properties prediction for porous materials

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.
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Granbarbiere wrote:

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
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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
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dave martin wrote:

<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
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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.

(i'm
values
Technology).
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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?
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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.
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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).
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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
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see:
http://ciks.cbt.nist.gov/~garbocz/geo/node10.html
It and related pages have good info for you.
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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.
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by
i
consider vibrating the mortar before it sets to decrease its porosity
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I usually rattle mortars as forseen in UNI EN norm. Are vibrations more effective than rattling? (i suppose so)
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