چكيده به لاتين
Nowadays, due to the increasing price of bitumen, the scarcity of aggregate resources, and the environmental problems caused by asphalt production, the use of Reclaimed Asphalt Pavement (RAP) has attracted the attention of researchers. The use of RAP in the asphalt mixture will increase its hardness and thus increase the potential for cracking. To address this issue, solutions such as the use of polymer-modified bitumen, warm mix asphalt additives, and rejuvenating agents have been proposed. In conditions where high amounts of RAP are used, rejuvenating agents are an effective method to reduce the stiffness of the mixture. Numerous studies have examined the impact of rejuvenating agents on the cracking properties of aged bitumen and asphalt mixtures. A common issue in most of these studies is that teh re-aging of the rejuvenated RAP mixtures has not been evaluated. Another important issue in the use of RAP and rejuvenating agents is that, due to the reduction in the stiffness of the rejuvenated RAP mixtures, their resistance to rutting decreases. Therefore, considering the existing research gap, this study investigates the rutting and cracking behavior of asphalt mixtures containing various amounts of RAP and rejuvenating agents, after short-term and long-term aging, using experimental and numerical methods.
In this study, asphalt samples containing 25%, 50%, and 75% RAP were reclaimed using recycled cooking oil and recycled engine oil as rejuvenating agents. Additionally, by mixing the recycled oils with 25% by weight of crumb rubber, two types of crumb rubber modified rejuvenating agents were prepared and used to rejuvenate the RAP mixture. The optimum content of bitumen and different rejuvenating agents was determined by the Marshall Mix design method and the penetration grade test, respectively. To prepare the asphalt mixture, bitumen with a penetration grade of 60/70 and limestone aggregates with a nominal maximum size of 12.5 mm were used. Furthermore, to investigate the cracking, rutting, and moisture susceptibility behavior of the RAP mixtures, various samples were subjected to dynamic modulus, dynamic creep, SCB fracture, and indirect tensile strength tests in both dry and wet conditions after short-term and long-term aging. For short-term aging simiulation, the various mixtures were kept at 135°C for 4 hours before compaction, and for long-term aging simiulation, the short-term aged samples were compacted and kept at 85°C for 4 days. After the experimental investigation of the various mixtures, the rutting and cracking behavior of the RAP mixtures was investigated using the finite element analysis method. For this purpose, the asphalt slab of the Wheel Track test and the SCB specimen were defined in the ABAQUS software, and to validate the model, the results obtained from the model analysis were compared with the results of the Wheel Track rutting test and the SCB fracture test. In the finite element analysis model, the behavior of the asphalt mixture was defined as viscoplastic for investigating rutting behavior and as viscoelastic for investigating cracking behavior. After validating the finite element analysis model, the effect of various parameters on the amount of rutting and the stress intensity factor at the crack tip of the RAP mixture was investigated using this model. Subsequently, based on the multiple linear regression method, the governing mathematical relationships between the parameters under investigation and the rut depth, as well as the stress intensity factor at the crack tip, were determined.
Based on the results obtained, before re-aging, the mixtures containing crumb rubber modified rejuvenating agents performed better than the samples containing pure rejuvenating agents and showed better resistance to rutting without changing the cracking resistance results. Also, after aging, the RAP mixtures containing crumb rubber modified rejuvenating agents are more resistant to cracking than the mixtures containing pure rejuvenating agents. Therefore, the crumb rubber modified rejuvenating agents not only increase the rutting resistance before re-aging, but also prevent a significant drop in cracking resistance in the RAP samples after re-aging due to their relatively high resistance to aging. According to the results of the tests, in the samples containing crumb rubber modified rejuvenating agent, the rutting resistance increases by about 40 to 70% before aging, and the cracking resistance increases by about 10 to 40% after aging. The results of the finite element analysis model show that parameters such as the type of rejuvenating agent, the percentage of RAP, the thickness of the asphalt layer, and the amount of pressure applied to the surface of the asphalt slab are among the effective parameters in the amount of rut depth of the asphalt slab, and parameters such as the pressure applied to the surface, the thickness of the existing asphalt layer, the thickness of the surface asphalt layer, and the crack width are among the effective parameters on the stress intensity factor at the crack tip in the overlay asphalt layer. Also, there is a high correlation between the rut depth obtained from the finite element analysis and the flow number obtained from the dynamic creep test at the end of the second phase. The mathematical relationship defined based on the results of the finite element analysis model for predicting the rut depth of the asphalt slab and the stress intensity factor at the crack tip in the asphalt layer has a coefficient of determination above 90%.
