Reinforced Concrete Design / How reinforced concrete works

Why concrete needs reinforcement

The one property that decides everything about how concrete is used.

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This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.

Checks needed for project use

A concrete-design reviewer must check the stated first-generation clauses, National Annex choices, detailing rules and factor sets. Numerical agreement alone does not establish code compliance.
First-generation EC2 flexural model and tensile-strength use require clause review; concrete tension is neglected for ordinary reinforced flexural ULS, not every concrete resistance mechanism.

These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.

Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.

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Why this matters

Concrete is strong in compression but has much lower tensile strength. Reinforcement carries tension across cracks, allowing concrete members to resist bending and other actions that would cause a plain section to fail.

By the end of this lesson you should be able to

State the approximate ratio of concrete's tensile to compressive strength
Explain why concrete has low and variable tensile strength
Say where reinforcement must go, and why
Describe what composite action requires

What you should already know

Bending moments and where tension occurs in a beam (Structural Analysis Fundamentals, Module 3)
Direct stress and strain (Structural Analysis Fundamentals, Module 7)
Bending stress and the neutral axis (Structural Analysis Fundamentals, Module 9)

Take a plain concrete beam and load it. Bending puts the top in compression and the bottom in tension — exactly as it would in a steel beam.

The compression side is fine. For C30/37, the characteristic cylinder compressive strength is 30 N/mm². That is a test property, not an allowable service stress.

The tension side is the problem. The same concrete has a tensile strength of roughly 3 N/mm², about a tenth of its compressive strength. So the bottom face reaches its limit long before the top face is anywhere near its own, and the beam fails at a small fraction of the load its compression side could have carried.

Worse, that 3 N/mm² is not a number you can rely on. Concrete fails in tension from the largest flaw present — a shrinkage crack, a void, a plane of weakness under a piece of aggregate. Test twenty nominally identical specimens in compression and the results cluster; test them in tension and they scatter. Concrete has low and variable tensile strength; the relevant design model must allow for this.

With no reinforcement, the first flexural crack has nothing to cross it. The beam fails suddenly, at a load governed by the weakest flaw on the tension face.

Note

Design ignores concrete in tension — deliberately

At the ultimate limit state, reinforced-concrete design assumes the concrete carries no tension at all.

This is not because concrete has no tensile strength. It is because that strength is small, variable, and already partly used up by shrinkage and thermal restraint before any load arrives. Counting on it would mean relying on the one property you cannot control.

So the design assumption is deliberately pessimistic: below the neutral axis, the concrete is treated as absent, and every newton of tension is carried by steel. Concrete's tensile strength does reappear later — it governs when the section first cracks, and it controls crack widths — and appears in other resistance models, including shear and anchorage; the zero-tension assumption here belongs to ordinary reinforced flexural section analysis.

Predict first

A plain concrete beam fails in bending. Where does the failure begin, and why?

The solution is to put a material that is good in tension exactly where the concrete is not: steel reinforcement, near the tension face.

For that to work, four things have to be true, and it is worth noticing how fortunate it is that they all are.

1.The steel must be strong in tension. Reinforcement has a characteristic yield strength of around 500 N/mm² — over a hundred times the concrete's tensile strength.
2.The two must act together. Steel embedded in concrete is gripped by it, through chemical adhesion, friction and, dominantly, the mechanical bearing of the ribs rolled onto the bar. This is bond, and it is what makes the pair behave as one member rather than two.
3.They must expand and contract alike. Steel and concrete have almost the same coefficient of thermal expansion, around 10 to 12 × 10⁻⁶ per °C. Had they differed appreciably, ordinary temperature change would have destroyed the bond and the idea would never have worked.
4.The concrete must protect the steel. Steel corrodes; the alkaline environment inside concrete passivates it, and the cover keeps corrosive agents away. Concrete also insulates the steel in a fire, which matters because steel loses strength rapidly with temperature.

Bond, suitable cover and compatible thermal behaviour allow the two materials to work together.

With reinforcement the beam still cracks — but the bars cross the cracks and carry the tension, so the member goes on carrying load. Many fine cracks form instead of one wide one.

Worked example

How much load does the tensile strength actually buy?

Given

Plain concrete beam, rectangular, 300 mm wide × 550 mm deep, spanning 5.00 m simply supported
Concrete of class C30/37, so mean tensile strength f_ctm ≈ 2.9 N/mm²
Compare with the same section reinforced with 3 no. 25 mm bars at an effective depth of 490 mm

Find

The moment at which the plain section cracks, and the moment the reinforced section can resist.

Practice

A plain concrete section is 300 mm wide and 550 mm deep. With a mean tensile strength of 2.90 N/mm², at what bending moment does it crack, in kNm?

Practice

For a concrete of class C30/37 with f_ck = 30 N/mm², the mean tensile strength is f_ctm = 0.30 f_ck^(2/3). What is f_ctm, in N/mm²?

Practice

Steel has a coefficient of thermal expansion of about 12 × 10⁻⁶ per °C and concrete about 10 × 10⁻⁶ per °C. Over a 30 °C temperature rise, what is the difference in free strain between them, in microstrain?

Summary

Concrete's tensile strength is roughly a tenth of its compressive strength, and far more variable
Reinforcement goes where the concrete would otherwise be in tension
Bond, matched thermal expansion, alkaline protection and fire cover make the partnership work
At the ultimate limit state, concrete in tension is assumed to carry nothing
Its tensile strength still matters — for when cracking starts, and how wide cracks become