Civil Engineering Reference
In-Depth Information
stiffness? Jacobs Babtie addressed the one aspect that if left unattended could
cause a problem. The relatively high stiffness that was now introduced into
the structure would attract load and cause peak stresses under wind loading.
As the beams were tied into the shell, they could cause cracking to the inside
of the tower.
A lightweight, strong reinforcing material, which could be applied
internally to the surface of the tower, was needed to counteract these stresses.
At this stage, construction chemical manufacturers and supplier BASF CC
Ltd advised the engineer on the materials available. Both carbon fibre and
aramid fibres were offered to the engineer. The material would be applied
from cradles to the inside of the tower and so it would need to be robust
enough to withstand application and future inspection, as the cradle wires
which guide the cradles along the contours of the internal face had the
potential to rub and cause mechanical damage.
Owing to the characteristics of carbon fibre, mainly its brittle nature, it
was decided that this option would not be viable for this project. Kevlar® is
an aramid fibre manufactured by DuPont, and has been used for more than
25 years in a wide variety of applications ranging from aerospace composites
and body armour to protective apparel. Using MBrace Kevlar represented the
obvious solution because of its low weight, high strength, and high stiffness
and damage tolerance. The fabric drapeability ensured that it would be easy
to apply, and would conform to the contours of the cooling tower wall.
The West Burton contract was the first application of MBrace Kevlar
brand fabric to a cooling tower anywhere in the world, and the first major
use of such fabric on a structure in Europe.
Before specialist subcontractor Balvac applied the Kevlar, several different
kinds of surface preparation were tried, from grit blasting, needle gunning
and concrete planing to keying the surface with a rotary wire brush. Concrete
planing with localised needle gunning where required (for example, in
existing depressions which planing could not reach), proved to be the most
successful method. Not only was it quicker than needle gunning on its own,
but it also avoided the disadvantages of grit blasting (dust, which would then
have to be removed).
In all cases, pull-off tests on the Kevlar after application to the prepared
surface led to the failure of the concrete, showing that the bond strength was
greater than the tensile strength of the concrete.
After surface preparation, the sheets of Kevlar fibre were encapsulated in
the two-part epoxy resin and applied to the wall of the tower using the wet
lay-up technique.
As well as using circumferential bands of Kevlar to counteract the stresses
caused by the circular strengthening beams, vertical strips were placed
between the relatively stiff rings to prevent cracking. These were applied in
deformed areas of the shell where analysis identified high stresses developing
under wind loading. Because the Kevlar was to be protected with a flexible
