A = area of void (πr2)
to blister. For example, if an air pocket beneath a
membrane adhered to a deck at 5 lbf/in. (875 N/
m) is heated from 70 to 140F (24 to 67C), a 5.2-
top of blister)
L = perimeter of void.
in. (13.2-cm) radius would be the smallest void that
could blister (Fig. 3a). However, if the air beneath
Equation 2 explains that the smaller the void,
the membrane is continually water saturated, the
the less likely it is to develop into a blister:
critical void would reduce to 2.25 in. (5.7-cm) ra-
dius (Fig. 3b). Of course, higher bond strengths
r = 2F/(P W)
(2)
are more resistant to blistering, but one must real-
ize that heat, the driving force of blisters, softens
the adhesive and diminishes peel strength. Thus,
where r is void radius. That is, it requires more
the 5-lbf/in. force used in the above analogy is
internal pressure (heat) to expand a small void
considered conservative, even though some mem-
than to expand a large one.
branes adhere more tightly to concrete at room
Figure 3, developed from eq 2, illustrates this
temperature.
concept. It consists of four graphs, each composed
The situation changes as soon as the membrane
of three curves, where each curve represents peel
strength plotted against temperature and critical
is topped with hot pavement. In this case the void
immediately heats up to 250F (146C) or more and
size. Each graph defines the smallest void expected
12
a.
b.
Dry Air Under Membrane
Saturated Air Under Membrane
10
8
6
10 lbf/in.
4
5 lbf/in.
2
1 lbf/in.
0
12
c.
d.
Dry Air Under
Saturated Air Under
10
Membrane and Pavement
Membrane and Pavement
8
6
4
2
0
100
150
200
250
300 100
150
200
250
Temperature (F)
Kh
001
Figure 3. Relationship between minimum void size, peel strength, and internal void temperature. Blister pressure,
which relates to temperature, was determined by considering dry and moist air to be ideal gases. The temperature of
70F (21C) represents atmospheric pressure.
5
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