Civil Engineering Reference
In-Depth Information
Figure 10.11
Arrangement of slab reinforcement for progressively shorter cast-in-situ section
10.3.6 Construction technology
Precast beams are suitable for very large, highly mechanised sites as well as for more
rural projects. Two contrasting sites on Benaim projects were the 630 m long bridge
across the Sungai Kelantan at Pasir Mas in northern Malaysia, described in
10.3.7
, and
the twin 17 km long GSZ Superhighway viaducts described in
10.3.8
.
If the economy of this form of construction is to be fully realised, it is necessary to
minimise the thickness of the webs, which in turn requires the use of steel shutters and
external vibrators.
The most common method of striking the side shutters of precast 'T' beams is by
rotation about their base. It must be ensured that the steel shutter cannot jam against
the concrete; indeed the movement of separation from the concrete should always
have a small component normal to the concrete surface, Figure 10.12. The shallow
haunch of the slab and its root fi llet help this striking action. The crossfall of the deck
must not be forgotten in the determination of this geometry.
If steam curing is adopted, it is feasible to achieve a concrete strength of 20-
25 MPa the morning after casting. Although this is adequate for the bending stresses
in the beam under the effect of its self weight and a fi rst phase of prestress, it would
not be enough beneath the prestress anchors. In order to allow early stressing of the
