6:19 PM
Types of Prestressed Concrete
Construction
1. Precast complete
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pre-tensioned factory products and
post-tensioned members.
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manufacture may take place on or off the site,
and the units are lifted into place with suitable tackle. In either case the
actual prestressed cannot be applied to the concrete until sufficient strength
has been developed, but economy in formwork is usually possible, particularly
if steam curing is applicable.
2. Precast in sections
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a prestressed member may be composed of a
series of sections united by prestressed. This allows precasting of units which
may then be transported to the site for assembly.
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the joints between the segments are most
important. In order to minimize delays in assembling the member, they may be
made with Ciment Fondu, but usually epoxy mortar or cement mortar or concrete
using high early strength Portland cement is satisfactory. segments must of
course be precisely aligned before jointing, to permit exact location of
tendons.
3. Cast in situ
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construction is similar to reinforced concrete
work, the usual falsework and formwork being employed. To enable early
stripping, the concrete is usually made with high early strength cement. With
bridges a methods for saving falsework is to cast one girder at a time across
the gap; after stressing, either the falsework or the girder is moved sideways
to the next position.
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4. Composite construction
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Prestressed concrete elements and plain or
conventionally reinforced concrete elements, interconnected structurally so
that the two act integrally, may be adopted in cases where such composite
construction provides an economical structure.
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The prestressed elements may be pre-tensioned
or post-tensioned,a nd may be precast or cast-in-place; the plain or reinforced
concrete elements are usually cast in place.
5. Circular structures
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Reservoirs and similar structures may be
economically built in prestressed concrete. The latter may be either wrapped
round the wall with a special wire winding machine which tensions the wire as
it is applied; or tendons may be laid round the wall or cast into the wall
sheaths, and subsequently stressed with jacks. When tendons are on the outside
of the wall, subsequent protection is obtained with a dense Gunite coating.
When the tendons are buried inside the concrete they emerge through special anchor blocks spaced
uniformly round the periphery, and are protected by grouting.
