Civil Engineering Reference
In-Depth Information
formed with SBR rubber, which is a natural rubber product and is highly
durable. The material can degrade under UV exposure and, to some degree,
due to oxygen presence. The environment of the seals means that neither of
these causes are of great concern. Suppliers of the seals can provide extensive
test data, including accelerated aging tests, and these are generally found to
be satisfactory. The current types of seals have been used for over 50 years
and the authors are not aware of any incidences of material degradation. The
creep behavior of the rubber materials is well documented and this is allowed
for in the design process. Occasionally, production defects occur, but these
are usually uncovered during the quality control checking processes.
Other important seals are the internal seals in the tunnel that are installed
in the structure joints, the ballast concrete, and the road surfacing. Suppliers
will not guarantee long life for such seals, and so, these have to be expected
to be replaced during the life of the tunnel. This is not easy beneath the bal-
last concrete, so the selection of those seals should be made, assuming they
will not be replaced, even if their design life is not underwritten by a sup-
plier. An important consideration is the resistance of the materials to cor-
rosive products that may be spilt in the tunnel and find their way to the joint
seals. Some high-grade materials, such as NBR rubber, are available that
are resistant to acids and alkalis, petrol-based products, and solvents. The
cost of these seals is relatively low compared to the remainder of the works,
and so, it is false economy to skimp on the specification of these items.
INTERNAL COATINGS
The internal coating of the tunnel performs several functions:
• Protection of the internal tunnel walls. If they are concrete, it protects
against deterioration from carbonation and saline ingress due to road
salts; if they are steel, then it prevents corrosion due to moisture in the
atmosphere and from the aggressive environment.
• Providing a relective coating to assist the lighting conditions in the
tunnel.
• Facilitates cleaning of the tunnel walls.
• Cosmetic, to provide the driver with a suitable driving environment.
The choice of coating may depend on the construction of the tunnel and
the options include just painting the interior surfaces, facing the walls with
blockwork or tiles, or fixing a secondary cladding system.
In a concrete tunnel, the calcium hydroxide formed during the hydration of
cement ensures that the concrete is alkaline with a high pH value. The pH is
normally above 12, which is sufficient to inhibit corrosion to the reinforcement.
Carbonation is the absorption of carbon dioxide by the concrete. This carbon
