Exposed geosynthetics are often employed in regions with varying temperatures and ultraviolet (UV) radiation, which are two environmental factors most likely to affect the total lifetime of the product. Having a better understanding of how differences in temperature and UV radiation alters geosynthetic performance is key to implementing appropriate designs, installation methods, and safety factors. In this article, we will review a testing report that can be referenced to approximate service life, discuss how temperature and UV radiation can affect the service life of geosynthetic products, and provide a performance overview of high-density polyethylene.
A laboratory test for predicting the service life of exposed geosynthetic products
The Geosynthetic Institute (GSI) published a report in 2019 for predicting the lifetime of exposed geosynthetics using laboratory weathering devices. Their report shows that the service life of exposed geosynthetic products can bepredicted by measuring the product’s strength and elongation half-life after exposure to elevated temperature and UV radiation. The half-life refers to the point in time when a product’s test property reaches half of its original manufactured value. Please note this is NOT the end of life. The material may still have some useful service time remaining. While these tests utilize either a Xenon Arc or the UV-Fluorescent weathering device to control temperature and UV exposure, the UV-Fluorescent approach (using the ASTM D7238 protocol) is most typically employed (1).
For those interested in creating a similar test, at least three weathering devices should be setup to generate enough data to create a plot. GSI recommends testing at 80°C, 70°C, and 60°C. When a test property reaches half-life, its incubation time is plotted. A trend line can then be created, allowing an extrapolation to a lower site-specific temperature.
Table 1. Six Geomembrane Field Half-life Predictions in Phoenix, Arizona Compared to Laboratory Predictions all at 20°C (from reference 1).
|Geomembrane (Various Resins)||Thickness (mm)||Laboratory Predicted Half-Life in Years||Phoenix, Arizona Predicted Half-life in Years|
How temperature and UV radiation affect exposed geosynthetics
The service life of exposed geosynthetic products is derived conventionally from the half-life of all relevant properties. In this case, strength and elongation. Laboratory tests by GSI using weathering devices have shown a linear relationship between temperature and the service life of a product. For example, in one GSI test of a standard geomembrane, every 10°C increase in temperature resulted in a ~6-year and ~5-year reduction of strength and elongation half-life, respectively.
Extending lab lifetime predictions to field conditions requires making assumptions based on latitude, altitude, ozone, environment, orientation, moisture, and more. The most significant property for exposed geosynthetics, however, is site-specific UV radiation. It’s important to note that laboratory UV radiation conditions are intentionally higher than typical field conditions, set at 42.42 W/m2 between 250–400 nm wavelength per the ASTM D7238 protocol. Converting lab conditions to field values requires looking at irradiance maps, which provides radiation values for various locations (1).
Performance overview of exposed high-density polyethylene
High-density polyethylene (HDPE) is among the highest performing geomembrane resins thanks to its increased density and crystallinity. The density and crystallinity of HDPE geomembranes can affect how oxygen diffuses through the material. The denser HDPE geomembrane, for instance, has a higher crystallinity and therefore reduced oxygen diffusion as well as less susceptibility to oxidation (2). When compared with other geomembrane resins, HDPE shows superior strength and elongation durability in both lab and field conditions (see Table 1).
While not every project calls for a geomembrane with a 50+ year service life or a liner able to perform well under exposed conditions, those that do would benefit from using an HDPE liner manufactured by AGRU. By incorporating a streamlined manufacturing process called flat-die extrusion and accurately regulating the temperature throughout the extrusion process, AGRU can produce a highly consistent liner with uniform thickness spanning the entire width and length of the sheet. Because thickness in geomembranes are an important factor that also affects its durability over time, a consistent thickness throughout the sheet is essential to minimize failure.
- 1. “GRI-GS20: Exposed Lifetime Prediction of Geosynthetics Using Laboratory Weathering Devices.” Geosynthetic Institute. (2019). Accessed online 5 June 2020 at https://geosynthetic-institute.org/grispecs/gs20.pdf.
- 2. G. Hsuan et al., “Long-term performance and lifetime prediction of geosynthetics.” EuroGeo4 Keynote Paper. (2008). Accessed online 5 June 2020 at https://pdfs.semanticscholar.org/5098/e4606b93a143ab69fd902ed3f50eac826bde.pdf.