A novel procedure for accurately measuring the Mode II fracture toughness of steel fiber reinforced self-compacting concrete

Most research on the mode II fracture toughness of fiber-reinforced concrete (FRC) has intentionally avoided bridging fibers at pre-notch surfaces by using a through-thickness crack (TTC) that cuts the entire thickness, including fibers in this region. The objective of the present research is to accurately measure mode II fracture toughness (K_IIC) using double-notched cube (DNC) specimens on steel fiber reinforced self-compacting concrete (SFRSCC). The effects of precrack-to-specimen width ratios (a/w), i.e., a/w = 0.3, 0.4, and 0.5, and fiber volume fraction percentage (Vf%), i.e., Vf% = 1% and 1.5% were investigated. A comparison between K_IIC measured through specimens having the TTC concept, i.e., the absence of fiber bridging on the surfaces of the pre-notch, and those with the presence of fiber bridging on the surfaces of the pre-notch, i.e., the matrix crack (MC) concept. For greater clarity, the SCC specimens were cast without fibers with (MC/C) or without fiber bridging on the pre-crack surfaces to determine the unique effect of the presence of fiber bridging on the pre-crack surfaces on enhancing K_IIC. The results showed that DNC specimens with MC consistently obtained the highest mode K_IIC for all values of a/w, indicating the greatest resistance to crack growth. K_IIC​ increased as the a/w ratio increased. MC/C method, i.e., the presence of fibers behind the crack front only, showed more effectiveness on the K_IIC than the TTC, i.e., the presence of fibers ahead of the crack front only. In general, the MC is an accurate method for measuring K_IIC of FRC.