Finnfoam Oy's XPS panels’ strength development and strength values
The below opinion is based on the stress analysis for XPS panels carried out at the Institute of Earth and Foundation Structures of Tampere University of Technology (Work No. 239, report dated January 15, 2003) and the results of the compressive strength self-monitoring provided by Finnfoam Oy.
The aging process of extruded polystyrene panels (i.e. XPS panels) entails that the compressive strength of the panels increases over the first few months or even years after manufacturing, depending on the strength class and thickness of the panel material. Thinner and lighter panels will reach their final strength faster than thicker and heavier panels. The final strength of the panels is, also depending on the strength class and thickness, roughly 50–100% greater than the compressive strength measured immediately after manufacturing.
The specification of the short-term compressive strength of the XPS panels by Finnfoam Oy's quality control is based on testing that is performed slightly later than the testing age of 45 days specified in the EN 13164:2008 standard. For this reason, the short-term compressive strength value of Finnfoam Oy's XPS panels may not satisfy the compressive strength value specified for the panel type at the age of 45 days. According to Finnfoam Oy, the observed deficits at the age of 45 days have however been below 20% of the reported short-term compressive strength of the panels.
With regard to feasibility related to the bearing capacity and stability of structures, the short-term compressive strength of XPS panels is in most cases a relatively insignificant factor. For most structural applications, such as industrial floors typically subjected to heavy loads and frost protection installed under slab-on-ground foundations, the feasibility of the panel material is, instead of short-term strength, determined by its strength against long-term or permanent load. In this case, the compressive strength of XPS panels in relation to long-term loads is considered to be 50% of the short-term compressive strength value, i.e. the maximum load allowed for panels with a strength class of 300 (short-term compressive strength 300 kPa) would be 150 kPa, for example. Based on the above, it can be stated that the long-term loads allowed for XPS panels in conventional structural use are so minute in relation to the short-term compressive strength of the panel that the panel material may reach the short-term compressive strength in accordance with its reported strength class without damage.
Cases where the short-term strength of XPS panel is a significant property in the structural sense are relatively rare. These situations may however occur, for example, when building scaffolds subjected to a heavy load on top of XPS panels, or when XPS insulation installed under a foundation is stressed by exceptional temporary restraint forces of the framework. In such cases it is possible that a relatively “fresh” XPS panel is subjected to load that is significant in relation to its short-term strength at a point where the non-aged panel has not yet reached its short-term compressive strength in accordance with its strength class.
If Finnfoam Oy's XPS panels are subjected to compressive loads that exceed their long-term compressive strength when they are less than 6 months old, a maximum short-term compressive load of 75% of their short-term strength in accordance with their strength class is permissible for the panels.
Short-term compressive strength as a function of the age of the panel
Building construction Opinion No. TRT/1754/2009
Tampere, April 17, 2009
TAMPERE UNIVERSITY OF TECHNOLOGY
Department of Civil Engineering