A pour in Cross Lanes had welded wire mesh specified for a garage slab addition, and when I got there to do the decorative finish work after the structural crew had left, the mesh was sitting on the subgrade, not suspended near mid-depth where it does something useful. Nobody had used chairs to hold it up, so the crew laid it down, poured concrete over it, and walked across it while placing the mix, pushing it straight to the bottom. That slab has wire mesh in it on paper. In practice it has a flat piece of steel doing almost nothing, buried where load never reaches it. This gap between what reinforcement is supposed to do and where it actually ends up is close to the most common misunderstanding I run into on residential concrete.
What rebar is actually for
Deformed steel rebar is designed to hold a slab together after it cracks, transferring load across the crack so the two sides keep working as one surface instead of becoming an uneven step. It also controls how wide a crack opens, keeping shrinkage and settlement cracking to a hairline rather than letting it spread. None of that works unless the bar sits at the correct height — for a typical residential driveway or patio, that usually means roughly mid-depth or the upper third, held there on chairs so the concrete has adequate cover above and below the steel. Rebar sitting on the ground, or stepped down to the bottom during the pour, gives up most of the benefit it was added for and can accelerate corrosion, since cover to the outside face is now too thin to protect the steel over time.
What wire mesh was supposed to do, and why it usually doesn’t
Welded wire mesh was a common residential standard for decades, intended to do a version of what rebar does — hold cracked concrete together and limit crack width — with a lighter, cheaper grid of wire instead of individual bars. The problem is almost entirely one of placement, not material. Mesh comes in flat rolled sheets that curl, and without chairs holding it at the right height, crews on a residential pour routinely walk it down to the bottom of the slab as they place and finish the concrete. Mesh sitting on the subgrade provides essentially no crack control, because it sits where the slab is in compression, not where it is in tension and actually needs steel holding a crack together.
What fiber actually controls
Synthetic or steel fibers mixed directly into the concrete are aimed at a different, earlier problem: plastic shrinkage cracking, the fine surface cracking that can appear in the first few hours after a pour, before the concrete has any real strength, usually from rapid surface drying. Fiber disperses that shrinkage stress across thousands of tiny fibers instead of letting it concentrate into a few visible cracks. What fiber does not do is transfer structural load across an existing crack the way rebar does, and it does not control the width of a crack once one has fully formed and the slab has separated on either side of it. Fiber is a mix-design addition that helps during and shortly after the pour; it is not a substitute for placed steel that does a different job later in the slab’s life.
The misconception that costs people twice
I hear “it’s got fiber in it, so we skipped the rebar” more often than I would expect from crews who otherwise know their trade. Fiber-reinforced concrete without rebar is common and appropriate for some flatwork, particularly lighter-duty slabs with stable subgrade and closely spaced control joints doing most of the crack-control work — but on a driveway carrying vehicle loads, or any slab over less than ideal ground, skipping rebar because the mix has fiber in it removes the one thing designed to hold a crack together after it forms. The American Concrete Institute publishes guidance distinguishing fiber reinforcement from structural reinforcement for exactly this reason — they solve different problems.
Why chairs and cover decide the outcome
All of this comes back to the same detail: reinforcement, whether bar or mesh, only works if it stays at the designed depth through the entire pour. Chairs are cheap relative to the concrete and steel they support, and skipping them is one of the most common ways a correctly specified slab ends up performing like an unreinforced one. Cover matters in the other direction too — steel too close to the surface is exposed to moisture and deicing salt faster, which is what causes rust-jacking and surface spalling years later.
Frequently asked questions
Does fiber-reinforced concrete still need rebar? On driveways or any slab carrying vehicle loads, yes — fiber controls early shrinkage cracking, not structural load transfer across a fully formed crack, which is rebar’s job.
Why does wire mesh sometimes seem to do nothing? It is usually a placement issue — mesh that gets walked down to the bottom of the slab during the pour, instead of held at the correct height on chairs, provides little to no real crack control.
How does reinforcement placement relate to control joints? Both control where and how a slab cracks; joints create planned weak points where cracking is meant to happen, while correctly placed steel manages what occurs at unplanned cracks — see control joints: where they go and how deep for how the two work together.