Construction and Building Materials 37 (2012) 562–572

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Construction and Building Materials

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Definition of a laboratory optimization protocol for road bitumen improvedwith recycled tire rubber

Bernardo Celauro a, Clara Celauro a,⇑, Davide Lo Presti b, Antonio Bevilacqua a

a Dipartimento di Ingegneria Civile, Ambientale, Aerospaziale, dei Materiali, Università degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italyb Nottingham Transportation Engineering Centre, University of Nottingham, UK

h i g h l i g h t s

" Performance improvement achievable when modifying bitumen with CRM from scrap tires is investigated." Authors define an optimization protocol that implies a reduced material consumption for laboratory needs." A low shear blending protocol proved to be comparable with the conventional one, with high shear mixing." The proposed protocol allows one to obtain AR bitumen with a reduced temperature susceptibility." This is extremely beneficial in countries with warm climates such as those in the Mediterranean area.

a r t i c l e i n f o

Article history:Received 14 March 2011Received in revised form 16 July 2012Accepted 23 July 2012

Keywords:BitumenTire recycled rubberLaboratory evaluationRotational viscometerDynamic shear rheometer

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.07.034

⇑ Corresponding author. Tel.: +39 09123899716; faE-mail address: clara.celauro@unipa.it (C. Celauro)

a b s t r a c t

Bearing in mind the need to answer one of the most frequent needs for civil construction, in general, andmajor project such as road infrastructures in particular, this paper presents the results of a laboratoryexperimental study in order to evaluate the performance improvement that is possible to achieve whenmodifying road bitumen with CRM (Crumb Rubber Modifier) from discarded tire rubber (TR), using a WetProcess.

The environmental advantage is double, since the aim of the research is to obtain high performanceroad bitumen thanks to the re-use of a discarded material, such as the rubber tires, which otherwisewould be dumped in scrap-yard.

In particular, this experimental study aimed to define a laboratory optimization protocol that allowedto discriminate amongst the different asphalt–rubber blends, in relation to the different content of rub-ber, as well as to the blending times, with the possibility of carefully controlling the production temper-ature throughout the production process. In order to do this, simple and widespread laboratoryequipments and procedures have been adopted, such as the Brookfield rotational viscometer, capableof ensuring the representativeness of the final product to be tested.

After the optimization study, a comparison was made between the conventional and rheological prop-erties of the optimized rubber–asphalt blend with those of two commercially available PMBs, modifiedwith synthetic rubber type Styrene–Butadiene–Styrene, with two different level of modification: a med-ium and a high level of polymer, respectively termed as ‘‘Medium’’ and ‘‘High’’ in this study. This com-parison allows one to appreciate to what extent control of the modification process is able to affectthe properties of the final product and shows that the preliminary optimization of the mixing parameterscarried out with the proposed protocol allows one to obtain a rubber–bitumen blend with improved per-formances, above all reduced temperature susceptibility, as proved by an increase in the Superpave™high temperature performance grading, up to a two grade jump, which it is possible to obtain only forSBS modified bitumen with high content of polymer.

Therefore, this study proves that, thanks to the appropriate re-use of crumb rubber form scrap tires,with the proposed optimization protocol it is possible obtain improved bitumen with high content ofrecycled materials that, with respect to specific needs of regions with warm climates such as the Medi-terranean area, can be considered as ‘‘high-performance blends’’.

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B. Celauro et al. / Construction and Building Materials 37 (2012) 562–572 563

1. Introduction

For sustainable development, the objectives of reducing con-sumption of valuable natural resources and, at the same time, reus-ing to the maximum those natural resources in any case involvedin road works have urged road technologists to consider severalrecycling techniques for new surface and/or structural layers ofroad bituminous pavement.

The use of ground tire rubber (TR) from scrap tires in systematicroad application may be dated back to the 1960s when CharlesMacDonald, during some maintenance work in urban areas, expe-rienced the advantages of using tire rubber as an additive in as-phalt cement for reparing potholes. MacDonals, in fact, foundthat accurately mixing the rubber into the bitumen and allowingthe blend a sufficient reaction time, it was possible to obtain aproduct with new and improved properties with respect to thoseof the original bitumen, and started the experimental phase thatled to the definition of a process that is known in the literatureas Wet Process [1].

In the same period, two Swedish companies produced a surfaceasphalt mixture with the addition of a small quantity of groundrubber form discarded tires as a substitute for a part of the mineralaggregate in the mixture, in order to obtain asphalt mixture withimproved resistance to studded tires as well as to snow chains,via a process known as Dry Process [2].

Hence, Crumb Rubber Modifier (CRM) can be added to bitumi-nous mixtures with two distinct procedures: Wet and Dry. In theWet Process, rubber and bitumen are made to react together athigh temperatures, so that the blend obtained is suitable for mak-ing bituminous mixtures. By contrast, in the Dry Process CRM isadded to the mineral aggregate before mixing with bitumen. In thisway, the rubber introduced acts both as an inert charge and as amodifying agent, in that during the mixing phase it reacts, thoughonly partially, with the bitumen [3].

The two processes differ not only in the quantity and gradationof the rubber used, but also in the quantity of the components(bitumen, in particular), as well as in the equipment required forproduction; the fact is that for the Wet Process the need to makethe bitumen and the rubber react together, so that the mixture ob-tained will show attainment of a condition of stable viscosity, in-volves the need for specific equipment (high shear mixers andextruders), which are not necessary for the Dry Process [4]. Hencethe Dry Process is easier to run, while the Wet Process, thoughmore complex, has the advantage of making it possible to governthe rheological properties of the AR binder obtained accurately.

Extensive literature [5–7] clearly shows the numerous successesobtained using AR mixtures produced with the Wet Process; in anycase, it is necessary to evaluate the extent of modification obtained,with easy laboratory tests that may be part of a mixing protocol foroptimizing asphalt–rubber blends, in order to guarantee the re-quired improvement in the mechanical performances as a resultof the introduction of rubber in the blend. This is also fundamentalfor estimating the pavement design life and extension, when crumbrubber is used for road application in flexible pavements [8]. There-fore, the aim of this study was to define a simple laboratory proto-col, easy to perform and consuming the smallest amount ofmaterial possible, able to provide useful information for controllingand optimizing the modification process of bitumen with rubber, aswell as the performance of the final product In order to prove theeffectiveness of the studied protocol as well as the representative-ness of the final product, the obtained rubber–asphalt blends werecharacterized via rheological tests and afterwards compared with:

� rubber–bitumen blends obtained via conventional high shearmixing, all the other process variables being kept constant;

� two commercially available modified bitumens, with differentcontent of elastomeric polymer type SBS, termed, as said before,‘‘Medium’’ e ‘‘Hard.’’

2. Background

The term Asphalt–Rubber’’ (AR) is defined by the ASTM [9] as ablend of paving grade asphalt cements, ground recycled tire rubberand some other additives, having at least 15% rubber by weight ofthe total blend, reacted at high temperature within the asphalt ce-ment to be modified, in order to provide swelling of the rubber par-ticles into the blend.

In the Wet Process, tire rubber – TR – and bitumen are made toreact together at high temperatures, (usually, 18 � 26% of rubberwith respect to the weight of the bitumen is added). This process,when the TR melts and swells into the blend, is strongly affectedby:

� mixing temperature;� blending time (at a constant temperature);� size, morphology and production of the TR;� amount of aromatics available in the bitumen;� type and energy of the mechanical mixing exerted.

The reaction itself is made up of two simultaneous processes:partial digestion of the rubber into the bitumen on one hand,and, on the other, adsorption of the aromatic oils available in thislatter within the polymeric chains that are the main componentsof the rubber, both natural and synthetic, contained in the TR. Thus,it is appropriate to use as base bitumen those that are particularlyrich in aromatics or, if necessary, to add aromatic oils in order tofavor swelling of the rubber granulate.

The rate of reaction between TR and bitumen is mainly affectedby the temperature and blending time, especially for devulcanizingscrap tire rubber in liquid asphalt [10], and may therefore be opti-mized by controlling these as well as the other variables listed be-fore of the process [11–14]. In order to achieve a rubber–asphaltblend with improved performances the rotational viscosity is thephysical parameter that is easiest to control during the modifica-tion process, from an operative point of view. In fact, the rubber–bitumen interaction, at a certain temperature, implies an increasein the blend viscosity up to a maximum value and a subsequentnoticeable decrease with the reaction time [15]. Thus, in this worka procedure that allows optimization of a rubber–bitumen blendvia continuous monitoring of its rotational viscosity wasdeveloped.

The overall working condition suggested by the technical liter-ature [16] and referring to the production of tire rubber modifiedbitumens (TR-MBs) are:

� reaction temperature: variable between 160 �C and 260 �C;� reaction time: starting from a minimum of 450 up to 8 h; in

order to produce a blend a suitable for producing hot asphaltmixtures;� imposed shear: usually not lower than 500 rpm.

The best performing asphalt mixtures are obtained when the ARbitumen is added to the selected aggregate gradation as soon aspossible, within 16 h from the production of the AR binder itself.It was suggested that the AR blends should not be kept at a hightemperature for long periods after production: the blends thatare not going to be used within a short period should be cooleddown and after that re-warmed before use. This is due to the ten-dency of the rubber to settle, when stored in silos at a high temper-ature [4].

Table 1Typical gradation of the CRM to be used in the Wet Process [18].

Sieve # Nominal size (mm) % Passing

10 2.00 10016 1.18 75–10030 600 lm 25–10050 300 lm 0–45

100 150 lm 0–10300 75 lm 0

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3. Materials and methods

Bituminous binders – A 50/70 pen bitumen was used as the base for the produc-tion of a tire–rubber modified bitumen (TR-MB) through a protocol involving pre-liminary estimation of rubber content optimum at low shear (TR-MB LS). Oncethe optimum rubber content was established, a high shear blending protocol wasalso performed, maintaining the same mixing temperature of 180 �C, in order tooptimize time and settling properties of the final product (TR-MB HS). At the end,also a comparison with two Styrene–Butadiene–Styrene modified bitumens (SBS-MBs with a medium and a high level of polymer), supplied by an Italian oil com-pany, was also performed in order to show the enhanced properties of the opti-mized TR-MB. All the binders were characterized performing physical andrheological tests. Moreover, base bitumen was also chemically characterizedthrough separations of the asphaltenes from the maltenes according to the standardASTM D4124 [17]. Results and comparisons are provided in the following sections(Table 1).

Summarizing, the following binders were considered for this study:

– neat bitumen: pen 50/70 bitumen.– TR-MB LS: tire rubber-modified bitumen (TR-MB) produced mixing at Low

Shear (LS) a base bitumen with the optimum content of 18% in weight of 30mesh rubber.

– TR-MB HS: tire rubber-modified bitumen (TR-MB) produced mixing at HighShear (HS) a base bitumen with 18% in weight of 30 mesh rubber.

– SBS-MB H: SBS modified bitumen (SBS-MB) with a high content of polymer(Hard – H).

– SBS-MB M: SBS modified bitumen (SBS-MB) with a medium content of polymer(Medium – M).

Tire rubber – The rubber used was provided by a Sicilian plant: it is obtained viamechanical grinding (at air temperature) of scrap tires; as regards the size of thegrains, in order to improve the reaction of the rubber with bitumen, only rubberpassing 30 mesh was used in this study (Fig. 1).

It should be noticed that the particularly fine gradation of rubber used in thisstudy satisfies the requirements for TR to be used for production of Wet mixtures(Table 2).

3.1. Physical and chemical characterization

The binders were characterized with the following conventional tests: penetra-tion [19], softening point [20], and rotational viscosity at 135� and 160 �C [21].Moreover, asphaltenes content was measured according to ASTM D4124 (ASTM,

Fig. 1. Gradation curve of the tire rubber use

2001), in order to have knowledge of the chemical composition of the base bitumen.This was done by determining the insoluble material that can be separated from as-phalt following digestion of the asphalt in Dry n-heptane, allowing the mixture torest in the dark for a night and then filtering the n-heptane insolubles (asphaltenes)with standard laboratory procedure.

3.2. Rheological analysis

Both optimization study of the asphalt rubber and performance comparisonswere mainly based on the evaluation of the rheological properties. Therefore, in or-der to obtain a complete rheological characterization, Dynamic Mechanical Analy-ses (DMA) have been performed by making frequency sweeps tests over a widerange of temperatures with an Anton Paar Physica MCR 101 dynamic shear rheom-eter (DSR) since it has been proved that SUPERPAVE DSR protocol can also be ap-plied to rubber modified binders [22]. The tests were performed under thefollowing conditions and making at least three repetitions:

� Loading mode: controlled-strain.� Temperatures: 0 �C to 80 �C at 5 or 10 �C intervals.� Frequencies: 0.10, 0.16, 0.25, 0.40, 0.63, 1, 1.6, 2.5, 4, 6.3 and 10 Hz.� Plate geometries: 8 mm / and 2 mm gap (0–50 �C); 25 mm / and 1 mm gap

(30–80 �C).� Strain amplitude: 0.1–2% with 8 mm plates (within LVE response dependent on

G�); 2–12% with 25 mm plates (within LVE response dependent on G�).

For each test, samples were prepared by means of a hot pour method, based onAlternative 1 of the AASHTO TP5 Standard [23]. The gap between the upper andlower plates of the DSR was chosen such that the rheological properties taken atwider gap widths (1.5 mm for 25 mm) were independent of the gap width whichmeans: |G�(12%)| > 0.95|G�(2%)|. Once the gap was set, a sufficient amount of hotbitumen (160 �C) was poured on the lower plate of the DSR to ensure a slight excessof material appropriate to the chosen testing geometry. The upper plate of the DSRwas then gradually lowered to the required nominal testing gap. The bitumen thatwas squeezed out between the plates was then trimmed flush to the edge using ahot blade. Finally, the gap was closed until there was a slight bulge around the cir-cumference of the testing geometry.

The rheological properties of the binders were measured in terms of their com-plex (shear) modulus, G�; and phase angle, d. Once the raw data were obtained, timetemperature superposition principle was applied in order also to produce mastercurves at 25 �C, isochronal plots and shift factor curves. Black diagrams and binderrutting parameters were used as the basis of all Anton Paar Physica the rheologicalanalyses in this paper.

4. Results and discussion

4.1. Estimation of the optimum rubber content by using the low shearblending protocol

A preliminary optimization study was carried out by adapting aBrookfield rotational viscometer as a low shear mixer. This practi-cal protocol was used with the main reason of constantly monitor-ing the viscosity of the binder in order to govern the modification.

d, as obtained via mechanical grinding.

Table 2Low shear mixer blending protocol (Brookfield viscometer with spindle 27).

Bitumen mass (g) Rubber mass (3–24%of bitumen masswith step of 3%) (g)

Rubber size(30 mesh) (mm)

Total weight (g) Mixing time (m) Mixing speed (rpm) Mixing temp. (�C)

10–12 (per each blend) 0.3–3 0–0.6 10–15 (per each blend) Optimized 200 180

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Many studies, indeed, have demonstrated that the applied shear isfundamental to improving the dispersion of the rubber and its set-tling tendency, but it also has almost negligible effects on the rhe-ology of the modified binders [13,14]. Rotational viscometer givesthe possibility of mixing bitumen with fine polymers at really lowshear rate and at the same time it constantly takes viscosity mea-surements by using a tool that is made to disturb the sample as lit-tle as possible. All this happens with accurate control of thetemperature and therefore offering the opportunity of understand-ing what is physically occurring during the process by monitoringthe key parameter, rotational viscosity. It is to be noticed that theBrookfield viscometer allows one to take measurements on only

Fig. 2. Results of constant monitoring using the

Fig. 3. Black diagrams of low shear TR-MBs with

10–15 g of material, therefore by using this protocol it is possibleto obtain a maximum of 15 g of modified bitumen for further tests.This quantity is not enough to perform conventional tests like pen-etration, but it results to be sufficient in order to perform DynamicMechanical Analyses (DMA) with a DSR.

In this study the low shear blending protocol, coupled withDMA, was used to establish the optimum rubber content in orderto obtain an optimized tire rubber modified bitumen (TR-MB) fromthe selected base bitumen. Table 2 summarizes the values of theparameters selected for the optimization study performed withthe low shear blending protocol.

Brookfield viscometer as a low shear mixer.

various rubber contents and neat bitumen.

Fig. 4. Master curves of G� (a) and d (b) at 30 �C of low shear TR-MBs with various rubber contents and neat bitumen.

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Each blend was replicated twice and considering that the inves-tigated range of rubber percentage was from 3% to 24% in weight ofbitumen, with a step of 3%, a total of 16 blends was performed byusing the following protocol:

1. a little tin of bitumen was rapidly heated at 180 �C in the ovenand then transferred into the container of the viscometer,weighted and inserted in the thermocell which was set at themixing temperature of 180 �C;

2. spindle n 27 was immersed into the sample and then slowly setto 200 RPM;

3. when the equilibration time reached, that is when viscosityreached 80 mPa s (viscosity of neat bitumen at 180 �C), rubberwas slowly fed in within 10–15 min and the spindle wasallowed to create a vortex on the surface of the sample to helpdispersion of the rubber;

4. once the rubber was all fed in, mixing was undertaken at a con-stant shear of 200 rpm for the time necessary to reach the bestperformance, that is when the viscosity did not increase morethan 5% within 10 min;

5. viscosity was constantly monitored and every 15 min the spin-dle was moved up and down in order to homogenize the sam-ple. Anytime that the spindle was raised from the measuringposition, speed was decreased in order to preserve theequipment.

A first important result coming from this preliminary study isrepresented by the maximum blending time to use, for each per-centage of rubber, in order to get the best performance at low shear(Fig. 2). This information is important for optimizing repetitionswith the same protocol and it could also give information to pre-dict the optimum blending time, which is expected to be less thanthe one needed in low shear mode, when a high shear mixer isused. Once the best performing material was obtained, it was takenout of the thermocell of the viscometer, immediately stirred with athin spatula and poured directly onto the preheated lower plate ofthe DSR, or stored in 10 ml vials for further tests. It is easy to real-ize how this protocol allows one to optimize laboratory work, giv-ing the chance to make a large number of blends with a really lowconsumption of materials, which in some cases are a preciousresource.

As is well known, rubber being an elastomer, the practice ofmodifying the bitumen with recycled tire rubber mainly improvesits rutting resistance at high service temperatures [24]. Moreover,the effect of rubber modification on the binder at the low servicetemperatures is less evident and of less importance in warm cli-mates. For this reason, the rheological analysis carried out for thispart of the study was performed at the high service temperatures(30 � 80 �C) with a step of 5 �C from 50 �C to 80 �C. The frequencydependence of complex modulus and phase angle for the modifiedbinders were assessed by means of the following Black diagrams

Fig. 5. Isochronal plots of G� (a) and d (b) of low shear TR-MBs with various rubbercontents and neat bitumen.

Fig. 6. Shenoy (a) and SHRP (b) rutting parameters of low shear TR-MBs withvarious rubber contents and neat bitumen.

Table 3High shear mixer blending protocol.

Bitumenmass (g)

Rubbermass (18%of bitumenmass) (g)

Rubbersize (30mesh)(mm)

Totalweight(g)

Mixingtime(m)

Mixingspeed(rpm)

Mixingtemp(�C)

2890 520 0–0.6 3410 120 5000 180

Fig. 7. Apparent viscosity of TR-MB HS at different blending time.

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(Fig. 3), rheological master curves at a reference temperature of30 �C (Fig. 4), isochronal plots (Fig. 5) and rutting parameters eval-uation (Fig. 6). All the master curves of the tire rubber modifiedbinders were obtained using the same shift factors as for their basebitumen (neat).

DMA allows one to have various kind of information on the rhe-ology of the tested material over the selected range of tempera-tures. Results shows that at low frequencies, which correspondsto high temperatures, modification with rubber makes the binderstiffer (increase of the complex modulus) and confers a better elas-tic behavior (decrease in the phase angle). This phenomenon is aclear effect of the formation of a polymer network, in which theelastic characteristics of the modifier agent (rubber) prevail overthe typical viscous behavior of the bitumen at high temperatures.As it is possible to see in the diagrams, this trend has a direct cor-relation with the rubber percentage: increasing the concentrationof rubber the binder became stiffer and more elastic (Fig. 4). Theisochronal plots show the clear improvements in thermal suscepti-bility and elastic properties of the asphalt rubber binders at hightemperatures (Fig. 5).

Other important information is given by evaluation of the rut-ting parameters as determined via the SHRP parameter |G�|/sin dand that suggested by Shenoy for modified binders [25–27](Fig. 6). From the former it is possible to notice that the high crit-ical temperature (higher value of the SHRP performance grade) ofthe un-aged neat bitumen increases from 66 �C to more than80 �C when at least 15% of rubber is added. Shenoy’s parameter|G�|/(1�(1/sin d� tan d)) better explains the differences reachedby means of addition of a polymeric modifier in neat binders andit shows some strange behavior for binders modified with morethan 18% of rubber.

All the diagrams highlight one fundamental datum, which wasalso one of the main aim of this part of the study, i.e. that the se-

lected base bitumen cannot be modified with more than 21% of tirerubber without using oil extender too. In fact, from the rheological

Fig. 8. Black diagrams of: TR-MB HS at different blending time, TR-MB LS and neat bitumen.

Fig. 9. Master curves of G� (a) and d (b) at 30 �C of: TR-MB HS at different blendingtimes, TR-MB LS and neat bitumen. Fig. 10. Isochronal plots of G� (a) and d (b) of: TR-MB HS at different blending times,

TR-MB LS and neat bitumen.

568 B. Celauro et al. / Construction and Building Materials 37 (2012) 562–572

analyses it appears that it is not possible to appreciate any furtherimprovement in terms of stiffness, elastic behavior, thermal sus-ceptibility and rutting resistance when passing from 21% to 24%of tire rubber in the blend.

4.2. Estimation of the optimum time by using the high shear blendingprotocol

In the first part of the optimization study, 21% of rubber contentwas assessed as the maximum percentage that the selected bitu-men can accept showing clear rheological improvements. Thus,

18% was fixed as the rubber content to be used in the second partof the study, where a high shear blending protocol was used tooptimize the mixing time. In fact, using high shear mixing rate re-duces the particle size of crumb rubber and provides enough en-ergy to breaks the particle–particle bonds, thus allowing theinteraction process to progress with greater speed [28]. The blend-ing of rubber and bitumen at high shear was carried out using aSilverson High shear mixer with a duplex head and using the fol-lowing protocol:

Fig. 11. Shenoy (a) and SHRP (b) rutting parameters of: TR-MB HS at differentblending times, TR-MB LS and neat bitumen.

Table 4Physical and mechanical characteristics of the binders studied.

Characteristic Neatbitumen

TR-MBHS40

SBS-MB H

SBS-MB M

Penetration 25 �C (mm/10) – EN1426

50 30 47 51

Softening point (�C) – EN 1427 51 63 85 64Apparent viscosity at 177 �C (Pa s)

– ASTM D6114– 3.150 – –

Viscosity at 135 �C, (Pa s) – EN13302

0.429 5.950 2.450 1.290

Viscosity at 160 �C (Pa s) – EN13302

0.139 2.300 0.742 0.472

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1. required amount of bitumen was heated at 180 �C in the ovenand then transferred into a thermostatic bath of 4 l (thereforeat least 3 l of bitumen per each blend in needed), provided withdouble walls containing oil to control the temperature, whichwas set at the mixing temperature of 180 �C.

2. after an equilibration time of about 30 min, the duplex head ofthe high shear mixer was totally immersed into the hot bitu-men and a constant shear of 3500 rpm was applied for the first10 min while the rubber was fed in.

3. once the rubber was all fed in, mixing was undertaken at a con-stant shear of 5000 rpm for 2 h.

4. every 20 min about 100 g of binder were sampled to control themodification process.

Table 3 summarizes the mixing parameters used in the blend-ing procedure.

All the samples, differing in terms of mixing time, were testedby estimating the so-called apparent viscosity at 177.5 �C (ASTM,2009) that is used in the asphalt rubber production as the keyparameter to assess the optimum performance. Rubber particlesreact with the bitumen improving its stiffness and elastic proper-ties until a peak performance is reached. In practical applicationsof asphalt rubber, this peak is considered to be achieved whenthe target of the apparent viscosity is obtained [29]. When bitumenis blended with tire rubber without any additional additive, thebest blending time is usually around 45–60 min [30]. Similar re-sults were obtained in this study, as shown by the curve in Fig. 7.

By analyzing the results shown in Fig. 7, all the samples col-lected after 40 min prove to have the highest apparent viscosities,between 3000 and 4000 mPa s, with a plateau at about 60 min, buta really high variability in the measurements between repetitionswas noticed. Therefore, from the apparent viscosity results it issurely possible to affirm that best performing blends are obtainedwhen the mixing time is more than 40 min. In order to go intomore depth, all the samples were also tested with DSR, which isable to indicate not only when the highest stiffness is reachedbut also the variability of the elastic properties of the binder.DMA at high service temperatures (30 � 80 �C) was performedfor all the TR-MB HS samples, and then followed by a comparisonwith the same rheological properties of the neat bitumen and thoseoffered by the best performing blend obtained using the low shearblending protocol: TR-MB LS.

From the comparison of the rheological properties assessedwith the DMA it is possible to notice that all the samples showimprovements in terms of stiffness and elastic properties at hightemperatures if compared with the neat bitumen.

As far as the optimization of the blending time is concerned, ablending time of 20 min provides a blend whose properties maybe improved, for sure, while a blending time equal to 120 minleads, in this case, to a blend with poorer properties, while thiswas exactly the optimum time for the low shear mode.

The TR-MB LS blend is the best performing amongst the onesproduced, in terms of both stiffness and elastic properties, eventhought it was obtained without applying high shear (Fig. 8).Therefore it is possible to affirm that the LS protocol can effectivelyprovide a correct indication about the reaction between bitumenand rubber.

This high performing blend can also be obtained, indeed, byusing the HS protocol. The results obtained show that the mini-mum blending time, in this case, is about 40 min that is about athird of that necessary to obtain the equivalent blend in low shearmode, in agreement to what was found by other authors [15].

In fact, as it is possible to notice from the master curves and iso-chronal plots (Figs. 9 and 10), the best performing blend producedwith high shear, TR-MB HS40, proves as stiff as the blend producedwith 60, 80, 100 min of blending time but with improved elasticproperties at high temperatures.

Finally, as can clearly be noticed from Fig. 11b, all the TR-MBshave a Performance Grade PG higher than 80 �C (only the un-agedstate was investigated). Moreover, it should be mentioned thatShenoy’s parameter (Fig. 11a) provides a more appropriate expla-nation of the differences between rutting resistance of the binders,in comparison with the SHRP method.

4.3. Comparison with SBS modified bitumens

In this part of the study the optimized TR-MB was comparedwith two SBS polymer modified bitumens with two different levelsof modification: SBS-MB Hard and SBS-MB Medium. Conventional

Fig. 12. Master curves of G� (a) and d (b) at 30 �C of: TR-MB HS40, TR-MB HS120, PMBs and neat bitumen.

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physical properties, i.e. Penetration (Pen), Softening Point (SP),Rotational Viscosity (RV) at 135 �C and 160 �C and rheology wereused as the basis of the comparison. Table 4 shows the results ofbinder characterization. From the classical identification tests, itcan be concluded that the TR-MB HS40 shows improvements interms of SP, closer to the ones found for the SBS-MB Medium thanto the one obtained for the SBS-MB Hard. Nevertheless, the rubbermodified binder proves to be much stiffer and more viscous thanthe the SBS-MB Hard. Therefore, it is difficult to make comparisonsonly based on conventional classification. Furthermore, it has to benoticed that the optimized TR-MB proves to have the characteris-tics (in terms of standard requirements for Pen, SP and apparentviscosity) to be classified as Asphalt Rubber type I or II [9].

The rheological analysis at high service temperatures confirmsthe results of the conventional tests and better demonstratesimprovements in the elastic properties as well as in the reductionof the binder’s temperature susceptibility. TR-MB HS120 (far abovethe optimum blending time) was included to emphasize theimportance of the mixing time. In terms of Stiffness (G�), both mas-ter curves and Black diagrams visibly show that both the TR-MBsare comparable to the SBS-MB H (Figs. 12 and 13). On the contrary,in terms of elastic properties, it is clear to what extent the optimi-zation of the blending time is fundamental in order to get a well-performing TR-MB. In fact, analysing the phase angle master curveand the isochronal plot (given in Fig. 14), the overblended TR-MB

HS120 ends up being less elastic and, at high service temperatures,it is comparable only with the PMB with a medium level of modi-fication (SBS-MB M).

In conclusion, evaluation of the rutting parameters (Fig. 15)confirms the results explained before and, again, this proves thatShenoy’s parameter is able to provide a more adequate measure-ment of the rutting resistance of the modified binders, with respectto the parameter originally provided by the SHRP protocol.

5. Conclusions

The aim of this paper was to evaluate the performance improve-ment that is possible to achieve when modifying road bitumenwith CRM (Crumb Rubber Modifier) from discarded tire rubber(TR), using a Wet Process.

Adapting a Brookfield viscometer as a low shear mixer allowedthe Authors to define a protocol that proved to be a really usefultool in order to properly optimize a blend of recycled rubber ob-tained by mechanical grinding of scrap tires with road bitumen(tire rubber modified bitumen, TR-MB), with beneficial reductionof material consumption for laboratory needs. A low shear blend-ing protocol, in fact, proved to be suitable for evaluating the rheol-ogy of the best performing TR-MB, obtainable with the selectedbase bitumen and rubber, giving results comparable with those ob-tained when high shear mixing is adopted.

Fig. 13. Black diagrams of: TR-MB HS40, TR-MB HS120, PMBs and neat bitumen.

Fig. 14. Isochronal plots of G� (a) and d (b) of: TR-MB HS40, TR-MB HS120, PMBsand neat bitumen.

Fig. 15. Shenoy (a) and SHRP (b) rutting parameters of: TR-MB HS40, TR-MB HS120,PMBs and neat bitumen.

B. Celauro et al. / Construction and Building Materials 37 (2012) 562–572 571

It is well known that in order to obtain the best performing TR-MB, the mixing time is a key parameter to be optimized. Thedescription protocol proved to be comparable with the one thatperforms blending at high shear: the best performing TR-MB HSwas obtained with the addition of the optimized amount of TR,with a mixing time about a third of that necessary to obtain theequivalent blend in low shear mode.

Simple evaluation of apparent viscosity seems not to be enoughin order to optimize the mixing time. In fact, even though viscositymeasurements provide important information about the stiffness

of the material, in order to correctly evaluate the elastic propertiesit is fundamental to complement the investigation by means ofDSR tests, as was done in this study.

The comparison of the results from testing the TR-MB and thoseoffered by two polymer modified bitumens, obtained by addingsynthetic elastomeric polymers (SBS), shows that it is possible to

572 B. Celauro et al. / Construction and Building Materials 37 (2012) 562–572

obtain a comparable level of modification, and thus the desiredimprovements of the mechanical performances once in mixture,as proved by an analogous increase in the high temperature PG, ob-tained by properly designing the TR-MB in the laboratory with theproposed low shear protocol.

Therefore, this study proves that with the proposed optimiza-tion protocol it is possible to obtain improved bitumen with highcontent of recycled materials that, with respect to specific needof binders with a reduced temperature susceptibility for use incountries with warm climates such as those in the Mediterraneanarea, can be considered as ‘‘high-performance blends.’’ The resultsof this study show, in fact, that the mixing protocol here presentedallows one to optimize these blends with high ecologic benefit,thanks to a laboratory method that is both time and materialsaving.

References

[1] Esch DC. Construction and benefits of rubber-modified asphalt pavements.Report N AK-RD-82-17, Alaska Department of Transportation and PublicFacilities, Fairbanks; 1982.

[2] Epps JA. Uses of recycled rubber tires in highways. NCHRP Report. In: Synthesisof highway practice no.198, TRB National Research Council. Washington, DC;1994.

[3] Rahman MM, Airey GD, Collop AC. The influence of ageing on dry processcrumb rubber modified asphalt mixtures. Int J Pav Eng Asph Technol2006;7(1):10–26.

[4] Celauro B, Celauro C, Di Francisca A, Giuffrè O. Effect of the constructionprocess on the performance of asphalt paving mixes with crumb rubber fromscrap tires. In: 3rd Eurasphalt & Eurobitume Proc., Vienna; 2004. p. 757–69.

[5] Airey GD, Collop AC, Mujibur MM. Mechanical properties of crumb rubbermodified asphalt mixtures. In: 3rd Eurasphalt & Eurobitume Proc., Vienna;2004. p. 800–12.

[6] Amirkhanian SN. Utilization of crumb rubber in asphaltic concrete mixtures-south carolina experience. South Carolina DOT, South Carolina, USA; 2001.

[7] Bahia HU, Davis R. Effect of Crumb Rubber Modifiers (CRM) on performancerelated properties of asphalt binders. J Assoc Asph Paving Technol1994;63:414–49.

[8] Sousa J, Way GB. Models for estimating treatment lives, pavement lifeextention and the cost effectiness of treatments on flexible pavements,vol. 2; 2009. <http://www.csuchico.edu/cp2c/documents/Library/sousa_vol_2.pdf>.

[9] ASTM standard specification for asphalt–rubber binder. Designation ASTMD6114/D6114M – 09, US, American Society for Testing and Materials; 2009.

[10] Zanzotto L, Kennepohl GJ. Development of rubber and asphalt binders bydepolymerization and devulcanization of scrap tires in asphalt. Transport ResRec 1996;1530:51–8.

[11] Pavlovich RD, Shuler TS, Rosner JC. Chemical & physical properties of asphaltrubber mixtures – Phase II: Product specifications and test procedures.Arizona. Arizona Department of Transportation, FHWA/AZ-79/121; 1979.

[12] Chehovits JD, Dunning RL, Morris GE. Characteristics of asphalt–rubber by thesliding plate microviscometer. J Assoc Asph Paving Technol 1982;51:240–61.

[13] Billiter TC, Chun JS, Davison RR, Glover CJ, Bullin JA. Investigation of the curingvariables of asphalt–rubber binder. Petrol Sci Technol 1997;15(5):445–69.

[14] Navarro FJ, Partal P, Martínez-Boza FJ, Gallegos C. Influence of processingconditions on the rheological behavior of crumb tire rubber-modified bitumen.Appl Polym Sci 2007;104(3):1683–91.

[15] Takallou HB, Sainton A. Advances in technology of asphalt paving materialscontaining used tire rubber. Transport Res Rec 1992;1339:23–9.

[16] Roberts FL, Kandhal PS, Brown ER, Dunning RL. Investigation and evaluation ofground tire rubber in hot-mix asphalt. NCAT, Auburn University, Alabama;1989.

[17] ASTM. Standard test method for separation of asphalt into four fractions.Designation ASTM D4124-01, US, American Society for Testing and Materials;2001.

[18] Hicks RG. Asphalt rubber design and construction guidelines. DesignGuidelines, vol. 1. NCRACTC and CIWMB; 2002.

[19] EN. European Standard 1426. Methods of tests for petroleum and its products.Bitumen and bituminous binders. Determination of needle penetration.Brussels CEN; 2000a.

[20] EN. European Standard 1427. Methods of test for petroleum and its products.Bitumen and bituminous binders. Determination of softening point. Ring andball method. Brussels CEN; 2000b.

[21] EN. European Standard 13302. Bitumen and bituminous binders -Determination of viscosity of bitumen using a rotating spindle apparatus.Brussels CEN; 2003.

[22] Tayebali AA, Vyas BB, Malpass GA. Effect of crumb rubber particle size andconcentration on performance grading of rubber modified asphalt binders. In:Jester RN editor, Progress of SUPERPAVE: Evaluation and ImplementationASTM STP 1322; 1997. p. 30–47.

[23] AASHTO TP5 Standard test method for determining the rheological propertiesof asphalt binder using a dynamic shear rheometer (DSR). AASHTO; 1998.

[24] Lo Presti D, Memon N, Airey G, Grenfell J. Comparison between bitumenmodified with crumb tire rubber and stirene butadiene styrene. MAIREPAV 6Proc., Turin, Italy; 2009. p. 54–63.

[25] Bahia HU, Zeng M, Zhai H, Khatri MA. Superpave protocols for modified asphaltbinders. In: 15th Quarterly Progress Report for NCHRP Project 9-10.Washington DC; 1999.

[26] Bahia HU, Hanson DI, Zeng M, Zhai H, Khatri MA, Anderson RM.Characterization of modified asphalt binders in Superpave mix design.NCHRP Report 459. Washington DC; 2001.

[27] Shenoy A. Refinement of the superpave specification parameter forperformance grading of asphalts. J Trans Eng 2001;127(5):357–62.

[28] Attia M, Abdelrahman M. Enhancing the performance of crumb rubber-modified binders through varying the interaction conditions. Int J Pave Eng2009;10(6):423–34.

[29] Kandhal PS. Quality control requirements for using crumb rubber modifiedbitumen (CRMB). J Ind Road Congr 2006;67(1). paper no. 522.

[30] Tabatabaee N, Karami S, Kamali Abyaneh Z. The effect of mixing time on theviscosity of CRM binders. In: 3rd Eurasphalt & Eurobitume Congress Proc.,Vienna; 2004. p. 881–8.