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Re: minimum fc for Residential slab on grade

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While I am no expert on the UBC code, the requirement (as far as I can
tell) only applies to "concrete members resisting earthquake-induced
forces." (i.e. section 1921 provisions)  To me, this would not apply to a
true slab-on-grade (i.e. non-structural where the concrete is strictly
there as a "transfer" material to the soil for the loads) nor would it
apply to any concrete member that is not considered part of the seismic
resisting system.

Of course, since I am not a building official in my neck of the woods nor
in California, my interpretation does not really mean too much, now does
it?  That still does not change the possibility that the code is being
mis-interpreted by code officials who do say that ALL concrete must be
greater than 2500 psi.

And, you are correct (at least in my experience) that the difference
between 2000 psi concrete and 2500 psi (or even 3000 psi) is rather minor.
Additionally, it has been my experience that you tend to get concrete that
is about 500 psi (or more) higher than what is typically specified.  When
I have specified 3000 psi concrete, the 28 day tests typically come back
at 3500 to 3700 and the strengths will only get higher from that date


Ypsilanti, MI

On Sun, 20 Jul 2003 JohnOttCE(--nospam--at) wrote:

> Dennis,
> Since the introduction of the 1997 UBC the requirement (and as important the
> enforcement) of the Code has been for all concrete to be Standard 2500 (f`c =
> 2,500 psi) and no Deputy Inspection unless the strength specified is greater
> than Standard 2500.
> This interpretation comes from the larger building departments and has spread
> to the outlying areas. I am surprised that it has taken this long to get to
> the jurisdictions that you work in.
> The builders complain, but in reality it only represents a small increase in
> the cement ratio to the rock and sand. Non-compliance is not an option.
> John Ott

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