The “4-out-of-5” question (Geosynthetics, Aug./Sept. 2013, page 8) is rather interesting and brings forth several quick thoughts. As a construction quality assurance (CQA) subcontractor supplying technician resources to engineering firms throughout the United States, I can say that this has been a topic of debate between myself and my clients. Uniformity in interpretation of the “4-out-of-5” strength requirement even varies among clients, where some will accept a 100% peel in the 5th coupon as long as it meets the 80% strength requirement.
My immediate thought on the “4-out-of-5” peel incursion focuses on extrusion welding. Over the years, measurement of peel incursion, especially in extrusion welds, has been one of the most highly debated issues. Extrusion welds are unique given the characteristic of the weld and the specification based around “original bonding area.” Much is open to interpretation as to where the weld actually begins and ends. This alone can influence whether a sample passes or fails, especially on narrower welds—the narrower the weld, the lower the original bonding area and the less incursion it takes to reach 25%. On samples such as these, it does not take much to have an error of 5% or more. Individual incursion interpretation on any extrusion coupon often varies between 5% and 10%, and I see this hold true even among well-known industry laboratories—meaning there is already significant margin/tolerance introduced into the pass/fail determination. While the eye can be calibrated somewhat for fusion peel incursion, due to the consistency of original bonding area (from consistent track width), extrusion incursion usually needs to be measured on an individual, time-consuming basis.
Shear elongation failures, while rare, do occur in both fusion and extrusion. Luckily, modern tensiometers directly measuring elongation take the guesswork out of the interpretation of the percent. While most of the failures I’ve observed occur in the 20–30% elongation range, I have seen failures between 40–50%, primarily in extrusion welds.
The general industry acknowledgement that extrusion welds are inferior to fusion welds and that extrusion welds are commonly thought to be the weakest link (higher probability of problems/leaks) of a welded containment system could warrant a more conservative approach in looking at the peel incursion and shear elongation requirements for extrusion welds. Additionally, a universal standardized method of assessing peel incursion throughout the industry (to complement ASTM D6392 and GM-19) may also be warranted to produce uniform, repeatable results across the board when assessing either fusion or extrusion peel incursion.
CQA Solutions Ltd. | Toledo, Ohio
To the editor
I have always been a bit suprised with the 4-out-of-5 pass requirement on the specimens extracted from a sample of geomembrane weld. I specify no failures permitted and have done this for years. In todays’ environment of good quality welding equipment available in the market, good quality geomembrane sheet accepted onto projects (where appropriate quality control is carried out), and with appropriate weld surface preparation, there is no reason why weld samples should not pass the strength requirements for all specimens. Our experience is we find failures where the welder unit was not working effectively or the sheet surface was dusty or moist during welding.
A 100% failure rate should be adopted, particularly when the industry is considering larger spacing between destructive sampling of completed welds, compared to the current default spacing of 100m.
Golder Associates | Melbourne, Australia