I need some correlations for mechanical properties of porous materials (i'm
working on areated concretes, in my thesis) that, starting from void

concentration and properties of matrix would give me some approximate values
for them.
I have found nothing about compressive strength.
Can anyone help me?
Thanks.

I believe that there was a thread on this subject (within the last three
months) either in sci.materials or sci.eng.metallurgy...
Do a google group search and you can discover what was said before.
Search http://www.google.com for "porosity mechanical properties
ceramics" and on the first page you will find a link to some good papers
and stuff from the NIST (National Institute for Standards and Technology).
You will learn a lot if you can imagine good search terms.
I haven't done much with the search terms "porosity mechanical
properties concrete", but you might try these terms and let us know what
you get.
Jim

This is the third time this poster has asked this or a very similar
question of this group.
Pervious answers were unacknowledged and questions designed to help
generate meaningful answers were ignored.
At least this time the poster had the courtesy of specifying which
mechanical property is of concern.
Granbarbiere, perhaps you will be kind enough to share some of what
you've learned in the past few months while working on your thesis?
Are you concerned about high or low porosity concentrations? Do you
seek theory or experimental results?
For the third time, How big are the pores compared to the aggregate?
A google search on "compressive strength"+concrete+porosity gives 4030
hits.
The second of which (from a civil 101 course) says:
Gel/Space Ratio -- In 1946, Powers and Brownyard published a work that
showed that the increase in compressive strength of Portland cement is
directly proportional to the increase in gel/space ratio, regardless
of age, w/c ratio, or type of cement. The gel/space ratio is the ratio
of solid products of hydration to the space available for these
hydration products, in other words, it is a measure of capillary pore
space. Before hydration this space is occupied by mixing water, after
hydration the space is the sum of the hydrated cement and the
remaining capillary pore space. The basic trend of this graph will be
the same if different cements or test shapes are used, however, the
data points will change. Even thought is this an important quantity
the gel/space ratio is difficult to determine.
RSVP

<snip>
Well, this fellow sounds like a finite element guy who has to model
concrete.
His interest in materials is probably zero or a good approximation of
that. That seems to be common among analytical types.
I suspected that it was he who kept repeating these requests.
It would be typical of the type of person who is a "finite element" guy
stuck with getting some information to support his analysis. Imagine,
actually having to try to talk to "materials people?.
Jim

In my thesis i'm working on the simulation of historical mortars. There's no
need to use materials with the same chemical composition of the old one.
Only the mechanical properties (young's moudulus, poisson ratio and
compressive strength) are investigated. The choice to use hydraulic paste
instead of slaked lime is justified by short hardening times never
achievable with the latter.
We are using aluminium powder and areating solutions to introduce increasing
grades of porosity into the matrix. In a second while we are going to
measure mechanical properties.
We would like to obtain a correlation between porosity in the hardened
matrix and young's modulus, poisson ratio and compressive strength. This
correlation, obtained from measures, should be supported by a theoretical
model. That's what i am interested in.
In the meanwhile i have found this article: "Computation of the linear
elastic properties of random porous materials with a wide variety of
microstructure" by A. P. Roberts and E. J. Garboczi. in which
Hashin-Shtrikman, 3-point bounds and Torquato's exact bounds and expansions
are discussed and validated thru a Finite element method.
In this article also overlapping solid spherical inclusion are discussed.
That should be enough, but i'm always open to new suggestion.
Thank you all.

Thanks for your response. From it I surmise that you have relatively
high concentrations of large pores.
You'll find that the properties you mention are not as tightly coupled
as you might like.
Young's or Bulk modulus are probably the least dependent on
microstructural details.
Poisson's ratio can highly dependent on foam structure see:
http://silver.neep.wisc.edu/~lakes/sci87.html
for a short discussion of a negative poisson ratio foam.
A google search on foam+poisson turns up many strange results
including copper foams with negative poisson's ratio and foams with
poisson's ratio greater than 1 (violating energy conservation).
These results imply that poisson's ratio for your cemented foams might
be sensitive to aggregate shape, surfactants, etc.
See: http://silver.neep.wisc.edu/~lakes/sci87.html for a short
presentation that includes a discussion of Poisson's ratio in
clellular materials and foams.
Compressive strength also depends strongly on microstructural details.
The strength at a particular volume fraction porosity will depend
strongly on details. A few isolated, spherical pores will have a much
different effect than
many finely divided pores with the same total volume. Pore shape will
also play an important role.
You will need more than %porosity to correlate with compressive
strength; perhaps pore size & pore shape descriptors.
Good luck in your work.
Is there an online version of the paper you mentioned?

Yes: http://ciks.cbt.nist.gov/~garbocz/paper127/b06.html
CEB states that young's modulus for cellular concrete young's modulus is
proportional to density^3/2 and compressive strength^1/2. (the same
statement occurs in "Lightweight Concrete and Aggregates" by Thomas A.
Holm -ASTM-).
But this relation seem so empirical... and i quiet skeptical about its use
for a porous concrete...
Thanks.

Thanks for the reference. It is informative and probably very useful
as it provides reasonable bounds.
I didn't see anything about strength in it. Is strength important to
you?
You say that you are conducting some kind of historical study of
mortar use. Why do you care about a detail like poisson's ratio?
I'd guess that strength is of most importance to you; in that context,
you might find good info in the rock physics literature. (strength or
porous & cracked rock).

I'm going to check it out. In the meanwhile i have a question.
In the article i've mentioned the properties of solid are supposed to be
known. But i will never obtain a pore-free mortar that could be used as
matrix. I will have a naturally areated mortar, whose pores are generated by
water, and some articially areated ones which i can compare to it. Should i
consider the naturally areated one as solid and then calculate porosity of
the others as difference between their porosity and the natural one?
For example, in your opinion, can i consider this system
NATURALLY areated porosity: 10%
ARTIFICIALLY areated porosity: 35%
equivalent to the following one
NATURALLY areated porosity: 0% (SOLID MATRIX)
ARTIFICIALLY areated porosity: 25%
and then use the results shown in that article?
Thanks

thanks.
i've checked the article you suggested; DEM method seemed to be good on the
very first sight, i only have to substitute zero-porosity boundary
conditions to given-porosity ones.
in these days i have been thinking about my previous question and came to
the conclusion that, since young's modulus has a no-linear relation to
porosity, then a 0%-25% jump differes from a 10-35% one. So, if i won't use
DEM relation, i think i should extrapolate solid (zero-porosity) properties
using my empirical points and a torquato-like curve.

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.