Rick Karlquist wrote:
>Let me make sure I understand this correctly. A piece of rebar
>poking out of the concrete below grade will rust. A piece of
>steel poking out above grade will not rust? Why is this?
Both will rust, outside the concrete. The alkalinity of the concrete
will delay corrosion of the steel inside the concrete, but this
protection will be lost if there is an opening below ground that permits
exchanging ions with the ground. The alkalinity of the concrete is
neutralized faster through an opening below grade where water, bearing
dissolved compounds, is exchanged between the ground and the interface
of concrete and steel. Complete closure prolongs the protection. I
don't know for how long, if it is measured in years, decades, or centuries.
It probably doesn't make much difference if the soil is strongly
alkaline. It will make a difference in the life of the steel if the
soil is acidic. Acid attacks iron.
I'm not sure what effect corrosion of the re-bar or other steel members
will have on the integrity of a tower base. The products of corrosion
occupy greater volume than the original steel and will exert great
pressure on the concrete, which is not particularly strong in tension.
That may crack the concrete. Continued corrosion will weaken the steel,
thus defeating the purpose of having it there. The base may or may not
fail because of either of these effects. However, I'd rather not take
Above ground, rust inhibiting paint will slow the exterior rusting and
there is less exchange of water and dissolved matter between the
steel-concrete interface and the ground that attacks the steel inside
>This brings up the question of anchor bolts: galvanized or black
>steel? I notice that UST supplies non-galvanized bolts. However,
>every streetlight I've seen uses galvanized bolts. According to
>what you say, the black steel is OK on account of it being above
>grade. Right? I noticed that the UST bolts are already rusty
>when you get them from UST, not to mention what happens after the first
>rain. Any advice here?
Sorry, I haven't researched the black steel characteristics. All of my
experience with steel, other than re-bar, in towers and tower bases has
been with galvanized steel.
Perhaps someone else will contribute better information for all of us.
73 de WOØW
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