its overburden increases more than 20-fold (a
ment--and peeled approximately another 2 in. (5
membrane weighs between 0.002 to 0.008 lb/in.2
cm). In interpreting the curves, one should recog-
whereas 2-in.-thick asphalt pavement weighs ap-
nize that the pulling force for each stage gradu-
proximately 0.168 lb/in.2). In this situation we see
ally built up, as the membrane and adhesive
that the critical size changes from a 2.1-in. (5.3-cm)
stretched and as the membrane seated in the grips,
radius when the void space is dry (Fig. 3c) to a 0.30-
until the force became large enough to progres-
in. radius when it is wet (Fig. 3d). Moreover, blis-
sively peel the membrane from the slab.
ters do not just expand once, they continually in-
The shape of each curve shows that adhesion
crease in size. Korhonen (1986) found this to be true
is a complex issue, difficult to describe with just
for roof blisters as did Hironaka and Holland (1986)
one number. Should a maximum, minimum, or
for pavement blisters. Thus, once a blister initiates,
average force be used to describe adhesion? Maxi-
no matter how small it may be, it eventually grows
mum values can be considered as the very best
large enough to become a big problem.
adhesion that one can hope to expect. It could be
Though Figure 3 represents idealized situations
argued that the resistance offered by a membrane
(a blister is not rigid and self-contained), clearly a
just prior to the onset of progressive peeling some-
nonporous membrane exposed to the sun will re-
times mimics that of a membrane against the ini-
main blisterless if its voids are smaller than 5.5 in.
tiation of a blister on a bridge. For some mem-
(14 cm) across (Fig. 3b). When exposed to the in-
branes in this study, however, progressive peeling
tense heat of freshly laid pavement, approximately
did not occur until the pulling force peaked; there-
quarter-sized voids (0.9 in., or 2.4 cm) (Fig. 3d) can
after, peeling occurred at a lower force (Fig. 4a,
lead to problems. Other scenarios are possible for
4b, 4c and individual results for Polyguard and
blisters but the quarter coin size should be useful
Soprema, App. B). This is not unlike what occurs
as a rule of thumb for bridge inspectors to distin-
on bridges where, once growth is initiated, blis-
guish when a membrane is being inadequately
ters seem to expand quite rapidly for a while. For
adhered to a deck. A permeable membrane can
other membranes, peak forces did not develop at
reduce blistering by allowing pressure build up
all (Fig. 4d, 4e and 4f) near the end of the test (Fig.
to escape through the membrane. However, the
4b and 4c). Average values, on the other hand,
section on water vapor permeance reveals that the
show a typical adhesion that can be expected. Av-
membranes in this study were not very breathable.
erages dampen out any of the extreme values dur-
ing testing and usually provide a reasonable basis
of comparison. However, by considering the dis-
Results and discussion
The entire data set for the adhesion tests con-
cussion on blister mechanics, clearly neither maxi-
sists of force-displacement diagrams for 157 strips
mum nor average values are adequate, because
of membrane peeled off mortar slabs (App. B). We
the root cause of all blisters is poor adhesion.
will not discuss each diagram but, rather, summa-
Therefore, minimum values are a revealing test
rize them in Figure 4 and make specific references
result, because a blister simply cannot form un-
to them in the following text to give the reader a
less a membrane is poorly bonded, at least in spots.
sense of their significance. The reader is encour-
With the foregoing in mind, Table 2 was devel-
aged to peruse Appendix B for added detail.
oped from Appendix B to compare the maximum,
Figure 4 shows six forcedisplacement dia-
average, and minimum adhesion values measured
grams, one for each membrane type where each is
in this study. (The values developed from the
composed of two curves (except for Figure 4f,
CRREL-made samples are differentiated from
where supplier samples were not available). As
those made by the supplier.) The maximum val-
can be seen by the difference between the two
ues in Table 2 were based on the entire loading
curves in each graph, the choice of applicator can
curve of each strip, whereas the average and mini-
influence results. Each curve represents the aver-
mum values came from the center portion of each
age of up to 15 strips peeled from three samples
testing stage (i.e., from approximately 1 to 2 in. of
fabricated by the membrane supplier compared
stage 1 and 3 to 4 in. of stage 2). Using only the
to three done by CRREL. The Figure 4 curves re-
center portion avoided effects caused by the start-
veal that testing was done in two stages. In the
up or end of each test. As can be seen, the
first stage, approximately 2 to 2.25 in. (5.7 cm) of
Polyguard and Soprema membranes have the best
the membrane was peeled off the slabs. The mem-
adhesion values when either maximum or aver-
brane was then unloaded, repositioned in the
age values are considered. However, the situation
grips--as the grips reached the end of their move-
changes when minimum values are considered.
6
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