During GeoAmericas 2016, K. Lin, A. Tang, and J. Wu detailed some ongoing exposure testing with geosynthetics in Taiwan. The results were somewhat unique in the field, due to the research being conducted in a warm, humid, and rainy subtropical region. Geosynthetica, which published the GeoAmericas 2016 proceedings, reprints the work here. NOTE: The feature image on this story is not from the testing protocol. It is from a multi-year ecological park project of ACE Geosynthetics, which was part of the exposure testing described in this article.
INTRODUCTION TO EXPOSURE TESTING
While geosynthetics have been used widely for numerous geotechnical engineering applications worldwide, questions remain regarding their outdoor exposed service lives. Many factors affect the material’s lifetime: sunlight, temperature, oxidation, hydrolysis, chemical, biological, etc. Decrease of safety performance due to various surrounding environments occurs when geosynthetics are applied in the construction field. Therefore, it is important to consider how serious the effects of outdoor environments may have to cause the degradations of property for the materials being used.
Among the outdoor harmful impacts, sunlight is an important cause of degradation for geosynthetics, which is made from polymers and very sensitive to ultraviolet from sunlight. Photo-degradation and thermal-degradation due to sunlight is well known to occur in polymer materials. The degradation is caused and initiated by photons of light or energy of thermal breaking the polymer’s chemical bonds. Several studies show that different types of polymer materials and chemical bounds have different sensitivity to the energy of a photon (wavelength).
Many research projects have explored the long-term performance of photo-degradation for polymer materials. There are two tests that have been commonly used for this purpose. Outdoor exposure testing is more trustworthy but requires much time (Lodi et al. 2008, Dierickx and Van Den Berghe 2004, Suits and Hsuan 2003, Grubb et al. 2000, Grubb et al. 1999). The accelerated exposure test is more efficient but difficult to interpret the complex correlation between the test result and actual weather condition (Rios-Soberanis et al. 2013, Bratton 2013, Liew and Brown 2012, Yang and Ding 2006, Suits and Hsuan 2003).
This outdoor test exposes the specimens to natural sunlight and weather conditions. ASTM D5970 has provided a standard procedure for conducting outdoor exposure testing for geotextiles. It also gives recommendations for the evaluation of photo-degradation at a particular site. However, because in-situ natural weather conditions vary, it is difficult to compare the test results from different sites. The intensity of total global solar radiation and solar UV radiation especially shall be measured during the testing for comparisons of different types of exposure testing results (Suits and Hsuan 2003).
For the real performance of photo-degradation, global solar radiation (UV radiation), temperature, and relative humidity are very important influence factors in UV degradation of polymer materials. Although the accelerated exposure test provide rapid test results for the comparison of relative durability for different materials, it is not useful for the same material on different sites due to the possible differences of UV radiation, temperature, relative humidity and other weather factors (Lodi et al. 2008).
Geosynthetics in Taiwan have seldom been tested for their degradation under local natural environment. It is important to evaluate the long-term performance of geosynthetics installed in such a warm, humid, and rainy subtropical region.
This study conducted outdoor exposure tests to observe the effect of environmental conditions for eight different geosynthetics in Taiwan for 12 months. The retained tensile strength properties of tested materials for each condition were then evaluated as percent strength change after the exposure test. The reported results might contribute valuable answers to the above questionable conditions.
The experiments were performed on eight different geosynthetics. One geogrid, three different woven geotextiles, and four different nonwoven geotextiles were evaluated in outdoor exposure test up to 12 months. As shown in Figure 1, each type of woven and nonwoven geotextiles was prepared in five duplicates with each piece cut to a dimension of 18cm wide and 30cm long. Geogrid was prepared in a dimension of 120cm wide and 240cm long. Table 1 presents the characteristics of the tested materials in this study. All materials are commercial products from three manufacturers and have been widely used for different geotechnical applications in Taiwan. As can be seen in Table 1, test materials were specifically selected for a wide range of characteristics to observe the effect of long-term exposure on different materials.
The test site is located in Wuchi, Taichung City, Taiwan (24° 14’ 25.51” N, 120° 30’ 29.47” E, 7m elevation). It is in the central area of the island where there is sufficient sunlight throughout the year. All samples were set up on the rooftop to expose themselves to the air totally (Figure 1). The experiment was commenced on 9 November 2010 for one year.
The outdoor exposure test was performed according to ASTM D5970. All test samples were attached to wooden frames oriented 45° from the horizontal. The surface of all wooden frames were prepared free from any paint and any shade.
They were placed in a manner such that no heat or sunlight was reflected on their surface from surrounding buildings. All samples were facing the equator at designated location and representative samples were taken for exposure times of 1, 2, 4, 8 and 12 months. When the predetermined exposure time was reached, one of the five samples for each type of geosynthetic was removed from the wooden frame and sent to laboratory for further examinations of their tensile strength and mass per unit area.
The tensile strength test for geogrid was performed according to ASTM D6637 method A, and those for woven and nowoven geotextiles were according to ASTM D5035. A percent strength retained and a percent elongation retained in terms of tensile test was calculated after several exposure time (1, 2, 4, 8 and 12 month).
RESULTS OF OUTDOOR EXPOSURE TESTING
The weather information was obtained from the Wuchi weather station (24° 15’ 31” N, 120° 30’ 54” E, 31.73 m elevation) which is close to the test site. Table 2 presents the weather information during the exposure period. Global solar radiation varied with seasons and higher amounts all concentrated in summer. The total solar radiation in Wuchi was 5,358.9 MJ/m2.
Retained strength properties
After the designated exposure time was reached, representative samples for each type of geosynthetics were taken to the laboratory for further examinations. Figures 2 and 3 show the test results for the percent strength retained and the percent elongation retained with the elapsed exposure time.
As can be seen in Figure 2, the geogrid showed virtually no reduction in strength throughout the 12 months of exposure in sunlight. However, the values of retained strength for all geotextiles generally decreased with the elapsed exposure time. The strength of nonwovens presented a greater reduction than those of wovens. NW1 and NW2, PP staple fibers geotextiles, appeared to have the worst results. The retained strength dropped to 10 to 20% of their initial condition after an exposure time of 12 months. NW3 and NW4, PET continuous filament geotextiles, have shown somewhat better results with a strength reduction of 40 to 70%. The retained strength of PP woven geotextiles exhibited much stronger performance than those of nonwovens. The greatest decrease was about 30% of its initial strength for GT3 whereas GT1 and GT2 only have shown little reduction in strength. Compared with GT1 and GT2, GT3 has the highest initial strength.
However, test results indicated that the geotextile with a higher strength in an exposed environment does not favorably warrant it has a better performance. In average, the rate of strength reduction was about 6.67% per month for nonwovens and 1.67% per month for those of wovens. When exposed in outdoor environment, the nonwoven geotextiles presented about 4 times faster in strength reduction than woven geotextiles.
Figure 3 shows the relationships of retained elongation with the elapsed exposure time. Generally, the tendencies were similar to those shown in Figure 2. However, nonwoven geotextiles immediately dropped their retained elongation within the first two months whereas woven geotextiles did not initiate their reductions until 4 months of exposure. Again, nonwovens showed the worst decrease with an averaged retained elongation only about 50% of their original values. For wovens, GT3 presented the greatest reduction among all samples tested.
The outdoor exposure test results indicated that long-term sunlight truly damaged the tensile strength properties of all geotextiles tested despite the types of the materials. The differences of reductions were probably because of fiber types, weaving structures, and amount of UV stabilizers etc. The PET geogrid has virtually no degradation for an exposure in outdoor environment for 12 months. It can be attributed to the protection of PVC coatings on the surface of geogrid. Based on the exposure testing studies, suitable protections are mandatory for geotextiles to be used in outdoor environment. Woven materials are also preferable for those applications exposed in sunlight. The test results collected in this study are valuable for the applications of those geosynthetics in the markets.
This research clearly indicates that the outdoor exposure testing can closely reflect natural weathering conditions and provide the most accurate local useable information. However, uncontrollable weathering variables limit the possibility of reproducible data among different sites. A longer test duration also frequently leads the engineers to the selection of an accelerated weathering method. Further studies are recommended to develop correlations of test results with other investigations to promote additional potential applications of such empirical local information.
The effects of outdoor environment on geosynthetics are important. This research conducted long-term exposure tests on geotextiles and geogrid to evaluate their possible degradations under local conditions. Based on the study, the following recommendations and conclusions can be drawn:
- The PET geogrid showed little reduction in strength throughout the 12 months of exposure in sunlight. It can be attributed to the protection of PVC coatings on the surface of geogrid.
- The values of retained strength and retained elongation for all geotextiles generally decreased with the elapsed exposure time. The strength of nonwovens presented a greater reduction than those of wovens. It also appears that the geotextile with a higher strength in an exposed environment does not favorably warrant it has a better performance.
- For nonwoven geotextiles, the retained strength of PP staple fibers geotextiles dropped to 10 to 20% of their initial condition after an exposure time of 12 months whereas PET continuous filament geotextiles have shown a little stronger results.
- When exposed to outdoor environment, the nonwoven geotextiles presented about 4 times faster in strength reduction than woven geotextiles. Nonwoven geotextiles also immediately dropped their retained elongation within the first two months whereas woven geotextiles did not initiate their reductions until 4 months of exposure.
- The outdoor exposure test results indicated that long-term sunlight truly damaged the tensile strength properties of all geotextiles tested despite the types of the materials. The differences of reductions were probably because of fiber types, weaving structures, and amount of UV stabilizers etc.
- Test results presented herein are valuable to the applications of geosynthetics in Taiwan. However, correlation database with other studies should be developed to increase more potential applications of such empirical local information.
Lin and A. Tang are with ACE Geosynthetics, Taiwan, R.O.C. J. Wu is with the Dept. of Civil Engineering, Chung Hua University, Taiwan, R.O.C.
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