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RE: Grout Strength

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Masonry is a composite material composed of masonry units, mortar, and
grout.  What you are ultimately after is the f'm of the composite.  The
testing methodology is best described in the ACI 530 as opposed to the UBC.
There are 2 ways of compression testing masonry (1. Prisms or 2 Unit
Strength).  The prism test has been recognized as the method that is
allowed, even if the unit testing fails.

I guess I don't understand why you have a problem is the f'm works in a
prism test.

It is legitimate to have f'm of 3000 psi and grout strengths of 2000 psi.
If you want to really get worried check the compressive strength of a
cylinder of mortar.  The point is that it doesn't matter what the unit
strengths are.  It is the prism that is critical. 

Keep in mind that even for concrete, you are allowed to have individual
tests less than the specified f'c.

I have had more problems with how the unit tests are performed.  Make sure
that they did the grout testing per ASTM C1019.

The problem with testing a core is that you can not extrapolate cylinders to
prisms well at all.   If you do anything, I would suggest cutting grout
prisms from the cores, but again, I would focus on the prism testing.

Harold Sprague

> -----Original Message-----
> From:	Brian K. Smith [SMTP:smitheng(--nospam--at)]
> Sent:	Friday, February 25, 2000 9:42 AM
> To:	seaint(--nospam--at)
> Subject:	Grout Strength
> 	I have a project currently under construction in which we have low
> grout
> strength.  I specified a grout strength of 3000 psi; however, we are
> getting
> 28 day strengths just over 2000 psi.  One break had a 28-day strength of
> 1970 psi which is lower than the 2000 psi minimum specified by UBC.
> Masonry
> prisms were specified, however, I have not seen the results.  Even if I
> surpass the f'm=2000 psi specified, I still do not meet the UBC minimum.
> 	The problem is, as always, the wall is partially in place.  I would
> like to
> determine what strength we actually have before the wall is completed.
> The
> wall is partially grouted, 32"oc.  However, I specified the bottom 4' of
> wall to be grouted solid.  Can I core this section of wall, and then run a
> compressive test on the core?  Do they need to saw off the face shells
> prior
> to the test?
> Any help will be appreciated.
> Brian K. Smith, P.E.