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RE: Mortar Testing

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Ah.

I had not responded to your other post (about the bad experience project)
cause I was not sure if I had any sage suggestions.  I was also not
completely clear if I understood the issue completely...I seemed to get
that you did have prism tests done that had results that confirmed a
masonry compressive strength at to or close to what you specified (which
this message that you just send confirmed) but then did not understand why
your structural observation report would cause the building official to
shutdown the project when said prism tests we getting you masonry
compressive strengths in line with what you specified.

To me, the moral of this situation is that it reinforces the knowledge
that knowing the individual strength of the masonry components (i.e. CMU
strength, mortar strength, grout strength) does really correlate well with
the overall masonry assembly strength, necessarily.

FWIW, the Unit Strength Method in the MSJC predicts that with your
situation (i.e. 3750 psi CMU units and type M mortar) you should be
getting f'm of 2500 psi and for the actual units used (i.e. roughly 1800
psi for CMU) you should have gotten slightly less than 1500 psi for the
overall masonry strength.  The only requirement on the grout per the Unit
Strength method is that it meet or exceed your specified f'm (i.e. 3000
psi) or 2000 psi which ever is worse...and that is it tested per ASTM
C1019.  It is interesting to note that Unit Strength Method does not seem
to "account" for the effect of the grout on the over all strength of the
masonry.  This is probably what made the difference in your case as I am
assuming that your wall is fully grouted as is typical in California.  If
so, then I have no problem seeing the grout becoming the dominate
component in determining the strength of your masonry...just as your prism
test seemed to prove out.

Now, the one item of caution that I would offer is that prism strength can
be influenced a lot by how the test was done (i.e. end platen restraint,
aspect ratio [height] of prism, end platen plate thickness/stiffness).
Presumably the testing company adjusted to such account for such things,
but it is something to watch for.

I am curious...why did the building official shut things down after your
structural observation report if the prism tests showed satifactory
masonry strength?  Or had the prism tests not been done yet?  Or am I
missing something else?

I did take a look at the pictures...which causes me to raise another
question.  Where the circular holes the "cuts" for the prisms?  I did not
see any other holes in the masonry that seemed to be the right size for
prism cutting.  If so, then how did they test those prism?  Did they lay
the masonry in horizontal so that the compressive force was applied to the
circular surface.  If those circular cuts are for the prisms, then I would
call those more like "cores" and I am not sure that you are getting a true
representative prism test.  The point of doing a prism test (as you are
likely aware) is to get the effect of the mortar joints in the test
specimen...and usually the more mortar joints (i.e. taller prism), the
more the test results will reflect the actual infield strength.  If they
did test those cores and applied the load to what was the vertical surface
in the actual wall (i.e. the circular surface), then I don't think that
would really count as a prism test and might make the test results
suspect.  Hopefully, I am missing something.

Scott
Adrian, MI


On Tue, 5 Apr 2005, Bill Allen, S.E. wrote:

> Scott -
>
> Sorry, I was referring to questioning what goes on in the field, quality of
> field tests (or lack thereof) and inspections (or lack thereof).
>
> What WAS interesting in my "experience" was the prism tests. I had
> originally designed for F'm =3,000 psi. I knew that 3,750 psi block was
> readily available and that, along with 2,500 psi grout would get me F'm =
> 2,500 psi (based on Tbl 21-D of the UBC/CBC). I needed a little more than
> that (3,000 psi), so I specified 4,000 psi grout, fairly sure that the
> combination of 3,750 psi block and 4,000 psi grout would get me F'm = 3,000
> psi. I also specified 3,000 psi mortar, but didn't get tests on that (even
> though I asked for them - now I know why from this thread). I did get Type M
> mortar with a specified minimum strength of 2,500 psi and a sample break of
> 7,500 psi (!) from a previous (2003), lab controlled test.
>
> Anyway, when the prisms were cut from the wall (including face shell, grout
> and joints), using normal strength block (compressive strength of 1,800
> psi), all of the tests but two (10 out of 12) came back in excess of 3,000
> psi. The averages on two of the buildings were over 3,000 psi and the one
> that wasn't was something like 2,960 psi which I called "close enough". One
> of the reasons is the grout tested at 5,500 psi (!) average. I can only
> imagine what 3,750 psi block and 4,000 psi grout would have produced. I wish
> I would have cause to ask for the remaining half of the 3rd building (the
> only one getting the 3,750 psi block) to be tested just to satisfy my
> curiousity.
>
> You can see photos, including the port holes where the prisms were cut, at:
>
> http://www.allendesigns.com/projects/20402/20402.htm
>
> Regards,
>
> T. William (Bill) Allen, S.E. (CA #2607)
> ALLEN DESIGNS
> Consulting Structural Engineers
> http://www.AllenDesigns.com
> V (949) 248-8588	 .	 F (949) 209-2509
>
> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
> Sent: Tuesday, April 05, 2005 4:05 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Mortar Testing
>
> Bill,
>
> Hey, questioning it good.  It keeps you on your toes and always learning.
>
>
>
>
>
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