Ordinary concrete rapidly loses its bearing capacity under impact, while the addition of steel fibers effectively enhances the overall integrity and matrix toughness of concrete, thus meeting the concrete performance requirements of high strength, high durability, and high toughness in practical engineering. The existing work on the strengthening and toughening mechanism and failure mechanism of steel fiber reinforced concrete is mainly based on the macroscopic experiments. It is thus difficult to reflect the randomness and non-uniformity of concrete. In this paper, the dynamic response of steel fiber reinforced concrete beam specimens at a low-speed impact was investigated by a drop hammer testing machine to comprehensively reveal the internal damage and failure process of concrete and the toughening mechanism of steel fibers. A three-dimensional microscopic model of steel fiber reinforced concrete beams was proposed, and the mechanical properties and crack development of steel fiber reinforced concrete beams under dynamic impact were analyzed. In addition, the effects of steel fiber volume fraction and impact velocity on the impact resistance and failure morphology of steel fiber reinforced concrete beams were also discussed.

Steel fiber reinforced concrete beams with different steel fiber volume fractions (i.e., 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%) were prepared based on the experimental mix proportion. The effect of steel fiber volume fraction on the impact performance of steel fiber reinforced concrete beams was analyzed via dynamic three-point bending performance in a model INSTRON-9530 drop hammer impact testing machine.

A steel fiber model was randomly proposed based on the Monte Carlo method, and a spring element was used to simulate the bond slip between steel fibers and concrete matrix. Based on its bond slip relationship, a three-dimensional fine calculation model for steel fiber concrete was established. The dynamic response and failure forms of steel fiber concrete were simulated at different volume fractions (i.e., 0%, 0.5%, 1.0%, 1.5%, 2.0%) and different impact rates (i.e., 1, 2, 3, 4, 5, 10, 20 m/s), respectively, The effects of steel fiber content and impact rate on the impact resistance and failure morphology of steel fiber reinforced concrete beams were analyzed.

The drop hammer impact test is widely used to test the dynamic bending performance of concrete beams, but the hammer impact force measured by a hammer force sensor is not an actual impact force due to its inclusion of structural inertia force. In the low-speed impact test of steel fiber reinforced concrete beams, this study integrated some force sensors with roller bearings to accurately obtain the support reaction force of the beam during the impact process and reduce the influence of inertial force, thereby revealing the true dynamic response of the tested specimens.

The key to improving the mechanical properties of fiber reinforced materials lies in the micro bridging effect of fibers on the concrete matrix. Steel fiber exhibit a large number and random distribution in the concrete matrix, making it difficult to quantitatively evaluate the fiber reinforcement effect at an impact load. This study used the Monte Carlo method to propose a fiber microscopic model and determine the range based on the unique three-dimensional position and direction information of each fiber. The numerical analysis methods were used to more intuitively clarify the deformation and damage situation of steel fibers and concrete matrix during the impact process to obtain the accurate energy quantification evaluation.

The key to improving the mechanical properties of steel fiber reinforced concrete lies in the bond slip between steel fibers and concrete matrix. Simulating the bond slip between concrete matrix and steel fibers accurately becomes a key to simulating steel fiber reinforced concrete. In this paper, spring elements were used to simulate steel fibers. The stiffness of the spring element adopts a "two-stage" model. The simulation data by the model are in reasonable agreement with the experimental results.

The simulation data of three-dimensional steel fiber reinforced concrete beams based on bonded slip spring elements were in reasonable agreement with the experimental results.

The bearing capacity and energy dissipation capacity of steel fiber reinforced concrete beams increased with the increase of steel fiber volume fraction when the fiber volume fraction was small. The peak impact force of the concrete beam was increased by 16.71% at the optimal dosage of steel fiber of 1.5%. However, the energy dissipation capacity and impact resistance of steel fiber reinforced concrete beams decreased as the volume fraction of steel fibers further increased.

The bearing capacity of steel fiber reinforced concrete beams was positively correlated to the rate when the impact rate was in the range of 1 m/s to 20 m/s. The peak impact force of steel fiber reinforced concrete beams increased, but the reinforcement effect of steel fiber on the steel fiber reinforced concrete gradually weakened as the rate continues to increased. The reinforcement effect of steel fiber was slight when the rate increased to 20 m/s. Therefore, for steel fibers with a length of 12.5 mm and an aspect ratio of 20, their enhancement effect on the dynamic mechanical properties of concrete beams was more significant in a low-speed impact range of less than 10 m/s.