Low Temperature Fracture Properties of Polyphosphoric Acid Modified Asphalt Mixtures

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  • Low Temperature Fracture Properties of PolyphosphoricAcid Modified Asphalt Mixtures

    Eyoab T. Zegeye1; Ki H. Moon2; Mugur Turos3; Timothy R. Clyne4; and Mihai O. Marasteanu5

    Abstract: The low temperature fracture properties of polyphosphoric acid (PPA) modified mixtures are evaluated and compared with those ofpolymer modified mixtures. The main objective is to determinewhether PPA can partially or completely substitute traditional polymer modifiers,without adversely affecting the mixtures resistance to thermal cracking. Laboratory compacted and field cored test samples from the MinnesotaRoad Research Project (MnROAD) were tested using traditional methods as well as newly developed fracture testing protocols: indirect tensiletest (IDT), semicircular bending test (SCB), and disk-shaped compact tension test (DCT). The effects of temperature, air-void content, commaand long-term aging on low temperature fracture properties were analyzed; and field performance observations of the test cells from MnROADwere discussed. On the basis of the analysis, the fracture resistance of the PPA modified mixture is less than that of the SBS modified mixture.However, when PPA is used to substitute part of the SBS, a mixture with fracture resistance comparable to that of the mixture modified with SBSalone is produced. DOI: 10.1061/(ASCE)MT.1943-5533.0000488. 2012 American Society of Civil Engineers.

    CE Database subject headings: Temperature effects; Cracking; Acids; Bending; Asphalts; Mixtures.

    Author keywords: Low temperature cracking; Polyphosphoric acid; Semicircular bending tests; Disk-shaped compact tension tests;Low temperature fracture properties; Modified asphalt mixtures.

    Introduction

    The increasing traffic loads to which asphalt pavements are sub-jected have accelerated the need to explore new paving materialsand new technologies that improve the performance of pavements.A common method used to improve the performance of asphaltbinders consists of increasing the high temperature limit of the per-formance grade (PG), thus expanding the temperature range with-out affecting the low temperature limit. For this reason, specialpolymer additives have been used to produce modified asphaltbinders with improved resistance to rutting and unaltered resistanceto thermal cracking. Recent investigations indicate that asphaltbinders modified with PPA, alone or in combination with tradi-tional polymers, can yield similar gains in PG limit for lower costof modification (Clyne et al. 2009). However, because of prematurefailures observed in some asphalt pavements containing PPA modi-fied binders, its application has been restricted by some highwayagencies (Arnold et al. 2009).

    In the research described in this paper, the effect of PPAmodification on the low temperature fracture properties of asphalt

    mixtures was investigated. Traditional and new fracture tests wereused to determine fracture properties for a set of laboratoryprepared specimens and field samples cored from the MinnesotaRoad Research Project (MnROAD).

    Background on Use of Acid Modifiers in AsphaltMixtures

    One of the earliest documented uses of PPA as an asphalt bindermodifier was in altering the viscositypenetration relationship ofasphalt [S. Alexander, Method of treating asphalt, U.S. Patent3,751,278 (1973)]. Unlike the air-blown oxidation process, whichstiffens the binder at high temperatures but severely damages itslow temperature properties, adding PPA increases resistance torutting while still meeting the low temperature requirements.

    In past decades, several researchers (Dickinson 1974; Giavariniet al. 2000; Bishara et al. 2001; Ho et al. 2001; Maldonado et al.2006; Arnold et al. 2009) have documented the mechanisms under-lying PPA modification. The strength of the effect may vary frombinder to binder, but it is generally observed that the PG higherservice temperature limit increases almost linearly with the amountof added PPA (Maldonado et al. 2006; Arnold et al. 2009).

    Giavarini et al. (2000)andArnoldet al. (2009)performed in-depthrheological andphysiochemical analysis, includingbinder fractiona-tion, to investigate whether the chemical composition of the binderaffects the outcome of PPA modification. Both studies reported anincrease in asphaltene and decrease in resin fraction with increasingPPA concentrations. Stiffening is more likely to occur in binders thatexhibit the high asphaltene-to-resin ratios caused by adding PPA.

    Kodrat et al. (2007) investigated the low temperature fractureproperties of PPA modified binders and compared them with thoseof plain and polymer modified (SBS, Elvaloy) binders of similargrades. Tests were performed on a large number of binder typesusing the extended bending beam rheometer (EBBR), compacttension (CT), and double edge-notched tension (DENT). The re-sults confirmed that addition of PPA increases the PG span withoutaffecting the lower temperature limit. The effects of PPA on fracture

    1Ph.D. Candidate, Univ. of Minnesota, 500 Pillsbury Dr. SE,Minneapolis, MN 55455 (corresponding author). E-mail: zegey001@umn.edu

    2Ph.D. Candidate, Univ. of Minnesota, 500 Pillsbury Dr. SE,Minneapolis, MN 55455.

    3Scientist, Univ. of Minnesota, 500 Pillsbury Dr. SE, Minneapolis,MN 55455.

    4MnROAD Operations Engineer, Mn/DOT Office of Materials andRoad Research, 1400 Gervais Ave., Maplewood, MN 55109.

    5Associate Professor, Univ. of Minnesota, 500 Pillsbury Dr. SE,Minneapolis, MN 55455.

    Note. This manuscript was submitted on March 31, 2011; approved onJanuary 24, 2012; published online on January 26, 2012. Discussion periodopen until January 1, 2013; separate discussions must be submitted for in-dividual papers. This paper is part of the Journal of Materials in CivilEngineering, Vol. 24, No. 8, August 1, 2012. ASCE, ISSN 0899-1561/2012/8-10891096/$25.00.

    JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / AUGUST 2012 / 1089

    J. Mater. Civ. Eng. 2012.24:1089-1096.

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    http://dx.doi.org/10.1061/(ASCE)MT.1943-5533.0000488http://dx.doi.org/10.1061/(ASCE)MT.1943-5533.0000488http://dx.doi.org/10.1061/(ASCE)MT.1943-5533.0000488http://dx.doi.org/10.1061/(ASCE)MT.1943-5533.0000488

  • properties in the brittle state and on the reversible aging processwere found to be insignificant. However, PPA modified binderssubjected to the DENT test at ambient temperature had significantlyreduced strain tolerance, thus increasing the probability of fatiguecracking in service.

    The combined effects of PPA and other additives such as hy-drated lime, SBS, and Elvaloy on some properties of asphalt mix-tures were also extensively discussed by Fee et al. (2010). Theseresearchers investigated the moisture susceptibility of mixturescomposed of limestone aggregates and PPA modified asphaltbinder. Hydrated lime used in combination with PPA did not affectthe PG grade gain and significantly improved mixture performance.However, they noted that if used in great amounts, the lime mayreact with the PPA and reduce or neutralize the expected gain inPG high temperature limit. Furthermore, the use of PPA in conjunc-tion with Elvaloy or SBS results in a greater improvement in mix-ture performance than the use of PPA alone.

    The considerably lower cost of PPA compared with polymermodifiers has made PPA modification a popular choice for pave-ment applications. In addition, using PPA in combination withother polymers can reduce the cost of modification (Clyne et al.2009) and improve the mixing and compaction characteristics ofthe mix (Arnold et al. 2009).

    Materials

    Four Superpave asphalt mixtures were used in this investigation (seeTable 1). The mixtures were used in Cells 33, 34, 35, and 77 of

    MnROAD and varied in the type and amount of binder modificationemployed; PPA (Cell 33), SBS (Cell 34), PPA SBS (Cell 35), andPPA Elvaloy (Cell 77). These modifications were used to up-grade a base binder of about PG 4934 to a binder of PG 5834.Accordingly, 0.75 PPA, 1% SBS 0:3 PPA, 2% SBS, and 1.1%Elvaloy 0:3 PPA were used for Cells 33, 34, 35, and 77, respec-tively. The amounts of modifiers used were varied to obtain the samegain in high temperature PG limit. The effects of the modificationson the rheological properties of the base binder are illustrated inFig. 1 (Clyne et al. 2009). The figure reports the direct shear rhe-ometer (DSR) and bending beam rheometer (BBR) test results. Priorto the BBR testing, the binders had been aged in a pressure agingvessel (PAV). It can be observed that the four different types andamounts of modification yielded similar gains in high temperaturePG without deteriorating the low temperature limit.

    The mixtures have a maximum aggregate of 12.5 mm, limited(< 10%) limestone, and 1% hydrated lime. In addition, three of themixtures also contained a liquid phosphate ester antistrip, as indi-cated in Table 1. The decision whether to add the liquid antistripwas based on Hamburg wheel track testing on laboratory mixturesperformed during the mix design stage.

    Sample Preparation

    The test samples for this study were prepared from loose mix com-pacted in the laboratory using the Superpave gyratory compactor(SGC). Mixtures were compacted at the University of Minnesota(UMN) with two target air-void contents: 4 and 7%. The latter

    Table 1. Description of Asphalt Mixtures Used

    MnRoad test cell Construction date Binder grade Asphalt modifiers Innovalt W liquid antistrip

    33 September 2007 PG 5834 0.75% PPA 0.5%34 September 2007 PG 5834 1:0%SBS 0:3% PPA 0.5%35 September 2007 PG 5834 2.0% SBS None77 September 2007 PG 5834 1:1%Elvaloy 0:3%PPA 0.5%

    Fig. 1. Performance grade (PG) of modified binders

    1090 / JOURNAL OF MATERIALS IN CIVIL ENGINEERING ASCE / AUGUST 2012

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  • is assumed to be more representative of typical asphalt pavementsimmediately after construction.

    In addition, field cores taken from MnROAD Cells 33, 34, 35,and 77 were also investigated. The cores were taken in June 2010,approximately 2 1/2 years after their construction. Eleven cylindri-cal field cores for each mixture, sampled from between wheelpaths, were delivered to UMN. The thickness of the cores rangedfrom 100 to 150 mm (4 to 6 in.). The cells from which the sampleswere collected had two identical 50-mm (2-in.) lifts, but only thetop layers were made of the mixtures of interest to this researchstudy. Therefore, the bottom layers were cut and discarded. In ad-dition, the upper 5 mm (0.20 in.) was also cut and discarded toproduce a smoother surface. Air-void contents of the field cores,determined according to AASHTO T166 (2005) are listed inTable 2.

    Experimental Variables

    Cylindrical SGC compacted and field cores were cut into indirecttensile (IDT), semicircular bending (SCB), and disk-shaped com-pact tension test specimens. Three replicates were used for eachfactor/level combination.

    The IDT, SCB, and DCT laboratory specimens obtained fromthe 4 and 7% SGC cylinders were tested at two temperatures:binder PG low temperature limit (PGLT), and PGLT 10C.

    An additional set of the 7% void content SGC cylinders waslong-term aged by placing the SGC cylinders an oven for 5 daysat 85C, according to the AASHTO R30-02 protocol (AASHTO2006). The long-term aged and field samples were tested onlyat PGLT.

    Test Methods

    The IDT and SCB tests were performed at the UMN, and the DCTtests were performed at the University of Illinois, Urbana-Champaign (UIUC).

    The IDT test was used to determine the creep compliance andstrength of the mixtures according to AASHTO T322(AASHTO 2003).

    The SCB test was used to determine the fracture toughness(KIC) and fracture energy (GF) of the mixtures. The test procedure,as well the computation of the SCB fracture parameters, is reportedelsewhere (Li and Marasteanu 2004, 2010). The SCB specimenwas a 150-mm-wide, 25-mm-thick semicircular slice. A verticalnotch 15 mm long and 2 mm wide was cut in the middle of thespan between the supports. Tests were performed in a MTS (EdenPrairie, MN) servohydraulic testing system equipped with an envi-ronmental chamber. The loading was controlled by the crack mouthopening displacement (CMOD) and varied to maintain a constantCMOD rate of 0:0005 mms for the entire duration of the test. Thisslow loading rate was selected to reflect the rate of temperaturechange that occurs in real field conditions.

    The DCT test was performed to determine the fracture energy ofthe mixtures according to ASTM D7313-07 (ASTM 2007). Thetest is performed on DCT specimens 150 mm wide, 145 high,and 50 mm thick. The initial fracture ligament length was82.5 mm. Tensile loading was applied at the holes at a constantCMOD rate of 0:017 mms.

    Test Results of Laboratory Compacted Samples

    Results of the IDT strength test are illustrated in Fig. 2. The figureshows the mean strength values calculated from three test repli-cates. The strength of the mixtures decreases as the temperaturedrops; however, the SBS and PPA Elvaloy modified mixtureswith 7% air-void content appear to be less sensitive to the changein temperature. As expected, mixtures with lower void content havehigher strengths than mixtures with higher void content. With re-spect to the effect of modification, several observations can bedrawn: SBS and PPA SBS modified mixtures have higherstrength values at all air-void and test temperature levels;

    Table 2. Air-Void Contents for Field Cores

    Cell Mean CV (%)

    33 5.3 1

    34 5.9 2

    35 6.4 2

    77 5.1 13

    Fig. 2. Results of IDT strength tests

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  • Elvaloy modified mixtures have about the same strength asPPA-modified mixtures.

    Fig. 3 shows the IDT stiffness test results at 60 and 500 s. It isshown that for all mixtures considered, stiffness increases dramati-cally when temperature drops. Similarly, stiffness is much higher inthe 4% void content mixtures. Overall, the highest increase in stiff-ness resulting from a temperature change is observed in the PPAElvaloy modified mixture. The PPA modified mixture is relativelyless responsive to change in temperature at 4% void content. On thecontrary, at 7% void content, the PPA modified mixture has thesecond highest increase in stiffness. With respect to the effect ofmodification, overall SBS alone and PP SBS modified mixtureshave higher stiffness values compared PPA modified mixtures.

    However, when only the highest test temperature is considered,the difference between the mixtures is reduced.

    Results of the SCB fracture tests are illustrated in Fig. 4. Thehighest fracture toughness v...

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