Kumar IronUpdated
In this guide
Light rust is usually fine
A thin, uniform orange film that rubs off with your hand or a rag is surface rust. It forms on all steel within days of rain. It does not reduce the strength of the bar in any meaningful way, and concrete bonds well to it once loose scale is removed.
Heavy, dark rust is different. If rust has started to eat into the bar, cross section is lost and the ribs that grip the concrete are worn.
Surface rust vs dangerous rust
| Condition | What you see | Verdict |
|---|---|---|
| Light surface rust | Thin orange film, ribs sharp, wipes off | Use after brushing |
| Scaly rust | Flakes that peel off in layers | Brush hard, then run the weight test below |
| Pitting | Small craters or rough pits after brushing | Reject or ask the engineer |
| Section loss | Ribs worn smooth, bar feels thinner | Reject |
The wire brush and weight test
- Cut a 1 m piece from a rusty bar, and keep a clean 1 m piece of the same size for comparison.
- Scrub the rusty piece hard with a stiff wire brush until no loose rust is left.
- Weigh both. Compare with the nominal weight for the size in our weight chart.
- If the cleaned piece is within the normal weight tolerance for the size, the loss is not significant.
- If it is clearly lighter than the tolerance allows, or the surface is pitted, do not use it for structural members.
You can also look at the ribs. If the cross ribs and longitudinal ribs are still sharp and clear after brushing, bond with concrete is intact.
Fixing and preparing rusted bars
Remove loose scale with a wire brush, and for larger quantities a wire wheel on a grinder. Do not use acid. Do not oil or paint the bars, because oil and paint reduce bond with concrete.
Brush right before the bars go into the formwork, not days earlier, so a fresh film does not form. Rust stains on the bars after the pour is a sign the cover was too thin or water was getting in.
Storing steel on site
- Keep bars off the ground on bricks, timber or concrete blocks, so they do not sit in mud.
- Cover with tarpaulin, but leave the sides open so air can pass. A sealed plastic wrap traps moisture.
- Keep different sizes and grades in separate stacks with the tags attached.
- Do not stack next to a wall that stays wet, or near salt, chemicals or cement bags.
- Use bars in the order they arrived.
Cover blocks protect the steel inside the slab
Once bars are in concrete, the cover is the layer of concrete between the steel and the outside. If the cover is too thin, moisture reaches the steel and rust spreads, cracking the concrete. Typical minimum cover for mild exposure is about 20 mm in slabs, 25 mm in beams, 40 mm in columns and 50 mm in footings, but your drawing governs.
Use proper cover blocks of concrete or plastic at the right spacing, not broken bricks or pebbles. Check the cover before the pour, and call the engineer if the bars are touching the shuttering.
Frequently asked questions
- Does rust reduce the strength of TMT bars?
- Light surface rust does not matter. Rust that causes pitting or loses section reduces strength and bond.
- Can I use rusted bars in a slab?
- If they pass the brush and weight test and look clean, usually yes. Ask your engineer when in doubt.
- Should I paint or oil the bars?
- No. Both reduce bond with concrete. Cement slurry coating is sometimes used on exposed starter bars, check with the engineer.
- How long can bars sit before they rust?
- Surface rust shows within days in the monsoon. Cover and lift them off the ground and they stay in good shape for months.