Influence of hindered phenol additives on the rheology of aged polymer-modified bitumen

Construction and Building Materials 38 (2013) 214–223

Contents lists available at SciVerse ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

Influence of hindered phenol additives on the rheology of agedpolymer-modified bitumen

Samer Dessouky a,⇑, David Contreras b,1, Jeremy Sanchez a, A.T. Papagiannakis a,2, Ala Abbas c,3

a One UTSA Circle, Department of Civil and Environmental Engineering, University of Texas at San Antonio, San Antonio, TX 78249, United Statesb Texas A&M Transportation Institute, Texas A&M University System, 3135 TAMU, College Station, TX 77843, United Statesc Department of Civil Engineering, University of Akron, Akron, OH 44325, United States

h i g h l i g h t s

" The stiffening index suggested thatSBR and HP2 are the most effectiveadditives.

" HP2 decreases bitumen stiffeningwhile polymers increase bitumenstiffening.

" Thermal stability analysis suggestedthat HP2 limited bitumen stiffeningincrease.

" HP2 with SBR combined reducedmoisture damage and increasedrutting resistance.

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.07.089

⇑ Corresponding author. Tel.: +1 210 458 7072; faxE-mail addresses: [email protected] (S. De

tamu.edu (D. Contreras), [email protected] (J.utsa.edu (A.T. Papagiannakis), [email protected] (A. A

1 Tel.: +1 979 862 4932; fax: +1 979 845 1701.2 Tel.: +1 210 458 7071; fax: +1 210 458 6475.3 Tel.: +1 330 972 8242; fax: +1 330 972 6020.

g r a p h i c a l a b s t r a c t

Hamburg test for bituminous mixes made with base, SBR-blend and HP2-enhanced (arrows show thestripping points).

a r t i c l e i n f o

Article history:Received 13 April 2012Received in revised form 12 July 2012Accepted 23 July 2012Available online 23 September 2012

Keywords:BitumenStiffening indexSBRRheologyHindered phenolsComplex modulus index

a b s t r a c t

Oxidation (aging) in polymer-modified bitumen occurs due to exposure in air at high temperatures. Theaging mechanism tends to alter the physical and chemical properties of bitumen as reflected by thechange in their rheological properties. In this study, a blend of SBR and hindered phenols (HPs) was usedto improve the rheological characteristics of base (unmodified) and polymer-modified bitumen. Rheolog-ical properties were measured using a Rotational Viscometer, a Dynamic Shear Rheometer and a BendingBeam Rheometer. Stiffening indices and complex modulus indices were used to assess the effectivenessof the additives in the bitumen and its thermal stability, respectively. It was found that the additives werecapable of increasing the stiffness at high temperature and reducing the hardening propensity of agedbitumen at intermediate and low temperatures. Bitumen morphology suggests that the degree of poly-mer network chain break-down was less severe when the HP was used in the bitumen. Performance test-ing of bituminous mixtures indicated that the HP additives enhanced the stripping resistance, improvedrutting performance and extended the mixture service life.

� 2012 Elsevier Ltd. All rights reserved.

ll rights reserved.

: +1 210 458 6475.ssouky), [email protected]), at.papagiannakis@bbas).

1. Introduction

Polymer-modified bitumen (PMB) are colloidal systems contain-ing several constituents, namely asphaltenes, resins, polymers andoils. Asphaltenes are polar with a higher molecular weight thanmaltenes. The maltenes form a continuous phase in which the

Table 1Description of bitumen additives (content 3% by weight of bitumen).

Additives ID Description

Co-polymers SBR Linear-random polystyrene block with 25%

S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223 215

asphaltenes are dispersed. The exact arrangement of the asphalteneparticles within the oily phase varies depending on the relativeamounts of resins, asphaltenes, and oils [22]. The concentration ofbitumen constituents and their arrangement in the oil phase alongwith the environmental conditions control their rheological charac-teristics. These rheological characteristics evolve with time as bitu-men undergo aging through mixing, placement and in-serviceexposure during their lives [17]. Bitumen aging contributes to thedeterioration of bituminous concrete mix pavements. Aging leadsto reduced thermal stability and increased hardening. This leads toreduced bonding to the aggregate particles, which increases the po-tential for stripping and moisture damage, decreases fatigue life andreduces resistance to thermal cracking.

Polymer modification has been proven by many studies to re-tard aging, improve bitumen thermal stability, decrease tempera-ture susceptibility, and increase ductility [29,16,6]. Thisultimately leads to increased concrete stiffness, improved resis-tance to deformation [27] and reduced reflection and thermalcracking [28,3]. Examples of polymers used in the production ofbitumen include natural rubber, polypropylene, styrene–ethyl-ene–butadiene–styrene (SEBS) co-polymers, and styrene–butadi-ene–styrene (SBS) co-polymers, polyolefins (polyethylene,ethylene/propylene diene co-polymer, chlorinated polyethylene),styrene–butadiene–rubber (SBR), and ethylene/vinyl acetate(EVA) co-polymers [5,20].

Antioxidant (AO) additives are alternative materials used to re-tard aging of bitumen and polymers. They are relatively new mate-rials that have been gaining popularity [3,21,18,4]. Degradation ofthe bitumen during aging occurs as a result of the generation offree radicals, which cause reactions compromising the bonds be-tween polymer segments. Li et al. [18] observed that antioxidantadditives work as scavengers of free radicals facilitating thedecomposition of hydro-peroxides, which improve the aging resis-tance of co-polymers. Ouyang et al. [21] found that zinc dialkyl-dithio-phosphate, zinc di-butyldithio-carbamate and naphthenoidoil tend to improve the aging resistance of PMB through by inhib-iting the formation of peroxides and by performing radical scav-enging. Apeagyei [4] used a blend of dilauryl-thio-dipro-pionateand furfural in the presence of a catalyst and concluded that theyare effective in reducing aging in base bitumen. Aksoy et al. [3]used rubber as an antioxidant to improve the aging resistance ofbitumen and reduce their stripping potential from aggregates.There are four AO compounds that contribute to inhibit the agingof polymers [24,12]:

1. hindered phenols which turn to terminate the chainthrough reaction with peroxyl radicals,

2. inhibitors to terminate chains through reaction with alkylradicals,

3. agents to decompose peroxides with no free radicals for-mation, and

4. agents to consume di-oxygen in fast rate to prevent oxidation.

There is a multitude of studies that address the aging of basebitumen [23,19,13,30] and PMB [21,7,9,25,31] and AO effect inaging retardation for base bitumen [4] and in co-polymers [21]but limited studies have considered the effect of AO as a stand-alone additive for PMBs. Moreover, there have been very few stud-ies addressing the effect of AOs on the mechanical properties of as-phalt bituminous mixes.

styreneSBS Linear polystyrene block with 32% styreneSEBS 68% Styrene ethylene–butylene–styrene

Hinderedphenols

HP1 Hindered phenolic, C42H82O4S (Vitamin E)HP2 Hindered phenolic, C35H62O3

HP3 Hindered phenolic, C73H08O12

2. Objective and methodology

The objective of this paper is to study the AOs effect on PMBrheological properties and to identify effective additives and addi-

tive blends to improve the thermal stability of bitumen and reducetheir rutting susceptibility and stripping under aged conditions.

The influence of the additives on the rheological behavior ofbitumen was studied using a stiffening index defined for this pur-pose as the ratio of the rheological properties of the enhanced PMBdivided by the properties of the control PMB. This index was usedto identify the most effective additives in reducing the aging ofbitumen. The AOs were introduced to the base bitumen in twostages. Firstly, through the pre-treated co-polymers with a second-ary AO system supplied by the manufacturer and secondly,through primary AO namely hindered phenols (HPs). After identi-fying the effective additive at each stage, a blend of co-polymerand HP was used to evaluate the bitumen rheology over a widespectrum of temperatures. Determination of the rheological char-acteristics of bitumen was accomplished using Rotational Viscom-eter (RV), Dynamic Shear Rheometer (DSR), and Bending BeamRheometer (BBR) testing. The characteristics of the bitumen hard-ening due to aging were studied using the stiffening index men-tioned above and a complex modulus index at in-servicetemperatures. The complex modulus index is defined as the ratioof the shear modulus for aged and non-aged bitumen. Optical fluo-rescent microscopy images of the bitumen with additives wereused to study their morphology. Hamburg testing on bituminousmixtures was also conducted to evaluate the effectiveness of theseadditives in reducing rutting and moisture damage.

3. Additive properties and blending process

A performance grade (PG) 64-22 bitumen was used as referencebase bitumen to prepare the PMBs tested. Two sets of additiveswere selected in the study as shown in Table 1, namely co-poly-mers and HP. The co-polymers used for bitumen modification in-cluded three types, namely SBR, SBS and SEBS. All co-polymerssupplied from the manufacturer were pre-treated by secondaryAO agents. As claimed by the manufacturer, the SBR is used as amodifier to improve the thermoplastic properties of the bitumen,the SBS is used to improve elasticity and adhesion properties andmaintain low viscosity and the SEBS is used to improve resistanceto weathering. The SBR and SBS are used more frequently thanSEBSs for bitumen polymer modification due to their lower cost.Sulfur was added for all co-polymers application to serve as across-linking agent in the amount of 3% by weight. Sulfur facilitatesthe crosslinking between polymer chains and other bitumen com-ponents, it promotes the formation of three-dimensional struc-tures, it prevents phase separation during hot storage, and itincreases the thermo-mechanical resistance of PMB to temperaturechanges [11].

Three types of HP were used as primary AO additives to miti-gate aging namely; HP1, HP2 and HP3. The main purpose of theHP is to hinder thermally induced oxidation, protect against ther-mo-oxidative degradation, and provide thermal stability underhigh temperature conditions. As noted by Apeagyei [4], the HP1,in essence Vitamin E, is a high molecular weight hindered light sta-

Table 2General rheological properties of the blends tested.

Bitumen/additives

RotationalViscometer

Dynamic Shear Rheometer Bending BeamRheometer

Non-aged Non-aged

RTFO RTFO + PAV RTFO + PAV

Viscosity(Pa s)

|G�|/sind(kPa)

|G�|/sind(kPa)

|G�|sind(kPa)

S (kPa) m-Value

135 �C 76 �C 76 �C 31 �C �12 �C

PG64-22 0.56 0.55 4.83 1211 144.67 0.32PG64 + SBR 3.56 4.06 8.14 1154 122.54 0.25PG64 + SBS 4.00 3.96 8.98 1400 137.70 0.25PG64 + SEBS 2.28 3.87 6.93 1741 124.80 0.28PG64 + HP1 0.63 0.82 2.84 966 89.38 0.32PG64 + HP2 0.32 1.67 4.16 851 69.80 0.32PG64 + HP3 0.39 1.27 3.92 2602 117.37 0.30PG64 + SBR + HP2 2.60 2.24 9.19 853 75.16 0.31

Table 3Phase angle of the blends tested.

Phase angle (�)

Non-aged RTFO RTFO + PAV(76 �C) (76 �C) (31 �C)

PG64-22 87.6 80.5 54.7PG64 + SBR 70.4 64.5 40.2PG64 + SBS 68.8 62.5 40.5PG64 + SEBS 62.2 59.3 45.6PG64 + HP1 79.6 74.4 52.2PG64 + HP2 71.2 63.1 45.2PG64 + HP3 75.5 66.0 44.0PG64 + SBR + HP2 70.7 57.9 44.7

216 S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223

bilizer known to provide excellent stability at temperatures be-tween 30 �C and 80 �C, which is the typical high-end in-servicetemperature for pavements. The HP2 is considered as one of themost effective AOs for elastomers and organic polymers stabiliza-tion, and it is used for reducing viscosity changes and controllingthermo-oxidative aging at high temperatures [2]. The HP3, as de-scribed by the manufacturer, is a long-term thermal stabilizer forSBS, SEBS and SBR polymers that provides protection against ther-mal degradation. It is compatible with most waxes and oils (com-mon substrates for bitumen) and provides high resistance toextraction and low volatility.

The mixing of the AOs was performed by adding 3% of eachadditive (e.g., co-polymers or HPs) by weight to 500 g of bitumen.Using a high shear stirrer and a hot plate, the bitumen was pre-heated to fluid state at 143 �C. The additives were introducedslowly to the bitumen and blended for 3 h at a 2500 revolutions/min rate until homogenization and consistency was achieved. Dur-ing stirring, the dissolution of the co-polymers into the bitumentakes place in three stages [10] as the polymer;

� becomes completely coiled,� begins to unfold and its blocks tend to react with bitumen,

and� completely dissolves causing its blocks to swell and makes

the dissolution irreversible at this stage.

After stirring, there were no signs of gel formation, which sug-gests that the bitumen and the additives were not degraded [26,1].Furthermore, there was no visible evidence of non-uniformity inthe additive-bitumen blends.

The bitumen were tested under three conditions: non-aged,short-term aged, and long-term aged. The aging protocols involvedthe rolling thin film oven (RTFO) and pressure aging vessel (PAV).Aging was performed in two stages using the RTFO at 163 �C for85 min for short-term aged samples (ASTM D2872). The long-termaged samples were conditioned by the RTFO then followed byaging in the PAV at 100 �C for 20 h (ASTM D6521). This aging ishence referred to as RTFO + PAV.

The rheological tests performed included the RV, DSR, and BBR.The RV and DSR were used to test the non-aged bitumen at 135and 76 �C, respectively. For short-term aged samples, the DSR test-ing was performed at 76 �C. For the long-term aged samples, theDSR and BBR tests were performed at 31 and �12 �C, respectively.Testing temperatures were selected according to ASTM D6373. Alltesting was conducted in the AASHTO-accredited Superpave facil-ity at the University of Texas at San Antonio. The PG rheologicalproperties of the control bitumen and the various blends testedat all aging states are summarized in Table 2.

4. Results and analysis

4.1. Brookfield Rotational Viscometer

RV viscosity was measured using a spindle spinning at a 20 rev/min rate after 30 min of thermal stabilization at 135 �C (ASTMD4402). Viscosity was measured for three replicates and the aver-age was recorded in Table 2. The co-polymer additives resulted in asignificant increase in the viscosity of the modified bitumen, whilethe HP2 and HP3 resulted in a reduction in the viscosity of themodified bitumen. This can be explained by the formation of athree-dimensional structure networks when co-polymers are dis-persed in the bitumen [11]. These networks develop strong bond-ing that tends to increase the shear stiffness and viscosity of thebitumen. On the contrary, the hindered phenols particularly HP2and HP3 exhibited a reduction in viscosity.

4.2. Dynamic Shear Rheometer (DSR)

The complex shear modulus, |G�|, and the phase angle, d, weremeasured using a SmartPave� DSR. Testing was conducted for eachof the three aging conditions, namely non-aged, RTFO aged andRTFO + PAV aged. The test was conducted in a stress-controlledmode at a frequency of 10 rad/s. For each aging state, three repli-cate tests were conducted and the average of the results was re-ported in Table 2. Non-aged and RTFO specimens were tested at76 �C using 25 mm diameter plates and a gap of 1 mm. AfterRTFO + PAV aging, specimens were tested at 31 �C using 8 mmdiameter plates and a 2 mm thickness sample. The average of thethree replicates was used in the analysis for each aging condition.Prior to testing, the specimens were kept at the testing tempera-ture until they reached thermal stability. The rutting parameter|G�|/sind and fatigue parameter |G�|sind were calculated for eachspecimen and the average value was reported in Table 2. As sug-gested by the polymer supplier, the addition of 3% co-polymersin the bitumen resulted in a PMB of PG76-22. Hence the tempera-tures mentioned previously are in line with the Superpave grade-specific testing conditions.

The effects of the additives on the rheological properties of bitu-men are summarized in Table 2. These results suggest that the typeof the additives and their compatibility with the particular basebitumen used affect the rheological properties of the resultingblends. For example, compared to the base bitumen, the HP blendsin general showed a marginal decrease in DSR properties for allaging conditions. The exception was the HP3 blend, which exhib-ited an increase in |G�|sind for the RTFO + PAV aged samples. Onthe contrary, the co-polymer blends showed a substantial increasein the DSR properties due to an increase in shear stiffness for allaging states. This can be attributed to the formation of a rigid net-work block created by the action of the polymers [26]. Isacsson andLu [15] also reported that the physical and cross-linking character-

S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223 217

istics of the polymer molecules tend to increase the bitumen stiff-ness due to the polystyrene end blocks and the viscosity of thepoly-butadiene rubbery blocks. This was indicated by the im-proved |G�|/sind parameter, which suggests higher rutting resis-tance at higher temperatures. However, the effect of the co-polymers on the |G�|sind parameter after RTFO + PAV aging sug-gests an increase in fatigue and low temperature cracking suscep-tibility, which is undesirable.

The phase angles, which represent the delayed response be-tween strain and stress, are summarized in Table 3. It is evidentthat the co-polymer additives produced blends with substantiallylower phase angles than the base bitumen, while the HP additivesproduced blends with a comparatively smaller reduction in phaseangles compared to the base bitumen.

4.3. Bending Beam Rheometer

BBR specimens were formed by pouring heated RTFO + PAVaged bitumen into aluminum beam molds. The beams dimensionswere 6.35 mm thick, 12.70 mm wide and 127 mm long (ASTMD6648). Four replicate tests were conducted at �12 �C and theiraverage was summarized in Table 2. The creep stiffness (S) of theblends was lower than that of the base bitumen. The averagereduction was 11% and 26% for the co-polymer blends and theHP blends, respectively. The creep rate (i.e., the m-value) of theco-polymer blends was lower than that of the base bitumen, whileno significant reduction was observed for the creep rate of the HPblends suggesting that the former has a lesser ability to relax ap-plied stresses. Overall, it is suggested that the co-polymers andHP blends are effective in reducing the temperature susceptibilityand improving thermal stability of bitumen at lower in-servicetemperatures.

5. Stiffening indices

A number of indices were defined to quantify the effect of theseadditives on the rheological properties of the resulting blends atdifferent aging levels. These are referred to as stiffening indices(SIs). Prior to establishing a comprehensive evaluation of additiveseffect in bitumen which consumes considerable amount of time, itwas decided to identify the effective additives at selected in-ser-vice temperatures. The indices use the traditional rutting, fatigueand creep stiffness parameters used at high, intermediate andlow in-service temperatures, respectively as follows:

0

0.5

1

1.5

2

PG

64-2

2SB

RSB

SSE

BS

HP

1H

P2

HP

3SB

R+H

P2

Stif

feni

ng I

ndex

(R

utti

ng)

(a)

0

0.5

1

1.5

2

2.5

PG

64-2

2SB

RSB

S

Stif

feni

ng I

ndex

(F

atig

ue)

(Fig. 1. Stiffening indices for additives blend at (a) RTFO-aged at high temperature, (btemperatures.

SIRutting ¼jG�j= sin djG�oj= sin do

� �RTFO

ð1Þ

SIFatigue ¼jG�j � sin djG�oj � sin do

� �RTFOþPAV

ð2Þ

SICreep ¼SSo

� �RTFOþPAV

ð3Þ

Eq. (1) indicates the effect of additives in short-term (RTFO) aging athigh temperatures, while Eqs. (2) and (3) represent the long-term(RTFO + PAV) aging at intermediate and low temperatures, respec-tively. The numerator represents the properties of the bitumen-additives blend, while the subscript ‘‘o’’ in the denominator denotesthe properties of the reference base bitumen, PG64-22. Effectiveadditives are those that result in blends with a higher stiffening in-dex values at high temperature and lower stiffening index values atintermediate and low temperatures. They suggest lower ruttingsusceptibility, and lower fatigue/cold temperature cracking propen-sity, respectively. Rutting is most likely to occur at high tempera-tures at early life of the pavement. On the other hand, fatiguecracking and thermal cracking are more likely to occur after in-ser-vice stiffening of the bitumen through aging under intermediate/lower temperatures. Hence, stiffer bitumen are more desirable earlyin the life of the pavement (RTFO aged), while softer (a viscous-like)bitumen are more desirable later in the life of the pavement(RTFO + PAV aged).

Fig. 1 suggests that at high temperatures, the SBS and SBR addi-tives produce the most effective blends by increasing the SI (rut-ting) by more than 50% (Fig. 1a). The HP1 and HP2 are the mosteffective additives in producing blends that resist hardenabilityat intermediate and low temperatures (Fig. 1b and c). The HP2 pro-duced a larger stiffness gain than the other HPs, while the SBRshowed a slight stiffness loss under aged conditions. This can beexplained by the influence of the co-polymer in stiffening the bitu-men as mentioned previously, and the HP’s antioxidant ability. As aresult, it was decided to examine whether combining the SBR andHP2 additives would improve the bitumen performance over therange of temperatures considered. This additives blend wouldcombine the desired high temperature characteristics of SBR alongwith the desired intermediate and low temperatures characteris-tics of HP2. The HP1 and HP3 were excluded due to the undesirablesoftness at high temperature and hardness at intermediate tem-peratures, respectively. The SBS and SEBS were excluded due to

SEB

SH

P1

HP

2H

P3

SBR

+HP

2

b)

0

0.5

1

1.5

2

PG

64-2

2

SBR

SBS

SEB

S

HP

1

HP

2

HP

3

SBR

+HP

2

Stif

feni

ng I

ndex

(C

reep

)

(c) ) RTFO + PAV-aged at intermediate temperature and (c) RTFO + PAV-aged at low

40

50

60

70

80

90

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

20 40 60 80 100

Pha

se A

ngle

(de

g)

|G*|

(P

a)

Temperature (°C)

PG64 PG64+SBR PG64+SBR+HP2

(a) Non-aged

40

50

60

70

80

90

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

20 40 60 80 100P

hase

Ang

le (

deg)

|G*|

(P

a)

Temperature (°C)

PG64 PG64+SBR PG64+SBR+HP2

(b) RTFO

40

50

60

70

80

90

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

20 40 60 80 100

Pha

se A

ngle

(de

g)

|G*|

(P

a)

Temperature (°C)

PG64 PG64+SBR PG64+SBR+HP2

(c) RTFO+PAV

Fig. 2. Isochronal plots of complex modulus and phase angle for tested bitumen at (a) non-aged, (b) RTFO and (c) RTFO + PAV-aged conditions.

218 S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223

the hardness gain at intermediate temperatures. A blend of 3% SBRand 2% HP2 of bitumen weight was chosen as recommended by theco-polymers manufacturer. It was elected to reduce the HP2 con-

tent to 2% because the SBR, as mentioned previously, was pre-trea-ted by secondary AO agents. Fig. 1 shows the stiffening indices forthis blend. The results suggest that the SBR + HP2 blend provided a

°°(a) Stiffening index for SBR-blend with respect to the base bitumen

°

(b) Stiffening index for HP2-enhanced with respect to the SBR-blend

S

Fig. 3. Stiffening indices as a function of temperature for RTFO and RTFO + PAV aged bitumen.

S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223 219

significant hardening at high temperatures, but only a slight soft-ening at low and intermediate temperatures compared to theHP2 only.

The rheology measurements obtained at a certain temperature(i.e., 76, 31 and �12 �C, as dictated by performance grade accordingto AASHTO 320) are not sufficient to fully capture the effect ofadditives on bitumen rheology. Therefore, a series of DSR temper-ature sweep tests were performed over a temperature range of 20–110 �C at a frequency of 10 rad/s for each of the aging conditionsconsidered. Comprehensive rheological properties of base, SBR-blend (PG64 + SBR) and HP2-enhanced (PG64 + SBR + HP2) bitu-men are shown in Fig. 2 in the form of isochronal plots. The iso-chronal plots represent the |G�| and phase angle changes withrespect to temperature. Fig. 2 suggests that the additives have asignificant effect on bitumen rheology, for each of the aging statestested.

As expected, temperature increases result in decreases in |G�|and increases in the phase angle. The effect of aging is evident bycomparing Fig. 2a–c. They suggest that the SBR decreased signifi-cantly the phase angle and increases the elasticity of the bitumen.The HP2 influence on the phase angle of the blends is more pro-nounced for aged bitumen. Moreover, both the SBR-blend andthe HP2-enhanced combination have a significant stiffening effecton the blends compared to the base bitumen for all aging condi-tions tested. This trend becomes more pronounced as temperatureincreases.

To assess the influence of each additive on the hardening ofbitumen due to aging, the stiffening indices defined by Eqs. (1)and (2) were plotted versus temperature (Fig. 3). To isolate the ef-fect of each additive, the indices were computed for the SBR-blendand the HP2-enhanced with reference to the base bitumen and the

SBR-blend, respectively. These comparisons are shown in Fig. 3aand b, respectively. Fig. 3a suggests that the SBR substantially in-creased the rutting and fatigue parameters as temperature in-creased. Although an increase in bitumen resistance to rutting upto seven times (at 100 �C) is highly desirable, it does have a nega-tive effect on fatigue resistance. The SBR also increased the harden-ing potential of the bitumen by a factor of 2 at intermediatetemperatures (20–40 �C), which could reduce the in-service fatiguelife of bituminous pavements. The influence of HP2 on the resis-tance of the blend is shown in Fig. 3B. Under RTFO conditions,the HP2 increased the bitumen stiffness at temperatures overabout 40 �C, while under RTFO + PAV aging conditions, it enhancedthe bitumen softness below about 80 �C. Hence, the HP2 is capableof creating two distinct zones; one where the stiffness indices arereduced compared to the SBR-blend and another zone where theseindices are increased compared to the SBR-blend. At higher tem-peratures the HP2 can improve rutting resistance up to 180% andat intermediate temperatures, it can reduce pavement fatigue sus-ceptibility by 25%.

6. Rheological characteristics and black diagram

The black diagram represents the rheological characteristics ofbitumen independently from the effect of temperature and fre-quency. A plot of the complex modulus |G�| against the phase anglefor the bitumen at different aging conditions is shown in Fig. 4. Dis-tinct rheological characteristics can be clearly seen for each bitu-men blend and aging state. Airey [1] attributed the differences inthe rheological characteristics of each blend to the physical effectof the additives and their compatibility with the bitumen. For in-

40

50

60

70

80

90

1.E+06

Pha

se a

ngle

(de

g)

|G*| (Pa)

non-aged RTFO RTFO+PAV

(a) Base

40

50

60

70

80

90

1.E+06

Pha

se a

ngle

(de

g)

|G*| (Pa)

non-aged RTFO RTFO+PAV

(b) SBR-blend

40

50

60

70

80

90

1.E+06

Pha

se a

ngle

(de

g)

|G*| (Pa)

non-aged RTFO RTFO+PAV

(c) HP2-enhanced

1.E+02 1.E+04

1.E+02 1.E+04

1.E+02 1.E+04

Fig. 4. Black diagrams for the base, SBR-blend and HP2-enhanced.

°°

°

Fig. 5. Thermal stability assessment using the complex modulus index for the base,SBR-blend and HP2-enhanced.

220 S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223

stance, the bitumen additives tend to lower the phase angles andimprove the elastic characteristics at all aging conditions with re-spect to the base bitumen.

Three distinct zones can be identified in these black diagrams.The first zone is observed at lower |G�| where the decrease rateof the base bitumen phase angle (Fig. 4a) is remarkably lower thanthat for the SBR-blend and the HP2-enhanced. At high tempera-ture, this behavior can be attributed to the polymeric network role

to alter the bitumen behavior. In the second zone, bound by |G�|values of 4.0E+4 and 6.0E+5, the PMB showed a plateau zonecaused by the formation of the rubber–elastic polymeric networkparticularly in non-aged and RTFO-aged conditions (Fig. 4b andc). Airey [1] remarked that this plateau is attributed to the elasticformation of polymers in the modified bitumen that tend to stabi-lize the phase angle. The nature of this polymer network is a func-tion of the chemical and physical properties of the polymer, itscontent in the blend and its compatibility with the bitumen [8].In this zone, the effect of the HP2-enhanced can be shown throughenhancing the rubber–elastic behavior of bitumen indicated by amore flattened plateau compared to the SBR-blend. The third zoneis observed at |G�| values of 6.0E+5 and higher. The PMB showed acontinued reduction in the phase angle (increase elastic response),while the base PG64-22 showed an increase in phase angle (de-crease elastic response).

7. Thermal stability

Thermal stability is defined as the capability of bitumen tomaintain steady rheological properties as it undergoes tempera-ture and oxidation changes. In this study, it has been assessedthrough the complex modulus index (CMI) defined as the ratio ofthe complex modulus of aged to non-aged bitumen using temper-ature sweep testing data [8]:

CMIRTFO ð%Þ ¼jG�RTFOjjG�ORIGj

� 100 ð4Þ

CMIPAV ð%Þ ¼jG�PAVj � jG

�RTFOj

jG�ORIGj� 100 ð5Þ

non-aged RTFO RTFO+PAV

HP2

-enh

ance

d SB

R-b

lend

Fig. 6. Fluorescence microscopy images magnified 40� for SBR-blend and HP2-enhanced at non-aged and aged conditions.

Table 4Gradation and volumetric properties of bituminous mix.

Sieve size % Passing

3/400 100.01/200 100.03/800 99.2No. 4 63.8No. 8 38.2No. 30 16.8No. 50 11.7No. 200 3.3

Mix propertiesOptimum asphalt content, AC (%) 5.0VMA @ optimum AC 15.44VFA @ optimum AC 74.10

Fig. 7. Hamburg test for bituminous mixes made with base, SBR-blend and HP2-enhanced (arrows show the stripping points).

S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223 221

Eqs. (4) and (5) present the hardening effect of bitumen due to theRTFO and PAV aging, respectively. The CMI provides a measure ofthe thermal stability as bitumen undergo aging. This index associ-ates the change in the aged properties to the non-aged propertiesas temperatures increases. Bitumen with high thermal sensitivityare indicated by high CMI value (i.e., values larger than 100%),and vice versa. Fig. 5 suggests that the SBR increased the stiffnessof the base bitumen by a factor of 10 after PAV aging, particularlyat elevated temperatures. However, the application of HP2 cappedthe stiffness increase to eight times its original value. This suggeststhat HP2 additives are more effective in controlling the hardeningand improving the thermal stability. It is suggested that controllingthe increase in bitumen hardening can substantially improve thepavement fatigue cracking resistance. However, no mechanical test-ing was performed in this study to verify the fatigue characteristicsof pavement mixes. On other hand, while SBR have increased thestiffness of RTFO aged base bitumen by a factor of three, HP2 re-sulted in further increasing the stiffness (e.g. by a factor of 4). Thissuggests also that, the HP2 additives are effective in increasing bitu-men stiffness after RTFO aging which may lead to improve the rut-

ting resistance of pavement. This is verified using the performancetesting explained later in the paper.

8. Bitumen morphology

The combined actions of HP2 and polymer suggest significantchanges in the rheology of the bitumen. The literature suggeststhat this is due to the formation of a continuous network of poly-mers within the bitumen [1]. After the network formation, thepolymers absorb the bitumen’s maltenes and swell up to ninetimes their initial volume [15]. Moreover, as polymers undergothermal oxidation, free radicals are generated. The HP2 role is toscavenge these radicals and prevent the polymers from degrading[18]. The interaction of the AO additives with the bitumen leads toalter its physical and chemical properties. Chemical analysis wasperformed by many researchers to study the influence of AO addi-tives on the bitumen behavior [19,7,30] and [18]. The chemicalinteraction between additives and bitumen leads to a change in

222 S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223

the bitumen morphology that can be captured by microscopicimaging. Researchers have used imaging in lieu of chemical testingto track the change in the bitumen morphology due to co-polymersapplication [26], evolution of structure due to aging [30] and sulfureffect with co-polymers [11]. The section below describes the mor-phological analysis of the influence of HP2 in the blended bitumen,namely those including the SBR-blend and the HP2-enhanced.

The morphology of these bitumen blends was studied using anoptical fluorescence microscope. Small bitumen drops were col-lected after oven heating and squeezed in-between two glassslices. The slices were scanned at room temperature to visualizethe bitumen phases at non-aged and aged conditions as shown inFig. 6. These images demonstrate the distinction in the bitumenmorphology at different oxidation levels. The white regions ofthe images correspond to the polymer and additives phase, whilethe gray areas represent the bitumen phase consisting of asphalt-enes and maltenes. To extract the distribution and concentrationof each phase in the image, image processing was performed. Thistechnique digitizes the image into numeric entities called pixels,improves the image resolution, enhances the visibility of the imagephases, and separates touching objects and thresholds them fromthe background. This was conducted using Image Pro Plus�, a com-mercial image processing software [14]. After processing, the num-ber of pixels corresponding to each phase was determined. Theconcentration of the additives was 24%, 33% and 15% for theHP2-enhanced and 27%, 30% and 36% for the SBR-blend in non-aged, RTFO and RTFO + PAV condition, respectively.

For the non-aged condition, the addition of SBR results in theformation of a continuous polymer phase in the bitumen. As shownin Fig. 6, polymer and additive compositions tend to cluster in thenon-aged phase, while after aging they tend to break apart. Duringthermal oxidation, degradation of the SBR molecules leads to dis-appearance of the composition structure and results in chainbreak-down in the network and increase in volume [30]. The in-crease in volume as remarked by Li et al. [18] is due to the antiox-idant additives including HP to work as scavengers of free radicals.This can be shown in the images of RTFO aging as the polymerlarge chains were broken in small areas and increased in concen-tration by 3% and 9% for SBR-blend and HP2-enhanced, respec-tively. While the polymer increase in volume is evident in theRTFO + PAV image for SBR it was not possible to obtain qualitymicroscopic images to verify the same phenomena with the HP2images resulting in diminishing polymer phase. Moreover, the de-gree of chains break-down was less severe in the case of the HP2-enhanced, as indicated by the size of the polymer phase in theRTFO images. The average chains area was 35 ± 29 and 70 ± 94(in pixel � pixel) for SBR blend and HP2-enhanced, respectively.

9. Performance testing of bituminous mixes

Hamburg wheel tracking (HWT) test was performed on Lime-stone fine-grained bituminous concrete mix to evaluate the effectof the additives against moisture stripping and rutting. This testis widely used to evaluate the mixes resistance to moisture dam-age. Mixes with oxidized bitumen can be easily debonded fromthe aggregate particles in the presence of water causing stripping.Once stripping occurs severe rutting will encounter in the mixcausing pavement failure.

The mix gradation and volumetric properties are listed in Ta-ble 4. Two mixes were molded with SBR-blend and HP2-enhancedbitumen. After oven aging for 3 h, the mix was compacted to an airvoid content of 7% using a gyratory compactor. The 150 mm spec-imen was cut to form two 75 mm height molds suitable for theHWT test. Two replicates were prepared for each mix design andthe recorded average was used in Fig. 7. During the test the mixes

were submerged in 50 �C water and loaded with two rolling steelwheels with vertical load of 72 kg to induce rutting. The specimenswere secured in circular molds while the wheels were rolling backand forth along the diameter span of two back-to-back specimens.The test was conducted for 10,000 cycles and the rutting was mon-itored at five points along the specimen surface.

Fig. 7 showed that HP2-enhanced mix exhibited the highest rut-ting resistance. At the termination of the test, a total reduction of2 mm in rutting was observed compared to the SBR-blend. Strip-ping is indicated by a drastic increase in the slope of the rut accu-mulation curve. The stripping was determined at 6000, 6000 and7000 cycles for the base, SBR-blend and HP2-enhaned mix, respec-tively. This suggests that HP2 improved the stripping resistanceand rutting performance of the bituminous mix. The rut depth de-creased 46% when SBR was used and 70% when the HP2 and SBRwere used in the bitumen. More comprehensive mixtures testingand expanded matrix design includes different mix design, aggre-gate types and mechanical testing is suggested.

10. Conclusions

This study presented the rheological properties of a base bitu-men PG64-22 modified with two types of additives; HP antioxi-dants and co-polymers. An optimum blend of SBR and HP wasidentified as an effective antioxidant additive for bitumen. The se-lected 3% SBR and 2% HP2 blend was capable to increase the bitu-men stiffness at high temperature and decrease the stiffness atintermediate and low temperature. The effectiveness of the addi-tives was also assessed through comprehensive evaluation usingtemperature sweep testing data.

The following additional conclusions are drawn:

� The HP additives resulted in a decrease in bitumen viscosity,complex modulus and creep stiffness and an increase in thephase angles. On the contrary, the co-polymers showed anincrease in viscosity, complex modulus and creep stiffness anda decrease in phase angles. The HP decreased bitumen stiffeningafter aging while the co-polymers increased bitumen stiffeningin non-aged and RTFO aging.� Three stiffening indices were introduced to evaluate the effec-

tiveness of the additives. These were expressed as the ratio ofthe rutting, fatigue and creep stiffness parameters for theenhanced versus the base bitumen. The stiffening index sug-gested that SBR and HP2 are the most effective additives. Thefirst had the highest RTFO stiffening index at high in-servicetemperatures amongst the co-polymer group. The second hasthe lowest RTFO + PAV stiffening index at low and intermediatetemperature amongst the HP group.� Thermal stability analysis using the CMI suggested that the HP2

limited the stiffening increase in bitumen after RTFO + PAVaging to 80% of the maximum stiffening gain from the SBR co-polymer alone.� Optical fluorescence microscopy images clearly distinguished

between the three phases of aging in the bitumen morphology.The break-down in the co-polymers chains was clearly lowerwhen the HP2 was used.� Hamburg testing indicated that the HP2 with the SBR co-poly-

mers reduced the moisture damage and increased the ruttingresistance in the bituminous mixtures.

Acknowledgements

The authors would like to thank Fred Ruiz, Mohammed Ilias andChris Reyes for their assistance in the laboratory testing. The addi-tives provided by Dynasol and Ciba are highly appreciated.

S. Dessouky et al. / Construction and Building Materials 38 (2013) 214–223 223

References

[1] Airey G. Rheological properties of styrene butadiene styrene polymer modifiedroad bitumens. Fuel 2003;82(14):1709–19.

[2] Al-Malaika S. Mechanism of antioxidant action and stabilization technology –the Aston experience. Polym Degrad Stab 1991;34.

[3] Aksoy A, Samlioglu K, Tayfur S, Ozen H. Effects of various additives on themoisture damage sensitivity of asphalt mixtures. Constr Build Mater2005;19:11–8.

[4] Apeagyei A. Laboratory evaluation of antioxidants for asphalt binders. ConstrBuild Mater 2011;25(1):47–53.

[5] Boutevin B, Pietrasanta Y, Robin J. Bitumen-polymer blends for coatingsapplied to roads and public constructions. Progr Org Coat 1989;17(3):221–49.

[6] Collins JH, Bouldin MG, Gelles R, Berker A. Improved performance of pavingasphalt by polymer modification. Proc Assoc Asphalt Paving Technol1991;60:43–79.

[7] Cortizo MS, Larsen DO, Bianchetto H, Alessandrinia JL. Effect of the thermaldegradation of SBS copolymers during the ageing of modified asphalts. PolymDegrad Stab 2004;86:275–82.

[8] Dessouky S, Reyes C, Ilias M, Contreras D, Papagiannakis AT. Effect of pre-heating duration and temperature conditioning on the rheological propertiesof bitumen. Constr Build Mater 2011;25:2785–92.

[9] Durrieua F, Farcas F, Mouillet V. The influence of UV aging of a styrene/butadiene/styrene modified bitumen: comparison between laboratory and onsite aging. Fuel 2007;86(10–11):1446–51.

[10] Dynasol. Pavement technical bulletin; 2004 [accessed throughdynasolelastomers.com].

[11] Estrada AM, Castellanos EC, Alonso MH, Najera RH. Comparative study of theeffect of sulfur on the morphology and rheological properties of SB- and SBS-modified asphalt. J Appl Polym Sci 2010;115:3409–22.

[12] Gladyshev GP, Ershov YA, Shustova OA. Stabilization of thermostablepolymers. Moscow; 1979. p. 1–271 [in Russian].

[13] Hrdlicka G, Tandon V, Prozzi J, Smit A, Yildrim Y. Evaluation of binder tests foridentifying rutting and cracking potential of modified asphalt binders. FHWA/TX-07/0-4824-1, Texas Department of Transportation and Center forTransportation Infrastructure Systems of The University of Texas at El Paso;2007.

[14] Image-Pro Plus version 6.2. Media cybernetics, Bethesda, MD; 2010.[15] Isacsson U, Lu X. Testing and appraisal of polymer modified road bitumens:

state of the art. Mater Struct 1995;28:139–59.[16] Kanitpong K, Bahia H. Relating adhesion and cohesion of asphalts to the effect

of moisture on laboratory performance of asphalt mixtures. Transport Res Rec2005;1901:33–43.

All in-text references underlined in blue are linked to publications on Rese

[17] Lamontagne J, Dumas P, Mouillet V, Kister J. Comparison by Fourier transforminfrared (FTIR) spectroscopy of different ageing techniques: application to roadbitumens. Fuel 2001;80:483–8.

[18] Li Y, Li L, Zhang Y, Zhao S, Xie L, Yao S. Improving the aging resistance ofstyrene–butadiene–styrene tri-block copolymer and application in polymer-modified asphalt. J Appl Polym Sci 2010;116(2):754–61.

[19] Lu XH, Isacsson U. Effect of ageing on bitumen chemistry and rheology. ConstrBuild Mater 2002;16:15–22.

[20] Morrison R, Lee K, Hesp M. Chlorinated polyolefins for asphalt bindermodification. J Appl Polym Sci 1994;54(2):231–40.

[21] Ouyang C, Wang S, Zhang Y, Zhang Y. Improving the aging resistance ofstyrene–butadiene–styrene tri-block copolymer modified asphalt by additionof antioxidants. Polym Degrad Stab 2006;91:795–804.

[22] Papagiannakis AT, Masad E. Pavement design and materials. Newark (NJ): JohnWiley and Sons Inc.; 2008, ISBN 978-0-471-21461-8.

[23] Scarsella M. The application of rheology to the evaluation of bitumen ageing.Fuel 2000;79:1005–15.

[24] Schwetlic K. In: Scott G, editor. Mechanism of polymer degradation andstabilisation. Elsevier Applied Science; 1990. p. 23.

[25] Sengoz B, Isikyakar G. Analysis of styrene–butadiene–styrene polymermodified bitumen using fluorescent microscopy and conventional testmethods. J Hazard Mater 2008;150(2):424–32.

[26] Sengoz B, Topal A, Isikyakar G. Influence of aging on the evolution of structure,morphology and rheology of base and SBS modified bitumen. Constr BuildMater 2009;23(2):1005–10.

[27] Sirin O, Kim H, Tia M, Choubane B. Comparison of rutting resistance ofunmodified and SBS-modified Superpave mixtures by accelerated pavementtesting. Constr Build Mater 2008;22(3):286–94.

[28] Tayfur S, Ozen H, Aksoy A. Investigation of rutting performance of asphaltmixtures containing polymer modifiers. Constr Build Mater2007;21(2):328–37.

[29] Woo W, Ofori-Abebresse E, Chowdhury A, Hilbrich J, Kraus Z, Martin A, et al.Polymer modified asphalt durability in pavements FHWA/TX-07/0-4688-1,Texas Transportation Institute, College Station, Texas; 2007.

[30] Wu S, Pang L, Moa L, Chen Y, Zhu G. Influence of aging on the evolution ofstructure, morphology and rheology of base and SBS modified bitumen. ConstrBuild Mater 2009;23:1005–10.

[31] Zhang F, Yu J, Han J. Effects of thermal oxidative ageing on dynamic viscosity,TG/DTG, DTA and FTIR of SBS- and SBS/sulfur-modified asphalts. Constr BuildMater 2011;25:129–37.

archGate, letting you access and read them immediately.