What fiber does inside the mix
Fresh concrete loses moisture fast in the first hours after it is placed, and the surface wants to shrink before the slab underneath it has any strength to resist that shrinking. That is when the finest, earliest cracking shows up, a web of hairlines across the top that were never a structural problem but are an ugly one.
Millions of short fiber strands, distributed evenly through the batch instead of sitting at one depth, give that early shrinkage something to resist in every direction at once. It is a different mechanism than a joint or a bar, and it works in the first hours, before either of those is doing anything.
Fiber is not wire
- Fiber mesh is mixed into the concrete at the plant and rides everywhere in the batch. Wire mesh is a welded steel grid, laid in the forms and held up to a set position before the pour.
- Fiber fights early shrinkage cracking at the surface. Wire mesh and rebar carry tension and hold a crack tight if the slab settles or moves.
- One does not replace the other. A slab that needs structural reinforcement still needs steel, whether or not fiber is also in the mix.
Where fiber gets specified instead of wire
Fiber shows up most on slabs where the risk is surface cracking rather than settlement, a garage floor or a shop floor on good undisturbed ground, or a patio where wire mesh would be one more thing to hold up while the concrete is placed. It is also easier to order than to fight with in the field, since there is no grid to lift into place or stake down before the pour.
It does not disqualify wire or rebar from the same slab. A job can call for both, fiber through the batch for the early surface behavior and steel at a set position for the structural load, and the mix ticket and the placement drawing carry each requirement separately.