Construction and Building Materials 28 (2012) 427–436

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Construction and Building Materials

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

Allex E. Alvarez a,⇑, Elvia M. Fernandez a, Amy Epps Martin b, Oscar J. Reyes c, Geoffrey S. Simate d,Lubinda F. Walubita e

a Department of Civil Engineering, University of Magdalena, Santa Marta, Colombiab Zachry Department of Civil Engineering, Texas A&M University, College Station, TX 77843, USAc Department of Civil Engineering, Nueva Granada Military University, Bogotá D.C., Colombiad Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africae TTI-The Texas A&M University System, College Station, TX 77843, USA

a r t i c l e i n f o

Article history: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 design

0950-0618/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.08.085

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

a b s t r a c t

Permeable friction course mixtures (PFC) are special hot mix asphalt (HMA) mixtures that are designed toimprove motorist safety and reduce traffic noise (i.e., tire–pavement 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 AR–PFC and PG–PFC mixtures, respectively. The analysis con-ducted provided evidence of differences between AR–PFC and PG–PFC mixtures that suggest the need fordifferentiation and refinement of the existing specifications to consider these mixtures as independentmaterials. Future research should, therefore, focus on defining particular specifications for mix designand control for AR–PFC and PG-PFC mixtures.

� 2011 Elsevier Ltd. All rights reserved.

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-ing—the 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 comparedto 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 PG–PFC and AR–PFC mixtures,respectively. Particular aggregate gradations are also specified foreach type of PFC mixture [3] to allocate different amounts of

ll rights reserved.

rez).

asphalt at similar total AV contents (e.g., 18%–22% in Texas). Theasphalts currently specified 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 fibers (at aminimum content of 0.2% by weight of the mixture) are alsospecified when using the PG76-XX asphalt,� a Type I- or II-AR: The corresponding minimum content of

crumb rubber (by weight of asphalt) specified is 15%. Particularspecifications for the Types I- and II-AR are reported elsewhere[3].

At present, PG–PFC and AR–PFC 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.,PG–PFC versus AR–PFC. As summarized by Alvarez et al. [5], sev-eral recent studies focused on improving the PFC mix design, bytreating both PG–PFC and AR–PFC mixtures as materials with sim-ilar properties and response. However, previous research [6] thatmeasured the acoustical absorption of various mixtures using a

Table 1Description of mixtures.

Mixture Asphalt type OAC (%) Aggregate type Other materials (L/CF) Number of SGC specimens

I-35-PG or 1-PG PG 76–22 6.1 Sandstone, Limestone 1%/0.3% 2IH-30-PG or 2-PG PG 76–22 6.6 Sandstone 1%/0.3% 2IH-20-PG or 3-PG PG 76–22 6.5 Limestone 1%/0.3% 2US-83-PG or 4-PG PG 76–22 6.4 Limestone 1%/0.3% 2US-59-PG or 5-PG PG 76–22 5.9 Granite, Limestone 1%/0.3% 2US-59Y-PG or 6-PG PG 76–22 5.8 Limestone 1%/0.3% 2US-281-AR or 1-AR Type II AR, grade B (AC-10 + 16% CR) 8.1 Sandstone, Limestone 0%/0% 2US-288-AR or 2-AR Type II AR, grade B (AC-10 + 17% CR) 8.0 Granite, Limestone 0%/0% 2US-290-AR or 3-AR Type II AR, grade B (AC-10 + 17% CR) 8.3 Sandstone 0%/0% 2SH-6-AR or 4-AR Type II AR, grade B (AC 10 + 17% CR) 8.2 Granite 0%/0% 0IH-35-AR or 5-AR Type II AR, grade B (AC-10 + 17% CR) 8.4 Sandstone, Limestone 0%/0% 0IH-10-AR or 6-AR Type I AR, grade C (AC-10 + 17% CR) 8.7 Gravel 0%/0% 1

Note: PG = Performance grade; AR = Asphalt rubber; AC = Asphalt cement; CR = Crumb rubber; OAC = Optimum asphalt content; L = Lime; CF = Cellulose fibers;SGC = Superpave Gyratory Compactor.

Table 2Gradation of aggregates used in both AR–PFC and PG–PFC mixtures (% Passing).

Sieve 1-PG 2-PG 3-PG 4-PG 5-PG 6-PG Specification-PG 1-AR 2-AR 3-AR 4-AR 5-AR 6-AR Specification-AR

3=4 100 100 100 100 100 100 100–100 100 100 100 100 100 100 100–100½ 90.3 81 85.3 90.5 80.2 84.5 80–100 99 95.6 99.7 95.7 98.9 84 95–100

3/8 59.5 43 59.4 50.9 57.7 52.8 35–60 54.6 54.9 75.7 68.7 54.6 57.6 50–80#4 10.1 15.5 18.6 3.2 15.9 6.6 1–20 5 4 7.9 6.5 5 14.7 0–8#8 5.2 6.7 2 1.5 6 4.2 1–10 1.9 2.1 1.1 2.2 2 3.2 0–4

#200 2.3 2.2 1.6 1.1 2.1 2.4 1–4 1 0.8 0.6 0.4 1 1 0–4

428 A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427–436

Bruel & Kjaer Type 4206A impedance tube showed relatively high-er absorption coefficients for the AR–PFC mixtures than for the PG–PFC mixtures. The acoustic (sound) absorption coefficient is oftenused to relate the noise reduction potential of a mixture. This coef-ficient is a function of the AV content and represents the propor-tion of acoustic energy not reflected 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 thePG–PFC and AR–PFC 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 PG–PFC and AR–PFC mixtures so as to providetechnical motivation as to the possible need to differentiate and re-fine the existing specifications.

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) specifications [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 TxDOTfield projects on different state and interstate highways.

At a target ‘‘design’’ total AV content of 20% [4] for all the mixtures, and as spec-ified by TxDOT [3], the optimum asphalt content of PG–PFC mixtures was between5.5% and 7.0% and for AR–PFC 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 films. 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 specified for each mix-ture type, i.e., differences in the asphalt type, aggregate gradation specification, andmodifiers 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 PG–PFC and AR–PFC mixtures were compacted at 149 �C. In addition,the assessed mixtures were used in field projects, where 152.4 mm in diameterroad cores were taken for laboratory analysis (i.e., drainability).

The assessment of PG–PFC and AR–PFC mixtures included laboratory testing todetermine 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 flow value [field 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 PG–PFCand AR–PFC 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 specificgravity (Gmm) for both the PG–PFC and AR–PFC 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 Specific 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 specific 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

2.26

2.31

2.36

2.41

2.46

2.51

3 4 5 6 7 8 9 10

Gmm

Asphalt Content (%)

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

2.26

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

(a) (b)

Fig. 1. Comparison of measured-Gmm and calculated-Gmm (I-35-PG and US-281-AR mixtures).

2.30

2.38

2.46

2.54

Gm

m

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

0.0

0.3

0.6

0.9

CO

V (%

)

2 3 4 5 6 7 8 9

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

6

10

14

18

22

Wat

er-A

cces

sib.

Air

Vo

ids

(%)

6 10 14 18 22

Total Air Voids (%)

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

A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427–436 429

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

Calculated� Gmm ¼100

100�PbGseþ Pb

Gb

ð1Þ

where Pb is the asphalt content in percentage and Gb is the mea-sured asphalt specific 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 specified asphaltcontent for PG–PFC mixtures in Texas [3]. A difference is, however,noted at higher asphalt contents of 5.5% and greater, with the dif-ference being more significantly pronounced for the AR–PFC mix-tures. Fig. 1b demonstrates this conclusion based on the distanceof the best fit 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 refinement 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 coefficient 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 AR–PFC mixtures compared to that of the PG–PFC mixtures, particularly for the higher asphalt contents of 5.5%or greater.

Figs. 1 and 2 provide evidence of the differences between PG–PFCand AR–PFC mixtures; most possibly related to less workability andmore loss of mastic during the testing process of the extremelysticky high asphalt content AR–PFC mixtures. Extreme caution(e.g., minimum mixture manipulation and careful air eliminationduring the measurement of the mixture volume) should thus beexercised when testing AR–PFC 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 isdefined 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 coefficient of correlation(CC) values (Fig. 3), the relationship between the total AV content

and water-accessible AV content values has a higher degree ofassociation for the PG–PFC mixtures than for the AR–PFC 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 PG–PFCmixtures (CC = 0.96) would be accepted as having a linear relation-ship. Data for the AR–PFC mixtures (CC = 0.69) did not show a lin-ear relationship.

In addition and as judged by the proximity to the equality linefor the PG–PFC data at the same total AV content value, the PG–PFCmixtures exhibit higher water-accessible AV content values ascompared to the AR–PFC mixtures. A high water-accessible AVcontent is desirable—considering the total AV content betweenan AV content design range to prevent durability problems—to pro-vide pathways for water and air flow. 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.

0

20

40

60

5.6 6.1 6.6 7.1 7.6 8.1 8.6 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

0

20

40

60

CO

V (%

)

Asphalt Content (%)

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

(a) (b)

Fig. 4. Comparison of (a) Cantabro Loss values and (b) corresponding variability (coefficient of variation (COV)) at different testing conditions.

430 A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427–436

3.2. 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 specified in Spain,for specimens tested in dry- and wet-conditions, subsequentlydefined, is 20% and 35%, respectively [2]. Fig. 4 compares the Cant-abro loss values and the corresponding variability (COV values) ofboth the PG–PFC and AR–PFC 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 and

subsequent 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

of stiffening and embrittlement on the durability response ofthe mixtures.� Aged for 6 months in a temperature controlled room at 60 �C to

investigate 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 PG–PFC mixturesas compared to that of the AR–PFC 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 AR–PFC and PG–PFC mix-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 infiltration; 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, and minimaldamage was noted for wet conditioning. In terms of wet condition-ing, the probable reason is that these mixtures have sufficient AV intheir internal structure that permit water to flow freely withoutcausing any damage or hydrostatic pressure at the bonding interfacebetween the asphalt and the aggregates during the correspondinglaboratory conditioning process. Thicker asphalt films 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 PG–PFC and AR–PFC 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 AR–PFC and PG–PFC 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 theAR–PFC mixture is again higher than that of PG–PFC mixtures. Inaddition, the response trend (defined in terms of the average val-ues) of cracking life as a function of asphalt content for PG–PFCmixtures is consistent with theoretical expectations, i.e., the higherthe asphalt content the longer the cracking life. A conclusiveresponse trend cannot be established for the AR–PFC 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

0

30

60

90

120

0

250

500

750

1000

5.5 6 6.5 7 7.5 8 8.5

CO

V (%

)

Cra

ckin

g Li

fe (C

ycle

s)

Asphalt Content (%)

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

Fig. 5. Comparison of cracking life for PG–PFC and AR–PFC mixtures.

10

100

1000

1 10 100 1000

Cor

e Pe

rmea

bilit

y (m

/day

)

Water Flow Value (s)

PG AR

NCAT minimum

Best fit lines

CC=-0.90, PGCC=-0.70, AR

Fig. 7. Relationship of water flow value and permeability values (adapted from[17]).

A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427–436 431

test results is higher for the AR–PFC mixture as compared to that ofthe PG–PFC mixture.

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

The data presented in Figs. 4–6 have shown that AR–PFC mix-tures exhibited higher variability as compared to PG–PFC mixtureswhen characterized in terms of laboratory durability using theCantabro test, the OT, and the HWTT test, respectively. These re-sults suggest that AR–PFC and PG–PFC 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 AR–PFC mixtures) is indicative of the limited pos-sibility of capturing the durability of AR–PFC mixtures by applyingthe same testing procedures and tests used to assess durability andperformance of the PG–PFC mixtures.

Note, however, that by Texas specifications, 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-ers for skid resistance improvement, noise reduction, surfacedrainage improvement, and water splash reduction [14].

4

8

12

16

5.5 5.7 5.9 6.1 6.3 6.5 6.7

Rut

Dep

th (m

m)

Asphalt Content (%)

Average I-35-PG COV

0

20

40

60

CO

V (%

)

(a)

Fig. 6. Comparison of Hamburg Wheel Tracking test (HWTT) results

3.3. Mixture drainability

Field drainability was assessed in terms of the water flow value(Tex-246-F test procedure) [4]. The water flow value is defined asthe time in seconds required by a given water volume to flowthrough a variable charge outflow meter of 152 mm in diameter.Complete sealing at the pavement-outflow meter interface was en-sured during testing through the use of plumber’s putty. Labora-tory permeability (or permeability) was measured in accordancewith ASTM PS 129-01[17]—falling head permeability test—for bothSGC specimens and road cores (or cores). As shown in Fig. 7, PG–PFC and AR–PFC mixtures exhibit different trends in the water flowvalue (field drainability)-permeability (laboratory) relationship.

These differences suggest discrepancies in the internal structureof the AR–PFC and PG–PFC field compacted mixtures, which is con-sistently reflected in the drainability measured in both the labora-tory and field. The water flow value has been suggested to controlboth mixture construction (i.e., compaction) and drainability [18].For example, a minimum water flow value can be specified to en-sure a minimum permeability value for the field compacted mix-tures. However, the differences shown in Fig. 7 suggest the needfor adapting different minimum water flow values for AR–PFCand PG–PFC 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 define the minimumwater flow values for these mixture types that are commonly usedin Texas.

Differences in laboratory permeability of SGC specimens androad cores of PG–PFC and AR–PFC mixtures are summarized inFig. 8. The total AV content values reported in this figure were as-sessed based on Gmb values determined by dimensional analysis

2000

6000

10000

14000

18000

7.5 7.7 7.9 8.1 8.3 8.5 8.7

HW

TT L

oad

Pass

es to

12.5

mm

Rut

Fai

lure

Asphalt Content (%)

Average US-281-AR COV

0

15

30

45

60

CO

V (%

)

(b)

for PG–PFC (a) and AR–PFC (b) mixtures (adapted from [11,12]).

0

100

200

16 20 24 28 32 36

Perm

eabi

lity

(m/d

ay)

Total Air Voids (%)

SGC PG

SGC AR

NCAT Minimum

Watson et al.

Pratico & Moro

Best fit lines

CC = 0.95, PGCC = 0.58, AR

0

100

200

16 20 24 28 32 36

Perm

eabi

lity

(m/d

ay)

Total Air Voids (%)

Cores PGCores ARNCAT Minimum

Best fit lines

(a) (b)

Fig. 8. Comparison of permeability and total air voids (AV) content for (a) SGC specimens and (b) road cores (adapted from [17]).

432 A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427–436

(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 Praticò and Moro [21] and Watsonet al. [22] on similar mixtures fabricated without using AR, whichallows comparison with the PG–PFC mixtures. This comparisonsuggests proper coincidence of the tendencies (best fit line ofPG–PFC 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 byPraticò and Moro, and Watson et al. as compared to those deter-mined using the best fit line of PG–PFC mixtures. Differences inthe aggregate gradation can partially explain these differences.

Based on Fig. 8a and considering similar AV content values, thePG–PFC mixtures exhibit higher permeability values as comparedto those of AR–PFC mixtures. Based on the best fit 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 fit 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 PG–PFC 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 fit lines presented inFig. 8b, and (iii) the laboratory and field 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 internalstructure 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 thefield compaction process of PFC mixtures due to inadequate fieldcompaction control [23], leading to different internal structures,and (ii) the lack of correlation between the internal structure oflaboratory- and field-compacted mixtures [24].

3.4. Stone-on-stone contact assessment

Assessment of stone-on-stone contact required application of thevoids in the coarse aggregate (VCA) procedure, suggested by TheNational Center for Asphalt Technology – NCAT [19], as modifiedin 2010 by recommendations made by Alvarez et al. [1]. Accordingto the modified 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]. The VCA ratio compares the VCA evaluated in the dry-roddedcondition (VCADRC) and the compacted PFC mixture (VCAmix). Thus,for a VCA ratio equal to 1.0 the coarse aggregate of a compacted

PFC 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 PG–PFC and AR–PFC 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 AR–PFC and PG–PFC mixtures. However, somedifferences are expected in the stone-on-stone contact of thesetwo types of mixture, since different gradations are specified totarget the same total AV content (average value 20%) despite theasphalt content of AR–PFC mixtures being higher compared to thatused in PG–PFC mixtures (Table 1). Additional research is requiredto capture these differences in the stone-on-stone contact configu-ration. In the future, application of X-ray CT and subsequent imageanalysis may be used for this purpose based on the quantificationof 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 providedelsewhere [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 configurationused 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 PG–PFC mixtures (Figs. 10a and 11a) and AR–PFC mixtures (Figs. 10b

0.6

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5-PG

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io Maximum VCA ratio (0.9)

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Fig. 9. VCA ratio for AR–PFC and PG–PFC mixtures.

A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427–436 433

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 inthis 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. 10. Vertical distribution of total air voids (AV) content for specime

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Fig. 11. Vertical distribution of air voids (AV) size (radius) for specime

portions of the SGC specimens are significant 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 PFC mix-tures. Modification of the compacted specimen height may also offeradvantages in obtaining an even more uniform vertical distributionof total AV content, based on a redistribution of the SGC energy in thespecimen [28]. Ultimately, as suggested in previous research [24],assessment of the internal structure of field compacted mixtures isstill required in the future to ensure a closer correlation of the inter-nal structure of laboratory- and field-compacted PFC mixtures.

Fig. 12 compares both the vertical distribution of the mean totalAV content and mean AV size values estimated for SGC specimensof PG–PFC and AR–PFC 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 AR–PFC mixtures evaluated have a highermean total AV content, along the entire specimen height, as com-pared to the PG–PFC mixtures. In addition, the AR–PFC mixturesshowed, in most of the specimen height, higher AV sizes (as indi-cated by the AV radius) than the PG–PFC mixtures. However, theAR–PFC mixtures exhibited smaller ratios of water-accessible AVcontent to total AV content (Fig. 3)—indicating less connected AVfor water and air flow—which can explain their smaller permeabil-

10 20 30 40

Air Voids Content (%)

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tion

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ns 146.1 mm in diameter of (a) PG–PFC and (b) AR–PFC mixtures.

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tion

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ns 146.1 mm in diameter of (a) PG–PFC and (b) AR–PFC mixtures.

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tion

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Mean -SD, AR

(a) (b)

Fig. 12. Comparison of vertical distribution of mean (a) total air voids (AV) content and (b) air voids (AV) radius for both PG–PFC and AR–PFC mixtures (based on images146.1 mm in diameter).

0

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tion

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(a) (b)

Fig. 13. Vertical distribution of total air voids (AV) content for specimens 101.6 mm in diameter of (a) PG–PFC and (b) AR–PFC mixtures.

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Mean, PGMean, ARMean -SD, PGMean -SD, ARTotal AV Content, PGTotal AV Content, AR

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Posi

tion

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Mean, AR

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Mean -SD, AR

(a) (b)

Fig. 14. Comparison of vertical distribution of mean (a) total air voids (AV) content and (b) air voids (AV) radius for both PG–PFC and AR–PFC mixtures (based on images101.6 mm in diameter).

434 A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427–436

ity values at similar total AV content values (Fig. 8a) compared tothe PG–PFC mixtures.

The differences in internal structure encountered for AR–PFCand PG–PFC 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

the 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 PG–PFC as well as AR–PFC 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

A.E. Alvarez et al. / Construction and Building Materials 28 (2012) 427–436 435

compared 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 structure—leading tochanges in the horizontal and vertical distribution of AV—duringcoring, 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 AR–PFC mixtures as comparedto those in PG–PFC mixtures.

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

4. Conclusions and recommendations

This paper presented an assessment of PG–PFC and AR–PFCmixtures, 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:

� PG–PFC and AR–PFC mixtures exhibited particular responseswhen the volumetric properties, durability, drainability (e.g.,field drainability and laboratory permeability), and mixtureinternal structure were evaluated.� Although in terms of stone-on-stone contact no differences

were observed between AR–PFC and PG–PFC 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 quantifi-cation for future mix design and specification development.� The analysis of mixture internal structure based on X-ray CT

and image analysis techniques revealed that the AR–PFC mix-tures evaluated have higher mean total AV content and meanAV size values, along the entire SGC specimen height, as com-pared to the PG–PFC mixtures. However, the smaller ratios ofwater-accessible AV content to total AV content of AR–PFC mix-tures, compared to PG–PFC mixtures, led to lower drainabilityvalues for the AR–PFC mixtures. The differences in internalstructure encountered for AR–PFC and PG–PFC 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 variability

in durability testing) as well as the magnitude of the differencesobserved in the indexes evaluated suggest that PG–PFC and AR–PFC 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 researchis still required to, (i) define particular specifications for mixdesign and control of each PFC mixture type (PG–PFC and AR–

PFC), and (ii) evaluate the field performance of both PG–PFCand AR–PFC 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 first author thanks the full supportprovided by the University of Magdalena to complete this researchstudy.

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