Effect of crack and fiber length on mode I fracture toughness of matrix-cracked FRC beams

•TTC approach is not suitable to predict the KIC of fibrous materials.•MC approach is a rational method to represent the actual crack field in FRC.•The effect of crack and fiber lengths on the real KIC of FRC was investigated.•Griffith's theory was adopted to determine the KIC of such matrix crack.•The proposed method for determining KIC of FRC was judged by the concept of dmax. Pre-cracking fiber-reinforced concrete (FRC) is the most challenging aspect of determining the fracture toughness of such materials. In this paper, a matrix-crack (MC) approach suggested recently by the authors was adopted to estimate the real mode I fracture toughness (KIC) of FRC beams. The effect of notch depth to beam depth ratios (a/d) of 0.1, 0.3, and 0.5 and fiber lengths of 35 mm, 50 mm, and a hybrid fiber composed of 50% from each length, on KIC was investigated. Similarly, traditional through-thickness cracked (TTC) beams were cast for comparison. Hooked-end steel fibers with 1% fiber volume fraction were used to produce FRC. A three-point bending test was performed on thirteen sets of FRC beams. The span/depth ratio was kept equal to four. The results indicated that the MC approach with different a/d successfully predicted KIC of FRC differently than the TTC approach. For MC beams, KIC of FRC decreased by increasing a/d. Furthermore, the length of debonding fibers has a detrimental impact on KIC. The beam with long fibers showed higher fracture energy than the short fibers based on the work of fracture method recommended by RILEM. However, increasing the length of the fibers reduced KIC because of increasing their debonding length and subsequently decreasing their efficiency.