International Journal of Adhesion & Adhesives 31 (2011) 238–247

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International Journal of Adhesion & Adhesives

0143-74

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/ijadhadh

Influence of the interfacial composition on the adhesion between aggregatesand bitumen: Investigations by EDX, XPS and peel tests

M. Horgnies a,n, E. Darque-Ceretti b, H. Fezai b, E. Felder b

a Lafarge Centre de Recherche, 95 rue du Montmurier, 38291 St Quentin-Fallavier, Franceb MINES ParisTech, CEMEF—Centre de Mise en Forme des Materiaux, CNRS UMR 7635, 1 rue Claude Daunesse, 06904 Sophia Antipolis cedex, France

a r t i c l e i n f o

Article history:

Accepted 25 January 2011The study of the adhesion between aggregates and bitumen is necessary to enhance the lifetime of the

roads. The purpose of this work concerns the interaction between the mineralogy of the aggregates and

Available online 1 February 2011

Keywords:

Interfaces

Peel

X-ray photoelectron spectroscopy

Bitumen

96/$ - see front matter & 2011 Elsevier Ltd. A

016/j.ijadhadh.2011.01.005

esponding author. Tel.: +33 474823340; fax:

ail address: matthieu.horgnies@lafarge.com (

a b s t r a c t

the adhesion force measured at the interface between bitumen and aggregate. The adhesion of bitumen

was studied according to the mineralogy of the aggregates, which were made of dolomite rock or

granite. A method was developed to measure the fracture energy during the peeling of the bitumen

layer from the aggregate surface. The specific manufacturing of the samples ensured reproducible

measurements using a constant thickness of the bitumen layer and by introducing a strengthened and

flexible membrane into the bulk of bitumen. The peeling results demonstrated that the locus of the

failure varied according to the mineralogy of the aggregate. The failure was cohesive during the peeling

of the dolomite/bitumen system while the failure was partly interfacial concerning the granite/bitumen

system. The interface between bitumen and minerals was characterized, before and after peeling. In

case of the granite, the detection of sulfur by X-ray Photoelectron Spectroscopy (XPS) highlighted only

the bitumen residues and allowed identifying the mineral compounds that weaken the interface

between bitumen and granite. Finally, XPS analyses showed that the alkali feldspars of the granite

induced a weak interface with bitumen.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The mechanism of adhesion between aggregates and bitumenshould be understood to improve the lifetime of the roads. Severalparameters influence this adhesion: porosity and roughness of theaggregates, wettability between bitumen and aggregates andchemical composition at the interface. Some authors demon-strated that moisture could damage the asphalt that lost instrength and durability due to the presence of water [1]. Otherstudies had correlated the adsorption of bitumen with theelemental composition of the minerals that compose the aggre-gates. For example, the alkali metals (ionic species made ofsodium and potassium) of several aggregates exhibit relative highmoisture sensitivity [2]. On the contrary, the ionic species madeof calcium or magnesium appear to be less soluble in water andcould promote the adhesion with bitumen [3,4]. Moreover, otherpapers demonstrated that the surface energy of the minerals andthe acid/base interactions could control directly the wettability ofthe crude oils [5–7].

ll rights reserved.

+33 474828011.

M. Horgnies).

The aim of this paper concerns the correlation between thesurface composition of the aggregates and the adhesion force ofbitumen. Two distinct families of aggregates were used (graniteand dolomite). Their compositions are very different due to theirorigins (igneous and sedimentary, respectively). The bitumen iscomposed of a colloidal structure made of asphaltenes micelles,aromatics and saturated hydrocarbons [8]. In this study, nomodifiers or fillers were used in its composition to enhance theadhesion on aggregates. The aggregate surfaces were previouslypolished before applying the bitumen to avoid any contribution ofthe mechanical anchorage. The adhesion between aggregates andbitumen was then investigated using several methods: peel tests,Energy Dispersive X-ray (EDX) spectrometry and XPS. Thesemethods of characterization have been already used to analyze thesurfaces of rocks [9,10], aggregates or bulk of bitumen [11–13].However, the interface between bitumen and minerals after peelinghas not been studied by chemical analysis before. The peel testshave been usually used to measure the energy dissipated duringthe debonding between adhesive and substrate [14–17]. Some ofthese publications have showed the use of strengthened membrane,liner or mesh sheet that were incorporated into the bulk of thesoft material to characterize. Giannis and Adams [18] had thenused a metal mesh sheet between two layers of sealant layer inorder to perform 1801-peel test. Due to the sensitivity of the

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247 239

rheological properties of bitumen [8], the same ambient conditions(temperature, relative humidity, speed of peeling) were used duringthe peel tests.

Firstly, the chemical composition of the different aggregateswas analyzed by EDX and XPS before applying bitumen. Thedistribution of several types of crystal was established at thesample surface. Secondly, a specific methodology was developedto measure the fracture energy during the peeling of the bitumenlayer. The preparation of the sample ensured a constant thicknessof the bitumen layer and prevented any deformation that couldaffect the measurement of the fracture energy [17,19]. The locusof the failure was also characterized to determine if the adhesionat the interface could be influenced by the mineralogy of theaggregate. Thirdly, the atomic ratios at the interface betweenaggregate and bitumen were quantified by XPS after peeling. Thedetection of specific elements was used to identify the mineralphases that induced the weakest adhesion with the bitumen.

Table 1Bulk composition in weight of the aggregates, analyzed by X-ray fluorescence.

Aggregates SiO2

(%)

Al2O3

(%)

Fe2O3

(%)

CaO

(%)

MgO

(%)

K2O

(%)

Na2O

(%)

TiO2

(%)

Dolomite

sample

0.7 0.4 0.1 50.3 3.7 – – –

Granite

sample

69.1 15.6 2.1 1.2 0.3 7.1 3.5 0.2

2. Experimental

2.1. Material

2.1.1. Aggregates

Two kinds of aggregates were studied in this work. Their originswere distinct and induced specific composition: the first aggregatewas an igneous sample (called ‘‘granite’’) and the second one was asedimentary sample (called ‘‘dolomite’’). The images of Fig. 1 allowcomparing their appearance under visible light. Three different phasesconstitute the granite: feldspar, mica and quartz. While quartz is onlymade of silica (SiO2), black micas, like biotite, are usually composedof magnesium, titanium and iron ions. On the other hand, feldsparscan contain potassium ions (orthoclase: KAlSi3O8), sodium ions(albite: NaAlSi3O8) or calcium ions (anorthite: CaAl2Si2O8). In thisstudy, specific granite composed of large orthoclase crystals (withmillimeter-size) was used. The ‘‘dolomite’’ sample was a dolomiterock made of CaCO3 and CaMg(CO3)2.

All the aggregates samples were cut using a fretsaw and waterlubricant. The sizes of the final samples were about 100�80 mmwith a thickness of 10 mm. Their surfaces were polished andcleaned with heptane solvent before the spectroscopic analyses of

Sample made of granite rock

Micas (in black) Large feldspar(in yellow)

Quartz(in white)

Fig. 1. Images of samples: (a) sample made of gran

reference or before the deposition of liquid bitumen. The rough-ness of the aggregates was measured with a Surftest SJ-201Mmechanical profilometer (from Mitutoyo, Japan) in order tocalculate the arithmetic mean of the profile deviations from themean line (Ra). The Ra value was then obtained by compiling thearithmetic mean of 5 profiles of 12.5 mm. Indeed, granite anddolomite samples exhibited a mean roughness of 0.7 and 1.3 mm(70.3) respectively.

The bulk composition in weight was analyzed by X-rayfluorescence, as described by Table 1. ‘‘Granite’’ sample wasmainly composed of silicon and aluminum oxides. Some otheroxides were also identified as Fe2O3, K2O and Na2O. ‘‘Dolomite’’sample was mainly composed of calcium oxide (CaO) with a fewpercents of a second oxide (MgO).

2.1.2. Bitumen

Bitumen is a mixture of hydrocarbon compounds (with high-molecular weight) produced during the oil refining process. Themajority of these compounds are aliphatic, naphtenic and aro-matic [20]. Bitumen is usually composed of 80�85% of carbon,10�15% hydrogen, 2�3% oxygen. Some minor compounds couldalso be detected like sulfur and nitrogen, depending of thegeographical origin. Bitumen could be presented like a colloidalsystem with different interactions between the several com-pounds [8]. The penetration grade characterized significantlythe properties of the bitumen. This parameter depends on thestandard EN12591:1999, which corresponds to the sinking with aforce of 1 N of a calibrated needle at 25 1C and for 5 s. We usedclassical bitumen with a penetration grade of 35/50, whichcorresponds to a penetration between 3.5 and 5 mm. In this

Sample made of dolomite rock

ite rock and (b) sample made of dolomite rock.

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247240

study, the bitumen used became liquid around 130 1C and it wasapplied on the surface of aggregates at 140 1C. Below 50 1C, theviscosity of bitumen did not affect the shape of the bitumen layer.

2.2. Methods of characterization

2.2.1. SEM-EDX

Samples of aggregates and bitumen were characterized byEnvironmental Scanning Electronic Microscope (ESEM XL 30 fromPhilips, using an accelerating voltage of 10 keV). Images ofaggregates were obtained after being polished. EDX spectrometryallows characterizing the first micrometers of the surface depth ofaggregates and bitumen. This technique was used to map thedistribution of specific elements (K, Na, Ca, S, etc.) that character-ized the different minerals of the aggregate. Software of analysis(INCA, from Oxford) and current ZAF calibration method wereused to quantify the elements.

2.2.2. XPS

XPS allows quantifying atomic ratios and chemical componentsof the first nanometers of the surface depth. XPS measurementswere done using a K-alpha spectrometer from ThermoFisher Scien-tific that was equipped with a monochromatized aluminum X-raysource (1.486 keV). The typical dimensions of the sample introducedin the XPS spectrometer were 5�5 mm with a thickness of 2 or3 mm. Samples of aggregates were performed after cutting, polish-ing and cleaning by heptane (to remove any impurities). Thereference spectrum of bitumen was recorded after the depositionof a drop of the liquid on a aluminum sheet. The bitumen wasanalyzed after solidification. Concerning the analyses of the interfaceafter peeling, the surfaces of aggregates were broken in littlesamples (5�5 mm with 2 mm of thickness). Whatever the samples(aggregates, bitumen), the analyzed area was an ellipse of

Fracture energy

Fig. 2. Experimental m

200�50 mm and a thickness of 5�10 nm was studied. The bindingenergies were determined with respect to the C-(C,H) component ofthe C 1s peak of carbon fixed at 284.8 eV. The peaks were decom-posed with software using a Gaussian/Lorentzian (85/15) productfunction, after subtraction of a Shirley baseline. The concentrationratios were calculated using peak areas normalized and based onacquisition parameters and elemental sensitivity factors.

2.2.3. Peel tests and specific method developed for

bitumen/aggregate sample

For a peel angle y, the adhesion force P and the width of thebitumen layer b, the fracture energy G could be calculatedaccording to [17]:

G¼P

bð1�cosyÞ

In the case of 901-peel, G is equal to the peeling force (P)divided by the adhesive width b.

Major difficulties occurred by manipulating liquid bitumenand by peeling a system made of a bitumen layer deposited onaggregates. At high temperature (4100 1C), the tack of the bitu-men was significant and the reproducibility of samples dimen-sions became difficult to ensure. Indeed, we had to control thethickness (h) of the bitumen layer and ensured that the width (b)was bigger than its thickness (b410�h, expected). As describedin Fig. 2, the bitumen layer was strengthened by inserting amembrane made of polyethylene terephtalate (PET), in order toproceed to the reproducible peel tests. The thickness of themembrane was 405 mm with a mesh opening of 300 mm to letthe bitumen impregnated the system. This PET membrane couldresist against high temperature (4200 1C).

Fig. 3 exhibits the different steps manufacturing of the aggre-gate/bitumen samples. First, we used a mold made of aluminum

Membrane

BitumenAggregate

ethod of 901-peel.

Fig. 3. Manufacturing steps of the samples: (a) aluminum mold; (b) aluminum mold protected by PTFE film; (c) fixation of the mold on the aggregate; (d) deposition of the

liquid bitumen at 140 1C; (e) application of the membrane with a second part of bitumen; (f) removing of the aluminum mold after 24 h of storage; (g) removing of small

pieces of bitumen and (h) final sample of aggregate covered by a layer of bitumen with strengthened membrane.

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247 241

Table 2Atomic ratios measured at the surface of the aggregates by EDX and XPS.

Atomic quantification

by EDX of the dolomite

sample (at.%)

Area no. 1 Area no. 2 Area no. 3

C 16.1 19.6 16.2

O 63.3 60.7 64.1

Si 0.1 0.3 0.3

Ca 19.7 17.8 12.1

Mg 0.5 1.4 7.5

Other elements

(Na, Cl)

0.3 0.2 0.1

Atomic quantification

by EDX of the granite

sample (at.%)

Area no. 1

(mica)

Area no. 2

(feldspar)

Area no. 3

(quartz)

Area no. 4

(quartz)

C 2.6 3.1 0.6 1.1

O 62.7 62.7 67.7 67.3

Si 14.2 21.4 30.2 30.8

Ca 0.5 – – –

Mg 4.3 – – –

Al 5.5 6.4 0.5 0.5

K 1.9 5.7 0.3 0.3

Fe 7.0 – – –

Ti 1.1 – – –

Na 0.2 0.7 – –

Atomic quantification of the surface of

the aggregates by XPS (at.%)

Dolomite

sample

Granite

sample

C 11.2 3.6

O 55.5 64.9

Si 1.6 15.7

Ca 26.9 3.6

Mg 4.3 –

Al – 9.5

K – 2.4

Na 0.3 –

N – 0.3

Cl 0.2 –

SE Picture Magnesium EDX-m

Area

Area

Area

Carbon EDX-ma

Fig. 4. SEM image and EDX ma

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247242

(dimensions: 80�17 mm, with 2 mm of thickness) to control thedimensions of the bitumen layer (Fig. 3(a)). A film made of poly-tetrafluoroethylene (PTFE) covered this aluminum mold (Fig. 3(b)) toavoid any adhesion with bitumen. The mold was fixed on theaggregate surface before the deposition of bitumen (Fig. 3(c)). Aggre-gate and mold were previously heated at the melting temperature ofthe bitumen (140 1C) for 30 min just before depositing the liquidbitumen (Fig. 3(d)). The membrane (dimensions: 100�16 mm with0.4 mm of thickness) was applied at the top surface of the liquid

apping Calcium EDX-mapping

pping Oxygen EDX-mapping

ps of the dolomite sample.

Area 3

Area 2

Area 4

Area 3

Area 2

Area 4Area 1

Fig. 5. SEM image of the granite sample.

Table 3EDX and XPS analyses of the bitumen.

Atomic ratio (at.%) Analyses by EDX Analyses by XPS

Ca 95.5 96.5

O 1.8 1.9

S 2.7 1.6

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247 243

bitumen layer. This membrane was encapsulated with a second partof bitumen (Fig. 3(e)). All the system was stored at 140 1C for 15 minto let the bitumen impregnated the membrane. Finally, the samplewas cooled for 24 h at ambient temperature before removing thealuminum mold (Fig. 3(f)) and before removing the small pieces ofbitumen next to the layer (Fig. 3(g)). The final bitumen samples hadreproducible dimensions (80�17�2 mm3 for length, width andthickness, respectively) with a specific strengthened layer of bitumen(Fig. 3(h)). Contrary to other publication [18], we did not use anymechanical step to initiate the crack at the interface between bitu-men and aggregate. The result of the peel tests included also theenergy used to bend and stretch the peel arm. However, thesecontributions were low because the membrane was flexible (veryeasy to bend) and difficult to stretch.

0.00E+00

1.00E+04

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9.00E+04

1.00E+05

1.10E+05

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1.30E+05

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1.60E+05

8009001000110012001300

Cou

nts

/ s

Bind

XPS spe

0

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1166168170172174

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nts

/ s

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XPS spectrum (su

Fig. 6. XPS spectra recorded on a reference sample of bitumen: (

The peel tests were undertaken under ambient conditions(25 1C, 50% relative humidity). Moreover, the speed of peelingwas adjusted to a constant 0.2 mm/s in order to avoid any stick-slip behavior.

0100200300400500600700ing Energy (eV)

C1s

O1s

S2p

ctrum (survey)

15816016264

)

lphur: S2p)

a) survey spectrum and (b) spectrum of sulfur S2p window.

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247244

3. Results

3.1. Composition of the aggregates surfaces

As presented in Table 2, EDX analyses highlight that thedolomite sample was mainly composed of calcium and magne-sium carbonates: CaCO3 and CaMg(CO3)2. Other minor oxideswere also detected by XRF and EDX (such as SiO2) but they couldnot be mapped by EDX. Fig. 4 shows a Second Electron (SE) imageand two X-rays maps of the main elements: calcium and magne-sium. The bright pixels symbolize each element and parts con-stituted of magnesium were clearly detected: the inclusions madeof CaMg(CO3)2 were surrounded by CaCO3 minerals.

Fig. 5 and Table 2 detail the composition of the ‘‘granite’’ sample.The analyses highlighted the three different phases that constitutedthe sample. The area no. 1 was based on mica phase, which wascomposed of oxides of silicon (Si) and aluminum (Al) mixed withspecific elements like iron (7.0% Fe) and magnesium (2.3% Mg). Thearea no. 2 was composed of oxides of silicon and aluminum andalso other specific elements such as potassium (5.7% K) and sodium(0.7% Na) that characterize alkali feldspar. Finally, area nos. 3 and 4were exclusively composed of quartz (more than 97% of SiO2).

Both the aggregates were also characterized by XPS to specify thecomposition of the nanometric outer-layers. Table 2 compares theatomic ratios detected at their surfaces. Concerning the ‘‘granite’’sample, oxides of aluminum and silicon were found to be the majorconstituents. Feldspars were clearly detected because the concentra-tions of calcium and potassium were significant. On the contrary, theatomic ratios measured at the outermost surface of the dolomite

0

100

200

300

400

500

600

700

800

900

1000

0

Frac

ture

ene

rgy

(N/m

)

(1)

(2)

(3)

(4)

Interfacial fa(2, 4, 6

Peeling d

BitumenResidues (1)

BitumenResidues (3)

10 20 30

Fig. 7. Peeling curve of the granite/bitumen sample, associ

sample were specific and characterized by a main proportion ofoxygen and calcium. Moreover, the detection of magnesium andcarbon confirmed the mineral composition (made of CaCO3 andCaMg(CO3)2).

3.2. Composition of the bitumen

The bitumen, previously melted and deposited on a thin substratemade of aluminum, was studied by EDX and XPS. SE image recordedon bitumen surface highlighted no specific detail (image not showed).Fig. 6(a) indicates the XPS spectrum of the bitumen where only threeelements were detected: carbon, oxygen and sulfur. As showed inTable 3, EDX and XPS gave the same composition: 96% of carbon, 2%of oxygen and 2% sulfur. These results confirm previous analysesfound in the literature [12]. Indeed, Bensebaa et al. [11] had perfor-med analyses of bitumen from different geographical origins andfound respective atomic ratios of 90–92% for carbon, 5–7% for oxygenand 2–3% for sulfur. This last element can be considered as a marker-element for detecting the bitumen residues because it was notdetected on the aggregates surfaces. Fig. 6(b) presents the spectrumrecorded on the sulfur window (around 164 eV). The two spin com-ponents of the S2p peak, separated by 1.25 eV, were easily detected.

3.3. Measurement of the fracture energy and investigation of the

locus of failure

The interface should be weakened to generate a debondingand to avoid any cohesive failure into the bulk of the bitumen

(5)

(6)

(7)

ilures)

istance (mm)

BitumenResidues (7)

40 50 60 70 80

ated to the image of the granite surface after peeling.

Studied area

10 cm

Granite surface after peel test

Picture recorded by the XPS camera

1

6

4

3

2

5

Fig. 8. Images recorded after the peeling of the bitumen/granite system: (a) by a

standard camera and (b) by the camera of the XPS spectrometer, where the

different loci of the analyses are underlined.

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247 245

during the peel test. Indeed, without any weakening, a cohesivefailure occurred exclusively into the bulk of the bitumen and nodata about the interface could be recorded. Some authors hadalready shown that water could induce a rapid debonding bydissolving the cationic species (K+, Na+) of the aggregate surfaceand/or modifying the rheological properties of bitumen [2,3]. Byusing the boiling water test (that determines the dewettingproperties of the bitumen) such as a model of accelerated ageing,we weakened the interface by soaking the bitumen/aggregatesamples into a water bath at 90 1C.

The sample was fixed horizontally during the ageing processand it was not removed from the water before decreasing itstemperature (o50 1C) to limit any modification of the shape andsize of the bitumen layer. The duration of this ageing processvaried according to the mineralogy of the aggregate. Concerning‘‘granite’’ sample, the samples were soaked for 30 min to obtainan interfacial failure during peel test. On the contrary, the failurewas always cohesive with dolomite sample (even after 300 min ofageing) and happened exclusively in the bulk of the bitumenlayer. This experiment demonstrated that the adhesion force ofbitumen with dolomite was higher than the one with granite.

During the peel test, the fracture energy was measuredaccording to the distance of peeling. The interface was observedafter debonding to determine if the failure was cohesive (in thebitumen layer) or interfacial (between aggregate and bitumen).Concerning the dolomite sample, the fracture energy (between800 and 1000 N/m) depended on the cohesive failure into thebulk of the bitumen layer. With a ‘‘granite’’ aggregate, the failurewas partly interfacial during the peeling of the bitumen. Fig. 7describes the curve of fracture energy according to the distance ofpeeling. For each up and down of the curve, the failure at theinterface could be located on the image of the granite surface(specified by arrows). The high levels of fracture energy (between750 and 900 N/m; part nos.1, 3, 5 and 7 on the adhesion curve)corresponded to a large amount of bitumen residues after peel-ing: the failure was cohesive in the bitumen layer. On the otherhand, the low levels of fracture energy (between 0 and 150 N/m;see part nos. 2, 4 and 6 on the fracture energy curve) were linked toa quasi-absence of bitumen residues: the failure was interfacial.

3.4. XPS analysis of the interface after peeling

The outermost surface of the ‘‘granite’’ sample was studied byXPS to detect bitumen residues. The aggregate was broken intosmall parts in order to be introduced into the XPS spectrometerand to keep a good quality of vacuum (too much bitumen coulddegas and deteriorate the ultra-high-vacuum, 10�6 Pa, whichmay limit the performances of the analysis). The XPS analysesdemonstrated that the reference bitumen is composed of 2% ofsulfur (see Table 3) but this element was not detected at thereference granite surface (see Table 2). Indeed, the sulfur wasused as a marker-element to detect invisible bitumen residues atthe aggregate surface after peeling. In XPS spectra, sulfur ischaracterized by two peaks S2s and S2p, which are detected at229 and 165 eV, respectively [21]. The second peak is alsocomposed of two components S2p3/2 and S2p1/2 separated by1.25 eV. Fig. 8 displays two optical images recorded after peelingof the granite/bitumen system. A standard camera recorded thefirst image (Fig. 8(a)) that was similar to the one of Fig. 7. Thecamera of the XPS spectrometer recorded the second image(Fig. 8(b)) to identify the analyzed areas: a great part of thesurface after peeling seemed to be interfacial because no blackresidues of bitumen could be observed.

However, Fig. 9 compares the sulfur peak S2p recorded in sixareas of Fig. 8(b). The area nos. 1 and 5 did not contain any traceof sulfur. However, the sulfur peak was detected (in area nos. 2, 3,

4 and 6) at the surface of the granite even if no black residues ofbitumen were directly observed. In area no. 4, the peak at 164 eVcorresponded to the zero valent sulfur, which was alreadydetected at the surface of the reference bitumen (see Fig. 6(b)).In area nos. 2, 3 and 6, a second component (corresponding tooxidized sulfur) could be also detected at 168 eV. Its presencecould not be linked to the composition of aggregates because notrace of sulfur was detected during the analysis of the referencegranite (see Table 4). The presence of oxidized sulfur could be dueto the aggression of the bitumen during its ageing under water.The presence of this second peak of sulfur could indicate that thelayer of the bitumen was very thin on the area nos. 2, 3 and 6(because only the extreme surface of the bitumen could bemodified during the ageing in water). On the contrary, the layerof bitumen seemed to be thick on area no. 4 because only areference peak of bitumen was detected, corresponding to itscomposition in the bulk.

These four areas of granite (nos. 2, 3, 4 and 6), where thebitumen was clearly detected, corresponded to the black andwhite crystals, characteristic of the mica and quartz minerals,respectively. On the contrary, the sulfur was not detected at the

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700

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900

1000

1100

1200

1300

Cou

nts

/ s

Area n°5 Area n°6

Area n°1 Area n°2

Area n°3 Area n°4

Fig. 9. XPS spectra of sulfur peak recorded on different loci of the failure interface (granite side after peel test).

Table 4Atomic ratio measured by XPS on the yellow-beige areas of the granite surface: on referent granite and after peeling of bitumen.

Elements C O Si Ca Mg Al K Fe Na N

Atomic ratio (at.%) of reference 20.2 50.1 14.0 2.7 0.0 6.6 2.9 0.0 1.2 2.3

Atomic ratio (at.%) after peeling 15.3 43.0 21.4 0.9 0.3 9.5 4.0 0.2 1.8 3.6

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247246

surface of the area nos. 1 and 5 of granite. Both these areasseemed to be beige-yellow and characteristic of the feldspars. XPSanalyses of these yellow-beige areas were performed after peelingto compare their composition to the one of reference granite.These surfaces were cleaned by heptane before XPS analysis inorder to remove the maximum of contamination (due to thesampling and the bitumen residues). Table 4 shows the atomicratios recorded on these surfaces. A part of carbon (15–20%) wasalways detected. This carbon could be induced by residual traces

of bitumen and solvent. In addition, results of Table 4 underlinedthat potassium (and sodium) in association with the oxides ofsilicon and aluminum were well detected on the yellow-beigesareas after peeling. On the contrary, the magnesium and iron(marker-elements of the micas crystals) were not detected onthese areas. Indeed, the presence of alkali feldspar was confirmedin these beige-yellow crystals.

These XPS analyses and the detection of a marker-element ofthe bitumen (sulfur) demonstrated that the failure was cohesive

M. Horgnies et al. / International Journal of Adhesion & Adhesives 31 (2011) 238–247 247

in specific loci of the granite sample composed of black micas andwhite quartz (color under visible light). Moreover, the absence ofbitumen residues (no sulfur detected) at the surface of the beige-yellow crystals confirmed that the failure was interfacial betweenbitumen and alkali feldspar. The adhesion of the bitumen onmicas and quartz seemed to be better than the one on alkalifeldspars.

4. Conclusions

Peel test and spectroscopic analyses were performed to studythe adhesion of bitumen according to the mineralogy of the rocks(granite or dolomite). The reference surfaces were firstly char-acterized by EDX and XPS. Sulfur was selected to identify thebitumen residues while calcium could characterize the dolomitesample. The distinct minerals of the granite were distinguished:quartz was characterized by silicon dioxide, micas by iron andtitanium elements, and alkali feldspars by potassium and sodiumelements. A peel methodology was developed to measure theadhesion force between aggregate and bitumen. A membrane wasinserted into the bulk of the bitumen to strengthen the systemand to assess reproducible peel tests. An ageing process underwater was used to initiate the failure at the interface. An ageingfor 30 min allowed peeling at the interface between granite andbitumen. On the contrary, more than 300 min of ageing could notinduce any interfacial failure between bitumen and dolomite: thedebonding occurred exclusively into the bulk of bitumen. Twodistinct levels of adhesion were measured during the peel of thebitumen/granite system. A high fracture energy (about 800 N/m)corresponded to large amounts of bitumen residues that partlycovered the granite after debonding. A low fracture energy (lessthan 150 N/m) did not correspond to any visible bitumen resi-dues. In this case, the failure appeared to be mainly interfacial.XPS investigations of this interfacial failure demonstrated that athin layer of bitumen (due to a cohesive failure) could cover themicas and quartz minerals. On the contrary, no bitumen residueswere detected on the alkali feldspars that seemed to induce aweak adhesion with the bitumen. In future, other grades of

bitumen and other compositions of granite should be studied.Indeed, the influence of anortithe (CaAl2Si2O8), which containsCa2 + ions, could be analyzed. Moreover, the impact of the ageingunder water on the rheological properties of the bitumen shouldbe studied.

Acknowledgements

The authors would like to thank firstly, Andre Hilary (fromMines-Paristech) for his help concerning the preparation of theaggregates samples and, secondly, Philippe Toubeau and NicolasRichard (from Lafarge) for their advices.

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