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Re: Masonry mortar

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I will expand a bit on Scott's reply. Scott is correct in his assessments of mortars. My response is from the experience one gains within spittin' range of the masons.

The N mortars have a higher lime content and have better self crack healing characteristics. The S and M mortars, while stronger, have a shorter board life. As such they will generally deveolp more plastic shrinkage cracks, and can have a bond problem if the masonry contractor is running behind. M mortars are best used when back parging cavities below grade.

When considering tuck pointing, match the mortar that is in place. When in doubt go to an N mortar for tuck pointing. S mortars in tuck pointing can cause problems. It is best to take a sample, send it in for petrographics and develop a match mortar. S is still your exposed normal mortar, and most used for exterior applications.

Regards,
Harold Sprague




From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: Re: Masonry mortar
Date: Mon, 11 Oct 2004 17:36:59 -0400 (EDT)

Michael:

From "Masonry Design and Detailing for Architects, Engineers and
Contractors" by Christine Beall (third edition):

"Type M is a high-strength mix recommedned for both reinforced and
unreinforced masorny with may be subject to high compressive loads.

Type S mortar produces tensile bond strengths which approach the maximum
obtainable with cement-lime mortar.  It is recommended for structures
subject to normal compressive loads but which require flecural bond
strength for high lateral loads from soil pressures, high winds, or
earthquakes.  Type S should also be used where mortar adhesion is the sole
bonding agent between facing and backing, such as application of
adhesion-type terra cotta veneer.  Because of its excellent durability,
Type S mortar is also recommended for structures at or below grade and in
contact with the soil, such as foundations, retaining walls, pavements,
sewers, and manholes.

Type N is a good general-purpose mortar for use in above-grade masonry.
It is recommeded for exterior masonry veneers and for interior and
exterior loadbearing walls.  This "medimum-strength" mortar represents the
best compromise among compressive strength and flexural strength,
workability, and economy and is, in fact, recommended for most masonry
applications.

Type O is a "high-lime," low-strength mortar.  It is recommended for
interior and exterior non-loading bearing walls and veneers which will not
be subject to freezing in the presence of moisture.  Type O mortar is
often used in one- and two-story residential work and is a favorite of
masons because of its excellent workability and economical cost.

Type K mortar has a very low compressive strength and a correspondingly
low tensile bond strength.  It is seldom used in new construction, and is
recommeded in ASTM C270 only for tuckpointing historic buildings
constructed orginally with lime-sand mortar."

HTH,

Scott
Adrian, MI

On Mon, 11 Oct 2004, Michael L. Hemstad wrote:

> All,
> I've decided to fill yet another annoying gap in my knowledge base.
> What is general practice for specifying different types of mortar (M, S,
> or N)?  Typically, I have specified these based on where they are
> located (exterior, below or above ground; or interior, respectively).
> Does the specified masonry strength play into the choice of mortar, or
> only the end use?  Amrhein's book has some information, but not much
> explanation.
>
> Thanks for any help.
>
> Mike Hemstad
>
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