Why is there steel reinforcement in concrete?
Concrete is strong in compression and weak in tension, so steel is placed where the tension is. Where that is, why cover matters more than quantity, and why reinforcement rusting is a structural problem rather than a cosmetic one.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 4 min
- Safety
- caution
Short answer
Concrete resists being squashed extremely well and resists being pulled apart very badly. Steel is the opposite, so bars or mesh are placed in the parts of an element that go into tension — the bottom of a beam or a slab spanning between supports, and the top over a support. The concrete around the steel is not decoration: it is what stops the steel rusting, and its thickness is specified.
This is the idea that explains a surprising number of things a householder actually meets: why a concrete lintel has a marked top face, why spalling concrete on a balcony is urgent, and why nobody should be drilling deep into a structural slab without knowing what is in it.
Safety
Step by step
- Separate compression from tension.Push on concrete and it is immensely strong. Pull it and it cracks at a small fraction of that. Every element that bends has one face being squashed and the opposite face being stretched.
- Put the steel where the stretching is.A beam or a slab spanning between two supports sags, so the bottom is in tension and the main bars go near the bottom. Over an intermediate support it hogs the other way, so there the bars go near the top.
- Understand why lintels are marked.A prestressed or reinforced concrete lintel has its steel concentrated near one face. Fitting it upside down puts the steel on the wrong side of the bending, which is why lintels carry a top mark and why it matters.
- Learn what cover means.Cover is the thickness of concrete between the steel and the surface. It protects the steel chemically — fresh concrete is alkaline and keeps steel passive — and physically. Cover is specified for the exposure, and it is not a tolerance to trim.
- Understand carbonation and why old concrete spalls.Over decades, carbon dioxide reacts with the surface concrete and reduces its alkalinity. Once that front reaches the steel, the steel can corrode. Rust occupies more space than steel does, so it splits the concrete off from inside. That is spalling, and the cracking is the symptom of a section already losing steel.
- Recognise mesh for what it is.Fabric reinforcement in a ground-bearing slab is usually there to control cracking and tie the slab together rather than to carry a span. That is a real job, and it is why the mesh has to sit at the right height in the slab rather than on the ground under it.
- Know when to stop and scan.Before drilling anything more than superficially into a concrete floor, ceiling, beam or lintel, find out what is in it. A cover meter or rebar scanner is a hire item, and the alternative is finding the steel with the drill.
Tips
- Mesh lying on the ground at the bottom of a slab does nothing useful. It needs supporting on spacers at the specified height, and this is one of the most common defects on small sites.
- Rust staining running out of a crack in concrete is a reason to get advice, not to paint over it. The crack is usually where the steel is.
- Stainless and galvanised reinforcement exist for aggressive exposures. If you are repairing exposed concrete near the sea, that is a question to ask.
Common mistakes
- Fitting a concrete lintel the wrong way up — The steel ends up in the compression face where it does nothing, and the tension face has none. The lintel is then a plain concrete beam, which is a very weak thing.
- Chasing a channel into a concrete floor or a lintel for a cable — Reinforcement sits close to the surface by design, at exactly the depth a chase reaches, and cutting a bar is not something that announces itself.
- Patching spalled concrete without treating the steel — The corrosion continues under the patch and pushes it off. Proper repair exposes the steel fully, cleans or replaces it, treats it and reinstates the cover.
If it doesn't work
A concrete lintel, canopy or balcony edge has cracked along its length with rust showing
Cause: Corroding reinforcement expanding and splitting the cover — Fix: Keep people from beneath it and get a structural engineer to look. Section loss in the steel is not visible from outside and this is not a filler job.
The drill has hit something hard and immovable in a concrete floor
Cause: Reinforcement at cover depth — Fix: Stop. Move the hole rather than persisting, and scan before the next one. A rebar cutting bit exists but using one on a structural element is a decision for an engineer.
Fine map cracking over a new slab surface
Cause: Plastic shrinkage while curing, not a reinforcement problem — Fix: This is a curing issue rather than a structural one. Keep new slabs damp and covered while they cure, particularly in wind and sun.
Questions people ask
Does a domestic garage or shed slab need mesh?
It depends on the sub-base, the thickness and what it carries. Mesh is common in slabs taking vehicle loads and in anything laid over ground that may settle unevenly. The specification should come from the design, not from a rule of thumb.
What are fibres in concrete?
Polypropylene or steel fibres mixed through the concrete to control shrinkage cracking and, for steel fibres, contribute some post-crack strength. They are not a direct substitute for designed structural reinforcement.
Why do bridges and car parks spall so much?
De-icing salt. Chlorides penetrate the concrete and break down the protection around the steel far faster than carbonation alone, which is why chloride-contaminated structures are treated seriously.