The mechanism of brittleness in high strength concrete (HSC) has been studied using tension softening evaluation. The formulation of a discrete equation to simulate a mode-I crack propagation with tension softening is described. Concrete with three different compressive strength (F_c) of 32.2, 60.3 and 96.4 Mpa, and with the same aggregate was tested by three point bend test on notched beams. The parameters of bilinear tension softening model were estimated by optimizing the load-deflection curve of the HSC. Fracture energy (G_f) did not monotonously increase with in-creasing F_c: however, the G_f value of the strongest one decreased. Tensile strength and final crack opening displacement estimated showed respectively monotonous increase and monotonous decrease with an increase in F_c. The estimation data indicates that strengthening mortar matrix reduces crack progress between matrix and particles and effects crack penetrating through particles. It is concluded that mechanism of brittleness in HSC is attributed to a reduction in bridging at interface, furthermore, tension softening has an advantage to evaluate relative variation in internal structure of concrete.
