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All geomembranes require some field seaming. One of the advantages of PVC geomembranes is that the amount of field seaming can be 70 to 80 percent less than that required for an HDPE geomembrane. The manufactured rolls of PVC are seamed under controlled conditions in a fabrication plant prior to field deployment. This fabrication results in large geomembrane panels that reduce the number of required field seams. (The photograph below shows large PVC geomembrane panels being placed in a liner system at an operating municipal solid waste facility.) It is generally recognized that field-prepared seams are the most problematic aspect of all lining systems and thus reducing the amount of field seaming is beneficial.

Presently, field PVC seams are being constructed with good results using chemical systems (adhesive or solvent). In recent years, thermal welding has also proven to be an efficient and cost-effective method of field seaming PVC geomembranes. The thermal fusion method of joining PVC geomembranes has been used for many years in the roofing industry and in the European geomembrane and tunneling industries. The use of wedge and hot air welding allows the same equipment and preferred QA/QC techniques for HDPE geomembranes to be used for PVC geomembranes. In addition thermal welding can be used in colder environmental conditions than chemical welds.

The principle of a thermal weld is that both surfaces to be joined come into intimate contact with the hot wedge. This melts the upper and lower layer surfaces of the viscous polymer sheets. Fusion is brought about by compressing the two melted surfaces together by nip rollers, causing an intermingling of the polymers from both sheets and a permanent bond. The relatively new use of thermal welds in the United States for PVC geomembranes has created the need for presenting some "standard" specifications for thermal welding of PVC geomembranes.

At present, two types of wedge welding are used in practice: solid and dual track wedge welding. Both types of wedge welding allow destructive and nondestructive testing to be carried out as soon as the seam has cooled. This rapid assessment of quality allows immediate changes to be made in the seaming process to ensure optimal productivity.

Solid thermal welding is frequently used in practice to field seam PVC geomembranes. Solid thermal welding provides a wide seam and is tested with the air lance test method, which is also used to test field chemical seams. The minimum seam width for a single thermal weld is a nominal one inch (25-mm). The seam is tested for continuity by air lance testing in accordance with ASTM Test Method D-4437. Air lance testing can be performed immediately after the wedge seam has been completed. The test utilizes a combination of air pressure (55 psi) and visual inspection. A pressurized stream of air is placed perpendicular to the seam edge and any unbonded or loose areas will visibly flap on the inside sheet showing an air channel or imperfection through the weld.  This test does not verify seam peel adhesion unlike air channel testing using ASTM D 7177.

Thermal dual track (split) welds consist of two nominal 0.5 inch (12.5 mm) wide welds running parallel and an unwelded 0.5 (l2.5 mm) inch air channel between them. The seam is tested for integrity by filling the channel with air, which is an efficient method of nondestructive testing currently used to evaluate field seams in other geomembranes. This facilitates the use of well-established QA/QC procedures developed for air channel testing.

The air channel testing is conducted in accordance with the ASTM D7177 Standard Specification for Evaluation of Polyvinyl Chloride (PVC) Dual Track Seamed Geomembranes.  This test method quickly shows a failure, since any hole will not allow the air channel to inflate the full length of the seam. The method also accounts for the increased flexibility of PVC by reducing the maximum air pressures used for testing more rigid geomembranes. The air pressure and sheet temperature requirements are as shown in Table 1.

It has been reported that extremely high temperatures may affect the performance and results of air channel testing in PVC geomembranes. As with all flexible geomembrane materials, PVC exhibits increased flexibility as the sheet  temperature increases. However, recent observations and data show that the air channel will develop the necessary air pressure even at high temperatures. In fact the air channel will not expand beyond a certain diameter at elevated temperatures and thus the air channel pressure can be achieved and maintained.  The values in Table one show that the air pressure requirements of ASTM D7177 vary according to ambient sheet temperature.

Temperature controllers on the welding device should be set according to type of geomembrane, thickness, ambient temperature, rate of seaming, and location of thermocouple within the device. Ambient factors such as clouds, wind, and sun require temperature and rate of travel settings to vary. As a result, it is necessary that the operator keep constant visual contact with the temperature controls, as well as the completed seam coming out of the machine.

A five-foot test strip should be fabricated and specimens manually tested prior to constructing each seam, or at the beginning of each shift change (e.g., morning or lunch). A minimum of one test strip should be made each morning and afternoon prior to commencement of welding. Single thermal welds must exceed minimum shear and peel specifications, and both tracks of a dual track weld must exceed minimum specifications.

Records for each test strip, seam, and each destructive test sample should include welder designation, technician, temperature, and travel rate.

On T Seams across ends of panels, it may be necessary to trim any loose edges of the factory or field seams prior to welding.

Any area of the weld that fails air lance testing or air channel testing should be patched using the same liner material with cold applied PVC to PVC welding adhesive, chemical fusion agent, or bodied chemical fusion agent. All repairs should be air lance tested in accordance with ASTM Test Method D-4437.

In summary the increased use of thermal welding techniques for the field seaming of PVC geomembranes has allowed seams to be checked immediately with measurable results using well established QAIQC techniques. PVC geomembrane seaming using the thermal method also offers higher seam strength values without the concerns of stress cracking in the seam area associated with other geomembranes.

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