Evaluating Real-World Performance of Field Aged TPO Roofs

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Sponsored by GAF
Presented by Jennifer Keegan, AAIA

Heat Aging and Weather Resistance

Surface cracking was evaluated by visual inspection of the field-aged roof membranes at the time the samples were collected, and via inspection at 7X magnification when bent over a 3-inch diameter mandrel. This evaluation is an important indicator of long-term performance.10 Surface cracking on a roof is not indicative of an immediate problem until such cracks propagated down to the reinforcing scrim, as seen in Figure 11. Surface cracks are suggestive of the stabilizer content being substantially depleted, which could lead to more rapid deterioration of the membrane. None of the samples—neither the 45- nor 60-mil membranes—exhibited any signs of surface cracking when bent over the mandrel and viewed at 7X magnification.

Figure 11: Testing performed by Rene Dupuis illustrates the reason why we look at cracking of the membrane. The test sample is from laboratory exposure and the real world sample is from an aged roof.

Brittleness Point

The aged roof samples were evaluated for brittleness point, an indicator as to whether the membranes become more susceptible to cracking during extreme cold conditions. Only those samples that had been mechanically attached were tested, with the results being shown in Figure 12.

Figure 12. Brittleness point of tested samples—nominal 45 mil in gray, nominal 60 mil in blue. Not all samples were tested for brittleness.

ASTM standard D6878 requires new membranes to have a brittleness point of -40F or lower. All except one of the 60-mil samples tested still meet cold temperature flexibility requirements after field aging. The 45 mil samples showed initial signs of cracking at -35F. The slight rise seen for these samples is indicative of low temperature stiffening due to oxidative crosslinking. Roofing membrane issues caused by rising brittleness point were previously observed with early versions of PVC, which exhibited cracking and shattering during winters.13 However, such issues were essentially eliminated by improving the formulations and by the use of reinforcing scrim. The scrim used in TPO is the same basic design and type as used in PVC and the small changes in brittleness point observed in some of the sampled TPO roofs are not a cause for concern. However, the data supports the use of thicker membranes for longer-term performance.

Aged Ply Adhesion

While failures associated with welded seam delamination are not widespread, ply adhesion or weld strength is used as a quality control tool in the field. However, there is not a clear industry consensus on the minimum strength requirement to evaluate weld strength. ASTM D6878 does not provide a minimum value for weld strength. Therefore, weld strength is compared to the values published in a study of new TPO membranes conducted by Structural Research, Inc. (SRI). This study included a broad TPO sampling of all industry manufacturers.14 This study which evaluated the weld strength of all thicknesses of TPO from all manufacturers averaged a ply adhesion (T-Peel) value of 40 lbf/inch, with a minimum value of 29.3 lbf/inch. Previously, however, Simmons et al. found that the ply adhesion tests typically failed adhesively, meaning there was not a strong bond between the welded TPO layers and the film-tearing bond was 0 percent, when the ply adhesion of the seam weld was 26 lbf/inch or less.15

The ply adhesion values of the aged TPO membrane seam welds were on average, 50 lbf/inch, which is above the average ply adhesion value from the SRI study on new TPO membranes, as shown in Figure 13. As expected, the aged welds appear to be performing well and are of adequate strength. Note that for Location 11, the value of 20.7 lbf is indicative of a suspect quality weld and would require further investigation.

Figure 13. Weld strengths of the aged roof samples.

Film tearing bond, as shown in Figures 14 and 15, was also analyzed as it is relied upon in the field to assess the qualitative integrity of a weld. If the sample fails and the scrim is not exposed, or the film tearing bond is not greater than 70 percent, the heat and/or speed of the welding equipment must be adjusted.

Figure 14. Film tearing bond of nearly 100 percent, from Location 6.

The film-tearing bond at Location 11 was of suspect quality, summarized in Figure 15. This is also suggested by the weld strength data at this location.

Figure 15. Percent film-tearing bond of the aged roof samples. Zero film-tearing bond at locations 4 and 11 suggest poor quality workmanship.

All of the samples except Locations 4 and 11 met the threshold requirements for percent film-tearing bond. Comparing the weld strengths and film-tearing bond percent, it is clear that the roofs at locations 4 and 11 might warrant further investigation due to the combination of low weld strength and film-tearing bond. In the case of location 2, the aged membrane has a weld strength of 53.9 lbf with a film-tearing bond less than 70 percent. This suggests that the issue on this roof could be localized, possibly to the installation time of day and ambient conditions.

The ply adhesion values of the aged TPO membrane seam welds were on average, 15 percent above the average ply adhesion value from the SRI study on new TPO membranes. Therefore, as expected, the aged welds appear to be performing well and are of adequate strength.

Aged Membrane Repair-Ability

To address questions around the ability to repair aged TPO membranes, this study examined the adhesion of new membrane to aged membrane. Two approaches were examined; welding the new membrane to the cap of the aged membrane, as well as new membrane welded to the core of the aged membrane.

The ply adhesion of the new membrane to the aged cap averaged 44.7 lbf/inch (standard deviation 8.9 lbf), is shown in Figure 16. For the new membrane welded to the aged core, the ply adhesion averaged 58.8 lbf (standard deviation 7.5 lbf), as shown in Figure 17. Both values represent the combined 45- and 60-mil thicknesses.

Figure 16. Ply adhesion of the new membrane to the aged cap. The average ply adhesion was 44.7 lbf/inch. Not all samples were tested for ply adhesion.


Figure 17. Ply adhesion of the new membrane to the aged core. The average ply adhesion was 58.8 lbf/inch. Adhered membranes were excluded from this evaluation.

The ply adhesion values of new repair membrane to the core of the aged TPO membrane are above the average ply adhesion value of 40 lbf/inch from the large independent study of new TPO membranes conducted by SRI. This provides validity to the integrity of repairs to aged TPO membranes and the ongoing maintainability of these roofs. It should be noted that ultimate ply adhesion is mainly a concern for wind uplift of mechanically fastened systems as shown in Figure 18.

Figure 18. Schematic showing the three main forces applied to a mechanically attached membrane during a high wind event.

Typically, repairs of single-ply membranes are made to punctures within the field of the membrane. As such, they usually represent a very small fraction of the membrane area and essentially the overall mechanical strength is not significantly impacted. The reinforcing scrim can redistribute loads around a small repair. For systems where the membrane is adhered to the substrate, ultimate membrane and ply adhesion are also not as critical.

 

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Originally published in September 2020

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