Construction and Building Materials 23 (2009) 3586–3590

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Construction and Building Materials

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Technical Note

Microstructure and performance of crumb rubber modified asphalt

Li Xiang a, Jian Cheng b,*, Guohe Que a

a State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, PR Chinab Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, PR China

a r t i c l e i n f o

Article history:Received 6 March 2009Received in revised form 15 May 2009Accepted 19 June 2009Available online 19 July 2009

Keywords:Crumb rubberModified asphaltMicrostructurePerformance

0950-0618/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2009.06.038

* Corresponding author. Tel.: +86 13886032109.E-mail address: wuhancengjian@163.com (J. Chen

a b s t r a c t

Crumb rubber modified asphalt (CRMA) was prepared by matrix asphalt preblending process (MAPP)which uses matrix asphalt as swelling agent in this study. Effects of the proportion of preblending matrixasphalt and properties of crumb rubber on performance of CRMA have been investigated. Scanning elec-tronic microscope (SEM) and fluorescence microscope were adopted to characterize the microstructuresof crumb rubber powder and CRMA, respectively. The experimental results indicated that the CRMA havebetter performance compared with matrix asphalt; the properties of crumb rubber display significanteffects on the performance of CRMA, performance of CRMA are improved with the decrease of ash contentand the increase of acetone extract. SEM graphs of crumb rubber pre and post preblending demonstratedthat the morphological characteristic of preblended crumb rubber powder is obviously different fromrubber powder without preblending, and the properties of rubber elastomeric are recovered andimproved significantly. Micrographs of CRMB prepared under different conditions showed that comparedwith CRMB prepared by the traditional preparation process, the scattering state of rubber powder andmatrix asphalt in CRMB prepared by MAPP is excellent, even and perfect polymer reticular structure isformed in the sample system.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Petroleum asphalt has been widely used in highways, airports,construction and other fields [1]. However, because of a numberof disadvantages, such as crack prone in low temperature, pooraging resistance and fatigue resistance, it cannot meet high-speedand heavy load of modern traffic on the road [1,2]. Therefore, themodified asphalt has become a research focus in recent decades[3–6]. Using waste crumb rubber powder as a modifier for modi-fied asphalt, on the one hand, can recycle waste rubber to easepressure on the environment [1,7,8]; on the other hand, can im-prove the penetration index, flexible, low-temperature ductility,deformation resistance performance of asphalt and enhance the as-phalt stability at high temperature, crack resistance at low temper-ature and fatigue resistance [4,9,10]; furthermore, driving comfortand safety can be improved and traffic noise can be reduced [2].Compared with unmodified asphalt roadway, the service life ofcrumb rubber modified asphalt (CRMA) roadway will be prolonged[11–13]. In addition, using crumb rubber as modifier instead ofexpensive polymer modifier (SBS, SBR, etc.) can also reduce the

ll rights reserved.

g).

cost of road asphalt. As a result, CRMA has attracted a close atten-tion of many countries in the world [14].

A large number of studies have shown that there are many fac-tors affecting the modified effects on CRMA, including the content,particle size and type of crumb rubber powder, and chemical/phys-ical properties of matrix asphalt, as well as its source, and modifiedasphalt processing technology and so on [4,15–18]. The authorshad used fluid catalytic cracking oil slurry with a high content ofaromatic component as swelling agent which promotes the crumbrubber to activate to prepare CRMA through traditional wet prep-aration process. According to the conventional performance test,the properties of CRMA did not achieve the desired results. Norwere the properties of CRMA which were obtained from crumbrubber with pretreatment of microwave irradiation and matrix as-phalt. Then, a new process named as matrix asphalt preblendingprocess (MAPP) has been proposed to produce CRMA in this study.Matrix asphalt is used as swelling agent instead of oil slurry in thenew process.

In this study, asphalt Zhonghai AH-70 was modified with crumbrubber through MAPP. Based on the performance of CRMA, effectsof the proportion of preblending matrix asphalt and properties ofcrumb rubber have been investigated. Furthermore, the micro-structures of crumb rubber powder and CRMA have been observedthrough scanning electronic microscope (SEM) and fluorescencemicroscope, respectively.

L. Xiang et al. / Construction and Building Materials 23 (2009) 3586–3590 3587

2. Materials and experimental

2.1. Materials

The selected matrix asphalt was AH-70, whose penetration (at 25 �C) was6.73 mm, softening point was 48.4 �C, and ductility (25 �C) was more than150 cm. The general crumb rubber was tyre crumb rubber from Huangshi SecondRubber Factory, China. The size of crumb rubber was 40–80 mesh. Several kindsof modifying agents included stabilizer and activator.

2.2. Preparation of CRMA

Preparation of CRMA was conducted by MAPP. Mixture of activator, crumb rub-ber and matrix asphalt with different proportions preblended at 160 �C for 1.5 h.After preblending, a given amount of fresh matrix asphalt was added to the mixture.The new mixture was high-speed shearing and stirring with 7000 r/min for 1 h at180 �C. Then, adding 1‰ stabilizer by the total content of the mixture, the CRMAwas made by breeding the mixture with 3000 r/min for 30 min at 160 �C.

2.3. Measurement of properties of CRMA

The softening point, penetration, ductility, elasticity recovery, separation testand thin-film oven test of matrix asphalt and CRMA were measured according toStandard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineer-ing (JTJ052-2000).

2.4. Observing of microstructure

The morphologies of crumb rubber powder pre and post preblending were ob-served by JSM-5510LV scanning electron microscopy (Hitachi Company, Japan).Prior to the observation, samples were pretreated by gold sputtering. The micro-structure of the matrix asphalt and CRMA were observed with a DMIRB fluores-cence microscope (Leica Company, German).

3. Results and discussion

3.1. Effects of the amount of preblending matrix asphalt

Performance analysis of CRMA prepared through MAPP wasconducted and the results were shown in Table 1 and Fig. 1. CRMAprepared by traditional preblending process was conducted bymixing with swelling agent and crumb rubber powder under cer-tain temperature for a period of time. Generally, swelling agentis oil slurry with high aromatic component. In this study, CRMAwas made by MAPP using matrix asphalt as swelling agent. Conse-quently, this process cannot only simplify the technological pro-cess, but also increase the blending efficiency between crumbrubber and matrix asphalt. Based on the importance of the prebl-ending process, CRMA was prepared by MAPP with changing theproportion of preblending matrix asphalt from 20% to 100 wt% ofmatrix asphalt binder, and the crumb rubber content was fixed25 wt% of matrix asphalt binder according to the previous work[19], activator content was 2 wt% of crumb rubber.

It was found that the aging properties of CRMA were good witha comparison with the standard of SBS modified asphalt and muchbetter than that of matrix asphalt from Table 1. The differences ofthe composition between matrix asphalt and CRMA determinedtheses results. Crumb rubber powder absorbed the light oil compo-nent of matrix asphalt in CRMA, which reduced the content of freeradicals; antioxidant and anti-ozone agent in crumb rubber raised

Table 1Aging properties of CRMA.

RTFOT reminder Matrix asphalt P2a P3 P4 P5

Mass loss (%) 0.07 �0.01 �0.03 �0.05 �0Penetration ratio (%) 69.2 79.3 83.3 87.4 92Ductility 5 �C (cm) 0 13.5 13.7 13.1 14

Preblending matrix asphalt: 20–100%, crumb rubber: 25%, activator: 2%, stabilizer: 1‰.a P2 :P2 represents the preblending matrix asphalt content was 20 wt% of total matrixb The standard is the standard of SBS modified asphalt.

the anti-oxidative capacity of CRMA, these two reasons contributeddirectly to improving of the anti-oxidative capacity of CRMA [20].Asphalt was oxidative and asphaltene content increased duringthe aging process. Added with crumb rubber, the content of stickyand elastic component could decrease and increase, respectively.Thus, the temperature sensitivity reduced and the high-tempera-ture performance of the CRMA after aging process were improved.

The conventional performance of CRMA prepared by MAPP wasshown in Fig. 1. As can be seen, a significant performance improve-ment of matrix asphalt was achieved through modifying. Each per-formance item displayed certain correlation with differentproportions of preblending matrix asphalt. Ductility at 5 �C pre-sented a minor reductive tend, which indicated that ductility at5 �C could be improved correspondingly in the presence of prebl-ending matrix asphalt. Softening point and elastic recovery firstlyincreased and then decreased with the increase of content of pre-blending matrix asphalt; nevertheless, penetration at 25 �C andseparation experiment softening point temperature differencefirstly decreased and then increased remarkably. The compatibleextent between crumb rubber and matrix asphalt enhanced signif-icantly under thermal energy after adding matrix asphalt. Crumbrubber delivered cohesive property and plasticity to CRMA [21].Those reasons contributed to increasing of softening point andelastic recovery and decreasing of penetration. However, with theincessant increase of the amount of preblending matrix asphalt,it displayed dilution effect of matrix asphalt on crumb rubber.Therefore, softening point and elastic recovery of CRMA starteddecreasing after increasing to a certain level, while penetrationstarted increasing. As to the overall trend, the appropriate prebl-ending matrix asphalt content was 40–60%.

3.2. Effects of crumb rubber properties

CRMA was prepared by MAPP with different types of crumbrubber and 50% preblending matrix asphalt. Basic properties of dif-ferent crumb rubber and performance analysis results of CRMAwere shown in Table 2 and Fig. 2.

As shown in Table 2 and Fig. 2, every performance item of 2-CRMA was optimal, especially softening point, penetration andelastic recovery, which suggested that the basic properties ofcrumb rubber display significant effects on the performance ofCRMA. The performance of 1-CRMA and 2-CRMA were close andboth were much better than 3-CRMA, which suggested that theacetone extract content played an important role in improving per-formance of CRMA. The higher content of the acetone extract iscorresponding to the higher content of resin-like (high fatty acid,steroid, etc.) in crumb rubber. Adding this type of crumb rubberto matrix asphalt, the resin-like could permeate through matrix as-phalt under thermal energy and mechanical force, while saturatesand aromatics in matrix asphalt could also wrap and swell macro-molecules in crumb rubber. Remarkable swelling and compatibil-ity were achieved through mutual infiltration of the componentsof crumb rubber and matrix asphalt, which improved obviouslythe performance of CRMA. The lower ash content of 2-crumb rub-ber may suggest the minor superiority of performance of 2-CRMA

P6 P7 P8 P9 P10 Standardb

.25 �0.04 �0.13 �0.03 �0.07 �0.17 ±1.0

.1 98.4 95.5 92.6 91.5 95.7 P65

.1 14.6 13.5 13.3 13.0 13.0 P15

asphalt. So were the other serial numbers.

Fig. 1. Effects of preblending matrix asphalt content on performances of CRMA.

Table 2Basic properties of different types of rubber powder.

Items 1 2 3

Ash content (%) 9.65 9.40 8.70Acetone extract (%) 13.35 12.97 9.41

3588 L. Xiang et al. / Construction and Building Materials 23 (2009) 3586–3590

comparing with 1-CRMA. Consequently, the ductility and cohesiveforce of 2-CRMA were better slightly, so were the ductility at lowtemperature and elastic recovery of 2-CRMA.

Aging property test of three different CRMA was conducted andthe results were shown in Table 3. It can be seen that CRMA had

Fig. 2. Effects of properties of rubber p

excellent aging properties, especially the penetration ratio, whichcould reach more than 90%. This could further illustrate the generalgood aging properties of CRMA.

3.3. Microstructure of crumb rubber

The micrographs of crumb rubber pre and post preblendingwere shown in Fig. 3. The interface of crumb rubber elastomerwas observed through SEM to find out effects of the preblendingof crumb rubber in matrix asphalt on the performance of CRMA.The interface of crumb rubber elastomer was clear and scatteredmany slot holes before preblending, which indicated poor compat-

owder on performances of CRMA.

Table 3Aging properties of different CRMA.

Items 1-CRMA 2-CRMA 3-CRMA Standarda

Mass loss (%) �0.13 �0.03 �0.09 ±1.0Penetration ratio (%) 96.4 96.2 90.4 P655 �C ductility (cm) 13.3 13.7 11.5 P15

Preblending matrix asphalt: 50%, crumb rubber: 25%, activator: 2%, stabilizer: 1‰.a The standard is the standard of SBS modified asphalt.

L. Xiang et al. / Construction and Building Materials 23 (2009) 3586–3590 3589

ibility and performance. Poor performance of CRMA would be ob-tained with directly mixing crumb rubber and matrix asphalt.However, the interface of crumb rubber with preblending treat-ment turned out to be indistinct, the slot holes disappeared, whichdemonstrated good compatibility and got good compatible matrixafter preblending treatment. The properties of crumb rubber elas-tomer were recovered and improved remarkably, and good perfor-mance of CRMA can be obtained with MAPP.

3.4. Microstructure of CRMA

Based on the main effects of the compatibility between matrixasphalt and modifier on the performance of CRMA, the morpholog-

Fig. 3. SEM micrographs of crumb rubber powder: (a) rubber powder witho

Fig. 4. Fluorescence microscope micrographs at 10 � 20 magnification of CRMA: (a) matrtechnology.

ical characteristics of matrix asphalt and CRMA prepared with dif-ferent conditions were examined by fluorescence microscope. Themicrographs were shown in Fig. 4, and the performances of CRMAin Fig. 4 were listed in Table 4.

As shown in Fig. 4, graphs of CRMA prepared through MAPP(Fig. 4b: CRMA with 25% crumb rubber, Fig. 4c: CRMA with crumbrubber and 2% SBS) exhibited well-dispersed punctate and floccu-lent structure, which could be interpreted by effective and efficientdegradation of crumb rubber in blending process under thermaland mechanical energy. Generative polymer segment homoge-neously dispersed in matrix asphalt and formed even and perfectpolymer reticular structure with the function of additives and sta-bilizers. Therefore, the performances (ductility at low temperature,softening point and elastic recovery, etc.) of CRMA were signifi-cantly improved.

Comparing graphs of Fig. 4b and c, it was found that graph ofCRMA with a small content of SBS distributed more uniformly.Corresponding items of performance (Table 4) were furtherimproved especially separation experiment softening point tem-perature difference, which was significant for storage and trans-portation of CRMA. Graph of CRMA prepared by traditional wettechnology (Fig. 4d) displayed disorderly morphological charac-teristics, which may be due to the less effective degradation of

ut preblending 3000� and (b) rubber powder with preblending 1000�.

ix asphalt; (b, c) CRMA prepared by MAPP and (d) CRMA prepared by traditional wet

Table 4Performances of matrix asphalt and CRMA.

Number 5 �C ductility (cm) Softening point (�C) 25 �C penetration (0.1 mm) PI Elastic recovery (%) Separation (�C)

a 6.0 48.4 67.3 �1.0 3 –b 18.4 57.0 66.4 1.0 65 1.9c 25.7 60.1 67.8 0.5 72 0.1d 11.5 49.8 71.1 �0.2 45 4.5

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crumb rubber and affected the improvement of performance ofCRMA.

4. Conclusions

Favorable properties of CRMA can be achieved through MAPP.The appropriate preblending matrix asphalt proportion in MAPPis 40–60% according to the variation tendency of CRMA perfor-mance. Effects of the properties of crumb rubber on the perfor-mance of CRMA are significant according to the analysis ofcharacteristics of crumb rubber. Performances of CRMA are im-proved with the decrease of ash content and the increase of ace-tone extract.

The morphological feature of crumb rubber pre and post prebl-ending has been observed through SEM. The results demonstratethat preblended crumb rubber powder is quite different from therubber powder without preblending; the properties of crumb rub-ber elastomeric are recovered and improved significantly, whichleads obvious improvement of the performance of CRMA. Fluores-cence microscope has been adopted to investigate the morpholog-ical characteristics of CRMB prepared with different conditions.Based on the micrographs, compared with the traditional prepara-tion process, the distribution state of rubber powder and matrixasphalt in CRMB prepared through MAPP is excellent. Fine and per-fect polymer reticular structure is formed in the sample system.

MAPP technology improves the compatibility of crumb rubberand matrix asphalt according to the morphological analysis ofcrumb rubber and CRMA. Good compatibility can be obtained withwell distribution of crumb rubber in matrix asphalt. In this study,CRMA prepared with using crumb rubber powder as a simplexmodifier can just achieve the I-C Standard of SBS Modified Asphalt.It is necessary to further improve the performance of matrix as-phalt with composite modification of crumb rubber powder andSBS which remains to be further studied.

Acknowledgements

This study was supported by State Key Laboratory of Heavy OilProcessing, China University of Petroleum, and the authors wouldlike to acknowledge their financial support.

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