Civil Engineering Reference
In-Depth Information
5.2
Membrane Characteristics
Bituminous Built-up Roofing (BUR).
Bituminous materials used
in BUR include
(a)
asphalt obtained in petroleum processing, and
(b)
a
product extracted from coal known as coal-tar pitch (CTP) (also referred to
as tar or pitch). In Canada, CTP is generally not available. Asphalt used for
saturating organic felts is commonly called No. 15 because the earlier types
weighed 15 lb/100 ft.
[2]
This and asphalt-impregnated glass fiber felts are
used as plies in the construction of BUR. Heated asphalt is mopped on each
felt layer to bind them together into the finished multi-ply membrane.
Many problems in BUR, like blistering, are related to moisture and
air voids in the membrane. The organic felts absorb moisture from improper
storage, and the lack of protection from rain during installation can result
in loss of strength due to wetness. Sometimes moisture is trapped where
there are skippings in the mopping of asphalt under the felt. Also, synthetic
fiber from rags sometimes gets included in the felts during manufacture,
resulting in poor asphalt saturation. Glass fiber felts are quite porous and
provide good adhesion. Their use in flashing is not recommended. During
installation, walking over felt on hot asphalt causes the asphalt to squeeze
out, which could result in a void and lack of adhesion in the membrane, a
potential source of moisture problems.
A very common source of problems is overheating of asphalt. This
apparently makes the mopping easy, but it hardens the asphalt and reduces
both its softening point and coefficient of linear thermal expansion. The
surface causes shrinkage cracks and alligatoring. Some other problems in
BUR include shrinkage of the membrane that pulls flashings away, caused
by a lack of adhesion of the roofing system to the deck. Membrane slippage
occurs if the softening point of the asphalt is too low with respect to the roof
slope, or the amount of interply asphalt is excessive.
Modified Bituminous (MB) Sheets.
This class of sheets is made
from bitumens and modifying polymers (synthetic rubbers or plastic
materials) together with fillers and special-property additives. Since the
process is basically mixing components, the amount of modifier can be
varied according to the required characteristics. The two most widely used
bitumen modifiers are SBS (styrene-butadiene-styrene) and APP (atactic
polypropylene). The average SBS content in the formulation is 12-15%.
Generally, more SBS means greater low temperature flexibility and fatigue
resistance as well as a higher softening point and wider temperature use.
There are about a dozen different SBS grades that accentuate one or the
