IBP / Software / WUFI / Basics / Measurement Methods / Porosimetry

Software / WUFI / Basics / Measurement Methods / Porosimetry

Liquid transport within a building material occurs in its pore space (exception: solution diffusion in plastics). For this reason information on the pore space of a building material is important for the description of its hygric behaviour.

Helium Pyknometry

The total porosity, i.e. the entire volume taken up by the pores, is an integral quantity that can be used to describe the pore space. It can be calculated if the bulk density and the true density of the specimen are given. The bulk density is known for most materials or can easily be measured. The true density can be measured with a helium pyknometer as follows:
The specimen is placed into a measuring cell; from the change of helium pressure in the known cell volume without and with the specimen the volume of the specimen's mineral matrix can be determined. Dividing the mass of the specimen by this volume yields the true density.

Mercury Intrusion Porosimetry

The institute's mercury intrusion porosimeter allows to determine the pore size distribution of a building material over a pore size range from ca. 0.4 mm to ca. 4 nm.
This is done by immersing the specimen in a mercury bath and then applying increasing pressure (up to 4000 bar), thus forcing mercury into smaller and smaller pores of the specimen. The non-wetting mercury does not enter the pores spontaneously, it only fills those pores for which the applied pressure is greater than the tension forces of the surface meniscus. The smaller the pores, the stronger is the curvature of the surface meniscus and the more pressure is necessary to fill the pore volume with mercury.

The measured quantities are the applied pressure and the corresponding volume of mercury forced into the specimen.

The pore size distribution has an effect on moisture retention as well as the vapor diffusion and liquid transport processes in the building material. Furthermore, it allows to assess the frost resistance or beginning microstructural damage which cannot be recognised from the specimen's outward appearance.

The principle of mercury intrusion porosimetry

Page created: 02 May 2001; last update: 30 Apr 2010.