The Design of Modern Steel Bridges - TEDI

194 The Design of Modern Steel Bridges
7.3.6 Parallel-wire cables
A bundle of parallel prestressing wires has been used as stays. Often the bundle
is passed inside a polyethelene tube which is later injected with cement grout
as protection against corrosion, a practice similar in concept to post-tensioning
cables in prestressed concrete construction. In the USA, 6.35 mm diameter
prestressing wires were used to form the stay cables of Pasco–Kennewick and
Luling Bridges. In Japan many bridges have been built with such cable-stays,
often with HiAm type of sockets for end anchorage.
7.3.7 Corrosion protection
Because of the small diameter and high stress of the wires in the cable stays, it
is essential that either corrosion of the wires does not take place at all or stays
can be replaced without undue trouble. Because of the high stress, onset of
corrosion can reduce the cross-sectional area of the stay rapidly, leading to
a dangerous condition of the bridge.
For spiral strands in early British cable-stayed bridges, corrosion protection
measure consisted of hot-dip galvanisation of the wires with zinc coating
weight of around 3 N/mm 2 , and subsequent painting. In Germany, the wires of
the locked-coil strands were not galvanised for the early cable-stayed bridges,
for fear of hydrogen-embrittlement. The inner spaces between wires were filled
with red lead during the formation of the cable, to prevent collection of moisture
that would initiate corrosion. After the stays were erected and tensioned
with the dead load of the bridge, the stay surfaces were throughly cleaned and
then coated with several coats of paint. The main aims of the cross-sectional
design of locked-coil cable were to reduce the volume of the inner spaces and
to produce an outer surface where painting would be durable. The locked-coil
strand cable-stays of the early German-built bridges, however, showed significant
corrosion within a few years of their building. The German practice then
changed to galvanising the wires, or at least the ones on the outer layers. In
Japan, all wires in locked-coil strands are galvanised. Instead of red lead in
linseed oil, polyurethane with zinc dust or ‘metalcoat’ (which is a suspension
of aluminium flakes in a resin carrier) is used to fill the interstices. The specialshaped
wires in locked-coil strands are sometimes given a final zinc-coating of
about 135 g/mm 2 by electroplating. Locked-coil strands have been tried with
stainless steel outer wires and ungalvanised steel wires in the inner layers, but
the corrosion–resistance of such strands was found to be no better than those
made with galvanised wires in outer layers.
Long-lay spiral strands have been manufactured with grease or polyurethane
to fill the interstices and then sheathed in black polyethylene.
The corrosion protection of parallel-wire strands relies mainly on enclosing
the cable in a protective tube or sheath. The cable is initially wrapped in
a polyester or polyethylene tape or film, and then enclosed in a plastic or