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Name: Nate
Status: educator
Grade: 9-12
Location: IL
Country: USA
Date: Summer 2012


Question:
I am in an aggregate materials teachers workshop. In order to evaluate the rocks for an aggregate (for concrete, asphalt, etc.), I am confused between permeability, absorption, and porosity. Would you please help me differentiate among these properties and how they are tested?



Replies:
One could throw a lot of definitions and some math at you , but conceptually:

Porosity is a measure of the void space in a rock given as as a percentage of total volume. A porosity of 25% means one quarter of the rock volume is void space (holes) A sponge has high porosity as most of it is made up of holes.

Permeability is a measure of how well a given fluids passes through the rock. Permeability can vary for the same rock depending on the fluid. An rock may be very permeable to air (small molecules), moderately to water and very impermeable to certain hydrocarbons (very large molecules). Permeability can also be affected by pore (void) size, interconnectedness and mineralogy as well as several other properties.

Absorption is essentially how easily or how much fluid a rock will take in under normal conditions. High permeability and high porosity should indicate high absorption (back to the sponge).

This is all from the point of view of a geophysicist with an oil and gas background and many industries have slightly different definitions of common technical terms. But it should hold up in generalities.

Hope this helps. Bob Avakian OSUIT Okmulgee, Oklahoma


Hi Nate! :)

Porosity is the volume of pores (air voids) in a concrete/aggregate/rock. Air voids can be due to bubbles in the concrete, or to air pockets within aggregate rocks, or to non-wetted portions of rocks (basically, a bubble created where the wet concrete doesn't contact the aggregate). Porosity is measured in percent (ratio of pore volume to material volume). Porosity can be hard to measure directly if pores are embedded in the material without being connected to the surface. Thus, you can also call certain experimental measurements the 'apparent porosity', reflecting the fraction of pores that are accessible. In the cases of incompletely hydrated concrete, the apparent porosity could even be higher than the actual, ultimate porosity of a concrete. To give you a visual device, soap suds have very HIGH porosity, while vegetable oil has very low (effectively zero) porosity.

Absorption is the ability of a concrete/aggregate/rock to hold water within it. Dry the concrete to some reference state, weigh it, then immerse in water until it reaches equilibrium, dry the surface and re-measure the weight. The percent change is its absorption. The apparent/actual discussion above also would apply to absorption.

Permeability is the ability for a fluid to pass through given concrete/aggregate/rock. Permeability can be thought of in terms of porosity and absorption -- the larger size and number of *connected* pores, the larger the permeability. The pores act as "channels" for water to move through. More channels = higher permeability. Units for permeability are inverse length squared -- based on a calculation of the ratio of fluid flow to pressure drop. Basically, you take a given piece of concrete, measure a fluid's flow through it (volume over surface area), multiple by the viscosity and the thickness of the concrete, and then divide by the pressure drop, and voila you have permeability. This includes a simplifying assumption, namely to treat the aggregate as a uniform material; you can classify different aggregates and measure their average properties. This is much easier (and usually sufficient) than doing a geometrically-aware formulation.

Here's a pretty simple reference I found as well, which references the ASTM test: http://www.masco.net/castintactweb/pdf/test_results/Water_Absorption_Test_ASTMC140.pdf And also a basic PPT on permeability, which has some example permeability values:

http://www.ce.berkeley.edu/~paulmont/241/permeability.pdf

If you want something more technical, here's one example:

http://onlinepubs.trb.org/onlinepubs/shrp/SHRP-C-628.pdf

Hope this helps! Burr Zimmerman


Hi Nate,

The confusion is understandable, they are all related.

Porosity is the ratio(or number) of the pores to the solid part of matrix. Permeability is how well a material may intercalate into those pores, there are ionic, polarity forces and size of pore considerations. Absorption is how well a material is held within the pores of the matrix, the staining of the material. Absorption is a property resulting from permeability and porosity.

Consider dyeing a carpet. The dyeing is dependent on the chemistry of material(matrix), how spongy(porosity) it is and the chemistry of the dyeing agent. Testing is gravimetric for porosity, permeability is tested using polar and non-polar substances. Absorption is performed exhaustively for fine stone materials and for building materials only environmental washes are tested. It really is that simple, even if the details of the materials and liquids are cumbersome.

The testing for it varies between the manufacturers and the materials, brick vs marble for example, but the consensus here is to go by the American Society of Civil Engineers testing, the first reference.

http://cedb.asce.org/cgi/WWWdisplay.cgi?132684

http://www.in.gov/indot/files/chapter_03.pdf http://www.ethosmarblecare.co.uk/petrology-technical/porosity.shtml

Stay solid! Peter E. Hughes, Ph.D. Milford, NH


You have not posed an easy question. Permeability and porosity are related. However, this venue makes it difficult to give a detailed accurate definition. Rather I am giving you a web site that addresses those definitions thoroughly and accurately:

http://shahharsh19.blogspot.com/2009/03/concrete-permeability.html

Absorption is defined as: The transport of liquids in unsaturated porous concrete due to surface tension acting in capillaries is called absorption. Absorption in concrete is related not only to the pore structure, but also to the moisture content of the concrete. This definition comes from a large discussion of properties of concrete.

http://discovery.ucl.ac.uk/1310144/1/1310144.pdf

All three of these important properties are difficult to measure, and difficult to get a good simple grasp of what is being measured!! The references cited above refer to concrete specifically, but the concepts are the same for other aggregates. You do not have to feel confused. Although I am a physical chemist, I read the citations carefully, but I could not find a simple definition. It is not clear that the definition of the three aggregate properties might overlap, that is, may not be independent variables.

A tough question. I hope that the references, both of which are large and detailed, can give you a starting point.

Vince Calder


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