Doug Miller wrote:
In article , RicodJour wrote:
On May 17, 8:00=A0pm, (Doug Miller) wrote:
How are they going to get bigger while they're encased in concrete?
It really surprises me that you have such problems understanding
this. Your personal observation of those funky brown stains on
concrete structures everywhere should indicate that something is
rusting. If you can't see what's rusting, and are keen enough to
realize that rust doesn't just magically appear on concrete, you'll
probably latch on that the rust is coming from inside the concrete.

In a nutshell. Concrete is not waterproof. It wicks up moisture.
The steel doesn't care that it is encased in concrete and will rust in
the presence of the H2O. Concrete sucks in tension - something on the
order of 1/10 its strength in compression. Constant tension on
concrete leads to cracking. Cracking allows in more moisture, and the
cycle continues until the structure falls apart.

I guess all those engineers who've been building things with steel-reinforced
concrete for all these years must be completely ignorant, huh?

I think it is a matter of construction and maintenance, not engineering.
On our condo, there have been several areas where rust caused bursting
of the concrete. Perhaps rebar was too close to the surface, or cracks
in stucco not patched and painted, allowing moisture and salt intrusion.
Another place was a concrete sill by a patio slider. We also have
iron railings on our balconies and atrium deck which are embedded in
structural concrete. They are the same railings used inside condo units
- the anchor is embedded and then the railings bolt onto the achors.
The exterior ones rusted badly years ago, to the extent that some rusted
through and the attachment was no longer identifiable. Along the atrium
deck (second story), several places where the anchors protrude busted
out the concrete. After grinding out the rust and patching and
painting, they hold for several years without further bursting of the
concrete. Enough anchors intact that nothing is loose. Several years
ago, it was a pretty big issue for condominium management companies -
can probably google up more info if you are interested. Perhaps it
isn't an issue at all if not near salt water.