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Asphalt is a solid or semi-solid mixture of bitumens commonly
used for paving, roads, and waterproofing. Its cost is dependent
on the world oil prices, and following the rise in world oil
prices during the 1970s the use of asphalt as a hard surface
lining declined because it was less durable and much more expensive
than concrete lining. Asphalt products became much thinner and
more flexible, and are now mainly classed as flexible membranes.
There are two types of flexible asphalt membrane liners: prefabricated
asphalt and spray-in-place asphalt.
Prefabricated asphalt
Prefabricated asphalt membrane linings were developed primarily
for small scale lining installations that did not warrant the
use of the minimum size truck and necessary skilled labour to
supply hot bitumen directly to the site. Prefabricated asphalt
linings are supplied in similar thickness to the range recommended
for the sprayed-in-place lining.
Prefabricated asphalt linings have been extensively trialled
by the USBR. Early trials experienced difficulty in maintaining
the life of the mastic joints used to join the prefabricated
blocks, especially above the water line. However, the lining
was reported as effective in seepage reduction (USBR, 1975).
Bitumen-based liners have also been combined with polyester reinforcement
to produce a combination-type liner. The liner used a styrene-butadiene-styrene
polymer combined with high-quality asphalt and a polyester reinforcement
to provide puncture reinforcement. Irrigation Districts in Washington
State and Okalahoma in the United States have used this technique
(Swihart and Haynes, 1999). The membrane is prefabricated into
rolls (4-5m wide), which eliminates the field quality control
problems with spray applied membranes. This material looks good
after 8 years of UV exposure. However, the seams are the weak
points. Seams are made with a propane torch, and the quality
of field seams is sometimes poor. The bitumen/polyester liner
is considered to be a more costly material than polyethylene.
This, however, may be offset by its comparative ease of installation,
with no specialised thermal welding equipment required, as seams
can be joined using an acetylene torch.
There has also been some Mexican experience in the early 1990s
in the use of asphaltic membranes (Swihart and Haynes, 1999).
Sheets generally 0.9m by 3m in size and 13mm thick had been used.
They were composed of 65% asphalt and 35% mineral fillers reinforced
with fibreglass. The sheets are spliced together with 100-150mm
overlap and bound together with hot applied oxidised asphalt
and asphaltic mastic. These sheets were used to control leakage
from aeration ponds at a wastewater treatment plant where a crack
had formed that resulted in several earthslides. After sealing
the crack with concrete the site was covered with 40,000m2 of
the material. After 2 years of operation, wave action caused
horizontal ripping of the liner. In addition leaks detected at
splices were not been stopped and intensive maintenance was required
in an area of lining exposed to sunlight.
Spray-in-place asphalt
Spray-in-place asphalt lining consists of a film of bitumen (possibly
mixed with synthetic polymers) spayed on at high temperature
(200°C) to form a waterproof barrier. The asphaltic liner
is mixed on or near the job and hauled by trucks to the point
where it is laid.
The USBR has used hot-mix sprayed in place asphalt linings extensively
on various projects. Most problems resulted from movement of
the subgrade and the development of weeds. Experiments have been
conducted to test various adaptations of asphalt lining, including
reinforcing, mixing with sand and other aggregates, and applying
soil sterilisers. It was noted that cracking develops within
a few years due to cold weather, weed eruption and exposure,
and therefore minor repairs are frequently required. Repairs
are made primarily by cement or mortar (USBR, 1975).
The most recent sprayed-in-place asphalt is an emulsion that
forms a thin coating that is applied to a firm subgrade like
steel or concrete. However, under field conditions it does not
bond well to the subgrade. When this method is used on an earthen
subgrade it is applied to a geotextile laid on the soil.
Spray asphalt emulsions applied to geotextiles were tested in
the United States as part of the Deschutes channel lining demonstration
project (Swihart et al., 1994). The application required minimal
subgrade preparation, was relatively simple to install and allowed
rapid construction. However, the geotextile was not anchored
to the invert and the test section washed out the first time
that the channel filled with water.
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Flexible membrane lining
techniques
Flexible
membrane materials
Covered liners
High-density
polyethylene (0.75mm)
Geosynthetic
clay liners
Exposed liners
High-density
polyethylene (2mm exposed)
High-density
polyethylene (1.5mm exposed)
Linear
low-density PE and very low-density PE
(1.5mm)
DamSeal
Unreinforced
polypropylene (1mm)
Unreinforced
polypropylene (0.75mm)
Reinforced
polypropylene (1.1mm)
Butyl
rubber |
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