چكيده به لاتين
The capability of production of high-performance asphalt containing reclaimed asphalt pavements (RAP) can be a great stride in the asphalt production industry both environmentally and economically. Nowadays, with the increase in the asphalt binder price and growing attention toward environmental issues, using RAP in asphalt mixtures is increasing. The mix design of the asphalt mixture is the first and most important step in the production of the asphalt mixture. As RAPs are used in the mixture, the question is "if the conventional mix design methods guarantee the proper performance of asphalt mixtures containing RAP?" The design of the asphalt mixture using the Marshall and Superpave (level one) mix design method relies on the volumetric properties of the asphalt mixture. Recently, a new mix design named balanced mix design (BMD) has been introduced, which considers the performance properties of the asphalt mixture in addition to its volumetric properties.
In BMD, the optimum binder content (OBC) is determined by balancing the rutting and cracking resistances of the mixture. The other point is that regarding the presence of aged asphalt binder in RAP, it is inevitable to use a recycling agent (RA) for recovering the properties of the aged asphalt binder. Concerning the variety of RAs and their different properties, determining the best step (addition method) to add RAs to obtain the best result is of great importance. This study aims to design the RAP-containing asphalt mixtures via the BMD approach and optimize the step of adding RAs. For this purpose, three RAs from different categories (vegetable oil, paraffinic oil, and aromatic extract oil) have been used in three different addition methods (RAP, asphalt binder, and mixture).
Also, two percent of RAP 25 and 50 are considered as the lower and upper limits. In the first section, the binder content of 21 different mixtures is determined using the Superpave method (level one), and the proper addition method to add three RAs is determined. Then, 63 mixtures are tested aiming to design the mix design of mixtures via BMD considering three percent of asphalt binder (OBC obtained from the Superpave method and 0.6 above and below). After determining the OBC using BMD method, this value is compared with the OBC obtained from the Superpave level one method. The Hamburg Wheel Tracking (HWT), Illinois Flexibility Index (I-FIT), and Indirect Tensile Strength (ITS) tests are used for evaluating the rutting performance, cracking performance, and moisture sensitivity of the asphalt mixture, respectively.
Also, the Pareto algorithm is used for multi-objective optimization of asphalt mixtures in this study. The summary of results and achievements obtained in this study are as follows: a) the best step (addition method) to add the RA is different for various RAs. Results of this study indicate that the best addition method to use the vegetable, paraffinic, and aromatic extract oils in this study is to add them to the mixture, asphalt binder, and RAP, respectively. When these RAs are used at the best addition method, the best performance in rutting, cracking, and moisture sensitivity is obtained. Concerning the high diversity of RAs and their different properties, such as classification, viscosity, and thermal sensitivity, the best addition method to use RAs before consumption must be determined by the producer, related labs and offices, or contracting companies. b) Results of mix design via BMD indicate that the OBC from the Superpave (level one) method for RAP-containing mixtures is not proper and does not guarantee the good performance of these mixtures, especially for those mixtures with no RAs.
