Cob: concrete is forever?

[DoNegard at aol.com]

Eric Straatsma
<< I have heard it said by at least one post,  that concrete is a forever
 material. I am here to give the opposing viewpoint.  Concrete does decay and
 degrade depending on many factors that cannot be controlled 100%.>>
I would like to know the source of your information, and the source of the 
information of the "concrete is forever" viewpoint.  I am going to be using 
some of the information I get here to build my dream house, and I have to 
have substantiated facts and knowledge before I begin.  Nothing less will 
satisfy me.

<< For example, lets assume that concrete is like a rock.>>
Is there a way that you could come up with this information withoug making 
any  assumptions?  Whenever I make assumptions, I believe I am taking quite a 
risk, sometimes with my personal safety, sometimes with my knowledge.

<< Rocks absorb moisture and concrete does too. As this moisture freezes and 
thaws, inside of concrete pores,  it breaks up the concrete from inside, 
expanding cracks and breaking off small pieces. All rocks go through the same 
process. Nature makes sand out of rocks eventually.>>
Are these statements supported by the actual testing of concrete?
 
<< Second, rocks are subject to heating and cooling stresses, that break up
 rocks also. If concrete is heated to 130 during the day and then cooled to
 50 degrees at night, it contracts and expands, which causes cracking and
 eventually destruction. It takes a long time, but it happens in rocks too.>>
Perhaps only the surfaces of the concrete go through the daily temperature 
swings.  Could it be that concrete has enough elasticity to allow for these 
daily temperature extremes without causing damage to the concrete?  I don't 
know, but would like to know.
 
<< Third, concrete is subject to chemical weathering, in acid soils especially
 or in acid rain areas, just as rocks are.>>
In my old area (Minneapolis, MN) it appears to me - from what I read in the 
paper, and what it looks like to me personally - that the main attacker of 
concrete (besides wear from traffic) is the salt used to melt ice and snow in 
the winter.

Don in Hot Springs, SD