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

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<ul><li><p>ma</p><p>43,mbiahann</p><p>a r t i c l e i n f o</p><p>Article history:</p><p>a b s t r a c t</p><p>to conventional dense-graded HMA mixtures, in terms of safety,economy, and the environment [2].</p><p>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</p><p>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</p><p> Corresponding author. Tel./fax: +57 5 4301292.</p><p>Construction and Building Materials 28 (2012) 427436</p><p>Contents lists available at</p><p>B</p><p>evE-mail address: allexalvarez@yahoo.com (A.E. Alvarez).1. Introduction</p><p>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</p><p>asphalt at similar total AV contents (e.g., 18%22% in Texas). Theasphalts currently specied in Texas [3] for PFC mixtures are:</p><p> 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,</p><p> 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</p><p>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.</p><p> 2011 Elsevier Ltd. All rights reserved.Comparison of permeable friction courseand performance-grade asphalt binders</p><p>Allex E. Alvarez a,, Elvia M. Fernandez a, Amy Epps MLubinda F. Walubita e</p><p>aDepartment of Civil Engineering, University of Magdalena, Santa Marta, Colombiab Zachry Department of Civil Engineering, Texas A&amp;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&amp;M University System, College Station, TX 77843, USA</p><p>Construction and</p><p>journal homepage: www.elsll rights reserved.ixtures fabricated using asphalt rubber</p><p>rtin b, Oscar J. Reyes c, Geoffrey S. Simate d,</p><p>USA</p><p>esburg, South Africa</p><p>SciVerse ScienceDirect</p><p>uilding Materials</p><p>ier .com/locate /conbui ldmat</p></li><li><p>Ag</p><p>SaSaLimLimGrLimSaGrSaGrSaGr</p><p>= Cru</p><p>-PG 1-AR 2-AR 3-AR 4-AR 5-AR 6-AR Specication-AR</p><p>BuiBruel &amp; 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-</p><p>Table 1Description of mixtures.</p><p>Mixture Asphalt type OAC (%)</p><p>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</p><p>Note: PG = Performance grade; AR = Asphalt rubber; AC = Asphalt cement; CRSGC = Superpave Gyratory Compactor.</p><p>Table 2Gradation of aggregates used in both ARPFC and PGPFC mixtures (% Passing).</p><p>Sieve 1-PG 2-PG 3-PG 4-PG 5-PG 6-PG Specication</p><p>3=4 100 100 100 100 100 100 100100 90.3 81 85.3 90.5 80.2 84.5 80100</p><p>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</p><p>#200 2.3 2.2 1.6 1.1 2.1 2.4 14</p><p>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.</p><p>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.</p><p>2. Materials and methods</p><p>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.</p><p>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</p><p>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</p><p>ndstone, Limestone 1%/0.3% 2ndstone 1%/0.3% 2estone 1%/0.3% 2estone 1%/0.3% 2</p><p>anite, Limestone 1%/0.3% 2estone 1%/0.3% 2</p><p>ndstone, Limestone 0%/0% 2anite, Limestone 0%/0% 2ndstone 0%/0% 2anite 0%/0% 0ndstone, Limestone 0%/0% 0avel 0%/0% 1</p><p>mb rubber; OAC = Optimum asphalt content; L = Lime; CF = Cellulose bers;</p><p>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</p><p>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.</p><p>3. Results and analysis</p><p>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.</p><p>3.1. Volumetric properties</p><p>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</p></li><li><p>cula</p><p>2.30</p><p>2.38</p><p>2.46</p><p>2.54</p><p>Gm</p><p>m </p><p>Asphalt Content (%)Gmm PG Gmm AR COV PG COV AR</p><p>0.0</p><p>0.3</p><p>0.6</p><p>0.9</p><p>CO</p><p>V (%</p><p>)</p><p>2 3 4 5 6 7 8 9</p><p>Fig. 2. Calculated-Gmm and coefcient of variation (COV) of measured-Gmm values(I-35-PG and US-281-AR mixtures).</p><p>6</p><p>10</p><p>14</p><p>18</p><p>22</p><p>Wat</p><p>er-A</p><p>cces</p><p>sib.</p><p> Air </p><p> Vo</p><p>ids </p><p>(%)</p><p>6 10 14 18 22</p><p>Total Air Voids (%)</p><p>Fig. 3. Comparison of water-accessible- and total-AV content for SGC specimens.</p><p>BuiAV content of PFC mixtures based on reduced variability and errorrelated to the asphalt loss during testing.</p><p>Calculated Gmm 100100PbGse</p><p> PbGb1</p><p>where Pb is the asphalt content in percentage and Gb is the mea-sured asphalt specic gravity.</p><p>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.</p><p>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.</p><p>Figs. 1 and 2 provide evidence of the differences between PGPFCand ARPFCmixtures; most possibly related to less workability and</p><p>2.26</p><p>2.31</p><p>2.36</p><p>2.41</p><p>2.46</p><p>2.51</p><p>3 4 5 6 7 8 9 10</p><p>Gmm</p><p>Asphalt Content (%)</p><p>Measured Gmm, ARCalculated Gmm, ARMeasured Gmm, PGCalculated Gmm, PG</p><p>(a)</p><p>Fig. 1. Comparison of measured-Gmm and cal</p><p>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.</p><p>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].</p><p>As shown in Fig. 3 and based on the coefcient of correlation(CC) values (Fig. 3), the relationship between the total AV content2.26</p><p>2.31</p><p>2.36</p><p>2.41</p><p>2.46</p><p>2.51</p><p>2.26 2.31 2.36 2.41 2.46 2.51</p><p>Cal</p><p>cula</p><p>ted Gmm</p><p>Measured Gmm </p><p>Line of equalityARPG</p><p>5.5% Asphalt content</p><p>Best fit lines</p><p>(b)</p><p>ted-Gmm (I-35-PG and US-281-AR mixtures).</p><p>ldin...</p></li></ul>

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