Microstructural and rheological investigation of asphalt mixtures containing recycled asphalt materials

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  • ti

    , Gnea

    ials in at (RAP),al backnot affestudy

    RAP and RAS on low temperature properties of asphalt mixtures is investigatedusing microstructural analysis and modeling of rheological data obtained on eight asphalt mixtures.

    porating RAP in new asphalt pavements signicantly reduces theusage of newmaterials, conserves natural resources and solves dis-posal problems [1]. To address this important issue, the NationalCooperative Highway Research Program (NCHRP) has funded anumber of projects, such as NCHRP 9-12 [2] and NCHRP 9-46 [3].

    the aged binders present in old pavements and roong shingles.

    2. Previous research efforts

    Several past studies were performed to investigate the effect ofRAP on recycled asphalt mixtures [58] and concluded that RAPcontent had a signicant inuence on mixture properties. More re-cently, research efforts focused on design procedures, forensicevaluation and modeling [1,9]; specications were also developedwith recommendations for selecting RAP content based on trafc

    Corresponding author. Tel.: +1 612 625 4579; fax: +1 612 626 7750.E-mail addresses: canno125@umn.edu (A. Cannone Falchetto), antonio.

    montepara@unipr.it (A. Montepara), gabriele.tebaldi@unipr.it (G. Tebaldi),

    Construction and Building Materials 35 (2012) 321329

    Contents lists available at

    Construction and B

    evmaras002@umn.edu (M.O. Marasteanu).Keywords:Reclaimed asphalt pavementManufacturer waste scrap shinglesTear-off scrap shinglesCreep stiffnessMicrostructureBack-calculation

    Detailed information on internal microstructure of asphalt mixtures is obtained from digital images ofasphalt mixtures beams and numerical estimations of spatial correlation functions. It is found that thatmicrostructure spatial distribution is not affected by RAP and RAS addition. Mechanical analog (Sharpe,2008 [18]) and semi empirical models are used to back-calculate binder creep stiffness from mixtureexperimental data. Differences between back-calculated results and experimental data suggest blendingbetween new and old binder may be only partial.

    2012 Elsevier Ltd. All rights reserved.

    1. Introduction

    Over the past 2030 years, the search for recycling alternativeshas led federal, state, and local agencies to consider a variety ofpotentially recyclable materials for pavement and other construc-tion applications. The list includes Reclaimed Asphalt Pavement(RAP), reclaimed Portland cement concrete, iron blast-furnace slag,y ash, waste tire rubber, waste glass, and roong shingles. Incor-

    Roong shingles represent another material that is available inlarge quantity for recycling. According to one estimate, about 10million tons of waste bituminous roong materials are generatedin United States each year [4]. The use of Recycled Asphalt Shingles(RAS) in hot mix asphalt has seen increased acceptance from gov-ernment agencies and construction contractors only in recentyears. The main obstacle is the potentially detrimental effect ofrecycled asphalt materials on asphalt pavement durability due toAccepted 25 April 2012emerged in Recycled Asphpaper, the effect of addingh i g h l i g h t s

    " We studied the use of recycled mater" We used Reclaimed Asphalt Pavemen" We used microstructural and analogic" Microstructure spatial distribution is" The analogical model is a good tool to

    a r t i c l e i n f o

    Article history:Received 27 October 2011Received in revised form 31 March 20120950-0618/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.04.016sphalt mixtures at low temperature.and Recycled Asphalt Shingles (RAS).-calculation modeling.cted by RAP and RAS addition.blending of new and recycled binders.

    a b s t r a c t

    Using increased proportions of Reclaimed Asphalt Pavement (RAP) in asphalt pavements construction hasbecome a top priority due to signicant economic and environmental benets. A similar trend has also

    alt Shingles (RAS), which represent the main roong material in US. In thisUniversity of Parma, Department of Civil and Environmental Engineering, Viale G.P. Usberti, 181/A, 43100 Parma, ItalyMicrostructural and rheological investigarecycled asphalt materials

    Augusto Cannone Falchetto a,, Antonio Montepara baUniversity of Minnesota, Department of Civil Engineering, 500 Pillsbury Drive, S.E., Minb

    journal homepage: www.elsll rights reserved.on of asphalt mixtures containing

    abriele Tebaldi b, Mihai O. Marasteanu a

    polis, MN 55455, USA

    SciVerse ScienceDirect

    uilding Materials

    ier .com/locate /conbui ldmat

  • asphalt binder in shingles ranges from 20 dmm to 70 dmm, while

    binders critical cracking temperature. In a recent study [16] on

    5. Microstructural analysis

    In most research efforts, asphalt mixture is considered either asa two-phase material [26,27], or a three phase material [28,29],and different micromechanical models are applied to describeand predict mixture behavior. Using low order microstructuralinformation, such as volume fraction, leads to poor prediction ofmixture properties [30]. More detailed information can be ob-tained from the estimation of spatial distribution of the micro-components of a material; this type of information is needed whenmodeling the effective properties of more complex microstruc-tures. In the case of random heterogeneous materials, higher-ordermicrostructural information is critical and statistical function suchas n-point correlation function, lineal path function, chord-lengthdensity function, are used.

    Two- and 3-point correlation functions were used in severalworks to describe random heterogeneous materials [3135]. In atwo-phase heterogeneous material, with the same volume fraction,the spatial distribution of its particles can dramatically affect themechanical properties as well as the failure properties [36]. Forthis reason, spatial correlation functions can be used as a tool toidentify uctuations in the microstructure of a heterogeneousmaterial.

    The n-point spatial correlation function measures the probabil-ity of nding n points all lying on the space occupied by one of thephases of the specic heterogeneous material [32]. In the case of atwo-phase heterogeneous material the 1-point correlation

    Fig. 1. Bending Beam Rheometer with thin asphalt mixture beam [25].

    andthe use of fractionated recycled asphalt pavement (FRAP) andRAS, it was found that the bers contained in RAS may be benecialto low temperature cracking resistance.

    3. Research approach

    In this research, the addition of RAP, MWSS and TOSS to asphaltmixtures used in pavement applications is investigated based onchanges in mixtures microstructure and on changes in mixturesand binders low temperature properties, respectively. The changesin material microstructure are evaluated using Digital Image Pro-cessing (DIP) and estimating 2 and 3 point correlation functionsof the aggregate phase. Huet [17] analogical model [1820] cou-pled with ENTPE transformation [2123] is used to investigatethe effect of adding recycled material on both asphalt binder andasphalt mixture creep stiffness. Back-calculation of the asphalt bin-der creep stiffness is performed using mixture creep stiffness dataobtained with the Bending Beam Rheometer (BBR) [24]. The goal isto determine if changes in mixture behavior are due to changes inmicrostructure from addition of recycled material, or to blending ofnew and old binder, or due to both.

    4. Materials and testing

    Eight different asphalt mixtures (Table 1) prepared with a PG58-28 binder wereused in this study. The virgin aggregate materials used in the mixtures consisted ofpit-run-sand, quarried 3=4 in. (19 mm) dolostone, and quarried dolostone manufac-tured sand. The recycled material consisted of different amounts of RAP, TOSSand MWSS. More details on these materials can be found elsewhere [15].

    Thin beams of asphalt mixtures (three replicates per mixture) were tested usingthe Bending Beam Rheometer (Fig. 1) following a procedure described elsewhere[25]. BBR tests [24] were also performed on the asphalt binder extracted from mix-tures 2, 3, 4, 5, 6, 7 and 8; extraction and asphalt binder tests were performed atMnDOT Ofce of Materials. BBR creep stiffness was also obtained on the originalPG58-28 binder after RTFOT aging; this value was assumed as control in the anal-traditional paving binders range from 50 dmm to 300 dmm [11].Reusing asphalt shingles in asphalt mixture poses signicant chal-lenges especially in cold climates, where good fracture resistance iscritical for good pavement performance. This is particularly true forTOSS, which contain highly oxidized binders that are more prone tobrittle failure. Unlike MWSS, only recently a provisional specica-tion on the use of TOSS was released and limits the content to 5%(by weight) with the provision that at least 70% (by weight) of therequired binder is new binder [12].

    In one of the rst comprehensive studies on the inuence of RASon HMA mixture properties, Newcomb et al. [11] found that up to5% of MWSS could be used in asphalt mixtures with a minimumimpact on the mixture properties. However, addition of TOSS re-sulted in an embrittlement or stiffening of the mixture that wasnot desirable for cracking resistance at low temperatures. The 5%limit for MWSS was also proposed by other authors [13,14].McGraw et al. [15] investigated the combined use of RAP andRAS showing the negative effect of TOSS on mixture strength andlevel. For example, Minnesota Department of Transportation(MnDOT) Specication 2350/2360 (2008) [10] allows up to 40%(by weight) RAP based on trafc level and binder grade.

    Regarding the use of Recycled Asphalt Shingles (RAS), two dis-tinct categories are available in the roong market: Tear-off ScrapShingles (TOSS), from old roofs that have been exposed to solarradiation and high temperatures for extended periods of time,and Manufacturer Waste Scrap Shingles (MWSS). Both TOSS andMWSS contain amuch harder asphalt binder compared to that usedin pavement applications: at 25 C, the penetration values for

    322 A. Cannone Falchetto et al. / Constructionysis. To avoid any errors associated with timetemperature superposition, binderand mixture experimental data were obtained at the same test temperature of6 C, which represents the binder PG low temperature limit +22 C.Table 1Mix design for the eight mixtures investigated.

    Mix Recycled material VMA VFA

    ID Description RAP(wt.%)

    TOSS(wt.%)

    MWSS(wt.%)

    1 PG58-28 Control 0 0 0 15.9 76.62 15% RAP 15 0 0 15.2 72.93 25% RAP 25 0 0 15.3 73.04 30% RAP 30 0 0 15.0 45.45 15% RAP 5%

    MWSS15 0 5 15.6 75.0

    6 15% RAP 5% TOSS 15 5 0 15.9 77.27 25% RAP 5% TOSS 25 5 0 15.4 73.98 25% RAP 5%

    MWSS25 0 5 14.8 72.5

    Building Materials 35 (2012) 321329function (S1) is the probability that any point lies on phase 1; thiscorrespond to the volumetric fraction of phase 1. The 2-point cor-relation function (S2) is the probability that two points separated

  • The 3-point correlation function for a heterogeneous materialcan be dened according to following equation:

    Si3 x1; x2; x3 hIix1 Iix2 Iix3i 5In the case of a translationally invariant isotropic material, and

    when there is no long-range order, the 3-point correlation functioncan be expressed as:

    lim Sijr1j; jr2j; u12 /3 6

    rr1

    r2r1

    r

    u12

    u12

    A. Cannone Falchetto et al. / Construction and Building Materials 35 (2012) 321329 323by a specic distance are located both in the phase of interest. The3-point correlation function (S3) is the probability that all the ver-tices of a triangle are all located in the same phase. Fig. 2 provides ageometric interpretation of the 1-, 2-, and 3-point correlation func-tions of aggregate phase; single points, segments, and triangles re-fer to S1, S2 and S3 respectively.

    n-point correlation functions of a two-phase random heteroge-neous material in i-dimensional Euclidian space can be expressedas [34,35]:

    Sin x1; x2; x3; ; xn hIix1 Iix2 Iix3 Iixni 1where hi is the ensemble averaging; I(i)(x) indicator function denedas:

    Iix 1; x 2 Vi0; x 2 Vi

    2

    The n-point correlation function is translationally invariant for astatistically homogeneous material. It means that the function de-pends on the differences in the coordinate values of the xi vectorsbut, not on their absolute position [34,35].

    The 1-point correlation represents the volume fraction /i of the

    r r

    2

    Fig. 2. Geometric interpretation of S1, S2 and S3 correlation functions.ith phase. S1 is constant and it is the probability that a randomlyselected point in the material belongs to ith phase:

    Si1 hIixi /i 3The 2-point correlation function Si2 x1; x2 is dened as:

    Si2 x1; x2 hIix1 Iix2i 4When microstructure of the material does not present long

    range order, the initial value of the 2-point correlation functionis /i(r = 0) and for very large r(r?1) it reaches the asymptoticlimit of /2i .

    Fig. 3. Matrix representation ojr1 j; jr2 j!1 3 i

    where r1 = x2 x1 is the vector; r2 = x3 x1 the vector; and u12 is thecosine of the angle h12 between vectors r1 and r2.

    The calculation of all the n-point correlation functions is re-quired when a complete description of the microstructure of a ran-dom heterogeneous material as asphalt mixture has to be assessed.However, this computation may become particularly complex bothanalytically and numerically [34,35]; for this reason alternativesolutions are required.

    5.1. Image processing

    Estimation of properties such as volumetric fraction of aggre-gates, particle size distribution, and n-point correlation functionscan be obtained from digital images of the material through DigitalImage Processing (DIP). Color (RGB) images of the asphalt mixturesBBR beams were acquired with a scanner at a resolution of 300 dpi(dots per inch) allowing for detection of aggregates larger than0.150 mm. MATLAB Image Processing Toolbox was used to obtainmastic and aggregate phases through a conversion procedure ofRGB images of asphalt mixture to binary images (Fig. 4); white(1) represents aggregates larger than 0.150 mm, and black (0) rep-resents voids + asphalt binder + aggregates smaller than 0.150 mm.

    The following procedure was used to obtain the binary images(Figs. 3 and 4): (a) conversion of RGB images to (b) gray scale, (c)equalization of the histogram of the image to enhance the contrast,(d) noise ltering and (e) nal conversion to binary image throughglobal thresholding.

    5.2. 2 and 3-Point correlation functions

    The 2-point correlation function can be computed using a dis-cretized expression of Eq. (4) accounting for the width and theheight of the digital binary image of interest. However, for highresolution images containing a large number of pixels, a bruteforce method is time consuming and sometimes even prohibitive.For this reason, 2- and 3-point correlation functions of the aggre-gate phase are hereafter calculated by means of a simplicationof the algorithm proposed by Velasquez [34,35]. This procedureis based on Monte Carlo simulations and on binary images of theasphalt mixture BBR specimens. For purpose of comparison withthe initial values of 2 and 3-point correlation functions (r = 0 andr...

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