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[SEAOC] Re: [SEAOC] Type M vs Type S mortar

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According to the 1993 "Masonry Designer's Guide" by The Masonry Society: "Do 
not use a higher compressive strength mortar than necessary....Unless dictated 
by the compressive strength requirement, avoid the selection of Type M mortar 
over Type S mortar.  However, when masonry is in direct contact with soil, 
e.g. in retaining walls, Type M mortar should be used regardless of strength 
requirements."  
 
According to PCA IS278.01M, "Factors affecting Bond Strength of Masonry": 
"...the finding that little or no increase in bond strength is achieved by the 
use of Type M mortars as compared to Type S mortars".  
 
Type N mortar is considered better in Colorado's dry climate since it has 
holds moisture better and doesn't dry out as quickly, although Type S can be 
used.  Type M mortar is a harsher mortar.  Bond may be more important than 
compressive strength and if the mortar dries out, it may not bond well. 
 
I recommend specifying/ allowing Type S mortar for most structural 
applications.  


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We routinely specify Type M mortar on jobs. No one has ever
complained. 

We recently completed drawings for a high school in Colorado. We used
our standard specification for mortar and have received a request for
change to Type S. The contractor must not see it often. But he *did*
bid the job from plans that included the Type M spec. He will need to
credit my client.

Aside from strength what are the relative benefits of the two types of
mortar?

Thanks,

Mark


Mark E. Deardorff, SE
Deardorff & Deardorff, Inc.
San Diego, CA
markedd(--nospam--at)ix.netcom.com

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