JOINT SU[1]-TUD[2] protocols for measuring hygric characteristics of porous construction materials: Introduction
A series of joint SU-TUD testing protocols describing determination and verification of hygric characteristics that are necessary for a material database is placed on this web site. In these documents we use the term “characteristics” rather than properties because the latter would imply a physical meaning of the measured variable, while many of the material characteristics depend not only on the material structure but also on the used test method. Therefore, instead of using “apparent” or “effective” properties of the material the term “material characteristics” is used.
1. The process of material characterization
These documents present steps and experiments necessary to develop the input to any HAM (heat, air and moisture) or CHAMPS (coupled heat, air, moisture, pollutant and salt transport) model. They are dealing with two aspects of the material characterization, 1) characterization needed for identification of a product batch and 2) measurement and tests defining hygric characteristics of the material.
The development process of material characteristics involves several stages:
· Sampling and specimen preparation,
· Generation of material properties (raw data),
· Evaluation of data consistency and control for probable errors,
· Curve fitting to the raw data and the optimization of the curve fit to the data,
· Generation of a pre-selected functional relationship, and
· Control of the precision in the transfer of the optimized curve to the database information.
Development of material characteristics is not a simple process because the separation of errors related to material variability (differences between various specimens) and those related to the uncertainty of the test methods are not yet fully realized. As one does not know what part of the variation in the test results is caused by the variability of specimens, one must be careful in ensuring that all these measurements are traceable to the material batch and sample. Measurements of hygric properties, thought well developed in the soil physics, are still in the infancy stage in the field of building physics. This particularly is the case when the durability of construction materials is concerned. Therefore, in the following test protocol one postulates a comprehensive test methodology to characterize an average property of the sample.
Requirements for material characterization (see the following section), are also included in the set of properties required for the input to the model. The degree of variability in these properties is estimated with two different measurements:
1. The water absorption coefficient
2. Either a dry cup water vapour permeability or saturated liquid conductivity (Darcy filtration coefficient)
These two measurements are performed on a larger number of specimens taken from one batch of the specified product. These measurements provide statistical limits for two basic parameters that are necessary to permit the discrimination between different batches of products.
2. Test protocols on the bedecanus site
The position paper (Bomberg et al., 2002) recommended a minimum set of hygric characteristics that would provide a unique description of the material. The precision of selected methods for determination of hygric properties of porous building materials has been investigated in the study performed under the EU-initiated HAMSTAD-project (Roels et al , 2004). Six laboratories measured the selected hygric properties of three porous building materials. For some measured properties an acceptable agreement was obtained, for others it was not so good. Deviations between the individual results increased when materials were either inhomogeneous or had a complex pore structure. It was found that some of the existing standards or commonly used measurement methods need improvement e.g., water vapour transmission tests.
To alleviate the situation the Syracuse University (SU, USA) and the Technical University of Dresden (TUD, Germany) decided to undertake a joint work with view to improving the test protocols used in determination of hygrothermal characteristics for construction materials. This is done in a series of short papers, short because all these test protocols are also subject to continuous improvements. This web site shall soon contain the following:
Protocol 1: Pore structure descriptors and measurements of moisture storage
Protocol 2: Moisture transport characteristics (state of the art 2003)
Protocol 3: Sampling procedures and sequence of testing for characterization of a product