Synthetic fibers can be divided into two categories based on their equivalent diameter: microfibers and macrofibers.

Microfibers typically have a diameter of less than 0.3 mm. They are primarily used to reduce premature cracking, control plastic shrinkage (cracking that can occur within 24 hours of concrete curing), provide impact and fire protection through their spalling resistance, and improve the uniformity of concrete during placement.

Macrofibers (also known as structural fibers) have a diameter greater than 0.3 mm and can be used to replace wire mesh and/or rebar in concrete. However, due to their lighter weight, non-corrosive nature, and high pumpability, they are generally easier to lay and handle.

Macrofibers also help reduce or eliminate premature and late cracking, improve durability and toughness, enhance fatigue and wear resistance, and have antioxidant properties.

Thus, the roles of microfibers and macrofibers in concrete complement each other. Optimum fiber addition, tailored to project needs, can achieve optimal results.