Civil Engineering Reference
In-Depth Information
In some types of bridge deck, the prestressing tendons may be housed in ducts
that are external to the concrete, generally within the void of a box section deck,
Figure 15.26. The ducts are usually made of high density polyethylene (HDPE). In
order to achieve the desired profi le of the prestress centroid, the ducts are defl ected at
pier diaphragms and intermediate deviators. Once the tendons have been stressed, the
ducts are grouted with cement, or with a petroleum jelly.
The principal advantage of this arrangement is that the tendons may be detailed
in such a way that they can be replaced in the event of their suffering corrosion. The
corollary to this benefi t is that as the tendons are external to the concrete, they are
inherently less well protected than internal tendons. They must be subjected to an
organised inspection regime that is capable of detecting corrosion occurring within the
ducts, and in particular in the most vulnerable areas which are immediately behind the
anchorage, at any junctions between plastic and steel duct and at the fi eld joints in the
ducts. This may impose a considerable burden on the maintaining authority. A further
advantage of external tendons is that the webs of the bridge are not encumbered with
the ducts and consequently are easier to cast and may be made thinner.
There are several practical disadvantages of external tendons. The need to avoid
a confusion of ducts within the box section deck generally leads to the use of large
prestress units. These large units require substantial deviators that generally take the
form of internal ribs or frames, and also require, for tendons that cannot be anchored
at pier diaphragms, large internal anchorage blisters. The volume of these deviators
and anchorage blisters generally more than cancels out any saving in concrete weight
achieved by making the webs thinner. Also, they generally require dense reinforcement
that signifi cantly increases the overall weight of passive reinforcing steel in the deck.
Furthermore, the deviators may locally stiffen the deck cross section and act as
diaphragms, attracting transverse bending moments.
The weight of prestressing steel is greater than for internal tendons. This is due to
the reduced eccentricity of the prestress as the tendons have to lie between the top
and bottom slabs of a box, and to the reduced fl exibility in stopping off tendons. In
order to provide adequate ultimate strength to the beam, the prestress may have to be
signifi cantly over-designed, or the shortfall in strength made up by additional passive
reinforcing steel.
The installation of the external ducts has to be carried out once the concrete has
been cast or erected, putting this activity on the critical path, and generally slowing
construction. This can be a critical consideration when speed of construction is of the
essence.
Finally, as the tendons are not bonded to the concrete, they do not signifi cantly
increase in stress when the deck is overloaded. This lack of composite action between
the main reinforcement and the concrete makes such decks less able to cope with
extreme situations.
External prestress is at its most cost effective when the prestress profi le is simple
and when the tendons can be anchored in the end diaphragms of a deck, such as in
statically determinate spans. External prestress may also be successful as continuity
prestress in bridges built by the balanced cantilever method, both cast in-situ and
precast, as the lack of internal tendons in the webs facilitates construction, and the
external tendons may be anchored in the pier diaphragms. However, as in such bridges
the negative moment tendons are internal, the main reason for using external prestress,
the facility to replace the tendons is not relevant.
