Reasons for Blister Formation

Mechanical properties, dimensional change with temperature, permeability, the ability to vent gas, etc., are major issues concerning blistering of waterproofing membranes. The main reason for blister formation is attributed to pressure pro­duced by air and water vapor under the pavement [4]. Blisters can be caused by the expansion of hot, humid air in the concrete after torching of the membrane with an open gas flame.

Short-term blisters occur when placing hot MA and result from humid air trapped in the asphalt mixture. These blisters are often removed in practice by punching a steel stick into the blister of the mixture to release pressure. This bad practice indicates a quality problem either in the material or construction process. Because it is practically impossible to release all pressure by punching through the blisters, some of the smaller blisters may get “frozen" during the cooling process and the trapped air may be the trigger for the formation of long-term blisters that may gradually grow under service condition. Reference 5 suggests that blisters on concrete with asphalt overlay are caused by thermal buckling of the pavement slab.

Waterproofing sheets on roofs may also show blister formation. In a broad analysis of blister phenomena [6], the authors indicate that blisters can only de­velop in initial voids or unbound areas because of poor membrane application during construction. After such blister initiation, gas expansion inside the blis­ter can cause blisters to grow. When the temperature rises, obviously, the pres­sure increases. However, from actual pressure measurements within blisters, it was found that the pressure is much lower than calculated for totally entrapped gases [6]. Because of temperature fluctuation in the surrounding environment, air is forced out during daytime and sucked in at night. It is supposed that tem­perature variation leads to breathing or pumping action, which changes the blister volume.