Reinforcement is the steel placed in concrete to give it tensile strength. Concrete is extremely strong in compression but weak in tension. Steel is strong in tension. By placing steel in concrete, you combine the best of both materials — this is called reinforced concrete.
Why is reinforcement needed?
A concrete slab or beam that is loaded will deflect. The top is compressed (compression), the bottom is stretched (tension). Concrete handles compression well but cracks under tension. The reinforcement sits on the tension side and takes that force.
Without reinforcement:
- A concrete slab cracks when it deflects
- A foundation can break with uneven settlement
- A lintel above a window opening can fail
Types of reinforcement
Rebar (reinforcing bars)
Round steel bars with ribs for better bond. Cut and bent to size.
| Diameter | Application |
|---|---|
| 6 mm | Light structures, stirrups |
| 8 mm | Floor slabs, foundations for garden rooms/sheds |
| 10 mm | Strip foundations, lintels |
| 12 mm | Heavier foundations, garages |
| 16 mm+ | Professional construction, large spans |
Welded mesh (steel mesh)
Prefabricated mesh of welded bars. Convenient for floor slabs:
| Designation | Bar diameter | Grid | Application |
|---|---|---|---|
| 6-150 | 6 mm | 15 cm | Light floor slabs |
| 8-150 | 8 mm | 15 cm | Standard garage floor |
| 10-150 | 10 mm | 15 cm | Heavier structures |
Fibreglass reinforcement
Plastic alternative to steel. Does not rust, but is less strong. Used in specific applications (swimming pools, chemical environments).
Placing reinforcement — basic rules
1. Cover — The reinforcement must be at least 3 cm from the outside of the concrete. This protects the steel against moisture and corrosion. Use spacers (plastic blocks) to ensure the correct cover.
2. Position — In a floor slab, the reinforcement sits at the bottom (where tension occurs). In a cantilever, it may be at the top.
3. Overlap — Where two bars or meshes are joined, they must overlap by at least 40x the bar diameter. For 8 mm bars, that means at least 32 cm overlap.
4. Tying — Bars and meshes are tied together with binding wire so they do not shift during pouring.
Reinforcement for DIY projects
Concrete slab (garage floor)
- Use welded mesh 8-150
- Place mesh on spacers (3 cm cover)
- Overlap at least 30 cm
- At edges and door openings: extra bars
Strip foundation
- Minimum 2 bars of 10 mm lengthwise
- Stirrups of 6 mm every 30 cm for shear
- Cover at least 3 cm
Concrete pad
- For a small pad (30×30 cm), reinforcement is not strictly necessary
- For a larger pad: bars crossed at 8 or 10 mm
Common mistakes
- Reinforcement on the ground — Without spacers, the reinforcement has 0 cm cover. The steel will rust and the concrete will crack.
- Too little overlap — If meshes do not overlap sufficiently, a weak point is created.
- Reinforcement displaced during pouring — Do not walk on the reinforcement. Check after pouring that the meshes have not shifted.
Related terms
- Concrete slab (floor slab)
- Strip foundation
- Shallow foundation
- Ring beam