How the two differ in kind
Both of these exist because concrete is superb in compression and close to useless in tension, and both get sold as the thing that stops your slab cracking. Neither is. Concrete shrinks as it dries, and a restrained shrinking material cracks — that is not a defect to be reinforced away, it is a property to be managed. What rebar and fibre genuinely differ on is WHEN they are working and what they are working against, and once that is clear the supplier and the inspector stop contradicting each other.
Synthetic micro-fibres — the product this calculator doses, at something around a kilogram of fibre to a cubic metre of concrete — are aimed at a window that closes before the first day is out. While the slab is still plastic and the surface is losing water faster than bleed water can replace it, the top few millimetres shrink against concrete that has not moved yet, and you get plastic shrinkage cracking: the fine random map of surface cracks that a windy, sunny, low-humidity pour produces. Millions of fibres give that plastic matrix a little tensile capacity at the one moment it has none. It is a real job, and it is done well. By the time the slab is hard, at that dosage, the manufacturers' own technical data sheets stop claiming a structural contribution.
Steel starts working where fibre stops. Its job in a slab-on-ground is not preventing the crack but holding it shut — keeping the aggregate interlocked across the fracture so the two sides stay one plane carrying load together, rather than two slabs free to settle differently and rock under a wheel. And where the slab has to span something — a soft patch of subgrade, a service trench backfilled across it, a thickened edge under a bearing wall — steel is the only one of the two supplying the tension that spanning needs. Which is why both statements you have been handed are true: the fibres do something real, and they do not do the thing the drawings are asking for.
The factors that actually differ
| Steel rebar / mesh grid | Fibre-reinforced concrete | |
|---|---|---|
| The failure it fights | The crack after it forms: holds it tight, keeps load transfer across it, stops the two sides working independently. | The crack before it forms — plastic shrinkage in the first hours, while the surface dries faster than the slab can bleed. |
| When it is working | From final set onward, for the whole life of the slab. | From placing until the concrete stiffens; its contribution is largely spent by the end of day one. |
| Structural capacity | Real and designable — spans soft spots, carries turndowns and thickened edges, dowels across joints, takes point loads. | Essentially none at shrinkage-control dosage. Macro fibre at several times that dosage can be designed to carry load, but that is a different product on a different specification. |
| Where the cost sits | Material plus a distinct crew-day before the pour. The mat scales with slab area, edge and turndown bars with perimeter, and ties multiply as spacing tightens: halve the grid and the bar length doubles while the tied intersections quadruple. | A line on the ready-mix ticket and no site labour at all. It is dosed by volume, so it scales with thickness as well as area — a thicker slab needs more fibre for the same footprint, while one mat of steel serves both. |
| How late you can change your mind | Before the truck. Once concrete is over it the decision is permanent — nothing retrofits reinforcement into a poured slab except breaking it out. | The morning of the pour, by phone. It is a change to the batch, not to the site. |
| What ruins it | Position. Steel lying on the subgrade — mesh hooked up mid-pour, chairs stepped down — contributes very little, and it is the usual finding when a reinforced slab cracks badly. | Dosing and mixing. Fibre needs its specified mixing time at full drum speed to disperse rather than ball, and the bags that never went in are invisible afterwards. |
| Corrosion and cover | Needs concrete cover top and bottom. In a thin exterior slab taking de-icing salt, short cover eventually means rust — and rusting steel expands and spalls off the surface above it. | Synthetic fibre does not corrode, which is its real edge in thin exterior slabs. Steel macro fibre near a broomed surface can rust-stain cosmetically. |
| Control joints | Continuous steel resists a sawcut opening, so joint layout and reinforcement have to be designed together — or the joint fails to activate and the slab cracks somewhere you did not choose. | No interaction: a fibre-only slab saws and opens cleanly. It also does nothing to let you space the joints further apart. |
Which one, and when
Choose steel rebar / mesh grid when…
- The slab does something structural — a bearing wall or post on it, a monolithic turndown acting as its own footing, a garage floor under vehicle wheel loads.
- Support underneath is not uniform: fill, a backfilled service trench crossing the slab, expansive clay, frost-susceptible ground. Steel is what lets a slab bridge a soft spot instead of hinging over it.
- The drawings or the local code prescribe reinforcement. Swapping fibre in for prescribed steel is an engineering decision with a signature on it, not a supplier's recommendation.
- You care what happens after the crack, because there will be one. Crack width and load transfer are steel's job, and nothing else in this comparison does them.
Choose fibre-reinforced concrete when…
- A genuinely non-structural slab on a properly compacted, uniform base: a shed floor, a path, a bonded topping, with nothing spanning and nothing bearing.
- The section is too thin to give a bar its cover top and bottom. Steel on short cover in an exterior slab taking de-icing salt eventually rusts and spalls the surface off above it, and synthetic fibre has nothing in it to corrode.
- You need the sawn joints to activate where you cut them. Continuous steel resists a joint opening and has to be laid out around it, whereas a fibre-only slab saws and opens cleanly at the line you chose.
- Site labour and programme are the constraint on a slab the design does not require steel in — no crew, no chairs, no pre-pour hold point, and a decision that can still be made the morning of the pour.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- My supplier says the fibres mean I do not need rebar. Are they right?
- Ask which fibre, at what dosage. Micro-synthetic fibre at shrinkage-control dosage does not replace reinforcement, and the manufacturers' own technical data sheets say so plainly — get the sheet for the product actually being supplied and read it. Structural macro fibre, steel or synthetic, dosed several times higher, genuinely can replace nominal slab-on-ground reinforcement in some designs — but that substitution is made on a specified residual strength measured by a standard beam test, by an engineer, on the drawings. If the answer to 'which product, what dosage, what residual strength' comes back vague, the answer to your original question is no.
- Can I use both?
- Yes, and on a great many slabs that is simply the correct answer rather than a fence-sitting one. They do not interfere with each other: the fibre works while the concrete is plastic and the steel takes over once it is hard. Fibre in the mix changes nothing about how you place, chair or lap the bars, and the steel changes nothing about the dosage. It earns its place most obviously when a slab that already has to have steel in it is going to be poured into a hard drying day — wind, sun, low humidity, a warm mix — because that is precisely the window steel is not yet awake for.
- Will either one stop my slab cracking?
- No. Drying shrinkage is a property of concrete, not a fault, and a slab held down by friction on its base will crack somewhere. Three things actually decide the outcome: the subgrade under it, the joints you cut and when you cut them, and the curing. Get those wrong and neither steel nor fibre rescues the slab. Joints decide where the crack goes, steel decides what it does afterwards, and fibre helps through the first few hours — none of them is a substitute for cutting the joints on time.
- Does it matter where the steel sits in the slab?
- Decisively, and it is where most reinforced domestic slabs are let down. Steel resting on the subgrade has almost no leverage against the tension it was put there to resist, and no cover protecting it from ground moisture. It belongs on chairs at the depth the design gives, and it has to still be there after the crew has walked the mat during the pour. Rolled mesh is the worst offender — it wants to return to a coil and it ends up on the ground. Sheet mesh or bar on chairs is the version that works.
- The inspector and the supplier disagree. How do I settle it?
- With paper, before the pour. The inspector is enforcing the approved drawings rather than an opinion, so a change from steel to fibre has to arrive as a revision to those drawings from whoever stamped them, with the fibre manufacturer's data sheet attached. There is a practical asymmetry worth knowing too: steel is visible, photographable and usually a pre-pour hold point, whereas once fibre is in the drum nobody can see it — the delivery ticket is the only evidence it was ever added. Keep the tickets whichever way this lands.
- Do fibres change the pour or the finish?
- Somewhat. Higher dosages reduce slump and workability, which is worth raising with the supplier before you specify one rather than discovering it at the chute — a water reducer is the right response there, not more water. Fibre also needs its full mixing time in the drum at speed to disperse instead of balling. On the surface, fine synthetic fibres can stay visible on a hard-trowelled finish for a while and generally wear or weather away; on a broom finish they are rarely noticed at all.