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FW: Mortar

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We just addressed this issue on a project.  U.B.C. 2105.3 compliance
with f'm, gives four possible options for meeting the specified f'm for
masonry.  These include 2105.3.2 Masonry prism testing, 2105.3.3 Masonry
prism test record, 2105.3.4 Unit strength method and 2105.3.5 Testing
prisms from constructed masonry.

Your question relates to 2105.3.4, Unit strength method.  When this
option is used for compliance with f'm,  both the masonry unit and the
grout must meet the 3750 psi criteria as you stated.  However, the
mortar does not.  The Reinforced Masonry Engineering Handbook Clay and
Concrete Masonry (James E. Amrhein) is a good reference as to why this
is the case.  Basically, the test cubes for the mortar are much thicker
and as a result the higher in-place mortar strength is attributed to the
inherent difficulty in failing the thin and wide mortar joint in
compression.  The completed wall strength is not greatly affected by the
mortar strength.  (See RMEH pages 14 and 15 for more detailed
information.)  Note that testing or letter of certification requirements
for masonry units, grout, and mortar using the Unit Strength Method are
addressed in U.B.C. 2105.3.4 (Depending on whether half or full
allowable stresses were used for design).
 ----------
From: Shake4bake(--nospam--at)aol.com
To: seaoc(--nospam--at)seaoc.org
Subject: Mortar
Date: Thursday, January 09, 1997 11:21AM


Greetings:

I've the necessity for f'm=2500 psi in the design of some slender walls
(cmu).

I'm attempting to write a spec which will result in f'm=2500 psi which
leads
to the following :

Am I reading Table 21-D correctly? For a specified compressive strength
of
masonry, f'm=2500 psi for Type M or S mortar, the corresponding
compressive
strength of concrete masonry units is 3750 psi. I am interpreting this
compressive strength to be that of the individual block, not the grouted
assembly, yes?

Footnote 4 of Table 21-D states that the compressive strength of grout
shall
be equal to or greater than the compressive strength of the concrete
masonry
units so... I should specify a grout compressive strength of 3750 psi,
yes?

Finally, Table 21-15-A of UBC Standards gives the minimum average
compressive
strength for Type S mortar as 1800 psi (read also Section 21.1505).
Should
the mortar compressive strength also be up at 3750 psi?

I have exhausted my office resources on this issue, any comments would
be
greatly appreciated.

Thanx,

Mark D. Baker
Baker Engineering