In an article recently published in the journal Construction and Building Materials, researchers discussed the rheology, mechanism, and self-healing characteristics of asphalt treated with carbon nanotubes (CNTs).
Study: Mechanism, rheology and self-healing properties of carbon nanotube modified asphalt. Image Credit: OlegRi/Shutterstock.com
The application of various techniques, such as regeneration and self-healing technologies, can significantly extend the service life of the pavement. When asphalt's temperature crosses a particular threshold, it will become a near-Newtonian fluid due to its viscoelastic nature. Currently, bitumen has the ability to repair damage caused by many failure scenarios. The ability of bitumen to mend itself has been extensively researched.
Due to their many fantastic features, CNTs are one of the most commonly employed carbon nanostructures in bitumen. Few researchers, however, have looked into how CNTs can enhance bitumen's natural healing abilities. Though it is acknowledged that the modification process between CNTs and bitumen is restricted, CNTs have been shown to have a reinforcing impact and enhance the self-healing capacity of bitumens. Getting a homogenous dispersion in asphalt while preventing agglomeration is still difficult. A stress concentration may result from the agglomeration brought on by an excess of CNTs. Therefore, it is important to keep the CNT content under control at an acceptable level such that the nanomodified asphalts can achieve the required fundamental qualities and hasten the self-healing processes.
About the Study
In this study, the authors used multi-walled carbon nanotubes (MCNT) to speed up bitumen's self-healing abilities. Fourier transform infrared (FTIR) spectroscopy and a fluorescence microscope (FM) were used to observe the compatibility and modification mechanism. Then, a dynamic shear rheometer (DSR) and a bending beam rheometer (BBR) machine were used to evaluate the rheological properties. Further, the self-healing capabilities and initial healing temperature were assessed.
The team assessed the self-healing capabilities and modification mechanism of asphalts incorporating CNTs. They looked into the MCNTs-modified asphalts' modification process and analyzed how MCNT-modified bitumen behaved rheologically. The self-healing characteristics of MCNT-modified asphalts were determined, and the ideal MCNT content was suggested. FM explored the compatibility of base asphalt with MCNT. To explore the modification mechanism, FTIR was used.
The researchers used a temperature and frequency sweep of DSR to assess the high-temperature rheological characteristics of MCNT-modified asphalts. A BBR was used to study the rheological characteristics of modified asphalts at low temperatures. Further, a DSR was used to determine the modified asphalts' initial healing temperature. Using a temporal sweep of DSR, the self-healing capabilities of each sample were examined at the corresponding healing temperature and at room temperature.
The specimens' recoverable moduli were 76.44, 76.63, 82.99, 93.12, and 82.92%, and their loading times were 0.71, 1.22, 0.92, 0.88, and 1.03 times longer than in the first stage. The healing index (HI) of specimens at healing temperatures was 2.09, 3.62, 2.94, 3.14, and 3.54 times higher than that at room temperature. At healing temperature, base asphalt's HI was 41.44%, whereas modified asphalt's HI increased by 49.92%, 73.43%, 57.53%, and 62.31%, respectively. The values of n increased from 0.7694 at 75 °C to 0.7859 at 30 °C. The asphalt transformed into a nearly Newtonian fluid at 50.13 °C and had the ability to self-heal.
At -6 °C, -12 °C, and -18 °C, the m/S of base asphalt was 0.00556, 0.00342, and 0.00124, respectively. The failure temperature of base asphalt was 50.36 °C, but it increased by 5.48 °C, 8.01 °C, 10.37 °C, and 13.59 °C for modified asphalts. At 58 C, G' decreased in each sample in the following percentages: 99.63%, 99.21%, 99.44%, 99.14%, and 98.74%, respectively.
The results showed that agglomeration happened when the MCNT level exceeded 1.5% and that MCNT and bitumen were modified by physical blending. According to the findings of master curves, the rheological performance at high and low temperatures would be increased and degraded, respectively, with MCNT. Additionally, the addition of MCNT would lower the flow behavior index and hence raise the starting healing temperature. There was no difference in the healing effect at room temperature, but when the materials were healed at their various healing temperatures, modified asphalt with a 1.5% MCNT content showed the best healing efficiency.
In conclusion, this study elucidated the development of MCNT-modified asphalt to examine its fundamental characteristics and self-healing capabilities. When asphalt and MCNT were physically combined, no new materials were created. The occurrence of agglomeration phenomena was dependent on the amount of MCNT, which increased above 1.5%. Asphalt's high-temperature performance could be considerably enhanced by MCNT, while the low-temperature crack resistance could be decreased.
Additionally, the findings from master curves supported the rheological variances mentioned above. Due to differences in the viscoelastic characteristics of the asphalt, the initial healing temperatures would increase as the MCNT content increases. When healed at its healing temperature, C-1.5 demonstrated the best healing performance.
The authors mentioned that it is possible to use MCNT to speed up asphalt's healing abilities. They advised using modified asphalt with a 1.5% composition, as this could both meet the minimum standards and exhibit the optimum healing effects.
Zhang, F., Cao, Y., Sha, A., et al. Mechanism, rheology and self-healing properties of carbon nanotube modified asphalt. Construction and Building Materials 128431 (2022). https://www.sciencedirect.com/science/article/abs/pii/S0950061822020918