TECHNICAL & RESEARCH
7.	Microscopy. One quadrant of each sample was 
cut through the impact site to view the membrane 
and cover board in cross-section with the use of a 
microscope (Figures 9 and 10). 
Results of Impact Testing
The following table describes the average results 
of the impact testing in terms of granule counts and 
exposed bitumen (expressed as a percentage of 
area) within the 3-inch diameter impact zone for 
each type of sample tested:
Table 1. Average manual granule counts and exposed 
asphalt area pre- and post-impact
Conclusions
The following conclusions were drawn from the 
results of the impact testing and analysis:
•	 In general, regardless of cover board type 
or impact, the quantity of granules on the 
membranes decreased with age, while the 
exposed bitumen area generally increased with 
membrane age. There was a larger increase in 
exposed bitumen area between five- and ten-year-
old membranes than there was between new and 
five-year-old membranes (Figure 11). 
	
■For each membrane age, the simulated hail 
impact resulted in minor granule loss that yielded 
slightly more exposed bitumen. The exposed 
bitumen area generally increased more upon 
impact with gypsum cover board than it did with 
wood fiber cover board. 
	
■The impacts did not fracture the membrane 
reinforcement in any of the samples.
	
■Cracking of the gypsum cover board was observed 
at the impact site in all gypsum cover board samples 
that were tested. The impacts also caused cover 
board facer separation from the gypsum core at 
the impact sites in two thirds of the gypsum cover 
board samples. The wood fiber cover 
board samples were visually unaffected 
by the impacts. 
	
■Comparison between image analysis 
and manual granule counts revealed 
that image analysis accuracy is 
variable due to differences in granule 
shape, color, texture, and size as well 
as features like granule overlap or 
fracturing. For this reason, manual 
counts were found to be more accurate 
and ultimately what was relied upon for 
the study. However, image analysis was 
found to be more useful in identifying 
exposed bitumen area due to the high 
color contrast between the granules 
and the bitumen, a property that image 
analysis is proficient in detecting.  
Figure 9. Cross-section of 
10-year-old gypsum cover board 
sample. Note separation of facer 
from gypsum (yellow arrow) 
and radial crack throughout 
thickness of gypsum (orange 
arrow). 
Figure 10. Cross-section 
of new wood fiber cover 
board sample. Note that no 
separations are visible. 
Figure 11. Comparison of new, five, and ten-year-old membranes 
over gypsum cover board. 

View this content as a flipbook by clicking here.