Keywords

Introduction

The physical and chemical characteristics of asphalt binders get worse when suffer from the aging process during storage, mixing, transportation and in-service life [1, 2]. Aged asphalt binders is more brittle and stiffer than fresh asphalt and noticeable changes in its composition which is mainly caused by the decrease of aromatic oil and increase of asphaltene [3]. The existence of aged asphalt can result in serious properties deterioration of pavement [4, 5]. Therefore, rejuvenators are widely used to restore the properties of aged asphalt and have been achieved excellent effect [6, 7].

There are more and more reports about the influence of asphalt rejuvenator on the properties of aged asphalt with the development of asphalt recycling technology, and various types of asphalt rejuvenators are emerging. With the addition of the rejuvenators, the stiffness of aged asphalt can be lower and the cracking resistance be improved [8,9,10]. Therefore, the rheological properties of aged asphalt binder are effectively improved [11, 12]. However, most of asphalt rejuvenators are kinds of mixtures of resin compounds and synthetic oils in petroleum industry, which are non-renewable, harmful to human body and restricted by petroleum energy. It is necessary to explore a non-petroleum base substitute as asphalt rejuvenator, which is environmental, renewable, and harmless to the human body. Citrus peel is rich in pigment, aromatic oil, pectin, bioflavonoid, etc. These organic compounds can be extracted and are similar to asphalt rejuvenator.

This study investigated using the waste peel extract to recycle the aged asphalt binder. The extraction process was carried out by steam distillation in laboratory, and the constituents of waste peel extract were analyzed by techniques of gas chromatography-mass spectrometry (GC-MS). Penetration, softening point, viscosity of cone/plate rheometer method, dynamic shear rheometer method and bending beam rheometer method was test to examine the rejuvenation effect of waste peel extract.

Experimental

  • Raw Materials.

  • Laboratory Aged Asphalt Binders. Virgin asphalts binders, base70# and base90#, from Ezhou City, Hubei Province, China, was treated by thin film oven test (TFOT) to imitate the short-term aged process, then was aged by pressure aging vessel test (PAV) to imitate the long-term aged process.

The TFOT test and PAV test were performed in accordance with the standards ASTM D1754 and ASTM D6521. The 70# short-term aging asphalt and 90# short-term aging asphalt was marked as T1 and T2; the 70# long-term aging asphalt and 90# long-term aging asphalt was marked as P1 and P2; the virgin asphalt binders, base70# and base90# was marked as V1 and V2.

  • Waste Peel Extract. The waste peel, come from fresh citrus produced in Yi Chang City, Hubei province, China, was cut into pieces and mashed into the distillation flask, with the addition of appropriate amount of NH4Cl and was then subjected to steam distillation. The organic compounds with aromatic odor in waste peel was extracted and prepared for the following recycling process.

  • Recycled Binders. Four realistic ratios, 1, 3, 5 and 7% by mass of waste peel extract were chosen to recycle the 70# long-term aging asphalt binder and 90# long-term aging asphalt binder by using laboratory shear mixer.

In total, 2 virgin binders, 4 aged binders and 8 recycled binders were prepared and their codes are given in Table 1.

Table 1 Codes of different types of asphalt binders
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  • Methods. GC-MS, which is an analytical method that combines the features of Gas Chromatography and Mass Spectrometer to carried out a qualitative and quantitative analysis of the components in mixture, was conducted in this research to analysis the components of waste peel extract.

Empirical rheological properties (Pen. And S.P.) were measured according to ASTM D5 and ASTM D26.

The viscosity was determined at three temperature 60, 100, and 150 °C. A cone/plate rheometer, with a cone angle of 1° and plate diameter of 25 mm, was used to characterize the viscosity of virgin, short-time aged, and long-term aging asphalt binders, at a shear rate of 0.05 s−1 under 60 °C and at a shear rate of 500 s−1 under both 100 and 150 °C.

The dynamic shear rheometer tests were used according to AASHTO T315. The parameters of the dynamic shear rheometer test are given in Table 2.

Table 2 Parameters of dynamic shear rheometer test
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Low temperature creep response properties of base, aged and recycled asphalt were measured by bending beam rheometer following ASTM D6648. This test was performed at –12 °C and at a constant bending load of 978 mN.

Results and Discussion

  • Chemical Analysis of Waste Peel Extract. The total ion chromatogram of GC-MS is shown in Fig. 1. The corresponding organic compounds were retrieved according to NIST 05 standard spectrogram and shown in Table 3. A total of 21 main organic compounds were detected from the waste peel extract, and the relative content could be represented by the ratio of peak area of each component to the total peak area.

    Fig. 1
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    Total ion chromatogram of GC-MS

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    Table 3 The main chemical component of waste peel extract
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The major components of waste peel extract are monocyclic monoterpenes, the others are aromatic hydrocarbons containing benzene ring. Monoterpene is a kind of the terpenoids, which polymerized by isoprene, and their oxygenated derivatives and unsaturated derivatives. Monocyclic monoterpenes are obtained by chain monoterpenes cyclization, and are the main sources of aromatic odor in the waste peel. The aromatic hydrocarbons measured in this paper are mainly m-xylene and m-cymene, polycyclic aromatic hydrocarbons were not detected. The chromatogram peaks of aromatic hydrocarbons have a smaller abundance and fewer species compared with monoterpenes. And the chemical structure of m-xylene and m-cymene are more similar to the aromatic component of asphalt compared with monoterpenes. The chemical construction of the components of waste peel extract are similar to the light oil, which consist of saturates and aromatics and lost during the asphalt aging process.

  • Empirical Rheological Properties. The penetration and softening points of 70# and 90# base, aged and recycled asphalt binders are shown in Tables 4 and 5. As we can seen from Table 4, the penetration decreased from 60.1 to 39.4 (dmm), softening point increased from 47.8 to 53.1°C after a short-term aging process. Then after a long-term aging, the penetration reduced to 24.2 (dmm), softening point increased to 60.8 °C. This is because the light oil lost and the colloidal structure changed during the aging process, and resulted in hardened asphalt, reduced penetration and increased softening point. After adding the waste peel extract to the long-term aging asphalt, the penetration increased in a certain degree and the softening point decreased. When 5% of waste peel extract was added, the penetration and softening points can be fully restored to the level of base asphalt, which indicated that the waste peel extract can significantly improve the empirical rheological properties. As can be seen from Table 5, the change rule of empirical rheological properties of 90# asphalt are similar to those of 70# asphalt, which shows that the waste peel extract has similar effect on different types of asphalt. The waste peel extract can increase the penetration and reduce the soft point of aged asphalt. This is because that the chemical components of the waste peel extract are similar to saturates and aromatics, which can improve the aged asphalt from gel colloidal structure to sol colloidal structure.

    Table 4 Empirical rheological properties of 70# base, aged and recycled binders
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    Table 5 Empirical rheological properties of 90# base, aged and recycled binders
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  • Viscosities. The viscosity can reflect an ability of binders to resist shear deformation, and is one of the most relevant properties with mechanical behavior of asphalt pavement. The viscosity of 70# and 90# base, aged and recycled asphalt at 60, 100 and 150 °C was measured by a cone/plate rheometer and shown in Tables 6 and 7. Due to aging process, the viscosity significantly increased at each temperature. The viscosity of aged binder decreases with the addition of waste peel extract. After adding 7% of the waste peel extract, the value was fully restored to the level of base asphalt. We can found that there was a similar change rule in viscosity when using the 90# asphalt as the base asphalt. The viscosity of asphalt binders has a great influence on workability and compaction of asphalt mixture during mixing and paving. Aged asphalt can’t satisfy the performance requirements of binders because a very high viscosity caused by long-term ageing process, and the engineering application of asphalt mixture can be significantly affected. Therefore, it is necessary to use the rejuvenator to decrease the viscosity. The waste peel extract can restore the viscosity of aged binder, and the recycled asphalt binders can satisfy the requirement of viscosity of binders during the asphalt mixture mixing and paving process.

    Table 6 Viscosities of 70# base, aged and recycled binders at three temperatures
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    Table 7 Viscosities of 90# base, aged and recycled binders at three temperatures
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  • Master Curves. Asphalt is a kind of typical viscoelastic material. It produces both elastic deformation and viscous flow deformation under the state of force. The complex shear modulus of asphalt are used to reflect the ability to resist the shear deformation. It consists of two parts, an elastic recoverable part and a viscous unrecoverable part. Phase angle is the relative index of recoverable and unrecoverable deformation.

The master curves of 70# and 90# base, aged and recycled asphalts were shown in Figs. 2 and 3. The long-term aging asphalt has the highest complex modulus than others. The complex modulus of recycled asphalt decreased after adding 1% of the waste peel extract, but is still bigger than that of the short-term ageing asphalt. The value of recycled asphalt with 3% of the waste peel extract is similar to short-term ageing asphalt. When the content of waste peel extract reached 5%, the value almost restored to the level of base asphalt, and the value is far smaller than that of base asphalt with the content of 7%. The phase angle shows completely different rule. The long-term aging asphalt has the smallest phase angle and the value of recycled asphalt with 1, 3 and 5% of the waste peel extract have a litter enhancement. The recycled asphalt with the addition of 7% of the waste peel extract has a smaller phase angle than base asphalt. The results shows that the complex modulus of aged asphalt can be reduced with adding the waste peel extract. The recycled asphalt has a smaller phase angle when has the same complex modulus with base asphalt. This indicates that the recycled asphalt have the more obvious elastic properties than base asphalt, and is easier to recover the deformation after the load is removed. This will be beneficial for the ability to resist permanent deformation, and makes the asphalt binder have a excellent high temperature stability.

Fig. 2
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Master curves of 70# base, aged and recycled binders

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Fig. 3
figure 3

Master curves of 90# base, aged and recycled binders

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  • Low Temperature Cracking Resistance. The low temperature stiffness modulus S and creep rate m-value represent the ability of asphalt to resist the low temperature cracking. Bending beam rheometer can accurately evaluate the creep stiffness modulus S and creep rate m-value of asphalt at low temperature. The measured stiffness modulus and creep rate m-value at –12 °C are shown in Tables 8 and 9. Due to the aging of asphalt, the stiffness modulus increased significantly, the rate m-value decreased. The more serious aging happened, the more seriously the stiffness modulus S increased and the rate m-value decreased. After adding the waste peel extract to aged asphalt, the stiffness modulus decreased and the rate m-value increased gradually with the increase of the amount of the waste peel extract. When 5% of waste peel extract was added, the stiffness modulus S and creep rate m-value of the recycled asphalt were restored to the level of the base asphalt. Table 9 shows that there is a same change rule of low temperature properties between the 70# asphalt binder and the 90# asphalt binder when added with the waste peel extract. And the change rule of low temperature properties is similar to that of regular rheological properties, complex modulus and phase angle. This indicated that there is a good correlation between each rheological property indexes.

    Table 8 Stiffness and m-value of 70# base, aged and recycled binders
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    Table 9 Stiffness and m-value of 90# base, aged and recycled binders
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At low temperature, a very high creep stiffness modulus S caused by ageing process can lead to asphalt brittleness, which makes the pavement easy to crack, thus the service life of asphalt pavement reduce. A bigger m-value means a better low temperature performance of the asphalt. This is because the response of asphalt binders will be similar to that of the material with lower stiffness when the temperature drops and pavement contraction. Consequently, the tensile stress in the asphalt mixture decreases, thus the possibility of cracking at low temperature of asphalt pavement decreases. Otherwise, the asphalt pavement is more easily to crack with a smaller m-value. The temperature cracking of asphalt pavement is usually caused by constantly increases of stiffness modulus S and decreases of m-value during asphalt aging process. Tables 8 and 9 shows that the waste peel extract can obviously improve the ability of asphalt to resist the crack at low temperature, and thus improve the service life of asphalt pavement.

Conclusions

In this study, the organic compound in waste peel was extracted and analyzed in laboratory. Recycled asphalt binder was prepared by using the waste peel extract as the bio-rejuvenator, and the rheological properties of base, aged and recycled asphalt were investigated. Three main results are found:

  1. 1.

    The main chemical compositions of the waste peel extract are light oil that similar to the aromatic component of asphalt.

  2. 2.

    After adding the waste peel extract to long-term aging asphalt, penetration, softening point, viscosity, complex modulus, phase angle, low temperature stiffness modulus and m-value, can be effectively improved.

  3. 3.

    The rheological properties of recycled asphalt can be fully restored to the level of base asphalt with adding the waste peel extract as bio-rejuvenator.