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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  • 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 Septemb