Compression Behavior

Compression behavior is of primary interest to design engineers. Much of the design experience to date has relied on data derived from testing 50 mm cube samples under unconfined compression at 10 percent strain per minute. Stress strain curves normally show an initial lag that is similar to effects of seating error in soil tests. This is in part due to difficulties associated with obtaining parallel and level specimen end areas and because sample edges are cut through expanded cells. As specified in ASTM C-303 and D-1622, this segment of the stress strain curve is ignored and the steep portion of the curve is extended backward to establish a correction for zero offset. Corrected and uncorrected unconfined compression test results for 50 mm cube EPS samples tested at a displacement rate of 5 mm/min are shown below (corrected curve in red).

Both initial modulus and strength increase with density. There is also a significant sample size effect on initial modulus. The general trend of increasing initial modulus with density for two sample sizes is shown below.

Compressive strength is usually specified as stress at 5 or 10 percent strain in unconfined compression. For the same density geofoam, the strength at 10 percent strain is higher than the strength at 5 percent strain by about 10 percent. The figure below shows trends of compression strength increases with geofoam density.

The compression behavior of geofoam is strain rate dependent. The figure below shows compression test results for comparable density EPS geofoam samples but tested with different strain rates.

Higher strain rates result in higher initial modulus and compressive strength. When the strain rate is changed during the course of a test, the stress strain curve shifts up or down accordingly, as shown below.

References

Frydenlund, T.E. (1991). "Expanded Polystyrene - A Lighter Way Across Soft Ground." Norwegian Road Research Laboratory

Negussey, D., Jahanandish, M., (1993). A Comparison of Some Engineering Properties of EPS to Soils.

Negussey, D.N., 1997, "Properties and Applications of Geofoam", Society of the Plastics Industry, Inc