Cementitious materials are widely used in construction due to their wide range of raw material sources and easy construction. However, the characteristics of brittleness and easy cracking restrict their application in ductile structures. Fiber toughening is an effective approach to enhance the toughness of cementitious materials. The damage of cementitious materials is a gradual, multi-scale occurrence of the process. The first micrometer-scale cracks emerge, gradually expand and become larger with the increase of the load, forming the microscopic cracks, following convergence, developing macroscopic cracks, and ultimately leading to the material failure. Therefore, the toughening and crack-resisting effect of single-scale fibers are difficult to meet the multi-scale cracking process of cementitious composites. To correspond to this process, the hybrid fiber is an effective method, and the synergistic effect between different types of fibers can achieve the effect of "1+1>2". There exist a few studies on the static flexural performance of hybrid fiber-toughing cementitious composites (HFTCC), but there is paucity of studies on the dynamic flexural behavior of HFTCC. To understand the hybrid effect of steel fiber and polyethylene fiber (PE fiber) on the static and dynamic flexural properties of cementitious composites, the static and dynamic flexural performances of HFTCC were investigated via three point bending test and drop weight impact test. In addition, the hybrid effect of steel fiber/PE fiber and the corresponding toughening mechanism were also analyzed.

The three-point bending tests were carried out based on a reference (GB/T 17671—2021). An electro-hydraulic servo universal testing machine (INSTRON 1342) was used to carry out the three-point bending test, at a loading rate of 0.2 mm/min. Load transducers and displacement sensor were used to record the load and mid-span deflection (δ) changes during the test, and the loading was stopped when the δ value reached 5 mm.

The impact flexural properties were determined by a model INSTRON-CEAST 9340 drop weight impact testing machine. The required velocities were obtained by releasing the impactor from a predetermined height. The mass of the impact hammer head used in this test was 4.08 kg, the set impact speeds were 3 m/s and 4 m/s, and the impact force was collected by a high-speed data acquisition system (within 22 kN) at a data acquisition frequency of 0.5 MHz.

The failure pattern of specimens indicates that fibers can effectively improve the dynamic flexural properties of specimens due to the fiber bridging and crack-blocking effect. Compared with PE fibers, steel fibers have a higher stiffness and a better ductility, which can resist the deformation of the specimen and limit the degree of opening of the main crack, thus improving the flexural properties of the specimen.

Comprehensive analysis of load-displacement curve of HFTCC specimens shows that the static flexural performance of HFTCC is positively correlated to the PE fiber content, and the dynamic flexural performance demonstrates a positive correlation with the steel fiber content. The specimen with 1.5% (in volume fraction) PE fiber and 0.5% steel fiber exhibits the optimal static flexural peak stress, while the specimen with 1.5% steel fiber and 0.5% PE fiber presents the optimal dynamic flexural energy absorption. Under static flexure, the peak strength and energy absorption of HFTCC are increased by 26.5%-31.7% and 14.8%-56.8%, respectively, compared to the specimen with 2.0% steel fiber content.

The dynamic increase factor (DIF) and energy absorption of HFTCC exhibit a significant strain rate effect. Under dynamic loading, the toughening effect of PE fibers is weaker than that of steel fibers, and the incorporation of a large number of PE fibers is not conducive to the enhancement of the dynamic energy absorption capacity of the specimen, which is related to the change in the failure mode of PE fibers. A small amount of PE fibers instead of steel fibers plays a toughening effect, which can exert the enhancement of the specimen dynamic energy absorption capacity.

Synergistic effect analysis reveals that the steel fiber plays the dual effect of enhancement and toughening on the higher rate of dynamic flexure when the steel-PE fiber system with a high dosage of steel fiber due to the high tensile strength, stiffness, and ductility of the steel fiber, effectively enhancing the energy absorption capacity of the HFTCC under the lager deformation.

The schematic diagram of HFTCC physical model was established via the analysis of SEM images. The steel-PE fibers effectively prevent the development of micro-cracks and the formation of macro-cracks, and the steel-PE fibers have a positive effect on the improvement of the mechanical properties and impact resistance of the cementitious materials during the static and dynamic loading process. Also, the micro-morphological analysis of HFTCC indicates that the steel-PE fibers form a three-dimensional mesh structure to have the effect of fiber toughening and crack-resisting on the matrix.

1) The steel-PE hybrid fibers improved the static and dynamic flexural properties of HFTCC, in which the 0.5% (in volume fraction) steel-1.5% PE hybrid fiber HFTCC exhibited the optimal static peak bending and tensile stresses, while the 1.5% steel-0.5% PE hybrid fiber HFTCC had the optimal energy absorption under the dynamic loads. The DIF and energy absorption of HFTCC had significant strain rate effects.

2) The steel-PE fibers in HFTCC showed a synergistic effect. For the peak load, static flexure mainly showed a positive synergistic effect, and the contribution of PE fibers to the peak stress of HFTCC was weaker than that of steel fibers under the dynamic impact loads. The specimen with 0.5% steel-1.5% PE hybrid fiber under the static flexure effectively improved the energy absorption of HFTCC, with the maximum hybrid coefficient S of 1.239, while the impact dynamic loading under the impact dynamic loading of the specimen with 1.5% steel-0.5% PE hybrid fiber effectively enhanced the energy absorption of HFTCC, with the maximum S of 1.086.

3) The toughening mechanism of HFTCC mainly originated from the synergistic crack-blocking and toughening effect between steel and PE fibers. Especially, the micro-fine steel fibers and PE fibers could fully utilized their respective mechanical properties at their appropriate dosages, thus effectively improving the static and dynamic flexural performances of HFTCC.