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RE: Mortar Testing

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Scott -

The reason why the building official shut the job down was that, when he
did, I had received very few inspection reports and test reports. The prism
tests are not a substitute for the inspection reports. For example, they
don't say if the steel was placed in accordance with the plans. The
contractor also thought that, since the prism tests were satisfactory, there
was no longer a need to submit the inspection reports.

With regards to the shape of the prism samples, this was discussed at
length. It was determined that the round holes were easier to repair than
square ones and my concern was getting something small enough to avoid
cutting bars. The "cores" are 12 inches in diameter and are taken so that
they represent face shell and mortar joints. Yes, the lab corrected the
results for the size of the prism. Yes, the sample was tested face-wise.

Regarding the solid grouting comment, at least as far as I'm concerned, not
all masonry is solid grouted. Solid grouting is not a specific requirement
just because the structure is in California. In this particular case it was
because of the large openings. On one of my other projects, I designed the
wall with grout at 32" to reduce the tributary mass to the roof. It also had
high strength block, but also had light weight grout. Of course, this was an
auto service building will very few openings, so reduced area for in-plane
shear was not a problem. On that particular job, I had no difficulty with
inspection and test reports or getting the correct block to the jobsite.
High strength, medium weight block is readily available here in Southern
California. It costs about 40% more than normal strength putting it about
the same as 12" block, but that cost is offset by lower transportation costs
and lower labor costs to install (over 12" normal strength block). 


T. William (Bill) Allen, S.E. (CA #2607)	
Consulting Structural Engineers	
V (949) 248-8588	 .	 F (949) 209-2509	

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)] 
Sent: Wednesday, April 06, 2005 12:35 AM
To: seaint(--nospam--at)
Subject: RE: Mortar Testing


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.

Adrian, MI


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