Short- and long-term compression strength ratings must be reported for all thermal insulation materials used in inverted roof structures and ground frost insulation and slab-on-ground base insulation applications as the materials used in these applications are subjected to considerable loads and stress during their lifetime.
Performing a long-term compression test (creep test) for operational life of 50 years takes 608 days. For this reason, a quicker short-term compression strength test is commonly used in the industry. Short-term compression strength is determined by compressing the material by 10% in relation to its original thickness over a duration of a few seconds, or by applying continuous compression until a fracture occurs. The test may also be used to determine a property that is much more important: the long-term compression strength (creep) of the material.
Structures must always be designed and dimensioned in accordance with the long-term compression strength (creep) of the material, which indicates how much the material compacts at specific loads. The long-term compression strength of Finnfoam F-300, for example, is reported as CC(2/1.5/50)130, which means 130 kPa, or approximately 13 tonnes per m². It also indicates that, at maximum load, the Finnfoam F300 compacts by no more than 1.5% over a duration of 50 years.
Short-term compression strengths are only reported for reference and must not be used as dimensioning values. Instead, long-term compression strength must be used, as this value indicates the maximum long-term load the material can bear.
Finnfoam's strength characteristics:
Short-term compression strength, 3 months (kPa)
Short-term compression strength, 45 days (kPa)
Long-term compression strength (kPa)
Modulus of elasticity (kPa)
Shearing strength (kPa)
Bending strength (kPa)
Tensile strength (kPa)
The unbroken completely homogenous and sealed structure formed by the very small cells makes Finnfoam a very strong material. In addition to this, the leather-like surface layer increases rigidity and creates a sandwich structure. The surface layers have excellent tensile strength, the core material has excellent compression strength and the core and the surface layers are securely attached to one another. The Finnfoam cell structure has no weak links, and there are no continuous air channels or glued connections between the cells.
The long-term compression strength of standard Finnfoam boards (F-200 to F-500) is 90–225 kPa depending on board type. The corresponding short-term compression strength is from 200 kPa to over 500 kPa. F-700 boards with short-term compression strength of over 700 kPa are also available by special order.
Our range of ground frost insulation materials includes the following products:
The F-200 has a short-term compression strength of over 200 kPa, which means approximately 20,000 kg/m², and a long-term compression strength of over 90 kPa, approximately 9000 kg/m². The F-200 is intended for use as a ground frost and base insulation material in applications with light loads, but can also be used as a wall and roof insulation material. The F-200 is also an excellent choice for load distribution structures in road construction.
The F-300 has a short-term compression strength of over 300 kPa, which means approximately 30,000 kg/m², and a long-term compression strength of over 130 kPa, approximately 13 000 kg/m². The F-300 can be used for virtually every part of a single-family home construction. The F-300 is usually also sufficient for ground frost protection of walkways and yard areas with no heavy traffic. In many cases, the F-300 is also suitable for installation under the footing of light buildings. For heavier structures, the F-400 and F-500 are commonly used.
The F-400 has a short-term compression strength of over 400 kPa, which means approximately 40,000 kg/m², and a long-term compression strength of over 180 kPa, approximately 18 000 kg/m². The F-400 is used in applications such as areas with heavy traffic, including roads, streets and lorry terminals. The F-400 is also used in parking areas, where no load-balancing topsoil layer (minimum thickness: 300 mm) is laid on top of the insulation boards. The F-400 is also commonly used as an insulation material for floor slabs in industrial buildings where the floor is subjected to heavy loads from lorries, forklift trucks, machinery, tall warehouse shelves, etc.
The F-500 has a short-term compression strength of over 500 kPa, which means approximately 50,000 kg/m², and a long-term compression strength of over 225 kPa, approximately 22 500 kg/m². The F-500 is used in applications where high compression strength is required, such as ground frost protection of railways. The F-500 is also used in industrial building floors that support tall shelves holding pallets or heavy machinery.
F-600 and F-700
The F-600 and F-700 are special-purpose products available by special order. They are used in applications such as the floors of industrial buildings. These extremely strong insulation boards are also used under railways where heavy axle loads are used.
Short-term compression strength as a function of board age
Building Technology, Statement No. TRT/1754/2009
Strength development and strength values of Finnfoam Oy's XPS boards
This statement is based on the XPS board strength analysis (Job no. 239, Report 15 January 2003) conducted by the Tampere University of Technology Soil Mechanics and Foundation Unit, and compression strength test results from internal monitoring conducted by Finnfoam Oy.
A factor that influences the aging behaviour of extruded polystyrene (XPS) boards significantly is that the compression strength of the boards increases during the first few months or even years after manufacture (the time depends on the weight rating and the thickness of the board). Thin, lightweight boards reach their final strength quicker than thick, heavy boards. Depending on the weight and thickness of the boards, the final compression strength increases by approximately 50 to 100% during this period.
The tests used by Finnfoam Oy's quality control department are conducted at an age point slightly later than the 45 days specified in standard EN 13164:2008. Therefore, the compression strength of Finnfoam XPS insulation boards may not fulfil the declared nominal compression strength values for the board type in question at the age point of 45 days. According to Finnfoam Oy, however, the measured compression strengths at the age point of 45 days have been less than 20% below the declared compression strengths of the boards.
In terms of the stability and the load-bearing capacity of structures, the compression strengths of XPS boards are, in most cases, relatively insignificant factors. For the majority of building applications, such as industrial floors with heavy loads and ground frost insulation installed under an earth-based footing, the suitability of the insulation material for the task is determined by the material's long-term (or permanent) resistance to compression rather than its short-term compression strength. In these instances, the long-term compression strengths of XPS boards should be considered as 50% of the short-term compression strength. The maximum load permitted for a strength 300 board (short-term compression strength 300 kPa), for example, would be 150 kPa. It can therefore be concluded that the long-term loads permitted for XPS boards in regular structural applications are so small compared with the board's short-term compression strength that the material will be able to reach its stated short-term compression strength value without being damaged.
The short-term compression strength of XPS boards is a significant factor in relatively few cases. These cases may, however, occur when building, for example, support structures with very heavy loads on top of the XPS sheets, or when XPS insulation boards under a building's footing are subjected to exceptional construction-time tensioning loads. In these cases, it is possible that an un-aged XPS board is subjected to a significant load (compared with its short-term strength) before having reached its full strength.
In cases where Finnfoam Oy's XPS boards aged less than 6 months are subjected to compression loads that exceed their long-term compression strength rating, the maximum compression load allowed is 75% of the board's nominal short-term compression strength.
Tampere, 17 April 2009
TAMPERE UNIVERSITY OF TECHNOLOGY
Department of Civil Engineering
Construction and Building Technology