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The fracture behavior of concrete with steel and macropolypropylene hybrid fiber blends (HyFRC) is evaluated and compared with steel fiber reinforced concrete (SFRC). Fiber blends at identical volume fraction as SFRC are shown to produce an improvement in the fracture response. At small crack separation, immediately after cracking, higher cohesive stress is produced in HyFRC compared to SFRC. Polypropylene fibers in concrete contribute cohesive stresses at larger crack separation. Polypropylene fibers in the concrete matrix improve the efficiency of the steel fibers by mobilizing a higher resistance at the initiation of the pullout. The initial stiffness, peak load, and the residual frictional resistance of the steel fiber pullout are increased with the addition of the polypropylene fibers. The higher reinforcing efficiency of steel fibers in the presence of polypropylene fibers is due to improved fiber-matrix bond, which results in a higher cohesive stress at smaller crack openings.

The fracture response of macro polypropylene fiber reinforced concrete (PPFRC) and hybrid blend of macro and micro polypropylene fiber reinforced concrete (HyFRC) are evaluated at 1, 3, 7 and 28 days. There is an improvement in the early-age fracture response of HyFRC compared to PPFRC. The changing cohesive stress-crack separation relationship produced by ageing of the concrete matrix is determined from the fracture test responses. An improved early-age cohesive stress response is obtained from the hybrid blend containing micro and macro fibers. The hybrid fiber blend also has a higher tensile strength at early age when compared to an identical volume fraction of macro polypropylene fibers.