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Effect of the Impact of Chemical and Environmental Factors on the Durability of the High Density Polyethylene (HDPE) Geogrid in a Sanitary Landfill

Effect of the Impact of Chemical and Environmental Factors on the Durability of the High Density... applied sciences Article Effect of the Impact of Chemical and Environmental Factors on the Durability of the High Density Polyethylene (HDPE) Geogrid in a Sanitary Landfill 1 , 1 2 3 Agnieszka Kiersnowska *, Eugeniusz Koda , Wojciech Fabianowski and Jacek Kawalec Department of Geotechnical Engineering, Faculty of Civil and Environmental Engineering, Warsaw University of Life Sciences, 02-787 Warsaw, Poland; eugeniusz_koda@sggw.pl Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland; wofab@ch.pw.edu.pl Faculty of Civil Engineering, Silesian University of Technology, Akademicka 5, 44-100 Gliwice, Poland; jacek.kawalec@vp.pl * Correspondence: agnieszka_kiersnowska@sggw.pl; Tel.: +48-22-59-35-226; Fax: +48-22-59-35-203 Academic Editor: Patrick A. Fairclough Received: 12 October 2016; Accepted: 15 December 2016; Published: 23 December 2016 Abstract: A high density polyethylene (HDPE) uniaxial geogrid was exhumed after twenty years of service in a sanitary landfill, and its properties were examined. A geogrid installed in a landfill is exposed to mechanical and chemical factors (e.g., a wide pH range and high temperatures), as well as different weather conditions. This paper presents the results of physical and mechanical analyses of virgin and aged HDPE geogrid samples. Structural changes observed by differential scanning calorimetry and Fourier transform infrared spectroscopy (FT-IR) spectroscopy correlate with the mechanical properties of the aged geogrid. The mechanical properties were found to have changed only slightly. In the FT-IR spectrum of the topmost layer of the aged geogrid samples, no significant changes were observed compared to the spectrum of the top layer of the virgin samples. This indicates the strong chemical resistance of the HDPE material, which is able to withstand environmental conditions for at least 20 years of service in a landfill. Keywords: durability; geogrids; polyethylene; reinforcement 1. Introduction Geogrids are widely used as reinforcements in slopes, walls, roads, and foundations where they are subjected to constant stress throughout their service life [1–3]. Uniaxial high density polyethylene (HDPE) geogrids are designed to be used in geotechnical structures where soil particles need support over long time periods. HDPE geogrids, due to their high strength and durability, are commonly used for the construction of steep slopes, where the rigid nodes of the geogrids are used to wedge soil in the mesh of the geogrids. Grain aggregates or soil particles pass through the geogrid mesh, partly clogging the spaces between the ribs. The strength and stiffness of the ribs prevent the displacement of soils on the sides but may lead to mechanical damage of the material [3–5]. The grid interaction with soil is a complex phenomenon and depends on several factors, such as soil type and density, grid geometry and mechanical properties, surface roughness, stress levels, and boundary and loading conditions. Geogrids embedded in soil both during the construction phase and during service life, used as reinforcements, can be exposed to different load conditions. The applied tensile loads on the geosynthetics can be permanent loads (dead weight of soil), repetitive loads (traffic loads), and loads applied during construction [6]. Appl. Sci. 2017, 7, 22; doi:10.3390/app7010022 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 22 2 of 15 Viscosity is recognized as a significant property of polymeric materials. The loading rate effects, creep deformation, and stress relaxation of polymeric geosynthetics are inherent responses to their viscous properties. The viscous properties of polymeric geosynthetics have been investigated by tensile tests with respect to the loading rate, where the deformation characteristics of polymeric geosynthetic reinforcements are more or less viscous, and the peak strength decreases noticeably with a decrease in the strain rate at failure [7,8]. These features have been studied by various experimental and theoretical methods [6–12]. However, these materials are exposed not only to mechanical effects but also to influences of the environment in which they are used, as well as the ageing process. Current standards claim up to 120 ears of design life for the structures; therefore, geogrids as components of such structures must also fulfil this age criterion [13–15]. The properties of geosynthetic material, including geogrids, generally depend on time. The decrease in the allowable tensile strength depends on short-term effects, such as installation damage, which reduces the maximum tensile strength but does not further affect long-term properties, as well as on effects such as creep and ageing by oxidation and abrasion, which result in a loss in long-term strength [16]. The service life of a structure with geosynthetic material largely depends on its durability over time. The ageing process of geosynthetic materials can be envisioned as the simultaneous combination of physical and chemical ageing [17–20]. As a result of ageing or degradation, several detrimental effects occur in the polymer: loss of additives and plasticizers, change in molecular weight, formation of free radicals and brittleness [21]. Physical ageing is related to degradation, which does not involve modifications in the molecular structure of the polymer chains. Some of these mechanisms involve mass transfer with the environment surrounding the material (extraction of additive, absorption of solvent, etc.). Others involve modifications of the organization of the internal chain in the material, i.e., change in morphology (chain orientation, crystallinity, etc.) [22]. In physical aging, the material attempts to establish an equilibrium from its as-manufactured non-equilibrium state. As a consequence, no primary (covalent) bonds are broken, and for semi-crystalline polymers such as HDPE, an increase in the material crystallinity occurs [19,23]. The mechanism of chemical ageing leads to changes in the molecular structure of the polymer chains [14]. This process eventually results in a decline in the mechanical properties and consequently damage to the material. Chemical attack can be launched directly in acidic and alkaline soils or indirectly by active waste, which is present in landfills. Depending on the chemical compound, a change in the structure of the polymer can be obtained by oxidation, chain scission, cross-linking, swelling or dissolution of the polymers. Furthermore, the effect of chemical degradation can be accelerated by temperature [24]. The predominant mechanism of degradation for most polymeric materials (geosynthetics) is chain cutting that occurs as a result of reactions of the polymer, which leads to the breakage of bonds in the backbone of the polymer chain, reduction in the chain length, and thus reduction in the molecular weight [25]. This phenomenon significantly alters the properties of polymeric materials, such as strength and elongation, and consequently affects the geotechnical structures. The oxidation process is initiated by heat (from temperature or UV (ultraviolet) radiation), mechanical stress, catalyst residues derived from the production of the geosynthetics, or reaction with impurities [26]. High density polyethylene (HDPE) geogrids are very resistant to chemical substances and do not easily deteriorate when exposed to alkaline and acid agents (except oxidizing acids), salt solutions, or microbes, because they are non-polar in nature [22]. However, in general there is more than one degradation mechanism operating at a given time, and synergistic effects (changes in pH, changes in temperature and changes in mechanical stress) can accelerate degradation [21]. Geosynthetics made from high density polyethylene have a potential for stress cracking, which is material failure caused by tensile stresses lower than the short-term mechanical strength. Appl. Sci. 2017, 7, 22 3 of 15 Appl. Sci. 2017, 7, 22  3 of 15  Cracking is usually formed in areas or regions with concentrated stress in the microstructure of local inhomogeneities. This phenomenon consists of two phases: crack initiation and crack growth. Environmental stress cracking (ESC) is the bursting of the polymer under tension when exposed to a  Environmental stress cracking (ESC) is the bursting of the polymer under tension when exposed to a chemical environment [26]. This failure mode can reduce the life of PE (polyethylene) used for critical  chemical environment [26]. This failure mode can reduce the life of PE (polyethylene) used for critical applications such as reinforcement applications in a landfill.  applications such as reinforcement applications in a landfill. The first reinforced soil structures in the world were built in France in 1970 and 1971 [27,28]. In  The first reinforced soil structures in the world were built in France in 1970 and 1971 [27,28]. subsequent years, the technique began to be used throughout the world; for example, in the United  In subsequent years, the technique began to be used throughout the world; for example, in the United States, where structures of this type have been built since 1974 [29]. The first permanent geogrid  States, where structures of this type have been built since 1974 [29]. The first permanent geogrid reinforced structure constructed in the U.S. was built in 1982 to support roadway access to the Devils  reinforced structure constructed in the U.S. was built in 1982 to support roadway access to the Devils Punch Bowl State Park along the central Oregon coast [30].  Punch Bowl State Park along the central Oregon coast [30]. This paper focuses on the durability of geogrids used for the reinforcement of a slope in an old  This paper focuses on the durability of geogrids used for the reinforcement of a slope in an old sanitary landfill. The analyzed geogrid was installed more than 20 years ago in an old sanitary landfill  sanitary landfill. The analyzed geogrid was installed more than 20 years ago in an old sanitary landfill located in Warsaw, Poland. The geogrid was excavated in October 2013 and then tested by several  located in Warsaw, Poland. The geogrid was excavated in October 2013 and then tested by several methods. The geosynthetics were installed in the landfill and exposed to mechanical and chemical  methods. The geosynthetics were installed in the landfill and exposed to mechanical and chemical factors (e.g., mechanical stress by load and trucks, a wide range of pH values and high temperatures)  factors (e.g., mechanical stress by load and trucks, a wide range of pH values and high temperatures) and were also subjected to changing weather conditions.  and were also subjected to changing weather conditions. 2. Materials and Methods  2. Materials and Methods 2.1. 2.1. Location Location an and d Descri Description ption of of th thee Study Study Site Site  The The Radiowo Radiowo land landfill fill covers covers an an are areaa of of appr approxima oximately tely 16 16 hectar hectares es at at an an altitude altitude exceeding exceeding 60 60 m m  above above  gr groou und nd  level. level.  The The  la landfill ndfill  is is  loc located ated  al along ong  the the  north north-western ‐western  border border  of of  Wa Warsaw rsaw  in in  Poland Poland  (Figur (Figure e 11) )..   Figure 1. View of the Radiowo landfill where the northern and western slopes were reinforced by a  Figure 1. View of the Radiowo landfill where the northern and western slopes were reinforced by a high density polyethylene (HDPE) geogrid in 1993.  high density polyethylene (HDPE) geogrid in 1993. From 1961 to 1991, mainly municipal waste was deposited there, and since 1992, it has become  From 1961 to 1991, mainly municipal waste was deposited there, and since 1992, it has become a structure that receives ballast waste from a composting plant. The ballast waste was composed of  a structure that receives ballast waste from a composting plant. The ballast waste was composed of 5% to 12% organic fraction (>10 mm) and approximately 4% mineral waste (>10 mm). The physical  5% to 12% organic fraction (>10 mm) and approximately 4% mineral waste (>10 mm). The physical characteristics of the ballast waste are as follows: the average density in non‐compacted waste is 0.15– characteristics of the ballast waste are as follows: the average density in non-compacted waste is 3 3 0.70 Mg/m ; the average density of compacted waste is 0.8–1.4 Mg/m ; and the waste humidity is  3 3 0.15–0.70 Mg/m ; the average density of compacted waste is 0.8–1.4 Mg/m ; and the waste humidity 15.5% to 28% [31].  is 15.5% to 28% [31]. For high and steep slopes, the key issue was to improve their stability. To this end, a number of  engineering procedures were required: comprehensive investigation of the mechanical properties of  the  waste  using  different  techniques,  mechanical  reinforcements  of  the  slopes,  changing  the  Appl. Sci. 2017, 7, 22 4 of 15 Appl. Sci. 2017, 7, 22  4 of 15  For high and steep slopes, the key issue was to improve their stability. To this end, a number of inclination of the slopes, determining the type of waste, provision of land next to the landfill and  engineering procedures were required: comprehensive investigation of the mechanical properties of clarifying the formal status of the landfill for further development [32–34].  the waste using different techniques, mechanical reinforcements of the slopes, changing the inclination Appl. Sci. 2017, 7, 22  4 of 15  Reclamation works in the landfill began in 1993 (Figure 2). They included safety improvements  of the slopes, determining the type of waste, provision of land next to the landfill and clarifying the in the geotechnical formations of the landfill body. To improve the conditions of the northern slope  inclination of the slopes, determining the type of waste, provision of land next to the landfill and  formal status of the landfill for further development [32–34]. stability  and  to  construct  the  main  access  road  to  the  landfill,  a  retaining  wall  was  constructed.  clarifying the formal status of the landfill for further development [32–34].  Reclamation works in the landfill began in 1993 (Figure 2). They included safety improvements Further  carving  of  the  slope  and  installation  of  a  horizontal  uniaxial  HDPE  geogrid  were  also  Reclamation works in the landfill began in 1993 (Figure 2). They included safety improvements  in the geotechnical formations of the landfill body. To improve the conditions of the northern slope proposed (Figure 3).  in the geotechnical formations of the landfill body. To improve the conditions of the northern slope  stability and to construct the main access road to the landfill, a retaining wall was constructed. The reason for such heavy modifications was that space was very limited on the northern slope  stability  and  to  construct  the  main  access  road  to  the  landfill,  a  retaining  wall  was  constructed.  Further carving of the slope and installation of a horizontal uniaxial HDPE geogrid were also proposed (land ownership issues).  Further  carving  of  the  slope  and  installation  of  a  horizontal  uniaxial  HDPE  geogrid  were  also  (Figure 3). proposed (Figure 3).  The reason for such heavy modifications was that space was very limited on the northern slope The reason for such heavy modifications was that space was very limited on the northern slope  (land (l ownership and ownership issues).  issues).  Figure 2. The process of geogrid installation in the landfill (1993).  Figure 2. The process of geogrid installation in the landfill (1993).  Figure 2. The process of geogrid installation in the landfill (1993). Figure 3. Cross-section and reinforcements in the northern slope of the landfill [31]. Figure 3. Cross‐section and reinforcements in the northern slope of the landfill [31].  Figure 3. Cross‐section and reinforcements in the northern slope of the landfill [31].  In November 2013, three samples of geogrids were collected from the landfill after 20 years of In November 2013, three samples of geogrids were collected from the landfill after 20 years of  In November 2013, three samples of geogrids were collected from the landfill after 20 years of  service (sample size: length approximately 1.20 m, width approximately 1.0 m). The samples were service (sample size: length approximately 1.20 m, width approximately 1.0 m). The samples were  service (sample size: length approximately 1.20 m, width approximately 1.0 m). The samples were  excavated from the first layer of the structure (Figure 3) located along the access road to the landfill. excavated from the first layer of the structure (Figure 3) located along the access road to the landfill.  excavated from the first layer of the structure (Figure 3) located along the access road to the landfill.  In this particular location, the HDPE geogrid was exposed not only to chemical and environmental In  this  particular  location,  the  HDPE  geogrid  was  exposed  not  only  to  chemical  and  In  this  particular  location,  the  HDPE  geogrid  was  exposed  not  only  to  chemical  and  impacts environmental but also to impact mechanical s but also loads  to m caused echanic by al lo the ads slope  caused itself  byand  the loading slope itsel off the andincoming  loading of tr the ucks  environmental impacts but also to mechanical loads caused by the slope itself and loading of the  filled incoming with waste.  trucks The  filled samples  with waste. were extracted The sample using s were mechanical  extracted us diggers. ing mecha They nical wer digger e remov s. They ed fr om were the  incoming trucks filled with waste. The samples were extracted using mechanical diggers. They were  removed from the edge of the road near the concrete slabs. The top layer of sand and waste were also  edge of the road near the concrete slabs. The top layer of sand and waste were also excavated in the removed from the edge of the road near the concrete slabs. The top layer of sand and waste were also  excavated in the same way.  same way. excavated in the same way.  Appl. Sci. 2017, 7, 22 5 of 15 Appl. Sci. 2017, 7, 22  5 of 15  However, in this case, the process was stopped when a distance of 0.3 m from the geosynthetics  However, in this case, the process was stopped when a distance of 0.3 m from the geosynthetics was reached. Then, the excavation was continued manually using a shovel to avoid damaging the  was reached. Then, the excavation was continued manually using a shovel to avoid damaging geogrid.  The  sampling  location  is  presented  in  Figure 4.  After  the  sample  was  excavated,  it  was  the geogrid. The sampling location is presented in Figure 4. After the sample was excavated, it carefully raised and laid between two films of black PE and transported to the laboratory for further  was carefully raised and laid between two films of black PE and transported to the laboratory for testing.  further testing. Figure 4. Geogrid HDPE samples collected after 20 years of service. Figure 4. Geogrid HDPE samples collected after 20 years of service.  2.2. Materials  2.2. Materials The The main main adv advantage antage of of ge geogrids ogrids is is thei theirr high high tensil tensilee strength. strength. Th This is ty type pe of of re reinfor inforc cement ement solution solution  began to be implemented in the late 1970s. The geogrid production process begins with an extruded  began to be implemented in the late 1970s. The geogrid production process begins with an extruded sheet sheet of of po polyethylene, lyethylene, which which isis perfora perforated ted inin a re a g regular ular papattern. ttern. In c In ont contr rolled olled  heati heating ng condit conditions, ions, the  sheet is stretched to a randomly oriented long chain. The molecules are drawn in an ordered and  the sheet is stretched to a randomly oriented long chain. The molecules are drawn in an ordered and aligned aligned state. state. The The who whole le process process is is performed performed to to incr increase ease the the tensil tensilee strength strength an and d tensile tensile stif stiffness fness of of  the polymer [35]. The main properties of the geogrid used in the landfill site are presented in Table  the polymer [35]. The main properties of the geogrid used in the landfill site are presented in Table 1. 1.  Table 1. Engineering properties of the uniaxial geogrid from Radiowo. Table 1. Engineering properties of the uniaxial geogrid from Radiowo.  Geometry Geometry  Aperture size (mm  mm) 16  140 Aperture size (mm × mm)  16 × 140  Rib thickness (mm) 0.95 Rib thickness (mm)  0.95  CMD bar thickness (mm) 2.5  2.7 Rib width (mm) 6.7 CMD bar thickness (mm)  2.5 ÷ 2.7  CMD bar width (mm) 16 Rib width (mm)  6.7  Weight (g/m ) 500 CMD bar width (mm)  16  Mechanical Properties Weight (g/m )  500  Tensile strength at 2% strain (kN/m) 19.0 Mechanical Properties Tensile strength at 5% strain (kN/m) 33.5 Tensile strength at 2% strain (kN/m)  19.0  Peak tensile strength (kN/m) 55 Tensile strength at 5% strain (kN/m)  33.5  Yield point elongation (%) 11.2 Peak tensile strength (kN/m)  55  CMD—Cross-Machine Direction. Yield point elongation (%)  11.2  CMD—Cross‐Machine Direction.  2.3. Tensile Strength Tests 2.3. Tensile Strength Tests  When the reinforcement functions as geosynthetic material, the tensile strength and elongation at When the reinforcement functions as geosynthetic material, the tensile strength and elongation  maximum load are the main challenges for appropriate assessment of the product stability, since the at maximum load are the main challenges for appropriate assessment of the product stability, since  action of elevated or reduced temperature and humidity changes their properties. Tensile properties of the  action  of  elevated  or  reduced  temperature  and  humidity  changes  their  properties.  Tensile  virgin and 20-year-old geogrid samples were evaluated according to the EN ISO 10319 [36] testing properties of virgin and 20‐year‐old geogrid samples were evaluated according to the EN ISO 10319  [36]  testing  method  using  an  Instron  universal  testing  machine  (Figure  5).  For  each  test,  five  Appl. Sci. 2017, 7, 22 6 of 15 Appl. Sci. 2017, 7, 22  6 of 15  method using an Instron universal testing machine (Figure 5). For each test, five specimens were used. The monotonic tensile tests were performed with a strain rate equal to 20%/min, as recommended by specimens  were  used.  The  monotonic  tensile  tests  were  performed  with  a  strain  rate  equal  to  EN ISO 10319:2010. 20%/min, as recommended by EN ISO 10319:2010.  Figure 5. Laboratory equipment for tensile testing. Figure 5. Laboratory equipment for tensile testing.  2.4. Fourier Transform Infrared (FT-IR) Spectroscopy 2.4. Fourier Transform Infrared (FT‐IR) Spectroscopy   FT-IR allows for the determination of the type of functional groups present in the structure of FT‐IR allows for the determination of the type of functional groups present in the structure of  the test compound, which allows for the specification of the qualitative composition of the sample. the test compound, which allows for the specification of the qualitative composition of the sample.  The chemical structures of the virgin and aged geogrid samples were analyzed with an FT-IR The  chemical  structures  of  the  virgin  and  aged  geogrid  samples  were  analyzed  with  an  FT‐IR  spectrometer Perkin Elmer 2000 (Waltham, MA, USA) with Pike Gladiator (Madison, WI, USA) spectrometer  Perkin  Elmer  2000 (Waltham,  MA,  USA)  with  Pike  Gladiator  (Madison,  WI,  USA)  equipped with a KBr beamsplitter and DTGS (deuterated triglycine sulphate) detector, adapted for equipped with a KBr beamsplitter and DTGS (deuterated triglycine sulphate) detector, adapted for  −1 measurements in reflective mode over the absorption range of 400–4000 cm . The spectral resolution measurements in reflective mode over the absorption range of 400–4000 cm . The spectral resolution  −1 was 2 cm . Each spectrum was averaged from 32 scans. was 2 cm . Each spectrum was averaged from 32 scans.  2.5. Characteristics of Differential Scanning Calorimetry (DSC) 2.5. Characteristics of Differential Scanning Calorimetry (DSC)  The change in crystallinity during ageing was measured using DSC. DSC measurements were The change in crystallinity during ageing was measured using DSC. DSC measurements were  carried out using a differential scanning calorimeter (DSC, Q2000 TA Instruments, New Castle, carried out using a differential scanning calorimeter (DSC, Q2000 TA Instruments, New Castle, DE,  DE, USA) over a temperature range of 60–200 C and a heating rate of 10 C/min in nitrogen USA) over a temperature range of −60–200 °C and a heating rate of 10 °C/min in nitrogen atmosphere  atmosphere (50  5 mL/min). The mass of the sample taken for the tests was approximately 4 mg. (50 ± 5 mL/min). The mass of the sample taken for the tests was approximately 4 mg. The test sample  The test sample was placed into an open DSC aluminum pan. For optimum heat flux, the highest was placed into an open DSC aluminum pan. For optimum heat flux, the highest possible contact  possible contact area between the sample and the pan bottom should be achieved. area between the sample and the pan bottom should be achieved.  The DSC method allowed for the determination of the melting temperature and the enthalpy of The DSC method allowed for the determination of the melting temperature and the enthalpy of  fusion and for the comparison of the degree of crystallinity in individual samples. fusion and for the comparison of the degree of crystallinity in individual samples.  2.6. Electron Microscopy Analysis 2.6. Electron Microscopy Analysis  Scanning electron microscopy (SEM) uses a focused beam of high-energy electrons to generate Scanning electron microscopy (SEM) uses a focused beam of high‐energy electrons to generate  a variety of signals on the surface of solid specimens. The signals derived from the electron-sample a variety of signals on the surface of solid specimens. The signals derived from the electron‐sample  interactions reveal information about the sample, including its external morphology (texture) and  chemical  composition.  SEM  is  capable  of  performing  analyses  at  selected  point  locations  in  the  Appl. Sci. 2017, 7, 22 7 of 15 Appl. Sci. 2017, 7, 22  7 of 15  interactions reveal information about the sample, including its external morphology (texture) and chemical composition. SEM is capable of performing analyses at selected point locations in the sample. sample. This approach is especially useful in the qualitative or semi‐quantitative determination of  This approach is especially useful in the qualitative or semi-quantitative determination of the chemical the chemical composition (using a microprobe energy dispersive spectrometer, EDS).  composition (using a microprobe energy dispersive spectrometer, EDS). Scanning  electron  microscopy  (SEM)  enables  the  observation  of  the  topography  of  the  test  Scanning electron microscopy (SEM) enables the observation of the topography of the test material. material. Likewise, the influence of chemical and environmental factors on the surface of the test  Likewise, the influence of chemical and environmental factors on the surface of the test material can material  can  be  determined.  The  morphological  structure  of  the  samples  was  characterized  by  be determined. The morphological structure of the samples was characterized by scanning electron scanning electron microscopy (SEM Zeiss Ultra plus, Oberkochen, Germany). Before SEM analysis,  microscopy (SEM Zeiss Ultra plus, Oberkochen, Germany). Before SEM analysis, the samples were the samples were coated with carbon by a sputter coater (SCD005, BAL‐TEC, Balzers, Liechtenstein)  coated with carbon by a sputter coater (SCD005, BAL-TEC, Balzers, Liechtenstein) under vacuum. under  vacuum.  The  magnification  of  the  images ranged  from  500×  to 5000×,  captured  with  2  kV  The magnification of the images ranged from 500 to 5000, captured with 2 kV accelerating voltage accelerating voltage to investigate the surface.  to investigate the surface. To detect the presence of different elements in the top coating of the aged geogrids a scanning  To detect the presence of different elements in the top coating of the aged geogrids a scanning electron  microscope  (SEM  Zeiss  Ultra  plus)  with  an  EDS  probe  (Bruker  Quantax  400,  Berlin,  electron microscope (SEM Zeiss Ultra plus) with an EDS probe (Bruker Quantax 400, Berlin, Germany) Germany) was used. The magnification of the images ranged from 1000× to 5000×, captured with 15  was used. The magnification of the images ranged from 1000 to 5000, captured with 15 kV kV accelerating voltage to investigate the surface.  accelerating voltage to investigate the surface. 3. Discussion  3. Discussion 3.1. Tensile Strength Tests  3.1. Tensile Strength Tests Figure 6 and Table 2 show the results of the monotonic tensile tests performed with a strain rate  Figure 6 and Table 2 show the results of the monotonic tensile tests performed with a strain rate equal to 20%/min, according to EN ISO 10319:2010.  equal to 20%/min, according to EN ISO 10319:2010. The sample of the geogrid in a tensile test must be selected using the standard recommendation  The sample of the geogrid in a tensile test must be selected using the standard recommendation (samples that had mechanical damage due to the exhuming work were rejected). Figure 6 shows  (samples that had mechanical damage due to the exhuming work were rejected). Figure 6 shows graphs graphs and values for five samples taken from the geogrid after 20 years of service, and the average  and values for five samples taken from the geogrid after 20 years of service, and the average value for value for the five samples is 48.92 kN/m. Given that the geogrid is mostly exposed to mechanical  the five samples is 48.92 kN/m. Given that the geogrid is mostly exposed to mechanical factors during factors  during  embedding,  this  measured  value  of  tensile  strength  should  be  considered  to  be  a  embedding, this measured value of tensile strength should be considered to be a relatively high value, relatively high value, preserving nearly 90% of the initial value (Table 2).  preserving nearly 90% of the initial value (Table 2). Figure 6. Tensile strength of an aged geogrid sample.  Figure 6. Tensile strength of an aged geogrid sample. The  summary  of  the  results  obtained  for  samples  of  new  (virgin)  geogrid  and  for  a  sample  The summary of the results obtained for samples of new (virgin) geogrid and for a sample removed from the landfill 20 years after installation is presented in Table 2.   removed from the landfill 20 years after installation is presented in Table 2. Twenty years ago, when geogrid reinforcements were planned and installed at the Radiowo site,  Twenty years ago, when geogrid reinforcements were planned and installed at the Radiowo site, a fairly conservative estimation of tensile strength of 22 kN/m (for the temperature of 10 °C) was  a fairly conservative estimation of tensile strength of 22 kN/m (for the temperature of 10 C) was given. This value was diminished by a security factor, which at that time, was assumed to be equal  to 1.35. The reduction factor due to mechanical damage during installation for size fractions greater  Appl. Sci. 2017, 7, 22 8 of 15 given. This value was diminished by a security factor, which at that time, was assumed to be equal to Appl. Sci. 2017, 7, 22  8 of 15  1.35. The reduction factor due to mechanical damage during installation for size fractions greater than 75 mm was 1.75. Therefore, the safe design strength of the geogrid for the SR55 (symbol assigned by than 75 mm was 1.75. Therefore, the safe design strength of the geogrid for the SR55 (symbol assigned  the manufacturer) fraction above 75 mm was accepted to be 22/(1.75  1.35) = 9.31 kN/m. by the manufacturer) fraction above 75 mm was accepted to be 22/(1.75 × 1.35) = 9.31 kN/m.  Table 2. Results from a wide range of tests for virgin (new) and aged geogrid samples. Table 2. Results from a wide range of tests for virgin (new) and aged geogrid samples.  Sample before Installing (Virgin)   Samples 20 Years after Installation(Aged)  Sample before Installing (Virgin) (Declared Strength 55 kN/m)  Samples 20 Years after Installation (Aged) Sample Number  (Declared Strength 55 kN/m) Sample Number Mean Tensile  Mean Strain at  Mean Tensile  Mean Strain at  Strength (kN/m)  Maximum Load (%)  Strength (kN/m)  Maximum Load (%)  Mean Tensile Mean Strain at Mean Tensile Mean Strain at 1  Strength 60.7 (kN/m) 7  Maximum 9.55 Load   (%) Strength 52.18 (kN/m)   Maximum 7.41  Load (%) 2  61.68  9.80  46.74  5.69  1 60.77 9.55 52.18 7.41 3  60.79  9.32  50.48  6.68  2 61.68 9.80 46.74 5.69 4  61.45  10.07  52.55  6.92  3 60.79 9.32 50.48 6.68 5  60.28  9.66  42.63  5.28  4 61.45 10.07 52.55 6.92 Mean  60.99  9.68  48.92  6.40  5 60.28 9.66 42.63 5.28 Standard Deviation  0.57  0.28  4.20  0.89  Mean 60.99 9.68 48.92 6.40 Coefficient of Variation (%)  0.93  2.89  8.58  13.86  Standard Deviation 0.57 0.28 4.20 0.89 Coefficient of Variation (%) 0.93 2.89 8.58 13.86 3.2. FT‐IR Spectroscopy  3.2. FT-IR Spectroscopy It  is  generally  accepted  that  the  rate  of  chemical  reactions  in  solid  polymers  may  change  significantly under the influence of external or internal stresses. On one hand, the rate may change  It is generally accepted that the rate of chemical reactions in solid polymers may change due to substantial changes in the structural and physical parameters of a polymer subjected to the  significantly under the influence of external or internal stresses. On one hand, the rate may change due action of mechanical stresses (molecular conformation, free volume in the polymer, changes in the  to substantial changes in the structural and physical parameters of a polymer subjected to the action of permeability and diffusion of the low‐molecular mass substances). On the other hand, stresses may  mechanical stresses (molecular conformation, free volume in the polymer, changes in the permeability directly affect the reactivity of deformed macromolecules, thereby altering the effective activation  and diffusion of the low-molecular mass substances). On the other hand, stresses may directly affect the energies  for  chemical  reactions  [37].  Furthermore,  the  high  testing  temperatures  may  induce  reactivity of deformed macromolecules, thereby altering the effective activation energies for chemical morphological  changes  in  the  polymeric  product,  which  can  affect  the  kinetics  of  oxidative  reactions [37]. Furthermore, the high testing temperatures may induce morphological changes in the degradation,  leading  to  the  formation  of  hydroxyls,  carbonyl  and  carboxylic  groups,  ethers,  polymeric product, which can affect the kinetics of oxidative degradation, leading to the formation of peroxides  and  hydroperoxides.  FT‐IR  spectroscopy  was  used  to  identify  oxygen  bearing  groups,  hydroxyls, carbonyl and carboxylic groups, ethers, peroxides and hydroperoxides. FT-IR spectroscopy which may be formed during localized ageing processes in the immediate environment of slowly  wasgrowing used to identify cracks inoxygen  the HDPE bearing  undergr stat oups, ic lowhich ads (Figure may 7) be.  formed during localized ageing processes in the immediate environment of slowly growing cracks in the HDPE under static loads (Figure 7). Figure 7. Collected Fourier transform infrared (FT-IR) spectra of virgin geogrid samples and aged Figure 7. Collected Fourier transform infrared (FT‐IR) spectra of virgin geogrid samples and aged  geogrid samples (Radiowo). geogrid samples (Radiowo).  Appl. Sci. 2017, 7, 22 9 of 15 Appl. Sci. 2017, 7, 22  9 of 15  Figure 7 shows the FT-IR spectra of new (virgin) and aged HDPE geogrids. For both materials Figure 7 shows the FT‐IR spectra of new (virgin) and aged HDPE geogrids. For both materials  (geogrid “virgin” and geogrid “Radiowo”), three absorption bands (characteristic of polyethylene) (geogrid “virgin” and geogrid “Radiowo”), three absorption bands (characteristic of polyethylene)  related to the vibrational modes of the C-H bond can be observed, namely, C-H stretching related to the vibrational modes of the C‐H bond can be observed, namely, C‐H stretching (2950–2850  1 1 1 (2950–2850 cm ), C-H bending (1350–1450 cm ) and C-H rocking (near 700 cm ). Besides the −1 −1 −1 cm ), C‐H bending (1350–1450 cm ) and C‐H rocking (near 700 cm ). Besides the aforementioned C‐ aforementioned C-H absorption bands, the FT-IR spectrum of the HDPE geogrid “Radiowo” shows an H absorption bands, the FT‐IR spectrum of the HDPE geogrid “Radiowo” shows an absorption band  absorption band at approximately 1030 cm . −1 at approximately 1030 cm .  In Figure 8, the FT-IR spectra of the extended region 2750–3000 cm clearly show that there are −1 In Figure 8, the FT‐IR spectra of the extended region 2750–3000 cm  clearly show that there are  no visible differences in the virgin and aged geogrid samples. no visible differences in the virgin and aged geogrid samples.  Figure 8. Collected FT-IR spectra (2750–3000 cm −1 region) of virgin geogrid samples and aged geogrid Figure 8. Collected FT‐IR spectra (2750–3000 cm  region) of virgin geogrid samples and aged geogrid  samples (Radiowo). samples (Radiowo).  −1 In Figure 9a shows the spectra in the region of 800–1350 cm  for the aged geogrid samples. Since  In Figure 9a shows the spectra in the region of 800–1350 cm for the aged geogrid samples. these Since spectr these spectra a were co wer llecte e collected d in reflective in reflective  mode,mode,  the topmost the topmost  spectrum spectr  show umsshows  the compos the composition ition of the  topmost layer of an aged geogrid sample. Subsequent spectra were collected downward from the  of the topmost layer of an aged geogrid sample. Subsequent spectra were collected downward from topmost the topmost  layelayers rs of the of the stud studied ied geogrids, geogrids,  which which  were wer reemove removed d syste systematically matically witwith h a cutti a cutting ng knife knife. . A  −1 decrease  in  the  intensity  of  the  band  can  be  observed  at  approximately  1050  cm ,  which  can  be  A decrease in the intensity of the band can be observed at approximately 1050 cm , which can be a attributed ttributed to to th thee Si Si-O ‐O bond bond present present in in a a typical typical fi fine ne sand sand (Figur (Figuree 99b b). ). These These spect spectra ra su suggest ggest the the sl slow ow  penetration of fine sand particles into the geogrid polymer structure. After removal of several layers,  penetration of fine sand particles into the geogrid polymer structure. After removal of several layers, −1 1 a small absorption band at approximately 1150 cm  can still be observed, which can be attributed to  a small absorption band at approximately 1150 cm can still be observed, which can be attributed to silica added as a filler to the HDPE matrix.  silica added as a filler to the HDPE matrix. Appl. Sci. 2017, 7, 22 10 of 15 Appl. Sci. 2017, 7, 22  10 of 15  (a)  (b)  Figure 9. (a) Corrected spectra for aged geogrids samples and (b) corrected spectra for “sample sand”  Figure 9. (a) Corrected spectra for aged geogrids samples and (b) corrected spectra for “sample sand” (silicates).  (silicates). Absorption bands originating from the oxidation of the HDPE matrix were not observed. Peaks  Absorption bands originating from the oxidation of the HDPE matrix were not observed. Peaks −1 from the OH groups (approximately 3300–3500 cm ) and strong absorptions from the carbonyl C =  from the OH groups (approximately 3300–3500 cm ) and strong absorptions from the carbonyl C = O −1 O bonds (1700–1750 cm  region) were not detected. This observation confirms the strong resistance  bonds (1700–1750 cm region) were not detected. This observation confirms the strong resistance of of the HDPE matrix against oxidation, and its high chemical resistance under the service conditions.  the HDPE matrix against oxidation, and its high chemical resistance under the service conditions. 3.3. Differential Scanning Calorimetry Results (DSC)  3.3. Differential Scanning Calorimetry Results (DSC) Crystallinity  influences  physical  and  mechanical  properties  such  as  yield  stress,  modulus  of  Crystallinity influences physical and mechanical properties such as yield stress, modulus of elasticity, impact resistance, density, permeability and melting point [38,39]. The melting point (Tm)  elasticity, impact resistance, density, permeability and melting point [38,39]. The melting point (T ) and the melting enthalpy (ΔH) were measured, and the percentages of crystallinity were determined  using the enthalpy of melting for polyethylene at 100% crystallinity, ΔH0 = 294 J/g [40]. The values  Appl. Sci. 2017, 7, 22 11 of 15 and the melting enthalpy (DH) were measured, and the percentages of crystallinity were determined using the enthalpy of melting for polyethylene at 100% crystallinity, DH = 294 J/g [40]. The values obtained Appl. Sci. 2017 from , 7, 22 the  DSC tests are collected in Table 3. This increase in crystallinity may be attributed 11 of 15  to the process of physical aging, in which the geogrids attempt to establish an equilibrium from its obtained from the DSC tests are collected in Table 3. This increase in crystallinity may be attributed  as-manufactured non-equilibrium state [38]. The results reveal that the degree of crystallinity for the to the process of physical aging, in which the geogrids attempt to establish an equilibrium from its  aged samples is higher than the crystallinity of the new (virgin) samples. as‐manufactured non‐equilibrium state [38]. The results reveal that the degree of crystallinity for the  aged samples is higher than the crystallinity of the new (virgin) samples.  Table 3. Differential scanning calorimetry (DSC) results for new (virgin-V) geogrid samples and aged (Radiowo-R) geogrid samples. DH: melting enthalpy; T : melting point; W : degree of crystallinity. Table 3. Differential scanning calorimetry (DSC) results for new (virgin‐V) geogrid samples and aged  (Radiowo‐R) geogrid samples. ΔH: melting enthalpy; Tm: melting point; Wk: degree of crystallinity.  Sample DH (J/g) T ( C) DH (J/g) W (%) m o Sample ΔH (J/g) Tm (°C)  ΔHo (J/g) Wk (%)  HDPE 153.3 131.15 293 52 V1 HDPE 154.1 130.13 293 52 HDPEV1  153.3  131.15  293  52  V2 HDPE 178.8 129.77 293 60 R1 HDPEV2  154.1  130.13  293  52  HDPE 184.1 130.22 293 62 R2 HDPER1  178.8  129.77  293  60  HDPE 174.7 129.70 293 59 R3 HDPER2  184.1  130.22  293  62  HDPE 185.0 129.60 293 62 R4 HDPER3  174.7  129.70  293  59  HDPER4  185.0  129.60  293  62  3.4. Electron Microscopy Analysis 3.4. Electron Microscopy Analysis  3.4.1. SEM-EDS Observations of Geogrids 3.4.1. SEM‐EDS Observations of Geogrids  SEM analyses were performed with an attached EDS probe to detect the presence of different elements in the top coating of the aged geogrids. EDS spectra were taken from selected (rectangular) SEM analyses were performed with an attached EDS probe to detect the presence of different  areas of the samples. elements in the top coating of the aged geogrids. EDS spectra were taken from selected (rectangular)  Figure 10 reveals the presence of mainly SiO (sand) and Al O (bauxite) and a minor contribution areas of the samples.  2 2 3 of other elements, such as iron, potassium, magnesium, sodium, calcium, and barium. These elements Figure  10  reveals  the  presence  of  mainly  SiO2  (sand)  and  Al2O3  (bauxite)  and  a  minor  occur in the form of oxides, chlorides and sulfates. Table 4 presents the composition of elements in the contribution of other elements, such as iron, potassium, magnesium, sodium, calcium, and barium.  coating These elements layer of aged  occurgeogrids  in the form (% of w /oxid w). (a)  Figure 10. Cont. Appl. Sci. 2017, 7, 22 12 of 15 Appl. Sci. 2017, 7, 22  12 of 15  (b)  (c)  Figure 10. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis  Figure 10. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis for for the coatings in the geogrid samples 1 (a), 2 (b), 3 (c).  the coatings in the geogrid samples 1 (a), 2 (b), 3 (c). Table 4. Percentage of elements in the coating layer of the geogrids.  Table 4. Percentage of elements in the coating layer of the geogrids. Characteristic Elemental  Elemental  Elemental  Elements  Percentage (%) (a)  Percentage (%) (b)  Percentage (%) (c)  Characteristic Elemental Elemental Elemental Elements Percentage (%) (a) Percentage (%) (b) Percentage (%) (c) Ba ‐  50.31   O  3.69  28.01  44.37  Ba - 50.31 Na  38.10  1.27  1.92  O 3.69 28.01 44.37 Mg Na  0.32 38.10   1.08 1.27   5.12 1.92  Mg 0.32 1.08 5.12 Al  1.31  1.57  8.77  Al 1.31 1.57 8.77 Si  3.13  2.97  18.40  Si 3.13 2.97 18.40 P ‐  0.07  0.29  P - 0.07 0.29 S  0.31  11.68  2.44  S 0.31 11.68 2.44 Cl  51.55  0.41  2.80  Cl 51.55 0.41 2.80 K  0.46  0.43  2.91  K 0.46 0.43 2.91 Ca  0.27  0.64  2.17  Ca 0.27 0.64 2.17 Ti ‐  ‐  1.04  Ti - - 1.04 Mn Mn ‐  - ‐  - 0.27 0.27  Fe 0.86 1.49 6.60 Fe  0.86  1.49  6.60  Cu - - 1.11 Cu ‐  ‐  1.11  Zn - - 1.78 Cd - 0.06 - TOTAL 100 100 100 Appl. Sci. 2017, 7, 22  13 of 15  Zn ‐  ‐  1.78  Cd ‐  0.06 ‐  Appl. Sci. 2017, 7, 22 13 of 15 TOTAL  100  100  100  3.4.2. 3.4.2. SEM SEM Ob Observations servations  Figur Figure e 11 11 presents presents SEM SEM ima image ges sof of vivir rgigin n and and age aged d geogr geogrid id sam samples. ples. SEM SEM  image images s of new of  new and  and agedaged  HDPE HDPE  sampl samples es did no did t reveal not  rsi eveal gnifica significant nt changes changes  in the su inrfa the ce an surface alysis analysis of the HDPE of the poHDPE lymer.  polymer These  observ . These ations observations   additionally additionally   confirm confirm   the  hithe gh  high resistance resistance   of  thi ofsthis   mamaterial terial  aft after er  20 20  ye years ars  of of  continuous continuous service. service.  (a)  (b) (c)  (d) Figure  11.  (a,b)  SEM  images  of  a  “virgin”  geogrid  sample  and  (c,d)  an  aged  “Radiowo”  geogrid  Figure 11. (a,b) SEM images of a “virgin” geogrid sample and (c,d) an aged “Radiowo” geogrid sample. sample.   4. Conclusions 4. Conclusions  Generally, HDPE geogrids analyzed after 20 years of continuous service in a municipal waste Generally, HDPE geogrids analyzed after 20 years of continuous service in a municipal waste  landfill display only minor changes compared to the virgin material. More detailed changes in the landfill display only minor changes compared to the virgin material. More detailed changes in the  mechanical and physicochemical properties are as follows. mechanical and physicochemical properties are as follows.  There is no significant deterioration of the geogrid mechanical parameters. The mechanical There  is  no  significant  deterioration  of  the  geogrid  mechanical  parameters.  The  mechanical  strength of the geogrid samples after 20 years of service decreased by approximately 10%–20% strength  of  the  geogrid  samples  after  20  years  of  service  decreased  by  approximately  10%–20%  compared to the virgin geogrid samples. compared to the virgin geogrid samples.  In the FT-IR spectra of the topmost layer of the aged samples, there are no significant changes In the FT‐IR spectra of the topmost layer of the aged samples, there are no significant changes  compared to the topmost layer of the virgin samples. This indicates the strong chemical resistance of compared to the topmost layer of the virgin samples. This indicates the strong chemical resistance of  the HDPE material, which is able to withstand environmental conditions during at least 20 years of service in a landfill. The DSC results indicate that a slow crystallization processes takes place within the aged HDPE geogrid samples. An increase in the degree of crystallinity for the aged samples can be observed. Appl. Sci. 2017, 7, 22 14 of 15 Elemental analysis of the coating on PE geogrids retrieved from the Radiowo landfill after 20 years of service indicates that salts of sodium, calcium, aluminum, potassium, iron, copper, zinc, and barium, mainly as oxides, sulfates and chlorides, are deposited on the surface of the material. SEM images of samples taken from the landfill show the influence of mechanical interactions on the surface of the geogrids, while there were no significant changes related to the impact of environmental and chemical factors. Author Contributions: A.K., E.K. and W.F. conceived and designed the experiments; A.K. and J.K. performed the experiments; E.K., A.K. and W.F. analyzed the data; A.K. and J.K. contributed reagents/materials/analysis tools; A.K. and E.K. wrote the paper. 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Geomembr. 2009, 27, 368–390. [CrossRef] 13. Geosynthetics—Guidelines for the Assessment of Durability; International Organization for Standardization: Geneva, Switzerland, 2008; ISO/TS 13434:2008. 14. Recommendations for Design and Analysis of Earth Structures Using Geosynthetic Reinforcements; Ernst & Sohn Verlag, EBGEO: Berlin, Germany, 2011. 15. Code of Practice for Strengthened/Reinforced Soils and Other Fills; British Standards Institution: London, UK, 2010; BS8006-1:2010. 16. Geogrids for Reinforced Soil Retaining Wall and Bridge Abutment System; British Board Agreement Technical Approval: Blackburn, UK, 2010; BBA 99/R109. 17. Hufenus, R.; Rüegger, R.; Flum, D.; Sterba, I.J. Strength reduction factors due to installation damage of reinforcing geosynthetics. Geotext. Geomembr. 2005, 23, 55–75. [CrossRef] 18. Hsuan, Y.G.; Koerner, R.M. Antioxidant depletion lifetime in high density polyethylene geomembranes. J. Geotech. Geoenviron. Eng. ASCE 1998, 124, 532–541. [CrossRef] 19. Rowe, R.K.; Sangam, H. Durability of HDPE geomembranes. Geotext. Geomembr. 2002, 20, 77–95. [CrossRef] 20. Carneiro, J.R.; Almeida, P.J.; Lopes, M.L. Resistance of high-density polyethylene geonets against chemical ageing. In Proceedings of the 6th International Congress on Environmental Geotechnics, New Delhi, India, 7–12 November 2010. Appl. Sci. 2017, 7, 22 15 of 15 21. Rowe, K.; Islam, M.Z.; Hsuan, Y.G. Leachate chemical composition effects on OIT depletion in an HDPE geomembrane. Geosinthetics Int. 2008, 15, 2. [CrossRef] 22. Kay, D.; Blond, E.; Mlynarek, J. Geosynthetics durability: A polymer chemistry. In Proceedings of the 57th Canadian Geotechnical Conference, Quebec City, QC, Canada, 24–26 October 2004. 23. Petermann, J.; Miles, M.; Gleiter, H. Growth of polymer crystals during annealing. J. Macromol. Sci. Phys. 1976, 12, 393–404. [CrossRef] 24. Mathur, A.; Netravali, A.N.; O Rourke, T.D. Chemical aging effect on the physico–mechanical properties of polyester and polypropylene geotextiles. Geotext. Geomembr. 1994, 13, 591–626. [CrossRef] 25. Koerner, R.M.; Hsuan, Y.D.; Lord, A.E. Remaining Technical Barriers to Obtain General Acceptance of Geosynthetics. In ASCE Geotechnical Special Publication; The American Society of Civil Engineers: New York, NY, USA, 1986. 26. Elias, V.; Kenneth, P.E.; Fishman, L.; Christopher, B.R.; Berg, R.R. Corrosion/Degradation of Soil Reinforcements for Mechanically Stabilized Earth Walls and Reinforced Soil Slopes; U.S. Department of Transportation Federal Highway Administration: Woodbury, MN, USA, 2009; Publication No. FHWA-NHI-09-087. 27. Leflaive, E. Durability of geotextiles: The French experience. Geotext. Geomembr. 1988, 7, 23–31. [CrossRef] 28. Pouig, J.; Blivet, J.C.; Pasquet, P. Earth reinforced fill with synthetic fabric. In Proceedings of the International Conference on use of fabrics in Geotechniques, Paris, France, 20–22 April 1977. 29. Bell, J.R.; Steward, J.E. Construction and observations of fabric soil walls. In Proceedings of the International Conference on use of fabrics in Geotechniques, Paris, France, 20–22 April 1977. 30. Bell, J.R.; Szymoniak, T.; Thommen, G.R. Construction of a steep sided geogrid retaining wall for an Oregon Coastal Highway. In Proceedings of the Polymer Grid Reinforcement Conference, London, UK, 22–23 March 1984. 31. Koda, E. Stability conditions improvement of the old sanitary landfills. In Proceedings of the 3th Internatonal Congress on Environment Geotechnics, Lisboa, Portugal, 7–11 September 1998. 32. Koda, E.; Osinski, ´ P. Application of alternative methods of slope stability improvements on landfills. In Proceedings of the XVI European Conference on Soil Mechanics and Geotechnical Engineering “Geotechnical Engineering for Infrastructure and Development”, Edinburgh, UK, 13–17 September 2015. 33. Koda, E.; Szymanski, ´ A.; Wolski, W. Field and laboratory experience with the use of strip drains in organic soils. Can. Geotech. J. 1993, 30, 308–318. [CrossRef] 34. Koda, E. Anthropogenic waste products utilization for old landfills rehabilitation. Ann. Wars. Univ. Life Sci. Land Reclam. 2012, 44, 75–88. [CrossRef] 35. Tensar. The Long-Term Performance of Tensar Geogrids; Tensar/Nelton LTD: Blackburn, UK, 1990. 36. Geosynthetics—Wide-Width Tensile Test; International Organization for Standardization: Geneva, Switzerland, 2010; ISO 10319:2010. 37. Pinter, G.; Haager, M.; Wolf, C.; Lang, R.W. Thermo-oxidative degradation during creep crack growth of PE-HD grades as assessed by FT-IR spectroscopy. Macromol. Symp. 2004, 217, 307–316. [CrossRef] 38. Rowe, R.K.; Rimal, S.; Sangam, H. Ageing of HDPE geomembrane exposed to air, water and leachate at different temperatures. Geotext. Geomembr. 2009, 27, 137–151. [CrossRef] 39. Kong, Y.; Hay, J.N. The measurement of the crystallinity of polymers by DSC. Polymer 2002, 43, 3873–3878. [CrossRef] 40. Wunderlich, B. Reversible crystallization and the rigid–amorphous phase in semicrystalline macromolecules. Prog. Polym. Sci. 2003, 28, 3383–3450. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Effect of the Impact of Chemical and Environmental Factors on the Durability of the High Density Polyethylene (HDPE) Geogrid in a Sanitary Landfill

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applied sciences Article Effect of the Impact of Chemical and Environmental Factors on the Durability of the High Density Polyethylene (HDPE) Geogrid in a Sanitary Landfill 1 , 1 2 3 Agnieszka Kiersnowska *, Eugeniusz Koda , Wojciech Fabianowski and Jacek Kawalec Department of Geotechnical Engineering, Faculty of Civil and Environmental Engineering, Warsaw University of Life Sciences, 02-787 Warsaw, Poland; eugeniusz_koda@sggw.pl Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland; wofab@ch.pw.edu.pl Faculty of Civil Engineering, Silesian University of Technology, Akademicka 5, 44-100 Gliwice, Poland; jacek.kawalec@vp.pl * Correspondence: agnieszka_kiersnowska@sggw.pl; Tel.: +48-22-59-35-226; Fax: +48-22-59-35-203 Academic Editor: Patrick A. Fairclough Received: 12 October 2016; Accepted: 15 December 2016; Published: 23 December 2016 Abstract: A high density polyethylene (HDPE) uniaxial geogrid was exhumed after twenty years of service in a sanitary landfill, and its properties were examined. A geogrid installed in a landfill is exposed to mechanical and chemical factors (e.g., a wide pH range and high temperatures), as well as different weather conditions. This paper presents the results of physical and mechanical analyses of virgin and aged HDPE geogrid samples. Structural changes observed by differential scanning calorimetry and Fourier transform infrared spectroscopy (FT-IR) spectroscopy correlate with the mechanical properties of the aged geogrid. The mechanical properties were found to have changed only slightly. In the FT-IR spectrum of the topmost layer of the aged geogrid samples, no significant changes were observed compared to the spectrum of the top layer of the virgin samples. This indicates the strong chemical resistance of the HDPE material, which is able to withstand environmental conditions for at least 20 years of service in a landfill. Keywords: durability; geogrids; polyethylene; reinforcement 1. Introduction Geogrids are widely used as reinforcements in slopes, walls, roads, and foundations where they are subjected to constant stress throughout their service life [1–3]. Uniaxial high density polyethylene (HDPE) geogrids are designed to be used in geotechnical structures where soil particles need support over long time periods. HDPE geogrids, due to their high strength and durability, are commonly used for the construction of steep slopes, where the rigid nodes of the geogrids are used to wedge soil in the mesh of the geogrids. Grain aggregates or soil particles pass through the geogrid mesh, partly clogging the spaces between the ribs. The strength and stiffness of the ribs prevent the displacement of soils on the sides but may lead to mechanical damage of the material [3–5]. The grid interaction with soil is a complex phenomenon and depends on several factors, such as soil type and density, grid geometry and mechanical properties, surface roughness, stress levels, and boundary and loading conditions. Geogrids embedded in soil both during the construction phase and during service life, used as reinforcements, can be exposed to different load conditions. The applied tensile loads on the geosynthetics can be permanent loads (dead weight of soil), repetitive loads (traffic loads), and loads applied during construction [6]. Appl. Sci. 2017, 7, 22; doi:10.3390/app7010022 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 22 2 of 15 Viscosity is recognized as a significant property of polymeric materials. The loading rate effects, creep deformation, and stress relaxation of polymeric geosynthetics are inherent responses to their viscous properties. The viscous properties of polymeric geosynthetics have been investigated by tensile tests with respect to the loading rate, where the deformation characteristics of polymeric geosynthetic reinforcements are more or less viscous, and the peak strength decreases noticeably with a decrease in the strain rate at failure [7,8]. These features have been studied by various experimental and theoretical methods [6–12]. However, these materials are exposed not only to mechanical effects but also to influences of the environment in which they are used, as well as the ageing process. Current standards claim up to 120 ears of design life for the structures; therefore, geogrids as components of such structures must also fulfil this age criterion [13–15]. The properties of geosynthetic material, including geogrids, generally depend on time. The decrease in the allowable tensile strength depends on short-term effects, such as installation damage, which reduces the maximum tensile strength but does not further affect long-term properties, as well as on effects such as creep and ageing by oxidation and abrasion, which result in a loss in long-term strength [16]. The service life of a structure with geosynthetic material largely depends on its durability over time. The ageing process of geosynthetic materials can be envisioned as the simultaneous combination of physical and chemical ageing [17–20]. As a result of ageing or degradation, several detrimental effects occur in the polymer: loss of additives and plasticizers, change in molecular weight, formation of free radicals and brittleness [21]. Physical ageing is related to degradation, which does not involve modifications in the molecular structure of the polymer chains. Some of these mechanisms involve mass transfer with the environment surrounding the material (extraction of additive, absorption of solvent, etc.). Others involve modifications of the organization of the internal chain in the material, i.e., change in morphology (chain orientation, crystallinity, etc.) [22]. In physical aging, the material attempts to establish an equilibrium from its as-manufactured non-equilibrium state. As a consequence, no primary (covalent) bonds are broken, and for semi-crystalline polymers such as HDPE, an increase in the material crystallinity occurs [19,23]. The mechanism of chemical ageing leads to changes in the molecular structure of the polymer chains [14]. This process eventually results in a decline in the mechanical properties and consequently damage to the material. Chemical attack can be launched directly in acidic and alkaline soils or indirectly by active waste, which is present in landfills. Depending on the chemical compound, a change in the structure of the polymer can be obtained by oxidation, chain scission, cross-linking, swelling or dissolution of the polymers. Furthermore, the effect of chemical degradation can be accelerated by temperature [24]. The predominant mechanism of degradation for most polymeric materials (geosynthetics) is chain cutting that occurs as a result of reactions of the polymer, which leads to the breakage of bonds in the backbone of the polymer chain, reduction in the chain length, and thus reduction in the molecular weight [25]. This phenomenon significantly alters the properties of polymeric materials, such as strength and elongation, and consequently affects the geotechnical structures. The oxidation process is initiated by heat (from temperature or UV (ultraviolet) radiation), mechanical stress, catalyst residues derived from the production of the geosynthetics, or reaction with impurities [26]. High density polyethylene (HDPE) geogrids are very resistant to chemical substances and do not easily deteriorate when exposed to alkaline and acid agents (except oxidizing acids), salt solutions, or microbes, because they are non-polar in nature [22]. However, in general there is more than one degradation mechanism operating at a given time, and synergistic effects (changes in pH, changes in temperature and changes in mechanical stress) can accelerate degradation [21]. Geosynthetics made from high density polyethylene have a potential for stress cracking, which is material failure caused by tensile stresses lower than the short-term mechanical strength. Appl. Sci. 2017, 7, 22 3 of 15 Appl. Sci. 2017, 7, 22  3 of 15  Cracking is usually formed in areas or regions with concentrated stress in the microstructure of local inhomogeneities. This phenomenon consists of two phases: crack initiation and crack growth. Environmental stress cracking (ESC) is the bursting of the polymer under tension when exposed to a  Environmental stress cracking (ESC) is the bursting of the polymer under tension when exposed to a chemical environment [26]. This failure mode can reduce the life of PE (polyethylene) used for critical  chemical environment [26]. This failure mode can reduce the life of PE (polyethylene) used for critical applications such as reinforcement applications in a landfill.  applications such as reinforcement applications in a landfill. The first reinforced soil structures in the world were built in France in 1970 and 1971 [27,28]. In  The first reinforced soil structures in the world were built in France in 1970 and 1971 [27,28]. subsequent years, the technique began to be used throughout the world; for example, in the United  In subsequent years, the technique began to be used throughout the world; for example, in the United States, where structures of this type have been built since 1974 [29]. The first permanent geogrid  States, where structures of this type have been built since 1974 [29]. The first permanent geogrid reinforced structure constructed in the U.S. was built in 1982 to support roadway access to the Devils  reinforced structure constructed in the U.S. was built in 1982 to support roadway access to the Devils Punch Bowl State Park along the central Oregon coast [30].  Punch Bowl State Park along the central Oregon coast [30]. This paper focuses on the durability of geogrids used for the reinforcement of a slope in an old  This paper focuses on the durability of geogrids used for the reinforcement of a slope in an old sanitary landfill. The analyzed geogrid was installed more than 20 years ago in an old sanitary landfill  sanitary landfill. The analyzed geogrid was installed more than 20 years ago in an old sanitary landfill located in Warsaw, Poland. The geogrid was excavated in October 2013 and then tested by several  located in Warsaw, Poland. The geogrid was excavated in October 2013 and then tested by several methods. The geosynthetics were installed in the landfill and exposed to mechanical and chemical  methods. The geosynthetics were installed in the landfill and exposed to mechanical and chemical factors (e.g., mechanical stress by load and trucks, a wide range of pH values and high temperatures)  factors (e.g., mechanical stress by load and trucks, a wide range of pH values and high temperatures) and were also subjected to changing weather conditions.  and were also subjected to changing weather conditions. 2. Materials and Methods  2. Materials and Methods 2.1. 2.1. Location Location an and d Descri Description ption of of th thee Study Study Site Site  The The Radiowo Radiowo land landfill fill covers covers an an are areaa of of appr approxima oximately tely 16 16 hectar hectares es at at an an altitude altitude exceeding exceeding 60 60 m m  above above  gr groou und nd  level. level.  The The  la landfill ndfill  is is  loc located ated  al along ong  the the  north north-western ‐western  border border  of of  Wa Warsaw rsaw  in in  Poland Poland  (Figur (Figure e 11) )..   Figure 1. View of the Radiowo landfill where the northern and western slopes were reinforced by a  Figure 1. View of the Radiowo landfill where the northern and western slopes were reinforced by a high density polyethylene (HDPE) geogrid in 1993.  high density polyethylene (HDPE) geogrid in 1993. From 1961 to 1991, mainly municipal waste was deposited there, and since 1992, it has become  From 1961 to 1991, mainly municipal waste was deposited there, and since 1992, it has become a structure that receives ballast waste from a composting plant. The ballast waste was composed of  a structure that receives ballast waste from a composting plant. The ballast waste was composed of 5% to 12% organic fraction (>10 mm) and approximately 4% mineral waste (>10 mm). The physical  5% to 12% organic fraction (>10 mm) and approximately 4% mineral waste (>10 mm). The physical characteristics of the ballast waste are as follows: the average density in non‐compacted waste is 0.15– characteristics of the ballast waste are as follows: the average density in non-compacted waste is 3 3 0.70 Mg/m ; the average density of compacted waste is 0.8–1.4 Mg/m ; and the waste humidity is  3 3 0.15–0.70 Mg/m ; the average density of compacted waste is 0.8–1.4 Mg/m ; and the waste humidity 15.5% to 28% [31].  is 15.5% to 28% [31]. For high and steep slopes, the key issue was to improve their stability. To this end, a number of  engineering procedures were required: comprehensive investigation of the mechanical properties of  the  waste  using  different  techniques,  mechanical  reinforcements  of  the  slopes,  changing  the  Appl. Sci. 2017, 7, 22 4 of 15 Appl. Sci. 2017, 7, 22  4 of 15  For high and steep slopes, the key issue was to improve their stability. To this end, a number of inclination of the slopes, determining the type of waste, provision of land next to the landfill and  engineering procedures were required: comprehensive investigation of the mechanical properties of clarifying the formal status of the landfill for further development [32–34].  the waste using different techniques, mechanical reinforcements of the slopes, changing the inclination Appl. Sci. 2017, 7, 22  4 of 15  Reclamation works in the landfill began in 1993 (Figure 2). They included safety improvements  of the slopes, determining the type of waste, provision of land next to the landfill and clarifying the in the geotechnical formations of the landfill body. To improve the conditions of the northern slope  inclination of the slopes, determining the type of waste, provision of land next to the landfill and  formal status of the landfill for further development [32–34]. stability  and  to  construct  the  main  access  road  to  the  landfill,  a  retaining  wall  was  constructed.  clarifying the formal status of the landfill for further development [32–34].  Reclamation works in the landfill began in 1993 (Figure 2). They included safety improvements Further  carving  of  the  slope  and  installation  of  a  horizontal  uniaxial  HDPE  geogrid  were  also  Reclamation works in the landfill began in 1993 (Figure 2). They included safety improvements  in the geotechnical formations of the landfill body. To improve the conditions of the northern slope proposed (Figure 3).  in the geotechnical formations of the landfill body. To improve the conditions of the northern slope  stability and to construct the main access road to the landfill, a retaining wall was constructed. The reason for such heavy modifications was that space was very limited on the northern slope  stability  and  to  construct  the  main  access  road  to  the  landfill,  a  retaining  wall  was  constructed.  Further carving of the slope and installation of a horizontal uniaxial HDPE geogrid were also proposed (land ownership issues).  Further  carving  of  the  slope  and  installation  of  a  horizontal  uniaxial  HDPE  geogrid  were  also  (Figure 3). proposed (Figure 3).  The reason for such heavy modifications was that space was very limited on the northern slope The reason for such heavy modifications was that space was very limited on the northern slope  (land (l ownership and ownership issues).  issues).  Figure 2. The process of geogrid installation in the landfill (1993).  Figure 2. The process of geogrid installation in the landfill (1993).  Figure 2. The process of geogrid installation in the landfill (1993). Figure 3. Cross-section and reinforcements in the northern slope of the landfill [31]. Figure 3. Cross‐section and reinforcements in the northern slope of the landfill [31].  Figure 3. Cross‐section and reinforcements in the northern slope of the landfill [31].  In November 2013, three samples of geogrids were collected from the landfill after 20 years of In November 2013, three samples of geogrids were collected from the landfill after 20 years of  In November 2013, three samples of geogrids were collected from the landfill after 20 years of  service (sample size: length approximately 1.20 m, width approximately 1.0 m). The samples were service (sample size: length approximately 1.20 m, width approximately 1.0 m). The samples were  service (sample size: length approximately 1.20 m, width approximately 1.0 m). The samples were  excavated from the first layer of the structure (Figure 3) located along the access road to the landfill. excavated from the first layer of the structure (Figure 3) located along the access road to the landfill.  excavated from the first layer of the structure (Figure 3) located along the access road to the landfill.  In this particular location, the HDPE geogrid was exposed not only to chemical and environmental In  this  particular  location,  the  HDPE  geogrid  was  exposed  not  only  to  chemical  and  In  this  particular  location,  the  HDPE  geogrid  was  exposed  not  only  to  chemical  and  impacts environmental but also to impact mechanical s but also loads  to m caused echanic by al lo the ads slope  caused itself  byand  the loading slope itsel off the andincoming  loading of tr the ucks  environmental impacts but also to mechanical loads caused by the slope itself and loading of the  filled incoming with waste.  trucks The  filled samples  with waste. were extracted The sample using s were mechanical  extracted us diggers. ing mecha They nical wer digger e remov s. They ed fr om were the  incoming trucks filled with waste. The samples were extracted using mechanical diggers. They were  removed from the edge of the road near the concrete slabs. The top layer of sand and waste were also  edge of the road near the concrete slabs. The top layer of sand and waste were also excavated in the removed from the edge of the road near the concrete slabs. The top layer of sand and waste were also  excavated in the same way.  same way. excavated in the same way.  Appl. Sci. 2017, 7, 22 5 of 15 Appl. Sci. 2017, 7, 22  5 of 15  However, in this case, the process was stopped when a distance of 0.3 m from the geosynthetics  However, in this case, the process was stopped when a distance of 0.3 m from the geosynthetics was reached. Then, the excavation was continued manually using a shovel to avoid damaging the  was reached. Then, the excavation was continued manually using a shovel to avoid damaging geogrid.  The  sampling  location  is  presented  in  Figure 4.  After  the  sample  was  excavated,  it  was  the geogrid. The sampling location is presented in Figure 4. After the sample was excavated, it carefully raised and laid between two films of black PE and transported to the laboratory for further  was carefully raised and laid between two films of black PE and transported to the laboratory for testing.  further testing. Figure 4. Geogrid HDPE samples collected after 20 years of service. Figure 4. Geogrid HDPE samples collected after 20 years of service.  2.2. Materials  2.2. Materials The The main main adv advantage antage of of ge geogrids ogrids is is thei theirr high high tensil tensilee strength. strength. Th This is ty type pe of of re reinfor inforc cement ement solution solution  began to be implemented in the late 1970s. The geogrid production process begins with an extruded  began to be implemented in the late 1970s. The geogrid production process begins with an extruded sheet sheet of of po polyethylene, lyethylene, which which isis perfora perforated ted inin a re a g regular ular papattern. ttern. In c In ont contr rolled olled  heati heating ng condit conditions, ions, the  sheet is stretched to a randomly oriented long chain. The molecules are drawn in an ordered and  the sheet is stretched to a randomly oriented long chain. The molecules are drawn in an ordered and aligned aligned state. state. The The who whole le process process is is performed performed to to incr increase ease the the tensil tensilee strength strength an and d tensile tensile stif stiffness fness of of  the polymer [35]. The main properties of the geogrid used in the landfill site are presented in Table  the polymer [35]. The main properties of the geogrid used in the landfill site are presented in Table 1. 1.  Table 1. Engineering properties of the uniaxial geogrid from Radiowo. Table 1. Engineering properties of the uniaxial geogrid from Radiowo.  Geometry Geometry  Aperture size (mm  mm) 16  140 Aperture size (mm × mm)  16 × 140  Rib thickness (mm) 0.95 Rib thickness (mm)  0.95  CMD bar thickness (mm) 2.5  2.7 Rib width (mm) 6.7 CMD bar thickness (mm)  2.5 ÷ 2.7  CMD bar width (mm) 16 Rib width (mm)  6.7  Weight (g/m ) 500 CMD bar width (mm)  16  Mechanical Properties Weight (g/m )  500  Tensile strength at 2% strain (kN/m) 19.0 Mechanical Properties Tensile strength at 5% strain (kN/m) 33.5 Tensile strength at 2% strain (kN/m)  19.0  Peak tensile strength (kN/m) 55 Tensile strength at 5% strain (kN/m)  33.5  Yield point elongation (%) 11.2 Peak tensile strength (kN/m)  55  CMD—Cross-Machine Direction. Yield point elongation (%)  11.2  CMD—Cross‐Machine Direction.  2.3. Tensile Strength Tests 2.3. Tensile Strength Tests  When the reinforcement functions as geosynthetic material, the tensile strength and elongation at When the reinforcement functions as geosynthetic material, the tensile strength and elongation  maximum load are the main challenges for appropriate assessment of the product stability, since the at maximum load are the main challenges for appropriate assessment of the product stability, since  action of elevated or reduced temperature and humidity changes their properties. Tensile properties of the  action  of  elevated  or  reduced  temperature  and  humidity  changes  their  properties.  Tensile  virgin and 20-year-old geogrid samples were evaluated according to the EN ISO 10319 [36] testing properties of virgin and 20‐year‐old geogrid samples were evaluated according to the EN ISO 10319  [36]  testing  method  using  an  Instron  universal  testing  machine  (Figure  5).  For  each  test,  five  Appl. Sci. 2017, 7, 22 6 of 15 Appl. Sci. 2017, 7, 22  6 of 15  method using an Instron universal testing machine (Figure 5). For each test, five specimens were used. The monotonic tensile tests were performed with a strain rate equal to 20%/min, as recommended by specimens  were  used.  The  monotonic  tensile  tests  were  performed  with  a  strain  rate  equal  to  EN ISO 10319:2010. 20%/min, as recommended by EN ISO 10319:2010.  Figure 5. Laboratory equipment for tensile testing. Figure 5. Laboratory equipment for tensile testing.  2.4. Fourier Transform Infrared (FT-IR) Spectroscopy 2.4. Fourier Transform Infrared (FT‐IR) Spectroscopy   FT-IR allows for the determination of the type of functional groups present in the structure of FT‐IR allows for the determination of the type of functional groups present in the structure of  the test compound, which allows for the specification of the qualitative composition of the sample. the test compound, which allows for the specification of the qualitative composition of the sample.  The chemical structures of the virgin and aged geogrid samples were analyzed with an FT-IR The  chemical  structures  of  the  virgin  and  aged  geogrid  samples  were  analyzed  with  an  FT‐IR  spectrometer Perkin Elmer 2000 (Waltham, MA, USA) with Pike Gladiator (Madison, WI, USA) spectrometer  Perkin  Elmer  2000 (Waltham,  MA,  USA)  with  Pike  Gladiator  (Madison,  WI,  USA)  equipped with a KBr beamsplitter and DTGS (deuterated triglycine sulphate) detector, adapted for equipped with a KBr beamsplitter and DTGS (deuterated triglycine sulphate) detector, adapted for  −1 measurements in reflective mode over the absorption range of 400–4000 cm . The spectral resolution measurements in reflective mode over the absorption range of 400–4000 cm . The spectral resolution  −1 was 2 cm . Each spectrum was averaged from 32 scans. was 2 cm . Each spectrum was averaged from 32 scans.  2.5. Characteristics of Differential Scanning Calorimetry (DSC) 2.5. Characteristics of Differential Scanning Calorimetry (DSC)  The change in crystallinity during ageing was measured using DSC. DSC measurements were The change in crystallinity during ageing was measured using DSC. DSC measurements were  carried out using a differential scanning calorimeter (DSC, Q2000 TA Instruments, New Castle, carried out using a differential scanning calorimeter (DSC, Q2000 TA Instruments, New Castle, DE,  DE, USA) over a temperature range of 60–200 C and a heating rate of 10 C/min in nitrogen USA) over a temperature range of −60–200 °C and a heating rate of 10 °C/min in nitrogen atmosphere  atmosphere (50  5 mL/min). The mass of the sample taken for the tests was approximately 4 mg. (50 ± 5 mL/min). The mass of the sample taken for the tests was approximately 4 mg. The test sample  The test sample was placed into an open DSC aluminum pan. For optimum heat flux, the highest was placed into an open DSC aluminum pan. For optimum heat flux, the highest possible contact  possible contact area between the sample and the pan bottom should be achieved. area between the sample and the pan bottom should be achieved.  The DSC method allowed for the determination of the melting temperature and the enthalpy of The DSC method allowed for the determination of the melting temperature and the enthalpy of  fusion and for the comparison of the degree of crystallinity in individual samples. fusion and for the comparison of the degree of crystallinity in individual samples.  2.6. Electron Microscopy Analysis 2.6. Electron Microscopy Analysis  Scanning electron microscopy (SEM) uses a focused beam of high-energy electrons to generate Scanning electron microscopy (SEM) uses a focused beam of high‐energy electrons to generate  a variety of signals on the surface of solid specimens. The signals derived from the electron-sample a variety of signals on the surface of solid specimens. The signals derived from the electron‐sample  interactions reveal information about the sample, including its external morphology (texture) and  chemical  composition.  SEM  is  capable  of  performing  analyses  at  selected  point  locations  in  the  Appl. Sci. 2017, 7, 22 7 of 15 Appl. Sci. 2017, 7, 22  7 of 15  interactions reveal information about the sample, including its external morphology (texture) and chemical composition. SEM is capable of performing analyses at selected point locations in the sample. sample. This approach is especially useful in the qualitative or semi‐quantitative determination of  This approach is especially useful in the qualitative or semi-quantitative determination of the chemical the chemical composition (using a microprobe energy dispersive spectrometer, EDS).  composition (using a microprobe energy dispersive spectrometer, EDS). Scanning  electron  microscopy  (SEM)  enables  the  observation  of  the  topography  of  the  test  Scanning electron microscopy (SEM) enables the observation of the topography of the test material. material. Likewise, the influence of chemical and environmental factors on the surface of the test  Likewise, the influence of chemical and environmental factors on the surface of the test material can material  can  be  determined.  The  morphological  structure  of  the  samples  was  characterized  by  be determined. The morphological structure of the samples was characterized by scanning electron scanning electron microscopy (SEM Zeiss Ultra plus, Oberkochen, Germany). Before SEM analysis,  microscopy (SEM Zeiss Ultra plus, Oberkochen, Germany). Before SEM analysis, the samples were the samples were coated with carbon by a sputter coater (SCD005, BAL‐TEC, Balzers, Liechtenstein)  coated with carbon by a sputter coater (SCD005, BAL-TEC, Balzers, Liechtenstein) under vacuum. under  vacuum.  The  magnification  of  the  images ranged  from  500×  to 5000×,  captured  with  2  kV  The magnification of the images ranged from 500 to 5000, captured with 2 kV accelerating voltage accelerating voltage to investigate the surface.  to investigate the surface. To detect the presence of different elements in the top coating of the aged geogrids a scanning  To detect the presence of different elements in the top coating of the aged geogrids a scanning electron  microscope  (SEM  Zeiss  Ultra  plus)  with  an  EDS  probe  (Bruker  Quantax  400,  Berlin,  electron microscope (SEM Zeiss Ultra plus) with an EDS probe (Bruker Quantax 400, Berlin, Germany) Germany) was used. The magnification of the images ranged from 1000× to 5000×, captured with 15  was used. The magnification of the images ranged from 1000 to 5000, captured with 15 kV kV accelerating voltage to investigate the surface.  accelerating voltage to investigate the surface. 3. Discussion  3. Discussion 3.1. Tensile Strength Tests  3.1. Tensile Strength Tests Figure 6 and Table 2 show the results of the monotonic tensile tests performed with a strain rate  Figure 6 and Table 2 show the results of the monotonic tensile tests performed with a strain rate equal to 20%/min, according to EN ISO 10319:2010.  equal to 20%/min, according to EN ISO 10319:2010. The sample of the geogrid in a tensile test must be selected using the standard recommendation  The sample of the geogrid in a tensile test must be selected using the standard recommendation (samples that had mechanical damage due to the exhuming work were rejected). Figure 6 shows  (samples that had mechanical damage due to the exhuming work were rejected). Figure 6 shows graphs graphs and values for five samples taken from the geogrid after 20 years of service, and the average  and values for five samples taken from the geogrid after 20 years of service, and the average value for value for the five samples is 48.92 kN/m. Given that the geogrid is mostly exposed to mechanical  the five samples is 48.92 kN/m. Given that the geogrid is mostly exposed to mechanical factors during factors  during  embedding,  this  measured  value  of  tensile  strength  should  be  considered  to  be  a  embedding, this measured value of tensile strength should be considered to be a relatively high value, relatively high value, preserving nearly 90% of the initial value (Table 2).  preserving nearly 90% of the initial value (Table 2). Figure 6. Tensile strength of an aged geogrid sample.  Figure 6. Tensile strength of an aged geogrid sample. The  summary  of  the  results  obtained  for  samples  of  new  (virgin)  geogrid  and  for  a  sample  The summary of the results obtained for samples of new (virgin) geogrid and for a sample removed from the landfill 20 years after installation is presented in Table 2.   removed from the landfill 20 years after installation is presented in Table 2. Twenty years ago, when geogrid reinforcements were planned and installed at the Radiowo site,  Twenty years ago, when geogrid reinforcements were planned and installed at the Radiowo site, a fairly conservative estimation of tensile strength of 22 kN/m (for the temperature of 10 °C) was  a fairly conservative estimation of tensile strength of 22 kN/m (for the temperature of 10 C) was given. This value was diminished by a security factor, which at that time, was assumed to be equal  to 1.35. The reduction factor due to mechanical damage during installation for size fractions greater  Appl. Sci. 2017, 7, 22 8 of 15 given. This value was diminished by a security factor, which at that time, was assumed to be equal to Appl. Sci. 2017, 7, 22  8 of 15  1.35. The reduction factor due to mechanical damage during installation for size fractions greater than 75 mm was 1.75. Therefore, the safe design strength of the geogrid for the SR55 (symbol assigned by than 75 mm was 1.75. Therefore, the safe design strength of the geogrid for the SR55 (symbol assigned  the manufacturer) fraction above 75 mm was accepted to be 22/(1.75  1.35) = 9.31 kN/m. by the manufacturer) fraction above 75 mm was accepted to be 22/(1.75 × 1.35) = 9.31 kN/m.  Table 2. Results from a wide range of tests for virgin (new) and aged geogrid samples. Table 2. Results from a wide range of tests for virgin (new) and aged geogrid samples.  Sample before Installing (Virgin)   Samples 20 Years after Installation(Aged)  Sample before Installing (Virgin) (Declared Strength 55 kN/m)  Samples 20 Years after Installation (Aged) Sample Number  (Declared Strength 55 kN/m) Sample Number Mean Tensile  Mean Strain at  Mean Tensile  Mean Strain at  Strength (kN/m)  Maximum Load (%)  Strength (kN/m)  Maximum Load (%)  Mean Tensile Mean Strain at Mean Tensile Mean Strain at 1  Strength 60.7 (kN/m) 7  Maximum 9.55 Load   (%) Strength 52.18 (kN/m)   Maximum 7.41  Load (%) 2  61.68  9.80  46.74  5.69  1 60.77 9.55 52.18 7.41 3  60.79  9.32  50.48  6.68  2 61.68 9.80 46.74 5.69 4  61.45  10.07  52.55  6.92  3 60.79 9.32 50.48 6.68 5  60.28  9.66  42.63  5.28  4 61.45 10.07 52.55 6.92 Mean  60.99  9.68  48.92  6.40  5 60.28 9.66 42.63 5.28 Standard Deviation  0.57  0.28  4.20  0.89  Mean 60.99 9.68 48.92 6.40 Coefficient of Variation (%)  0.93  2.89  8.58  13.86  Standard Deviation 0.57 0.28 4.20 0.89 Coefficient of Variation (%) 0.93 2.89 8.58 13.86 3.2. FT‐IR Spectroscopy  3.2. FT-IR Spectroscopy It  is  generally  accepted  that  the  rate  of  chemical  reactions  in  solid  polymers  may  change  significantly under the influence of external or internal stresses. On one hand, the rate may change  It is generally accepted that the rate of chemical reactions in solid polymers may change due to substantial changes in the structural and physical parameters of a polymer subjected to the  significantly under the influence of external or internal stresses. On one hand, the rate may change due action of mechanical stresses (molecular conformation, free volume in the polymer, changes in the  to substantial changes in the structural and physical parameters of a polymer subjected to the action of permeability and diffusion of the low‐molecular mass substances). On the other hand, stresses may  mechanical stresses (molecular conformation, free volume in the polymer, changes in the permeability directly affect the reactivity of deformed macromolecules, thereby altering the effective activation  and diffusion of the low-molecular mass substances). On the other hand, stresses may directly affect the energies  for  chemical  reactions  [37].  Furthermore,  the  high  testing  temperatures  may  induce  reactivity of deformed macromolecules, thereby altering the effective activation energies for chemical morphological  changes  in  the  polymeric  product,  which  can  affect  the  kinetics  of  oxidative  reactions [37]. Furthermore, the high testing temperatures may induce morphological changes in the degradation,  leading  to  the  formation  of  hydroxyls,  carbonyl  and  carboxylic  groups,  ethers,  polymeric product, which can affect the kinetics of oxidative degradation, leading to the formation of peroxides  and  hydroperoxides.  FT‐IR  spectroscopy  was  used  to  identify  oxygen  bearing  groups,  hydroxyls, carbonyl and carboxylic groups, ethers, peroxides and hydroperoxides. FT-IR spectroscopy which may be formed during localized ageing processes in the immediate environment of slowly  wasgrowing used to identify cracks inoxygen  the HDPE bearing  undergr stat oups, ic lowhich ads (Figure may 7) be.  formed during localized ageing processes in the immediate environment of slowly growing cracks in the HDPE under static loads (Figure 7). Figure 7. Collected Fourier transform infrared (FT-IR) spectra of virgin geogrid samples and aged Figure 7. Collected Fourier transform infrared (FT‐IR) spectra of virgin geogrid samples and aged  geogrid samples (Radiowo). geogrid samples (Radiowo).  Appl. Sci. 2017, 7, 22 9 of 15 Appl. Sci. 2017, 7, 22  9 of 15  Figure 7 shows the FT-IR spectra of new (virgin) and aged HDPE geogrids. For both materials Figure 7 shows the FT‐IR spectra of new (virgin) and aged HDPE geogrids. For both materials  (geogrid “virgin” and geogrid “Radiowo”), three absorption bands (characteristic of polyethylene) (geogrid “virgin” and geogrid “Radiowo”), three absorption bands (characteristic of polyethylene)  related to the vibrational modes of the C-H bond can be observed, namely, C-H stretching related to the vibrational modes of the C‐H bond can be observed, namely, C‐H stretching (2950–2850  1 1 1 (2950–2850 cm ), C-H bending (1350–1450 cm ) and C-H rocking (near 700 cm ). Besides the −1 −1 −1 cm ), C‐H bending (1350–1450 cm ) and C‐H rocking (near 700 cm ). Besides the aforementioned C‐ aforementioned C-H absorption bands, the FT-IR spectrum of the HDPE geogrid “Radiowo” shows an H absorption bands, the FT‐IR spectrum of the HDPE geogrid “Radiowo” shows an absorption band  absorption band at approximately 1030 cm . −1 at approximately 1030 cm .  In Figure 8, the FT-IR spectra of the extended region 2750–3000 cm clearly show that there are −1 In Figure 8, the FT‐IR spectra of the extended region 2750–3000 cm  clearly show that there are  no visible differences in the virgin and aged geogrid samples. no visible differences in the virgin and aged geogrid samples.  Figure 8. Collected FT-IR spectra (2750–3000 cm −1 region) of virgin geogrid samples and aged geogrid Figure 8. Collected FT‐IR spectra (2750–3000 cm  region) of virgin geogrid samples and aged geogrid  samples (Radiowo). samples (Radiowo).  −1 In Figure 9a shows the spectra in the region of 800–1350 cm  for the aged geogrid samples. Since  In Figure 9a shows the spectra in the region of 800–1350 cm for the aged geogrid samples. these Since spectr these spectra a were co wer llecte e collected d in reflective in reflective  mode,mode,  the topmost the topmost  spectrum spectr  show umsshows  the compos the composition ition of the  topmost layer of an aged geogrid sample. Subsequent spectra were collected downward from the  of the topmost layer of an aged geogrid sample. Subsequent spectra were collected downward from topmost the topmost  layelayers rs of the of the stud studied ied geogrids, geogrids,  which which  were wer reemove removed d syste systematically matically witwith h a cutti a cutting ng knife knife. . A  −1 decrease  in  the  intensity  of  the  band  can  be  observed  at  approximately  1050  cm ,  which  can  be  A decrease in the intensity of the band can be observed at approximately 1050 cm , which can be a attributed ttributed to to th thee Si Si-O ‐O bond bond present present in in a a typical typical fi fine ne sand sand (Figur (Figuree 99b b). ). These These spect spectra ra su suggest ggest the the sl slow ow  penetration of fine sand particles into the geogrid polymer structure. After removal of several layers,  penetration of fine sand particles into the geogrid polymer structure. After removal of several layers, −1 1 a small absorption band at approximately 1150 cm  can still be observed, which can be attributed to  a small absorption band at approximately 1150 cm can still be observed, which can be attributed to silica added as a filler to the HDPE matrix.  silica added as a filler to the HDPE matrix. Appl. Sci. 2017, 7, 22 10 of 15 Appl. Sci. 2017, 7, 22  10 of 15  (a)  (b)  Figure 9. (a) Corrected spectra for aged geogrids samples and (b) corrected spectra for “sample sand”  Figure 9. (a) Corrected spectra for aged geogrids samples and (b) corrected spectra for “sample sand” (silicates).  (silicates). Absorption bands originating from the oxidation of the HDPE matrix were not observed. Peaks  Absorption bands originating from the oxidation of the HDPE matrix were not observed. Peaks −1 from the OH groups (approximately 3300–3500 cm ) and strong absorptions from the carbonyl C =  from the OH groups (approximately 3300–3500 cm ) and strong absorptions from the carbonyl C = O −1 O bonds (1700–1750 cm  region) were not detected. This observation confirms the strong resistance  bonds (1700–1750 cm region) were not detected. This observation confirms the strong resistance of of the HDPE matrix against oxidation, and its high chemical resistance under the service conditions.  the HDPE matrix against oxidation, and its high chemical resistance under the service conditions. 3.3. Differential Scanning Calorimetry Results (DSC)  3.3. Differential Scanning Calorimetry Results (DSC) Crystallinity  influences  physical  and  mechanical  properties  such  as  yield  stress,  modulus  of  Crystallinity influences physical and mechanical properties such as yield stress, modulus of elasticity, impact resistance, density, permeability and melting point [38,39]. The melting point (Tm)  elasticity, impact resistance, density, permeability and melting point [38,39]. The melting point (T ) and the melting enthalpy (ΔH) were measured, and the percentages of crystallinity were determined  using the enthalpy of melting for polyethylene at 100% crystallinity, ΔH0 = 294 J/g [40]. The values  Appl. Sci. 2017, 7, 22 11 of 15 and the melting enthalpy (DH) were measured, and the percentages of crystallinity were determined using the enthalpy of melting for polyethylene at 100% crystallinity, DH = 294 J/g [40]. The values obtained Appl. Sci. 2017 from , 7, 22 the  DSC tests are collected in Table 3. This increase in crystallinity may be attributed 11 of 15  to the process of physical aging, in which the geogrids attempt to establish an equilibrium from its obtained from the DSC tests are collected in Table 3. This increase in crystallinity may be attributed  as-manufactured non-equilibrium state [38]. The results reveal that the degree of crystallinity for the to the process of physical aging, in which the geogrids attempt to establish an equilibrium from its  aged samples is higher than the crystallinity of the new (virgin) samples. as‐manufactured non‐equilibrium state [38]. The results reveal that the degree of crystallinity for the  aged samples is higher than the crystallinity of the new (virgin) samples.  Table 3. Differential scanning calorimetry (DSC) results for new (virgin-V) geogrid samples and aged (Radiowo-R) geogrid samples. DH: melting enthalpy; T : melting point; W : degree of crystallinity. Table 3. Differential scanning calorimetry (DSC) results for new (virgin‐V) geogrid samples and aged  (Radiowo‐R) geogrid samples. ΔH: melting enthalpy; Tm: melting point; Wk: degree of crystallinity.  Sample DH (J/g) T ( C) DH (J/g) W (%) m o Sample ΔH (J/g) Tm (°C)  ΔHo (J/g) Wk (%)  HDPE 153.3 131.15 293 52 V1 HDPE 154.1 130.13 293 52 HDPEV1  153.3  131.15  293  52  V2 HDPE 178.8 129.77 293 60 R1 HDPEV2  154.1  130.13  293  52  HDPE 184.1 130.22 293 62 R2 HDPER1  178.8  129.77  293  60  HDPE 174.7 129.70 293 59 R3 HDPER2  184.1  130.22  293  62  HDPE 185.0 129.60 293 62 R4 HDPER3  174.7  129.70  293  59  HDPER4  185.0  129.60  293  62  3.4. Electron Microscopy Analysis 3.4. Electron Microscopy Analysis  3.4.1. SEM-EDS Observations of Geogrids 3.4.1. SEM‐EDS Observations of Geogrids  SEM analyses were performed with an attached EDS probe to detect the presence of different elements in the top coating of the aged geogrids. EDS spectra were taken from selected (rectangular) SEM analyses were performed with an attached EDS probe to detect the presence of different  areas of the samples. elements in the top coating of the aged geogrids. EDS spectra were taken from selected (rectangular)  Figure 10 reveals the presence of mainly SiO (sand) and Al O (bauxite) and a minor contribution areas of the samples.  2 2 3 of other elements, such as iron, potassium, magnesium, sodium, calcium, and barium. These elements Figure  10  reveals  the  presence  of  mainly  SiO2  (sand)  and  Al2O3  (bauxite)  and  a  minor  occur in the form of oxides, chlorides and sulfates. Table 4 presents the composition of elements in the contribution of other elements, such as iron, potassium, magnesium, sodium, calcium, and barium.  coating These elements layer of aged  occurgeogrids  in the form (% of w /oxid w). (a)  Figure 10. Cont. Appl. Sci. 2017, 7, 22 12 of 15 Appl. Sci. 2017, 7, 22  12 of 15  (b)  (c)  Figure 10. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis  Figure 10. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis for for the coatings in the geogrid samples 1 (a), 2 (b), 3 (c).  the coatings in the geogrid samples 1 (a), 2 (b), 3 (c). Table 4. Percentage of elements in the coating layer of the geogrids.  Table 4. Percentage of elements in the coating layer of the geogrids. Characteristic Elemental  Elemental  Elemental  Elements  Percentage (%) (a)  Percentage (%) (b)  Percentage (%) (c)  Characteristic Elemental Elemental Elemental Elements Percentage (%) (a) Percentage (%) (b) Percentage (%) (c) Ba ‐  50.31   O  3.69  28.01  44.37  Ba - 50.31 Na  38.10  1.27  1.92  O 3.69 28.01 44.37 Mg Na  0.32 38.10   1.08 1.27   5.12 1.92  Mg 0.32 1.08 5.12 Al  1.31  1.57  8.77  Al 1.31 1.57 8.77 Si  3.13  2.97  18.40  Si 3.13 2.97 18.40 P ‐  0.07  0.29  P - 0.07 0.29 S  0.31  11.68  2.44  S 0.31 11.68 2.44 Cl  51.55  0.41  2.80  Cl 51.55 0.41 2.80 K  0.46  0.43  2.91  K 0.46 0.43 2.91 Ca  0.27  0.64  2.17  Ca 0.27 0.64 2.17 Ti ‐  ‐  1.04  Ti - - 1.04 Mn Mn ‐  - ‐  - 0.27 0.27  Fe 0.86 1.49 6.60 Fe  0.86  1.49  6.60  Cu - - 1.11 Cu ‐  ‐  1.11  Zn - - 1.78 Cd - 0.06 - TOTAL 100 100 100 Appl. Sci. 2017, 7, 22  13 of 15  Zn ‐  ‐  1.78  Cd ‐  0.06 ‐  Appl. Sci. 2017, 7, 22 13 of 15 TOTAL  100  100  100  3.4.2. 3.4.2. SEM SEM Ob Observations servations  Figur Figure e 11 11 presents presents SEM SEM ima image ges sof of vivir rgigin n and and age aged d geogr geogrid id sam samples. ples. SEM SEM  image images s of new of  new and  and agedaged  HDPE HDPE  sampl samples es did no did t reveal not  rsi eveal gnifica significant nt changes changes  in the su inrfa the ce an surface alysis analysis of the HDPE of the poHDPE lymer.  polymer These  observ . These ations observations   additionally additionally   confirm confirm   the  hithe gh  high resistance resistance   of  thi ofsthis   mamaterial terial  aft after er  20 20  ye years ars  of of  continuous continuous service. service.  (a)  (b) (c)  (d) Figure  11.  (a,b)  SEM  images  of  a  “virgin”  geogrid  sample  and  (c,d)  an  aged  “Radiowo”  geogrid  Figure 11. (a,b) SEM images of a “virgin” geogrid sample and (c,d) an aged “Radiowo” geogrid sample. sample.   4. Conclusions 4. Conclusions  Generally, HDPE geogrids analyzed after 20 years of continuous service in a municipal waste Generally, HDPE geogrids analyzed after 20 years of continuous service in a municipal waste  landfill display only minor changes compared to the virgin material. More detailed changes in the landfill display only minor changes compared to the virgin material. More detailed changes in the  mechanical and physicochemical properties are as follows. mechanical and physicochemical properties are as follows.  There is no significant deterioration of the geogrid mechanical parameters. The mechanical There  is  no  significant  deterioration  of  the  geogrid  mechanical  parameters.  The  mechanical  strength of the geogrid samples after 20 years of service decreased by approximately 10%–20% strength  of  the  geogrid  samples  after  20  years  of  service  decreased  by  approximately  10%–20%  compared to the virgin geogrid samples. compared to the virgin geogrid samples.  In the FT-IR spectra of the topmost layer of the aged samples, there are no significant changes In the FT‐IR spectra of the topmost layer of the aged samples, there are no significant changes  compared to the topmost layer of the virgin samples. This indicates the strong chemical resistance of compared to the topmost layer of the virgin samples. This indicates the strong chemical resistance of  the HDPE material, which is able to withstand environmental conditions during at least 20 years of service in a landfill. The DSC results indicate that a slow crystallization processes takes place within the aged HDPE geogrid samples. An increase in the degree of crystallinity for the aged samples can be observed. Appl. Sci. 2017, 7, 22 14 of 15 Elemental analysis of the coating on PE geogrids retrieved from the Radiowo landfill after 20 years of service indicates that salts of sodium, calcium, aluminum, potassium, iron, copper, zinc, and barium, mainly as oxides, sulfates and chlorides, are deposited on the surface of the material. SEM images of samples taken from the landfill show the influence of mechanical interactions on the surface of the geogrids, while there were no significant changes related to the impact of environmental and chemical factors. Author Contributions: A.K., E.K. and W.F. conceived and designed the experiments; A.K. and J.K. performed the experiments; E.K., A.K. and W.F. analyzed the data; A.K. and J.K. contributed reagents/materials/analysis tools; A.K. and E.K. wrote the paper. 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Published: Dec 23, 2016

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