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The principle of HFRC work is explained in Fig. 1. The short fibers bridge microcracks when they appear in the concrete. They are more effective than macrofibers because their number in the mixture is greater for the same volume. As a result, the strength of the concrete increases in the initial tensile phase. When the microcracks connect and as a result, the macrocracks develop, the longer fibers start to be active and begin to counteract the crack growth. In this way, the concrete is able to resist a significant load, despite the constantly increasing deformations, thus the ductility of the concrete is improved.

The effectiveness of combining microfibers and macrofibers has been confirmed, among others, in the research, where after combining microfibers with macrofibers: the toughness, compressive, tensile, and flexural strength improved, and drying shrinkage strain decreased. THE mixture of fibers had a positive effect on the post-cracking behavior of concrete. Additionally concluded that combination resulted in the enhancement of compressive strength and modulus of elasticity. It is important to note, that the loading rate has an influence on the stress-strain curves.

The compressive strength, toughness, critical strain and elastic modulus dynamic factors increased with the strain rate. ALSO reported that FRC members showed a higher performance when loaded at high speed or under impact loading

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