Reinforcing steel: what Grade 60 means, and how to inspect rebar on site

Reinforcing steel gives concrete its ability to resist tension. Concrete alone is strong in compression and weak in tension, and the steel makes up for that weakness.

1) Grades under ASTM A615

The American standard ASTM A615 covers deformed and plain carbon-steel reinforcing bars in several grades1. The most common:

Source: ASTM A6151. Grade 60 is sometimes written in metric as "Grade 420".

• Yield strength: the stress at which steel starts to deform permanently; structures are designed on it.
• Tensile strength: the maximum stress before breaking.

2) What to check on site

1. Mill certificate for each shipment, matching the required grade.
2. Diameters, numbers and spacing matching the structural drawings for each element.
3. Lap lengths and their locations per the drawings.
4. Concrete cover: spacers/chairs between steel and formwork at the specified thickness.
5. Cleanliness: steel free of oil, mud and flaking rust before pouring.
6. Approval of the steel by the supervising engineer before pouring, documented with photos.

3) Bar identification marks, certificates and storage

In the U.S. system, bars carry rolled-on marks in this order2:

1. The producing mill's symbol.
2. The bar size.
3. The steel type: usually S for carbon steel (ASTM A615) or W for low-alloy steel (ASTM A706).
4. The grade mark: a number (60, 75, 80...) or longitudinal lines; one line for grade 60, two for 75, three for 80 and 100.

Mills producing to other standards may use a different marking system; the reference is the mill certificate for the standard the steel was produced to.

1. Photograph the marks on a sample from each shipment and compare them with the mill certificate (practical advice).
2. Keep the mill certificate for each shipment in the quality file. See construction quality checks.
3. Storage: keep steel off the ground on supports, covered from rain and damp, and away from mud and oil (practical advice); mud, oil and flaking rust are removed before pouring, per the checklist above.
4. No sizes or substitutes of your own: every diameter, grade and spacing comes from the approved structural drawing.

4) Corrosion: the silent enemy

Steel in concrete is protected by concrete's alkaline environment and the cover. If the cover is thin or the concrete porous and poorly cured, water and salts reach the steel, which rusts, expands and bursts the concrete around it. So steel durability depends on the quality of concrete and its curing and on the right cement type for the environment. See rebar corrosion.