J. Cent. South Univ. (2020) 27: 3888-3898

DOI: https://doi.org/10.1007/s11771-020-4578-5

Laboratory investigation on effects of organic montmorillonite on performance of crumb rubber modified asphalt

TAN Zhi-hua(谭志华)¹, WANG Ji-jing(王觊婧)², SHI Zhen-ning(史振宁)²

1. School of Physical Education, Changsha University of Science & Technology, Changsha 410004, China;

2. School of Traffic & Transportation Engineering, Changsha University of Science & Technology,Changsha 410004, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: In this paper, organic montmorillonite (OMMT) was added into crumb rubber modified asphalt（CRMA） to improve its high temperature performance, anti-aging performance and storage stability. The effects of different OMMT content on properties of CRMA were studied. The rutting factor obtained by dynamic shear rheological (DSR) test was adopted to evaluate the high-temperature performance. The creep stiffness and m value determined by the bending beam rheometer (BBR) test were employed to evaluate the low-temperature performance. The softening point, ductility, rutting factor before and after rolling thin film ovens test (RTFOT) and pressure aging vessel test (PAV) were compared to characterize the aging properties. Moreover, the segregation test after being reserved for 48 h and 7 d was conducted, and the softening point and rutting factor of upper and lower layers of segregation pipe were adopted to evaluate the storage stability. The results indicated that the high-temperature performance and anti-aging performance were developed with the increasing content of OMMT, while the low-temperature performance deteriorated. The storage stability was improved with the increasing content of OMMT before the content exceeded 4%, after which the storage stability declined. Taking account of all factors, it is suggested that the optimum content of OMMT is 3%-4%.

Key words: crumb rubber modified asphalt; organic montmorillonite; high-temperature performance; low-temperature performance; storage stability.

Cite this article as: TAN Zhi-hua, WANG Ji-jing, SHI Zhen-ning. Laboratory investigation on effects of organic montmorillonite on performance of crumb rubber modified asphalt [J]. Journal of Central South University, 2020, 27(12): 3888-3898. DOI: https://doi.org/10.1007/s11771-020-4578-5.

1 Introduction

The production of tires increases dramatically with the booming of traffic. Consequently, the number of abandoned tires increases, which causes a series of problems to the ecological environment. Waste tire rubber powder mainly contains styrene-butadiene rubber, isoprene rubber, cis-butadiene rubber and other polymers [1, 2]. Such polymers are difficult to be decomposed in the natural state, and cannot be recycled to re-manufacture tires. They will be placed directly in the natural world, causing great “black pollution” to the environment and ecology. However, after the special treatment of tires in asphalt mixture by highway researchers [3], not only is the ecological environment problems treated, but the pavement performance of asphalt is improved, and the construction cost of asphalt pavement is reduced. A lot of research results have been obtained, which contributes to the application of waste rubber powder modified asphalt. The compatibilizer was added to the modified asphalt of waste rubber powder after it was treated by AIREY with hydrogen peroxide [4]. It was found that the high temperature performance and thermal stability of modified asphalt of waste rubber powder were better. WEN et al [5] found that the larger the specific surface area of waste rubber powder particles, the rougher the surface of waste rubber powder particles, and the higher the elastic recovery of modified asphalt.

Although the properties of crumb rubber modified asphalt are superior to the base asphalt, there are still some problems [6-8]. Waste rubber powder dispersed in matrix asphalt as dispersed phase is in suspension state. It cannot be dissolved in asphalt to achieve real modified state [9-10]. It also cannot be well stored, resulting in segregation state easily [11, 12]. And its high temperature performance and thermal stability are poor [13]. To solve above all, many researchers [13-16] have carried out the compound modification of waste rubber powder to build a “bridge” between the base asphalt and waste rubber powder, so that the waste rubber powder modified asphalt has better road performance. Among many compound modifiers, nano-materials can improve the performance of asphalt on micro-scale by their unique micro-size effect. The mentioned problems of polymer modified asphalt above can be solved effectively by using nano-materials and polymer modifiers to prepare compound modified asphalt [17]. As a layered silicate nano-clay material, montmorillonite is one of the research hotspots of nano-polymer compound modified asphalt [18]. Montmorillonite can not only make the polymer embed into the interlayer to form the embedded nano-material, but also make the sheet layer evenly disperse in the polymer to form the laminated nano-composite material. When montmorillonite used for asphalt modification, it is usually two kinds of structures: intercalated nano-composite structure (mostly inorganic montmorillonite) and exfoliated nano-composite structure (mostly organic montmorillonite). In these two structures, the layer of montmorillonite dispersed in the asphalt will restrict the movement of the asphalt molecular chain to a certain extent, so as to improve the performance of the asphalt [19-21].

In this paper, organic montmorillonite (OMMT) was used as a compound modifier to modify the crumb rubber modified asphalt. The high- temperature performance, low-temperature performance, rheological property, anti-aging performance and thermal stability of compound modified asphalt were tested.

2 Experimental

2.1 Material

PG70 asphalt, produced by Dongguan Taihe Asphalt Co. Ltd., was selected in the test, and its main technical properties met the specifications, as shown in Table 1. The organic montmorillonite was supplied by Fenghong Clay Chemical Factory, Zhejiang, China [22]. It has a cation exchange capacity (CEC) of 130 meq/100 g, maximum particle size of 120 nm, specific surface area of 750 m²/g, and its diameter-thickness ratio is more than 200.

The penetration index and a series of road performance indicators, including Brookfield viscosity, high-temperature performance, low- temperature performance, rheological property and anti-aging performance, of crumb rubber modified asphalt, styrene-butadiene-styrene block copolymer modified asphalt and matrix asphalt were tested and compared. The results are shown in Table 2. According to the test results of the three indicators, all the indicators of modified asphalt met the JTG F40-2004, technical specifications for construction of highway asphalt pavements [23] requirements.

2.2 Preparation of specimens

The instrument used for preparing crumb rubber asphalt is high-shear Mulser MBE-10, following specifications JTG E20-2011, T0602- 2011, standard test methods of bitumen and bituminous mixtures for highway engineering [24]. The preparation process is shown in Figure 1. After the preparation, the sample was placed in a constant temperature oven for heat preservation for subsequent tests.

Table 1 Main technical properties of PG70 asphalt

Table 2 Performance indexes of matrix asphalt, crumb rubber modified asphalt and SBS modified asphalt

Figure 1 Flowchart of preparation of asphalt modified by organic montmorillonite OMMT and crumb rubber

2.3 Road performance tests

The effects of different OMMT content on properties of CRMA were studied. In order to evaluate the high-temperature performance of asphalt binder, Brookfield viscosity and DSR tests were implemented. The creep stiffness and m value determined by the BBR test were employed to evaluate the low-temperature performance. The softening point, ductility, rutting factor before and after RTFOT and PAV were compared to characterize the aging properties. Moreover, the segregation test after being reserved for 48 h and 7 d was conducted, and the softening point and rutting factor of upper and lower layers of segregation pipe were adopted to evaluate the storage stability.

The Brookfield viscosity test was adopted to determine the apparent viscosity of asphalt. The test temperature was 135 ℃. For the used instrument, the control accuracy of temperature was 1 °C, the sub-model was 28, and the rotational speed was 20 r/min. The Brookfield viscosity test was carried out on different content of organic montmorillonite in compound modified asphalt.

The purpose of DSR test was to reveal the viscoelastic characteristics of the asphalt binder, following specifications JTG E20-2011, T0628- 2011, standard test methods of bitumen and bituminous mixtures for highway engineering. The test temperature was selected to be 52, 58, 64, 70 ℃, and the rotational speed was (10±0.1) rad/s. DSR test was carried out for the compound modified asphalt with different content of organic montmorillonite; the control mode was strain- controlled and the value was 12%.

BBR can better simulate the low-temperature cracking of asphalt pavement, so the BBR test was implemented to analyze the anti-deformation ability of asphalt binder at low temperature. The test temperatures were -12, -18 and -24 ℃, and the incubator solution was anhydrous ethanol. The reference specifications were JTG E20-2011, T0627-2011, standard test methods of bitumen and bituminous mixtures for highway engineering.

RTFOT and PAV were used to study the short-term and long-term anti-aging performance of asphalt, and the evaluation indicator was aging index A_I. The RTFOT reference specifications were JTG E20-2011, T0610-2011, standard test methods of bitumen and bituminous mixtures for highway engineering; the test temperature was 163 ℃, the holding time was 5 h, and the rotating speed was 5.5 r/min. The PAV reference specifications were JTG E20-2011, T0630-2011, standard test methods of bitumen and bituminous mixtures for highway engineering; the test temperature was 100 ℃, the holding time was 20 h, the pressure value was 2.1 MPa, and the test sample was the residue after RTFOT.

The segregation test was selected to evaluate the compatibility of modifiers and pure asphalt binders. The thermal stability was carried out by statically storing the asphalt in an environment with a constant temperature of 163 ℃, and the storage time was 48 h and 7 d. After being taken out, it was separated by a separation tube; after being frozen in a refrigerator, segregation pipe upper (top) and (bot) asphalts were poured and tested. The reference specifications were: JTG E20-2011, T0661-2011, standard test methods of bitumen and bituminous mixtures for highway engineering.

3 Results and discussion

3.1 Basic performance index

The penetration, ductility and soft point of different compound modified asphalts were fitted as shown in Figure 2.

In Figure 2, S is the softening point value; P is the penetration; D is the ductility value; x is the content of OMMT. As shown in Figure 2, as the content of organic montmorillonite increases, the penetration of compound modified asphalt decreases, the softening point increases and the ductility decreases. When the content of organic montmorillonite exceeds 4%, the softening point decreases. Based on the purpose of improving the high-temperature performance of asphalt in this paper and considering the variation trend of the three indicators with the dosage, it was considered that the content of organic montmorillonite is between 3% and 4%. It could be seen from the Brookfield viscosity test that the viscosity of the crumb rubber modified asphalt was much larger than that of the matrix asphalt or the SBS modified asphalt, and the crumb rubber modified asphalt was high-viscosity asphalt.

Figure 2 Relationship between content of OMMT and three indicators of OMMT-crumb rubber compound modified asphalt

It could be found from the high-temperature performance test that the addition of crumb rubber reduced the rutting factor of asphalt, which makes the high-temperature performance of asphalt became worse.

When the three major asphalts were at the same temperature, the m value of the crumb rubber modified asphalt was greater than that of the matrix asphalt or SBS modified asphalt, and the stiffness modulus was less than that of the two. It could be observed that the low-temperature performance of the crumb rubber modified asphalt was better than that of the matrix asphalt or SBS modified asphalt. Compared with SBS modified asphalt, the anti-aging property of crumb rubber modified asphalt was still inadequate.

In summary, the addition of crumb rubber could effectively improve the low-temperature performance of the matrix asphalt, but its high-temperature performance, thermal stability and anti-aging performance were weaker than SBS modified asphalt. In this paper, considering various factors, organic montmorillonite was used to modify the crumb rubber modified asphalt to improve its high-temperature performance, thermal stability and anti-aging performance, and the best dosage of organic montmorillonite was obtained.

3.2 High-temperature performance

1) Viscosity

The test results of viscosity of compound modified asphalts are shown in Figure 3.

Figure 3 Effect of content of OMMT on Brookfield viscosity of asphalt modified by OMMT-crumb rubber (B is the Brookfield viscosity value; x is the content of OMMT)

It can be seen from Figure 3 that as the content of organic montmorillonite in the compound modified asphalt increased, the Brookfield viscosity of the asphalt increased dramatically. This indicated that the introduction of organic montmorillonite increased the friction coefficient inside the asphalt and the resistance of the rotor during rotational shearing, resulting in an increase in Brookfield viscosity. The higher the viscosity of asphalt is, the higher the mixing and compacting temperature is. When the content of organic modified montmorillonite increased, the viscosity of the compound modified asphalt also increased, while the rheological property deteriorated, and the workability dropped further. Therefore, the content of organic montmorillonite should be controlled in a certain range.

2) Rutting factor

Rutting factor was used as an indicator to judge the high-temperature performance in DSR test, which is calculated by the measured complex modulus G* and phase angle δ, that is, the rutting factor=. The test results are shown in Figure 4.

Figure 4 Relationship between rutting factor and content of OMMT (y₁ is rutting factor value at 52 °C; y₂ is rutting factor value at 58 °C; y₃ is rutting factor value at 64 °C; y₄ is rutting factor value at 70 °C; x is content of OMMT)

The larger the rutting factor, the better the high-temperature performance. At the same temperature, with the increase of the content of organic montmorillonite, the rutting factor also increased, which indicated that the incorporation of organic montmorillonite could improve the high-temperature performance of the crumb rubber modified asphalt, and the higher the dosage is, the better the high-temperature performance is. However, after the dosage exceeded 3%, the improvement of high-temperature performance was small. Therefore, the content of organic montmorillonite of 3% to 5% improved the high- temperature performance of asphalt more effectively.

3.3 Low-temperature performance

The creep stiffness S of the asphalt beam at 60 s is calculated according to Eq.(1).

                           (1)

where S(t) is the creep stiffness at time t; P is the constant load, here P=988 mN; L is the beam fulcrum spacing, here L=102 mm; b is the beam width, here b=12.5 mm; h is the beam thickness,here h=6.25 mm; δ(t) is the real-time deflection, here t=60 s.

The BBR test was carried out on different organic montmorillonite in compound modified asphalts as shown in Figure 5.

Figure 5 Effects of content of OMMT on creep stiffness and m value

1) At the same temperature, with the change of the content of organic montmorillonite, the changes of the creep stiffness and m value were significantly different. At -12 ℃, as the content of organic montmorillonite increased, the creep stiffness also increased, while the m value showed an irregular wave shape. At -18 ℃ and -24 ℃, creep stiffness and m value fluctuated with the increase of organic montmorillonite.

2) Creep stiffness and m value were used as indicators for evaluating low-temperature performance. The greater the creep stiffness is, the greater the temperature-shrinkage stress generated on the pavement is and the more the cracks generated are. The smaller the value of m, the lower the stress relaxation, the lower the slope of the asphalt stiffness curve, the worse the deformation ability of the asphalt at low temperatures, and the easier it was to crack at low temperatures. Based on the changes of creep stiffness and m value with the content of organic montmorillonite at three temperatures, it could be found that when the content of organic montmorillonite was 3%, there was a value other than the partial value, and the m value reached the maximum. The point where the creep stiffness got the smallest value was the best combination of low-temperature performance at three temperatures.

3.4 Aging characteristic performance

The test results of softening point, ductility and rutting factor before and after aging (including RTFOT and PAV) were used to evaluate the short-term and long-term aging performance of asphalt. The aging index (A_I) was defined as the ratio of original asphalt performance to asphalt performance after aging.

The A_I results of different dosages of organic montmorillonite compound modified asphalt are shown in Figure 6.

1) Compared with the original asphalt, the asphalt after RTFOT has no obvious change in softening point and ductility. The asphalt after PAV had obvious softening point and ductility compared with the original asphalt, and the increase of softening point was smaller with the increase of the content of organic montmorillonite.

2) The aging index is used as an indicator to judge the anti-aging performance of asphalt. The closers the value to 1, the better the anti-aging performance of asphalt. With the increasing of the content of organic montmorillonite, the aging index of softening point also increased, closer to 1. The aging index of crumb rubber modified asphalt was 3.273, and the aging index of SBS modified asphalt was 1.572. The aging index of compound modified asphalt decreased with the increasing of dosage, and the larger the dosage is, the closer the aging index is to that of SBS modified asphalt. With the increase of dosage, the aging index curve of rutting factor is basically the same at different temperatures. This eliminates the temperature interference, and the aging factor of the rutting factor after RTFOT and PAV will continue to approach 1 when the dosage was increasing. At the same time, combined with the crumb rubber modified asphalt PAV aging index and the SBS modified asphalt PAV aging index listed above, it could be found that when the content of organic montmorillonite was 3%, its aging index and that of SBS modified asphalt very are close. When the dosage exceeds 3%, its aging index was between 0.5 and 1, and the anti-aging performance was better than that of SBS modified asphalt.

Figure 6 Relationship between aging index of ductility, softening point and content of OMMT (a), RTFOT aging index of rutting factor and content of OMMT (b), PAV aging index of rutting factor and content of OMMT (c)

In summary, the effect of organic montmorillonite with the content of 3% to 5% was better.

3.5 Storage stability

The thermal stability test results of compound modified asphalts are shown in Figure 7.

1) At the top of the 48 h storage tube, the softening point of the asphalt increased as a whole, and the softening point of the bottom asphalt decreased slightly. The softening point of the asphalt at the top of the 7 d storage tube and the softening point of the bottom asphalt both increased. At the four test temperatures, the top asphalt rutting factor and the bottom rutting factor of the 48 h storage tube showed a wave shape, and the top rutting factor and bottom rutting factor of the 7 d storage tube showed an upward trend.

2) The content of organic montmorillonite in compound modified asphalt increased. At each temperature, the softening point and rutting factor curve of the top and bottom asphalt of the two types of storage tubes gradually approached. The difference between softening point and rutting factor gradually decreased and separation was getting smaller, indicating that the improvement of thermal stability was better.

3) The minimum difference between the softening point of the top and bottom asphalt of the 48 h and 7 d storage tubes occurred at 5% and 3% of the organic montmorillonite, respectively. For the difference between the top and bottom rutting factors of the 48 h and 7 d storage tubes at the four test temperatures, the minimum values occurred when the contents of organic montmorillonite were 4% and 3%, respectively. Therefore, the content of organic montmorillonite of 3% to 4% improves the thermal stability of asphalt.

3.6 Optimal dosage of OMMT

The technical standards of crumb rubber modified asphalt are shown in Table 3. According to the above test results, the softening point of five contents of OMMT can meet the requirement of Class III. And when the OMMT content is 2% to 5%, the compound modified asphalt can meet the requirement of Class II. If the requirement of Class I is needed to be achieve, that is, the soft point reached 60 °C, the OMMT content of the modified asphalt should be 3%-4%. As to the ductility measured at 5 °C and viscosity, the three grades require a value of more than 10 cm, and the viscosity should be within 1.5-7.5 Pa·s. All the ductility values of compound modified asphalts containing OMMT met the requirements of Classes I, II, and III. In terms of viscosity, only the value of compound modified asphalt with OMMT content of 5% exceeded the specification. In terms of anti-aging performance, the compound modified asphalt with OMMT content higher than 3% met the specification requirements. Considering the high-temperature performance and low-temperature performance comprehensively, the best content of OMMT recommended to prepare compound modified asphalt is 3%-4%.

Figure 7 Relationship between storage softening point at 48 h and 7 d and content of OMMT (a), storage rutting factor at 48 h (b) and storage rutting factor at 7 d (c) and content of OMMT

Through the three indicators tests, Brookfield viscosity, DSR, RTFOT, PAV, BBR and other orthogonal test research, the experimental data were analyzed. The optimal dosage of OMMT was 3%-4%. The indicators of the compound modified asphalt under the optimum dosage (3%) are compared with those of the matrix asphalt, SBS modified asphalt and crumb rubber modified asphalt. The results are shown in Table 4. Based on the previous research and the data in Table 4, following conclusions have been drawn.

1) Comparing the performance of crumb rubber modified asphalt with SBS modified asphalt, it was found that the low-temperature performance of crumb rubber modified asphalt was superior to that of SBS modified asphalt. The high-temperature performance and anti-aging performance were worse than SBS modified asphalt, and the viscosity was larger than that of SBS modified asphalt, so the organic montmorillonite compound modification was applied to the crumb rubber modified asphalt.

2) At the same temperature, as the content of organic montmorillonite increased, the rutting factor became larger. As the content of organic montmorillonite increased, the softening point also increased. Therefore, it could be explained that as the content of organic montmorillonite increases, the high-temperature performance of the asphalt becomes better.

3) At -12, -18, and -24 °C, the creep curves of organic montmorillonite-crumb rubber compound modified asphalt with different contents were inconsistent after stress. However, as the content of organic montmorillonite increased, the low temperature performance deteriorated.

4) The higher the content of organic montmorillonite in compound modified asphalt, the greater the viscosity of the compound modified asphalt.

Table 3 Technical standard of crumb rubber modified asphalt

Table 4 Performance indexes of asphalt, crumb rubber modified asphalt, SBS modified asphalt and OMMT-crumb rubber modified asphalt

5) The higher the content of organic montmorillonite in compound modified asphalt, the closer the aging index to 1, the better the anti-aging performance. The aging index was between 0.5 and 1 when the dosage was higher than 3%, and the aging index had exceeded that of SBS modified asphalt.

6) The smaller the degree of segregation at 48 h, the smaller the degree of segregation at 7 d, and the greater the degree of segregation when The content of organic montmorillonite in compound modified asphalt increased, was higher than 4%.

7) The introduction of organic modified montmorillonite into compound modified asphalt can significantly improve high-temperature performance, anti-aging performance and storage performance, but the improvement of these properties was based on the premise of lowering low-temperature performance and increasing rheological properties. Therefore, the content of organic montmorillonite was recommended to be 3% to 4%.

4 Conclusions

In this paper, the organic montmorillonite (OMMT) was used as a compound modifier to modify the crumb rubber modified asphalt. The effects of content of OMMT on the road performance of crumb rubber modified asphalt were studied. Some conclusions could be drawn as follows.

1) The high-temperature performance and anti-aging performance of compound modified asphalt was developed with organic montmorillonite, while the low-temperature performance deteriorated. Thus, the content of OMMT should be controlled.

2) The storage stability improved with the organic montmorillonite when the addition of OMMT is in a certain range of content. However, when the content of OMMT was over 4%, the storage stability became worse, which might be due to the hard dispersion of OMMT.

3) Taking account of all factors, it is suggested that the optimum content of organic montmorillonite is 3% to 4%. In further study, the micro-experiments will be done to explain the microstructural change in the OMMT-asphalt system upon adding OMMT and analyze the modification mechanism of OMMT.

Contributors

TAN Zhi-hua and WANG Ji-jing came up with the frame and designed the experiments; All authors carried out the experiments; TAN Zhi-hua did the original draft preparation; TAN Zhi-hua, WANG Ji-jing and SHI Zhen-ning reviewed and edited the paper. All authors have read and agreed to the published version of the manuscript.

Conflict of interest

The authors declare no conflicts of interest.

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(Edited by YANG Hua)

中文导读

基于室内试验有机蒙脱土对胶粉改性沥青性能的影响

摘要：本文将有机蒙脱土掺入胶粉改性沥青中，以期提高沥青的高温性能、抗老化性能及储存稳定性能，并研究不同有机蒙脱土掺量对胶粉改性沥青性能的影响。通过开展动态剪切流变试验测车辙因子、弯梁流变仪试验测劲度模量及蠕变速率、长期及短期老化前后3个指标的变化情况，来研究不同有机蒙脱土掺量对胶粉改性沥青高温性能的影响，48 h及7 d离析试验来综合评价有机蒙脱土及其掺量对路用性能及储存稳定性能的影响。试验结果表明：有机蒙脱土的掺入提高了胶粉改性沥青的高温性能与抗老化性能，且随着有机蒙脱土掺量的增加，改善效果也越明显。但有机蒙脱土的掺入降低了胶粉改性沥青的低温性能。当有机蒙脱土掺量不高于4%时，改性沥青的储存稳定性得到显著改善。综合各方面性能，建议有机蒙脱土掺量控制在3%~4%。本研究成果可助力废旧橡胶(如橡胶跑道、废旧轮胎)的绿色处理。

关键词：胶粉改性沥青；有机蒙脱土；高温性能；低温性能；储存稳定性

Foundation item: Projects(51838001, 51878070, 51908069) supported by the National Natural Science Foundation of China

Received date: 2019-12-09; Accepted date: 2020-10-10

Corresponding author: WANG Ji-jing, PhD Candidate, Lecturer; Tel: +86-13755040336; E-mail: wangjijing@csust.edu.cn; ORCID: https://orcid.org/0000-0002-3950-1262