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Re: Mortar

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In a message dated 97-01-09 12:22:51 EST, you write:

<< Subj:	Mortar
 Date:	97-01-09 12:22:51 EST
 From:	Shake4bake(--nospam--at)
 Reply-to:	seaoc(--nospam--at)
 To:	seaoc(--nospam--at)
 I've the necessity for f'm=2500 psi in the design of some slender walls
 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
 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.
 Mark D. Baker
 Baker Engineering


When designing a wall for a strength of 2500 psi, you are correct, the
specifications require that the minimum unit strength of the block is to be
f'm = 3570 psi and the grout must be designed for the same minimum strength.
 The mortar does not need to be designed for this strength, 1800 psi is ok.
 I recommend that you include in the specification that the grout design
shall be stamped by a licensed civil engineer responsible for the mix design
unless you feel comfortrable in designing your own grout.   You can also try
contacting John Chrysler of the Masonry Institute of America in Los Angeles
at (213) 388-0472 for furthur information.

As a side note,  I have been told that masonry design strengths above
f'm=3000 psi using concrete block are unreliable, that maximum prisim tests
typically come in around 3100 to 3300 psi.

Hope this helps.

Michael Cochran 
Brian L Cochran Associates
Los Angeles, CA