The addition of fibers brings benefits to both plastic and hardened concrete.

When the concrete is still in its plastic state, fibers provided an internal support system for the concrete. It has been likened to a “bird’s nest” supporting the coarse aggregate throughout the concrete. The fibers also improve the cohesiveness of the concrete, reducing the tendency of the coarse aggregate to segregate. If you look at a cross-section of fiber-reinforced concrete, you can observe uniform distribution of the coarse aggregate, especially near the top surface. Due to this internal support and cohesion, plastic-shrinkage cracking and plastic-settlement cracking are reduced. It also explains why bleeding in more uniform for fiber-reinforced concrete. Furthermore, fiber-reinforced concrete pumps easier since rock pockets in the elbows of concrete pumps are eliminated.

 

Fibers bring other benefits to concrete in the hardened state. Since more coarse aggregate is near the top of the concrete, abrasion resistance is often improved. Also, the fibers anchor in the concrete paste, diminishing the effect of shattering forces by tightly holding the concrete together. This can be observed when breaking compressive strength cylinders. Typical concrete just falls apart after reaching the peak load. Fiber-reinforced cylinders, however, become an assembly of broken pieces that linked together by the fibers. If a crack does occur in a slab, the reinforcing provided by the fibers will help keep the crack tight. This crack-holding ability is termed toughness or residual strength. Toughness can be observed when breaking flexural strength beams. As soon as the crack forms, a plain concrete beam just falls in two. On the other hand, the fiber-reinforced beam will still carry a load after the initial break. The higher the load carrying capability, the higher the residual strength or toughness. The military takes advantage of the toughness for exotic applications such as building protection or explosive force mitigation.

 

For clarification of the original question, fibers will increase the peak flexural strength of the concrete when used at higher dosage rates, typically greater than about 60 lb/yd³ for steel fibers, and greater than about 10 lb/yd³ for macro synthetic fibers. This is well above typical dosage rates.