Highly hydraulic mortars with greater tensile
strength (ie. Portland cements) require
proportionately higher tri-axial stress to induce
crystal deformation and pore collapse. In some
cases the excessive tri-axial stress required to
achieve pore collapse in Portland cement mortars
produces shear strain at the masonry's bond that
can exceed the tensile tolerance of historic
masonry units and the units themselves may fail.
By contrast, softer lime mortars require much less
stress to induce deformation and very little strain
is imposed on surrounding masonry units.
(Wiggins 2018) (Insert picture of sheer at bond)
ELASTICITY VS. PLASTIC DEFORMATION
Under stress a marshmallow compresses and,
when relieved of that pressure, it returns to its
original volume and shape. A mortar’s capacity
to deform and rebound without being damaged is
characterized by its modulus of elasticity. Highly
hydraulic mortars that experience only partial
pore collapse as a result of crystal deformation
have a comparatively high modulus of elasticity.
Mortars with higher elasticity exert additional
sheer stress on surrounding masonry units under
deformation. Although highly elastic mortar has
some benefits in new construction, its unyielding
characteristics make it less desirable for
restoration work where soft masonry units may
be susceptible to damage caused by shear strain.
Less hydraulic lime mortars are more prone to
permanent deformation (plastic deformation)
as a result of micro cracking and greater pore
collapse. Under plastic deformation a mortar
experiences greater overall volume reduction
as it compresses under stress. Ultimately,
masonry structures constructed with low strength
lime mortars that exhibit plastic deformation are
better able to adapt to differential rates of
settlement and thermal movement than stronger
hydraulic mortars. This ability is best exemplified
by nearly all-historic masonry structures built prior
to 1900, which were constructed using lime
mortars without employing movement joints.
(Costigan & Pavia 2013)
32 | Masonry Design
CONCLUSION
Mortar selection determined principally by high
compressive strength ultimately disregards other
more significant performance characteristics that
are critical to the preservation of historic masonry
structures. In restoration, a mortar's mechanical
role does not lie in its strength but rather its ability
to deform under stress and accommodate
movements within the structure.
Masonry construction is responsible for the
world's most impressive historic structures and
Milan's Duomo is just one of many examples.
The scale of the Duomo and its breathtaking
artistry inspires awe among people from all
cultures and generations. Yet, what is just as
impressive as the Duomo's intricately carved
facade is how effective its mortar has served to
preserve the structure for over 600 years. That
fact that Milan's Duomo has withstood centuries
of stress from settling and thermal expansion is a
real testament to the mechanical elements of the
building's mortar. So the next time you find
yourself gazing at beautiful historic masonry
remember to also admire its mortar.
Masonry Restoration Specialists
-offering a comprehensive line of historic masonry materials-
Products
· JAHN Restoration Mortars ·
· KEIM Mineral Coatings ·
· St. Astier Natural Hydraulic Limes·
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