Fiber reinforced concrete (FRC) contains fibers physically mixed with gravel, sand, cement, and water. So far, adequate mechanical performance of FRC has been obtained at high cost and using complex technologies; important here is the geometry and surface character istics of the polymers. We have modified polymeric-fiber surfaces by using gamma radiation. Irradiated polypropylene (PP) fibers were submitted to 0, 5,10, 50, and 100 kGy of gamma irradiation dosages. First, tensile strength of PP fibers was evaluated, and then fibers blended at 0, 1.0, 1.5, and 2.0% in volume with Portland cement, gravel, sand, and water. The highest values of compressive strength were obtained with irradiated-fibers at 10 kGy and 1.5% in volume of fiber. The result is 101 MPa, as compared to 35 MPa for simple concrete without fibers.