The following description reflects a current industry quality control method. It is presented as a general format, not as a direct substitute of a relevant test procedure.
Significance - The geosynthetic clay liner (GCL) peel test provides a means of evaluating the needling strength of Bentofix® Thermal Lock GCLs and is an index procedure which is used to monitor the bonding strength as well as the wear of the needles during production. Needles are changed as peel test results begin to approach the minimum acceptable strength. The relationship between the internal peel strength and the peel bonding strength is described by Heerten et al. (1995), von Maubeuge et al. (2000).
Description -
In the past tensile strength test methods, such as ISO 10319, ASTM D4632 and ASTM D4595 were used as a basis for determining specimen size, testing equipment and the general test procedure subject to modifications in the rate of expansion, grip size as well as grip orientation. Since 1999 a test method is available which describes a procedure to determine the peel bond strength of needle punched Geosynthetic Clay Liners, ASTM D6496 "Standard Test Method for Determining Average Bonding Peel Strength Between the Top and Bottom Layers of Needled GCLs".
In this peel test, a GCL specimen is partially de-laminated by cutting the needlepunched bonds between the geotextiles just enough to allow each geotextile to be separately inserted into the grips of the tensometer. The specimen is peeled at 300 mm per minute (12 in./min.) and the average peel bonding strength over a peeling distance (grip separation) of 100 mm (4 in.) is recorded as the average peel bonding strength for that individual specimen. Five specimens are tested to determine the average peel strength of a sample.
QC Test Frequency - Bentofix® peel testing is performed at a minimum frequency of one test series every 4,000 m² (40,000 square feet) of material produced. Bentofix® GCLs are tested at a much higher frequency than recommended by ASTM D5889 "Standard Practice for Quality Control of Geosynthetic Clay Liners", which recommends the test to be carried out every 20,000 m² (200,000 ft²). The Bentofix® testing frequency is typically increased when peel strengths approach the minimal acceptable level.
Test Procedure 1. A 300 mm (12 in.) long by full roll-width sample is cut from each designated production Bentofix® roll. This sample is termed the testing roll.
2. Using a template, five 100 mm x 200 mm (4 in. x 8 in.) specimens are cut from the testing roll with the longer length oriented in the machine direction. Specimens not exhibiting proper orientation are rejected and a new specimen is cut.
3. A utility knife is used to slice the needlepunched bonds between the two geotextiles from the designated end of each specimen and the geotextiles are separated 50 mm (2 in.) apart.
4. The grips from the testing device are oriented so that the wide width of the grip is parallel to the 100 mm (4 in.) width of the test specimen and perpendicular to the applied stress. The grips must clamp the entire width of the specimen. The specimen is mounted in the testing device such that the upper grips contain the one geotextile flap and the lower grips contain the second geotextile flap. The grips should be set at a baseline distance of 50 mm (2 in.) apart. As mounted the grip will apply a 100 mm (4 in.) wide stress.
5. The tensile testing device is set to a constant rate of expansion of 300 mm/min (12 in./min). Readings of force at a rate of 20 per second are taken from 25 mm (1 in.) until 125 mm (5 in.) grip separation.
6. The average force [N (lbf)] for each specimen is then recorded and divided by the specimen width. The average peel bonding strength of all specimen is then calculated in N/m (lbf/in.).
Comparison of ASTM D6496 with the Past Peel Bond Method(s) -
The main difference with ASTM D6496 compared to other methods is the sample size and the final value. In the past the most test methods required that a maximum or peak value is reported. This value was then reported in N (lbs) or in N/10 cm (lbf/in.). With ASTM D6496 the reported value is an average value, because all bonding values are summarized in the average value and are not based on only one very high peel bond value.
Acceptance Criteria - Project specific Bentofix® products can have different peel bond values. However, the normal average of the five average values for Bentofix® is usually greater than or equal to 400 N/m (2,3 lbs/in.). Individual specimen results may fall below the specified Bentofix® value if the average of the five specimens is equal or greater than the specified Bentofix® value. If the tested value falls below the specified Bentofix® value, the test is repeated on 5 additional specimens taken from adjacent locations on the same testing roll. If the average of the individual values of the 5 specimens exceeds the specified Bentofix® value, the roll and the representative production material is deemed to be acceptable.
If the average is less than the specified Bentofix® value, the roll is not accepted for shipment. The Bentofix® rolls manufactured immediately before and after the rejected roll are then sampled and tested as described above in steps 1 - 6 and the same acceptance/rejection criteria are applied to these rolls. This process is repeated until the acceptance criteria are achieved to isolate non-conforming rolls.
Bentofix Specified Value - A minimum acceptable peel bonding strength is used to qualify material as peel values will vary depending upon the sample roll's proximity to the last needleboard change. Bentofix® is usually specified with a minimum index peel bonding strength of 400 N/m (2,3 lbs/in.) under the above test procedure. This is helpful for the manufacturing process and is also a quick method of ensuring thorough needlepunching characteristics.
Bentofix® is a registered trademark of NAUE GmbH & Co. KG, GmbH.
GCL 204 03/2003