Adding fiber to concrete

Fibers are sometimes added to concrete mix. Why is this done?

Concrete is a mixture of aggregate (rocks), sand , Portland cement, and water. Water combines with cement to form hydration products, which glue the mix together.

Upon initial set, there is a volume shrinkage of the cement paste. This small shrinkage may result in micro-cracks; tiny little flaws on the surface of the curing concrete. These cracks occur because wet cement paste has a low tensile strength, and cannot resist the small shrinkage from the initial cure.

Once cement has cured, it forms a brittle material. When any brittle material cracks, it always begins cracking at a small surface flaw, such as those produced on initial set, due to weak cement paste shrinkage. If these micro-cracks can be eliminated, a much stronger and longer-lived concrete is produced.

Micro fibers have a tendril-like structure, where tiny little spirals come out of a larger fiber (commonly polypropylene). When properly mixed, this type of fiber provides anchorage and locking into a wet cement paste. It will provide the necessary tensile strength for the wet cement paste to resist micro-cracking from shrinkage on initial set.  This results in a much stronger and tougher concrete.

Fiber added to concrete mix reduces the workability of the wet concrete to a small degree. It can be difficult to float a surface due to the fibers.

The specifications for fiber concrete show no increase in tensile strength, but those who really know concrete realize that these microfibers provide an actual slight increase in tensile strength of concrete, and also increase its toughness (resistance to crack propagation). Fibered concrete is a little tougher and has more flexural rigidity.

Fiber added to concrete mix makes a better concrete.