In this example, the marshmallow exhibits a high
degree of elasticity and is able to deform
significantly without breaking apart.
Although mortar has significantly greater rigidity
than a marshmallow, it behaves similarly when
under compression. ASTM C109, The Standard
Test Method for Compressive Strength of
Hydraulic Cement Mortars, determines the
compressive strength of mortars in the same way
as our marshmallow example. In ASTM C109
a compressive force, measured in pounds per
square inch (PSI), is applied to a cured two-inch
cube of mortar until the mortar distends and fails
under tensile strain. The compressive strength of
the mortar is determined by the PSI that was
applied to the cube of mortar at the moment it
failed. (Insert picture of compressive testing)
The tensile strength of a mortar is proportionate
to its compressive strength; therefore the
stronger a mortar is under compression the more
resistant it is to tensile strain. Lime binders, on
the other hand, generally have substantially lower
compressive, and therefore, tensile strength and
require comparatively low compressive stress
30 | Masonry Design
to generate deformation. Rather than a sudden
rupture characteristic of Portland cement failures,
softer limes compact and eventually crumble
at their point of failure. This very gradual and
controlled failure is one example of lime mortar
performing its job as a sacrificial element in a
building.
TRI-AXIAL STRESS
Let’s revisit our marshmallow experiment and
envision how it would behave if the marshmallow
was contained in a cylinder. As you compress the
marshmallow from above you would feel
resistance. The compressive pressure of you
pushing downward on the marshmallow forces
the mass to press against the walls of the
cylinder. The tensile strain formed within the
marshmallow induced by its desire to stretch
outward is now counteracted by its confinement
in the cylinder. Under confinement, tensile stress
is converted into horizontal compressive forces
redirected towards the center of the mass in a
mechanical function known as tri-axial stress.
Like the marshmallow confined in a cylinder,
mortar is confined by and bonded to surrounding
masonry units. As mortar distends from the joint
under compression it is restrained by the friction
caused at the bond interface between the mortar
and adjacent masonry units resulting in tri-axial
stress and shear strain at the bond. The point of
failure under shear strain in well-bonded masonry
will occur within the material that has the least
tensile strength. When high strength hydraulic
lime or Portland cement rich mortars are used,
the point of shear failure under tri-axial stress
may occur in the weaker masonry unit and not
the more easily replaceable mortar joint.
(Hansen, Navarro & Van Balen 2008)