thermal properties of concrete(24.03.10)
TRANSCRIPT
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THERMAL PROPERTIES OF CONCRETE
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CONTENT
INCREMENTAL PUSH LAUNCHING METHOD
CANTILEVER CONSTRUCTION METHOD
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INCREMENTAL PUSH LAUNCHING METHOD
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CONCEPT
The basic concept of INCREIMENTAL PUSH LAUNCHING
METHOD (ILM) is to prefabricate approximately 10m to 30m long
units of the bridge under factory conditions behind an abutment and
launch the units by sliding them on Teflon bearings into final position
without the aid of scaffolding.
The total deck slab of the bridge is subdivided longitudinally intosection of 10m to 30m length, depending upon the length of the spanand time available for construction.
Each section is concreted immediately behind the abutment ofthe bridge in steel framed forms. These steel frames which remain insame place for concreting all sections.
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The forms are designed in such a way that it can be moved transversely or
rotated on hinges to facilitate easy stripping after sufficient hardening of
the concrete.
The forms are hydraulically or mechanically activated to ensure rapid
stripping.
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This is the launching equipment,
which consists of a combination
of hydraulic jacks acting
vertically and horizontally onsliding bearings.
The sliding bearing consists of
concrete blocks covered with
stainless steel and Teflon-coated
reinforced elastometic pads.
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A section is manufactured in formwork in lengths corresponding to one-
third to one-half of the normal span length. After the stripping of the forms,
tendons are inserted and stressed.
The section is then jacked forward, sliding on the Teflon, which, in turn, is
affixed to stainless steel plates on top of the piers.
A new section is then constructed immediately adjoining the rear end of thefirst. Pre-stressing tendons are spliced so that the two segments are joined
as one longer pre-stressed unit and then jacked forward another length.
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At the head of the first section, a steel nose consisting of a light truss is
attached to facilitate reaching of the first and subsequent piers without
including a too large cantilever moment during construction.
The second and following sections are concreted directly on the face of the
hardened portion and the longitudinal reinforcement can continue across
the construction joint.
The pushing is achieved by hydraulic jack which acts against the abutment.
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Launching Process
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Low capacity hydraulic jack would be enough to move the bridge decks.
Intermediate pier supports or towers may be used to give temporary support
across longer spans since the practical limit of a span length for this type of
construction is about 60 meters.
When launching from both ends of the bridge, different curvatures may be
used, but this must be constant for each launching point. Similarly, the super-
elevation must be constant.
After the launched bridge is in place, the bearing seats may be jacked up to
remove the Teflon, and the permanent bearings are installed. Jacking points
should have the required supplemental reinforcing to permit this application
of force at a location offset from the permanent bearings.
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FEATURES OF THE INCREMENTAL
LAUNCHING METHOD
Construction is carried out completely without falsework, so that there is no
problem in passing over obstacles below, such as roads, railways, rivers,
buildings areas.
The superstructure is made up of units of 15 to 25 m length, each
completed in one week; there are no joints, since each unit is concreted
directly against the preceding one.
It is a economic construction technique.
To achieve quality work with reduced amount of labour.
It is a speedy construction technique.
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CANTILEVER CONSTRUCTION METHOD
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SEGMENTAL CANTILEVER CONSTRUCTION
The Economy of continuous construction method is enhanced by the
adoption of the segmental cantilever method by M/s.Dyckehof &
Widman in Germany.
This method is particularly economical where construction of
superstructure using staging from the river bed is not feasible.
The segmental cantilever construction may adopt cast-in-situ place
segments or precast segments, and is considered suitable for the span
range of 50 to 200m, though a few bridges have been built beyond
200m Span.
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The box section is the best suited cross section for cantilever
construction, in view of the torsional rigidity and the availability of the
top and bottom slab portion for accommodating the tendons.
The single cell box is used in many bridge construction in India for
deck width less than 13m.
The Two webs may be vertical or sloping. Box sections with two or
three cells are used for wider decks.
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Types Cast-in-Place Segments
Precast Segments
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CAST-IN-PLACE SEGMENTS
The Structure is built from a pier or support across the open span
without the temporary support in sections of about 3.5 m length. The
construction should proceed on both sides of piers such that at any
stage ,the unbalanced moment kept minimum.
Each section is cast in the cantilever form traveler(CFT) which is
attached to the end o constructed portion. When the concrete attains
specified strength, the segment is pressed against the previous one by
means of prestressing tendons ending at the front of the section.
Other prestressing tendons are carried through unstressed at a lengthcorresponding to the static requirements of the construction. At each
section, the tendons ending there may be bonded by grouting the duct,
so that at every stage the construction is safe against rupture.
This process is continued till mid span
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PRECAST SEGMENTS
Long span prestressed concrete bridge can also be built using precast
segments. The segments, precast in a nearby casting yard are about
3.5m long covering the full width of bridge.
Each segment is transported by a trolley system from the plant to the
launching truss, from where picked up a traveling frame hoist
suspended from the bottom chord of the truss.
The traveling frame has a spreader beam which can be raised, loweredor rotated so that wide section can pass between the supporting legs of
launching truss and can be lowered into the proper position.
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The section is then fastened in place with epoxy and prestressing
tendons.
One problem with the use of precast segments is in joining segmentsprior to stressing. In order to ensure precise matching of the mating
faces, each segment is cast directly against the face of previous
segment.
This process is called as Match-Casting.
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ADVANTAGES
Centering and false work are avoided, enabling construction of
structures with tall piers and over deep valleys.
The speed of construction is enhanced, typically at 1 m/day/CFT for
cast-in-place construction and possibly 3m/day with the use ofprefabricated segments.
Enhanced levels of quality and workmanship or facilitated due to
mechanization repetitive tasks.
The cost of construction permits competition with alternative design of
a steel structure of long span.
Creep in concrete is less due to age at time of initial loading.
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