By Fred P. Rohe and Sam Lewis
Chemical welding and heat welding are the two most common methods of producing seams in geomembrane fabrication. While heat welded seams reach full strength almost as soon as they cool, chemically welded seams require a period of time for curing in order to reach full strength. Since heat welding is not always practical for use on thin gauge geomembrane material, chemical welding has been used successfully on thin gauge geomembrane materials since the 1950's.
Currently, there is an increasing requirement for destructive testing of geomembrane seams at the earliest possible time. The curing of chemically welded PVC seams takes place over a long period of time. The age of the sample will affect the test results and should affect the engineer's interpretation of these results.
On July 25, 1988, Environmental Protection, Inc. began a study of the curing of chemically welded PVC seams. A 100 foot long seam of 20 mil PVC was fabricated using normal EPI factory fabrication techniques. This seam was then cut into 24" long test sample blanks. These samples were then tested at the following intervals: 20 minutes, 1 hour, 2 hours, 4 hours, 7.5 hours, 23 hours, 30.5 hours, 46.5 hours, and then at a rate of one per day until the end of one month. The samples were tested for bonded seam strength using ASTM D-3083 NSF modified, and they were tested for seam peel adhesion following ASTM D-413 as modified by NSF.
The results from these tests were then plotted on two graphs: shear vs. age, and peel vs. age. The natural logarithm was taken of the data and was also plotted on two graphs: ln shear vs. In age, and ln peel vs. ln age. These graphs were found to be highly linear with a correlation coefficient of .95 for ln shear vs. ln age, and .80 for ln peel vs. In age.
A linear regression analysis was then performed for both log-log graphs to find the equations for the best fit lines. These lines were then superimposed on the log-log plots. The equations for the best fit lines was then exponentiated to find the best fit curves for the real data.
While the applicability of these results is limited to the materials and methods used by EPI, they do show that chemically welded PVC seams will increase in shear and peel strengths over time. ASTM requires 40 hours of conditioning time in the laboratory prior to testing PVC seams for shear and peel. As can be seen from the data presented here, this may not be sufficient time for the seam to reach its ultimate strength. However, based on the age of the sample, a prediction could be made using this curve as to what the ultimate strength of the seam will be.
While it is sometimes necessary to test the seams when still fairly new, the results of these tests should not be taken as representative of the ultimate strength of a chemically welded PVC seam. Caution should be used in evaluating the data on the testing of seams that are not completely cured.
Figure 1.) Shows the best fit curve for the shear strength vs. time of this test. The minimum shear strength of 36.8 lbs. per inch reach in approximately 2 hours. The shear strength to increase over time to a strength of 53.3 lbs. per inch width.
Figure 2.) Shows the best fit curve for peel strength vs. time. The peel strength reached the minimum requirements in approximately 96 hours. The strength continued to increase throughout the time of the test reaching an ultimate peel strength of 12.1 IBS. per inch width at the conclusion of the test.
As part of EPI's quality control process a second long term test on the curing of a chemically welded 20 mil PVC seam was conducted in December of 1988. Again, a l00 foot seam was fabricated using normal EPI processes. The seam was cut into 24 inch samples immediately after fabrication, and tested at the same intervals as the July long term test. Calculation of the data to plot the best fit curve was performed in the identical manner of the original test. Figure 3 illustrates the shear strength of the seam vs. its age. The minimum required bonded seam strength of 36.8 lbs. per inch was achieved in approximately 2 hours. The seam continued to cure and increase in strength to a final value of 57 lbs. at the end of the 30 day test.
Figure 4.) Illustrates the best fit curve of the peel strength vs. the age of the seam.
The ultimate peel strength of this seam is approximately 2.5 lbs. per inch width higher than the original test. Also, the seam reached its initial minimum requirements for peel strength much sooner than the original test. Although the ultimate value of the second test was slightly higher, when superimposed on each other, the best fit curves for the increase of peel strength are virtually identical. This observation also holds true for the shear strength of the seam in that, when superimposed, the shear strength best fit curves are also almost identical from these two tests.
Please direct any questions to Fred Rohe or Mark Wolschon at EPI. They will be pleased to answer any questions regarding the curing of chemical fusion welded seams, accelerated testing, and factory fabrication quality control.
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