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Construction and Building Materials 22 (2008) 1037–1042

and Building

MATERIALS

Laboratory investigation of the properties of asphalt and itsmixtures modified with flame retardant

Peiliang Cong *, Jianying Yu, Shaopeng Wu, Xiaofeng Luo

Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology, Ministry of Education, Wuhan 430070, China

Received 25 November 2006; received in revised form 14 March 2007; accepted 15 March 2007Available online 1 May 2007

Abstract

The purpose of this study was to evaluate the properties of asphalt and its mixtures modified with Decarbomodiphenyl ether(EBPED), Antimony trioxide and zinc borate (ZB). Thermal analysis and Infrared spectroscopy (IR) were used to asses the effects ofthe flame retardants on physical and chemical properties of asphalt binders. The pavement performances of flame retardant modifiedasphalt mixture were estimated by Marshall stability, Marshall stability loss, flow value, indirect tensile strength (ITS), ITS loss anddynamic stability. To investigate the flame 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 flameretardant 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 flame retardant. Flame retardant modified asphalt mixtures have betterflame retardancy and pavement performances. Therefore, flame retardant modified asphalt mixture is a novel road functional materialto meet demands as flame 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

Asphalt binders with excellent flame retardancy havebeen prepared successfully by adding various flame retar-dants such as antimony trioxide, decarbomodiphenyl etherand zinc borate [1]. The introduction of flam retardantsinto asphalt pavement has the potential to decrease fireaccident in tunnels [2–8]. 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 filler con-tent, and therefore SMA using flame retardant modifiedasphalt binders were broadly representative of the asphaltmixture to investigate the flame retardancy and pavement

0950-0618/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2007.03.012

* Corresponding author. Tel.: +86 27 62981108.E-mail address: congpl@whut.edu.cn (P.L. Cong).

performances. The flame retardancy and pavement perfor-mance of asphalt binder containing various flame retar-dants have been demonstrated in the previous research,but the flame retardant modified asphalt mixture perfor-mance is not reported. It is difficult to test the flame 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 flame retardancy and pavement perfor-mance of flame retardant modified 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 flame retardancy of the asphalt mix-ture. Marshall tests, indirect tensile tests and wheeltracking tests were employed to investigate the effect of

Table 2Properties of aggregate

Test name Standard Measured values

Abrasion loss (%) (Los Angeles) ASTM DC-131 12.6Frost action (%) (with Na2SO4) ASTM C-88 8.15Specific gravity (g/cm3) ASTM C-127 2.838Water absorption (%) ASTM C-127 0.61

Polishing value BS-813 0.60

1038 P.L. Cong et al. / Construction and Building Materials 22 (2008) 1037–1042

flame retardants on the pavement performance of asphaltmixture.

2. Experiments

2.1. 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-ified by zinc borate (ZB), Antimony trioxide andDecarbomodiphenyl ether (EBPED). Based on previousresearch reports, asphalt binders were modified by adding6% mixed flame 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 filler), 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 0–0.3 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 fibre, added 3& by thetotal weight of the mixture, was used as a drain downstabilizer.

2.2. Methods

Flame retardant modified asphalt binder was preparedby Silverson L4R Homoginizer 000650 (SilversonMachines Ltd, Chesham Bucks, UK) at 1800 rpm. Thementioned mixed flame retardants were gradually addedinto bitumen at 170 �C, the mixture was processed underhigh shear about 30–45 min to ensure the well dispersionof flame retardant in asphalt binders.

Limiting oxygen index (LOI) methods, which describethe tendency of a material to sustain a flame and providea convenient, reproducible, means of determining a numer-ical measure of flammability, are widely used as a tool toinvestigate the flammability 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 80–150 mm length, 10 mm thickness and10 mm width is prepared to study the flame retardancy ofasphalt mixture. The test procedure was as follows: theupper part of the sample was ignited by a hydrogen flame

Table 1Combined aggregate gradation

Sieve size (mm) 16 13.2 9.5 4.75Total cumulative passing (%) 100 92.1 56.6 25.2

which was withdrawn once ignition had occurred, and thenthe lowest oxygen concentration in a flowing mixture ofnitrogen and oxygen which just supports sustained burningcan be determined.

Differential 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 flowing 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 film onto a potassium bromide (KBr) thin plate,and the spectra were obtained by 1.9 cm�1 resolution. Thenthe peak areas of relevant wavenumbers were calculatedover wavenumbers ranging.

The Marshall method (ASTM D1559) was used fordetermining optimal asphalt content of flame retardantmodified 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 deflections were measured per 20 sand dynamic stability (DS) was calculated as

DS ¼ 15Nd60 � d45

¼ 42� 15

d60 � d45

ð1Þ

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

2.36 1.18 0.6 0.3 0.15 0.07518.9 16.4 14.7 12.2 10.8 9.1

P.L. Cong et al. / Construction and Building Materials 22 (2008) 1037–1042 1039

3. Results and discussion

3.1. Thermal analysis

Fig. 1 shows DSC–TG test results of original and flameretardant modified 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 effect is then observed when test tem-perature is below 300 �C. The exothermic peaks of declineand DSC master curves are lower than original asphalt’swhen flame retardants are added into asphalt. The Differ-ential Scanning Calorimetry master curve of originalasphalt appears as a wide exothermic peak at temperaturerange of 350–450 �C, but flame retardant modified asphaltexhibits this exothermic peak from 420 �C to 500 �C andthe peak value reduces apparently. That means flame retar-dant reduces the peak value, postpones the appearance ofexothermic peak and improves the flame resistance ofasphalt binder. Moreover, a larger endothermic peak repre-sents a benefit in enhancing flame retardancy at 540 �C. Itis important to note that the thermal behaviour of flameretardant modified asphalt binder is quite different fromthat of original asphalt binder.

Thermogravimetry master curves of original and flameretardant modified asphalt binders are also show inFig. 1. As for original asphalt binder, first stage degrada-tion with 56% weight loss is apparently observed at 250–490 �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, finally only 28% char formation at

0 100 200 300 400 500 600-2

-1

0

1

2

3

4

5

20

40

60

80

100

Original asphalt Flame retardant modified asphalt

TG (%

)

DSC

(mW

/mg)

Temperature (ºC)

exo

Fig. 1. Results of the DSC–TG test of original and flame retardantmodified asphalt binder.

600 �C. But flame retardant modified asphalt binder showsfirst stage degradation in temperature range between 195and 500 �C, 24% char formation at 600 �C. The observa-tion that the weight loss for flame retardant modifiedasphalt binder starts at a slightly lower temperature thanthat for the original asphalt suggests that some flame retar-dant components might be yielded prior to the maindecomposition of asphalt. Nevertheless, it is stronglybelieved that no decomposition of flame retardant occursduring mixing in plant and paving and compacting in fieldbecause asphalt mixture processing temperature of 160–180 �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.

From the above results, it is implied that addition offlame retardant into asphalt did drastically change the ther-mal degradation behaviour of asphalt binder and delay firefailure, thus gives time for fire fighter to exterminate thefire.

3.2. Infrared spectroscopy analysis

Fig. 2 gives Infrared spectroscopy analysis master curvesof the flame retardant modified and original asphalt bind-ers at 25 �C. The strong peaks within 2850–2960 cm�1

region are typical C–H stretching vibrations in aliphaticchains. The C–H asymmetric deforming in CH2 andCH3, and C–H symmetric deforming in CH3 vibrationsare observed at 1400–1500 cm�1 and 1370–1390 cm�1.The characteristic absorption peak around 1545–1640 cm�1 is attributed to C@C stretching vibrations inaromatics. Besides this normal to C@C absorption peak,the flame retardant modified asphalt binder also shows atiny peak between 1650 and 1725 cm�1, which is C@Ostretching vibrations in carbonyl or carboxylic. This region

4000 3500 3000 2500 2000 1500 1000 500

Tran

smitt

ance

Wavenumber (cm-1)

Original asphaltFlame retardant modified asphalt

2850-2960

1545-16401650-1725

1400-15001370-1390

Fig. 2. Results of the IR test of original and flame retardant modifiedasphalt binder.

Table 4Marshall test results

Mix type Air Marshall stability Marshall stability Loss

1040 P.L. Cong et al. / Construction and Building Materials 22 (2008) 1037–1042

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 flame retardant duringpreparation of the flame retardant modified asphalt. Allasphalt binders studied contain aliphatic chain, but thecontent of some functional group varies, especially the con-tent of C–H stretching vibrations in aliphatic chains. Asindicated, flame retardant modified 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 aimin the design of asphalt mixtures is to optimize the proper-ties of the mixture [11–14], for example the stability, dura-bility, flexibility, 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 flow values are generally measured forinformation but not used for acceptance [15–17]. 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 maximumdeflection of 4.189 mm. The flame retardant modifiedasphalt mixtures are of 5797 cycles/mm dynamic stabilityand 3.809 mm maximum deflection, 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 effect 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 specific gravity

(g/cm3)2.546 2.549

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

RuttingDynamic stability (cycles/mm) 5080 5797Maximum deflection (mm) 4.189 3.809

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 flame retardancyasphalt mixture has good water stability. The reasonsbehind these properties are that the flame retardant modi-fied 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 final ITS of controlasphalt mixture is 1.02 MPa and 0.91 MPa, it is larger thanone of the flame retardancy asphalt mixtures. Both the ITSloss and Marshall stability loss are less than 20%, thoughrelated values are different. The indirect tensile strengthloss 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 modified by flame retardantdoes not significantly affect the moisture susceptibility com-paring with the control one. Thus, we can conclude that theuse of flame retardant modified asphalt cannot destroy thepavement performances.

3.4. Flame retardancy

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

void(%)

at 60 �C, 30 minwater immersion(kN)

at 60 �C, 24 hwater immersion(kN)

(%)

Control 3.5 11.8 10.5 11.0Flame

retardantmodifiedasphaltmixture

3.6 12.6 11.7 7.2

Table 5Indirect tensile strength (ITS) test results

Mix type Airvoid(%)

Initial ITS(MPa)

Final ITS(MPa)

Loss(%)

Control 3.6 1.02 0.91 10.8Flame retardant modified

asphalt mixture3.8 0.98 0.87 11.2

05

101520253035404550556065

Lim

iting

oxy

gen

inde

x

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

Fig. 3. Results of the LOI test of samples.

P.L. Cong et al. / Construction and Building Materials 22 (2008) 1037–1042 1041

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

SA ¼ 0:41þ 0:01X

P iF 0SAi ð2Þ

where SA, Pi and F 0SAi are the surface area, percentagepassing and the surface area factor, respectively.

The surface area factor and surface area of combinedaggregate 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 flameretardancy of asphalt mixture. Fig. 3 shows the LOI testresults of asphalt binders and its mixture, flame retardantmodified asphalt binder and its mixture. As a result, themineral powder can increase flame retardancy of asphaltbinder, and LOI increases with the increasing content ofmineral powder. Original asphalt binder exhibits poorflame 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 importanteffect on flame retardancy of asphalt mixture because theaggregate proportion by weight is over 90%. Limited oxy-gen index value of flame retardant modified asphalt binder(containing 6% blends flame retardants) is 25.8. It meansthat the flame retardant has more significant effect onimproving flame retardancy of asphalt binder comparingwith mineral powder. The mixture of flame retardant mod-ified asphalt binder with 100% mineral powder shows ahigher LOI value of 36.4, which has better flame 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 partsflame retardant modified asphalt binder with 100 partsmineral powder, which is the LOI of the stone matrixasphalt mixture. The results indicated that the flame retar-dant modified 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 56.6Surface area factor (m2/kg) 0.41 – –Surface area of combined aggregate (m2/kg) 0.41 – –P

¼ 6:42 m2=kgSurface area of mineral powder (m2/kg) 100 100 100P

¼ 53:36 m2=kg

flame retardancy and mineral powder also can improvethe flame retardancy of asphalt binder. This is becausethe higher filler content is sufficient to suppress the flamefrom spreading along the specimen. But testing resultreveals that the addition of mixed flame retardants is moreeffective in promoting flame retardancy than the additionof mineral powder for asphalt binder. Therefore, the intro-duction of the flame retardant modified asphalt binder intotunnel asphalt pavement can dramatically improve flameretardancy of asphalt concrete and small reduction of thepavement performances of asphalt mixture.

4. Conclusions

Asphalt is an inflammable material. By blending theflame retardant with SBS-modified asphalt, a new materialwith better flame retardancy and pavement performance isobtained. The adding of 6% mixed flame retardants can sig-nificantly improve asphalt binder thermal properties byconsuming the released heat of asphalt in a temperaturerange of 195–600 �C, and enhance its flame resistance.The flame retardant modified asphalt binder shows firststage degradation at lower temperature than that of origi-nal asphalt binder, but the flame retardant has no decom-position during processing, paving and compacting inplant. A tiny peak between 1650 and 1725 cm�1 presentin infrared spectra of flame retardant modified asphalt bin-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 flame retar-dant modified 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 0.07525.2 18.9 16.4 14.7 12.2 10.8 9.10.41 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

100 100 100 100 99.4 97.4 88.9

1042 P.L. Cong et al. / Construction and Building Materials 22 (2008) 1037–1042

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

Acknowledgements

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 financialsupport of this work.

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