Reinforced Concrete Design / How reinforced concrete works
Bond, cracking and what changes at first crack
The moment a reinforced section stops behaving like a lump of concrete.
Lesson review: expert verification remains open
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.
- Crack width requires integrated strain difference and appropriate crack spacing; crack assessment depends on function, exposure and crack 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.
Read the lesson register and outstanding review notesWhy this matters
An ordinary reinforced beam may crack under service loading. Reinforcement carries tension across those cracks, while the concrete between them can still carry tension through bond. Understanding that change is needed for stiffness and crack-width calculations.
By the end of this lesson you should be able to
- Explain how bond transfers force between concrete and steel
- State the strain-compatibility assumption and its limits
- Describe what changes at first cracking
- Explain why many fine cracks are better than one wide one
What you should already know
- Why concrete needs reinforcement (this module)
- Strain compatibility in a bent section (Structural Analysis Fundamentals, Module 9)
A bar cast into concrete is gripped by it. Three mechanisms contribute, in ascending order of importance:
- Adhesion — a chemical bond at the interface, which breaks at very small slip.
- Friction — resistance to the bar sliding once adhesion has gone.
- Mechanical interlock — the ribs rolled onto a deformed bar bear directly on the concrete between them.
For a modern ribbed bar the third dominates so completely that the first two are effectively a bonus. The ribs push against the concrete, which pushes back; that bearing generates a ring of tension around the bar, which is why bond is improved by cover and by links, and why a bar too close to a face can split the concrete off rather than develop its force.
The consequence of bond is strain compatibility: at any section, the strain in a bar equals the strain in the concrete immediately around it. Not the stress — the two materials have very different moduli, so their stresses differ by that ratio. The strain is what they share.
That single statement is what allows a section to be analysed at all, and it appears in every derivation in this course.
Now follow a beam from zero load and watch what happens.
Uncracked. At low load the whole section acts, including the concrete in tension. The neutral axis sits close to mid-depth. The section is stiff, and the steel carries very little — it has almost the same strain as the concrete next to it, and concrete at that strain carries very little stress, so the steel does too.
First crack. The tensile stress at the extreme fibre reaches the concrete's tensile strength. A crack opens. At that section the concrete below the neutral axis stops carrying tension — instantly and completely.
What changes. The tension that concrete was carrying has to go somewhere, and there is only one place left: the bar. Steel stress jumps sharply at the cracked section. Because the tension zone has been lost, the neutral axis rises and the section becomes markedly less stiff.
What does not change. Equilibrium. The compression resultant still equals the tension resultant, and their couple still equals the applied moment. Cracking changes the section, not the physics.
Beyond first crack. More cracks form, spaced along the tension zone. Between cracks the concrete still carries some tension, picked up from the bar through bond — this is tension stiffening, and it is why a real cracked beam is a little stiffer than a fully cracked calculation predicts.
Predict first
When a reinforced beam first cracks, what happens to the neutral axis?
One more idea completes the picture, and it explains a lot of detailing practice.
A crack is not a failure. What matters is how wide it is, because width controls whether moisture and chlorides can reach the steel.
Given a fixed amount of extension to accommodate, you can have it as one wide crack or many fine ones. Many fine ones are far better, and what produces them is well-distributed reinforcement: several smaller bars rather than a few large ones, at a modest spacing.
The reason is bond. Each bar can only transfer force back into the concrete over a certain length. Closely spaced bars, with more total surface area for the same steel area, re-anchor the concrete more often, so a new crack forms sooner and the extension is shared between more cracks.
This is why crack-control rules limit bar spacing and bar diameter rather than simply demanding more steel. Two 32 mm bars provide about 1608 mm²; four 25 mm bars provide about 1963 mm², 22% more. To isolate the effect of diameter on crack control, compare arrangements with similar steel area and service steel stress.
Worked example
Steel stress before and after cracking
Given
- Rectangular section 300 × 550 mm, effective depth 490 mm, with 1470 mm² of tension steel
- Concrete C30/37: Ecm ≈ 33 GPa. Steel: Es = 200 GPa
- Applied moment 40 kNm, just below the cracking moment of about 44 kNm
- Then the same section at 120 kNm, comfortably cracked
Find
The steel stress in each state, to see the size of the jump.
Practice
Concrete has Ecm = 33 GPa and steel Es = 200 GPa. What is the modular ratio Es/Ecm?
Practice
A cracked section carries a moment of 120 kNm with 1470 mm² of steel at an effective depth of 490 mm. Taking the lever arm as 0.87d, what is the steel stress, in N/mm²?
Practice
Bond depends on bonded surface area, and what matters is how much surface you get for each square millimetre of steel. Compare 25 mm bars with 32 mm bars: by what factor do the smaller bars give more perimeter per unit of steel area? Give the ratio to two decimal places.
Summary
- Bond comes mainly from mechanical interlock of the ribs, not adhesion
- Bond gives strain compatibility: steel and adjacent concrete share strain, not stress
- At first crack, the concrete tension disappears, the steel stress jumps and the neutral axis rises
- Equilibrium is unchanged — cracking changes the section, not the physics
- Tension stiffening: between cracks the concrete still helps, so real beams are stiffer than a fully cracked calculation
- Many fine cracks beat one wide one, which is why rules limit bar spacing and diameter