Composites are advanced materials composed of at least two components. They are aimed at developing new materials with superior mechanical and physical properties. The components are not soluble and retain their unique properties after mixing. Concrete is a type of composite with a low tensile strength. The presence of voids in the concrete causes poor mechanical properties. Voids are generated due to the concrete shrinkage after drying. Nowadays, fiber reinforced concretes (FRC) are promising candidates to be used in the building industry. FRCs are concretes which have been reinforced by fibers in order to improve their tensile and compression properties. Although polypropylene (PP) fibers as reinforcing component in concrete are advantageous, however, they suffer from weak bonding to concrete. This drawback stems from PP smooth surface and weak affinity between PP fibers and concrete. The current study is aimed at addressing this problem. This is implemented through coating the surface of PP fibers with Silica nano-powders to create a stronger bond between PP fibers and concrete. First, Silica nano-powders were coated on PP fibers through various methods. Then the presence of nano-powders on the surface of PP fibers was investigated by optical microscope and scanning electron microscope (SEM). For further verification, FTIR-ATR test was also conducted on the samples. At second step, pull-out test was performed on silica-coated PP fibers. Initial PP fibers were considered as standard sample. The coated fibers were regarded to make FRCs. Since concrete structures are exposed to various loadings caused by earthquake, wind, shrinkage and temperature changes, the prepared FRCs were subjected to compression, bending and impact tests. Two concrete samples, i.e. an unreinforced concrete and FRC containing PP fibers without surface modification, were prepared as standard samples. The results indicate that the FRC containing coated PP fibers has more pull out force than that containing unmodified PP fibers. This can be interpreted by the fact that the shear strength between PP fibers and concrete is enhanced due to the presence of nano-powders. The flexural, compression and impact performance of the FRCs containing coated PP fibers were superior than unreinforced concrete and concrete reinforced with PP fibers. Ultimately, due to the significance of maintaining concrete properties after exposure to heat, flexural and 2 compression tests were also conducted on FRCs after exposing to heat (300 ?C) for an hour. The results also showed that the compression and flexural strengths of FRCs containing coated PP fibers were the highest .
