Construction and Building Materials 35 (2012) 385–392

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Construction and Building Materials

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Comparison of asphalt rubber-aggregate and polymer modifiedasphalt–aggregate systems in terms of surface free energy and energy indices

Allex E. Alvarez a,⇑, Evelyn Ovalles a, Amy Epps Martin b

a Department of Civil Engineering, University of Magdalena, Santa Marta, Colombiab Zachry Department of Civil Engineering, Texas A&M University, 3136 TAMU, College Station, TX, USA

h i g h l i g h t s

" Surface free energy and energy indices served to characterize asphalt–aggregate systems (AAS)." Asphalt rubber (AR)- and polymer modified (PM)-asphalts produced comparable AAS." The AAS evaluated are representative of permeable friction course (PFC) mixtures." The energy indices computed can be used to improve the design of PFC mixtures.

a r t i c l e i n f o

Article history:Received 11 January 2012Received in revised form 27 March 2012Accepted 25 April 2012

Keywords:Asphalt rubber (AR)Polymer modified (PM) asphaltSurface free energy (SFE)Work of adhesionEnergy indicesHot mix asphalt (HMA)Pavements

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.04.029

⇑ Corresponding author. Tel./fax: +57 5 4301292.E-mail address: allexalvarez@yahoo.com (A.E. Alva

a b s t r a c t

The surface free energy (SFE) is a material property that can be used to identify optimum asphalt–aggre-gate combinations (i.e., interfaces) for improved design of hot mix asphalt (HMA) and to characterize theHMA performance through the use of micromechanical models. Based on SFE measurements, and subse-quent calculation of energy indices, this paper compares asphalt–aggregate interfaces formed withasphalt rubber (AR) and polymer modified (PM) asphalt binders (or asphalts) specified for fabricationof permeable friction course (PFC) mixtures in Texas. Six PM asphalts and four AR asphalts with five dif-ferent aggregates were assessed. Corresponding results suggest that, in terms of the energy indices com-puted, the fracture resistance, moisture damage susceptibility, and the wettability of the asphalt over theaggregate of AR asphalt–aggregate systems can be comparable to that developed by PM asphalt–aggre-gate systems. However, this conclusion is restricted by the variability encountered for both asphaltgroups analyzed (i.e., AR and PM). Additional research is recommended to analyze the effect of lime inPM asphalts (since PFC mixtures fabricated with PM asphalts often include lime) and the effect of mineralfillers on both PM- and AR-aggregate systems.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Adhesion can be defined as the interfacial strength between twomaterials (e.g., asphalt and aggregate) [1]. The physical adhesioncomponent probably contributes more to the overall adhesion ofasphalt–aggregate interfaces than mechanical interlocking andchemical interactions [2]. This physical adhesion, and the loss ofit due to the presence of water (i.e., debonding), of asphalt–aggre-gate interfaces in hot mix asphalt (HMA) mixtures can be calculatedbased on the surface free energy (SFE) components of the corre-sponding asphalt and aggregate. In addition, the wettability of theasphalt over the aggregate can be quantitatively evaluated basedon the SFE components of these materials.

ll rights reserved.

rez).

Measurements of SFE—a fundamental material property—andcomputation of both the physical adhesion and wettability haveonly been recently applied for characterization of materials usedin paving HMA mixtures and subsequent improvement of HMAmix design and performance [1]. Specific previous applicationsincluded analysis of the effect of diverse asphalt [3,4] andaggregate [5] modification processes, evaluation of the mineralfiller effect [6] and aging effects [1] on HMA mixtures, and char-acterization of additives [7] and fatigue resistance [8] of warmmix asphalt. However, at present there is limited informationon the assessment of the quality of adhesion and wettabilitydeveloped by asphalt rubber (AR) and polymer modified (PM) as-phalt binders (or asphalts).

Therefore, based on SFE measurements, and subsequentcalculation of energy parameters (or indices), this paper comparesasphalt–aggregate interfaces formed with both asphalt rubber(AR) and polymer modified (PM) asphalts specified for fabrication

386 A.E. Alvarez et al. / Construction and Building Materials 35 (2012) 385–392

of permeable friction course (PFC) mixtures. These mixtures arealso termed new generation open-graded friction course, andanalogous European mixtures are identified as Porous Asphalt.PFC mixtures are a special type of HMA mixture used as surfacingcourse and characterized by a high total air voids content (i.e.,18–22% by design in Texas PFC mixtures) and interconnectedair voids content—also termed effective air voids content. Thehigh air voids content generates high permeability for water trav-eling through the mixture to prevent hydroplaning and enhancesafety for traffic traveling in wet weather conditions. The con-nected air voids structure also provides noise reduction capacityto the PFC mixture [9], and the higher macrotexture of these mix-tures, as compared to that of conventional dense-graded HMA,leads to improved safety for users.

In addition, as compared to conventional dense-graded HMA,the high connected air voids content of PFC mixtures allow oxy-gen and water more access into the mixture microstructure,and therefore, implies the need of high quality asphalt–aggregatesystems to ensure mixture durability in terms of proper resis-tance to aging, moisture damage, and adhesive fracture to preventraveling. Raveling—mixture disintegration starting at the pave-ment surface—is the distress most frequently reported in PFCmixtures [10]. To minimize the development of these phenomenawith a favors on raveling, PFC mix design practice includes: (i)use of high asphalt contents to provide proper aggregate coatingwith thick asphalt films, (ii) addition of mineral- or cellulose-fi-bers to successfully prevent draindown in the thick asphalt films,and (iii) use of modified asphalts.

Previous studies [11,12] reported that thick asphalt films areassociated with durable HMA mixtures that are less susceptibleto moisture damage. Correspondingly, the specified asphalt con-tent for Texas PFC mixtures fabricated using AR asphalts (or AR-PFC mixtures) is in the range of 8–10%, and that for mixtures con-structed using PM asphalts (or PM-PFC mixtures) is between 5.5%and 7% [13]. Inclusion of fibers in PM-PFC mixtures is now a com-mon practice, whereas AR-PFC mixtures do not include any typeof fibers. In addition, although some states still use unmodifiedasphalts, the use of modified asphalts (i.e., PM- and AR-asphalts)is currently the most common practice in The United States [9]for PFC mixture fabrication. This practice is supported by exten-sive research that highlighted the advantages of the modified as-phalts over neat asphalts for PFC mixtures [14–16]. Despite thesePFC design practices, adequate response of the asphalt–aggregateinterface at the micro-level (i.e., adhesion and wettability) is stillcritical to ensure appropriate PFC mixture durability.

Therefore, based on SFE measurements of asphalts and aggre-gates and computation of energy indices to characterize asphalt–aggregate interfaces, this paper provides additional insight to bet-ter understand some differences in the durability exhibited byAR-PFC mixtures and PM-PFC mixtures. Previous researchdocumented differences between AR-PFC and PM-PFC mix-tures—based on macroscopic evaluations—in terms of mixtureresistance to disintegration (i.e., raveling) [17–19], permanentdeformation [17–20], fatigue, and moisture damage [21]. How-ever, PFC mixtures fabricated with both AR- and PM-asphaltsare currently designed following similar specifications [18]. Theanalysis conducted in this research is also useful to advance thecharacterization of both AR- and PM-asphalts and explore, usinga fundamental material property (i.e., SFE), the differences be-tween these two types of asphalts. In the paper, after this intro-ductory section, the objectives and methodology are followedby a section describing the basis of SFE and energy indices usedfor evaluating the asphalt (AR and PM)-aggregate interfaces. Thenthe materials and methods are presented, followed by results andanalysis. Conclusions and recommendations complete the paper.

2. Objective and methodology

The main objective of this paper is to provide a comparison ofasphalt–aggregate interfaces formed with AR- and PM-asphalts(specified for fabrication of PFC mixtures in Texas) in terms ofenergy indices to assess the expected durability of PFC mixturesfabricated with these two asphalt types and optimize the corre-sponding materials selection.

Achievement of this objective included these main tasks:

� Laboratory testing for SFE measurement of aggregates, ARasphalts, and PM asphalts and calculation of correspondingSFE components.� Calculation of energy parameters for quantitative comparison of

AR asphalt–aggregate and PM asphalt–aggregate systems.

3. Surface free energy (SFE) and indices used for assessment ofthe AR- and PM-asphalts

The total SFE (or SFE) is defined as the amount of energy re-quired to create a new surface of unit area in a given material un-der vacuum [22]. As indicated by the Good–Van Oss–Chaudhurytheory, for a given material this total SFE can be decomposed inthe following components: (i) non-polar, CLW, (ii) monopolar basic,C�, and (iii) monopolar acid, C+ [2].

Using the SFE components values for asphalts, aggregates, andwater, which are subsequently represented by the subscripts A, S,and W, respectively, several energy parameters were calculated toquantitatively compare the AR asphalt–aggregate and PMasphalt–aggregate interfaces. These energy parameters are summa-rized in Table 1. The AW and SW components in Eq. (2) are calcu-lated using Eq. (1) and applying, respectively, the SFE componentsof asphalt (A) and water (W), and aggregate (S) and water (W).The term WAA corresponds to the asphalt work of cohesion—energyto be supplied for propagating an existing crack at an asphalt–as-phalt interface creating two new surfaces of unit area—and iscalculated by replacing twice in Eq. (1) the SFE components deter-mined for the asphalt. Previous research provided evidence of goodcorrelation between both the work of adhesion and the energy ratio1 index and the field- and laboratory-performance of HMA [3,23,24]and served as criterion for application of these energy parametersin this study.

The work of adhesion corresponds to a quantitative index ofphysical adhesion and is defined as the amount of energy thatshould be supplied to create two new surfaces of unit area by prop-agating an existing crack at the interface of two materials, forexample, an aggregate–asphalt interface [23]. The energy ratio 1index (ER1) allows identification of asphalt–aggregate systemswith low moisture damage susceptibility—based on low absolutevalues of work of adhesion in wet condition—and high resistanceto fracture—based on high values of work of adhesion in dry condi-tion. The spreading coefficient (SC) is a quantitative index of thewettability of the asphalt over the aggregate and the energy ratio2 index (ER2) permits identification of asphalt–aggregate systemswith high wettability and low moisture damage susceptibility—based on low absolute values of work of adhesion in wet conditionðWwet

WASÞ.

4. Materials and methods

Table 2 summarizes the main characteristics of both the asphalts and aggre-gates evaluated in terms of SFE for subsequent assessment of the corresponding as-phalt–aggregate interfaces. In terms of the asphalts, the table includes thenomenclature used to identify them, observations on its classification, code forthe neat base asphalt, and corresponding field section were the asphalt was usedto produce the PFC mixture. A common value of the code for the neat base asphalt

Table 1Energy indices evaluated.

Energy index Equation Units

Work of adhesion in dry condition (i.e., absence of water

at the asphalt–aggregate interface); WdryAS

WdryAS ¼ cAS ¼ 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCLW

A CLWs

q¼ 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiCþA C�s

qþ 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiC�A Cþs

q(1) Ergs/cm2

Work of adhesion in wet condition; WwetWAS Wwet

WAS ¼ cAW þ cSW ¼ cAS(2) Ergs/cm2

Energy ratio 1 index (or ER1) [4] ER1 ¼Wdry

AS

jWwetWAS j

(3) –

Spreading coefficient (or SC) [7] SC ¼WdryAS �WAA

(4) Ergs/cm2

Energy ratio 2 index (or ER2) [3] ER2 ¼Wdry

AS �WAA

jWwetWAS j

(5) –

Table 2Asphalts and aggregates evaluated.

Asphalts Aggregates

Modified asphalt type Nomenclature Observations Code of neat base asphalt PFC field section

Polymer Modified (PM) 1-PM PG 76-22 1 I-35-PG2-PM PG 76-22 1 Not available3-PM PG 76-22 2 IH-20-PG Limestone I4-PM PG 76-22 US 59Y-PG5-PM PG 76-22 IH 30-PG Limestone II6-PM PG 76-22 US 59-PG Sandstone I

Asphalt Rubber (AR) TR – 2 IH-20 Granite1-AR Type II AR, Grade B (AC-10 w/16% CR) 1 US-281 Quartzite2-AR Type II AR, Grade B (AC-10 w/17% CR) 3 US 2903-AR Type II AR, Grade B (AC-10 w/17% CR) 3 US-288-AR

CR = Crumb rubber.

A.E. Alvarez et al. / Construction and Building Materials 35 (2012) 385–392 387

identifies the modified asphalts that were produced using the same source of neatasphalt. Each neat base asphalt coming from a common source, however, was ob-tained from different production batches (i.e., there exists batch variability, forexample, between the 1-PM and 2-PM asphalts).

As per Texas specification, fabrication of PFC mixtures requires either a Type I-or II-AR asphalt or a Performance Grade (PG) asphalt with a minimum high temper-ature grade of PG 76-XX [13]. The three AR asphalts evaluated were industriallyprepared (i.e., reacted at elevated temperature) by addition of grade B crumb rubberto the neat base asphalt. These AR asphalts differed in the neat base asphalt used toprepare them (i.e., in terms of its source or its production batch). In addition, inaccordance with the specification, these AR asphalts should contain a minimumof 15%, by weight of virgin asphalt, of grade B or grade C crumb rubber. The gradeB (or C) refers to the crumb rubber gradation as defined in Item 300.2.G of theTxDOT specifications [13]. These gradations are presented in Table 3.

In addition, an asphalt modified with inclusion of crumb rubber at 7% (termedin Table 2 as TR) was evaluated for comparison purposes. All the PM asphalts eval-uated in this research classified as PG-76–22. These asphalts were also industriallyprepared based on the addition of polymers to different neat base asphalts distilledfrom different sources. Particular information on the amount and polymer typeused to produce the PG asphalts was not disclosed by the asphalt producers.

The Wilhelmy plate method was applied in this study to determine the dynamiccontact angle (h) between a probe liquid and a solid surface (i.e., thin regular asphaltfilm at 25 �C coating a thin glass slide that serves as support for the asphalt film),which allowed subsequent computation of the SFE components of the asphalts. De-tails on this computation are reported elsewhere [25,26]. Corresponding laboratorytesting was conducted in accordance with the procedure and probe liquids—se-lected based on analysis of the ‘‘condition number’’—suggested by Hefer et al.[25]. The condition number, calculated based on a matrix of the SFE components

Table 3Gradation of crumb rubber (TxDOT specification).

Sieve size (% passing) Grade B Grade C

Min. Max. Min. Max.

#8 (2.36 mm) – – – –#10 (2 mm) 100 – – –#16 (1.18 mm) 70 100 100 –#30 (0.6 mm) 25 60 90 100#40 (0.425 mm) – – 45 100#50 (0.3 mm) – – – –#200 (0.075 mm) 0 5 – –

of the selected probe liquids, is a mathematical index of the sensitivity of the com-puted SFE components to small experimental errors in the measurement of contactangles. The probe liquids indicated by Hefer et al. [25] corresponded to distilledwater, glycerol, formamide, ethylene glycol, and methylene iodide (diiodometh-ane). The SFE computations were based on the advancing contact angle formed be-tween each of the five probe liquids and the asphalt evaluated.

Although only three probe liquids are required for computation of the asphaltSFE components, five probe liquids were used in the laboratory to improve the reli-ability of the computation. Measurement of the advancing contact angle with eachprobe liquid included a minimum of four replicate specimens. In all cases, thesereplicate measurements exhibited a coefficient of variation smaller than 8.6% (meancoefficient of variation equal to 1.38%). Final selection of the probe liquids includedin the SFE computation required the analysis of the CLcosh versus CL plot, where CL

is the total SFE of the probe liquid. As recommended in previous research [25], aprobe liquid deviating from a smooth curve plot of CLcosh versus CL should be ex-cluded for the SFE calculation. Previous research [25,27] documents additional de-tails on the equipment and laboratory testing procedure for the Wilhelmy platemethod.

The 1-AR, 2-AR, and 3-AR asphalts were heated at the specified mixing temper-ature (i.e., 163 �C), passed through the no. 80 (0.2 mm) sieve, and immediately sam-pled at this temperature—while the asphalt temperature was kept at 163 �C using ahot plate—for the SFE specimens (i.e., thin glass slides coated with asphalt). Thisprocess was required to separate the coarser fractions of rubber. These fractionswere removed, since testing with the original AR asphalt proved inadequate dueto the formation of irregular, thick asphalt profiles on the glass slides, whereas thin,smooth asphalt profiles—obtained based on this procedure—are required to meetthe operation principle of the Wilhelmy plate method. The TR- and PM-asphaltsdid not require any specific treatment to successfully prepare the SFE specimens.

Evaluation of asphalt–aggregate interfaces included five aggregates, used in ac-tual fabrication of HMA mixtures, of different mineralogical composition (Table 2).These aggregates were previously characterized [28] in terms of their SFE compo-nents by means of the Universal Sorption Device in accordance with the procedurepresented by Bhasin and Little [29]. Corresponding testing used three probe vapors,namely water, nhexane, and methyl propyl ketone. Details on this equipment andthe corresponding testing procedure can be found in previous work [29,30].

5. Results and analysis

This section presents the results and comparison analysis of theenergy indices computed for the asphalt–aggregate interfacescharacterized. The results are presented in terms of: (i) work ofadhesion in dry condition (i.e., resistance to fracture), (ii) work of

388 A.E. Alvarez et al. / Construction and Building Materials 35 (2012) 385–392

adhesion in wet condition and energy ratio 1 index (i.e., moisturedamage susceptibility), and (iii) spreading coefficient and energyratio 2 index (wettability of the asphalt over the aggregate).

Results of the testing program conducted provided evidence ofsimilar orders of magnitude for the components and total values ofSFE computed for both the AR- and PM-asphalts characterized.Therefore, these SFE values led to comparable values of the energyparameters as subsequently discussed. Previous research [1] indi-cated the limitations of comparing materials based on values ofSFE components, since the magnitudes of the acid and base SFEcomponents (C+ and C�, respectively) are calculated using a rela-tive scale of acid–base components. Therefore, further comparisonsof the AR- and PM-asphalts was not conducted based on the corre-sponding SFE information, but in terms of the energy parametersusing the selected aggregates as reference materials for potentialasphalt–aggregate combinations.

In addition, future research is suggested to evaluate and com-pare additional methods for the extraction of the coarser fractionsof crumb rubber in the AR to enable standardization of properspecimen fabrication for SFE measurement using the Wilhelmyplate method. At this point, mechanical separation (sieving) is be-lieved to provide asphalt specimens exhibiting sufficiently repre-sentative chemical composition of the AR for SFE measurementpurposes.

5.1. Comparison of resistance to fracture (work of adhesion in drycondition)

Resistance to fracture under repeated traffic load (i.e., fatiguelife) is an important feature of HMA mixtures required to ensuretheir durability. Although conventional fatigue cracking is not themain concern in PFC mixtures, sufficient resistance to fracture atthe asphalt–aggregate interface can contribute to prevent ravelingin properly designed and constructed PFC mixtures. In this context,Fig. 1 shows the values of work of adhesion in dry condition (i.e.,without water at the asphalt–aggregate interface) for the asphalt–aggregate combinations of both PM- and AR-asphalts. Asphalt–aggregate interfaces with high resistance to fracture—and longerexpected fatigue life—develop high values of work of adhesion.

As shown in Fig. 1, the values of work of adhesion computed forthe set of AR asphalt–aggregate interfaces are comparable to thosedeveloped by the set of PM asphalt–aggregate interfaces, providingevidence of comparable resistance to fracture in both systems. Arigorous statistical comparison of both sets was not pursued, sincethe results presented in Fig. 1 are representative of a group ofaggregates conventionally used for fabrication of HMA, but the val-ues of work of adhesion computed will change as different materi-als combinations are evaluated.

A specific comparison of the 1-PM, 2-PM, and 1-AR asphalts ispossible, since they were modified based on the same source ofneat asphalt obtained from different production batches (Table2). This comparison suggests that the modification via rubber addi-tion can lead to production of an AR that develops smaller values ofwork of adhesion in dry condition as compared to the asphalt mod-ified by polymer addition. For example, for the asphalt–granitecombination, the reduction in the work of adhesion in dry condi-tion was approximately 20% when comparing the 1-PM and 1-ARasphalts. However, this comparison can be limited, since the differ-ences in the values of work of adhesion can also be related to thebatch variability of the base asphalt properties, which are reflectedon differences in the SFE component values. Examples of thisvariability can be observed by comparing the 1-PM and 2-PM as-phalts as well as the 2-AR and 3-AR asphalts. Each of these pairswere fabricated using a common base—neat—asphalt as indicatein the Table 2. The differences in the values of work of adhesion

for these pairs are comparable to those discussed for the 1-PMand 1-AR asphalts (Fig. 1).

In addition, the variability of the work of adhesion in dry condi-tion values computed for the set of five aggregates combined witheach of the PM asphalts was evaluated in terms of the coefficient ofvariation (COV). The maximum COV value (14%) was computed forthe 3-PM-aggregate interfaces, while the minimum COV value (8%)was computed for the 5-PM-aggregate interfaces. For the set of ARasphalts, the maximum (14%) and minimum (12%) COV valueswere computed, respectively, for the 1-AR-aggregate and 3-AR-aggregate material combinations. These COV values provide evi-dence of similar variability in the asphalt–aggregate interfaces ob-tained with the AR- and PM-asphalts evaluated.

As indicated, all the PM asphalts evaluated in this research clas-sified as PG-76-22. Therefore, asphalt binders classified as similarin terms of their rheological properties can exhibit significant dif-ferences in terms of their thermodynamic properties (i.e., SFE)and consequently, in terms of the resistance to fracture whenforming asphalt–aggregate systems. The differences in SFE can berelated to discrepancies in the chemical composition of the modi-fied asphalts, since previous research [31] concluded the existenceof correlation between the SFE and chemical composition ofasphalts.

The values of work of adhesion in dry condition computed forthe TR asphalt (with inclusion of crumb rubber at 7%)-aggregatecombinations are comparable to those computed for the PM andAR asphalt–aggregate combinations, which suggests analogous re-sponses in term of resistance to fracture for the asphalt–aggregateinterfaces of mixtures fabricated using the TR asphalt and some ofthe PM- and AR-asphalts evaluated (i.e., TR, 6-PM, and 2-AR).

5.2. Comparison of susceptibility to moisture damage (work ofadhesion in wet condition and energy ratio 1 index, ER1)

Moisture damage is one of the most recurrent phenomenonaffecting the performance and service life of HMA and resultingdistresses lead to increased maintenance and rehabilitation costsfor pavement structures [32]. Several aspects have been high-lighted in previous studies to reduce the susceptibility to moisturedamage in HMA, among them, proper selection of asphalts andaggregates [11,32]. Given the high total and connected air voidscontent in PFC mixtures (i.e., 18–22% total air voids content by de-sign in Texas [33]), water travels through the mixture and drainsalong the bottom surface of the PFC course.

However, a portion of the water flowing throughout the con-nected air voids in the compacted PFC mixture can be retainedand then contribute to the moisture damage process. Comparisonof weight measurements on compacted PFC specimens subjectedto immersion for at least 4 min and then dried at room tempera-ture for a minimum time of 24 h—and up to 60 h—under forcedventilation, were conducted as part of a parallel study and pro-vided evidence of water trapped in PFC mixtures after this dryingprocess. Most probably, this response can be replicated in fieldcompacted PFC mixtures after rain events and is due to the diffi-culty in draining water from small air voids in the mixture, wherehigh suction values can be developed. In addition, irregular lateral-and longitudinal-profiles can generate temporal or permanentpresence of water at the bottom of the PFC courses. Therefore, de-spite the high permeability of PFC mixtures, moisture damage canbe a critical distress affecting the durability of these mixtures.

In this research, the susceptibility to moisture damage for theasphalt–aggregate combinations of both PM- and AR-asphaltswas analyzed in terms of both the work of adhesion in wet condi-tion (i.e., with water at the asphalt–aggregate interface) and ER1 in-dex values. Corresponding values are presented in Figs. 2 and 3.The work of adhesion in wet condition consistently showed nega-

Fig. 1. Values of work of adhesion in dry condition for PM- and AR-asphalts.

A.E. Alvarez et al. / Construction and Building Materials 35 (2012) 385–392 389

tive values, which indicated thermodynamic potential for thewater to disrupt the asphalt–aggregate interfaces. Therefore, evenin the absence of external energy, the asphalt–aggregate interfacecan be disrupted by the water. Asphalt–aggregate systems withsmall absolute values of work of adhesion in wet condition arecharacterized by reduced susceptibility to moisture damage [30].

In addition, high values of the ER1 index are required to simul-taneously assess resistance to fracture and moisture damage.Based on a comparison of field performance (i.e., moisture damageresistance) and ER1 index values, Bhasin et al. [34] proposed threecategories for moisture damage resistance in the field for dense-graded HMA mixtures and corresponding threshold values forthe ER1 index. ER1 index values between 1.5 and 0.5 are associatedwith medium resistance to moisture damage. Higher and lowervalues of the index are related, respectively, to HMA mixtures withhigh and low resistance to moisture damage. These reference val-ues were used in this research to comparatively evaluate the po-tential moisture damage resistance of asphalt–aggregateinterfaces in PFC mixtures. However, future research should beconducted to corroborate the applicability of the same thresholdvalues defined for dense-graded HMA mixtures to PFC mixtures.

As indicated in terms of the work of adhesion in dry condition,the values of both work of adhesion in wet condition and the ER1

index computed for the AR asphalt–aggregate interfaces are com-parable to those developed by the PM asphalt–aggregate interfaces(Figs. 2 and 3), providing evidence of comparable resistance to frac-ture and moisture damage in both types of systems. An evaluationof the ER1 index data (Fig. 3) based on the threshold values sug-gested by Bhasin et al. [34] leads to the conclusion that most ofthe asphalt–aggregate interfaces evaluated (including all the AR-and PM-asphalts) fall in the category of medium resistance tomoisture damage. Direct inspection of the values in this categoryprovides additional evidence of the variability that can be expectedin the response of similar PM asphalts based on PG grade (i.e., PG-76-22 asphalts).

The comparison of the 1-PM, 2-PM, and 1-AR asphalt—fabri-cated using the same neat base asphalt—leads to the conclusionthat the AR asphalt–aggregate system evaluated has a higher sus-ceptibility to moisture damage as compared to the systems formedwith the two PM asphalts (Figs. 2 and 3). However, as previouslydiscussed, this comparison can be limited, because the differencesin the values of work of adhesion in wet condition can also be re-lated to the variability of the base asphalt properties. As previouslyillustrated, examples of this variability can be observed by compar-ing the 1-PM and 2-PM asphalts as well as the 2-AR and 3-AR as-phalts in Fig. 2.

Comparable values of COV were determined for the work ofadhesion in wet condition values computed for the set of fiveaggregates combined with each of the PM- and AR-asphalts (i.e.,the COV for each PM asphalt was computed by grouping the values

of work of adhesion in wet condition computed for each asphaltwith the five aggregates evaluated). This comparable COV valuessuggests similar variability in the resistance to moisture damage,evaluated in terms of the work of adhesion in wet condition, ofAR asphalt–aggregate interfaces as compared to the PM asphalt–aggregate interfaces.

The data presented in Figs. 2 and 3 reinforce the previous dis-cussion on the important differences in terms of resistance to frac-ture and moisture damage that can be exhibited by asphaltsclassified as similar based on their rheological properties. Forexample, the 3-PM and 4-PM asphalts—both classified as PG 76-22—combined with all the aggregates evaluated led to values ofthe ER1 index differing by more than 100%. In addition, the valuesof work of adhesion in wet condition computed for the TR asphalt–aggregate combinations are again comparable to those computedfor the PM and AR asphalt–aggregate combinations, which sug-gests analogous responses in term of resistance to moisture dam-age for the asphalt–aggregate interfaces of mixtures fabricatedusing the TR asphalt and some of the PM- and AR-asphalts evalu-ated (i.e., TR, 3-PM, 6-PM, and 2-AR as shown in Fig. 2).

Specific information to fundamentally explain the differencesand similarities in the response captured for the AR- and PM-as-phalts, as discussed in terms of the energy indices evaluated to as-sess the resistance to both fracture and moisture damage, is out ofthe scope of this paper. However, future research should focus onexploring the differences in chemical composition of the modifiedasphalts and the effect of both rubber and polymer in terms of theSFE component values of the modified asphalts.

The conclusions previously discussed are particular and applyonly to the material combinations assessed. However, the data in-cluded in Figs. 1–3 showed the potential similarities and differ-ences of AR- and PM-asphalts in terms of the quality of adhesion(i.e., resistance to fracture) and susceptibility to moisture damagethat corresponding asphalt–aggregate systems can develop. Theanalysis of the energy parameters discussed can also be usefulfor selection of the optimum material combinations (i.e., maximumresistance to fracture and minimum susceptibility to moisturedamage) for mix design of HMA mixtures, including PFC mixtures,as discussed in previous literature [24].

5.3. Comparison of wettability (spreading coefficient and energy ratio2 index, ER2)

As previously discussed, proper aggregate coating with thick as-phalt films is critical in PFC mixtures to minimize raveling issues.Figs. 4 and 5 show, respectively, the values of the spreading coeffi-cient and ER2 index computed for all the asphalts and aggregatesanalyzed. The spreading coefficient defined in Eq. (4) is a measureof the wettability—ability of the asphalt to wet, or coat, the aggre-gate surface—of a liquid (hot asphalt) over a solid (aggregate) [3].

Fig. 2. Values of work of adhesion in wet condition for PM- and AR-asphalts.

Fig. 3. Values of energy ratio 1 index (ER1) for PM- and AR-asphalts.

Fig. 4. Values of spreading coefficient for PM- and AR-asphalts.

Fig. 5. Values of energy ratio 2 index (ER2) for PM- and AR-asphalts.

390 A.E. Alvarez et al. / Construction and Building Materials 35 (2012) 385–392

Improved ability of the asphalt to coat the aggregate surface is re-lated to high values of the spreading coefficient. In addition, as sug-gested in previous research, proper asphalt coating is related tobetter asphalt–aggregate mechanical interlocking [3]. High values

of the ER2 index (Eq. (5)) are related to asphalt–aggregate systemswith high wettability and low moisture damage susceptibility.

The data shown in Figs. 4 and 5 provided evidence of compa-rable wettability and moisture damage susceptibility for both the

A.E. Alvarez et al. / Construction and Building Materials 35 (2012) 385–392 391

PM- and AR-aggregate interfaces evaluated. As discussed for theenergy indices previously presented, a wide range of wettabilityvalues—spreading coefficients varying from 38 to 133 ergs/cm2—was also identified for the PM asphalts classified as PG-76-22 as-phalts. Additional evidence of the variability of the PM- and AR-asphalts is shown by the spreading coefficient values (Fig. 4)and ER2 index values (Fig. 5) reported for each pair: 1-PM and2-PM asphalts as well as 2-AR and 3-AR asphalts. Although thesepairs were fabricated using a neat base asphalt from the samesource, the differences in wettability (SC) and moisture damagesusceptibility (ER2 index) are evident.

The SC values computed for the TR asphalt–aggregate combi-nations are also comparable to that computed for the PM andAR asphalt–aggregate combinations evaluated. This response sug-gests that the different modifications (i.e., TR, AR, PM) led to com-parable wettability properties, although the variability in eachgroup of asphalts makes it difficult to differentiate.

As previously discussed, based on macroscopic evaluation, pub-lished research [17–21] documented differences between AR-PFCand PM-PFC mixtures. The energy parameters analyzed in this re-search can be applied to partially explain the differences betweenthese PFC mixtures. However, a systematic approach, based onSFE measurements, computation of the energy indices included inthis study, and a comprehensive study of the PFC mixtures perfor-mance in the laboratory and field should be conducted to permitthe practical application of the concepts herein discussed. This ap-proach can lead to the enhancement of PFC mix design.

6. Conclusions and recommendations

This paper presents a comparison of asphalt–aggregate inter-faces formed with asphalt rubber (AR) and polymer modified(PM) asphalt binders specified for fabrication of permeable frictioncourse (PFC) mixtures in Texas. The comparison was accomplishedin terms of energy indices (e.g., work of adhesion in both dry- andwet-conditions, energy ratio 1 index (ER1), energy ratio 2 index(ER2), and spreading coefficient) computed based on SFE measure-ments of the asphalts and aggregates used in the asphalt mixtures.The analysis and discussion presented suggested the followingconclusions and recommendations:

� The energy indices computed for the AR- and PM-aggregate sys-tems suggest that these systems can present similar responsesin terms of fracture resistance, moisture damage susceptibility,and wettability of the asphalt over the aggregate. However, thisconclusion is restricted by the variability encountered for par-ticular asphalts grouped as AR based on their composition(i.e., fabricated from similar AC-10 asphalts and rubber contentaddition) and grouped as PM based on their performance grade(i.e., PG 76-22).� In terms of variability, more specifically, asphalt binders classi-

fied as similar in terms of their rheological properties (e.g., PMasphalts classified as PG 76-22) exhibited significant differencesin terms of their thermodynamic properties (i.e., SFE) and con-sequently, in terms of the resistance to fracture, moisture dam-age resistance and wettability as evaluated in terms of theenergy indices computed for the asphalt–aggregate systemsevaluated. Therefore, PFC mix design—and in general HMAmix design—can be improved based on the calculation of energyindices based on SFE measurements of asphalts and aggregates.� The differences in SFE can be related to discrepancies in the

chemical composition of the modified asphalts, and additionalresearch is required to further explore this relation and funda-mentally explain the differences here reported in terms of theenergy indices. In addition, a systematic approach, based on

SFE measurements, computation of the energy indices includedin this study, and a comprehensive study of the PFC mixturesperformance should be conducted to permit the practical appli-cation of the concepts herein discussed, and the subsequentimprovement of PFC mix design.� A simple technique was applied in this research to successfully

produce the specimens required for testing the SFE of AR bind-ers based on the Wilhelmy plate method principle. However,alternative technique(s) may be required to further validatethese determinations. Additional research should also be con-ducted to determine the effect of lime addition on the responseof the PM asphalts, since the PM-PFC mixtures are often fabri-cated with inclusion of lime to minimize the susceptibility tomoisture damage. The effect of the mineral filler on PM-andAR-asphalts is another factor to be explored in the future, sincethis mixture component can be critical in the performance ofPFC mixtures.

Acknowledgements

This study was based on work originally conducted for theTexas Department of Transportation (TxDOT), and the authorsthank TxDOT and the Federal Highway Administration for theirsupport in funding the initial study. The first author, as AssociateProfessor of the University of Magdalena (Colombia), also thanksthe full support provided by the University of Magdalena, throughFONCIENCIAS, to complete this research study.

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