Foto: Nachhaltiges Wohnungsangebot-Sandgrubenweg

Basic research on the load bearing capacity and high thermal insulation properties of foam glass granulate

The aim of the scientific investigations is to find the characteristic parameters of foam glass granulate applied as load transfer layer and thermal insulation, to provide the base for the application of a multi functional and economical insulation material at the interface between building and ground for buildings of the future.

Short Description

Status

completed

Summary

Starting point / motivation

Up to now foam glass granulate can be used only for a narrow range of application, because its thermal and mechanical characteristic parameters are not known. In order to evaluate the insulation of the building envelope as precisely as possible, the knowledge of the effective thermal conductivity of the insulation in use is essential. Currently for foam glass granulate layers no practical values are available, because there is no custom measurement procedure. Without scientific basics, concerning the thermal and mechanical behavior, the application of this material as a standard solution is not possible.

Contents and objectives

The aim of the planned scientific investigations was to find the characteristic parameters of foam glass granulate applied as load transfer layer and thermal insulation. A successful completion of the research program would establish the product foam glass granulate as standard solution at the interface between building and ground.

Methods of treatment:

To determine (soil-)mechanical properties, at the university of Innsbruck an experimental setup was developed, able to measure the load deformation relationship of glass foam granulate layers. In this process glass foam granulate layers with different strength and compression are loaded up to 800 kN/m² in steps of 100 kN/m². In course of these experiments the force of the hydraulic jack and the deformation are measured. Based on these information moduli for initial and subsequent loads can be calculated.

In addition to the laboratory studies at the University of Innsbruck the Institute for Geotechnical Engineering at the Technical University of Vienna performs the following tests:

  • grain size distribution of raw as well as sampled materials
  • compression tests in steel cylinders (Ø250) to investigate the load-deformation behavior
  • large shear tests to study the shear properties
  • permeability tests
  • Proctor tests
  • frost resistance analysis

The innovative one-plate measuring equipment developed in the framework of the project enables the study of the influence of the degree of compaction, temperature, and water content on the thermal conductivity of bulk insulation. The sample can be compressed inside the device and investigated under vacuum. The direction of heat flow is from top to bottom to allow the measurement under realistic conditions (as of the installation in below the basement of a building). Convection in the bulk is thus excluded. The measurement of thermal conductivity in the one-plate apparatus is carried out in a dry state for three different bulk densities, which are achieved through compression by using the hydro pulse machine.

Results

Producers of glass foam granulate up to now classify their products often on the basis of the value of the bulk density. Test results within this research project show clearly, that not the bulk density but the compression strength sck of the single glass foam grain is suitable for describing the material, regarding the load deformation behaviour of glass foam granulate layers.

The level of compression on the other hand represents the leading factor concerning the vertical stiffness and sequentially the vertical deformations of glass foam granulate layers.

Hence, using glass foam as as load transfer layer and thermal insulation, great importance has to be attached on an efficient compression of the glass foam granulate layers.

In course of the performed research project, based on various tests, values within the range of 1.0 to 6.0 MPa can be observed for the stiffness modulus Es1 (initial loading), which strongly depends on the type of glass foam granulate and the corresponding load.

The values of the thermal conductivity of the glass foam granules measured with the new oneplate apparatus are in the range of 0.08 W /(m K) to 0.1 W /(m K) and thus show significant scattering. The scattering decreases with increasing degree of compression. The thermal conductivity is in the range of 0.08 W/(m K) for compacted foam glass granules (measured value of the thermal conductivity). The thermal conductivity of the glass foam granules increases slightly with increasing temperature. The thermal conductivity increases approximately linearly with increasing moisture content at low temperatures, i.e. 20 ° C and less. With a moisture content of 20 Vol.% the thermal conductivity increased to values between 0.15 W/(m K) and 0.2 W/(m K).

The thermal conductivity of the glass foam granules decreases slightly with increasing degree of compression, however the thermal resistance decreases (given a constant volume of the material sample), as the thickness of the sample decreases with increasing decree of compression. From the thermal point of view the compaction should accordingly be as low as possible. With the presented mathematical models good agreement between predicted and measured results can be obtained for dry samples and for low water contents (i.e. 10 Vol.% and lower). For higher water contents the model suggested in Ochs f., 2010 delivers better results.

Prospects / Suggestions for future research

The knowledge gained, can be considered as the basis of a future development of a standardized procedure for the classification of GSG. The findings also encourage further research on this special material (e.g. within a PhD-thesis).

As the glass foam granulate layers showed divergent characteristics concerning the load deformation behaviour, it is recommended to focus on an improvement of the production process in order to guarantee a constant quality of the material.

Further studies with a focus on the use of foam glass granules as an insulating material for large underground heat stores will follow in the framework of the FFG project store4grid. The one-plate measuring equipment has been extended such that the direction of heat flow can be changed (by rotation of the whole apparatus). This allows studying the influence of convection on the thermal conductivity of bulk insulation. This represents an advantage in comparison to previous works (e.g. Ochs 2010), where a two-plate apparatus was used.

In the framework of the FFG, project store4grid the influence of moisture will be further examined. Furthermore, the influence of convection on the thermal conductivity in bulks with the direction of the heat flow from bottom to top and horizontal heat flow will be investigated. For the prediction of the effective thermal conductivity of foam glass granules installed in the basement the development of the relative humidity or equilibrium moisture content of the foam glass granulate with time is crucial. In-situ measurements over a period of minimum 3 years in a relevant number would be necessary for this. This should be examined in future projects.

Project Partners

Project management

Dr. Andreas Andreatta
University Innsbruck / Institute for Structural Engineering and Material Sciences / Unit for concrete structures and bridge design

Project or cooperation partners

Contact Address

University Innsbruck / Institute for Structural Engineering and Material Sciences / Unit for concrete structures and bridge design
Dr. Andreas Andreatta
Technikerstrasse 13
6020 Innsbruck, Austria
Tel.: +43 (512) 507 6638
E-Mail: andreas.andreatta@uibk.ac.at
Web: www.uibk.ac.at/massiv-und-brueckenbau/

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