Durability of geosynthetics: The basics
This article is based on a training presentation delivered on Sept. 24, 2014, at the 10th International Conference on Geosynthetics in Berlin, Germany. That talk was itself based on courses given at ERA Technology, Leatherhead, U.K., with Alan Friday, and later at SKZ in Würzburg, Germany, with the participation of Hartmut Schröder, Peter Trubiroha, Helmut Zanzinger, and Bob Koerner. The lectures were then compiled as a 275-page e-book published by SBRCURnet in the Netherlands with an extra chapter by Wim Voskamp. A revised and updated Second Edition is scheduled for publication in 2015 by SBRCURnet/CRC Press in London, U.K.
This article is not a review of the latest research but it describes the basic information needed whether one is a manufacturer, user, or legislator. For more detail, diagrams, and a full list of references, see this book.
Geosynthetics have been in use for about 60 years and have proven themselves remarkably durable. The market, however, requires lifetimes of up to 100 years from polymers that were themselves only invented 80 years ago or less. Polymer scientists have to use their knowledge and experience to assess how these materials could degrade over a variety of uses and environments, what measures can be taken to prevent this, and how durability can be tested in a short time. The words could degrade are carefully chosen: they emphatically do not mean that the materials must degrade in this way.
Apart from those geotextiles that are intended to degrade, such as those based on jute, geosynthetics are not susceptible to biological degradation. The molecular weights of the polymers used are too high. Should a test be required, EN (European Standard) 12225 provides a method where the material is placed in a soil under optimum conditions for biological degradation.
Tests also exist for assessing the resistance of geomembranes, in particular geosynthetic clay liners, to penetration by roots, for which the materials are placed in a soil below plants with aggressive taproots such as lupins or pyracantha (EN 14416, EN 13948). Enjoyable though these tests may appear, they are slow to perform and do not allow for any accelerated lifetime.
Chemical effects on geosynthetics are undoubtedly the most complex type of degradation. The following are some important fundamentals:
- The reactions involve liquids or gases.
- In partially saturated soils, the particles are surrounded by a layer of moisture and many reactions take place within this layer.
- Most reactions occur at the surface of the polymer.
- Where small molecules such as water and oxygen can diffuse into the polymer, the reaction takes place throughout the whole volume.
- In semicrystalline polymers, diffusion and reaction take place in the amorphous rather than the crystalline regions.
- Rupture of the polymer chains is critical for the strength of the geosynthetic, as is rupture of the tie molecules that connect crystalline regions in semicrystalline polymers.
The hydrolysis of polyester (polyethylene terephthalate) fibers is easier to predict, particularly as most polyester fibers used in geotextiles and geogrids are of a similar high quality. The polyester reacts with water to split the long molecules into two, the reverse of how the polymer was formed in the first place. This leads to a slow reduction in molecular weight, which, in turn, lowers the strength. As mentioned above, water in the form of hydroxyl ions can diffuse into the polymer so the reaction can occur throughout the cross section, which is why it is termed internal hydrolysis. This process is very slow at normal ambient temperature. Protons from carboxyl end groups act as a catalyst.
Numerical prediction of the rate of degradation and its effect on strength at service temperatures makes use of Arrhenius’ formula. Svante Arrhenius, who received the Nobel Prize for Chemistry in 1903, established from thermodynamic principles a relation between the logarithm of the rate at which a process occurs and the reciprocal of the absolute temperature.
Using this formula we can perform tests at higher temperatures, confirm the relation between rate and temperature and establish the constant of proportionality, and then use it to predict the much slower rate at the design temperature. This must, of course, be coupled with an end-of-life criterion such as a minimum acceptable molecular weight or a minimum strength.
For example, the Arrhenius diagram may typically predict a proportional reduction in strength per day of 0.00000102, leading to a residual strength of 96.2% after 100 years or RFCH = 1.04. In general, polyester fibers are predicted to have >90% retained strength after 100 years at an ambient temperature of 20 C. The loss of strength will proceed much faster at 35 C or above; or slower in dry soils.
In North America, resistance to hydrolysis is assured by specifying a low carboxyl end group count (<30 meq/mol) and a high molecular weight (MN >25000) to limit the number of end groups present. In Europe there is a specific test (EN 12447) in which a sample of geotextile is immersed in water at 80 C for 14, 28, or 56 days with a requirement of 50% minimum retained strength to assure for 25, 50, or 100 years’ durability respectively.
In alkaline environments an additional reaction takes place at the fiber surface, thus the term external hydrolysis. This reaction progressively reduces the cross section of the fibers and, as a result, their strength. Caution is required when specifying a geotextile for use in a soil environment of pH 9 or above, such as close to new concrete walls.
Oxidation of polyolefins
There are no specific additives for polyesters to increase their lifetime; the opposite is the case for polyolefins such as PP and PE and their blends. Polypropylene (PP) and polyethylene (PE) are used in a variety of forms such as highly oriented fibers, partially oriented geogrids, and extruded sheets such as geomembranes with little orientation. The principal mechanism of degradation is oxidation and all polyolefin geosynthetics will contain additives to retard or prevent this occurring.
A masterbatch of additives in a typical polyolefin might include:
- antioxidants effective at the high temperatures used in processing the polymer.
- antioxidants effective at the low temperatures typical in service.
- ultraviolet stabilizers: UV absorbers, UV quenchers, hindered amine light stabilizer (HALS).
- deactivators for metal catalyst residues left over after processing, or metal ions in the soil.
The effect of these additives on the durability of the geosynthetic is huge, and the emphasis now shifts from measuring the intrinsic rate of degradation to assessing the lifetime of the additives themselves.
The simplest method for predicting the rate of degradation and lifetime is to expose the geosynthetic in air at higher temperatures and to measure the time to the point at which the retained strength is 50% of its initial value, by which time the additives will have become ineffective. This is the basis of the index test EN 13438 Method A in which a specimen of geosynthetic is immersed in water at 80 C for 28 days and then exposed to air at 90 C for 56, 112, or 224 days, according to whether the required lifetime is 25, 50, or 100 years (50% retained strength is required).
A more sophisticated procedure is to split the exposure time into three, namely an incubation phase in which the antioxidant additive depletes but there is no change in oxidation induction time (OIT); a second phase during which the OIT decreases due to further depletion of additives, but where there is no change in strength; and a final phase during which the strength decreases until it reaches a minimum acceptable level. By performing tests at multiple temperatures and determining the length of each phase it is possible to establish Arrhenius diagrams for each phase and to extrapolate each to the service temperature to predict the duration of the respective phase. These are then added to predict the total lifetime or can be used to predict the state of the polymer at a set service lifetime. Both of these methods are slow and the second method is valid only for stronger antioxidants.
Heating the geotextile any further in an effort to speed up oxidation would melt the polymer. Instead, the process can be accelerated by increasing the concentration of oxygen, first by using oxygen instead of air (which contains 21% oxygen) and then by increasing the pressure.
Methods have been developed both in the USA and in Germany, in which the German method immerses the specimen in alkaline water to include the effect of leaching and to reduce the possibility of explosion. An index test based on this procedure requires initial immersion of the specimen in water at 80 C for 28 days, the application of oxygen at 30 barometer at the same temperature for 28, 56, or 112 days depending on the intended lifetime, and the measurement of the retained strength with a passmark of 50%.
Alternatively, multiple tests can be performed at different temperatures and pressures to determine the relation between these two parameters and strength and thus predict lifetime. These methods are offered as an alternative to oven ageing, but still lack sufficient correlation with each other and with real durability lifetime for them to be fully accepted.
Geomembranes and landfills
Oxidation is the ultimate stage of degradation for polyolefin geomembranes. The various stages of degradation for geomembranes used in landfills have been summarized as follows:
damage on installation, which can be located and repaired.
- damage during waste filling.
- dormant period.
- stress cracking.
- stress cracking and oxidation.
- oxidation, leading to embrittlement, loss of strength, and the generation of holes.
The importance of limiting mechanical stresses during installation cannot be overemphasized; most holes are formed at this stage. Indentations, welds, folds, and joints can be locations of high mechanical stress, which can initiate stress cracking. Where the geomembranes are used to line landfills for municipal waste, they can be exposed to fatty acids and inorganic salts and to surfactants that can cause antioxidant depletion or environmental stress cracking (ESC). Transition metals can act as a catalyst for oxidation, while the heat generated by the waste can raise the local temperature, providing further acceleration. Much information is available in the literature and corresponding environmental regulations.
Other polymers used less commonly in geosynthetics include polyamides (PA), polyaramids, polyvinyl alcohol, and polyvinyl chloride (PVC). Polyamides are sensitive to weathering and to acid environments, and can be subject to hydrolysis and oxidation, both of which are slow and likely to be affected by thickness, orientation, and crystallinity, as well as the water content, which can be as high as 10%.
Information on PA geosynthetics is scarce, but one can draw on experience with other uses of the material. Polyaramids are used for their high modulus and strength, and can be sensitive to weathering and hydrolysis. Again, more information is available for other uses, particularly for fiber-reinforced plastics.
Polyvinyl alcohol fibers are offered as an alternative to polyester in highly alkaline environments. They are potentially susceptible to slow hydrolysis and oxidation but information on their durability is only now becoming available. PVC has been widely used as an early geomembrane, for example in the protection of dams. Plasticized PVC contains about 35% plasticizers, and the depletion of these plasticizers is the primary reason for degradation, with 15% as the end-of-life criterion. Choice of a high molecular weight plasticizer (>400) is essential. Antioxidants are also required to prevent slow oxidation. The lifetimes are quoted at 20–30 years.
There is much debate concerning the use of recycled polymers in geosynthetics. Current European standards suggest that rework material may be used, but that recycled material from other sources, whether industrial or consumer, should only be acceptable for applications with lifetimes up to five years.
The European standards for geotextiles in various applications contain a common annex B, specifying the index tests to be performed according design lifetimes of 5, 25, or 100 years. The tests apply to all of the most common polymers used and to a general environment with a soil pH between 4 and 9 and a temperature not exceeding 25 C. Such index tests use extreme conditions, while strength is generally used as the criterion for durability because it can be measured simply and with good precision.
Where there is a range of products, it is acceptable to perform tests on the lightest product rather than having to test each one separately. A similar annex for geomembranes applies principally to polyethylene and does not specify any particular lifetime. It sets out tests for weathering, biological resistance, ESC, leaching, oxidation, and chemical resistance with both retained strength and elongation at break as the normal pass criteria. These standards are currently under review and the user is strongly recommended to check for the latest published editions.
Experience: Evidence from exhumed material
This article does not survey the evidence for durability, of which there is an increasing amount of published evidence. It is, however, essential to determine the significance of any such observation. If a piece is retrieved after 20 years, and shows no change in strength, the first question is: Was any change expected in the environment that the geosynthetic actually experienced? If not, all it proves is there were no unanticipated effects. The evidence cannot be used to justify the original prediction.
To establish whether an observation on past durability is relevant to a new application, we must be able to answer the questions:
- How similar was the exposed material to the one we are planning to use?
- What environment did the exposed material actually experience (often milder than the environment assumed in the original design)?
- What changes were observed?
…Were these changes due to installation damage?
…Were these changes better or worse than expected (if necessary, recalculate the original expectations for the actual environment)?
…Is there any archive material for comparison?
Use of this data together with the predictions from accelerated tests should enable us to refine our predictions of durability in future applications.
Geotextiles have proven themselves very durable. Of the failures that have been reported, nearly all have been due to poor material selection, poor design, or poor construction, and not to actual durability failure. In many cases the environment turns out to be less severe than what was designed for; thus the importance of knowing the environment and recalculating the prediction of lifetime to compare with observation.
However, any prediction of durability or lifetime is an estimate. Paraphrasing former U.S. Defense Secretary Donald Rumsfeld, there are known knowns, known unknowns, and unknown unknowns. The known knowns we allow for using our existing test regimes. Known unknowns—effects we know about but cannot quantify sufficiently—can be allowed for by a safety factor. But there are also unknown unknowns—the ones we don’t know we don’t know. A prediction can never take into account the wholly unexpected—the “unknown unknowns.” For example, climate change will change the environment, but we are not sure how.
As we conclude in the book, the prediction of durability is the best we can do with what we know, and those who follow us will either praise or disown our efforts long after we have joined the company of geosynthetics deep under the soil.
John Greenwood is a consultant based in Ewhurst, U.K.
Durability of Geosynthetics. CUR Report 243. CUR, Gouda, Netherlands, 2012, pp. 275. Due for publication (2015) in revised and updated form as Durability of Geosynthetics, Second Edition by John H. Greenwood, Hartmut F. Schröder, and Wim Voskamp. SBRCURnet/CRC Press, London, U.K., approx. 275 pages, ISBN 9789053675991.