Hot Wedge Welding for PVC Geomembranes
The need to have field seams that can be checked quickly with measurable results has caused geosynthetic engineers and Quality Assurance/Quality Control (QC/QA) technicians to be reluctant to use field chemical seams for PVC geomembranes. In recent years, however, hot air and hot wedge welding has proven to be an efficient and cost–effective method of field seaming PVC geomembrane liners. The use of hot air and wedge welding also allows preferred QA/QC techniques to be used for PVC geomembranes.
Before the advent of wedge welding, field PVC seams were made using either an adhesive or chemical fusion agent. Adhesive seams utilize a bonding agent that remains as an additional element in the seam after curing. Chemical fusion seams employ a solution that actually dissolves the surface of the materials to be joined, allowing them to be fused together. Both methods have limitations during field application. In addition, QA/QC procedures for chemical welds can be difficult due to long curing times. This can delay peel and shear testing of seams up to 24 hours and also delay non-destructive testing until the seam has cured properly. This can result in a delay in placing cover material and meeting project deadlines. Surface preparation in order to keep the seam area dry and clean is required in the field as with any good seaming procedure. Surface temperature of the geomembrane needs to be monitored to assure the temperature is not too low or too high to assure good chemical bonding.
Thermal welding PVC geomembranes, on the other hand, has the benefit that it can extend liner construction into the cooler weather when chemical seams would not be possible.
In light of the limitations of field chemical welds, the benefits of thermal welding of PVC become apparent. PVC possesses 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 systems can thermal weld PVC geomembrane as thin as 20 mil. These systems allow the operator to adjust both wedge speed and temperature to create the best quality seam. During installation, temperature and speed are set according to geomembrane thickness. It should also be adjusted to account for large variations in ambient temperature. Thermal welders have been used successfully to seam 20 mil PVC in ambient temperatures as low as -8° C (18°F). Machine temperature depends chiefly on the type of welder being used. Manufacturers may also specify a temperature based on the thickness of the PVC geomembrane. Depending upon the manufacturer, recommended wedge temperatures vary from 360° to 750° F.
At present, two types of thermal welding are used in practice: dual track and solid seam welding. Both types of thermal 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. Both types of thermal welding will be discussed in the following paragraphs.
Dual track thermal welding allows air channel testing to be performed. This facilitates the use of well-established QA/QC procedures developed for air channel testing. Therefore, QAlQC personnel familiar with these test methods can be utilized for PVC using the PVC related specifications for air channel testing that are described subsequently.
Dual track air channel testing is an efficient method of nondestructive testing currently used to evaluate field seam peel strength and continuity in PVC geomembranes. ASTM D7177 Standard Specification for Air Channel Evaluation of Polyvinyl Chloride (PVC) Dual Track Seamed Geomembranes is the accepted standard for testing of PVC 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 accounts for the increased flexibility of PVC by reducing the minimum and maximum air pressures used for testing at higher temperatures.
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 ambient temperature increases. The pressure chart in ASTM D7177 shows that the air channel pressure variation corresponding with the sheet temperature change.
Solid wedge welding is also frequently used in practice to field seam PVC geomembranes. Solid wedge welding provides a wider seam and is tested with the air lance test method that was used to test field chemical seams. 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. However, air lance testing does not verify the peel strength of the weld as is done by ASTM D 7177 Air Channel Testing.
Successful dual track and solid wedge welding of PVC requires the use of a wedge surface that is compatible with PVC. During the welding process, an acidic environment can be created by the polymer softening of PVC material which may accumulate on the wedge welding equipment. Traditional copper wedges are susceptible to attack by this acidic environment, which could result in a rough wedge surface. This problem has long been recognized and today there are several alternatives available to installers of PVC geomembranes. For example, protective coatings have been developed to shield copper wedges from acidic by-products of welding and polished stainless steel wedges have also been used successfully. Hot air welders use a stainless steel nozzel which is not affected by the PVC material.
Installers of PVC geomembranes must be aware of the same key installation guidelines that are required for other geomembranes in order to avoid common problems. Machine temperature needs to be adjusted to reflect the proper temperature range used for PVC. Excessive melting will weaken the geomembrane and insufficient melting will lead to low seam strength. Welder burnout is also a concern as the thickness of the geomembrane decreases. The intrusion of soil particles along the weld track must be minimized in order to ensure an integral bond between sheets. Also steps should be taken to assure that excessive moisture is kept from the welding area.
The increased use of thermal welding techniques for the field seaming of PVC geomembranes has eliminated the drawbacks associated with field chemical methods. PVC geomembrane seaming using the thermal welding method offers higher seam strength values without the concerns of stress cracking in the seam area associated with other geomembranes. In addition, the use of thermal welding allows preferred QA/QC techniques to be used for PVC geomembranes.
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