What was said about freezing and acids is true. Add earthquakes to that 
if not properly reinforced. A lot depends on the original quality of 
materials and workmanship, but if it is still in good condition today, 
in particular a rather un-severe usage like a dwelling, then it is 
likely to survive many more years. Signs of distress would include 
spalling (loss of surface materials), cracks with movement, and other 
obvious to the eye defects. For protection, there are coatings that have 
some flexibility like urethanes, and pore sealers like silanes. Check 
with a commercial/industrial contractor's sealant supplier. Brand names 
include Tremco, Sika, Meadows, Euclid Chemical.  Generally one of their 
distributors will guide you correctly., and a few others.

The American Concrete Institute has numerous publications on 
deterioration/repair of concrete

 http://www.concrete.org/bookstorenet/default.aspx

My qualifications include registered engineer (Civil) in Ohio, where one 
of my areas of expertise is concrete.

AGSCalabrese wrote:
> A contractor told me today that concrete has a service life of 80 years.
> He said 40 years getting stronger and then 40 years of decline.
> This seems odd to me.   I am looking at my dwelling that was built in
> 1908.  The concrete foundation does not look like it is deteriorating.
> I recently cut a hole into the foundation and it was a b**ch.  
> ( difficult )
>
> What kind of lifetime can one expect from concrete that is exposed to  
> temperature cycling such as Denver, CO ?
> What kind of lifetime can one expect from concrete that is underground  
> ( 10 degrees C ) or inside a heated basement ?
> What sayest the august members of the PIClist ?
>
> What can be done to extend the life of existing concrete ?
>
> Best
> Gus
>
>   
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