>
> 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
>
> --
> To terminate puerile preservation prattling among pals and the
> uncoffee-ed, or to change your settings, go to:
> <http://maelstrom.stjohns.edu/archives/bullamanka-pinheads.html>
>
>
I will not claim any great expertise in the testing methods used for
determining Modulus of Elasticity, but the reference standards I have
seen usually involve either a compressive or flexural stress, rather
than tensile.
I think there is a potential for getting into trouble when materials
are characterized as "High" or "Low" modulus in too general a fashion.
To characterize concrete as low in Modulus would be unusual for normal
concrete, and repair mortars are usually characterized as High, Medium
or Low Modulus in reference to typical concrete. Thus, latex-modified
concrete is considered to be low in modulus compared with normal
concrete, but would undoubtedly be much higher than the highest
modulus polyurethane sealants.
The significance of Modulus of Elasticity with respect to mortars, as
I have seen it discussed, is that low modulus mortars can relieve
stresses by deforming, and yet return to their original dimensions
when the stress is eliminated or reversed, such as will occur in
temperature and wet-dry cycling. By deforming, the mortar avoids
transmitting stress to the substrate, which may be low in compressive
strength when we are dealing with some masonry materials. By
recovering, the mortar's tendency to crack is reduced. Modulus of
Elasticity is discussed in the ICRI guidelines for selection of
concrete repair materials.
I have not seen "Plasticity" defined in this regard, and in my mind
the lack of a standard definition would render this term unusable.
There are standard definitions and ASTM test methods for Modulus of
Elasticity and even though they are not all the same and do not yield
directly comparable results, this allows the property to be
objectively defined and tested.
As for Portland mortar destroying soft masonry, there may be more than
one reason. Portland mortar is higher in Modulus than portland-lime or
lime mortar and transmits the forces caused by masonry unit expansion
and contraction back to the units themselves. I have also seen masons
adjust Portland mortars for better workability by adding way too much
cement to the mix, making it less permeable than the masonry units.
This forces moisture to transfer through the masonry, which can have
disastrous effects on soft brick and sandstone.
Edison Coatings, Inc.
M. P. Edison
President
3 Northwest Drive
Plainville, CT 06062 USA
Phone: (860)747-2220
Fax: (860)747-2280
email: [log in to unmask]
Internet: www.edisoncoatings.com
--
To terminate puerile preservation prattling among pals and the
uncoffee-ed, or to change your settings, go to:
<http://maelstrom.stjohns.edu/archives/bullamanka-pinheads.html>
|
