Checking in with PGI
by Richard W. Thomas and Timothy D. Stark
One of the current research topics being pursued by the PVC Geomembrane Institute (PGI) is the thermal welding and subsequent air channel testing of PVC geomembrane seams, which is the focus of this column. Other active research projects include the long-term performance of PVC geomembranes using field case histories, shear behavior of PVC geomembrane interfaces, and the chemical resistance of PVC geomembranes. These projects will be discussed in future columns.
This article focuses on the use of a new seam burst test to evaluate field PVC geomembrane seams. This topic is part of a larger study on the thermal welding of PVC geomembranes and some results from this study are also presented herein. The seam burst test is an excellent indicator of seam quality and has been directly related to seam peel strength (ASTM D6392). These results allow the burst test to be used in the field to test the entire length of a PVC seam for the specified peel strength instead of using destructive peel tests over a limited portion of the seam. It is anticipated that the use of the burst test will lead to a reduction in the destructive testing required during field installations.
Thermal welding has proven to be an efficient and cost-effective method of field seaming PVC geomembrane liners since 1991 and has been used for factory fabrication since 1982. PVC geomembranes possess excellent thermal welding characteristics such as a wide thermal seaming range, lack of residual stress or stress cracking, and no required surface preparation, such as grinding. Fully automated welding systems can thermally weld PVC geomembranes as thin as 0.5 mm (20 mil). These welding systems allow the operator to adjust welding speed, nip-roller pressure, and welding temperature to create the best quality seam. In addition, thermal welding can be used to create an air-channel using a dual track wedge and thus conduct air-channel testing of the seam.
An important a difference between air-channel testing of PVC geomembranes seams versus high-density polyethylene (HDPE) geomembranes is the flexible nature of a PVC geomembrane, which allows the field technician to see the air channel inflate as the air pressure migrates down the seam. The inflated air channel somewhat resembles an inflated inner tube and this distinctive behavior has been referred to as “inner tubing” of PVC seams. If a weak spot is encountered and leaks, the air pressure may not be fully inflated at this weak spot. Figure 1 presents a photograph of a 0.75 mm (30 mil) geomembrane with an inflated air channel in the field. It can be seen that the air channel is inflated (inner tubing), readily visible, and maintaining the air pressure.
The thermally welded seams used in this study were created in a single day in Austin, Texas at TRI/Environmental on an asphalt subgrade. Two installation crews from two different companies (Colorado Lining International and Environmental Protection, Inc.), one using a hot air welder and the other using a hot wedge welder, created the seventy-two (72) 9.2 m (30 foot) long thermal seams used in this study. The hot air machine is a Leister Twinnie Model CH6056. The hot wedge machine is a Mini-Wedge made by Plastic Welding Technologies (formerly Columbine, Inc.). The 0.75 to 1.00 mm (30 and 40 mil) PVC geomembrane used in the thermal seam testing was provided by Canadian General-Tower, Ltd. of Cambridge, Ontario, Canada.
Figure 1: Inflated air channel in 0.75 mm (30 mil) geomembrane in the field (Photo courtesy of Fred Rohe, Environmental Protection Inc.)
The seams were evaluated by the standard peel test at 20 in/min at 73ºF (ASTM D 6392, 1999) and by the seam burst test developed during this project. The burst test was performed by sealing off one end of a seam length and pressurizing the other end with compressed air. The seam length tested in the burst test was 2 m (6 feet). The basic test procedure involved selecting a starting air pressure, holding that air pressure constant for 30 seconds, then increasing the air pressure 5 psi at a time, and holding the new air pressure another 30 seconds. This was repeated for each 34.4 kPa (5 psi) air pressure increment. The 34.4 kPa (5 psi) air pressure increment was achieved in a 5 second time period. This procedure of increasing the air channel pressure, holding the air pressure, and then increasing the air pressure by 34.4 kPa (5 psi) continued until the seam “burst”. Most of the burst failures involved the peel mode, which occurred during the 30 second holding period. However, some seams burst during the 34.4 kPa (5 psi) air pressure increase step.
The seam peel strength is compared to the burst pressure because pressurizing the air channel results in the seam being challenged more in a peel mode than a shear mode. Therefore the seam peel strength, and not the shear strength, was used for comparative purposes. It is important to note that the burst test fails the seam from the inside towards the outside of the seam whereas the peel test fails the seam from the outside towards the inside of the seam. The impact of this difference, if any, is a subject of the ongoing research but it does not impact the results herein because PVC seam requirements are specified in terms of peel strength and the burst pressure is simply being correlated to this specified parameter. The specified value for the peel strength of both 0.75 and 1.00 mm (30 and 40 mil) thick PVC seams according to the PGI material specification (PGI 1997) is 2.6 N/mm (15 ppi).
The main contribution of the air-channel testing research is the development of a relationship between peel strength at room temperature (22.8ºC; 73ºF) and the burst pressure at sheet temperatures ranging from 22.8 to 46.7ºC (73 to 116ºF) as shown in Figure 2. This relationship was developed from the test results obtained during the research program and from Arrehnius modeling that allows the data to be extended to sheet temperatures below 22.8ºC and above 46.7ºC. This relationship allows field personnel to perform seam QA/QC operations without conducting destructive tests because the seam peel strength can be measured indirectly by applying air pressure to the air channel in a dual track weld. This field air channel test can be used instead of destructive seam testing, which has the advantages of NOT cutting holes in the geomembrane, geomembrane surface preparation such as grinding, and patching the resulting geomembrane. The main advantage of the peel strength/burst pressure relationship in Figure 2 is the ability to test the entire seam length instead of a 1 m coupon. This procedure coupled with the flexibility of PVC geomembranes which allows the air channel to expand (inner tube) so field personnel can visually inspect the seam as the air pressure migrates along the channel and the fact that the presence of any defect may not allow the air channel to fully expand or inflate in the vicinity of the defect, results in a excellent means for ensuring the integrity of field thermal seams in PVC geomembranes.
Figure 2: Burst pressure required to ensure a specified peel strength of 2.6 N/mm (15 ppi) at various sheet temperatures.
Some of the results from the larger study on the thermal welding of PVC geomembranes include understanding the effects of welding temperature, welding speed, and sheet temperature on seam performance. These variables were evaluated for two geomembrane thicknesses and two types of welder, i.e., hot air and wedge welders, using the results of seam peel tests (ASTM D 6392) and the new burst test. The seam test results show that welding speed has a greater impact on the measured peel strength than welding temperature. Therefore, welding personnel can increase the seam peel strength for a given sheet temperature and welding temperature simply by reducing the speed of the welder. A welding speed in the range of 0.9 to 2.1 m/min provides the best seams under the widest range of sheet temperature, geomembrane thickness, and welding temperature. Welding speeds as high as 3.1 m/min can produce good seams especially if the sheet temperature is high via sunshine and/or the welding temperature is high.
The seam test results also show that a welding temperature of 315.6ºC (600ºF) is too low and a welding temperature of 482.2ºC (900ºF) is too high for this 0.75 mm (30 mil) thick PVC geomembrane. Therefore, an optimal welding temperature to initiate welding is 398.9ºC (750ºF) for this 0.75 mm (30 mil) PVC geomembrane. The test results also suggest that an optimal welding temperature might range from 454.4 to 468.3ºC (850 to 875ºF) for a welding speed of 3.1 m/min for 1.00 mm (40 mil) seams.
ASTM D 6392, 1999, “Standard Test Method for Determining the Geomembrane Seams Produced using Thermo-Fusion Methods”, American Society for Testing and Materials, Vol. 04.09, West Conshohocken, Pennsylvania, USA, pp. 1311-1315.
PVC Geomembrane Institute (PGI), 1997, “PVC Geomembrane Material Specification 1197”, University of Illinois, Urbana, IL, www.pvcgeomembrane.com, January 1, 1997.