Rebar Explained: Sizes, Grades and How It Is Specified

Rebar, short for reinforcement bar, is the steel placed inside concrete to give it the tensile strength concrete lacks on its own. Concrete is excellent at resisting compression but weak in tension, so almost every structural concrete element — slab, beam, column, wall and foundation — contains steel bars positioned to carry the tension. Rebar is the quiet partner that makes reinforced concrete one of the most important materials in modern construction.

This article explains what rebar does, why it is placed where it is, how it bonds to the concrete, and why correct placement and cover are not just details but the difference between a sound structure and a defective one.

Why concrete needs steel

When a concrete member bends, one face goes into compression and the opposite face goes into tension. Concrete handles the compression well but cracks under even modest tension. Without reinforcement, a concrete beam would crack and fail the moment it was loaded. Rebar solves this by sitting in the tension zone and carrying the tensile force across the cracks, holding the member together and giving it strength and ductility.

This partnership works because steel and concrete bond together and expand at similar rates with temperature, so they act as one composite material. The full principle is set out in reinforced concrete explained; rebar is the steel half of that partnership.

Where rebar goes — and why it matters

Rebar must sit where the tension is, and that location changes with the member:

In a normal beam, the main bars sit near the bottom, because the underside is in tension.
In a cantilever, the main bars sit near the top at the support, because that face is in tension — the opposite of a beam.
In a column, bars run vertically to help carry load and resist bending, tied with links to hold them and prevent buckling.

Getting this placement wrong is one of the most serious errors in concrete construction. Bars laid in the wrong face, or at the wrong depth, leave the tension zone unreinforced and the member dangerously weak — even though it may look identical from the outside. This is why reinforcement is set out on drawings by an engineer and why its placement is inspected before any pour, a responsibility tied to what a structural engineer does.

Bond, anchorage and laps

Rebar only works if it is firmly bonded to the surrounding concrete, so that force passes between the two. Modern bars are rolled with ribs (deformations) on their surface to grip the concrete mechanically. For a bar to develop its full strength, it must be embedded a sufficient length — its anchorage length — into the concrete; if it is cut short, it can pull out under load.

Where bars are too long to be a single piece, they are joined by overlapping two bars — a lap — long enough to transfer the force from one to the other through the concrete between them. Anchorage and lap lengths are calculated, not eyeballed, and short laps are a common defect that quietly undermines a structure.

Concrete cover: protecting the steel

The layer of concrete between the rebar and the outside surface is called the cover, and it does two vital jobs. It protects the steel from corrosion by keeping moisture and air away, and it provides the bond and fire protection the bars need. Too little cover and the steel rusts; rust expands, cracks the concrete, and the damage accelerates — a frequent cause of the deterioration described in types of concrete cracks.

Cover requirements increase in aggressive environments — near the sea, in wet areas, or where concrete is exposed to weather. On site, cover is maintained using small spacers that hold the bars off the formwork at the correct distance. Inadequate cover is one of the most common reasons older buildings suffer reinforcement corrosion decades later.

Grades, sizes and fixing

Rebar comes in standard diameters and strength grades, and the engineer specifies which to use, how many, and at what spacing. On site, the bars are cut, bent and fixed into cages tied together with wire, then positioned in the formwork with the correct cover before concrete is poured. The accuracy of this fixing — right bars, right place, right cover, properly tied so they do not move during the pour — determines whether the finished member matches its design.

Because so much depends on it, reinforcement is normally checked and approved before concrete is placed. Once the concrete is poured, the steel is hidden forever, so any error becomes permanent and invisible. That single fact is why rebar inspection is taken so seriously.

Deformed bars versus plain round bars

Rebar comes in two basic forms. Deformed bars have ribs rolled onto their surface, which lock mechanically into the concrete and give far better bond. Almost all main reinforcement today is deformed, high-yield steel. Plain round bars are smooth, mild steel with lower strength. They rely on friction and adhesion alone, so they need longer anchorage and hooked ends. These days they are used mainly for small links, dowels and fixings, and they are common in older buildings.

The difference matters when assessing an existing structure. Plain bars of lower strength in an older building cannot be treated as if they were modern high-yield steel, which is one reason an engineer checks the original drawings, or exposes and tests the bars, before allowing new loads.

Rebar sizes and grades

Rebar is made in standard nominal diameters, typically 6, 8, 10, 12, 16, 20, 25, 32 and 40 mm. Slabs and walls tend to use the smaller sizes, beams and columns the larger. A useful rule of thumb gives the mass per metre as the diameter squared divided by 162: a 12 mm bar weighs about 0.89 kg/m and a 25 mm bar about 3.85 kg/m.

The grade describes strength and ductility. In Singapore, weldable ribbed reinforcement is covered by SS 560, a modified adoption of the British standard BS 4449. Its grades B500A, B500B and B500C share a characteristic yield strength of 500 MPa and differ in ductility, from A (lowest) to C (highest). The 2024 amendment also introduced 600 MPa grades. Other markets use other systems: Japan specifies bars under JIS G 3112 with grades such as SD345 and SD390 and sizes named D10, D13, D16; the United States uses ASTM A615 bars numbered in eighths of an inch. The general principles behind steel grades are covered in our guide to steel grades in construction.

How rebar is specified on drawings

On a structural drawing, rebar is described in a compact notation that gives type, size, spacing and position. A slab note such as H12-150 B1 means high-yield 12 mm bars at 150 mm centres in the bottom outer layer. Singapore drawings often use T instead of H for high-yield bars, and R for plain round bars. Beam and column bars are listed by number and size, for example 4H25, with links called out separately.

Every bar is given a bar mark and scheduled on a bar bending schedule, which lists its shape, dimensions, quantity and mass so it can be cut and bent off site. On delivery, the steel should come with mill certificates showing it meets the specified standard. On larger projects, samples are also tested. Getting the specification right is the engineer’s job; seeing it built as drawn is the job of site inspection, as set out in what a structural engineer does.

Frequently asked questions

What is rebar?

Rebar, or reinforcement bar, is the steel placed inside concrete to carry the tensile forces that concrete cannot resist on its own, allowing the two materials to work together as reinforced concrete.

Why does rebar go at the bottom of a beam but the top of a cantilever?

Because the main bars must sit where the member is in tension; a normal beam is in tension on its underside, while a cantilever is in tension on its top face at the support, so the steel positions are reversed.

What is concrete cover and why does it matter?

Cover is the layer of concrete between the rebar and the surface; it protects the steel from corrosion and fire, and too little cover lets the steel rust, expand and crack the concrete, shortening the structure’s life.

Why are reinforcement laps and anchorage important?

Because a bar only develops its strength if it is embedded enough length into the concrete, and where bars are joined they must overlap sufficiently to transfer force; short laps or anchorage can let bars pull out under load.

Why is rebar inspected before concrete is poured?

Because once concrete is poured the steel is hidden permanently, any error in the bars’ size, position or cover becomes invisible and impossible to correct, so the reinforcement is checked against the drawings beforehand.

What does T12 or H12 mean on a structural drawing?

It denotes a high-yield deformed reinforcing bar of 12 mm nominal diameter, with T and H both used for high-yield steel and R used for plain round mild-steel bars; it is usually followed by the spacing in millimetres and the layer, such as T12-150 B1 for bars at 150 mm centres in the bottom outer layer.

What is the difference between B500B and B500C rebar?

Both have a characteristic yield strength of 500 MPa, but B500C has higher ductility, with a greater required elongation at maximum force and a higher tensile-to-yield ratio than B500B, which makes it the usual choice where a structure must deform significantly without fracturing.

Concerned about reinforcement in a structure or a concrete defect you have noticed? Talk to our consultancy or get in touch.