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Effect of particle mobility on aggregatestructure formation in asphalt mixturesNima Roohi Sefidmazgi a , Pouya Teymourpour a & Hussain U. Bahiaa

a Department of Civil and Environmental Engineering , Universityof Wisconsin-Madison , Madison , WI , 53706 , USAPublished online: 23 Sep 2013.

To cite this article: Nima Roohi Sefidmazgi , Pouya Teymourpour & Hussain U. Bahia (2013) Effectof particle mobility on aggregate structure formation in asphalt mixtures, Road Materials andPavement Design, 14:sup2, 16-34, DOI: 10.1080/14680629.2013.812844

To link to this article: http://dx.doi.org/10.1080/14680629.2013.812844

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Road Materials and Pavement Design, 2013Vol. 14, No. S2, 16–34, http://dx.doi.org/10.1080/14680629.2013.812844

Effect of particle mobility on aggregate structure formation inasphalt mixtures

Nima Roohi Sefidmazgi*, Pouya Teymourpour and Hussain U. Bahia

Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison,WI 53706, USA

During compaction of asphalt mixtures, aggregate structure starts building up by proximityand direct contact of aggregates. In the previous studies, it has been shown that the aggregatestructure directly affects the service performance. However, the mechanisms of the aggregatestructure formation are not clearly understood. This study is focused on the mechanisms affect-ing aggregate mobility during compaction and the effect of material properties on the aggregatestructure formation. At the initial stages of compaction, there is a relatively thick layer of mastic(i.e. mix of binder and filler) between aggregates, which allows for a shearing mobility in themix, if the mastic viscosity is sufficiently low. However, as compaction proceeds, the masticlayer at proximity zone of aggregates becomes thinner due to high stress intensity and thehigher viscosity of thin mastic film or the aggregates dry contact effect increases the shearingresistance against compaction (i.e. mix becomes locked). In this study, mixes are compacted atdifferent temperatures using one base binder and three different modified binders. The qualityof the aggregate structure and packing throughout the compaction is characterised using two-dimensional imaging of mixture sections and the total aggregate on aggregate proximity lengthis measured as an indication of the aggregate-packing level. It is shown that for mixtures toobtain the maximum packing, the compaction temperature should be picked based on masticviscosity. The viscosity of mastic should be low enough for lubrication, but high enough toprovide sufficient film thickness at proximity zones and prevent locking of mixture at the earlystages of compaction.

Keywords: compaction; imaging; asphalt mixture; viscosity; film thickness; mastic; proximityzone

1. IntroductionIn construction of asphalt pavements, workability is defined as the ease of placement and consoli-dation (i.e. compaction) of the asphalt mixture in the field (Bennert, Reinke, Mogawer, & Mooney,2010). Several studies have characterised the workability of asphalt mixtures from diverse per-spectives (Austerman, Mogawer, & Bonaquist, 2009; Bahia, Friemel, Peterson, & Russell, 1998;Bahia et al., 2001; Bennert et al., 2010; Delgadilo & Bahia, 2008). In the Superpave mix designprocedure, mixing and compaction temperatures are selected based on rotational bulk viscosity ofbinders (American Association of State Highway and Transportation Officials [AASHTO] T 316,2007). Relying on the selection of mixing and compaction temperatures based on binder viscosityalone has been expanded upon in other studies (Delgadillo & Bahia, 2008). Bennert et al. (2010)suggested that bulk viscosity may not be sufficient and thus investigated the application of thin filmrheology to ranking asphalt mixture workability and binder selection. Hanz, Mahmoud, and Bahia(2011) also investigated the role of viscosity, and his group reported that viscosity alone cannotexplain the effect of compaction temperatures on density growth. Hanz et al. suggested lubricity

*Corresponding author. Email: roohisefidma@wisc.edu

© 2013 Taylor & Francis

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Road Materials and Pavement Design 17

Figure 1. The Stribeck curve and lubrication regimes: Z , N , and p are lubricant viscosity, shearing speed,and normal pressure.

as a possibly better or complimentary binder property to explain density growth and introduceda lubricity testing fixture for the measurement of coefficient of friction between steel balls with athin film of binder in between to quantify the effect of warm mix additives on the workability.

Review of the tribology literature, however, indicates that the lubrication phenomenon is notsimple and depends on the interaction of lubricant film thickness, pressure in the interface zone,viscosity, and surface texture (Lu, Khonsari, & Gelinck, 2003). Stribeck developed the concept oflubrication and the effect of surface roughness and lubricant properties on the lubrication mecha-nism. The “Stribeck curve”, or “Stribeck–Hersey curve” (named after Richard Stribeck and MayoD. Hersey), used to categorise the friction properties between two surfaces, was developed in thefirst half of the twentieth century. According to the Stribeck curve, friction regimes for slidinglubricated surfaces have been broadly categorised into three main regimes: (1) boundary lubri-cation, (2) mixed lubrication, and (3) hydro-dynamic lubrication (Figure 1). Boundary regime isdefined as lubrication mechanism in which two surfaces are mainly in direct contact with maxi-mum asperities interlock providing a high coefficient of friction. However, in mixed lubricationregime, increased lubricant film thickness between two surfaces decreases the interlocking ofsurface asperities and the coefficient of friction in between as a result. The hydro-dynamic regimestarts with complete separation of two surfaces when the lubricant film is thick enough comparingto surface texture. The coefficient of friction increases again in hydro-dynamic regime as depictedin the Stribeck curve (Figure 1) due to a “viscous drag” phenomenon (Johnson, Greenwood, &Poon, 1972; Lu et al., 2003).

Johnson et al. (1972) demonstrated that the applied load on a contact of solid–liquid–solidis carried by a pressure distribution on the contact which is dependent on the distribution ofasperities, asperity heights (i.e. texture), liquid film thickness, and mechanical properties.

Therefore, it can be inferred from the literature that the explanation of the asphalt mixturecompaction process and workability ranking using binder rheological properties as a single factorcan be misleading, especially for polymer modified binders.

Another challenge to using binder viscosity as the controlling factor is the inclusion of mineraldust, which is a requirement in mixture design. Adding mineral filler changes the rheologicalproperties of asphalt binders significantly due to particle interaction and absorption of asphaltbinder (Faheem, 2009). The change in rheological properties such as viscosity is dependent onseveral factors: filler type and gradation (i.e. surface area), binder type, and filler concentration(Richardson, 1915). A fundamental challenge in characterising the mixture workability in terms

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of mastic viscosity is that a direct correlation between binder viscosity and mastic viscosity maynot exist, although limited research has been conducted on the subject.

To quantify the workability and study the effects of temperature, several researchers havefocused on understanding the compaction process using mixture testing (Bahia et al., 1998;Gudimettla, Cooley, & Brown, 2003). Bahia et al. (1998) quantified the workability of asphaltmixtures using indices that can be obtained from the densification curve (i.e. Gmm versus numberof gyrations) with an energy concept which successfully characterised different mixtures based onthe required applied energy during compaction to achieve a target density. Gudimettla et al. (2003)used a bucket mixer to measure the torque required for mixing of material at different temperaturesto select the appropriate mixing and compaction temperatures. Although the general belief is thathigher temperature should provide a better workability for asphalt mixtures (since the viscositiesof binder and mastic decrease with an increase in temperature), several studies showed that theshear resistance and density are not a simple nor consistent function of temperature. In a number ofstudies, it is reported that the profile of density versus compaction temperature follows a parabolictrend (Anderson, 2002; Bennert et al., 2010; Gudimettla et al., 2003; Hurley and Prowell, 2005a,2005b, 2005c). In these studies, it is clear that increasing the compaction temperature causes anincrease in density to a specific level as shear resistance of mixture decreases, after which highertemperatures could cause shear resistance to increase, and thus density to decrease. Therefore,the results shown in these studies indicate that there is a zone of minimum shear resistance formany mixtures (if not all) at which density is the highest.

The zone of minimum internal shear resistance of mixtures is commonly referred to as the ‘Ten-der Zone’; difficulties in achieving the desired density in the field are reported for compactionwithin this range of temperatures (Brown, Lord, Decker, & Newcomb, 2000; Buchanan & Coo-ley, 2002; Crawford, 1986; Marker, 1977). Buchanan et al. demonstrated that for some materialsthe ‘Tender Zone’ may occur in the typical range of compaction temperatures commonly expe-rienced in the field, and recommended to let the mat cool down and use heavier rollers at lowertemperatures. This phenomenon could not be explained with binder viscosity testing at differenttemperatures.

The majority of mix designs and compaction literatures considers a specific range of densityor air void content (i.e. 3.5–4.5% for the Superpave mix design, AASHTO M 323-07, 2007)as a target for compaction and quantify the workability based on that metric alone. The basicassumption is that the performance of the mixture can be indicated using density. However, sev-eral studies have demonstrated that materials with the same density may perform significantlydifferent under service loading (Coenen, Kutay, Roohi Sefidmazgi, & Bahia, 2012; Olard, 2012;Roohi Sefidmazgi, Tashman, & Bahia, 2012). Based on the contact mechanism analysis, Zhu andNodes (2000) demonstrated that the transmission of load in the asphalt mixture is mainly deter-mined by the interaction of aggregates and binder at the proximity zones of adjacent aggregates.According to this concept, Roohi Sefidmazgi et al. showed that specific microstructural indices,such as total aggregate proximity length in the aggregate skeleton (i.e. in two-dimensional sectionimages of mixtures), can be a better indicator of mixture load bearing aggregate structure andperformance. The aggregate structure indices show sensitivity to compaction conditions such ascompaction temperature, method, and mix design properties (Bahia and Roohi, 2012; Tashman,Masad, Peterson, & Saleh, 2001).

Therefore, there is a need for a more comprehensive understanding of the mechanics governingthe compaction process to better understand the aggregate on aggregate lubrication, referred hereas “aggregate mobility.” The mechanisms resisting aggregate mobility control the formationof the aggregate skeleton, the main load bearing structure in asphalt mixtures, thus a deeperunderstanding of this process can be invaluable for the control and optimisation of the constructionprocess and enhancement of the pavement performance.

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Road Materials and Pavement Design 19

2. Mechanisms controlling particle mobility during compactionWhen the compaction load is initially applied on loose mix, packing of aggregates increases rapidlyas the applied stress can overcome the flow resistance of the mastic. In this stage, mastic viscositycould be the dominant factor since aggregate interlock is minimal. However, with the subsequentaggregate skeleton formation, the rate of material bulk deformation decreases significantly andtends towards zero. Li and Gibson (2011) used the gyratory compactor to show that the number ofgyrations needed for locking of mixtures is dependent on the material properties and mix design.In addition, Li and Gibson reported that most of mixes compacted in the study locked at thenumber of gyrations of 60 or less and the continued compaction to design gyrations of 100 did notchange the density of mixes significantly. However, there is not a clear fundamental explanationfor the difference in locking point of different mixes.

This study is focused on mixture compaction mechanisms based on the geometrical andmechanical properties of aggregate on aggregate proximity zones. For the purposes of this study,aggregates are determined to be in contact if the perimeters of two adjacent aggregates are withina predefined spacing in a two-dimensional section. It is assumed that when the shearing stressapplied on the mixture is higher than the shearing strength of aggregate proximity zones (i.e.contact friction) or the internal friction of the lubricant in between, the mixture plastically deforms.

Contact friction of two surfaces with a lubricant in between is known to be a function of normalload applied, film thickness, viscosity of lubricant, and morphological properties of surfaces, suchas texture (Lu et al., 2003). In a contact, given other factors to be constant, by decreasing theviscosity of lubricant (such as increasing temperature of asphalt binder), it is hypothesised thattwo phenomena can happen simultaneously:

• Reduction of friction between two surfaces as a result of reduction in viscosity of thelubricant, as the energy needed to overcome the internal friction of lubricant is lower.

• Reduction of film thickness and increase in proximity of two surfaces due to the lubricantbeing squeezed out in the case of low or zero confinement. When surfaces of particles aresmooth, the reduction of film thickness to a very thin layer (i.e. in the nano-meter orderof magnitude) causes the effective viscosity of lubricant to increase, which increases thefriction between two surfaces (Luengo, Schmitt, Hill, & Israelachvili, 1997). In the caseof rough surfaces such as natural crushed aggregates, high stress concentrations at theasperities remove the lubricant, and the solid surfaces with a high coefficient of frictionwill be directly in contact. This process is called “dry contacts” in this study. This conditionis also termed as boundary lubrication in the tribology science studies (Lu et al., 2003,Figure 1). The rate of squeezing out of lubricant is directly related to viscosity of material(Macosko, 1994). Therefore, if the viscosity of the lubricant is not sufficiently high to resistsqueezing out, the friction between two surfaces may increase significantly as a result ofthis dry contact effect.

Therefore, it can be hypothesised that the effective friction between two surfaces in a proximityzone is a parabolic function of lubricant viscosity as shown in Figure 2. This trend is caused bythe competing mechanisms of decreasing film thickness and an increase in viscosity. The initialreduction in friction is caused by increased film thickness and the final increase in friction iscaused by an increase in lubricant viscosity as shown in Figure 1.

Compaction of asphalt mixtures can be envisioned as packing of aggregates that are lubricatedby the asphalt mastic. Therefore, for a better packing of aggregates, the viscosity of mastic shouldbe low enough to allow sufficient workability, but high enough to maintain adequate film thicknessat proximity zones to avoid dry contact or thin film viscosity effect.

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Figure 2. Proposed schematic of change in friction at proximity zones as a function of temperature.

The focus of this study is on the packing of aggregate particles during compaction. However,there are some other concerns in compaction temperature selection such as binder drain down atvery high temperatures, aggregates’ segregation and low mastic film thickness (which may causedurability problems). These concerns are outside the scope of this study.

In this study, it is hypothesised that viscosity of mastic, assuming other factors to be con-stant, controls the mobility and structure formation of aggregate skeleton. Therefore, as shownin Figure 2, changing the compaction temperature will result in different aggregate packing dueto the change in friction at aggregate proximity zones. This implies the existence of an optimumpacking at the lowest level of proximity zone friction.

3. Study objectivesThe main objectives of this study are to determine the possible mechanisms of the aggregatestructure formation during compaction of asphalt mixtures and the effect of mastic rheology andfilm thickness on aggregates mobility.

4. Materials and test methods4.1. Mixture sample preparation and testingTo isolate the effect of mastic as the lubricant phase, mixes with significantly different masticviscosities at a specified compaction temperature were produced. The gradation (Figure 3) hasbeen designed for the purposes of this study from aggregate sources commonly used for hot mixasphalt (HMA) production in Wisconsin (i.e. Granite).

To cover a wide range of mastic viscosities, four different binders were used for compaction: abase neat binder and three modified binders with three different modifiers (i.e. a styrene butadienestyrene-based elastomer modifier (S), a polyethylene-based plastomer (C), and the mix of elas-tomer and plastomer modifiers (H)). The level of modification was selected to obtain high truegrade of 77 ± 1◦C for all modified binders. The binders’ designations and performance gradesare given in Table 1.

The reference mixing and compaction temperatures were selected as 155◦C and 145◦C, respec-tively, based on the viscosity–temperature profile (i.e. using the Superpave mix design procedure)

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Road Materials and Pavement Design 21

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Figure 3. Aggregate gradation used in this study for mix design.

Table 1. Binders’ designations andperformance grades (PGs).

Binder PG

Neat 64–22S 76–22C 76–22H 76–22

Table 2. Compaction temperatures.

Mix Compaction temperature (◦C)

Neat 65, 115, 145S 115, 145, 165C 65, 95, 145, 165H 65, 115, 145, 165

for the neat binder and was kept consistent for all mixes. Additionally, the samples are mixedand compacted at several temperatures providing significantly different orders of magnitude ofmastic viscosity. The mixing temperature was always kept 10◦C higher than desired compactiontemperature. The compaction temperatures are given in Table 2. All mixes are compacted with100 gy, which is the Wisconsin E-10 mix design required level of gyrations (Ndes) with 5.5%asphalt content by mass.

After compaction, the samples were cut in three sections leading to the attainment of six two-dimensional images (Figure 4), with one cutting section at the middle of the sample and twoin the 25.4 mm distance from the middle. All mixes are scanned according to the procedure ofRoohi Sefidmazgi et al. (2012) and image analysis has been performed for the aggregate structuralcharacterisation (i.e. using Image Processing and Analysis Software (IPAS2), Roohi Sefidmazgiet al., 2012). The index as a result of the image analysis procedure to represent the packingcondition in mixtures is the aggregate total proximity zone length in two-dimensional images. Inthe imaging software, proximity is defined when two aggregates’ perimeter pixels are within adistance specified by the user (i.e. 0.1 mm in this study) and all of the pixels of the two aggregates’perimeters within this distance are captured. These pixels form a line referred to as “contact lines”in Roohi Sefidmazgi et al. (2012) (Figure 5). Higher total contact length (i.e. proximity length)

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Figure 4. Cutting sections (Roohi Sefidmazgi et al., 2012).

Figure 5. Contact line (proximity line) (Roohi Sefidmazgi et al., 2012).

represents higher aggregate interlock and packing in the mixture and better rutting performance asa result. Based on image analysis results, the total aggregate proximity length profile for differentcompaction temperatures (i.e. different mastic viscosities) was obtained.

In addition to compaction at different temperatures, mixes are compacted for different levels ofeffort (i.e. 10, 20, 40, 80, and 100 gy) at the temperature of 145◦C and image analysis is performed.The objective is to investigate the difference in the trend of aggregate packing attainment for

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Table 3. Air void% of mixes compactedat different number of gyrations (compactiontemperature = 145◦C).

Air void (%)

Mix 10 gy 20 gy 40 gy 80 gy 100 gy

Neat 12.1 8.8 6.5 6.1 4.5C 10.0 7.0 5.8 3.8 3.5S 11.5 7.8 6.3 3.8 4.0H 11.3 7.2 4.9 3.8 3.0

Figure 6. Bob and Cup fixture.

materials with different viscosities during compaction. The air void content of mixes at differentgyrations is given in Table 3.

4.2. Binder and mastic preparation and testingIt is hypothesised that viscosity of unconfined/semi-confined lubricant between two surfacescontrols the internal frictional behaviour in asphalt mixtures through the two-aforementionedmechanisms before the boundary lubrication phase (i.e. dry contact) at which the locking ofmixture occurs. In order to test this hypothesis and study the effect of lubricant viscosity onaggregate packing, viscosity testing of binders and mastics was performed.

Four binders were aged using the Rolling Thin-Film Oven for the short-term ageing duringmixing and compaction. The viscosity testing is performed at all compaction temperatures withshear rates of 0.01, 0.1, 1, 10 and 100 (s−1) to cover a wide range of shear rates that were proposedin different studies (Bahia et al., 2001; Delgadillo & Bahia, 2008) with rheometer limitationconsiderations. Binders were tested using a Bob and Cup fixture (Figure 6) with a binder filmthickness of 1 mm. The samples are conditioned at the compaction temperatures for 15 min in thefixture. The viscosity measurements are performed for 3 min for each shear rate starting with thelowest shear rate (i.e. 0.01 s−1).

Many of the previous studies on asphalt mixture workability and compaction mechanisms arefocused on rheological behaviour (e.g. viscosity) of binder as the most prominent mix constituentthat affects workability. Yet, several studies have shown that adding filler (i.e. passing sieve no.200) to binder may significantly change the mechanical and rheological behaviours of binder(Clopotel, 2012; Faheem, 2009), since filler particles reinforce the binder and absorb specific

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molecular components of the binder (e.g. asphaltenes) depending on type, gradation, concentra-tion, and specific surface area of the filler particles. Therefore, for the purposes of this study,mastics with the same fillers used in mixture production were prepared for viscosity testing. Thebinder content of mastic samples was determined based on specific surface area of aggregates.With the constant binder film thickness assumption for aggregates of different sizes in the mix, theportion of binder content of mixture that coated the filler particles was determined (i.e. depend-ing on the ratio of surface area of filler particles to the surface area of whole aggregate blend).Based on such calculations, the binder content of mastics was 55.3% by weight. All mastics aremixed using un-aged binders and conditioned at the same mixing and compaction temperaturesof mixtures.

The viscosity testing procedures performed on mastics are the same as binder viscosity testingusing Bob and Cup fixture with the same shear rates. Considering that the size of particles inmastic is finer than 0.075 mm, which is less than 10% of the sample film thickness in Bob andCup fixture, the same fixture used for binder testing can be used for mastic testing since there isno interference expected with the sample geometry.

5. Results and discussionA pervious study (Roohi Sefidmazgi et al., 2012) has demonstrated that mixtures with the samegradation design but different binders may have significantly different aggregate structures. It ishypothesised in this study that the difference in structures can be explained with mastic viscosity.Therefore, mixtures were produced at the same temperature (i.e. 145◦C) and different compactionefforts (i.e. 10, 20, 40, 80, and 100 gy) to monitor the aggregate structure formation for differentbinders. All mixes are cut, scanned; and image analysis is performed. The total aggregate onaggregate proximity zone lengths, as an indication of aggregate internal structure and packing forimages of each mix, were determined and depicted in Figure 7.

The results show that the aggregate packing in the neat mixture is mostly obtained during thefirst 10 gy and the change in aggregate structure with application of more compaction effort (i.e.up to 100 gy) is insignificant. However, for ‘C’ and ‘S’ mixes, the aggregate structure has been

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Figure 7. Total proximity zone length for mixes in different number of gyrations.

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Road Materials and Pavement Design 25

0.1

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Figure 8. Viscosity at 145◦C: (a) binder and (b) mastic.

formed up to 20 gy. The number of gyrations for the aggregate structure formation in the mixturedesignated as ‘H’ was 40 and the structure was formed in a more gradual manner and locked latercompared to the other mixtures.

To help explain the trends of Figure 7, all four binders and mastics were tested for viscosity usingBob and Cup fixture at the temperature of 145◦C (i.e. compaction temperature) and at differentshear rates. The results are shown in Figure 8. It should be noted that the average standard deviationof the viscosity measurement in the rheometer using the bob and cup geometry is ±0.005 Pa.s).

The viscosity results show different ranking of material for binders compared to mastics.According to the binder results at 145◦C, neat binder showed the lowest viscosity and ‘C’, ‘H’,and ‘S’ have higher viscosity, in respective order. However, mastics are shear rate dependent andthe ranking at low shear rates is different than the binder ranking: neat, S, C, and H from low tohigh viscosity.

As it was noted, the lubricant phase in mixture is mastic; therefore, it is more reliable toconsider mastic viscosity for workability properties of mixtures. In addition, it was shown in theNCHRP 9-10 study (Bahia et al., 2001) that the average shear rate occurring in mixtures, afterinitial gyrations, is low according to the axial strain rate. The mastic viscous behaviour is not easilypredicted from the binder viscosity alone at low shear rates. Therefore, for a better characterisationof the aggregate structure formation during compaction, the low shear rate viscosity values ofmastics (i.e. 0.01–0.1 (s−1)) are chosen.

Mastics with higher viscosity can resist the stress condition at proximity zones of aggregatesand resist the squeeze out from between proximity zones longer (more gyrations) than soft mastics,which provide aggregate mobility in a longer period during compaction. In other words, locking ofmixtures due to dry contacts (i.e. direct contact of aggregates) or thin film high viscosity happenssooner in mixes with very soft mastics. However, the mastic viscosity should be low enough toallow for lubrication and shearing deformation between aggregates with the compaction effortapplied. This theory is in agreement with the results. The ‘H’ mastic showed the highest viscosityamong others at the shear rate of 0.01 and 0.1 (s−1) and neat mastic was the lowest, which explainsthe gradual formation of structure in the ‘H’ mixture and rapid locking in the neat mixture.

It is further believed that the film thickness of mastic (or mortar) between coarse aggregatesdecreases rapidly in the neat mix relative to the other mixtures as it is a function of mastic or mortarviscosity (Macosko, 1994). To prove this hypothesis, mortar film thickness in four mixtures isestimated at the same number of gyrations depicted in Figure 7 (Mortar is defined here as bindermixed with aggregates passing sieve no. 16, since aggregates finer than 1.18 mm are not clearlyvisible in the images used for the analysis in this study.)

Aggregates are naturally rough particles. Therefore, the film thickness of material (e.g. binder,mastic, or mortar) between surfaces of two aggregates is not constant (Figure 9). In addition,the friction between two aggregates increases significantly when asperities on the two surfaces

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Figure 9. Black and white image of a mixture representing the minimum film thickness between aggregates1 and 2.

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Figure 10. Evolution of mortar film thickness index at proximity zones of aggregates for different numberof gyrations.

begin contacting each other as the coefficient of friction between bare aggregates is considerablyhigher than the internal friction of material in between (simply meaning that more shearing loadis needed to drag an aggregate on another aggregate when there is no lubricant between them).Therefore, the minimum mortar film thickness between each pair of aggregates (i.e. at proximityzones) is calculated as an index to represent the trend of the aggregate contact formation duringcompaction.

Histograms of mortar minimum film thickness distribution between aggregates for the ‘H’ mixduring compaction are shown in Figure 10. The histogram for the 10 gy level of compaction isflatly distributed, however, by increasing the number of gyrations, the film thickness of mortarbetween aggregates was decreased and the histogram is distributed in a narrower range of filmthickness (i.e. less than 0.1 mm which is considered as contact in image analysis).

The same analysis shown in Figure 10 is performed on other three mixtures. For a more appro-priate comparison of different mixtures in terms of film thickness at proximity zones evolution

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0.0

0.5

1.0

1.5

2.0

2.5

10 20 40 80 100

Film

Thi

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)

Gyrations

H C S Neat

Figure 11. Mortar film thickness index.

R² = 0.95 R² = 0.90 R² = 0.78

0.1

1

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100

1000(a) (b)

0.0 0.5 1.0 1.5 2.0 2.5

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Film Thickness (mm)

0.01 (sec-1) shear rate - 145 °C

10 Gyrations

20 Gyrations

40 Gyrations

80 Gyrations

100 Gyrations

R² = 0.95R² = 0.88R² = 0.80

1

10

100

0.0 0.5 1.0 1.5 2.0 2.5

Vis

cosi

ty (

Pa.s

)

Film Thickness (mm)

0.1 (sec-1) shear rate - 145 °C

10 Gyrations

20 Gyrations

40 Gyrations

80 Gyrations

100 Gyrations

Figure 12. Low shear viscosity versus film thickness at different compaction levels: (a) 0.01 s−1 shear rateand (b) 0.1 s−1 shear rate.

during the compaction process, a weighted average of film thickness for each histogram (i.e. ateach number of gyrations) is calculated. Results show that (Figure 11) the film thickness of mor-tar between aggregates is gradually decreasing for the ‘H’ mix during compaction up to 40 gy;however, the neat mix lost its mortar film between coarse aggregates before 10 gy which is inagreement with the total proximity zone length and mastic viscosity data.

Mastic viscosities at 145◦C and shear rates of 0.01 (s−1) and 0.1 (s−1) are plotted in Figure 12against film thickness at each gyration level. There is a correlation between film thickness and lowshear viscosity of mastics at compaction efforts of 40 gy or less, that is considered the aggregatestructure formation stage at which the controlling lubrication mechanisms are hydro-dynamic andmixed lubrication. This confirms the hypothesis that mastics (or mortars) with higher viscositieskeep thicker mastic layers between aggregates and it takes a higher number of gyrations (i.e.longer time) to squeeze mastic out of proximity zones under the stress conditions.

To further validate the hypothesis, mastic viscosity values at different shear rates are plottedversus total proximity length of the four mixes compacted to 100 gy in Figure 13. The results showthat there is a good correlation between total proximity length as a representation of aggregatepacking with mastic viscosity at low shear rates (i.e. 0.01 and 0.1 s−1). However, there is nosignificant correlation at high shear rates (i.e. 10 and 100 (s−1)).

It can be concluded that among the four binders used for this study, ‘H’ provided the best work-ability and packing of aggregates since viscosity of mastic ‘H’ at the compaction temperature washigh enough to provide sufficient film of mastic between aggregates and consequently aggregatemobility to a higher number of gyrations. However, in the neat mixture with the lowest mastic

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R² = 0.75 R² = 0.13

R² = 0.99

R² = 1.00

1

21

41

61

81

101

121

500 600 700 800 900 1000 1100

Vis

cosi

ty (

Pa.s

)

Total Proximity Zone Length (mm/ 100 cm2)

1 (1/sec) 10 (1/sec)0.1 (1/sec) 0.01 (1/sec)

Figure 13. Viscosity versus total aggregate proximity zone length at 145◦C.

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

60 80 100 120 140 160

Tot

al P

roxi

mity

Zon

e L

engt

h(m

m/1

00cm

2 )

Compaction Temp. (°C)

Control C S H

Figure 14. Total proximity zone length – compaction temperature profile.

viscosity, the film of mastic at aggregate proximities is squeezed out at less than 10 gy whichpossibly resulted in dry contacts or a very thin layer of mastic with high viscosity that increasedfriction between aggregates significantly. The ‘C’ and ‘S’ mixes showed a transition from the ‘H’mix (i.e. high mastic viscosity, gradual locking of mix) to the neat mix (i.e. low mastic viscosity,rapid locking of mix) from the mastic viscosity and aggregate packing point of view.

In contrast, using mastics with extremely high viscosities is expected to result in lower worka-bility and lower mobility of particles. Thus, there should be a high and low limit for viscosity ofmastics within which sufficient aggregate mobility during compaction is achieved. Compactionoutside this range of viscosities should be avoided. To investigate this concept, mixes were pro-duced and compacted at different compaction temperatures as given in Table 2. Compaction ofmixes at temperatures from 65◦C to 165◦C provided a wide range of mastic and binder viscosities.

The image analysis results of mixes compacted in different temperatures is depicted in Figure 14.The analysis for the ‘S’ mix compacted at 65◦C was not performed since it had significantaggregate coating problems.

As shown in Figure 14, there is a maximum proximity zone length (i.e. optimum packing)at a unique temperature for each mixture. The maximum values of total proximity length fordifferent mixes are approximately the same; possibly due to the fact that the same gradation isused and thus according to the mastic viscosities at different temperatures, ultimately a maximum

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Road Materials and Pavement Design 29

0.1

1

10

100

1000

10000(a) (b)

(c) (d)

0.01 0.1 1 10 100

Vis

cosi

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Pa.s

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Shear Rate (sec-1) Shear Rate (sec-1)

Neat-binder

Neat 65 C Neat 115 C Neat 145 C

0

1

10

100

1,000

10,000

0.01 0.1 1 10 100

Vis

cosi

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C-binder

C 65 C C 95 C C 145 C C 165 C

0

1

10

100

1,000

10,000

0.01 0.1 1 10 100

Vis

cosi

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Shear Rate ( sec-1)

S-binder

S 65 C S 115 C S 145 C S 165 C

0

1

10

100

1,000

10,000

0.01 0.1 1 10 100V

isco

sity

(Pa

.s)

Shear Rate ( sec-1)

H-binder

H 65 C H 115 C H 145 C H 165 C

Figure 15. Bob and Cup binder viscosity results at different temperatures and shear rates: (a) neat binder,(b) C binder, (c) S binder, and (d) H binder.

packing of particles is achieved. It is possible that different mastics provided similar viscosityranges at different temperatures and at that specific range for the gradation used in the four mixesoptimum packing has been achieved. It has been shown in a previous study that there is a goodrelation between total proximity length and rutting performance (Roohi Sefidmazgi et al., 2012).Therefore, this finding shows that the structure needed to resist the service loading can be obtainedby compaction of any of the four mixes only by selecting the correct compaction temperature.

To determine the effect of viscosity on total proximity zone length at different compactiontemperatures, binders and mastics were tested using the Bob and Cup geometry in the dynamicshear rheometer (DSR). The viscosity results for the binders in different temperatures and shearrates are shown in Figure 15. According to the results, all binders show Newtonian (shear rateindependent viscosity), specifically at high temperatures, although for ‘H’ and ‘C’ at 65◦C, thematerials showed slight shear rate dependency of viscosity. For all practical purposes, the viscosityranking of binders at temperatures above 115◦C can be considered independent of shear rate.

Mastics were tested at the same shear rates and temperatures as binders using the Bob andCup fixture (Figure 16). The test results for modified mastics show higher shear rate dependency(specifically for ‘C’ and ‘H’ mastics) compared to binders at different temperatures. The otherimportant difference of mastic behaviour compared to binders is that the viscosity ranking ofmastics does not follow that of binders. Based on the tests performed on the four mastics, ‘H’ isthe most viscous material and ‘neat’ showed the lowest viscosity at different temperatures. Exceptfor the neat mastic, viscosity measurements at 65◦C were out of the torque limit of the rheometer.

As explained in the previous sections, low shear viscosity of mastics is proposed as a factoraffecting mixture workability at different temperatures because it correlates better with aggregatepacking in addition to the fact that there is always a blend of binder and filler (i.e. mastic) amongcoarser aggregates acting as a lubricant phase. However, it is shown in this study that the totalaggregate proximity zone length values do not demonstrate a clear trend with increasing viscosityas caused by reducing the compaction temperature. The total proximity zone length results versus

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0.1

1

10

100

1000

10000

100000

(a) (b)

(c) (d)

0.01 0.1 1 10 100

Vis

cosi

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Neat-mastic

65 115 145

0

1

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0.01 0.1 1 10 100

Vis

cosi

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Pa.s

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Shear Rate (sec-1)Shear Rate (sec-1)

Shear Rate (sec-1)Shear Rate (sec-1)

C-mastic

95 145 165

0.1

1

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1000

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100000

0.01 0.1 1 10 100

Vis

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S-mastic

115 145 165

0.1

1

10

100

1000

10000

100000

0.01 0.1 1 10 100V

isco

sity

(Pa

.s)

H-mastic

115 145 165

Figure 16. Bob and Cup mastic viscosity results at different temperatures and shear rates: (a) neat mastic,(b) C mastic, (c) S mastic, and (d) H mastic.

1,000

1,500

2,000

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3,000

3,500

4,000

4,500

0.1 1 10 100 1000 10000 100000Tot

al P

roxi

mity

Zon

e L

engt

h (m

m/1

00cm

2 )

Mastic Viscosity (Pa.s)

Control C S H

Figure 17. Total proximity zone length as a function of mastic viscosity at different compactiontemperatures at shear rate of 0.01 s−1.

mastic viscosity for mixes compacted at different temperatures are plotted in Figure 17. In additionto mastic viscosity, film thickness at proximity zones (which is also a function of viscosity) isanother important factor affecting friction between aggregates. By increasing the compactiontemperature, viscosity is decreased providing sufficient lubrication between aggregates and highertotal proximity zone length (as long as the viscosity is higher than a specific range or, in otherwords, mastic film thickness is still sufficient to avoid dry contacts). This is demonstrated inFigure 17. However, for viscosities lower than a specific range, film thickness of mastic becomesvery thin within the proximity zones of aggregates, as shown in Figure 18. Therefore, due to thedry contacts and/or the thin film viscosity effect, the workability of the mixture and aggregate

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Road Materials and Pavement Design 31

0.0

1.0

2.0

3.0

4.0

5.0

60 80 100 120 140 160

Film

Thi

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ss (

mm

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Compaction Temp (°C)

Mortar Film Thickness

H C S Neat

Figure 18. Mortar average film thickness at different compaction temperatures.

mobility decreases, leading to a decrease in total proximity zone length (i.e. packing of aggregates)after a maximum value for mixes compacted in higher temperatures.

The data points from the film thicknesses plots at which the maximum total proximity zonelength is attained are circled in Figure 18.

In this study, limited numbers of compaction temperatures are selected for proof of concept.However, to determine the viscosity range providing sufficient film thickness and lubrication foroptimum workability, compaction of mixes at additional temperatures is needed to accuratelydetermine the compaction temperature (or mastic viscosity as material property) leading to max-imum packing. In addition, change in aggregate gradation design changes the stress distributionin the mix and at the aggregates proximity zones. Therefore, with the results of this study, it canbe concluded that different aggregate gradations may lead to different ranges of viscosity to resistthe squeezing out due to the contact normal stress condition which needs more verifications. Thisis in agreement with recent findings in the literature that the viscosity as a single factor is notenough to predict the workability of different mixtures (Hanz et al., 2011).

6. ConclusionThis study was focused on understanding the factors that can critically control aggregate mobilityand packing during compaction. The study is limited to one gradation and four different binders.This was done to challenge the concept that binder viscosity as a function of temperature is asufficient parameter to recommend minimum limits for compaction temperatures. In addition tomeasuring density of mixes compacted at various temperatures, viscosities of binders and masticswere measured, and estimates of film thickness at aggregate proximity zones using advancedimaging tools were performed. Although many previous studies had considered a desired densityas a compaction process target, this study was focused on the evolution of aggregate structureduring compaction and effect of lubricant rheology (i.e. viscosity) on the aggregate packing as amore fundamental indication of final product’s performance. The aggregate total proximity zonelength as result of two-dimensional image analysis has been used to characterise the aggregatestructure of asphalt mixtures. The main findings of this study are:

• Most design specifications use binder viscosity as an indication of mixture workability atthe compaction temperature. This study demonstrated that binder viscosity is not sufficientand that the workability of the mixture is more dependent on viscosity and film thickness ofthe actual lubricant, which is the binder and filler mastic. The film thickness that is important

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is not the overall average, but the thickness within the proximity zones of aggregates. Thisfinding is supported by many studies in the field of tribology and lubrication mechanics.

• Two competing mechanisms are proposed as controlling factors of the compaction processand aggregate structure formation. Lubricants with lower viscosity provide easier slidingbetween the lubricated surfaces. On the other hand, lower viscosity results in a highersqueezing out rate of lubricant for a constant normal force within the proximity zonesbetween two surfaces, which increases the chance of dry contact interaction, or thin filmhigh viscosity effect. The result is an increase in friction between two surfaces. Therefore,by increasing the compaction temperature, the aggregate mobility and structural packingusually begin to increase to a peak value and then decrease as dry contact or thin filmviscosity effects become significant at higher temperatures.

• Mixes were compacted to different number of gyrations with the same gradation but differentbinders to demonstrate the effect of mastic viscosity on the aggregate structure formation.The rates of increase in total aggregate proximity zone lengths were significantly different fordifferent mixes during compaction. Additionally, binders and mastics were tested at differentshear rates. The viscosity behaviour of mastic at low shear rates could not be predictedfrom binder viscosity, and since mastic is the lubricant phase in asphalt mixtures, masticviscosity has been used to explain the behaviour of mixture. With the aid of image analysis,the film thickness of mortar at proximity zones of aggregates at different compaction stages(i.e. number of gyrations) has been determined. The mastic viscosity at low shear rates inaddition to mortar film thickness between coarse aggregate results show that the mastic withhigher viscosity at compaction temperatures squeezed out of aggregate proximity zones ina more gradual manner comparing to mastics with lower viscosity. Therefore, compactionof mixes with higher viscous mastics resulted in a better packing of aggregates and the mixwith the lowest mastic viscosity was locked at the initial stages of compaction and the totalproximity zone length was unchanged for higher number of gyrations.

• In order to study the effect of mastic viscosity on the aggregate structure, mixes werecompacted at temperatures from 65◦C to 165◦C using four binders providing significantlydifferent viscosity levels. The results showed that with an increase in compaction temper-ature or decrease in mastic viscosity, total proximity zone length in the aggregate skeletonincreased up to a certain temperature, and then began to decrease. The temperature at whichthe optimum total proximity zone length was achieved varies for different materials. How-ever, the value of optimum total proximity zone length was approximately the same for allmixtures. This finding shows that the particle arrangement and packing level for the samecompaction pressure and aggregate gradation is related to mastic viscosity as a fundamentalmaterial property which can be the same for different materials at different temperatures.However, the number of mixes and compaction temperatures in this study are not sufficientto define a meaningful range of mastic viscosity for the optimum packing.

AcknowledgementsAs members of the Asphalt Research Consortium, funding and support from the FHWA and WRI is acknowl-edged. The help and comments from Dr Raul Velasquez, Dr Andrew Hanz, and Dan Swiertz are greatlyappreciated. The testing and sample preparation of Trevor Schultz is also acknowledged. Partial fundingfrom Honeywell Inc., USA is acknowledged.

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