<<Ralph is right. The lower the modulus, the greater the ability to
deform under stress. Caulk is much much lower in modulus than mortar.>>


Just to confuse the issue, Young's modulus (modulus of elasticity {E})
is the ratio of stress to strain a material can withstand and remain
"elastic" or return to its original shape. The values are normally based
on tensile reactions, and a high E value indicates a very large amount
of stress can be applied to a material with limited strain. Most
materials become plastic when the amount of stress applied to them
passes their elastic limit (which can be determined using Young's
modulus) and will continue to deform but will not return to their
original shape when the stress is removed. Very brittle materials
(versus ductile materials) however, like concrete have low elastic
limits, low plasticity and typically have low values of E (3,150,000 PSI
for 3000 PSI concrete at 28 days cured), however extremely flexible
materials may also have low E values due to their low strength. In other
words the amount of strain is extremely high relative to the amount of
stress. This is why steel is used to reinforce concrete. It's very high
E (29,000,000 for A36) allows it to absorb the high tensile stress that
the concrete cannot with very limited strain.

That said, it would seem using Young's modulus to determine the
effectiveness of lime vs Portland mortar vs caulk in reaction to implied
stresses as a result of building movement has limited value. The
elasticity of the materials is less of an issue than the plasticity.
Plasticity is the ability of a material to deform without rupture. In
the case of Portland cement the plasticity sucks, which is why it
destroys (literally) softer materials in a stress/strain competition. It
is strong in compression (often too strong) but has low plasticity. Lime
mortar on the other hand has much better plasticity, while still having
high relative strength. Elastic sealants have very high flexibility, but
often low compressive strength.

I'm sure I'm in trouble now.

Rudy

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