Study of Mechanical and Fracture Properties of Concrete with Different Lengths of Polypropylene Fibers

This study investigates the effect of polypropylene fibers of different lengths (54 mm, 38 mm, 19 mm) on the mechanical and fracture properties of high-strength concrete. Unlike most existing research focusing on a single fiber length, this work evaluates a fixed hybrid ratio of 4:1:1, thereby addressing the synergistic action of macro- and microfibers. Three dosages were tested and compared to a reference mixture without fibers. Validation was performed by repeated testing of multiple specimens and statistical evaluation of mean values and standard deviations. The results showed that the optimal hybrid mixture (2.0/0.5/0.5 kg/m3) increased compressive strength by 28.7% and splitting tensile strength by 30.1% relative to the reference. Fracture toughness and specific fracture energy also improved significantly, demonstrating enhanced crack resistance and energy absorption. The main contribution of this work is to provide experimental evidence that a hybrid combination of polypropylene fibers at a fixed ratio can improve both mechanical strength and fracture resistance, with direct implications for durability and service life.