Determination of Thickness of Smooth Geomembranes

Tests were conducted to determine thickness of smooth, nonreinforced geomembranes using three methods: mechanical (according to ASTM and European standards), ultrasonic, and magnetic methods. The mechanical method is the standard procedure used for determining thickness of geomembranes. The ultrasonic and magnetic methods are not commonly used for geomembranes; however, they are used for testing other materials such as metals. Tests were conducted on 15 geomembranes representing five types of polymers (HDPE, LLDPE, PVC, PP, and EPDM). The results of the testing program indicated that the level of pressures applied affected the thickness measurements in mechanical tests. While the low pressures were not sufficient to flatten particularly the rigid geomembranes, the high pressures tended to compress the geomembranes excessively. Both high and low pressures prevented obtaining representative measurements. The measurements obtained using the ASTM method were more reliable than the measurements obtained with the European method, although it is believed that the most reliable measurements can be obtained by the nondestructive methods (ultrasonic and magnetic). These techniques are sensitive only to the thickness of the materials due to the inherent properties of the test procedures, and they work equally well for rigid and flexible geomembranes. Of the two nondestructive methods, ultrasonic testing is better due to several advantages: it allows for testing from the top surface of geomembranes in the laboratory or in the field, and it can be used on coupons of geomembranes as well as on whole sheets without the need for removing test samples. Both nondestructive methods can be improved for application to geomembranes. Thickness is a basic property of geomembranes that is used for general identification and classification of these materials (Koerner 1997). Thickness is used in all phases of production and lifetime of geomembranes including manufacturing, design, and postfailure forensic analysis. Manufacturing quality control procedures include determination and verification of thickness (Daniel and Koerner 1995). Thickness measurements are needed to calculate the numerical values of properties, such as tensile strength. Thickness measurements are also used in the evaluation of degradation and endurance properties of geomembranes. In addition, properties of geomembranes including mechanical properties and resistance to transmission of fluids are affected by thickness (Giroud et al. 1994; Park et al. 1995). Minimum required thicknesses are included in specifications for all applications of geomembranes. This study was conducted to evaluate the effectiveness of existing mechanical methods and newly adapted ultrasonic and magnetic methods to determine the thickness of smooth, nonreinforced geomembranes. Reproducibility and repeatability of measurements obtained using different methods were determined. Comparisons were made between the thicknesses determined using the various methods, as well as thicknesses measured at various pressures using mechanical methods. The methods were rated using several parameters, and a comparison was provided which included comments about practical use of the thickness measurement setups and test procedures. Recommendations for determination of thickness are provided based on the results of the study. Thickness Measurement Techniques Mechanical Thickness Measurements Mechanical thickness measurements consist of determination of the thickness of a geomembrane under a specific pressure. Geomembranes are placed horizontally in a thickness gage over a flat surface and a load is applied through a loading tip placed on the geomembrane. The magnitude of the applied load and the dimensions of the loading tip are set to induce a specific pressure on the geomembrane. Generally, the load is applied using a dead-weight loading mechanism. A schematic depiction of thickness measurement using a mechanical gage is presented in Fig. 1. Mechanical thickness measurements are described in ASTM D5199—Standard Test Method for Measuring Nominal Thickness of Geotextiles and Geomembranes. The ASTM requirement is a pressure of 20 kPa applied through a circular loading tip with a diameter of 6.35 mm. A high pressure in the range of 50 kPa to 200 kPa is recommended to be used for HDPE geomembranes to overcome the rigidity of the material and obtain representative measurements. Tests are conducted on samples with a minimum dimension of 75 mm in diameter. Guidelines for mechanical thickness measurements are also provided in European standards (EN 964-1). In this case, pressures of 2 kPa, 20 kPa, and 200 kPa are applied using a circular tip with a diameter of 56.41 mm. This arrangement requires significantly higher loads compared to the ASTM standard procedure, due to the increased loading tip area. Tests are conducted on samples with a minimum dimension of 1.75 times the diameter of the loading tip. Determination of thickness of smooth geomembranes is usually simple and fast with mechanical thickness gages. The mechanical thickness gages are generally built to conduct tests on precut samples of geomembranes with relatively small dimensions (minimum 75-mm diameter for ASTM D5199, minimum 98-mm diameter for EN 964-1). Although larger gages that can accommodate sample sizes up to a few hundred millimeters can be constructed, the designs become impractical for larger sample sizes. Mechanical thickness gages are generally bench-scale devices with a relatively smooth, horizontal surface required for the placement of the instrument. The mobility of these devices (for in-situ measurements) can be limited particularly when high loads to induce high pressures are required. Ultrasonic Thickness Measurements Determination of thickness is one of the most common applications of ultrasonic testing (McIntire 1991). Ultrasonic thickness gages are available that can be used for determination of thicknesses from fractions of a millimeter to more than a meter. Portable ultrasonic testing equipment is available that allows for testing in situ in addition to bench-scale devices. Thickness measurements are used for quality control and quality assurance purposes, as well as for monitoring the quality of materials during or after use. Ultrasonic testing refers to mechanical wave propagation analyses conducted at frequencies higher than the audible sound range (>20 kHz). Generally, transmission of compression waves (primary or Pwaves) is used for thickness measurements. The thickness is determined as the multiplication of the Pwave propagation velocity in the test material and the travel time of the P-wave through the height (thickness) of the sample. The ultrasonic pulse-echo method is used for testing materials such as metals, composites, and plastics. A schematic depiction of thickness measurement using the pulse-echo method is presented in Fig. 2. This method requires access to only one surface of the test material. Waves are sent and received by a single transducer placed on one surface of the test material. Tests can be conducted on precut samples of materials or at locations on rolls or sheets of materials without cutting and removing samples. The P-wave velocity in the test sample needs to be known to determine the thickness of the material. Published values are available for common materials including air, water, liquids, metals, and plastics. The velocity can be predetermined for materials tested less commonly (such as geomembranes) using preliminary ultrasonic measurements. An ultrasonic test method to evaluate the condition of geomembranes was reported by Yesiller and Sungur (2001). The test method was partially based on ultrasonic thickness measurements that were used to detect damage and degradation in the geomembranes. This method was developed and tested in the laboratory. An application of ultrasonic testing to evaluate thickness of a geomembrane in the field was reported by Steffen and Asmus (1993). Problems were encountered during the installation of a polyethylene geomembrane in a waste disposal facility in Germany. The geomembrane had expanded and contracted due to temperature variations and was overstretched at locations near the anchor trenches. Local reductions in the thickness of the geomembrane were detected using ultrasonic tests. While details were not provided in the paper, it is believed that the pulse-echo inspection technique was used for thickness measurements. Velocity of wave transmission in the geomembrane was known, and this was used together with wave travel times obtained during testing to determine the thicknesses. Magnetic Thickness Measurements Determination of thickness is also a common application of magnetic testing. Test equipment is available for measuring thicknesses of both ferromagnetic and nonferromagnetic materials. Similar to ultrasonic applications, the thickness measurements are used for quality control and monitoring purposes. Magnetic thickness measurements can be conducted using a variety of devices. Emphasis is placed on Hall Effect devices in this paper, since a magnetic thickness gage that operates on this principle is used for this study. The Hall Effect is a physical phenomenon that occurs in a material carrying an electric current subjected to a magnetic field. A potential difference is generated when the material carrying the electric current is placed in a magnetic field acting in the perpendicular direction. The potential difference occurs in a direction perpendicular to the directions of both the electrical current and the magnetic field (Bray and McBride 1992). A schematic depiction of thickness measurement using the magnetic method is presented in Fig. 3. The thickness is determined using the voltage difference generated by the presence of the test material placed between the probe and the target (small metal object). The presence of the test material affects the magnetic flux density generated between the magnet in the probe and the target. This difference is detected by the Hall Effect cell as a voltage differential and converted to a thickness measurement using results of preliminary calibration tests. Materials Tests were conducted on 15 geomembrane samples representing five polymer types with nominal thicknesses ranging from 0.76 mm to 2.5 mm (Table 1). Multiple samples of each geomembrane type were tested, except for EPDM. All of the samples were smooth, nonreinforced geomembranes. The test materials represent the most commonly used geomembranes (Koerner 1997). Testing Program The testing program consisted of determination of thickness of smooth geomembranes using mechanical, ultrasonic, and magnetic methods. Tests were conducted on specimens cut to the dimensions of 100 mm X 100 mm to meet the requirements provided in both the ASTM and the European standards. The measurements were taken at the centerpoint of the specimens. The specimens were conditioned for 24 h at a temperature of 21°C and a relative humidity of 60% prior to testing. Equipment, Test Setups, and Procedures The mechanical tests were conducted using a dead-weight loading system with interchangeable loads and loading tips. A photograph of the test setup is presented in Fig. 4a. The resolution of the dial gage was 0.001 mm. For ASTM D5199 tests, pressures of 20 kPa, 50 kPa, 100 kPa, 150 kPa, and 200 kPa were used. For EN 964-1 tests, pressures of 2 kPa, 20 kPa, and 200 kPa were used. The test setup was originally built for testing at 20 kPa pressure according to the ASTM standard. Modifications were made to allow testing at higher pressures, and using the European standard. The modifications significantly increased the size and weight of the instrument and rendered the setup essentially immobile. The ultrasonic tests were conducted using a commercially available thickness measurement system. The equipment consisted of a thickness gage and an ultrasonic transducer. A photograph of the setup is presented in Fig. 4b. Details for the components and the use of the equipment are described by Sungur (1999). Thicknesses can be determined with a resolution of 0.001 mm using this setup. In the ultrasonic tests, a measurement is made by placing the transducer on the top surface of a test specimen. A small amount of coupling material (in this case, water) is applied between the transducer and the specimen to eliminate air gaps and provide good contact between the sensor and the test material. A custom-made weight is placed on top of the transducer to ensure that the transducer is stable on the specimen and that it is in good contact with the geomembrane. A pressure of 19 kPa is applied on a specimen by the ultrasonic setup. The ultrasonic setup that consists of the thickness gage and the transducer is small and mobile (Fig. 4b). The P-wave velocities of the 15 geomembranes were determined prior to ultrasonic thickness measurements. A total of 864 tests were conducted on each geomembrane sample. This included measurements at a number of points on various specimens of a geomembrane sample and also repeated measurements at a given location by replacing the transducer at the location various times. Each ultrasonic measurement consisted of an average of 10 measurements increasing the data used for determination of the velocities to 8640 measurements per sample. While this is a significant number, it must be noted that the P-wave propagation occurs at time intervals measured in microseconds and the 10 average measurements are automatically recorded by the thickness gage. Generally, a single measurement (average of 10 readings) was completed in 30 s, including the time required to move the