Construction and Building Materials 25 (2011) 14–20

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Construction and Building Materials

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Starch as a modifier for asphalt paving materials

Al-Hadidy AI a,*, Tan Yi-qiu b, Ayman Talib Hameed a

a Civil Engineering Department, University of Mosul, Mosul, Iraqb School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150090, PR China

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

Article history:Received 13 January 2010Received in revised form 6 May 2010Accepted 19 June 2010Available online 13 July 2010

Keywords:StarchStyrene–butadiene–styreneStone matrix asphaltPerformance tests

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.06.062

* Corresponding author. Tel.: +86 413946033276.E-mail address: abd_et76@yahoo.com (A.-H. AI).

This paper investigates the viability of using starch (ST) as a new modifier for asphalt paving materials.Different ratios of ST (2.5, 5.0, and 7.5% by weight of asphalt) were blended with 70/100 paving gradeasphalt. Unmodified and modified asphalt binders were subjected to physicochemical, alkali, acid andfuel resistance tests. The performance tests including, Marshall stability, Marshall Quotient (MQ), tensilestrength, tensile strength ratio, flexural strength, rutting resistance and resilient modulus (MR) were car-ried out on unmodified and modified stone matrix asphalt (SMA) mixtures. The analyses of test resultsshow that the performance of ST-modified asphalt mixtures are better than conventional and styrene–butadiene–styrene (SBS)-modified mixtures. The rutting potential, moisture susceptibility and tempera-ture susceptibility can be reduced by the inclusion of ST in the asphalt mixture. The laboratory MR valuesare lower than the calculated ones using the empirical equations. The results also revealed that this mod-ifier can be used as anti-stripping agent. It also shows resistance to fuels and most common chemicals. AST content of 5% by weight of asphalt is recommended for the improvement of the performance of asphaltconcrete mixtures similar to that investigated in this study.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The use of asphalt in road building has gradually increasedthrough the years and reached its peak in 1979. It is speculatedthat the current annual worldwide consumption of asphalt is over100 million tons. Currently in the United States, the annual con-sumption is at least 25 million tons. Approximately 94% of USroads are paved with asphalt [1].

Asphalt cement may be modified by the addition of componentsthat increase the strength of the material or otherwise alter itsproperties. In particular, it has become known in the art to add car-bon black to asphalt as a filler material. It has also become knownthat one may add polymers to an asphalt blend in order to improveits strength and reduce its temperature susceptibility. Modificationof asphalt with a variety of different polymers, including polyeth-ylene (PE), polypropylene (PP) and styrene–butadiene–styrene(SBS) has been described [2–4]. Various additives, polymers, etc.,have been utilized for the purpose of improving the high and lowtemperature characteristics of asphalt compositions, as well as toimprove their toughness and durability. Additives such as styrenebased polymers, polyethylene based polymers, polychloroprene,gilsonite, various oils, and many other modifiers including tall oilhave been added to asphalt to enhance various engineering prop-erties of asphalt [5].

ll rights reserved.

Because the polymer modifiers that have been employed bene-ficially as asphalt modifiers are rather expensive, a need exists foralternative, lower-cost modifiers that nonetheless impart im-proved properties comparable to those achieved by using the moreexpensive polymers.

Starch (ST) is lighter in weight and generally much cheaper(0.5 $/kg) than other conventional polymers such as PE, SBS andPP. ST available widely in the plant kingdom, and exists as micro-scopic white grains that are insoluble in alcohol, ether, and coldwater. It is a highly organized mixture of two carbohydrate poly-mers, amylose and amylopectin. ST comes from its source as a fine,free-flowing powder, which obviates any need for preprocessingthe ST before mixing it into the asphalt.

ST granules when heated in water gradually absorb water andswell in size, causing the mixture to thicken. With continued heat-ing however, the swollen granule fragments become less thick, andamylose and amylopectin become soluble in the hot mixture. Thisprocess of granules swelling and fragmenting is called ‘‘gelatiniza-tion”. Because of the larger size of the swollen granules comparedto the size of amylose and amylopectin, the viscosity of the swollengranules mixture is much higher than that of the amylose/amylo-pectin mixtures [6]. This makes it especially desirable to developalternative and more-cost effective modifier for road building.

On the other hand, and based on many research reports andengineering case studies [7,8] it has been shown that the use ofstone matrix asphalt (SMA) on road surfaces can achieve betterrut resistance and durability. The SMA mixtures are designed to

Table 2Composition of STa.

Property Result

Moisture (%) 12.88Ash (%) 0.20Lipid (%) 0.80Phosphorus (%) 0.06Amylose 27.49Price ($/kg) 0.5

a As provided from supplier.

0

20

40

60

80

100

120

0.01 0.1 1 10 100Seive size (mm)

% p

assi

ng

Lower limitUpper limitMiddle limit

Fig. 1. SMA gradation limits.

Table 3Source and consensus properties of aggregates.

Property Coarseaggregate

Fineaggregate

Filler(CaCO3) Test method(ASTM)

Bulk sp. gr. 2.721 2.652 0.477a C-127 & C-128Apparent sp. gr. 2.763 2.691 2.734Water

absorption(%)

0.278 1.4877 –

Angularity (%) >100 44.863 –Toughness (%) 22.07 – – C-131Soundness (%,

Na2SO4)1.784 1.205 – C-88

a N.B.: Bulk density in toluene g/cc.

A.-H. AI et al. / Construction and Building Materials 25 (2011) 14–20 15

have high aggregate content, high asphalt content typically 5.5–7%and high filler content. For ordinary SMA, the use of regular asphaltcement together with fibrous material as a drainage inhibitor issufficient. Under high temperatures and heavy loading, a harderasphalt grade will also suffice. A polymer (such as PE, PP or SBS)modified binder may be used to substitute the fibrous material.It is possible to increase the capability of resistance to permanentdeformation at the expense of a higher price and greater instabil-ity. The demand for higher pavement quality from users is ever-increasing. The cost of a pavement failure is also mounting higher;hence there is a strong desire to have better asphalt mixture fromhighway agencies.

However, this study is an attempt to satisfy the followingobjectives:

1. To find out a suitable method for producing a ST-modifiedasphalt concrete.

2. To determine the physiochemical properties of ST-modifiedasphalt binder (STMAB) and compare the results with 70/100grade asphalt cement.

3. To investigate the alkali, acid and fuel resistance of STMAB andcompare the results with 70/100 grade asphalt cement.

4. To evaluate temperature susceptibility of STMAB and comparethe results with asphalt cement.

5. Possibility of using the South Carolina Department of Transpor-tation (SCDOT) 13 mm SMA gradation on China pavingmaterials.

6. To investigate the moisture and temperature susceptibility ofST-modified SMA (STMSMA) mixtures.

7. To investigate the rutting potential of ST–MSMA mixtures.8. To evaluate the low temperature performance of ST–MSMA

mixtures.9. To determine the resilient modulus of ST–MSMA mixtures from

the repeated load indirect tensile test and compare the resultswith those obtained using empirical equations mentioned by[9,10].

2. Materials and methods

2.1. Materials used

Asphalt cement of 70/100-penetration grade was used. Polymer modified as-phalt binder of 70 grades (PMAB-70) with SBS of readily blended type in commer-cial form using 70/100 asphalt cement as base binder was used in the investigation.For the modification of base binder, 5% of SBS copolymer in solid form was mixed.The basic properties of asphalt cement and PMAB-70 with SBS are shown in Table 1.

ST has a chemical unit [C6H10O5]n = 100�1000 and purity of 99.2% was used tomodify the 70/100-paving grade asphalt cement. The composition of ST is reportedin Table 2.

Aggregate was obtained from one asphalt plant in Heilongjiang Province locatedin the north part of China. Fig. 1 shows the recommended gradation limits by the SCDOT [11] for SMA mixtures and the selected gradation in this research was in themiddle of the limits.

The properties of the aggregates, such as angularity value, toughness value,soundness value, water absorption value, and specific gravities were determined,and the test results are presented in Table 3.

The filler used was calcium carbonate (CaCO3) and it was brought from one as-phalt plant. Calcium carbonate was passed through a 200 sieve and had specificgravity of 2.734.

Table 1Physical properties of base asphalt and PMAB.

Property Asphalt 70/100 PMAB

Penetration (25 �C, 100 g, 5 s, dmm) 78 47Softening point (�C) 42 54Ductility (25 �C, 5 cm/min, cm) 150+ 100+

Sp. gr. 1.02 0.954Flash point (�C) 241.5 –Loss on heat (163 �C, 5 h, %) 0.905 0.587Asphaltene (%) 9.11 –

2.2. Preparation of ST-modified binder

There are two generally distinct approaches for producing a ST-modified as-phalt concrete. The principal difference between the two approaches is the choiceof the concrete component to be modified. On one hand, it might be thought desir-able to premix the powdery ST with the aggregate, which already contains a mix-ture of materials of different particle sizes including a substantial quantity offines. However, the ST particles are still significantly smaller than the fine aggre-gate: the granule sizes for ST vary from (1–100) microns in diameter, while fineaggregate suitable for asphalt concrete typically contains not more than 5–10%passing a No. 100 sieve. As a result, the mixing operation generates considerableST dust. Moreover, when ST is mixed with the aggregate it tends to function primar-ily as an inert filler rather than as a modifier.

While inert fillers do improve the stability and strength of hot mix asphalt con-crete, their inclusion does not alter the properties of the concrete to the same extentthat polymer-based modifiers added to the asphalt do. As used herein, ‘‘filler” isconsidered to include material passing the No. 200 (0.075 mm) sieve. In general,a filler material is introduced into the hot mix asphalt because it can provide addedstability and strength. It is generally believed that the main action of a filler is to fillthe voids between the coarse aggregates in the mixture. The effect of the filler onthe binder depends on its geometric irregularities such as shape, angularity, andsurface texture. The surface texture affects the surface activity – that is, the capacityof the filler surface to absorb binder. Another very important factor is the size dis-tribution of the filler material. The larger filler particles probably serve to fill thevoids between the coarse aggregates as described above. Very fine particles of filler

16 A.-H. AI et al. / Construction and Building Materials 25 (2011) 14–20

may become suspended in the asphalt, forming a mastic. In general, a filler does notinteract chemically with asphalt, and its effect is mainly mechanical. On the otherhand, a modifier is expected to have a chemical interaction with the binder, chang-ing its properties through such an interaction. Major binder properties that areinfluenced by modification include viscosity and stiffness. In general, the strategybehind modifying binders is to depend on the base asphalt to provide good lowtemperature properties while depending on the modifier to provide good high tem-perature properties.

ST that is mixed into the asphalt concrete after the asphalt and aggregate havebeen combined also tends to act as a filler rather than as a modifier. A far preferableapproach, therefore, is to blend ST into the asphalt before it is combined with aggre-gate. This may be accomplished by using oil or water to disperse ST into the asphalt.Water slurry blends are preferred over oil slurries because the latter tend to signif-icantly increase the penetration value measured for the cement. However, the ST-modified asphalt samples were prepared in the following procedure:

Asphalt was heated to the desired temperature in a three-neck flask providedwith stirrer and contact thermometer. The temperature was held constant by anautomatic control system while stirring intensively. The ST was dispersed intothe asphalt by using hot water (60 ± 1 �C) with 25–27% by mass of the ST. The as-phalt was treated with different percentages of ST (2.5, 5, and 7.5 wt.% of neat as-phalt). The best results were achieved when the blending temperature wasmaintained above about (135–142 �C).

2.3. Laboratory testing

A series of tests were carried out on modified binders according to ASTM [12]methods to characterize the mixtures designed for different percentages of ST asa modifier. The tests that were conducted include the following:

– Physicochemical tests, such as penetration, softening point, solubility, asphalt-enes, etc.

– Temperature susceptibility.– Alkali and acid resistance.– Fuel resistance.

Alkali and acid resistance test according to ASTM D147 was performed on theSTMAB samples. Two groups for each STMAB were prepared. The first group wasplaced in 40% NaOH at 21 �C solution for 1000 h. The second group was immersedin 95% HCl at 21 �C solution for 1000 h. The samples were then brought to weightloss determination as designated as alkali and acid resistance.

Fuel resistance test according to ASTM D5329 was performed on the STMABsamples. Three groups for each STMAB were prepared. The first group was im-mersed in JP-4 fuel bath at 40 ± 1 �C for 24 h. The second group was placed inhydraulic bath at 25 ± 1 �C for 7-days. The third group was immersed in glycol/water (50/50) bath at 25 ± 1 �C for 7-days. The samples were then brought toweight loss determination as designated as fuel resistance.

The performance tests conducted on SMA mixture modified with ST and SBSinclude:

– Drain down test.– Marshall test.– Static indirect tensile strength test.– Moisture susceptibility test.– Flexural beam test.– Rutting resistance test.– Resilient modulus test.

Fig. 2. Method of determining rate of rutting on a typical plot of rut depth data.

2.4. Mixture design

The mix design procedure for SMA as proposed in NCHRP Report No.425 [8] wasfollowed in performing the mix design to be used. Locally available materials thatmeet the normal SMA specifications were used to produce the reference mix. A70/100 penetration grade asphalt without mineral fiber was used in this referencemix. Laboratory specimens were prepared using 50 blows of the Marshall hammerper side. Seventy-five compaction blows were not used since they would not resultin a significant increase in density over that provided by 50 blows. SMA mixtureshave been more easily compacted on the roadway to the desired density than theeffort required for conventional hot mix asphalt mixtures [12]. The optimum as-phalt content for SMA mixtures is usually selected to produce 4% air voids and draindown of less than 0.3%.

In this research, compaction of all the SMA samples was performed using fiftyblows of the Marshall hammer per side. The optimum asphalt content for the con-trol mixture was found to be 6.12% at 4% air voids, and it was used in preparing allother ST and SBS-modified mixtures to maintain consistency through the study.Three identical samples for each mixture were fabricated.

2.5. Performance tests used

Four performance tests in the laboratory were adopted. The tests performedwere moisture susceptibility, rutting resistance, flexural strength and resilientmodulus tests. The moisture susceptibility test according to AASHTO T283 proce-dure was performed on the three SMA mixes, which were compacted to an averageair void content of 7.0%. Three Marshall specimens for the dry group and three spec-imens for the wet group were prepared. A tensile strength ratio (TSR) of wet groupto dry group was computed from the results of the indirect tensile strength test at25 �C. The higher the TSR value, the less the strength should be influenced by thewater soaking condition, or the more water-resistant it should be. Normal SMAspecification requires a TSR value of 70% or more.

Rutting resistance test was done according to the wheel tracking test. In thistest, the sample was a square slab with a side 30 cm long and 5 cm thick. The spec-ified air voids content of the slab specimen was controlled by controlling theamount of mixture in the specified steel mold. The specified air voids content ofthe slab specimen was controlled by controlling the amount of mixture in the spec-ified steel mold. The target air voids content for the slab specimens was 4%. The fin-ished slab was allowed to cool at room temperature for 12 h. It was then put into anoven at 60 �C for another 5 h before being placed into the wheel tracking device. Thewheel tracking device was maintained at 60 �C. The loading wheel was a piece ofsolid, hard rubber; 20 cm in diameter, with a width of 5 cm. The wheel weighed70 kgf and traveled back and forth at 21 rounds per minute. This is equivalent topressing the slab 42 times per minute. We then recorded the depth of the trackdepression (rut depth) at various times. Calculation of the rate of rutting (RR)was based on a time interval between 45 and 60 min as shown in Fig. 2. The RRcan be computed as (d2–d1)/15 with a unit of mm/min. A lower RR means betterresistance to permanent deformation. For each of the three kinds of mixtures eval-uated, we made two samples and took their averages to represent the rut resistancefor each mixture.

Flexural test was done on small beams of the dimension 300 � 50 � 48 mm cut-ting from a compacted square slab specimen (30 � 30 � 5 cm). Three series of beamspecimens were prepared and tested at �10 �C temperature using a universal test-ing machine and temperature control cabinet [Materials Testing System (MTS-810)]. This was to monitor the low temperature behavior of SMA mixes.

Resilient modulus (MR) is one of the most important mechanical properties ofAC. When elastic-layered system theory is used to design asphalt pavement struc-tures, the modulus of AC is a basic design parameter. The current asphalt pavementperformance prediction models also use the modulus as a critical material param-eter. Therefore, it is desirable that the modulus of AC be predicted during the ACmixture design stage to improve mixture design and pavement performanceprediction.

The MR represents the ratio of an applied stress to the recoverable strain thattakes place after the applied stress has been removed. The MR was determinedusing the following techniques:

� Based on the properties of binder and the volume concentration of the aggre-gate using Eqs. (1) and (2) mentioned by [9,10].

Eb ¼ 1:157� 10�7 � t�0:368 � 2:718�P:I:ðTR&B � TaspÞ5 ð1Þ

where Eb is the elastic modulus of the bituminous binder (MPa), TR&B is the recoveredbitumen ‘Ring and Ball’ softening temperature (�C), Tasp is the temperature of the as-phalt layer (�C), P.I. is the recovered bitumen ‘Penetration Index’ and, t is the loadingtime (s).Eq. (3.5) is only applicable when:0.01 s < t < 0.1 s, �1.0 < P.I. < 1.0,20 �C < (TR&B � Tasp) < 60 �C.

Em ¼ Eb½1þ ð257:5� 2:5VMAÞ=ðn� ðVMA� 3ÞÞ�n ð2Þ

Fig. 3. Physiochemical properties of STMABS.

Table 4Test results of STMABS.

Property ST (%)

A.-H. AI et al. / Construction and Building Materials 25 (2011) 14–20 17

n ¼ 0:83 log½4� 104=Eb�

where Em is the elastic modulus of the asphalt mixture (MPa), VMA percent voids inmixed aggregate.Eq. (1) is only applicable when 12% < VMA < 30%, Eb > 5 MPa.

� From tests on cylindrical specimens for each mixture at designed asphalt con-tents in the indirect tension mode. About 15% of the indirect tensile strengthof each mixture was applied on the vertical diameter for conventional, SBS-modified and ST-modified specimens.

For the repeated load indirect tensile test, Marshall specimens prepared with aparticular binder were subjected to repeated compressive loads in equipment fab-ricated for this purpose. The frequency of load application used was 1 Hz, with aload duration of 0.1 s to represent field conditions and a resting period of 0.9 s.

The tests were conducted at 25 �C; the test procedure adopted was as per ASTM4123 [13]. Constant test temperature was maintained using an environmental airchamber. Each specimen was placed inside the chamber at the set temperaturefor 3 h before testing. For each of the three kinds of mixtures evaluated, three sam-ples were made and took their averages to represent the MR for each mixture.

0 2.5 5 7.5

HCl, %loss N/Aa N/A 3.73 3.20NaOH (%loss) N/A N/A 0.72 0.56JP-4 (%loss) N/A N/A 5.44 4.80Hydraulic (%loss) 2.83 1.95 1.21 0.93Glycol/water (%loss) 0 0 0 0

a N/A: Not available.

3. Results and discussion

3.1. Physicochemical tests

The physicochemical properties of the STMABS were evaluatedand the results are shown in Fig. 3. The results indicate that ST iseffective in improving the physicochemical properties of asphalt

Fig. 4. Penetration index of STMABS.

cement. Examining Fig. 3, it can be seen that the addition of ST con-tent to neat asphalt reduces the penetration and solubility values,whereas increases in softening point and asphaltenes values wereobserved with the addition of ST modifier. The study showed thatthe softening point of virgin asphalt was raised by 33.4%, whereasthe penetration was decreased by 41% with the addition of 5% ST inasphalt.

3.2. Temperature susceptibility

The temperature susceptibility of asphalt binders is quantifiedby penetration index (P.I). Calculated values of P.I for STMAB and

Fig. 5. Drain down of SBS and ST–MSMA.

Table 5Marshall test results of SBS and ST–MSMA mixtures.

Mixture type BSG (kg/m3) TMD (kg/m3) VTM (%) VMA (%) VFB (%) VCAmix (%) Stability (kN) Flow (mm) MQ (kN/mm)

Asphalt 2377 2476 4.00 17.63 77.31 41.37 6.81 3.12 2.18SBS 2376 2469 3.76 17.66 78.70 41.38 7.14 2.65 2.69ST 2371 2454 3.38 17.82 81.03 41.49 7.57 2.14 3.53

VCADRC = 41.679% > VCAmix.

18 A.-H. AI et al. / Construction and Building Materials 25 (2011) 14–20

asphalt cement are given in Fig. 4. A higher value of P.I indicatesthe lower temperature susceptibility of binder. From Fig. 4, it canbe observed that the values of P.I. for virgin and 5% STMAB samplesare �2.447 and 0, respectively. This significant fact demonstratesthat ST will lead to make this binder less susceptible to tempera-ture and more favourable for hot climates.

3.3. Alkali, acid and fuel resistance

Table 4 shows the effect of chemical solvents; fuel and oil onSTMABS. From Table 4, it can be seen that the modified sampleswith 5 and 7.5% ST comply with ASTM and Pacific specifications(ASTM and Pacific specifications requires that the percentage lossof sealant due to HCl and JP-4 solvents does not exceed 5%)[12,14], and showed resistance to fuel, oil and chemicals in a rangeof conditions and provide a high level of asphalt properties for ex-tended periods of time. It is therefore recommended to use ST-as-phalt binder in special road construction.

3.4. Engineering properties of SMA mixtures

3.4.1. Statistical considerationsResults of the drain down, Marshall, indirect tensile, tensile

strength ratio, flexural strength, rutting resistance and resilientmodulus tests were statistically analyzed with a 5% level of signif-icance using SPSS software V.16. For these comparisons, it shouldbe noted that all specimens were produced at optimum asphaltcontent.

3.4.2. Drain downDrain down test using wire basket method as proposed in the

NCHRP Report No. 425 [8] was run on all the mixes evaluated. Inthis test, the laboratory-prepared loose mix was placed in a forceddraft oven for 1 h at a pre-selected temperature. At the end of 1 h,the basket containing the sample is removed from the oven alongwith the plate and the mass of the plate was determined. Theamount of increased weight of the plate is the amount of drain

Fig. 6. ITS of SBS and ST–MSMA.

down of the mix. As specified in NCHRP [8], the oven temperaturefor performing the drain down test should be at the mixing tem-perature and/or the mixing temperature plus 15 �C. Thus, an oventemperature of 150 �C was used for the reference and ST–MSMAmixtures and an oven temperature of 165 �C was used for theSBS–SMA mixture. All the SMA mixtures in this study passed thedrain down test.

Fig. 5 shows the drain down test results for three types of SMAmixtures. From Fig. 5, it can be observed that the values of draindown for conventional, 5%SBS and 5%ST-modified asphalt concretesamples are 0.3026, 0.2663 and 0.205, respectively. The potentialeffects of the inclusion of ST in base asphalt are therefore beneficialin preventing bleeding phenomenon of the SMA mixtures. No fiberwas needed to prevent drain down when this asphalt-ST was used.Hence, it should be mentioned that a 5 wt.% of both SBS and ST wasselected that complies with polymer loaded in concentrations ofabout 4–6% by weight with respect to the asphalt as mentionedby [15]. Also, higher concentrations of polymers are consideredto be economically less viable and also may cause other problemsrelated to the material properties.

3.4.3. Marshall propertiesThe Marshall properties of SBS and ST–MSMA mixtures were

evaluated and the results are presented in Table 5. From Table 5,it can be seen that the addition of 5% SBS and ST raises the Marshall

Fig. 7. TSR of SBS and ST–MSMA.

Fig. 8. RR of SBS and ST–MSMA.

Fig. 9. Flexural strength properties of SBS and ST–MSMA.

A.-H. AI et al. / Construction and Building Materials 25 (2011) 14–20 19

stability of control mix by 4.8% and 11%, respectively, whereas de-creases in flow value was observed with the addition of SBS and STmodifiers. The results also revealed that Marshall Quotient (MQ)value increased by 23.5% and 62% at 5% SBS and 5% ST content,respectively. It can be said that STMABS provide better resistanceagainst permanent deformations due to their high stability andhigh MQ.

3.4.4. Moisture susceptibilityTensile strength and TSR for SBS and ST–MSMA mixtures are gi-

ven in Figs. 6 and 7, respectively. For SBS-modified, ST-modifiedand control mixtures, average static indirect tensile strength (ITS)values of the conditioned specimens are 0.465, 0.882 and0.356 MPa, respectively. For these, standard deviations of resultsobserved were 4.87 � 10�3, 1.25 � 10�2 and 5.77 � 10�3, respec-tively. Average ITS values obtained for unconditioned specimensare 0.591, 0.801 and 0.501 MPa, respectively. For these, standarddeviations of results observed were 1.49 � 10�2, 4.05 � 10�2 and5.9 � 10�3, respectively. From these, value% TSR obtained forSBS-modified, ST-modified and control mixtures are 78.5, 110and 71%, respectively. It can be seen that the STMAB improvesthe resistance to moisture susceptibility of the asphalt mixtures.From Fig. 7, it was also shown that the stripping value of STMSMAmixture is negative (i.e. �9%). This is related to that ST granuleswhen heated in water gradually absorb water and swell in size,causing the mixture to thicken. With continued heating however,the swollen granule fragments become less thick, and amyloseand amylopectin become soluble in the hot mixture [6]. This pro-cess of granules swelling and fragmenting is called ‘‘gelatiniza-tion”. These results revealed that this modifier can be used asanti-stripping agent instead of other known types such as coconutoil ethanolamine, Wetfix I, Lilamin VP75P, Chemcrete, and hy-drated lime.

3.4.5. Rutting resistanceFig. 8 shows the wheel tracking test results for three types of

SMA mixtures. The RR values obtained for control, SBS-modifiedand ST-modified mixtures are 0.094, 0.02 and 0.0349, respectively.For these, standard deviations of results observed were

5.66 � 10�3, 8.485 � 10�4 and 2.72 � 10�3, respectively. FromFig. 8, it can be seen that the addition of 5% SBS and 5% ST reducesthe RR of the control SMA mixture by 78.7% and 63%, respectively.This significant fact demonstrates that SBS and ST will lead to makethis mixture less susceptible to permanent deformation.

3.4.6. Flexural strengthThe flexural test results of mixtures containing SBS and ST are

given in Fig. 9 for �10 �C testing temperatures. They show an in-crease in both stiffness and modulus of rupture of the mixturesat low temperatures with the addition of SBS and ST.

The modulus of rupture values obtained for control, SBS-modi-fied and ST-modified mixtures are 0.366, 0.455 and 0.522 MPa,respectively. For these, standard deviations of results observedwere 1.91 � 10�2, 7 � 10�3 and 0.212, respectively. Similarly, aver-age stiffness modulus values obtained for control, SBS-modifiedand ST-modified mixtures are 85.225, 106.857 and 120.47 MPa,respectively. Fig. 9 indicates that the addition of ST content to neatasphalt increases the stiffness, modulus of rupture and strain val-ues of the control mixture by 41, 43 and 13%, respectively. Fromthe results, it is evident that ST-modified mixtures perform betterthan control mixture at low temperature, which is a desirable char-acteristic of SBS and ST-modified mixtures. This may probably les-sen the chances of premature cracking.

3.4.7. Resilient modulusThe MR test results are presented in Fig. 10. From Fig. 10, it can

be observed that the MR for conventional, 5% SBS-modified and 5%ST-modified asphalt concrete samples are 1468 MPa, 2046 MPa and2021 MPa, respectively. For these, standard deviations of resultsobserved were 48.542, 112.023 and 107.611, respectively. The per-centage increase in the average MR is found to be comparativelysignificant. The study showed that the percentage increase in MRvalue with the addition of 5% SBS and 5% ST in asphalt were foundto be 39.45 and 37.7%, respectively. Fig. 10, also compares the MRvalues obtained from the repeated load indirect tensile test withthose determined based on the properties of binder and the volumeconcentration of the aggregate using Eqs. (1) and (2) mentionedearlier. From this Figure, it can be seen that the calculated MR

Fig. 10. MR of SBS and ST–MSMA.

20 A.-H. AI et al. / Construction and Building Materials 25 (2011) 14–20

values for conventional, 5% SBS-modified and 5% ST-modifiedasphalt concrete samples are 1531 MPa, 2501 MPa and 2594 MPa,respectively. Analysis of data presented in Fig. 10 indicates thatthe laboratory MR values are lower than the calculated ones.

4. Conclusions

Based on the testing and analysis presented herein, the conclu-sions of the study are summarized as follows:

(A) A review of the standard binder testing and grading resultsindicated the following:

1. Penetration at 25 �C will generally decrease as ST contentincreases, which indicates an improved shear resistance inmedium to high temperatures.

2. Softening point tend to increase with the addition of STcontent, which indicates improvement in resistance todeformation.

3. Penetration index values indicated that ST reduced the tem-perature susceptibility of asphalt.

4. The ST-asphalt binder is resistant to fuels and most commonchemicals. Therefore it can be used in special paving con-struction such as fuel station.

5. It was found in this study that 5–7.5% ST is a good range toproduce high level of asphalt properties.

6. Low material price of ST-asphalt binder will extend their usein large volume applications.

7. ST-asphalt binder can be blend by hand or with a slow-speed drill, whereas SBS–asphalt binder requires specialmetering equipment; such as high shear mixer.

8. SBS is destined to separate from the asphalt when stored athigh temperature, which is the major obstacle to theapplication of SBS-modified asphalts in paving, whereasST–asphalt binders do not have this problem.

(B) A review of the drain down, Marshall, indirect tensilestrength, flexural strength, wheel tracking and resilient mod-ulus results indicated the following:

1. The ST was effective in preventing excessive drain down ofthe SMA mixtures (i.e. bleeding phenomenon).

2. Marshall results indicated that ST raised the stability ofcontrol mix by 11%.

3. The tensile strength ratios for the mixes containing the STwere greater than 100%. This indicates that this type of addi-tive does not cause the mixture to weaken when exposed tomoisture, and at the same time can be used as anti-strippingagent.

4. The wheel tracking and flexural strength results indicatedthat rutting resistance and low temperature cracking ofthe ST–SMA mixture were much better than that of a con-trol mixture.

5. The use of ST in asphalt concrete mixtures caused anincrease in resilient modulus values at high (25 �C) temper-atures. Analysis of data presented herein indicates that thelaboratory MR values are lower than the calculated ones.

6. Based on the indirect tensile strength, rutting, flexuralstrength and resilient modulus results, ST can satisfy theperformance requirements of moisture damage sensitivity,high temperature, and low temperature, therefore, it is pos-sible to use it as an alternative (new) modifier for asphaltpaving materials.

7. Asphalt concrete containing asphalt cement modified withST requires no special technique for application to a roadbedas compared with unmodified and SBS-modified asphaltconcrete mixtures.

Acknowledgments

This research was part of a project (Project No. 50778057) sup-ported by National Natural Science Foundation (NSFC) and the Re-search Fund for the Doctoral Program of Higher Education (RFDP)of China.

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