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Re: concrete of forty years old

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Rpuebla,

	The increase of f'c with time was discussed on this list about December
20, 2001.  Many people contributed at that time.  You should access
those through the archives.

	Syed has provided good advice (as always).

	Concrete strength increases with age.  Deterioration of concrete would
normally start at the outside.  Spalling, chemical corrosion, or other
deteriorating effects should normally be clearly visible.  Sulfate
attack is a possibility and others on the list can advise you better
than I can.  Chemical analysis of small chips of concrete and/or nearby
soil can give you a lot of information on this possibility.

	Deterioration of the reinforcing steel is another matter.  Reinforced
concrete structures exposed to some chemicals (the most common is salt,
NaCl, from the ocean or when used to remove ice, but there are others)
can deteriorate very quickly.  These chemicals cause the reinforcing to
rust; the rusting steel grows larger like a tree root and breaks the
surface concrete away causing a delamination type failure.  If you are
within 1 mile of the ocean this is something to check, otherwise I think
it's unlikely.

	I do think you need to do some testing of both the concrete and the
steel both for strength and for chemical contamination.

	I hope this is helpful.

				Regards,

				H. Daryl Richardson

syed faiz ahmad wrote:
>
> Rpuebla,
>
> Daryl put the question which I also wanted to put.
>
> As, I understand from your description (ur english is far better than my
> Spanish/Portugues is?), you have two parts of ur question:
>
>      1. Do oncrete deteriorate in terms of its
>         compressive strengths over a period of forty
>         years?
>
>      2. The Code requirements for design of columns
>         were not the same as it is now?
>
> My response to your first question is the compressive strengths of concrete
> continue to progressively increase as the time period elapse; for an idea, a
> 365 day strength becomes approximately 1.56 times its 28-day strength.
>
> But having said that, you must understand that this is the progressive
> increase in compressive strenghts which is because of the reason that
> HYDRATION PROCESS (chemical reaction b/w cement & water) which continues to
> go on for an indefinite period. But at the same time if things do not work
> well there could be deterioration of concrete due to exposure to aggressive
> environments; the greater the severity of aggressiveness of the exposed
> environment, the greater the deterioration . This deterioratio of concrete
> could be spalling of concrete due to Corrosion of rebars or just cracks &
> rupture due to alkali-silica reaction, so on  & so forth.
>
> But the fact remains that the strength of concrete do continue to
> progressively increase with the passage of time.
>
> As for the second question; yes you are absolutely right Codal provisions
> have changed considerably over last forty years. Earlier the column sections
> used to be thick & robust (due to working stress design probably & poor
> analytical tools!?...) and now the sections are slimmer comparatively. Also
> there is a minimum/maximum steel ratio requirement which should be different
> from those forty years ago.
>
> So, if you are into a problem of analysing the capacity of the column
> section my advise to you is:
>
>     -  you can safely assume that the strenghts
>        have increased since then; in case of doubt
>        you can go for some samples of core tests.
>
>     -  for column section & its reinforcement, i
>        don't know what Codes of Practice is
>        applicable in your country; if it were
>        ACI-318, you can certainly assume it was
>        based on ACI-318/63.
>
>     -  Just in case you do not know it already,
>        please be informed 1963 was the year which
>        saw the TRANSITION from the Working Stress
>        method of Design to the modern day Strength
>        Design. That is why this Code is basically a
>        Code based on woking Stress Theory & has an
>        appendix on the Strength Design methods as
>        well.
>
> I hope I have been of some help to you & now over to our friend Daryl.
>
> Regards,
>
> SYED FAIZ AHMAD; MENGG, MASCE
> SENIOR STRUCTURAL ENGINEER
> SAUDI OGER LTD
> RIYADH, SAUDI ARABIA.
>

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