Construction and Building Materials 23 (2009) 2277–2282

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Construction and Building Materials

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Investigation of the properties of asphalt and its mixtures containing flameretardant modifier

Jianying Yu *, Peiliang Cong, Shaopeng WuKey Laboratory of Silicate Materials Science and Engineering of Ministry of Education, Wuhan University of Technology, Wuhan 430070, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 June 2008Received in revised form 19 October 2008Accepted 19 November 2008Available online 4 January 2009

Keywords:AsphaltFlame retardancyAluminium hydroxide (ATH)Rut resistance

0950-0618/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2008.11.013

* Corresponding author. Tel./fax: +86 27 87873892E-mail addresses: jyyu@whut.edu.cn (Y. Jianyin

Peiliang), wusp@whut.edu.cn (W. Shaopeng).

The flame retardancy of asphalt binder was improved by adding flame retardants in previous research.The research was conducted to assess the performance of two kinds of flame resistant asphalt mixturecompared with the control mixture in laboratory. Differential scanning calorimetry (DSC) and thermo-gravimetry (TG) were employed to investigate the effect of aluminium hydroxide (ATH) on the thermalproperties of asphalt binder. The mixture was designed using Superpave (Superior Performing Pavement)methods and related pavement performance was evaluated. The wheel-tracking was employed toresearch the dynamic plastic deformation performance. Experimental results illustrated that two kindsof flame resistant asphalt binders have better flame retardancy and all the pavement performances oftwo kinds of modified asphalt mixture can meet the Superpave design requirements. Therefore, it wasfeasible to prepare asphalt mixture with the excellent flame retardancy and pavement performance usingATH modified asphalt binders.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Asphalt is a complex mixture of organic molecules that varies inchemical composition and molecular weight. Asphalt concrete isroad pavement material with excellent property and is widely usedin the constructing pavement [1–3]. But it is dangerous to apply as-phalt mixture in tunnel or gas station for flammability [4]. Over thepast few years, many investigations have been done in road fireareas [5–8]. Especially, some researchers have been investigatingthe flame resistant asphalt mixture for tunnels pavement [9,10].Previous studies have reported that a kinds of mixed flame retar-dants modified asphalt binder exhibits better flame retardancyand pavement performance [11–13]. The mixed flame retardantswere added to the asphalt binder before introducing it in the as-phalt concrete. Thus, specialized equipment is required for themanufacture or application of mixed flame retardants modified as-phalt mixture. There have little studied on the flame retardants di-rectly added to asphalt mixture. However, existing literaturediscusses the use of fillers, such as limestone powders, rubbercrumb, silica and carbon black, as modifier for asphalt mixture[14–17]. Basic results obtained on fillers should provide the basisand technical background for understanding the effect of flameretardants on pavement performance. It is also believe that flameretardants added into asphalt mixture figure as fillers will be of vi-tal importance in the practical application of flame retardants

ll rights reserved.

.g), congpl@whut.edu.cn (C.

modified asphalt mixture. Some literature discusses the funda-mental properties of flame retardants and its mixture using Mar-shall mix design methods [18,19]. The Superpave (SuperiorPerforming Pavement) mixture design method provides a com-plete, comprehensive means for the design of asphalt mixture thatwill achieve a level of performance commensurate with the uniquedemands of traffic, climate, pavement structure and reliability forthe project. It facilitates the selection of asphalt binder, aggregateand modifier to achieve the required level of pavement perfor-mance [20,21].

The overall objective of this study is to investigate the effect ofaluminium hydroxide (ATH) on the performance of asphalt and as-phalt mixture using Superpave mix design procedures includedoptimal asphalt content, aggregate gradation and mixes mechani-cal performance. Mechanical performance evaluation consisted ofMarshall stability, loss of Marshall stability, indirect tensilestrength, loss of indirect tensile strength and rutting resistanceare compared to unmodified and mixed flame retardants (EBPED:antimony trioxide:zinc borate = 3:1:1 by mass) modified asphaltmixture.

2. Experiments

2.1. Raw materials

SBS Modified asphalt with performance grade PG76-22 was employed to pre-pare flame retardants modified asphalt because of extreme temperature with hightemperature over 40 �C in summer and low temperature below-16 �C in winter inHubei Province, China. All of flame retardants were commercial product. The re-lated properties of asphalt and flame retardants are showed in Table 1.

Table 1Properties of the used asphalt and flame retardants.

Raw materials Properties Values

Asphalt Penetration (25 �C, 100 g, 5 s, 0.1 mm) 67Ductility (5 cm/min, 5 �C, mm) 45Softening point (�C) 81.5

Aluminium trihydroxide (ATH) Density (g/cm3) 2.42Maximum particle size (lm) 10

Zinc borate (ZB) Density (g/cm3) 2.67Maximum particle size (lm) 5Maximum ignition loss (%) 15.5

Antimony trioxide Density (g/cm3) 5.17Maximum particle size (lm) 1.6Melting point (�C) 656

Decabromodiphenyl ether(EBPED)

Density (g/cm3) 3.25Maximum particle size (lm) 5Minimum bromine concentration (%) 82.0Maximum free bromine concentration(%)

10

Pyrolysis temperature (�C) 320

Table 3The parameters of the Superpave gyratory model.

Parameters Value

The diameter of cylinder (mm) 150Axial load (kPa) 600Angle of gyration (�) 1.25Revolutions per minute (r/min) 30

2278 Y. Jianying et al. / Construction and Building Materials 23 (2009) 2277–2282

The dolerite aggregate was used for preparing asphalt mixture and the crushedlimestone powders were applied as mineral powder (or filler). Table 2 exhibits thesummary of the aggregate and mineral powder properties.

2.2. Methods

The flame resistant asphalt binders were prepared as follows: asphalt washeated to 170 ± 5 �C in an oil-bath heating container until it flowed fully. The appro-priate amounts of flame retardants (ATH or mixed flame retardants) were addedinto the blend and mixed for 30 min with a lab mixer set fast enough to create asmall vortex, without whipping excessive air into the sample. Then the tests sam-ples are molded.

Two kinds of Flame retardants modified asphalt mixture were prepared asfollows:

Mixed flame retardants modified asphalt mixture: the mixed flame retardantsincluding EBPED, ZB and antimony trioxide are added to the asphalt binder beforeintroducing it in the asphalt concrete. Based on previous research reports, asphaltbinder is modified by adding 6% mixed flame retardants (EBPED: antimony triox-ide:zinc borate = 3:1:1) by mass in this study.

ATH modified asphalt mixture: after mix design, the desired amount of fillerwas removed and replaced by an equal amount of ATH to prepare ATH modified as-phalt mixture.

Limiting oxygen index (LOI) methods are used to measure the flammability ofasphalt binders and to investigate the effectiveness of ATH according to ASTMD-2863-77. Test procedures were as following: the top of the sample was ignitedby a gas flame which is stopped once ignition has occurred, and then the lowestoxygen concentration in a flowing mixture of nitrogen and oxygen which just sup-ports sustained burning can be determined.

Table 2Properties of the used aggregate and mineral powder.

Raw materials Test properties Test results

Aggregate Coarse aggregate angularity (%) 100Fine aggregate angularity (%) 52Flat/elongated particles (%) 9.8Clay content (%) 0.3Coarse aggregate specific gravity (g/cm3) 2.838Coarse aggregate absorption (%) 2.3Fine aggregate specific gravity (g/cm3) 2.801Fine aggregate absorption (%) 4.3Sand equivalent (%) 65Combined aggregate specific gravity (g/cm3) 2.825Combined aggregate apparent specific gravity (g/cm3)

3.022

Abrasion loss (%) (Los Angeles) 12.6Frost action (%) (with Na2SO4) 8.15Polishing value 0.60

Mineral powder Specific gravity (g/cm3) 2.727

Major chemical compounds CaO content (%) 52.3SiO2 content (%) 1.68

Percent passing (%) 0.3 mm 99.40.15 mm 97.40.075 mm 88.9

Differential scanning calorimetry (DSC) and thermogravimetry (TG) were per-formed using STA449C (NETZSCH Instruments Co., Ltd., Germany) to studying thethermal properties of asphalt. The test was carried out under flowing air of20 ml/min and a heating rate of 10 �C/min, about 10.0 mg of sample was taken ineach case and respective master curves were recorded.

A nominal maximum size 12.5-mm (0.5 in) Superpave mixture was used for themix design in this study because it represents the most common nominal size fordense-graded wearing courses. The procedures described in AASHTO TP4 [20]regarding the preparation of asphalt specimens were followed. Based on the specificgravity of selected aggregates, asphalt binder content was calculated to be 5.0% bymass of total mix. The Superpave gyratory compactor (EP-31111model, America)was employed to prepared asphalt mixture specimens. The traffic volume was as-sumed to be at 30,000,000 equivalent single axle loads (ESALs). Therefore, thenumber of gyrations for initial compaction (Ninitial), design compaction (Ndesign),and maximum compaction (Nmax) can be determined as Ninitial = 9 gyrations,Ndesign = 125 gyrations, and Nmax = 205 gyrations. The parameters are shown inTable 3.

The loose asphalt mixtures were aged at the compaction temperature in ovenfor 2 h prior to the compaction which are commonly used as short-term over agingtemperatures in the laboratory to simulate binder aging and absorption during theconstruction of hot mixture asphalt pavements.

To evaluate mechanical performance of flame retardants modified asphalt mix-ture, the cylinder specimens (101.6 mm diameter and 63.5 ± 1.3 mm high) wereproduced with 75 blows compacting energy per side by Marshall Compactor, andthe square slab specimens were prepared using the slab compactor. The size ofthe slab samples was 300 mm in length, 300 mm in width and 50 mm in thickness.

The wheel-tracking test was employed to measure rutting resistance of asphaltmixture. The experiment condition were as follows, the square slab specimens with300 mm in length, 300 mm in width and 50 mm in thickness are immersed in dryatmosphere at 60 ± 0.5 �C for 6 h and then a wheel pressure of 0.7 MPa, the wheeltraveling distance of the wheel was 230 ± 10 mm at a speed of 42 ± 1cycles/min,is load to test for 60 min by a special solid rubber tire. Rut deflections were mea-sured per 20 s and dynamic stability (DS) was defined as

DS ¼ 15Nd60 � d45

¼ 42� 15d60 � d45

ð1Þ

where N is wheel traveling speed, generally, N = 42 cycles/min, d60 and d45 is thetracking depth at 45 and 60 min, respectively.

3. Results and discussion

3.1. Flame retardancy

Fig. 1 displays the effect of ATH and mixed flame retardantscontents on limiting oxygen index (LOI), respectively. Experimen-tal results of LOI indicate that two kinds of flame retardants mod-ified asphalt possess better flame retardancy when adequatequantity of flame retardant was added. Mixed flame retardantscan obviously increase the LOI of asphalt at low content, and thehigher ATH content was required to achieve equal LOI. For examplethe LOI of 25.9 and 25.7 was achieved when 6% mixed flame retar-dants and 30% ATH was added to asphalt, respectively. Variationsof ATH content within the selected ranges have the most signifi-cant effect in increasing the flame retardancy of asphalt binder.LOI of original asphalt binder is only 20.4. But the LOI of 27.5 isachieved for asphalt containing 40% ATH. Asphalt is transformedfrom inflammable material into flame retardant material.

3.2. Thermal analysis

The thermal behaviour of the mixed flame retardants modifiedhas been investigated in previous research [19]. The mixed flameretardants reduce remarkably the peak value. Fig. 2 illustrates

15.0

17.5

20.0

22.5

25.0

27.5

30.0

8%

6%

4%

40%

30%

20%

Lim

itted

Oxy

gen

Inde

x

ATH Mixed flame retardants

Contents(%)

0

10% 2%

Fig. 1. Effect of ATH and mixed flame retardants contents on limiting oxygen index.

Fig. 2. Results of the DSC–TG test of ATH, original and ATH modified asphalt binder.

Fig. 3. The Superpave gradation of the selected mixtures.

Y. Jianying et al. / Construction and Building Materials 23 (2009) 2277–2282 2279

the results of DSC–TG test results of ATH, original and ATH modi-fied asphalt binder at a wide temperature range from 50 �C to800 �C. Asphalt is complex mixtures of hydrocarbons containingoxygen, sulfur and nitrogen atoms, so a great number of exother-mic peaks are observed on the DSC master curve, which indicatethat plenty of ingredient transform during test. A large endother-mic peak was observed at 300 �C, which is linked with the releaseof crystallization water of ATH. Compared with original asphaltbinder, ATH modified asphalt binder has a similar trend when testtemperature is below around 300 �C. It is means ATH appearsexcellent thermal stability in such a temperature range. Therefore,it is strongly believed that no decomposition of ATH will occursduring mixing, paving and compacting in field because the relatedworking temperature rang, around 160–180 �C, are much less than300 �C. The DSC master curve of original asphalt appears a wideexothermic peak at temperature range of 400–450 �C and ATHmodified asphalt binder exhibits this exothermic peak from375 �C to 425 �C, but the peak value of ATH modified asphalt bin-der reduces apparently. Two endothermic peaks appear at ATHmodified asphalt binder’s DSC master curve when temper rangfrom 300 �C to 400 �C. For original asphalt binder, a larger exother-mic peak presents at temperature range 550–650 �C. But the peakvalue decline when ATH added to asphalt binder and the exother-mic peak appear at 500–625 �C. It is important to note that the DSCmaster curve of ATH modified asphalt binder is quite differentfrom that of original asphalt binders and more often the exother-mic peak lower than original asphalt binder’s exothermic peak.So ATH modified asphalt binders shows better flame retardancythan original asphalt binder.

Thermogravimetry master curves of ATH, original and ATHmodified asphalt binder also presents in Fig. 2. As for ATH, firststage degradation is apparently observed in a temperature rangebetween 250 �C and 350 �C with 28% weight loss, and slowerweight loss continues at higher temperatures, finally resulting in70% char formation at 800 �C. The first stage degradation tempera-ture begins at 250 �C for Original and ATH modified asphalt binder.But the weight loss for ATH modified asphalt is larger than originalasphalt binder’s and the first stage degradation is end at 480 �C. Itis suggests that the crystallization water retained and continued tobe removed with temperature increasing when ATH added to as-phalt binder. Thus ATH can increase the flame retardancy at highertemperature. The second stage degradation for original asphaltbinder is from 500 �C to 800 �C, the weight loss continued to in-creased, finally only 1.14% char formation at 800 �C. But ATH mod-ified asphalt binder exhibits second stage degradation starts atslightly lower temperature than that for the original asphalt and19.75% char formation at 800 �C. It suggests that ATH releasedcrystallization water releasing was delayed. From data obtained

experimentally, the addition of adequate ATH did drasticallychange the thermal degradation behaviors and increase the flameretardancy of asphalt binder.

3.3. Superpave mix design results

Asphalt binder and aggregate have been selected for the asphaltmixture and various combinations of these materials have beenevaluated to determine the optimum mixture using the Superpavegyratory compactor. An acceptable final blended gradation wasproduced as Fig. 3 display. The control asphalt mixture was testedto evaluate the effect of mixed flame retardants and ATH on as-phalt mixture.

The void in the mineral aggregate (VMA), voids filled with as-phalt (VFA), air void (Va), Effective asphalt content (Pbe) and dustproportion (DP) are very important to asphalt mixtures, becausethese volumetric properties significantly affect asphalt mixtureperformances. The % Gmm at Ninitial and Nmax can be used to iden-tify the potential tender mixture behaviors and similar compactionproblem. The initial trial asphalt binder content (Pb) for blendedgradation was calculated to be 5.0% by mass of total mix basedon the specific gravity of selected aggregates. Table 4 shows the re-sults of the estimated blend properties (Pb, VMA, VFA, % Gmm atNinitial and Nmax, Pbe and DP) at the estimated asphalt content toachieve 4% air voids at Ndesign. The results indicates that the

Table 6Marshall stability test results for both asphalt mixtures.

Mix type FRMAM Controlmixture

ATH modified asphaltmixture with differenceof ATH content

10% 20% 30% 40%

Air void (%) 4.4 4.3 4.1 4.2 4.4 4.5Initial stability (kN) 12.5 11.2 12.1 11.8 11.6 11.4Conditioned stability

(kN)10.8 9.6 10.6 10.3 10.0 9.7

Loss (%) 13.6 14.3 12.4 12.7 13.8 14.9

2280 Y. Jianying et al. / Construction and Building Materials 23 (2009) 2277–2282

selected blended gradation satisfied all the specification limitsusing original asphalt binder and mixed flame retardants or ATHmodified asphalt binders. Superpave design procedure recom-mended asphalt content is 5.0% and 4.8% for mixed flame retar-dants modified asphalt mixture and control mixture, respectively.The optimum asphalt content, selected as that which satisfies thespecified volumetric criteria at 4% air void, of mixed flame retar-dants modified asphalt mixture is greater than the one of the con-trol. It is maybe flame retardants appear as asphalt binder, soeffective asphalt content is not greater than that of control mix-ture. The results of Superpave mix design at optimum asphalt con-tent are showed in Table 5. The results indicated that mixed flameretardants modified asphalt mixture with 4.9% asphalt content canmeet the Superpave mixture specified requirement. Therefore,mixed flame retardants modified asphalt binder does not signifi-cantly affect the mixture performance comparing with the controlone.

3.4. Marshall stability and loss of Marshall stability test results

In order to investigate the effect of water on compacted asphaltmixture, the Marshall stability and loss of Marshall stability wasmeasured in accordance with Marshall procedures. Eight samplesfrom each mixture were immersed in the water bath at 60 �C. After30 min immersion in the water bath, four samples from each mix-ture were tested for Marshall stability values. The other four sam-ples were tested for Marshall stability after 24 h immersion in thewater bath. Table 6 exhibits that the Marshall stability test resultsand the percent loss in Marshall stability of three asphalt mixtures.The results disclosed that mixed flame retardants modified asphaltmixture has the best Marshall stability and the lower of the per-cent loss in Marshall stability after 30 min and/or 24 h immersionsin water bath. Marshall stability of ATH modified asphalt mixturedecrease with the ATH contents increasing. And the percent lossin Marshall stability increase when the ATH contents increased.

Table 4Estimated results at target air void (4% at Ndesign).

Estimated results Mix type Criteria

FRMAM Control mixture

Trial Pb 5.0 5.0 –Pb (Est.) 5.0 4.8 –% VMA (Est. @ Ndesign) 15.8 15.2 P13.0% VFA (Est. @ Ndesign) 75.1 73.7 65–75% Gmm (Est. @ Ninitial) 88.1 88.4 <89% Gmm (Est. @ Nmax) 97.6 97.9 <98Pbe 4.9 4.7 –DP 1.10 1.15 06–1.2

Note: FRMAM is flame resistance modified asphalt mixture.

Table 5Results of Superpave mix design.

Mix properties Mix type

FRMAM Control mixture

Pb (%) 4.9 4.8% Gmm at Ninitial (%) 87.8 88.3% Gmm at Ndesign (%) 96 96% Gmm at Nmax (%) 97.2 97.5Va @ Ndesign (Air voids) (%) 4 4VMA @ Ndesign (%) 15.5 15.2VFA @ Ndesign (%) 74.2 73.7Pbe (%) 4.7 4.7DP 1.15 1.15

These findings suggest that the use of ATH modified asphalt mix-ture can increase the Marshall stability and improve the moistureresistance of these mixtures, although the comparison of these per-formances shows no apparent difference. Thus, the mixed flameretardants and ATH can improve the Marshall stability and de-crease loss of Marshall stability of asphalt mixture compared withcontrol asphalt mixture.

3.5. Indirect tensile strength and loss of indirect tensile strength

Eight samples with air void of 6–8% made by Marshall Compac-tor to test indirect tensile strength (ITS) and loss of ITS. Four sam-ples from each mixture were placed in vacuum container with98.5 kPa. After 15 min vacuum saturation, the samples are eachwrapped with plastic bag containing 10 ± 0.5 ml of water andsealed. The plastic bags filling specimens are placed in a freezerat a temperature of �18 ± 3 �C for 16 ± 1 h. After frozen, they werethawed in 60 �C water for 24 ± 1 h. Then, eight samples from eachmixture are placed in a water bath at 25 ± 0.5 �C for 2 h ± 10 min.Finally, a load was applied to all specimens by forcing the bearingplates together at a constant rate of 50 mm/min. The maximumload is recorded, and the load continued until the specimen cracks.The test results are displayed in Table 7.

Initial and final ITS of control asphalt mix is larger than one ofthe mixed flame retardants modified asphalt mixture, but lowerthan that of the ATH modified asphalt mixture. The loss of indirecttensile strength is 11.3%, 12.4% for control and mixed flame retar-dants modified asphalt mixture. ATH can improve the initial ITS ofasphalt mixture, and the ITS is increased with the ATH contentsincreasing. The average values of the final ITS of ATH modified as-phalt mixture are slightly higher than that of the control asphaltmixture, except asphalt mixture with 10% ATH show a slightdecreasing. However, mixed flame retardants and ATH can increasethe loss of indirect tensile strength. For example, asphalt mixturewith mixed flame retardants modified asphalt, the value of lossof indirect tensile strength is 12.4% and 14.3% of loss of indirecttensile strength for 10% ATH modified asphalt mixture. The lossof indirect tensile strength increase with the ATH content increas-ing. But the loss of indirect tensile strength is within the 20% allow-able loss limit specified in Superpave specifications. The resultsindicated that asphalt mixture modified by mixed flame retardants

Table 7Indirect tensile strength (ITS) test results.

Mix type FRMAM Controlmixture

ATH modified asphalt mixturewith difference of ATH content

10% 20% 30% 40%

Air void (%) 6.5 6.6 6.2 6.5 6.3 6.4Initial ITS (MPa) 0.89 0.97 0.98 1.00 1.01 1.03Conditioned ITS

(MPa)0.78 0.86 0.84 0.86 0.87 0.89

Loss (%) 12.4 11.3 14.3 14.0 13.9 13.6

10% 2 0% 30% 40%

3.6

3.8

4.0

4.2

4.4

Dyn

amic

Sta

bilit

y(cy

cles

/mm

)

Rut

Dep

th(m

m)

ATH Contents (%)

3800

4000

4200

4400

4600

4800

Fig. 5. Effect of ATH content on the wheel-tracking test results of asphalt mixture.

Y. Jianying et al. / Construction and Building Materials 23 (2009) 2277–2282 2281

and /or ATH does not destroy the indirect tensile strength of as-phalt mixture comparing with the control one. The loss of ITScan meet specified requirements, although there are a little ofdecreasing when mixed flame retardants or ATH present to asphaltmixture.

3.6. Wheel-tracking test results

Wheel-tracking test results for control mixture, mixed flameretardants and ATH modified asphalt mixtures are showed inFig. 4. The two stages that the asphalt concrete passes throughin rutting testing (densification and steady state) clearly appearin this figure. In comparing the performance of asphalt mixture,the control mixture, mixed flame retardants and 30% ATH modi-fied asphalt mixture has the maximum deflection of 4.091 mm,3.810 mm and 3.965 mm, respectively. Dynamic stability (DS)was calculated as Eq. (1). The dynamic stability of 4117 cycles/mm, 4921 cycles/mm and 4375 cycles/mm was obtained for con-trol mixture, mixed flame retardants modified asphalt mixtureand 30% ATH modified asphalt mixture. The results indicate thatflame retardant and ATH can improve the rutting resistance of as-phalt mixture. Fig. 5 shows the effect of ATH percentage on therut depth and dynamic stability of the asphalt mixture. The re-sults indicated that the rut depth decreases and the dynamic sta-bility increases as the percent of ATH increases. Thus, thepermanent rutting resistance increases as the contents of ATH in-creases in the asphalt mixture. Furthermore, the rut depth anddynamic stability of mixtures does not decrease markedly. ATHhave little effect on the wheel-tracking test results of asphaltmixture.

The higher asphalt contents may lead to the high permanentdeformation, due to too much asphalt binder in the mixture thatcauses the loss of internal friction between aggregate particles.Optimum asphalt contents for mixed flame retardants modified as-phalt mixture are higher than control mixture. But flame retar-dants modified asphalt mixture reveal better resistance to rut. Itis thought that flame retardants contribute much to cohesion abil-ity in asphalt mixtures. The particle of ATH is small, and the surfacearea factor is larger. So it adsorbs more asphalt binder than filler.The ATH modified asphalt mixture was prepared when the desiredamount of filler is replaced by an equal amount of ATH. ATH adsorbasphalt and effective asphalt content in asphalt mixture decrease.The rut resistance of ATH modified asphalt mixture may attributethe asphalt contents decreasing and asphalt binder’s cohesionincreasing.

Fig. 4. Wheel-tracking test results of asphalt mixture.

4. Conclusions

On the basis of the results of this laboratory test on mixed flameretardants modified asphalt mixture and ATH modified bindersand it mixtures compared with control mixture, the following con-clusions can be drawn.

(a) Mixed flame retardants can reduce significantly the flamma-bility of the asphalt binder at low content. However, ATHmodified asphalt binder can obtain equal flame retardancywhen adequate amount of ATH was added.

(b) The thermal decomposition of asphalt binder is affected bythe presence of ATH. The decomposition shifts to lower tem-perature by about 40 �C and the DSC master curves appeartwo endothermic peak at temperature 300–400 �C. The exo-thermic peak is lower than one of original asphalt binder forATH modified asphalt binder. So ATH can improve asphaltbinder’s flame retardancy and not decomposes during mix-ing, paving and compacting.

(c) The ATH contents have a little effect on the performance ofasphalt mixture. Increasing the percentages of ATH in themixtures leads to higher initial stability and conditioned sta-bility, this increase is also very effective in improving ruttingresistance over the conventional mixtures.

(d) The effect of mixed flame retardants and ATH on ITS and lossof ITS is different. The ITS and conditioned ITS is lower thanone of control mixture for mixed flame retardants modifiedasphalt mixture. ATH can increase ITS and conditioned ITS.But the loss of ITS of ATH modified asphalt mixture is largest.

In short, the successful utilization of ATH modified asphalt mix-ture in pavement construction can provide a new and safer roadmaterial, especially in tunnel. It should be noted that these resultswere based on limited test data in the lab and should be furtherexamined in field.

Acknowledgments

The author is grateful to the Department of Transportation inHubei Province, China and Headquarters of Shi-Man Expresswayin Hubei Province for its financial support of this work.

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