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Thermal welding geomembrane - how it began. In 1990, field seams for wedge-welded PVC geomembranes appeared. The earlier goal was to extend the construction period by developing a PVC welding technology that could be used in colder weather. This would allow earlier spring starts and delayed closures until late autumn.
After several years of trials and multiple negotiations with equipment manufacturers, people made large-scale changes to hot air welders used for welding PVC. This received great help from major equipment manufacturing companies. Developing a program for using hot air welders on thinner, more flexible PVC materials was a challenging task. But after the problems were overcome, welding technicians began to develop the skills to professionally weld geomembranes of any thickness in almost any weather condition.
Evolved from these problem-solving meetings and the large amount of test data developed was the absolute belief that air channel testing can be used to verify the physical strength of PVC welds. In 2001, a company agreed to conduct some tests and research on PVC thermal welds. There was also interest in whether PVC seams could use air channel testing to test peel strength. In 2002, a burst test study was conducted on hot air and hot wedge welded PVC seams of 30 and 40 mil PVC. The result of this test and other studies was a graph of the pressure of air channel tested PVC geomembrane versus sheet temperature, verifying that the minimum peel strength over the entire length of the test section had met the usage standards.
In 2002, the company manager managing quality control introduced the idea of the PVC standard air channel test. Since then, a special working group has been established to develop new standards. After more than two years of extensive discussions, the standard specification for air channel evaluation of geomembrane double-track seams was adopted in 2005 and is still the recognized standard for PVC field seam air channel testing today. Simply put, according to the air channel test, poorly made thermal welds in geomembrane will peel off. This test applies pressure to the entire length of the seam, so any weak area, no matter how small, will be immediately located. Any failed seam should be replaced. Field welding was also experienced, by removing destructive samples, a small area of the same weld failed the air channel test. This type of seam must be re-welded to ensure that customers receive the best product. Moreover, relevant institutions also compiled qualitative test results and conducted statistical analysis on the data. We found that through this process, the peel strength of seams is usually stronger, and seams can be verified for compliance within minutes rather than days after production. Air channel testing is used for seam continuity as well as verifying the peel strength of the entire seam length!
"T" Seams
All field seams must be tested, and if not properly welded, T-shaped seams may be difficult to air channel test. A T-shaped seam is defined as the point where three layers of material overlap each other in the seam. This occurs when a double-track weld crosses a factory weld, usually at a 90-degree angle. We have received hundreds of questions from engineers and customers about air channel testing of "T" shaped seams, which are formed along the ends of PVC geomembrane panels. Air channel test all field seams, including seams at the ends of factory panels.
Although panels can be made square, panels are usually made rectangular, longer than wide. Panels are made using a large number of individual strips from rolls, welded edge to edge together. These edge welds terminate at the end of each panel and overlap with adjacent panels when deployed in the field. This end panel overlap must be properly welded so that the resulting weld can be air channel tested.
At each "T" there is a possibility of a very small hole at the junction of the three layers of material. This is another key reason why air channel testing of each seam is critical to the integrity of the liner system, to find and eliminate these holes. Welding technicians take special care when setting up the welder to ensure that this overlap is completely sealed, so air channel testing can also be used to verify the strength and continuity of these seams.
Factory seams do not have loose edges, so the process of welding T-shaped seams is relatively easy. Slowing down the travel speed of the welding machine will allow the molten PVC material to flow together at the junction of the three layers of material, thereby providing the necessary seal and weld strength. For manufacturers that leave loose edges on factory seams, each loose edge needs to be trimmed, similar to the process used in field welds that intersect with other seams.
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