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Thrace-LINQ TECH NOTE #8
ADDRESSING UV STABILITY WITH Thrace-LINQ GEOSYNTHETICS
The test method ASTM D-4355 recently came up for reapproval during a biannual meeting held by ASTM D-35 Committee on Geosynthetics. Concerns surfaced about the test method and its uses. ASTM D-35 has begun working toward solutions.
ASTM D-4355 should be used as part of a manufacturer's quality control program only. We should discourage the use of ASTM D-4355 as an ASTM test method for specification (outdoor exposure is being discussed within ASTM D-35.)
Geosynthetic products are exposed to potentially harmful ultraviolet radiation while being installed for most civil engineering applications. This necessary evil means that the question of a geosynthetic's UV stability needs to be answered. A geosynthetic's UV stability, or the geosynthetic's ability to withstand UV exposure while retaining a reasonable amount of its relevant properties, has been routinely determined by using the index-test method, ASTM D-4355-92 (Deterioration of Geotextiles from Exposure to Ultraviolet Light and Water) xenon-arc type apparatus.
The xenon arc's light spectrum closely simulates the sun's UV rays that cause the greatest damage to polymers. As the test method states, however, ASTM D-4355 should not be used to correlate to site-specific conditions:
"Ultraviolet radiation from the sun varies with the duration of exposure, angle of inclination of the sun, atmospheric conditions, topography and geography. The xenon-arc test cannot simulate all these variables, so it is not likely that xenon-arc test results will relate directly to sunlight exposure test results for a specific fabric at a given site."Although ASTM D-4355's use as a site-specific test method is unacceptable, it unfortunately has been used for index specification. The variation between each xenon-arc apparatus is so high, however, that its validity as an index-test method also is questionable.
In June 1991, a representative from one xenon-arc weatherometer manufacturer gave a presentation to ASTM. The representative explained that interlaboratory variability can be significant with this equipment. Interlaboratory testing performed by manufacturers, departments of transportation and private testing facilities confirm this. To illustrate, the following table shows various strength retention values on the same nonwoven geotextile from three different labs using the same brand and type of xenon-arc weatherometer test equipment:
|ASTM D-4355 Strength Retention Test Results|
|Nonwoven Geotextile (after 500 hours)|
|Lab I||Lab II||Lab III|
In January 1992, the reapproval task group suggested the removal of ASTM D-4355 because of this high interlaboratory variability. The main committee rejected the task group's proposal of removing ASTM D-4355 because no substitute existed at that time, but the following was added to the current ASTM D-4355-92 test method, published in 1993:
"This procedure has been found to produce highly variable results depending on the calibration of the equipment, and as such, is not recommended for acceptance/rejection of materials."Suggestions for future test methods include an outdoor exposure method. As of the January 1993 ASTM D-35 meeting, a draft for outdoor exposure testing was being shaped.
Outdoor exposure measures the real thing--sunlight--instead of a simulated conditions. More importantly, the test method is going in the direction of focusing strength retained as a result of exposure intensity in place of a set time period. From a performance standpoint, one can relate a sunlight intensity quantity to a global map on annual radiation (Van Wijk and Stoerzer1).
Although exposure testing ideally should be performed in the location where the geosynthetic will be installed to account for all variables (humidity, temperature, wind, etc.), this approach is costly and time prohibitive. Focusing on sunlight intensity as the most important variable affecting geosynthetic UV degradation provides a realistic and relevant approach.
Unless extreme attention is paid to operator proficiency and equipment maintenance and calibration, an unacceptable environment surrounds ASTM D-4355, because no manufacturer, department of transportation or private lab consistently produces similar results on the same geosynthetic.
An alternative for the specifier to consider is to require that geosynthetics be covered within a minimal time period, say 14 days or less. This obviates the need for UV exposure testing for the vast majority of geotextile applications. Outdoor exposure testing then would be applicable only to those specific situations in which exposure is a necessary material requirement (for example, silt fence).
The use of ASTM D-4355 as part of an internal quality assurance program is the only acceptable role for the xenon-arc test method. The variability within each machine is acceptable, giving a manufacturer a tool to control his internal processes. Without ASTM D-4355, an unsatisfactory void exists where UV degradation specification is required. The advent of outdoor exposure testing for specification purposes is needed for those critical applications, and ASTM D-4355 should not be used for specification. Contact your Thrace-LINQ distributor for actual outdoor testing results on Thrace-LINQ geosynthetics.
1 Van Wijk, W. and, Stoerzer, M., "UV Stability of Polypropylene," Third International Conference on Geotextiles, 1986, pp. 851-855