Synthetic fibers help control plastic shrinkage cracking which can occur in the very early stages of concrete drying. Steel fibers reinforce the concrete in its' hardened state, thereby improving its' strength and durability.

The major difference between steel and synthetics is their respective Youngs modulus and tensile yield strength. Steel fibers have a sufficient high modulus of elasticity and tensile strength to assume excess strain across a crack and hold it tightly.