Rigid Foam as an Engineered Material

The traditional use of extruded polystyrene (XPS) rigid foam insulation has been to control heat flow between building living space and outside environment; however, new information reveals additional ways XPS rigid foam insulation can be used in heavy load roadway, railbed, and airfield applications. The objective of this paper is to provide engineers with a better understanding of the behavior of rigid foam as an engineered insulation material to support transportation infrastructure in cold regions. Numerous aspects of foam application and performance are discussed in this paper. Aspects include equivalent thermal resistance of foam vs. soil, normalized stress-strain curves of various rigid foam ultimate compressive strengths, cost comparisons between rigid foam and fill, and recent testing results of rigid foam when applied loadings exceed the ultimate compressive strength under repeated loadings. Testing approach was designed to consider loading associated with heavy equipment active in oil and mining operations. Historic design recommendations for use of rigid foam in bearing considerations has been the inclusion of a sustained dead load from which a reduced live load is then utilized. The reduced live load presents challenges in roadway and runway applciations as more fill is required to reduce applied surface loads to lower values, thus disqualifying XPS as a viable insulation solution to support pavement structures under heavy live loads. With the introduction of new data, analysis, and interpretation, XPS is shown to provide a cost-effective thermal barrier between the subgrade and road surface for heavy loads when compared to fill that is sourced far from the construction site. The recent testing of both extruded and expanded polystyrene under heavy and repeated loadings, with thermal resistance comparisons, leads to additional understanding of rigid foam as an engineering material.