Comparison of permeable friction course mixtures fabricated using asphalt rubber and performance-grade asphalt binders

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    Article history:

    a b s t r a c t

    to conventional dense-graded HMA mixtures, in terms of safety,economy, and the environment [2].

    PFC mixtures are currently fabricated using either performancegrade (PG) or asphalt rubber (AR) asphalt-binders (or asphalts),which in this paper are termed as PGPFC and ARPFC mixtures,respectively. Particular aggregate gradations are also specied foreach type of PFC mixture [3] to allocate different amounts of

    At present, PGPFC and ARPFC mixtures are assessed anddesigned using the same laboratory methods, and similar criteriaare applied for the mix design and performance evaluation [4].Despite the fact that some differences could exist, little has beendone to comprehensively evaluate and discretely differentiate thematerial characteristic properties of these two mixture types, i.e.,PGPFC versus ARPFC. As summarized by Alvarez et al. [5], sev-eral recent studies focused on improving the PFC mix design, bytreating both PGPFC and ARPFC mixtures as materials with sim-ilar properties and response. However, previous research [6] thatmeasured the acoustical absorption of various mixtures using a

    Corresponding author. Tel./fax: +57 5 4301292.

    Construction and Building Materials 28 (2012) 427436

    Contents lists available at

    B

    evE-mail address: allexalvarez@yahoo.com (A.E. Alvarez).1. Introduction

    Permeable friction course (PFC) mixtures constitute a particulartype of hot mix asphalt (HMA) characterized by a high intercon-nected air voids (AV) content and a coarse granular skeleton withstone-on-stone contact. These mixtures are also termed as newgeneration open graded friction course mixtures. The high AV con-tent provides high permeability and the stone-on-stone contactensures proper resistance to permanent deformation and ravel-ingthe distress most frequently reported as the primary causeof failure in PFC mixtures [1]. Use of PFC mixtures is guaranteedbased on the advantages that these mixtures offer, as compared

    asphalt at similar total AV contents (e.g., 18%22% in Texas). Theasphalts currently specied in Texas [3] for PFC mixtures are:

    a PG76-XX asphalt: Addition of a minimum of 1.0% of lime (byweight of dry aggregate) and mineral or cellulose bers (at aminimum content of 0.2% by weight of the mixture) are alsospecied when using the PG76-XX asphalt,

    a Type I- or II-AR: The corresponding minimum content ofcrumb rubber (by weight of asphalt) specied is 15%. Particularspecications for the Types I- and II-AR are reported elsewhere[3].Received 13 April 2011Received in revised form 30 August 2011Accepted 30 August 2011Available online 4 November 2011

    Keywords:Permeable friction course mixture (PFC)Performance grade (PG)Asphalt rubber (AR)Hot mix asphalt (HMA)Mix design0950-0618/$ - see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.08.085Permeable friction course mixtures (PFC) are special hot mix asphalt (HMA) mixtures that are designed toimprove motorist safety and reduce trafc noise (i.e., tirepavement noise). In terms of pavement struc-tural design and construction, PFC mixtures are typically used as the surfacing course layer where inaddition to improving the skid resistance, also serve as the surface drainage layer. This paper comparesPFC mixtures designed and fabricated using performance grade (PG) and asphalt rubber (AR) asphalt-binders (or asphalts). The experimental design included assessment of total air voids (AV) content, dura-bility, drainability, stone-on-stone contact, and internal structure of the PFC mixtures fabricated usingboth AR and PG asphalts; denoted herein as ARPFC and PGPFC mixtures, respectively. The analysis con-ducted provided evidence of differences between ARPFC and PGPFC mixtures that suggest the need fordifferentiation and renement of the existing specications to consider these mixtures as independentmaterials. Future research should, therefore, focus on dening particular specications for mix designand control for ARPFC and PG-PFC mixtures.

    2011 Elsevier Ltd. All rights reserved.Comparison of permeable friction courseand performance-grade asphalt binders

    Allex E. Alvarez a,, Elvia M. Fernandez a, Amy Epps MLubinda F. Walubita e

    aDepartment of Civil Engineering, University of Magdalena, Santa Marta, Colombiab Zachry Department of Civil Engineering, Texas A&M University, College Station, TX 778cDepartment of Civil Engineering, Nueva Granada Military University, Bogot D.C., Colod Faculty of Engineering and the Built Environment, University of the Witwatersrand, Joe TTI-The Texas A&M University System, College Station, TX 77843, USA

    Construction and

    journal homepage: www.elsll rights reserved.ixtures fabricated using asphalt rubber

    rtin b, Oscar J. Reyes c, Geoffrey S. Simate d,

    USA

    esburg, South Africa

    SciVerse ScienceDirect

    uilding Materials

    ier .com/locate /conbui ldmat

  • Ag

    SaSaLimLimGrLimSaGrSaGrSaGr

    = Cru

    -PG 1-AR 2-AR 3-AR 4-AR 5-AR 6-AR Specication-AR

    BuiBruel & Kjaer Type 4206A impedance tube showed relatively high-er absorption coefcients for the ARPFC mixtures than for the PGPFC mixtures. The acoustic (sound) absorption coefcient is oftenused to relate the noise reduction potential of a mixture. This coef-

    Table 1Description of mixtures.

    Mixture Asphalt type OAC (%)

    I-35-PG or 1-PG PG 7622 6.1IH-30-PG or 2-PG PG 7622 6.6IH-20-PG or 3-PG PG 7622 6.5US-83-PG or 4-PG PG 7622 6.4US-59-PG or 5-PG PG 7622 5.9US-59Y-PG or 6-PG PG 7622 5.8US-281-AR or 1-AR Type II AR, grade B (AC-10 + 16% CR) 8.1US-288-AR or 2-AR Type II AR, grade B (AC-10 + 17% CR) 8.0US-290-AR or 3-AR Type II AR, grade B (AC-10 + 17% CR) 8.3SH-6-AR or 4-AR Type II AR, grade B (AC 10 + 17% CR) 8.2IH-35-AR or 5-AR Type II AR, grade B (AC-10 + 17% CR) 8.4IH-10-AR or 6-AR Type I AR, grade C (AC-10 + 17% CR) 8.7

    Note: PG = Performance grade; AR = Asphalt rubber; AC = Asphalt cement; CRSGC = Superpave Gyratory Compactor.

    Table 2Gradation of aggregates used in both ARPFC and PGPFC mixtures (% Passing).

    Sieve 1-PG 2-PG 3-PG 4-PG 5-PG 6-PG Specication

    3=4 100 100 100 100 100 100 100100 90.3 81 85.3 90.5 80.2 84.5 80100

    3/8 59.5 43 59.4 50.9 57.7 52.8 3560#4 10.1 15.5 18.6 3.2 15.9 6.6 120#8 5.2 6.7 2 1.5 6 4.2 110

    #200 2.3 2.2 1.6 1.1 2.1 2.4 14

    428 A.E. Alvarez et al. / Construction andcient is a function of the AV content and represents the propor-tion of acoustic energy not reected by the surface of thematerial for a normal incidence plane wave [6]. The noise reduc-tion potential of PFC surfacing in pavements arises predominantlyfrom their high total AV content; i.e., the higher the total AV con-tent the greater the potential to absorb sound and thus minimizenoise.

    With this background, this paper provides a comparison of thePGPFC and ARPFC mixtures, in terms of the volumetric proper-ties, durability, drainability, stone-on-stone contact, and internalstructure. Primarily, the objective of the study was to identify thedifferences between PGPFC and ARPFC mixtures so as to providetechnical motivation as to the possible need to differentiate and re-ne the existing specications.

    2. Materials and methods

    This section summarizes the characteristics of the mixtures evaluated in thisstudy including the laboratory test methods used to assess these PFC mixtures.Mix design of these PFC mixtures was conducted in accordance with the TexasDepartment of Transportation (TxDOT) specications [4]. Table 1 summarizes themain characteristics of the mixtures that were evaluated in this study and the num-ber of specimens, compacted using the superpave gyratory compactor (SGC),scanned for the analysis of mixture internal structure. Table 2 presents the corre-sponding aggregate gradations. The mixtures included were used in actual TxDOTeld projects on different state and interstate highways.

    At a target design total AV content of 20% [4] for all the mixtures, and as spec-ied by TxDOT [3], the optimum asphalt content of PGPFC mixtures was between5.5% and 7.0% and for ARPFC mixtures between 8.0% and 10.0% (Table 2). The min-imum asphalt contents (5.5% and 8.0%) are intended to ensure durability of theseopen-graded mixtures in terms of both aging resistance and moisture damage byproviding thick asphalt lms. As shown in Tables 1 and 2, the differences in theoptimum asphalt content range between these two PFC mixture types is primarilyattributed to the differences in the material characteristics specied for each mix-ture type, i.e., differences in the asphalt type, aggregate gradation specication, andmodiers included.The mix design as well as the entire evaluation of the PFC mixtures included inTable 1 was conducted on specimens, 152.4 mm in diameter and 115 5 mm inheight, compacted using a ServoPac SGC (at 50 gyrations; 1.25; 600 kPa; 30 rev/min) [4]. Both PGPFC and ARPFC mixtures were compacted at 149 C. In addition,the assessed mixtures were used in eld projects, where 152.4 mm in diameter

    100 100 100 100 100 100 10010099 95.6 99.7 95.7 98.9 84 9510054.6 54.9 75.7 68.7 54.6 57.6 50805 4 7.9 6.5 5 14.7 081.9 2.1 1.1 2.2 2 3.2 041 0.8 0.6 0.4 1 1 04gregate type Other materials (L/CF) Number of SGC specimens

    ndstone, Limestone 1%/0.3% 2ndstone 1%/0.3% 2estone 1%/0.3% 2estone 1%/0.3% 2

    anite, Limestone 1%/0.3% 2estone 1%/0.3% 2

    ndstone, Limestone 0%/0% 2anite, Limestone 0%/0% 2ndstone 0%/0% 2anite 0%/0% 0ndstone, Limestone 0%/0% 0avel 0%/0% 1

    mb rubber; OAC = Optimum asphalt content; L = Lime; CF = Cellulose bers;

    lding Materials 28 (2012) 427436road cores were taken for laboratory analysis (i.e., drainability).The assessment of PGPFC and ARPFC mixtures included laboratory testing to

    determine both the total and interconnected AV content, durability (using the Over-lay test [OT], Hamburg Wheel-Tracking test [HWTT] and the Cantabro loss test[Cantabro test]), and drainability (in terms of laboratory permeability values andwater ow value [eld drainability]). In addition, the study included evaluation ofstone-on-stone contact (based on the computation of voids in the coarse aggregate).The internal structure of the mixtures was also assessed using X-ray ComputedTomography (X-ray CT) and image analysis techniques.

    3. Results and analysis

    This section presents the results of a comparison of the PGPFCand ARPFC mixtures in terms of the volumetric properties, dura-bility, drainability, stone-on-stone contact, and internal structure.

    3.1. Volumetric properties

    Fig. 1 compares the values of the theoretical maximum specicgravity (Gmm) for both the PGPFC and ARPFC mixtures computedusing two methods, namely the calculated-Gmm and the measured-Gmm. For any asphalt content, the measured-Gmm values weredetermined in the laboratory in accordance with the Tex-227-F testprocedure (Theoretical Maximum Specic Gravity of BituminousMixtures) [4]. The Tex-227-F procedure also includes a methodfor the calculation of Gmm (or calculated-Gmm), which can be usedto obtain the Gmm value at a high asphalt content (i.e., in the PFCmix design asphalt content range, 5.5%10.0%). This calculation,based on Eq. (1), assumes a constant effective specic gravity forthe aggregate (Gse), which is measured at low asphalt contents (lessthan 5.0%). Additional details on these procedures to determineGmm are documented by Alvarez et al. [7]. The same study [7] rec-ommended the calculated-Gmm method for estimation of the total

  • cula

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    Asphalt Content (%)Gmm PG Gmm AR COV PG COV AR

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    Fig. 2. Calculated-Gmm and coefcient of variation (COV) of measured-Gmm values(I-35-PG and US-281-AR mixtures).

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    Fig. 3. Comparison of water-accessible- and total-AV content for SGC specimens.

    BuiAV content of PFC mixtures based on reduced variability and errorrelated to the asphalt loss during testing.

    Calculated Gmm 100100PbGse

    PbGb1

    where Pb is the asphalt content in percentage and Gb is the mea-sured asphalt specic gravity.

    As expected and as shown in Fig. 1, the values of the calculated-and measured-Gmm are equivalent for asphalt contents less than5.5%, which also corresponds to the minimum specied asphaltcontent for PGPFC mixtures in Texas [3]. A difference is, however,noted at higher asphalt contents of 5.5% and greater, with the dif-ference being more signicantly pronounced for the ARPFC mix-tures. Fig. 1b demonstrates this conclusion based on the distanceof the best t lines to the Gmm equality line. Based on the datashown in Fig. 1 and considering the mixtures evaluated in thisstudy, this observation may suggest that Eq. (1) is more applicablefor asphalt contents equal to or higher than 5.5% (i.e., in the designasphalt content range). While further renement in the applicationof the model in Eq. (1) may be necessary, inherent laboratory test-ing errors associated with measured-Gmm values cannot be ruledout as one of the contributing factors to the differences betweencalculated- and measured-Gmm values.

    Fig. 2 shows both the calculated-Gmm values and coefcient ofvariation (COV) of the replicate measured-Gmm tests for both theI-35-PG and US-281-AR mixtures at different asphalt contents.Based on the COV magnitude, the results indicate relatively highervariability for the ARPFC mixtures compared to that of the PGPFC mixtures, particularly for the higher asphalt contents of 5.5%or greater.

    Figs. 1 and 2 provide evidence of the differences between PGPFCand ARPFCmixtures; most possibly related to less workability and

    2.26

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    2.51

    3 4 5 6 7 8 9 10

    Gmm

    Asphalt Content (%)

    Measured Gmm, ARCalculated Gmm, ARMeasured Gmm, PGCalculated Gmm, PG

    (a)

    Fig. 1. Comparison of measured-Gmm and cal

    A.E. Alvarez et al. / Construction andmore loss of mastic during the testing process of the extremelysticky high asphalt content ARPFC mixtures. Extreme caution(e.g., minimum mixture manipulation and careful air eliminationduring the measurement of the mixture volume) should thus beexercised when testing ARPFC mixtures to minimize the errorsduring the Gmm determination process.

    Fig. 3 shows the relationship between water-accessible AV con-tent and total AV content values calculated by applying the doublelarge bag vacuum method using the Corelock device [8] asdescribed by Alvarez et al. [9]. The saturated weight measurementof the specimen in water, for calculation of water-accessible AVcontent, was conducted with a minimum saturation time of4 min, but this time is variable for each mixture [7,9]. As an alter-native volumetric parameter, the water-accessible AV content isdened as the volume fraction, with respect to the compacted mix-ture total volume, that is accessible to water [9].

    As shown in Fig. 3 and based on the coefcient of correlation(CC) values (Fig. 3), the relationship between the total AV content2.26

    2.31

    2.36

    2.41

    2.46

    2.51

    2.26 2.31 2.36 2.41 2.46 2.51

    Cal

    cula

    ted Gmm

    Measured Gmm

    Line of equalityARPG

    5.5% Asphalt content

    Best fit lines

    (b)

    ted-Gmm (I-35-PG and US-281-AR mixtures).

    lding Materials 28 (2012) 427436 429and water-accessible AV content values has a higher degree ofassociation for the PGPFC mixtures than for the ARPFC mixtures.In fact, if a 95% reliability level and a minimum CC value of 0.95 arearbitrarily used as criteria of correlation, only data for the PGPFCmixtures (CC = 0.96) would be accepted as having a linear relation-ship. Data for the ARPFC mixtures (CC = 0.69) did not show a lin-ear relationship.

    In addition and as judged by the proximity to the equality linefor the PGPFC data at the same total AV content value, the PGPFCmixtures exhibit higher water-accessible AV content values ascompared to the ARPFC mixtures. A high water-accessible AVcontent is desirableconsidering the total AV content betweenan AV content design range to prevent durability problemsto pro-vide pathways for water and air ow. The particular responses re-ported suggest differences in the mixture internal structure (e.g.,size, distribution, and connectivity of AV) of each mixture type assubsequently discussed.

  • (vari

    Bui3.2. Durability assessment

    The standard Cantabro test (Tex-245-F), the OT (Tex-248-F), andthe HWTT (Tex-242-F) [10] were used to characterize the mixturedurability in terms of the Cantabro loss (%), cracking life (numberof cycles), and rutting resistance (permanent deformation depthin mm). These are all standardized tests and their detailed proce-dures are documented elsewhere [10]. The Cantabro test is primar-ily used to assess PFC mixtures, while the OT and HWTT are mostoften used to assess dense-graded HMA. Thus, the OT and HWTTwere included in this durability assessment to comparatively judgethe PFC mixture response when subjected to different repetitiveloading conditions.

    The Cantabro loss is an index of the PFC mixture resistance todisintegration (i.e., resistance to raveling), which can be quicklyassessed using the Los Angeles machine without abrasive loading[11]. Additional details on the testing methodology are reportedelsewhere [12]. The maximum Cantabro loss specied in Spain,for specimens tested in dry- and wet-conditions, subsequentlydened, is 20% and 35%, respectively [2]. Fig. 4 compares the Cant-abro loss values and the corresponding variability (COV values) ofboth the PGPFC and ARPFC mixtures (I-35-PG and US-281-AR)assessed for the SGC compacted specimens subjected to four differ-ent conditioning processes. These conditioning processes were:

    Dry, tested after estimation of the total AV content. This condi-tioning process served as the control for all the other condition-ing processes.

    Wet, subjected to water immersion at 60 C for 24 0.5 h andsubsequent drying for a minimum of 24 h, to evaluate theeffects of moisture damage and wetting on the durability prop-erties of the mixtures.

    Low-temperature (24 h, minimum, at 3 C) to assess the effects

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    5.6 6.1 6.6 7.1 7.6 8.1 8.6

    Can

    tabr

    o Lo

    ss (%

    )

    Asphalt Content (%)

    Dry PG Dry ARWet PG Wet ARLow Temp. PG Low Temp. AR6 Months Aged PG 6 Months Aged AR

    (a)

    Fig. 4. Comparison of (a) Cantabro Loss values and (b) corresponding

    430 A.E. Alvarez et al. / Construction andof stiffening and embrittlement on the durability response ofthe mixtures.

    Aged for 6 months in a temperature controlled room at 60 C toinvestigate the effects of oxidative aging on the durability prop-erties of the mixtures.

    The results presented in Fig. 4 suggest that: (i) the variability(COV value) is higher in dry- and wet-conditions and smaller inlow temperature- and aged-conditions for the PGPFC mixturesas compared to that of the ARPFC mixtures assessed, and (ii)the Cantabro loss captured the differences in the mixtureresponses as related to conditioning induced by temperature,moisture damage, and asphalt (oxidative) aging. The magnitudeof both the Cantabro loss and corresponding COV providedevidence of differences in the response of ARPFC and PGPFCmix-tures and the expected durability of each of these mixture types.

    In general, asphalt aging seems to have the highest negative im-pact on the durability of the mixtures as measured by the percent-age of the Cantabro loss, followed by the effects of lowtemperatures. This was not unexpected because of the high AVcontent in the mixtures that permit easy air inltration; thus facil-itating rapid oxidative aging of the asphalt in the mixtures [13].Lower temperatures make the asphalt more brittle and susceptibleto cracking; and thus, another detrimental impact on durability isindicated by the Cantabro loss in Fig. 4.

    The least damage occurred at ambient temperature, andminimaldamage was noted for wet conditioning. In terms of wet condition-ing, the probable reason is that these mixtures have sufcient AV intheir internal structure that permit water to ow freely withoutcausing anydamage or hydrostatic pressure at the bonding interfacebetween the asphalt and the aggregates during the correspondinglaboratory conditioning process. Thicker asphalt lms in the PFCmixtures, as compared to those developed in dense-graded HMA,can also reduce the moisture damage induced through the labora-tory conditioning process. For each conditioning process, however,Fig. 4 also indicates that there is an optimum level of asphalt contentat which the Cantabro loss is minimum or maximum. For instance,the maximum Cantabro loss with respect to wetting appears tooccur at 6.1% and 8.1% for the PGPFC and ARPFC mixturesanalyzed, respectively.

    Longer cracking life (i.e., higher number of cycles to failure asassessed in the OT) is indicative of higher HMA resistance to crack-ing [14]. Additional information on the test protocol is reported byZhou and Scullion [14]. Fig. 5 summarizes the cracking life andcorresponding variability (COV) of ARPFC and PGPFC mixtures(I-35-PG and US-281-AR). Individual values of cracking life mea-sured on replicate specimens are also included and are indicatedby the vertical lines in Fig. 5.

    As indicated in Fig. 5 and accounting for the differences inasphalt contents, the variability of the cracking life values for theARPFC mixture is again higher than that of PGPFC mixtures. In

    5.6 6.1 6.6 7.1 7.6 8.1 8.60

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    Asphalt Content (%)

    Dry PG Dry ARWet PG Wet ARLow Temp. PG Low Temp. AR6 Months Aged PG 6 Months Aged AR

    b)

    ability (coefcient of variation (COV)) at different testing conditions.

    lding Materials 28 (2012) 427436addition, the response trend (dened in terms of the average val-ues) of cracking life as a function of asphalt content for PGPFCmixtures is consistent with theoretical expectations, i.e., the higherthe asphalt content the longer the cracking life. A conclusiveresponse trend cannot be established for the ARPFC mixturesgiven the high variability in the OT results at the high asphalt con-tents used. In both mixtures, however, the variability trend as mea-sured in terms of the COV magnitude is the same and appears to beincreasing with an increase in asphalt content.

    The HWTT test was successfully used in assessing both resis-tance to permanent deformation and moisture damage of HMA[15]. Additional details on the HWTT testing procedure arereported by Alvarez et al. [12]. Fig. 6a shows the rut depth at20,000 load passes in the HWTT and the respective COV valuesfor the I-35-PG mixture. Fig. 6b presents the number of HWTT loadpasses applied to attain the rut failure depth of 12.5 mm and thecorresponding COV values for the US-281-AR mixture. As expectedand consistent with previous results, the variability of the HWTT

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    Average, I -35-PGAverage, US-281-ARCOV, I-35-PGCOV, US-281 -AR

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    A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427436 431test results is higher for the ARPFC mixture as compared to that ofthe PGPFC mixture.

    In terms of performance, the PGPFC mixture seems to exhibitbetter laboratory rutting resistance under the HWTT than theARPFC mixture that failed at all asphalt content levels evalu-atedbased on a minimum number of repetitions required of10,000 for dense-graded HMA [3], since a corresponding specica-tion is not available for PFC mixtures. Compared to the PGPFCmixtures, the poor performance of the ARPFC mixture in theHWTT could be related to the high asphalt content that ranges be-tween 8.0% and 10.0%. Additionally, the PGPFC specication in Ta-ble 2 suggests a relatively coarser aggregate gradation compared tothe ARPFC specication; which could have also contributed to thesuperior performance of the PGPFC mixture in the HWTT.

    The data presented in Figs. 46 have shown that ARPFC mix-tures exhibited higher variability as compared to PGPFC mixtureswhen characterized in terms of laboratory durability using theCantabro test, the OT, and the HWTT test, respectively. These re-sults suggest that ARPFC and PGPFC mixtures have different re-sponses when subjected to deterioration through application ofdifferent modes of repetitive loading and testing conditions. As dis-cussed in previous research [12], the higher variability (COV valuesup to 120% in the ARPFCmixtures) is indicative of the limited pos-sibility of capturing the durability of ARPFC mixtures by applyingthe same testing procedures and tests used to assess durability andperformance of the PGPFC mixtures.

    Note, however, that by Texas specications, the OT and HWTTare generally not conducted for PFC mixtures [16]. This is becausePFC mixtures are rarely used as structural layers. As stated previ-ously, they are predominantly used as non-structural surfacing lay-

    Fig. 5. Comparison of cracking life for PGPFC and ARPFC mixtures.ers for skid resistance improvement, noise reduction, surfacedrainage improvement, and water splash reduction [14].

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    Fig. 6. Comparison of Hamburg Wheel Tracking test (HWTT) results3.3. Mixture drainability

    Field drainability was assessed in terms of the water ow value(Tex-246-F test procedure) [4]. The water ow value is dened asthe time in seconds required by a given water volume to owthrough a variable charge outow meter of 152 mm in diameter.Complete sealing at the pavement-outowmeter interface was en-sured during testing through the use of plumbers putty. Labora-tory permeability (or permeability) was measured in accordancewith ASTM PS 129-01[17]falling head permeability testfor bothSGC specimens and road cores (or cores). As shown in Fig. 7, PGPFC and ARPFC mixtures exhibit different trends in the water owvalue (eld drainability)-permeability (laboratory) relationship.

    These differences suggest discrepancies in the internal structureof the ARPFC and PGPFC eld compacted mixtures, which is con-sistently reected in the drainability measured in both the labora-tory and eld. The water ow value has been suggested to controlboth mixture construction (i.e., compaction) and drainability [18].For example, a minimum water ow value can be specied to en-sure a minimum permeability value for the eld compacted mix-tures. However, the differences shown in Fig. 7 suggest the needfor adapting different minimum water ow values for ARPFCand PGPFC mixtures. Although Fig. 7 provides a preliminary indi-cation of the minimum values based on a minimum permeabilityof 100 m/day recommended in previous research [19,20], more re-search and additional data are required to dene the minimumwater ow values for these mixture types that are commonly usedin Texas.

    Differences in laboratory permeability of SGC specimens androad cores of PGPFC and ARPFC mixtures are summarized in

    Fig. 7. Relationship of water ow value and permeability values (adapted from[17]).Fig. 8. The total AV content values reported in this gure were as-sessed based on Gmb values determined by dimensional analysis

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    for PGPFC (a) and ARPFC (b) mixtures (adapted from [11,12]).

  • (b

    tent

    Bui(i.e., geometrical estimation of volume assuming that the gyratorycompacted specimen is a regular cylinder with smooth faces).Fig. 8a also presents the relationships of total AV content and per-meability values reported by Pratic and Moro [21] and Watsonet al. [22] on similar mixtures fabricated without using AR, whichallows comparison with the PGPFC mixtures. This comparisonsuggests proper coincidence of the tendencies (best t line ofPGPFC mixtures) reported in this research and in previous litera-ture. However, for a particular total AV content, smaller permeabil-ity values were obtained based on the relationships reported byPratic and Moro, and Watson et al. as compared to those deter-mined using the best t line of PGPFC mixtures. Differences inthe aggregate gradation can partially explain these differences.

    Based on Fig. 8a and considering similar AV content values, thePGPFC mixtures exhibit higher permeability values as comparedto those of ARPFC mixtures. Based on the best t lines illustratedat 20% total AV content, for example, the difference in drainabilitybetween these mixtures is more than 50 m/day. Greater differ-ences are obtained in terms of the road core permeability valuesas shown in Fig. 8b. Opposite trends (as shown by the best t linesin the Fig. 8b) for the association of total AV content and perme-ability values are obtained for the two mixture types. In particular,the PGPFC cores revealed an unexpected tendency of decreasingpermeability when the total AV content increases.

    As discussed in previous research [23,24], the differences in, (i)permeability values, (ii) trends of the best t lines presented inFig. 8b, and (iii) the laboratory and eld responses (Fig. 8a and b)are most probably associated with discrepancies in the internalstructure (AV distribution, size, and connectivity) of the two typesof PFC mixtures due to differences in the workability of the twotypes of asphalt used (AR and PG). These differences in the internal

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    432 A.E. Alvarez et al. / Construction andstructure are further discussed subsequently for the SGC speci-mens. In addition, the discrepancy in the response of SGC and roadcores are most probably related to, (i) limited consistency in theeld compaction process of PFC mixtures due to inadequate eldcompaction control [23], leading to different internal structures,and (ii) the lack of correlation between the internal structure oflaboratory- and eld-compacted mixtures [24].

    3.4. Stone-on-stone contact assessment

    Assessmentof stone-on-stone contact requiredapplicationof thevoids in the coarse aggregate (VCA) procedure, suggested by TheNational Center for Asphalt Technology NCAT [19], as modiedin 2010 by recommendations made by Alvarez et al. [1]. Accordingto the modied VCA procedure, a VCA ratio equal to or less than0.9 implies the existence of stone-on-stone contact in the PFC mix-ture [1]. TheVCA ratio compares theVCA evaluated in thedry-roddedcondition (VCADRC) and the compacted PFC mixture (VCAmix). Thus,for a VCA ratio equal to 1.0 the coarse aggregate of a compactedPFC mixture would develop a stone-on-stone contact conditionequivalent to that existing in the dry-rodded aggregate.

    Fig. 9 shows a comparison of the mean and range values of VCAratio for the PGPFC and ARPFC mixtures included in Table 1.These computations are based on Gmb values determined usingdimensional analysis. The range in Fig. 9 indicates the variabilityof the VCA ratio associated with the variability of the Gmb estima-tions based on the number of replicate specimens indicated ontop of each data bar.

    The macroscopic assessment conducted based on the VCA ratiodoes not reveal major differences in the stone-on-stone contactconditions of the ARPFC and PGPFC mixtures. However, somedifferences are expected in the stone-on-stone contact of thesetwo types of mixture, since different gradations are specied totarget the same total AV content (average value 20%) despite theasphalt content of ARPFC mixtures being higher compared to thatused in PGPFC mixtures (Table 1). Additional research is requiredto capture these differences in the stone-on-stone contact congu-ration. In the future, application of X-ray CT and subsequent imageanalysis may be used for this purpose based on the quanticationof the contact points or contact areas and corresponding spatialdistribution.

    3.5. Evaluation of the mixture internal structure

    The internal structure of HMA can be estimated in terms of theorientation, distribution, and contact of aggregates and the AV dis-tribution, size, and connectivity [24]. The internal structure of PFCmixtures included in this study was determined using the non-destructive X-ray CT test and subsequent image analysis tech-niques. Details on the fundamentals of this technique are provided

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    lding Materials 28 (2012) 427436elsewhere [9,25]. For this particular study, each specimen wasscanned using the X-ray CT to obtain gray scale digital images, with1 mm of vertical gap, along the specimen height. The congurationused led to a voxel size of 0.17 by 0.17 by 1 mm (pixel size of0.17 mm). The grayscale images were then processed using amacro developed for Image-Pro Plus [26]. This macro generatesblack (AV) and white (aggregates and the mastic) images and al-lows subsequent estimation of the total AV content and AV sizefor each image and, therefore, for the specimen. Additional detailson the image analysis are reported in previous research [9,26].

    Figs. 10 and 11 show, respectively, the vertical distribution oftotal AV content and AV size mean values computed for SGC spec-imens using X-ray CT and image analysis techniques. These resultsare presented in terms of mean values, since all the SGC specimensanalyzed consistently exhibit a C shape distribution similar tothat illustrated in Figs. 10 and 11 for the mean values. Therefore,the distributions were determined using the mean values of the to-tal AV content and AV size estimated for all the specimens of PGPFC mixtures (Figs. 10a and 11a) and ARPFC mixtures (Figs. 10b

  • and 11b) analyzed. Based on the same data set, the standard devi-ation of total AV content and AV size were estimated and includedin Fig. 10 as the mean values of total AV content plus (Mean + SD)and less (Mean SD) one (1.0) times the standard deviation. Sim-ilarly, Fig. 11 includes the standard deviation values obtained forthe AV size. Data presented in Figs. 10 and 11 were estimated forimages electronically cored from 152.4 mm in diameter (originaldiameter of images representing 152.4 mm in diameter specimensobtained from the SGC) to 146.1 mm. This additional processing ofthe images was required to minimize the possible error induced byinclusion or exclusion of AV in contact with the specimen surface.The 146.1 mm in diameter images are considered a better repre-sentation of the specimens for the particular analysis pursued in

    portions of the SGC specimens are signicant as compared to thetarget total AV content used for mix design (20%).

    The strongly heterogeneous vertical distribution of the total AVcontent (Fig. 10) suggests trimming the SGC specimens to better as-sess the mixture properties and response. A more uniform AV con-tent distribution can be obtained by cutting the top and bottom20 mm of the specimens. A similar recommendation was suggestedin previous research on dense-graded HMA [27]. However, addi-tional adjustments would be required for applying this recommen-dation in the mix design process, since the mean total AV contentvalue of the specimen left after cutting would likely be smaller thanthe 20% total AV content typically targeted for the design of PFCmix-tures.Modicationof the compacted specimenheightmayalso offeradvantages in obtaining an even more uniform vertical distributionof total AVcontent, basedona redistributionof the SGCenergy in thespecimen [28]. Ultimately, as suggested in previous research [24],assessment of the internal structure of eld compacted mixtures isstill required in the future to ensure a closer correlation of the inter-nal structure of laboratory- and eld-compacted PFC mixtures.

    Fig. 12 compares both the vertical distribution of the mean totalAV content and mean AV size values estimated for SGC specimensof PGPFC and ARPFC mixtures. In addition, for comparison pur-poses, Fig. 12a includes the total AV content mean values calcu-lated by macroscopic measurements in the laboratory (based ondimensional analysis to compute Gmb) for the same specimensscanned using the X-ray CT. As previously indicated, the targettotal AV content for all the mixtures analyzed was 20%. Theseresults show that the ARPFC mixtures evaluated have a higher

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    (

    A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427436 433this study.Data presented in Figs. 10 and 11 suggest that laboratory com-

    pacted PFC mixtures, using the SGC at 50 gyrations, consistentlyresemble a C shape distribution of the total AV content and AVsize. More and bigger AV are concentrated at the top and bottomsections of the SGC specimens. The differences in the total AV con-tent values (approximately 20% points) of the central and extreme

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    Fig. 11. Vertical distribution of air voids (AV) size (radius) for specimemean total AV content, along the entire specimen height, as com-pared to the PGPFC mixtures. In addition, the ARPFC mixturesshowed, in most of the specimen height, higher AV sizes (as indi-cated by the AV radius) than the PGPFC mixtures. However, theARPFC mixtures exhibited smaller ratios of water-accessible AVcontent to total AV content (Fig. 3)indicating less connected AVfor water and air owwhich can explain their smaller permeabil-

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    434 A.E. Alvarez et al. / Construction andity values at similar total AV content values (Fig. 8a) compared tothe PGPFC mixtures.

    The differences in internal structure encountered for ARPFCand PGPFC mixtures are believed to be related to discrepanciesin the mixture workability during compaction. The workabilitydiverges in the two types of PFC mixtures analyzed due to discrep-ancies in both the asphalt binder rheological properties and aggre-gate gradation. Different rheological properties for the AR and PGasphalts used to fabricate these PFC mixtures were reported in aprevious study [18].

    Previous research [24] recommended coring of SGC specimensfrom 152.4 mm to 101.6 mm in diameter to reduce the horizontalheterogeneity of the total AV content. To further explore this rec-ommendation, the same specimens analyzed to produce Fig. 10were electronically cored to 101.6 mm in diameter. Fig. 13 shows

    Fig. 12. Comparison of vertical distribution of mean (a) total air voids (AV) content and146.1 mm in diameter).

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    Fig. 14. Comparison of vertical distribution of mean (a) total air voids (AV) content and101.6 mm in diameter).0

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    lding Materials 28 (2012) 427436the corresponding vertical distribution of total AV content meanvalues, which was estimated following the same procedure dis-cussed for Fig. 10. For comparison purposes, Fig. 13 also includesthe vertical distribution of total AV content mean values for thespecimens 146.1 mm in diameter. In addition, Fig. 14 shows thecomparison of the vertical distribution of both total AV contentand AV radius mean values for PGPFC as well as ARPFC mixtures,based on images 101.6 mm in diameter.

    Although coring the SGC specimens from 146.1 mm to 101.6 mm in diameter can reduce the horizontal heterogeneity of thetotal AV content [24], data presented in Fig. 13 suggest that a moreheterogeneous vertical distribution of total AV content is obtained(given the higher differences between the total AV content of thecentral section and the top- and bottom-sections of the SGC spec-imens) from this process. In addition, as shown in Fig. 14b and as

    (b) air voids (AV) radius for both PGPFC and ARPFC mixtures (based on images

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  • Buicompared to Fig. 12b, coring the SGC specimens can induce addi-tional variability in the vertical distribution of AV size as evaluatedin terms of the differences in AV size between the top and centralsections of 146.1 mm and 101.6 mm in diameter SGC specimens.Additional research may also be required to assess other potentialeffects of coring the SGC specimens, which can include, but notlimited to: rearrangement of the internal structureleading tochanges in the horizontal and vertical distribution of AVduringcoring, and sealing of surface AV.

    Data shown in Fig. 14 are comparable with data shown inFig. 12 and provide additional evidence of the existence of higherAV content and bigger AV sizes in the inner core (101.6 mm indiameter) of the SGC specimens of ARPFC mixtures as comparedto those in PGPFC mixtures.

    Results from Lu and Harvey [6] were consistent with the obser-vations made in this study that the ARPFC mixtures have a highervertical distribution of mean total AV content values and would,therefore, be more sound absorptive with a higher acoustic coef-cient. Thus, the ARPFC mixtures would likely provide greaternoise reduction as compared to the PGPFC mixtures.

    4. Conclusions and recommendations

    This paper presented an assessment of PGPFC and ARPFCmixtures, conducted in terms of volumetric properties, durability,drainability, stone-on-stone contact, and mixture internal struc-ture. Based on the results and discussion presented herein, themain conclusions and recommendations are:

    PGPFC and ARPFC mixtures exhibited particular responseswhen the volumetric properties, durability, drainability (e.g.,eld drainability and laboratory permeability), and mixtureinternal structure were evaluated.

    Although in terms of stone-on-stone contact no differenceswere observed between ARPFC and PGPFC mixtures, this con-clusion is limited by the index available (VCA ratio) for this par-ticular assessment. Additional research based on X-ray CT andimage analysis techniques can provide a better understandingof the stone-on-stone contact in PFC mixtures and its quanti-cation for future mix design and specication development.

    The analysis of mixture internal structure based on X-ray CTand image analysis techniques revealed that the ARPFC mix-tures evaluated have higher mean total AV content and meanAV size values, along the entire SGC specimen height, as com-pared to the PGPFC mixtures. However, the smaller ratios ofwater-accessible AV content to total AV content of ARPFC mix-tures, compared to PGPFC mixtures, led to lower drainabilityvalues for the ARPFC mixtures. The differences in internalstructure encountered for ARPFC and PGPFC mixtures areassumed to be related to discrepancies in the mixture workabil-ity during compaction. The workability diverges in the twotypes of PFC mixtures analyzed due to differences in both theasphalt binder rheological properties and aggregate gradation.

    The particular responses (e.g., drainability trends and variabilityin durability testing) as well as the magnitude of the differencesobserved in the indexes evaluated suggest that PGPFC and ARPFC are mixtures with different, (i) mechanical responses (dueto differences in the asphalt binder rheological properties andaggregate gradation (stone-on-stone contact)), and (ii) func-tional responses (due to differences in internal structure-AVcontent, size, and connectivity). Ultimately, the analysis con-ducted suggests the need to recommend different criteria forassessing each of these PFC mixture types. Additional research

    A.E. Alvarez et al. / Construction andis still required to, (i) dene particular specications for mixdesign and control of each PFC mixture type (PGPFC and ARPFC), and (ii) evaluate the eld performance of both PGPFCand ARPFC mixtures to set performance thresholds and vali-date the conclusions obtained in this study based on the labora-tory characterization conducted and discussed.

    Acknowledgements

    Part of this study was originally conducted for the TexasDepartment of Transportation (TxDOT), and the authors thankTxDOT and the Federal Highway Administration for their supportin funding this study. Special thanks are also due to the NationalScience Foundation for providing funds, under the Major ResearchImplementation (MRI) program, for the acquisition of the X-rayComputed Tomography equipment at Texas A&M University. Theauthors are also thankful for the laboratory assistance providedby Rick Canatella, Lee Gustavus, and David Zeig at Texas A&MUniversity to complete the laboratory testing. The authorswould like also to acknowledge the valuable input from Dr. CharlesGlover from the Chemical Engineering Department at Texas A&MUniversity. In addition, the rst author thanks the full supportprovided by the University of Magdalena to complete this researchstudy.

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    lding Materials 28 (2012) 427436 435friction courses. Information series 115. Lanham, MD: National AsphaltPavement Association; 2002.

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    [25] Masad E. X-ray computed tomography of aggregates and asphalt mixes. MaterEval J 2004;62(7):77583.

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    436 A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427436

    Comparison of permeable friction course mixtures fabricated using asphalt rubber and performance-grade asphalt binders1 Introduction2 Materials and methods3 Results and analysis3.1 Volumetric properties3.2 Durability assessment3.3 Mixture drainability3.4 Stone-on-stone contact assessment3.5 Evaluation of the mixture internal structure

    4 Conclusions and recommendationsAcknowledgementsReferences

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