Fiber cracking in concrete theory there are two main and Swamy's "composite theory" proposed by the United States Romualdi "fiber spacing theory."

A, fiber spacing theory: According to fracture mechanics to explain fiber concrete crack resistance effect of cracks on that to improve the strength of concrete, the need to reduce the concrete original defects, cracks the number and scale, when the average center cement matrix fibers distance less than 7.6mm, tensile or flexural strength of fiber reinforced concrete first crack were able to significantly improve. The average distance between the fibers by the following formula.
- The average distance between the fibers, - respectively, the diameter and the volume ratio of the fiber.

As can be seen from the above formula: average spacing of fibers with fiber volume fraction decreases with an increase, with the fiber diameter decreases.

B, composite theory: the principle of composite material from a mixed starting to be seen as fiber reinforced concrete system, the principle of presumption of Hybrid fiber reinforced concrete tensile strength, tensile strength and fiber reinforced concrete proposed incorporation of fiber orientation the relationship between aspect ratio and adhesion.