Barry Kutner wrote:
>Can someone explain to me why the rebar WITHIN the concrete doesn't
>rust? Concrete always retains moisutre and absords it from surrounding
>soil, doesn't it?
>Rust is oxidation. Being underground, whether in concrete or soil,
>doesn't the minimal oxygen content under ground retard the rust
>formation?
Rusting is a combination of oxidation and hydration, so it requires both
an oxidizing environment and free water.
When the cement paste cures by forming an interlocking crystal structure
(which gives its strength), it both shuts out oxygen and takes up water
into its crystal structure. There is always some free water in the
pores, but the surrounding cement makes that water highly alkaline which
inhibits rusting.
The ideal condition for rebar is very lightly rusted, which helps to
form a stronger chemical bond to the cement paste than slick bare steel.
If there is any further chemical attack when the concrete is curing,
think of it as bonding between the cement paste and the metal.
The aim is to place the rebar sufficiently deep in the concrete that it
inhibits oxygen diffusion, and deep enough to prevent oxygen reaching
the rebar through surface cracking.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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