Hybrid Reinforcement of Asphalt-Concrete Mixtures Using Glass and Polypropylene Fibers

In recent years, research has been devoted to modify the properties of bitumen and improve the performance of the flexible pavements. Use of different fibers in mixtures is known as beneficial HMA (hot mix asphalt) modifier. Although applying these modifiers increases the initial cost, they may increase pavement resistance for rutting therefore, postpone the rehabilitations and decrease maintenance cost. In this research, effect of polypropylene (pp) additive at two lengths (6 and 12mm) on properties of asphalt cement was examined. Three percent of pp were used: 2, 4 and 6% by weight of asphalt were added to unmodified asphalt (wet base) at optimum asphalt content of 4.3%. Penetration, softening point and ductility tests were applied to pp modified asphalt cement and the results were compared with unmodified asphalt cement. Also, three different percent of glass fiber: 0.05, 0.1 and 0.2% by weight of aggregate with 12mm length was selected as a second fiber for the pp modified bitumen mixture (dry base). Since glass fiber has smooth surface area with extreme tensile strength potential (more than 60000MPa) and pp provides good adhesion with asphalt cement, glass fiber was added to pp modified asphalt mix to increase the internal friction of glass fiber with other materials. All of the specimens were made and compacted by Superpave Gyratory Compactor (SGC) apparatus and then analyzed by Marshall Method and finally tested by Marshall Stability test. Results indicate that pp modified bitumen reduced penetration value and increased softening point value compared to unmodified asphalt, which may result in increased rutting resistance of the modified mixtures and resistance to traffic-induced deformation at high temperatures. Also, pp additive caused ductility value to decrease. Marshall test indicated that pp additive can affect the properties of the mix. Use of 0.1% glass fiber plus 6%pp presented the best hybrid reinforcement by increasing stability and decreasing flow value for both of pp lengths.