggregates can be characterized by a similar method. In this method aggregates that pass through a ⅜ inch (0.95 cm) sieve and that are retained on