A micromechanics-based equivalent elastoplastic damage model for both notch-sensitive and multiple cracking hybrid fiber reinforced composite is proposed in this study. In this model, the elastic modulus, first cracking strength, and ultimate strength are estimated based on micromechanics. To quantify strain after matrix cracks, a novel characteristic length is defined based on the damage mechanics. The effects of the fiber length, diameter and modulus, and interfacial bond stress on the characteristic length of hybrid fiber reinforced composite are presented. In order to avoid the difficulty of determining the traditional damage and plastic potential function, this model is developed from the behavior of single fiber at mesolevel to the response of hybrid fiber reinforced composite at macrolevel. Then the calculated results are verified with several published experimental results of fiber reinforced composites and hybrid fiber reinforced composite, including notch-sensitive cracking fiber reinforced composite, multiple cracking fiber reinforced composite, and multiple cracking hybrid fiber reinforced composite reinforced with two types of fibers (steel fiber and polyethylene fiber). A parametric study has been performed to investigate the effects of the fiber properties, including the fiber volume fraction, length, diameter, and interfacial bond stress, on the tensile performance of hybrid fiber reinforced composite reinforced with steel fiber-like and polyethylene fiber-like fibers. The results indicate that enhancement of the tensile performance can be achieved more effectively by improving the polyethylene fiber-like fiber than steel fiber-like fiber.

In this paper, the mechanical performance of an ultra-high-performance concrete (UHPC) repaired cementitious composite system, including the old matrix and the new reinforcement (UHPC), under various high temperature levels (20 °C, 100 °C, 300 °C, and 500 °C) was studied. In this system, UHPC reinforced with different contents of steel fibers and polypropylene (PP) fibers was utilized. Moreover, the physical, compressive, bonding, and flexural behaviors of the UHPC repaired system after being exposed to different high temperatures were investigated. Meanwhile, X-ray diffraction (XRD), baseline evaluation test (BET), and scanning electron microscope (SEM) tests were conducted to analyze the effect of high temperature on the microstructural changes in a UHPC repaired cementitious composite system. Results indicate that the appearance of the bonded system changed, and its mass decreased slightly. The average percentage of residual mass of the system was 99.5%, 96%, and 94–95% at 100 °C, 300 °C, and 500 °C, respectively. The residual compressive strength, bonding strength, and flexural performance improved first and then deteriorated with the increase of temperature. When the temperature reached 500 °C, the compressive strength, bonding strength, and flexural strength decreased by about 20%, 30%, and 15% for the UHPC bonded system, respectively. Under high temperature, the original components of UHPC decreased and the pore structure deteriorated. The cumulative pore volume at 500 °C could reach more than three times that at room temperature (about 20 °C). The bonding showed obvious deterioration, and the interfacial structure became looser after exposure to high temperature.