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RE: Determining F'm from Tests

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Paul already offered some thoughts along the lines of what I was
contemplating.  I echo his comment that even if the face shell had not
fallen off, the test of the cylinder load perpendicular to the face shell
is not inline with any standardized/accepted masonry tests that I am aware
of.  As Paul pointed out, prism tests are done on a rectangular specimen
made up of mortar and units (and grout if desired) that is laid out in the
pattern of the fully assembled masonry (i.e. stacked or running bond, etc)
and with the loading parallel to the face shells.  Such testing is done
per ASTM C 1314, as Bill P. pointed out.

You other option in general is the unit strength method, as may or may not
be aware.  In this method, you basically test the various components (i.e.
masonry units, mortar, grout, etc) and from that can then using the MSJC
(masonry) code tables determine a f'm.  This method is "aimed" at being
done/used during construction, not really post-construction.  The big
difference will be the curing characteristics...the effect of the mortar
curing with the CMU as opposed to being cured in a lab environment in cube
forms (i.e. the CMU "sucks" some water from the mortar and grout if
present).  In this method, the f'm is really dependent on the compressive
strength the CMU units and the type of mortar, but not the grout.  The
grout compressive strength must be greater than the f'm that you get from
the tables based upon the tested compressive strenght of the units and mortar,
but not less than 2000 psi.  Based upon this method (assuming that the
testing of the units was done per ASTM C 140, which might not be too
likely if they just tested the face shell and assuming that you want to
gloss over the differences that might arrise due to testing a unit that
has not been put in place rather than an in place unit), with the
compressive strengths of the units that you cited, you would have a f'm of
about 1600 psi with type M or S mortar or about 1500 psi with type N
mortar, per the unit strength method.  And you grout seems to meet the
requirement of being greater than f'm and 2000 psi (but may not have been
tested per ASTM C 1019).

In general, the best way to test masonry is to use a prism test.  As Paul
kind of pointed out, cylinder tests for masonry is not really appropriate.
It is more than just the invasive nature of the coring process, but also a
function of the fact that masonry is comprised of descrete elements
"melded" together that while in many ways behave together much like
concrete (we basically use the same design calculation methods) but in
reality behave quite differently than concrete in other ways.


Adrian, MI

On Fri, 28 Jul 2006, Paul Feather wrote:

> Bill,
> The only really appropriate way to test the compressive strength of in
> place masonry is to saw-cut a square prism from the wall.  This is
> supported in both Amrhein and Schneider.
> Core testing is appropriate for concrete, however as you pointed out the
> invasive nature of core testing usually results in separation of the
> face shell.  Add to this the fact that the wall is not loaded
> perpendicular to the face shells, and standard procedure for masonry
> strength testing is a prism.  Core testing can tell you about the
> relative soundness of constituent materials, or whether the materials
> were placed and compacted properly, but that is about it.
> Paul Feather PE, SE
> pfeather(--nospam--at)
> ________________________________
> From: Bill Allen [mailto:T.W.Allen(--nospam--at)]
> Sent: Friday, July 28, 2006 7:14 AM
> To: seaint(--nospam--at)
> Subject: Determining F'm from Tests
> I am trying to analyze an existing reinforced masonry wall. The wall
> consists of 10" thick CMU. A testing lab took two, five inch diameter
> cores. Due to the fact that one face shell separated from the grout, the
> testing lab tested the grout and face shells separately. The compressive
> strength of the grout was 1,990 psi and 2,290 psi for an average of
> 2,140 psi. The lab then conducted five cube tests of the face shells.
> The results are 2,170 psi, 1,550 psi, 2,640 psi and 2,820 psi for an
> average of 2,295 psi.
> While the lab (a reputable one) has given me a method to determine F'm,
> I want to check to see if their method is the correct/best method by
> querying this list. I would be most appreciative for any guidance on
> determining F'm based on the data provided above as well as documented
> references for such methods.
> TIA,
> T. William (Bill) Allen, S.E.
> Consulting Structural Engineers
>  V (949) 248-8588 * F(949) 209-2509

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