2014Novel Method to Prepare Activated Crumb Rubber Used for

Novel Method to Prepare Activated Crumb Rubber Used for Synthesis of Activated Crumb Rubber Modified AsphaltMaozhen Xu 1;Jinjing Liu 2;Wenzhi Li 3;and Wenfeng Duan 4Abstract:This work introduced a novel method to prepare activated crumb rubber by using ultrasonic focusing apparatus,which could more significantly improve the compatibility between the activated crumb rubber and virgin asphalt compared with ordinary crumb rubber.The obtained activated crumb rubber was used for the synthesis of activated crumb rubber modified asphalt (ACRMA)by blending virgin asphalt,activated crumb rubber,and certain additives (e.g.,SBS,antiaging agent,filler).The performance test results indicated that ACRMA had better performance than the ordinary crumb rubber modified asphalt,in terms of high-temperature property,low-temperature stiffness,duc-tility,especially fatigue resistance,and aging resistance.In this case,the application of novel method of activated crumb rubber synthesis could accelerate the material acceptance rate in pavement engineering.DOI:10.1061/(ASCE)MT.1943-5533.0001115.©2014American Society of Civil Engineers.Author keywords:Crumb rubber;Modification;Performance;Interaction effect;Bituminous/flexible pavements.IntroductionPetroleum asphalt,a complex,heterogeneous mixture of various hydrocarbons,has been widely used as a binder for aggregate material in pavement engineering.However,due to its disadvantage of crack prone in low temperature,poor aging resistance and fatigue resistance,it could not meet high requirement and heavy load of modern traffic on the road (Endres et al.1951;Navarro et al.2004,2007;Upadhyay et al.2008).Therefore,the development of modified asphalt has become a hot topic in recent decades.Crumb rubber modified asphalt (CRMA)has become more and more popular for its well-known advantages including a noticeable reduction in temperature susceptibility,increased aging property and pavement service life,retarded reflection cracking,decreased traffic noise,reduced maintenance cost,decreased pollution,and increased environmental quality (Morrison and Hesp 1995).How-ever,there is still some limitation on the wide application of the CRMA due to the following reasons:the concentration of crumb rubber must be higher than 20%to ensure the desired property,which resulted in the increase of CRMA viscosity.To achieve the processing feasibility,the viscosity of CRMA should be reduced.Increasing process temperature could help the viscosity decrease,but this change may cause CRMA to be aged easily and create high-processing risk and environment pollution.Therefore,a new approach of activation was proposed to improve the compat-ibility between virgin asphalt and crumb rubber and change their interface combination.In this work,we developed a novel method to prepare the activated crumb rubber using ultrasonic focusing apparatus and se-lected the activated crumb rubber as an asphalt pared with the current existing method (dynamic desulphurizion),it had many advantages as subsequently shown.First of all,this method had the better production efficiency compared with the current existing method.Then,it would not generate any industrial waste-water and gases in the making stly,the modification effect was quite obvious.With the application of this method,the mixing amount of ac-tivated crumb rubber in asphalt system was improved significantly from 16to 25%by weight.Based on the performance of ACRMA,we determined the effect of activated crumb rubber structure on diffusion and clarified the type of rubber-asphalt interaction by scanning electronic microscope (SEM)and fluorescence micro-scope,respectively.It was expected that the better understanding of asphalt-rubber interaction would be essential to optimizing the performance of activated crumb rubber modified binder (ACRMB),thus,to accelerating the acceptance rate of these ma-terials in construction field.Materials and ExperimentalMaterialsVirgin asphalt (AH-70,pen)was offered by China Petroleum &Chemical Corporation.Table 1showed its relevant performance indexes.The ordinary crumb rubber (40∼80mesh)was sourced from truck tire and purchased from Huangshi Second Rubber Powder Factory;several kinds of modifying agents included stabi-lizer and activator.Sample PreparationFig.1schematically illustrated the full preparation of activated crumb rubber.First of all,the crumb rubber was charged into storage silo of ultrasonic focusing apparatus and the ultrasonic1Junior Engineer,Beijing Oriental Yuhong Waterproof Technology Co.,Ltd.,Concha 2,Shaling,Shunping Rd.,Shunyi District,Beijing 101309,China (corresponding author).E-mail:xumz@ 2Senior Engineer,Beijing Oriental Yuhong Waterproof Technology Co.,Ltd.,Concha 2,Shaling,Shunping Rd.,Shunyi District,Beijing 101309,China.E-mail:liujj@ 3Senior Engineer,Beijing Oriental Yuhong Waterproof Technology Co.,Ltd.,Concha 2,Shaling,Shunping Rd.,Shunyi District,Beijing 101309,China.E-mail:liwz@ 4Senior Engineer,Beijing Oriental Yuhong Waterproof Technology Co.,Ltd.,Concha 2,Shaling,Shunping Rd.,Shunyi District,Beijing 101309,China.E-mail:duanwf@Note.This manuscript was submitted on August 22,2013;approved on April 25,2014;published online on August 8,2014.Discussion period open until January 8,2015;separate discussions must be submitted for in-dividual papers.This paper is part of the Journal of Materials in Civil Engineering ,©ASCE,ISSN 0899-1561/04014173(7)/$25.00.©ASCE04014173-1J.Mater.Civ.Eng.D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y C h o n g q i n g J i a o t o n g U n i v e r s i t y o n 09/29/15. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .piezoelectric transducer was controlled to treat on the crumb rub-ber.In the treatment process,the out power and frequency of ultra-sonic focusing apparatus was set up as 1,200W and 20KHz,respectively.The activated crumb rubber was obtained after treat-ment for 12∼15s.The whole process was carried out in a close system without releasing polluted gas or water.Fig.1described the detailed information about this machine.The ACRMB prepared in the laboratory was produced by add-ing 15%crumb rubber by weight of binder to a preheated sample of asphalt binder.The ACRMB was preheated to 180°C and upon ad-dition of the crumb rubber an impellor rotating at 750rpm was used for 1.5h to mix the blend.Characterization of Activated Crumb Rubber Properties The morphologies of crumb rubber powder pre-and postmodifying were observed by scanning electron microscopy (JSM-5510LV ,Hitachi Company,Japan).Samples were pretreated by gold sput-tering prior to the observation.The microstructure of ACRMB was inspected with a fluorescence microscope (DMIRB,Leica Company,German).The analyses of crumb rubber were done accordance to ASTM D 297and the crumb rubber gradation was determined using ASTM D 5644.The viscosity of ACRMB was tested under theBrookfield viscometer at seven different temperature conditions (190,180,170,160,150,140,and 130°C).A 35%torque was applied and the rotation speed was set at 100rpm.The #29spindle was adopted in the tests in favor of the #27spindle due to the high viscosity of ACRMB.The viscosity test followed the AASHTO T316standard test specification.The softening point,fraass breaking point penetration,ductility,elasticity recovery,and separation test of ACRMB were measured according to “Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering ”(JTG E20-2011).Result and DiscussionACRMA was fabricated with the blending of virgin asphalt,acti-vated crumb rubber,furfural extract oil,activator,and stabilizer at 180°C for 2h.ACRMA showed good storage stability because of the improved compatibility between the activated crumb rubber and virgin asphalt.According to the asphalt storage stability test,the difference of softening points between two extreme parts of the aluminum tube could be decreased to 2.5°C.Moreover,ACRMA showed excellent performance,including high-temperature toler-ance property,low-temperature stiffness,ductility,especially fatigue resistance and antiaging,as summarized in Table 1,from which it could be ranked as performance grade (PG)82-28.Table 1showed the softening point and fraass breaking point of ACRMA were obviously increased by the modification.Therefore,we dis-cussed the processing of ultrasonic focusing apparatus,crumb rub-ber content,processing temperature,and swelling time on the high and low temperature performance.Processing of Ultrasonic Focusing ApparatusCrumb rubber devulcanization by using ultrasonic energy was first discussed in this paper.Ultrasonic devulcanization technology was actually composed of devulcanization system that utilized mechanical extrusion and ultrasonic energy to devulcanize crumb rubber.In this type of devulcanization system,crumb rubber par-ticles were loaded into a hopper and subsequently fed into anTable 1.Test Index of the Virgin Asphalt,CRMA,and ACRMA (the Ordinary Crumb Rubber and Activated Crumb Rubber Account for 22%by Weight of CRMA and ACRMA,Respectively)PropertiesVirgin asphalt CRMA ACRMA Softening point (°C)47.959.173Fraass breaking point (°C)−6−14−2525°C penetration (0.1mm)7110376Elastic recovery (%)—67935°C ductility (cm)1.315.546After segregation,difference of the softening point/°C — 5.80.7135°C viscosity (Pa ·S)0.484.622.35Fig.1.Ultrasonic focusing apparatus used in this study (image courtesy of Quanyi Ultrasonic Equipment Co.,Ltd)D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y C h o n g q i n g J i a o t o n g U n i v e r s i t y o n 09/29/15. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .extruder.The extruder mechanically pushed and pulled the crumb rubber to be exposed to ultrasonic energy.The ultrasonic waves,at certain levels,in the presence of pressure and heat,could quickly break up the three-dimensional network in cross-linked vulcanized rubber.It was an applicable process in which the vulcanized rubber was devulcanized based on the use of high-power ultra-sound electromagnetic radiation,using 1,200W and 20KHz ultra-sonic waves after treatment for 12∼15s.Structural studies of ultrasonically-treated rubber showed that the breakup of crumb rubber vulcanization network structures was accompanied by the partial degradation of the rubber chain.The process apparently could break down C-S and S-S bonds by comparing FTIR spectra of crumb rubber before and after modifying.Because the degree of degradation of C-C bonds was substantial,the process could not break down C-C bonds.As a consequence of the process,ultrasoni-cally devulcanized crumb rubber became soft,enabling this material to be reprocessed in virgin bitumen in the same way as virgin rubber.Moreover,Table 1showed the activated ACRMA exhibited good mechanical properties.The process of ultrasonic devulcanization was very fast,simple,and efficient.This new tech-nology has been used successfully in the laboratory to devulcanize truck tire rubber.On the other hand,crumb rubber particles in ultrasonic focusing apparatus near the solid surface experienced two kinds of forces of particular interest to this study:adhesion forces —linear and non-linear interactions that arise through interactions of ultrasound withcrumb rubber particles.In general,adhesion forces comprised capillary forces,van der Waals forces,electrostatic image forces,and electrical double-layer forces.During the operation,the adhe-sion forces of most importance were van der Waals forces and elec-trical double layer forces.Linear interaction forces included added mass,drag,lift,and basset forces while nonlinear ones included radiation pressure forces and drag forces due to acoustic streaming.Linear forces were time dependent and had zero means while non-linear ones had a time-independent component and took nonzero values.Linear interaction forces were usually much larger than nonlinear counterparts in conventional applications at moderate frequencies.Therefore,the crumb rubber vulcanization network structures were destroyed and the morphology of crumb rubber changed under the interaction.Fig.2showed the gradation of crumb rubber and activated crumb rubber was determined using ASTM D 5644.This phe-nomenon indicated that the crumb rubber vulcanization network structures were destroyed through the physical processing,corre-sponding to the increasing of the covering area of activity units,which revealed the small activity units and chemical reactivity increased in the activated crumb rubber.Therefore,Fig.4showed the compatibility between the activated crumb rubber and virgin asphalt could be largely improved.It has been reported that the crumb rubber type and size have significant impact on the high temperature viscosity of ACRMA (Upadhyay et al.2008;Sabbagh and Lesser 1998;Xiang et al.2009).Herein,we selected 40∼80mesh truck tire crumb rubber in diameter,which was beneficial for asphalt swelling.Moreover,the ordinary truck tire crumb rubber must be modified to obtain the activated crumb rubber by ultrasonic focusing apparatus (observed in Fig.1).During making process,the activated crumb rubber with popcorn appearance (observed in Fig.3)was obtained due to ultra-sonic cavitation,resulting in a much higher complex modulus and phase angle of ACRMA by DSR frequency sweep test.The com-plex modulus,defined as the ratio of maximum stress to maximum strain,provided a measure of the total resistance to deformation due to repeated shearing force.The phase angle,defined as the phase difference between the peak stress and strain,represented the time delay between the applied stress and measured strain with limiting values of 0°for purely elastic behavior and π=2for purely viscous behavior.In other words,it was a direct measure of the ratio of the viscous to elastic contribution to the overall response in the CRMA and ACRMA.Thus the ACRMA could keep better elasticity to resistant rutting than the CRMA according to DSRresults.Fig.2.Passing percent gradation of crumb rubber and activated crumbrubberFig.3.SEM images of (a)preactivated crumb rubber;(b)postactivated crumb rubberD o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y C h o n g q i n g J i a o t o n g U n i v e r s i t y o n 09/29/15. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .Table 1showed the comparison of the properties of ordinary CRMA and ACRMA a significant performance improvement of ACRMA was achieved through the modification.Based on the main effects of compatibility between the activated crumb rubber and virgin asphalt on the performance of ACRMA,the morphologi-cal characteristics of crumb rubber and ACRMA were examined by SEM and fluorescence microscope,respectively.We found that the apparent morphology of activated crumb rubber showed a strong influence on the performance of ACRMA.Fig.3(a)showed the interface of traditional crumb rubber was clear,which indicated poor compatibility between virgin asphalt and ordinary crumb rub-ber as shown in Fig.4(a).It showed that the swelling of crumb rubber was very poor,and the light components of asphalt could not ultimately permeate into crumb rubber.In the perspective of thermodynamics,the CRMA system was very unstable,and two-phase separation occurred easily (Khaldoun et al.2012).Thus Table 1summarized CRMA obtained with ordinary crumb rubber had poorer performance.However,the interface of activated crumb rubber turned out to be indistinct,which provided higher surface area than ordinary crumb rubber.Herein,the activated crumb rubber provided a large area to create chemical bonds and homogeneously dispersed in bi-tumen phase due to the good compatibility between virgin asphalt and the activated crumb rubber as seen in Fig.4(b).Meanwhile,the presence of swollen activated crumb rubber particles would be helpful to reinforce the friction and embedding of aggregates to form a high-strength mixture structure,which increased the cohe-sion and flexibility,and limited the temperature sensitivity of as-phalt.Obviously,the dispersion degree and swelling capacity of crumb rubber had been improved by the activation,and the crumb rubber swelling could promote the formation of elastic network of ACRMA,as well as the improving properties of ACRMA dis-played in Table 1(Upadhyay et al.2008;Xiang et al.2009).In the making process of ACRMA,the activated crumb rubber could easily absorb the light oil component in bituminous phase,which reduced the content of free radicals and indicated a significant increasing in storage stability.Moreover,because of devulcanization and depolymerization,the activated crumb rubber homogeneously dispersed in virgin asphalt and formed perfect polymer reticular structure with the function of additives and sta-bilizers,which contributed to increasing softening point,fraass breaking point,elastic recovery of ACRMA,and decreasing pen-etration of ACRMA.In addition,there were many antioxidant and improving the antioxidative capacity of ACRMA (Sabbagh and Lesser 1998;Navarro et al.2005).Table 2showed the basic chemical properties of crumb rubber displayed significant effects on the performance of CRMA.The extract content (acetone and chloroform)played an important role in improving CRMA properties.The higher content of the extract was corresponding to the higher content of resinlike (high fatty acid,steroid,etc.)in crumb rubber.Adding this type of crumb rub-ber to matrix asphalt,the resinlike could permeate through matrix asphalt under thermal energy and mechanical force,while saturates and aromatics in matrix asphalt could also wrap and swell macro-molecules in crumb rubber.Remarkable swelling and compatibility were achieved through mutual infiltration of the components of crumb rubber and matrix asphalt,which improved the performance of CRMA obviously.The lower ash content of crumb rubber could indicate the minor superiority;the ductility at low temperature,cohesive force,and elastic recovery of CRMA would be better.Fig.5shows the viscosity graphical plots for the CRMB and ACRMB,respectively.It was clear that the viscosity of CRMB and ACRMB decreased with increasing test temperature,with the same trend holding true for the virgin asphalt.The addition of crumb rubber could greatly increase the binder viscosity,which was vital in increasing the binder film thickness for coating aggre-gates in the hot mixture.As expected,the viscosity of the ACRMB was significantly lower than the CRMB.In this respect,it was also evident that the activation of crumb rubber could significantly im-prove the compatibility between the activated crumb rubber and virgin asphalt compared with ordinary crumb rubber.Moreover,the viscosity of CRMB and ACRMB decreased with the increased test temperature.Effect of Crumb Rubber ContentCrumb rubber was a special flexible powder,which was prepared by mechanical grinding from waste truck tire;consisting ofrubber,Fig.4.Fluorescence micrographs at 400magnification of modified asphalt by (a)ordinary crumb rubber;(b)activated crumb rubberTable 2.Chemical Composition of Crumb Rubber Used in This Study Major rubber components (%)Crumb rubber (%)Natural and synthetic rubber55.7Extract content (acetone and chloroform)(wt%)14.6Ash content (wt%)7.2Carbon black content (wt%)32.1D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y C h o n g q i n g J i a o t o n g U n i v e r s i t y o n 09/29/15. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .carbon black,softener,vulcanization promoting agent,and some other ingredients as shown in Table 2,whose surface was inert with cross-linked structure (Navarro et al.2005;Miknis and Michon 1998).Hence,we treated ordinary crumb rubber to obtain the activated crumb rubber,which could significantly enhance the performance of ACRMA,including high-temperature property,low-temperature stiffness,ductility,especially fatigue resistance and aging resistance.Compared with ordinary crumb rubber (max 20%of the weight of virgin asphalt),the activated crumb rubber content was found to be the main factor that affected the performance of ACRMA in the preparation of ACRMA.The concentration analysis of activated crumb rubber demonstrated that it had a significant effect on the properties of ACRMA.Fig.6illustrated that when the concentra-tion of activated crumb rubber was very low,the activated crumb rubber could not form network structure in ACRMA,the modified effects were relatively poor,and the high-and low-temperature performances of ACRMA got minor improvement.While with the activated crumb rubber content increasing to a proper value,the ACRMA system would be stable homogeneously.The soften-ing point,fraass breaking point,the ductility,and elastic recovery of ACRMA was obviously improved (Liu et al.2009;Thodesen et al.2009).However,the higher activated crumb rubber content resulted in a significant decrease in compatibility degree between the activated crumb rubber and bituminous phase.It was assumed that insoluble activated crumb rubber particles remained in ACRMA and led to a heterogeneous substance and an uneven system.That meant the activated crumb rubber modification was a physical modification rather than a polymeric modification (Navarro et al.2005;Liu et al.2009).Therefore,compared with the softening point,fraass break-ing point,viscosity,and penetration,the addition of activated crumb rubber must be controlled in a proper range (15∼25%of the weight of virgin asphalt).Effect of Processing TemperatureIt was well known that an increase in processing temperature led to an increase in both the amount of crumb rubber digestion and the asphalt aging phenomenon (Miknis and Michon 1998;Ruan et al.2003;Chipps et al.2001;Akisetty et al.2009).Fig.7showed the evolution of high-and low-temperature properties as a function of processing temperature.As expected,the increase up to180°C led to a significant increase in softening point and fraass breaking point due to a higher amount of activated crumb rubber digestion and lower interaction among solid particles in ACRMA.Under the processing condition,the effect of aging on ACRMA was less sig-nificant than the effect of digesting crumb rubber.Nevertheless,increasing the processing temperature to 200°C changed the evo-lution of this balance.At 200°C,the effect of aging on ACRMA dominated the effect of digestion,leading to the decrease of fraass breaking point observed in Fig.7.But then,the softening point and penetration grade remained effectively independent of processing temperature,showing little aging effect on ACRMA illustrated in Fig.7.In this respect,the evolution of certain technological properties with processing temperature was also discovered (results not shown).Along with the increase of processing temperature,the ac-tivated crumb rubber was digested in asphalt accompanied by the partial depolymerization and devocalization of crumb rubber net-work,which led to an increase in the number of components that were incorporated to the bituminous phase.Thus,activated crumb rubber delivered cohesive property and plasticity to ACRMA,which contributed to increasing of elastic recovery and decreasing of penetration.However,when the processing temperature ex-ceeded the optimum temperature (190°C),the digestion ofactivatedFig.5.Viscosities of CRMB andACRMBFig.6.Effects of activated crumb rubber content on the performances ofACRMAFig.7.Effects of processing temperature on the performances of ACRMAD o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y C h o n g q i n g J i a o t o n g U n i v e r s i t y o n 09/29/15. C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .crumb rubber and aging process was improved.Thus,the global effect of these conflicting phenomena was a decrease in mechanical properties,followed by an increase in viscosity due to aging (Upadhyay et al.2008;Chipps et al.2001;Akisetty et al.2009;Jeong et al.2010;Wang et al.2012).Effect of Swelling TimeAt 180°C,crumb rubber could not be fully swollen in short blending time,but the excessive blending time led to excessive decomposition and aging.Consequently,it was possible to set a high-optimum processing temperature to ensure a higher quan-tity of ACRMA (Dong et al.2012).Fig.8showed the single factor experimental results of swelling time for the blending of activated crumb rubber,virgin asphalt,and certain additives.As can be seen,the swelling time effect did not manifest significant differ-ence.In processing prophase,progressively prolonging swelling time could digest crumb rubber into ACRMA system,thus improving the softening point and fraass breaking point of ACRMA.When the length of swelling time exceeded a certain threshold (2h)at 180°C,the result was just the opposite and mix properties were impoverished.It led to an exponential increase in viscosity due to primary aging of the bituminous base,with a consequent loss of improvement in the mechanical properties of ACRMA.The low-temperature stiffness would slowly degrade due to the aging of ACRMA observed in Fig.8.At the same time,a decrease in viscosity took place as a result of the increase in the amount of activated crumb rubber digestion.Nevertheless,the ductility and elastic recovery of ACRMA was independent of swelling time in processing (Jeong et al.2010;Gawel et al.2006).Therefore,the swelling time of ACRMA must be controlled in a suitable range (1∼2h).ConclusionsThe activated method could destroy the crossing-linked network structure of crumb rubber in a close system without pollution gas or water.The activated crumb rubber was used as a modifier for asphalt to modify asphalt,which achieved the favorable proper-ties of ACRMA and the substitution of SBS.The combined modi-fication with activated crumb rubber and certain additives was an effective method to improve the performance of ACRMA.The influential factors on performance include:crumb rubberstructure,activated crumb rubber content,processing temperature,and swelling time.Based on laboratory investigations,single factor analyses on the processing of ACRMA were carried out in this study.Herein,the topgallant activated crumb rubber content was 25%of the weight of virgin asphalt,the optimum processing temperature was 180°C,and the adequate swelling time was 2h.Thus,ACRMA possessed excellent performance including high-temperature stability,low-temperature stiffness,deformation re-sistance,elastic recovery,viscosity,and ductility.The ACRMA separation could completely meet the standard of modified asphalt and applied in pavement 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C o p y r i g h t A S CE .F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .。