Laboratory investigation of the properties of asphalt and its mixtures modified with flame retardant

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    The purpose of this study was to evaluate the properties of asphalt and its mixtures modied with Decarbomodiphenyl ether

    Asphalt binders with excellent ame retardancy have

    tent, and therefore SMA using ame retardant modiedasphalt binders were broadly representative of the asphaltmixture to investigate the ame retardancy and pavement

    dants have been demonstrated in the previous research,

    mance of ame retardant modied asphalt mixture. To pre-pare the LOI test sample of the asphalt mixture, themineral powder was substituted with aggregate in the mix-ture to evaluate the ame retardancy of the asphalt mix-ture. Marshall tests, indirect tensile tests and wheeltracking tests were employed to investigate the eect of

    * Corresponding author. Tel.: +86 27 62981108.E-mail address: (P.L. Cong).

    Available online at

    Construction and Building Materia

    Constructionbeen prepared successfully by adding various ame retar-dants such as antimony trioxide, decarbomodiphenyl etherand zinc borate [1]. The introduction of am retardantsinto asphalt pavement has the potential to decrease reaccident in tunnels [28]. Many research reports and engi-neering case studies have suggested that the use of stonematrix asphalt (SMA) on road surface can achieve betterpavement performances [9,10]. Stone matrix asphalt mix-tures are hot asphalt mixtures consisting of high coarseaggregate content, high asphalt content and high ller con-

    but the ame retardant modied asphalt mixture perfor-mance is not reported. It is dicult to test the ame retar-dancy of asphalt mixture by the limited oxygen index (LOI)because large stone grains exist. The sample for LOI test issmall in size and coarse aggregate grains exists in asphaltmixture, the misdate of LOI proceeded from the samplebecame loose and large aggregate grains drop as the samplewas ignited.

    The main objective of this research study is to evaluatethe favourable ame retardancy and pavement perfor-(EBPED), Antimony trioxide and zinc borate (ZB). Thermal analysis and Infrared spectroscopy (IR) were used to asses the eects ofthe ame retardants on physical and chemical properties of asphalt binders. The pavement performances of ame retardant modiedasphalt mixture were estimated by Marshall stability, Marshall stability loss, ow value, indirect tensile strength (ITS), ITS loss anddynamic stability. To investigate the ame retardancy of asphalt mixture, the limited oxygen index (LOI) test sample of asphalt mixturewas prepared by the substitution of mineral powder for aggregate in mixture. Experimental results indicated that the addition of ameretardant did drastically change the thermal degradation behavior of asphalt binder, but the distribution of functional group and struc-ture of asphalt binders did not appear to alter radically upon ame retardant. Flame retardant modied asphalt mixtures have betterame retardancy and pavement performances. Therefore, ame retardant modied asphalt mixture is a novel road functional materialto meet demands as ame retardant materials and road materials at the same time. 2007 Elsevier Ltd. All rights reserved.

    Keywords: Asphalt; Flame retardancy; Thermal analysis; Marshall stability; Indirect tensile strength; Limited oxygen index (LOI)

    1. Introduction performances. The ame retardancy and pavement perfor-mance of asphalt binder containing various ame retar-Laboratory investigation of thmixtures modied

    Peiliang Cong *, Jianying Yu

    Key Laboratory of Silicate Materials Science and Engineering, Wuhan

    Received 25 November 2006; received in revisAvailable on

    Abstract0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2007.03.012properties of asphalt and itsth ame retardant

    haopeng Wu, Xiaofeng Luo

    iversity of Technology, Ministry of Education, Wuhan 430070, China

    orm 14 March 2007; accepted 15 March 20071 May 2007

    ls 22 (2008) 10371042

    and Building


  • ame retardants on the pavement performance of asphaltmixture.

    2. Experiments

    DS d60 d45 d60 d45 1

    where N is the wheel traveling speed, generally, N = 42 cy-cles/min, d 60 and d45 are the deection at 45 and 60 min,respectively.

    Table 2Properties of aggregate

    Test name Standard Measured values

    Abrasion loss (%) (Los Angeles) ASTM DC-131 12.6Frost action (%) (with Na2SO4) ASTM C-88 8.15

    1038 P.L. Cong et al. / Construction and Building Materials 22 (2008) 103710422.1. Raw materials

    Asphalt of Superpave performance grade PG76-22 witha penetration of 69 (0.1 mm at 25 C 100 g and 5 s), soften-ing point of 82 C and ductility of 43 cm (at 5 C) was mod-ied by zinc borate (ZB), Antimony trioxide andDecarbomodiphenyl ether (EBPED). Based on previousresearch reports, asphalt binders were modied by adding6% mixed ame retardants (EBPED:antimony trioxide:zincborate = 3:1:1) by mass in this study.

    The dolerite aggregate was used for preparing asphaltmixture. The combined aggregate gradation and summaryof the aggregate properties are given in Tables 1 and 2,respectively. The crushed limestone powders were appliedas mineral powder (or ller), with a density of 2.727 g/cm3, major chemical compounds, CaO content 52.3% andSiO2 content 1.68%. The particle size of the mineral powderwas in the range of 00.3 mm, with percent being 99.4% in0.3 mm, 97.4% in 0.15 mm and 88.9% by mass smaller than0.075 mm. The short chopped wood bre, added 3& by thetotal weight of the mixture, was used as a drain downstabilizer.

    2.2. Methods

    Flame retardant modied asphalt binder was preparedby Silverson L4R Homoginizer 000650 (SilversonMachines Ltd, Chesham Bucks, UK) at 1800 rpm. Thementioned mixed ame retardants were gradually addedinto bitumen at 170 C, the mixture was processed underhigh shear about 3045 min to ensure the well dispersionof ame retardant in asphalt binders.

    Limiting oxygen index (LOI) methods, which describethe tendency of a material to sustain a ame and providea convenient, reproducible, means of determining a numer-ical measure of ammability, are widely used as a tool toinvestigate the ammability of polymers. Flame retardancyof asphalt binders and its mixture was assessed by the lim-iting oxygen index tester (HC-2, Jiangning InstrumentsCo., Ltd, China) according to ASTM D-2863-77. The testsamples are molded and machined to the proper size. Thesample with 80150 mm length, 10 mm thickness and10 mm width is prepared to study the ame retardancy ofasphalt mixture. The test procedure was as follows: theupper part of the sample was ignited by a hydrogen ame

    Table 1Combined aggregate gradationSieve size (mm) 16 13.2 9.5 4.75Total cumulative passing (%) 100 92.1 56.6 25.2which was withdrawn once ignition had occurred, and thenthe lowest oxygen concentration in a owing mixture ofnitrogen and oxygen which just supports sustained burningcan be determined.

    Dierential scanning calorimetry (DSC) and thermo-gravimetry (TG) were performed using STA449C (NET-ZSCH Instruments Co., Ltd, Germany) to studying thethermal properties of asphalt. The test was carried outunder owing air of 20 ml/min and a heating rate of10 C /min. About 10.0 mg of sample was taken in eachcase and respective master curves were recorded.

    Fourier transform infrared (FT-IR) Spectroscopy(NEXUS, Thermo Nicolet, USA) was used to obtain theIR spectra of asphalt binders. Sample was prepared bycasting lm onto a potassium bromide (KBr) thin plate,and the spectra were obtained by 1.9 cm1 resolution. Thenthe peak areas of relevant wavenumbers were calculatedover wavenumbers ranging.

    The Marshall method (ASTM D1559) was used fordetermining optimal asphalt content of ame retardantmodied asphalt mixtures. Three identical samples(101.6 mm diameter and 63.5 1.3 mm high) were pro-duced with 50 blows compacting energy per side for Mar-shall stability, loss of Marshall stability, indirect tensilestrength and loss of indirect tensile strength tests. Thewheel tracking test was employed to measure rutting resis-tance of sample. The experiment condition was as follows,the square slab sample with 300 mm length, 300 mm widthand 50 mm thickness are immersed in dry atmosphere at60 0.5 C for 6 h and then a wheel pressure of0.7 MPa, the wheel traveling distance of the wheel was230 10 mm at a speed of 42 1cycles/min, is loaded totest for 60 min. Rut deections were measured per 20 sand dynamic stability (DS) was calculated as

    15N 42 15

    Specic gravity (g/cm3) ASTM C-127 2.838Water absorption (%) ASTM C-127 0.61

    Polishing value BS-813 0.602.36 1.18 0.6 0.3 0.15 0.07518.9 16.4 14.7 12.2 10.8 9.1

  • 3. Results and discussion

    3.1. Thermal analysis

    Fig. 1 shows DSCTG test results of original and ameretardant modied asphalt binder at a wide temperaturerange from 25 C to 600 C. Asphalt is complex mixturesof hydrocarbons containing oxygen, sulfur and nitrogenatoms, so a great number of exothermic peaks is observedon the DSC master curve, which indicate that plenty ofingredients are transformed during test. At low tempera-ture, a small exothermic peak was observed around43 C, which is probably linked with the dissolution ofcrystallization fraction which precipitated on cooling. Abroad endothermic eect is then observed when test tem-perature is below 300 C. The exothermic peaks of declineand DSC master curves are lower than original asphalts

    600 C. But ame retardant modied asphalt binder showsrst stage degradation in temperature range between 195and 500 C, 24% char formation at 600 C. The observa-tion that the weight loss for ame retardant modiedasphalt binder starts at a slightly lower temperature thanthat for the original asphalt suggests that some ame retar-dant components might be yielded prior to the maindecomposition of asphalt. Nevertheless, it is stronglybelieved that no decomposition of ame retardant occursduring mixing in plant and paving and compacting in eldbecause asphalt mixture processing temperature of 160180 C are less than 195 C. As thermogravimetry mastercurve of two kinds of asphalt binder shows, the degrada-tion curve apparently superposes at temperature rangefrom 430 C to 460 C. Those suggest that the asphalt mol-ecules cracking is faster and the stronger chemical bondsare broken at high temperature.

    P.L. Cong et al. / Construction and Building Materials 22 (2008) 10371042 1039when ame retardants are added into asphalt. The Dier-ential Scanning Calorimetry master curve of originalasphalt appears as a wide exothermic peak at temperaturerange of 350450 C, but ame retardant modied asphaltexhibits this exothermic peak from 420 C to 500 C andthe peak value reduces apparently. That means ame retar-dant reduces the peak value, postpones the appearance ofexothermic peak and improves the ame resistance ofasphalt binder. Moreover, a larger endothermic peak repre-sents a benet in enhancing ame retardancy at 540 C. Itis important to note that the thermal behaviour of ameretardant modied asphalt binder is quite dierent fromthat of original asphalt binder.

    Thermogravimetry master curves of original and ameretardant modied asphalt binders are also show inFig. 1. As for original asphalt binder, rst stage degrada-tion with 56% weight loss is apparently observed at 250490 C, which indicates that there are physical or chemicalreactions taking place. The second stage is from 490 C to600 C, the larger molecules decompose into small mole-cules in the gas phase, nally only 28% char formation at

    0 100 200 300 400 500 600-2













    Original asphalt Flame retardant modified asphalt

    TG (%





    Temperature (C)

    exoFig. 1. Results of the DSCTG test of original and ame retardantmodied asphalt binder.From the above results, it is implied that addition ofame retardant into asphalt did drastically change the ther-mal degradation behaviour of asphalt binder and delay refailure, thus gives time for re ghter to exterminate there.

    3.2. Infrared spectroscopy analysis

    Fig. 2 gives Infrared spectroscopy analysis master curvesof the ame retardant modied and original asphalt bind-ers at 25 C. The strong peaks within 28502960 cm1

    region are typical CH stretching vibrations in aliphaticchains. The CH asymmetric deforming in CH2 andCH3, and CH symmetric deforming in CH3 vibrationsare observed at 14001500 cm1 and 13701390 cm1.The characteristic absorption peak around 15451640 cm1 is attributed to C@C stretching vibrations inaromatics. Besides this normal to C@C absorption peak,the ame retardant modied asphalt binder also shows atiny peak between 1650 and 1725 cm1, which is C@Ostretching vibrations in carbonyl or carboxylic. This region

    4000 3500 3000 2500 2000 1500 1000 500




    Wavenumber (cm-1)

    Original asphaltFlame retardant modified asphalt



    1400-15001370-1390Fig. 2. Results of the IR test of original and ame retardant modiedasphalt binder.

  • contains the absorption peaks for carboxylic acids, ketonesand anhydrides. Ketones and anhydrides form on oxidativeaging. So the results indicated that asphalt has a slightaging and/or chemical action with ame retardant duringpreparation of the ame retardant modied asphalt. Allasphalt binders studied contain aliphatic chain, but thecontent of some functional group varies, especially the con-tent of CH stretching vibrations in aliphatic chains. Asindicated, ame retardant modied asphalt binder containthe functional groups like the original asphalt binderbesides carbonyl or carboxylic.

    3.3. Conventional physical properties of asphalt mixture

    Hot asphalt mixture is a combination of aggregatemixed with asphalt binder and/or other additive. The aim

    stability were measured in accordance with Marshall proce-dures. Eight samples from each mixture were immersed inthe water bath at 60 C. The Marshall stability values forfour samples from each mixture were obtained after35 min of water immersion. In addition, the Marshall sta-bility value after 24 h water immersion were also obtained.Table 4 shows the two kinds of Marshall stability testresults and the percent loss in Marshall stability of bothmixtures. The results indicate that the ame retardancyasphalt mixture has good water stability. The reasonsbehind these properties are that the ame retardant modi-ed asphalt mixtures have a higher viscosity asphalt bindergiving them higher stability.

    Table 5 shows the test results of Indirect Tensile strengthtest (ITS). The value of initial and nal ITS of controlasphalt mixture is 1.02 MPa and 0.91 MPa, it is larger thanone of the ame retardancy asphalt mixtures. Both the ITSloss and Marshall stability loss are less than 20%, thoughrelated values are dierent. The indirect tensile strength

    1040 P.L. Cong et al. / Construction and Building Materials 22 (2008) 10371042in the design of asphalt mixtures is to optimize the proper-ties of the mixture [1114], for example the stability, dura-bility, exibility, fatigue resistance, skid resistance,permeability, and workability. This is often accomplishedwith the evaluation of the volumetric properties of theasphalt mixture. Table 3 shows the volumetric propertiesof two types of asphalt mixtures at the optimum asphaltcontent. The optimum asphalt content for SMA mixturesis usually selected to produce 3.0%4.0% air voids. Mar-shall stability and ow values are generally measured forinformation but not used for acceptance [1517]. So thewheel tracking test was employed to evaluate the ruttingresistant of two kinds of asphalt mixture. The controlasphalt mixture, which was tested for comparison purpose,has the dynamic stability of 5080 cycles/mm and maximumdeection of 4.189 mm. The ame retardant modiedasphalt mixtures are of 5797 cycles/mm dynamic stabilityand 3.809 mm maximum deection, respectively. Thedesign results indicate that two kinds of asphalt mixturesshow similar volumetric properties within the same sourceof aggregate, gradation and test condition.

    In order to investigate the eect of water on compactedasphalt mixture, the Marshall stability and loss of Marshall

    Table 3Marshall design results

    Properties Control Flameretardancy

    Percent binder by wt. aggregate (%) 6.5 6.5Percent binder by wt. mixture (%) 6.1 6.1Mix theoretical maximum specic gravity

    (g/cm3)2.546 2.549

    Mix bulk specic gravity (g/cm3) 2.456 2.455Voids in mineral aggregate (%) 17.5 17.5Voids lled with asphalt (%) 79.6 79.0Air void (%) 3.5 3.7Marshall stability (kN) 11.8 12.6Flow (0.1 mm) 44 37


    Dynamic stability (cycles/mm) 5080 5797Maximum deection (mm) 4.189 3.809loss and Marshall stability loss are usually used to predictthe stripping susceptibility of asphalt mixtures. The maxi-mum values necessary to ensure good pavement perfor-mance and therefore 25% and 20% for Marshall stabilityloss and ITS loss are generally considered to be reasonableand acceptable minimum values. The test results demon-strate that asphalt mixture modied by ame retardantdoes not signicantly aect the moisture susceptibility com-paring with the control one. Thus, we can conclude that theuse of ame retardant modied asphalt cannot destroy thepavement performances.

    3.4. Flame retardancy

    Flame retardancy of asphalt mixture was evaluated bythe limited oxygen index. The eective surface area method

    Table 4Marshall test results

    Mix type Airvoid(%)

    Marshall stabilityat 60 C, 30 minwater immersion(kN)

    Marshall stabilityat 60 C, 24 hwater immersion(kN)


    Control 3.5 11.8 10.5 11.0Flame


    3.6 12.6 11.7 7.2

    Table 5Indirect tensile strength (ITS) test results

    Mix type Airvoid(%)

    Initial ITS(MPa)

    Final ITS(MPa)


    Control 3.6 1.02 0.91 10.8

    Flame retardant modied

    asphalt mixture3.8 0.98 0.87 11.2

  • was employed to simulate the content of aggregate becauselarge aggregate grains in asphalt mixture fall down as theupper part of the sample is ignited. In accordance withthe asphalt institute (AI) equation for calculating surfacearea of aggregate [18,19], the limited oxygen index test sam-ple of asphalt mixture was prepared by blending suitablemineral powder. The related equation was expressed as


    der, is produced due to the asphalt binder aging. Comparedwith the control asphalt mixture, the pavement perfor-mance results indicated that it is feasible to prepare theexcellent pavement of asphalt mixture using ame retar-dant modied asphalt binder. The mineral powder replacescoarse aggregate to prepare the asphalt mixture sample for

    4.75 2.36 1.18 0.6 0.3 0.15 0.0756 25.2 18.9 16.4 14.7 12.2 10.8 9.1

    0.41 0.82 1.64 2.87 6.14 12.29 32.770.10 0.15 0.27 0.42 0.75 1.33 2.98


    g ox




    Original asphaltOriginal asphalt :mineral powder=1:1Flame retardant modified asphaltFlame retardant modified asphalt :mineral powder=1:1Original asphalt :mineral powder=54:100Flame retardant modified asphalt :mineral powder=54:100

    P.L. Cong et al. / Construction and Building Materials 22 (2008) 10371042 1041SA 0:41 0:01 P iF 0SAi 2where SA, Pi and F

    0SAi are the surface area, percentage

    passing and the surface area factor, respectively.The surface area factor and surface area of combined

    aggregate gradation are given in Table 6. The results indi-cated that mineral powder is of large surface area, as com-pared with the combined aggregate. According to theabove combined aggregate gradation, the ratio of asphaltbinder and mineral powder is 0.54 in stone matrix asphaltmixture. The smaller mineral particle in mixture is not easyto fall down from the LOI test sample during testing. Thus,it is a simple and feasible method to investigate the ameretardancy of asphalt mixture. Fig. 3 shows the LOI testresults of asphalt binders and its mixture, ame retardantmodied asphalt binder and its mixture. As a result, themineral powder can increase ame retardancy of asphaltbinder, and LOI increases with the increasing content ofmineral powder. Original asphalt binder exhibits poorame retardancy with an LOI value of 19.8, the asphaltbinder containing 100% mineral powder shows the LOIvalue of 28.6. When the ratio of original asphalt binder/mineral powder is 0.54, the relevant LOI value reaches34.2, which means the aggregate in mixture has importanteect on ame retardancy of asphalt mixture because theaggregate proportion by weight is over 90%. Limited oxy-gen index value of ame retardant modied asphalt binder(containing 6% blends ame retardants) is 25.8. It meansthat the ame retardant has more signicant eect onimproving ame retardancy of asphalt binder comparingwith mineral powder. The mixture of ame retardant mod-ied asphalt binder with 100% mineral powder shows ahigher LOI value of 36.4, which has better ame retardancythan that of the mixture of 54 parts original asphalt binderwith 100 parts mineral powder. However, Limited oxygenindex value of 42.3 was obtained by the mixture of 54 partsame retardant modied asphalt binder with 100 partsmineral powder, which is the LOI of the stone matrixasphalt mixture. The results indicated that the ame retar-dant modied asphalt binder and its mixture have better

    Table 6Calculating surface area from combined aggregate gradation

    Sieve size (mm) 16 13.2 9.5Total cumulative passing (%) 100 92.1 56.Surface area factor (m2/kg) 0.41 Surface area of combined aggregate (m2/kg) 0.41 P 6:42 m2=kg

    2Surface area of mineral powder (m /kg) 100 100 100P 53:36 m2=kgame retardancy and mineral powder also can improvethe ame retardancy of asphalt binder. This is becausethe higher ller content is sucient to suppress the amefrom spreading along the specimen. But testing resultreveals that the addition of mixed ame retardants is moreeective in promoting ame retardancy than the additionof mineral powder for asphalt binder. Therefore, the intro-duction of the ame retardant modied asphalt binder intotunnel asphalt pavement can dramatically improve ameretardancy of asphalt concrete and small reduction of thepavement performances of asphalt mixture.

    4. Conclusions

    Asphalt is an inammable material. By blending theame retardant with SBS-modied asphalt, a new materialwith better ame retardancy and pavement performance isobtained. The adding of 6% mixed ame retardants can sig-nicantly improve asphalt binder thermal properties byconsuming the released heat of asphalt in a temperaturerange of 195600 C, and enhance its ame resistance.The ame retardant modied asphalt binder shows rststage degradation at lower temperature than that of origi-nal asphalt binder, but the ame retardant has no decom-position during processing, paving and compacting inplant. A tiny peak between 1650 and 1725 cm1 presentin infrared spectra of ame retardant modied asphalt bin-





    Fig. 3. Results of the LOI test of samples.100 100 100 100 99.4 97.4 88.9

  • the LOI test. This method provides a convenient, reproduc-ible test method of ame retardancy for asphalt mixture.However, laboratory evaluations are still controversial,eld trials are to be carried out to assess the ame resis-tance and pavement performance of the ame retardantmodied asphalt and its mixture.


    The author is grateful to the Department of Transporta-tion in Hubei Province, China and Headquarters of Hur-ong-Xi Expressway in Hubei Province for its nancialsupport of this work.


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    Laboratory investigation of the properties of asphalt and its mixtures modified with flame retardantIntroductionExperimentsRaw materialsMethods

    Results and discussionThermal analysisInfrared spectroscopy analysisConventional physical properties of asphalt mixtureFlame retardancy



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