Effect of polypropylene fibers on the mode I, mode II, and mixed-mode fracture toughness and crack propagation in fiber-reinforced concrete

Concrete is the most commonly used material in civil engineering and also the most conventional and cheap material available in the market. Concrete cracking and fracturing may cause irreparable damages to concrete structures. Hence, fiber-reinforced concretes have been introduced in recent years as a strategy to eliminate these drawbacks to a large extent. In this study, crack propagation and the fracture toughness of concrete specimens without polypropylene (PP) fibers and those containing 0.2, 0.35, and 0.5 vol% of PP fibers were investigated through testing the straight notched Brazilian disc specimens. Furthermore, crack propagation from the pre-existing cracks in the specimens was investigated, and the mode I, mode II, and mixed-mode I/II fracture toughness were also calculated. The Brazilian disc (BD) test was performed on the specimens at different inclination angles of 0, 15, 28.83, 60, 75, and 90 degrees relative to the direction of pre-existing cracks. According to the experimental results, at angles less than 75 degrees (0 < alpha less than 75), the wing cracks initiated from the tip of pre-existing cracks, and as the loading continues, the path of crack propagation and growth approaches the loading direction. At angles equal to or greater than 75 degrees, the crack initiated with a distance d from the crack tip. This distance was larger in the fiber-free specimens than in the fiber-reinforced specimens. The results showed that the mode I, mode II, and mixed-mode I/II fracture toughness of the specimen containing 0.35 vol% of PP fibers were higher than the fracture toughness of the fiber-free specimens or those containing other PP fiber levels.