The objective of this study is to develop a highly permeable rejuvenating agent for the recycling of the asphalt pavement. The rheological properties and permeability of recycled asphalt after adding the self-developed rejuvenating agent, as well as two other agents, were compared and evaluated. An improved softening point method was devised to evaluate the permeability. In addition, the recycled asphalt was analyzed using Fourier transform infrared spectroscopy (FTIR). The results showed that the self-developed rejuvenating agent had high permeability, could effectively restore the performance of the aged asphalt, and could improve the aging-resistant property of the recycled asphalt. FTIR analysis showed that the matrix asphalt experienced oxygen absorption and dehydrogenation during the aging process. The aging of the SBS-modified asphalt was achieved via dual aging of the matrix asphalt and SBS-modified components. In addition, the rejuvenating agent CA had an inhibitory effect on asphalt aging, and its recycling efficiency was better than that of the rejuvenating agent A for the aged SBS-modified asphalt. Finally, a relationship between the microscopic functional group index and the macroscopic test index was established.
Currently, the circular economy and green economy are being vigorously promoted during development of the world. The recycling of waste asphalt mixtures has attracted more and more attention because it is beneficial to environmental protection. Rejuvenating agents are used for recycling waste asphalt mixtures. Rejuvenating agents with good properties could be compatible with aged asphalt to improve its performance, thus extending the service life of the pavement [
Carpenter and Wolosick [
Most studies have focused on restoring a certain property of aged asphalt, the recycling effect, the mechanism, and the diffusion mechanism of rejuvenating agents, while fewer studies have investigated rejuvenating agents with high permeability. Meanwhile, numerous studies have investigated the macroperformances and microstructures of recycled asphalt, but the correlation between them has rarely been studied.
This research aimed to develop a highly permeable rejuvenating agent for asphalt pavement recycling. A rejuvenating agent was developed, and the rheological properties and permeability of the recycled asphalt after adding the agent were systematically investigated. The recycling mechanism was analyzed via FTIR. Two other rejuvenating agents were also used for comparison. The self-developed rejuvenating agent had high permeability, had an inhibitory effect on asphalt aging, and effectively restored the properties of aging asphalt. Finally, the recycling mechanism of recycled asphalt was qualitatively and quantitatively analyzed by FTIR, and a relationship between the microscopic functional group index and macroscopic test indicators was established to further reveal the recycling mechanism of the aged asphalt.
A highly permeable rejuvenating agent (CA) was developed for the hot recycling of the asphalt pavement in this study. Based on the concepts of adding missing components, repairing the aging structure, and improving permeability, our research team developed CA. The raw materials of CA are composed of base oil, a plasticizer, an antiager, a structure-repairing agent, and a penetrant, as listed in Table
Raw materials and dosages of CA.
Ingredient | Proportion (%) |
---|---|
Base oil X | 76.64 |
Plasticizer Y | 11.21 |
Antiager Z | 5.61 |
Structure-repairing agent K | 5.61 |
Penetrant J | 0.93 |
Because of adding the accelerating permeation component (Penetrant J) to CA, the mixing time and energy consumption in the mixing process are reduced, which makes it more environmentally friendly. Furthermore, CA merges with aging asphalt more quickly to form new asphalt structures in the mixing process, resulting in less volatile substances under the accelerated heating in the course of the mixing processes and field aging. The aromatic hydrocarbons in CA exist in the form of new asphalt components, so its thermal stability is greatly improved, which is close to the thermal stability of the components of original asphalt. Meanwhile, in the current laboratory research stage, the use of CA in recycling asphalt imposed less additional burden on the environment.
The other two rejuvenating agents included RA-100 (A) produced by Jiangsu and an ordinary rejuvenating agent (B) produced by Guangdong. #70 matrix asphalt and SBS-modified asphalt I-C were used in this study. The parameters of the matrix asphalt and SBS-modified asphalt are listed in Table
Main indicators of the matrix asphalt and SBS-modified asphalt.
Parameters | Matrix asphalt | SBS-modified asphalt I-C | Normative value | Measurements1 |
---|---|---|---|---|
Penetration (25°C, 100 g, 5 s) (dmm) | 66.7 | 67.2 | 60–80 | T0604 |
Ductility (5 cm/min) (cm) | >100 | 34.5 | ≥100 | T0605 |
Softening point (ring and ball) (°C) | 46.7 | 62.7 | ≥43 | T0606 |
Viscosity (135°C) (mPa·s) | 257.8 | 3100 | — | T0625 |
1The test methods described are provided in reference [
The extracted aged asphalt from the reclaimed asphalt pavement is generally impure, as it contains mineral powder and volatile trichloroethylene. Therefore, this approach cannot truly present the performance of actual aged asphalt, and it has a lower extraction efficiency. Considering the limitations of the extraction method, aged asphalt samples were prepared using the rotating thin film oven test (RTFOT) [
Dynamic rheological properties, which are chief indicators for evaluating asphalt pavement performance in the Strategic Highway Research Program (SHRP), are measured by DSR. The complex shear modulus (
The Anton Paar MCR 102 advanced rheometer was used for DSR tests. The shear strain control mode was applied in the tests. DSR tests were conducted to measure
Matrix asphalt and SBS-modified asphalt samples.
Samples | Asphalt types |
---|---|
1 | Original matrix asphalt |
2 | Aged matrix asphalt via RTFOT |
3 | Original SBS-modified asphalt |
4 | Aged SBS-modified asphalt via RTFOT |
5 | Aged matrix asphalt + CA (7 wt.%) |
6 | Aged matrix asphalt + A (7 wt.%) |
7 | Aged SBS-modified asphalt via RTFOT + CA (7 wt.%) |
8 | Aged SBS-modified asphalt via RTFOT + A (7 wt.%) |
The penetration and softening point methods are common methods for evaluating the macroscopic properties of recycled asphalt. The former is widely applied in testing the permeability of rejuvenating agents, but the latter is used less frequently. In this study, these two test methods were used to evaluate the permeability of the rejuvenating agents.
Test molds of the improved softening point method.
After cooling at room temperature, the residual plasticine on the test molds was scraped off and the molds were cut with a thin blade. Finally, the softening point of each sample was measured at three points, and the average value was taken as the result. In this study, H1, H2, H3, and H4 are assumed to be different penetration depths h1, h2, h3, and h4, respectively, and the softening point of each mold corresponds to that of different depths.
The contents of special functional groups in aged asphalt, such as carbonyl and sulfoxide groups, can be quantitatively analyzed by characteristic absorption peaks using Fourier transform infrared spectroscopy. The diffusion effect among the original asphalt, rejuvenating agents, and aged asphalt can also be investigated via FTIR-ATR [
Recent studies have shown that the degrees of aging and recycling of asphalt can be assessed by the specific gravity of the functional groups. An aliphatic functional group index (BI), an aromatic functional group index (AI), a hydroxyl functional group index (CI), and a sulfoxide functional group index (DI) were introduced to quantitatively analyze aging and recycling phenomena by calculating the areas of the relevant functional groups. The four functional groups represent the long-chain alkyl group, the benzene ring substitute, and the asphalt aging product content, respectively. The relevant calculations are expressed as formulas (
As shown in Figure
Schematic diagram of the functional group area at X cm−1.
The measured penetration, viscosity, ductility, and softening point of the samples are shown in Figures
Penetration of the samples.
Viscosity of the samples.
Ductility of the samples.
Softening point of the samples.
As shown in Figures
The results of
Variation in
As shown in Figure
Figure
As shown in Figure
Penetration values were tested after adding different rejuvenating agents to aged asphalt samples. The results are shown in Figure
Penetration curves of aged asphalt after adding rejuvenating agents at a certain holding time: (a) recycled matrix asphalt; (b) recycled SBS-modified asphalt.
As shown in Figure
The curves in Figure
Change curves of penetration of two regenerated asphalt samples.
Asphalt type | Rejuvenating agents | Fitting formula |
|
---|---|---|---|
Aged SBS-modified asphalt | B |
|
0.9795 |
A |
|
0.9822 | |
CA |
|
0.9157 | |
|
|||
Aged matrix asphalt | B |
|
0.9399 |
A |
|
0.9567 | |
CA |
|
0.912 |
The permeability was studied by testing the softening points of samples after adding different rejuvenating agents. The test results are plotted in Figure
Softening point curves of aged asphalt after adding rejuvenating agents at a certain penetration depth: (a) recycled matrix asphalt; (b) recycled SBS-modified asphalt.
Permeability curves featuring the improved softening point method are shown in Figure
Softening point fitting formulas.
Asphalt type | Rejuvenating agents | Fitting formula |
|
---|---|---|---|
Aged matrix asphalt | CA |
|
0.9617 |
B |
|
0.9554 | |
A |
|
0.9617 | |
|
|||
Aged SBS-modified asphalt | CA |
|
0.9647 |
B |
|
0.967 | |
A |
|
0.9626 |
Figure
FTIR spectra of original and aged matrix asphalt.
After aging, the vibration peaks of the carbonyl group and the benzene ring skeleton at wavenumbers 1640 cm−1 and 1456 cm−1 showed slightly increased intensities, and the trends moved toward lower wavenumbers. The other peaks showed little change in intensity. However, the overall transmittance increased, indicating that the amount of saturated and aromatic hydrocarbons decreased and the aging of asphalt was obvious.
Figure
Comparison of FTIR spectra of several matrix asphalt samples.
The infrared spectrogram of the SBS-modified asphalt is shown in Figure
FTIR spectra of original and aged SBS-modified asphalt.
As shown in Figure
Comparison of FTIR spectra of several SBS-modified asphalt samples.
If
Functional group indices for different states of asphalt.
Recycled asphalt | BI | AI | CI | DI |
---|---|---|---|---|
Matrix asphalt | 0.2342 | 0.03654 | 0.0067 | 0.0038 |
Aged matrix asphalt | 0.2243 | 0.04331 | 0.0983 | 0.0047 |
Recycled matrix asphalt + CA | 0.2431 | 0.02534 | 0.00591 | 0.0037 |
Recycled matrix asphalt + A | 0.2371 | 0.03893 | 0.00602 | 0.0035 |
SBS-modified asphalt | 0.2401 | 0.04663 | 0.02361 | 0.0039 |
Aged SBS-modified asphalt | 0.2324 | 0.05738 | 0.07907 | 0.0059 |
Recycled SBS-modified asphalt + CA | 0.2548 | 0.04269 | 0.01334 | 0.0036 |
Recycled SBS-modified asphalt + A | 0.2524 | 0.04476 | 0.03086 | 0.0035 |
The BI represents the content of long-chain alkyl groups, the AI represents the content of benzene ring substituents, and the CI and DI, respectively, represent the contents of hydroxyl- and sulfoxide-based functional groups of asphalt aging products. As listed in Table
The four functional group indices of the two types of recycled asphalt were close to those of the original asphalt and were significantly different from those of the aged asphalt. This indicated that the composition and structure of the aged asphalt changed after recycling. For instance, the increase in the AI indicated that the light components in the recycled asphalt increased. Therefore, it may be conducive to investigating the recycling mechanism by establishing a relationship between the functional group index and the macroscopic indicator of the recycled asphalt. According to Figures
Relationships between the functional group indices and penetration.
Relationships between the functional group indices and softening points.
Relationships between the functional group indices and ductility.
Relationships between various functional group indices and viscosity.
As shown in Figure
Figure
As shown in Figure
The relationship between viscosity and the index of regenerated asphalt is shown in Figure
From the above analysis, it is observed that the long-chain alkyl decreases after aging of the asphalt. This is because the long chain breaks under the action of environmental factors, and the resulting free radicals are easily oxidized. The free radicals combine with oxygen to form various types of functional groups at the breaking points. The free radicals could combine with free sulfur to form sulfoxides under the action of oxygen. The alkane at the break combines with elemental oxygen in the air to form a carbonyl-containing aldehyde and a ketone-like substance. After the long chain of the benzene ring is cleaved, the hydrogen atom in the external group is easily substituted by a macromolecule to form a benzene ring substituent. After the asphalt is recycled, the added light components replenish the long-chain alkyl group and inhibit the formation of free radicals; the macromolecular benzene ring substituents are cleaved and replaced by hydrogen atoms, which reduces the content of aromatic compounds. The C=O and S=O bonds generated by the aging process are reduced to the C=C bond, S-R bond, etc. Thus, the molecular weight is reduced. The recycling process is the reverse process of asphalt aging, and components of rejuvenating agents have important roles in asphalt recycling.
Compared to rejuvenating agents A and B, the penetration, softening point, viscosity, and ductility of the recycled asphalt with the rejuvenating agent CA could be recycled significantly, with performances close to those of the original asphalt. The DSR tests showed that the compound shear modulus ( The permeability performance of the rejuvenating agent could be evaluated by the penetration and improved softening point methods. Using these two methods, the rejuvenating agent CA was proved to be highly permeable. Fourier transform infrared spectroscopy showed that the matrix asphalt experienced oxygen absorption and dehydrogenation during the aging process. The aging of SBS-modified asphalt was the result of dual aging of the matrix asphalt and SBS-modified components. After incorporating rejuvenating agents, the components were supplemented. Rejuvenating agents CA and A had similar effects on the recycling of the matrix asphalt. The rejuvenating agent CA had an inhibitory effect on the aging of SBS-modified asphalt, and its recycling capacity was better than that of the rejuvenating agent A for the aged SBS-modified asphalt.
Because the data in this paper are still a project of the Fundamental Research Funds for the Central Universities of China, the data need to be used in the follow-up study of this project. So all the data (figures and tables) used to support the findings of this study are supplied by the corresponding author under license and cannot be made freely available. Requests for access to these data should be made to Yanjuan Tian, Key Laboratory for Special Area Highway Engineering of Ministry of Education, Chang’an University, Middle Section of South Second Ring Road, 710064, Xi’an, Shaanxi, China (Tel: +86-298-233-4846; email:
The authors declare no conflicts of interest.
This research was sponsored by the Fundamental Research Funds for the Central Universities (Grant No. 310821163502), the Transportation Department of Shandong Province (Grant Nos. Lujiaokeji [2017] 28 and 2013A01-01), the Xixian New District Management Committee of Shaanxi Province (Grant No. 2017 44), and the Science and Technology Bureau of Pingdingshan of China (Grant No. 2018610002000604). We express our gratitude to Mr. Hongyin Li for his support to this paper and many suggestions and help in the writing process.