Sorry to hear about trip problems - Quit flying our plane 3 years ago, 
haven't been on an commercial plane in 20 years. Enjoy driving, helps to 
have big engine :)

Nothing that goes against my thinking below. :) Most reinforced (and 
prestressed) deteriorate due to the steel rusting from chlorine atoms 
(think road deicing salt - NACL) reaching the steel. The chlorine atoms 
can migrate through good solid concrete. When the chlorine atoms get to 
the steel, they combine with the iron to make FECL (my chemistry is a 
little rusty with the subscripts being off, but the idea is here). The 
FECL breaks down to make rust (FEO?) and free chlorine (CL2). The rust 
occupies more space than the previous items, plus that combination 
reaction generates considerable force outward, and as was pointed out, 
tension is weak in tension, causing cracks, and eventually say on a 
bridge deck, popping a large area loose. There have been recent 
developments, including epoxy coated steel, an cathodic protection to 
help this.

Concrete in the routine construction project will have a compressive 
strength in the 3,000 - 5,000 psi. ultimate compressive strength range, 
and a ultimate tensile strength of only in the 100 psi. range. High 
strength concretes can go to 10,000 psi., and I recently read of 15,000 
psi. being used for the first time in a commercial high rise project. 
Factors of safety usually are in the 1.4 - 2.0 range, but the building 
codes define much more exact. Generally the allowable tensile strength 
is considered zero, some low number.

Other causes of deterioration include aggregates (stones) that don't 
resist freeze/thaw, chemicals in the concrete mix like sea water or 
other chemicals with the chlorine/steel reaction as above. Generally if 
the water is drinkable it is OK to use in concrete, but use that comment 
with care!

Russell McMahon wrote:
> In Hong Kong.
> Know nothing formal about concrete.
> Choose any two.
>
> Email letter should happen sometime for those 'subscribed'- too busy so far.
> Having our fares all cancelled one day out and having to rebook whole
> trip helped increase the fun level.
> Anon.
> Snippets - worth what you paid for them Accuracy variable.YMMV.
>
> CO2 increases concrete strength by converting it back to the Calcium
> Carbonate it started out as.
> Airborne CO2 does this over decades.
>
> Romans made stuff from burned limestone that wasn't concrete. Some
> structures made with this still remain. The great discovery was that
> adding fly-ash made a vast difference to the properties.
>
> Many Roman structures were concrete with brick being added as facings
> for cosmetic appearance. Much of the Colosseum is concrete.
>
> Roman bricks have an aspect ratio that offends my sense of how a brick
> should look. Much longer and thinner than the current norm.
>
> WW2 concrete structures will now be around 60-70 years old. Those
> remaining here seem very solid but with surface degradation. Load
> bearing structures no doubt a different matter.
>
> We have a large road viaduct that carries the busiest section of the
> NZ highway system across a city shopping area. Mayhaps 40-50 years
> old. It's mechanically worn out and due for replacement. Looks fine to
> the uninitiated eye.
>
> Concrete has about zero tensile strength. Pull on the cement glue that
> holds all the rocks and sand to eac other and it crumbles. Push on it
> and you force the parts together iup to a limit. SO concrete in
> tension can be thought of as a means of resisting the tension of the
> reinforcing material which does the actual structural work.
>
> Carl and others expert may now tear this apart :-).
>
>     Russell
>   
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