Civil Engineering Reference
In-Depth Information
about the bearing can cause additional movements at the level of the expansion joint.
This can also affect the design of the measures to inhibit cracking of the road surfacing
at the fi xed end of short spans. See
6.13.3
for a full discussion of the importance of
temperature gradients in the design of bridge decks.
7.2.4 Shrinkage of concrete
As concrete matures, it gradually reduces in volume. The amount and rate of shrinkage
depends on the type of cement, the aggregate, the mix proportions, the size and shape
of the specimen, and on the relative humidity of the air. The shrinkage to be considered
when designing the articulation of a bridge may be defi ned by the local code of practice,
or it may be necessary to refer to specialist literature [1]. There are large variations in
the shrinkage values called for by the different national codes of practice.
Shrinkage is restrained by the presence of bonded reinforcement, which is stressed
in compression by the shortening of the concrete. This is graphically illustrated by
the early prestressed concrete balanced cantilever bridges which had a hinge at mid-
span. Some of these bridges exhibited unexpected downward defl ection of the mid-
span hinge over a period of years. Part of the explanation for this behaviour was
differential shrinkage between the top and bottom fl anges of the box section. In the
top fl ange, the shrinkage was restrained by the large number of bonded prestressing
cables, while the bottom fl ange was only very lightly reinforced in the longitudinal
direction.
BS5400: Part 4: 1990, Appendix C gives values of shrinkage for a typical bridge
deck in UK conditions, of some 180 × 10
-6
, or 18 mm per 100 m length, of which
10 per cent is completed in 10 days, and 35 per cent in 100 days. The actual values
depend on the criteria mentioned above, and should be calculated for each case. If the
section is signifi cantly reinforced with bonded steel the shrinkage will be less.
The amount of shrinkage that affects the design of the articulation of any bridge will
depend on the method and programme of construction. For instance, when a single-
span bridge is cast in-situ in one pour the bearings will have to cope with 100 per
cent of the shrinkage. However, if the bridge consists of several continuous spans,
each being cast at three-weekly intervals, a considerable portion of the shrinkage of
earlier spans will have taken place before the last span is cast and consequently the
bearings may be designed for reduced movements. When a bridge is made of precast
components which will be several weeks old when they are incorporated into the deck,
the shrinkage to be used for the design of the substructure is signifi cantly reduced.
The amount of shrinkage to be considered in the design of the expansion joints will
depend on the average age of the deck when they are installed.
The codes of practice may give useful guidance for typical structures. However
for very long structures where movements of expansion joints and bearings must be
known with some certainty, they should not be relied upon.
7.2.5 Elastic shortening under compression
Prestressed concrete decks shorten during the tensioning of the cables, and this is
known as elastic shortening. The elastic modulus for concrete to be used to calculate
this shortening depends principally on the strength of the concrete, its age when the
prestress is applied and also on the aggregate type. Generally the modulus to use is
