|Frequently Asked Questions (FAQs)
Q: How should mortar strength be chosen?
Isn’t the strongest mortar best for every project?
Quite simply, stronger is not always better when it comes to mortar.
The rule of thumb is that a Type N mortar, “normal strength”
mortar, should be the first choice. Mortars having moderate or lower
strength are preferred because they have the ability to deform slightly
under load, handling small movements with minimal cracking. Only
if there is a compelling reason to choose otherwise should a different
mortar type be specified.
The property that describes a material’s deformation under
loading is called the modulus of elasticity. Generally, higher strength
materials have a higher modulus of elasticity. Therefore, higher
strength mortars tend to crack more than lower strength mortars.
In the U.S., mortar types are designated by ASTM C270, Specification
for Mortar for Unit Masonry, as M, S, and N in order of decreasing
strength. (In Canada, mortars are specified by CSA A179, Mortar
and Grout for Unit Masonry.) The property table of C270 lists the
strength for each mortar type as follows:
||Minimum average compressive strength,
28 days, psi (MPa)*
|*Note that the strengths shown
are for mortars made and tested in a laboratory. Field sampled
mortars will likely exhibit different strengths due to differences
in field water content, molding, and curing. This is normal
and expected but does not generally indicate any problems with
Ingredients and the Role of Water
Mortar is made with aggregates, water, and a binder. Cementitious
materials, which react with water to set and harden, are the binders
in mortar, and include portland cement, blended cement, masonry
cement, mortar cement, and lime, alone or in combination with each
other. Each material has its own ASTM specification.
Mortar bonds units together, and in order to do this effectively,
it should be mixed with the maximum amount of water consistent with
the ambient conditions and placing needs. A mason and the person
mixing mortar will decide together on the best consistency for the
job they are doing.
Reasons for Choosing a Different Mortar Type
The “compelling” reasons most often cited for specifying
increased mortar strength are structural integrity or durability.
Where the wall will be load-bearing masonry, a Type M or S mortar
may be required. In reinforced masonry, the system (of units, grout,
and steel) dominates structural behavior, and the mortar’s
contribution is minimized. Even though mortar strength does not
have a major impact on the wall’s compressive strength, higher
strength mortars do support greater loads.
Mortar durability refers primarily to resistance to freeze-thaw
cycles or chemical attack. In each case, performance is generally
improved as the cementitious materials content of mortar is increased;
thus, the mortar has higher strength.
ASTM C270 should be consulted for additional guidance in selecting
proper mortar type based on strength and other characteristics.
Water Contents of Other Cement-Based Materials
For comparison, concrete, grout, and plaster are also made with
the same main ingredients as mortar but they are formulated for
different purposes, and their consistency (water content) is related
to that purpose:
- Concrete must have enough plasticity to fill forms. It usually
has a limitation on relative proportions of water to cement (or
cementitious materials) denoted as w/c or w/cm, and this should
not be exceeded. The limit is there for structural integrity (strength)
and durability (often freeze-thaw resistance).
- Grout is placed in constricted spaces that may contain reinforcement
between units that may be absorbing water out of the mix. It is
mixed to a very fluid consistency, so it contains the highest
water content of the materials described here. (A notable exception
to the water content is the newer grout formulation called self-consolidating
- Plaster has to be wet enough to be readily placed on the
wall and spread over its surface, yet be dry enough so that it doesn’t
sag under its own weight. It is very similar to mortar in material
proportions other than water. Regarding water, it may be slightly
drier than mortar because it does not come into contact with units
above and below it, but is placed against a substrate, sometimes
absorptive, sometimes not.
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