This paper presents an experiment to investigate the influence of moisture on the static mechanical properties of concrete, and prediction equations for strength and fracture toughness of concrete at different strength grades, relative to water saturation, were established respectively. The research results show that all of the compressive strength, splitting tensile strength, and fracture toughness of concrete exhibit an approximately linearly decreasing trend with the increase in water saturation. For saturated concrete specimens with w/c 0.65, 0.55, 0.42 compared with dry ones, compressive strength decreases by 40.08%, 36.08%, and 33.73%, respectively, splitting tensile strength decreases by 45.39%, 42.61%, and 35.18%, respectively, and fracture toughness decreases by 57.31%, 49.92%, and 46.76%, respectively. The higher the water saturation of concrete, the larger the slope of the ascending part of the uniaxial compressive stress-strain curve, and the smaller the peak strain corresponding to the peak compressive stress, then in this case, both crack mouth opening displacement and loading point deflection corresponding to the critical load on three-point bending beam, decrease. Ingress of water causes the deformation capacity to decrease, and the toughness to weaken, which are unfavorable to the mechanical properties of concrete.

A new fiber cocktail reinforced cement–based materials (NFCRCM) with steel, polyvinyl alcohol (PVA) fiber, and calcium carbonate whisker (CCW) was developed to obtain powerful fire resistance in this paper. The flexural properties, synergistic effects, and microscopic mechanism of NFCRCM at elevated temperature were particularly studied. With the introduction of CCW, the degradation of flexural strength and toughness of steel-PVA fiber cocktail reinforced cement–based materials below 800°C was decreased. Below 800°C, the NFCRCM with 1.5% steel fiber $+0.5\%$ PVA fiber $+3\%$ CCW showed a significant positive hybrid effect in flexural strength and toughness resulting from microscopic crack inhibition and nucleation effect of CCW. When the temperature reached 800°C or above, a negative hybrid effect was generally observed due to thermal decomposition of CCW.