Experimental study of improving hot mix asphalt reinforced with carbon fibers

Using hot asphalt mixtures (HMAs) is very common in flexible pavement construction due to its mechanical properties of durability, water resistance, and riding comfort. It proved to substantially reduce the applied stresses from vehicular traffic to subgrade. However, these types of pavements can undergo different types of distress such as cracks and rutting, which could be the result of weak mixing of the asphalt mixture and/or increasing traffic volumes. Therefore, the need to improve the mechanical properties of hot mix asphalt mixtures using different additives and modifiers was raised. Different materials have been recently used to reinforce asphalt concrete. For the construction of road pavements, using fibers in a HMA has emerged as a significantly more appealing option due to their strengthening properties. The alteration of asphalt binder is expected to benefit from carbon fibers (CFs) more than other types of fiber. In this study, an experimental study is carried out to investigate the effects of adding CFs to the asphalt mixtures with grades of (40–50) which comply with the Iraqi specifications. Twelve Marshall specimens were tested for stability and flow, 24 specimens for moisture failure testing, and two specimens for the rutting resistance of the asphalt mixture. The optimum bitumen content of 5.0% was used to prepare all the specimens. Different percentages of CFs by weight of asphalt (0, 0.2, 0.4, and 0.6%) were implemented to study the performance of paving mixtures. The way to mix the disperse the CFs homogeneously is by dry state (after separating the fibers manually) for 10 min, and then the asphalt was added; the mixture was heated to 165°C and mixed for at least 20 min. The results showed that the optimum properties of the HMA were reached by adding 0.4% CF to the HMA. Marshall stability was increased by 48%, the flow was decreased by 23%, and in addition, rut depth was reduced by 50%. The unit weight and the percentage of air voids in the mix were maintained and, along with other mix properties, are preserved within acceptable limits. Moreover, the indirect tensile strength has increased by 88.6%. Furthermore, moisture susceptibility reached 88%. Therefore, the addition of 0.4% CFs improved the performance of HMA at a high temperature of 60°C, which is close to the paving temperature in Iraq, by reducing the rutting of the asphalt mixture, which is considered the most dangerous tiling problem in Iraq. By improving the performance characteristics of asphalt mixture, the durability of the pavement will increase, and the riding will be more comfortable.