pretensioning systems
DESCRIPTION
prestress,pretension,post tension, pre-tension, RCC,PSCTRANSCRIPT
PRE-TENSIONING
SYSTEMS
Principle of Prestressing
Prestressing is a method by which predetermined
compression force is applied to the concrete section
before loading.
The effect of prestressing is to reduce the tensile stress
in the section to the point that the tensile stress is below
the cracking stress. Thus, the concrete does not crack!
RC vs. PSC
Reinforcement stops cracking in tension zone from opening
Prestressing stops the concrete going into tension
Advantages of PC over RC
Take full advantages of high strength concrete and high strength steel
Need less materials
Smaller and lighter structure
No cracks
Use the entire section to resist the load
Better corrosion resistance
Good for water tanks and nuclear plant
Very effective for deflection control
Better shear resistance
Applications of Prestressed Concrete
Bridges
Slabs in buildings
Water Tank
Concrete Pile
Thin Shell Structures
Offshore Platform
Nuclear Power Plant
Repair and Rehabilitations
Classification and Types
Pretensioning vs. Post tensioning
External vs. Internal
Linear vs. Circular
End-Anchored vs. Non End-Anchored
Bonded vs. Unbonded Tendon
Precast vs. Cast-In-Place vs. Composite
Partial vs. Full Prestressing
Prestressing Types
Pre-tensioning
• Tendons are stressed prior to concrete being placed
• Force transferred by action of bond
Post-tensioning
• Tendons are stressed after concrete has been placed
• Can be bonded or unbonded
Prestressing Applications
Pretensioning
Pre-tensioning
Tensioning of the tendons to a predetermined level,
after which the concrete is placed
After the concrete is hardened, the tension force is
released.
The tendon tries to shrink back to the initial length but
the concrete resists it through the bond between them,
thus, compression force is induced in concrete.
Pretension is usually done with precast members
Pre-tensioning
Mechanism of transfer
Tension in the steel is transferred to the concrete
entirely by bond-“prestress transfer bond. ”
Three factors which contribute pretension bond
Adhesion between concrete and steel
Hoyer’s Effect
Mechanical interlocking
Adhesion
Adhesive bond is a chemical mechanism, between the
prestressing strand and concrete.
Easily destroyed in the presence of any strand slip or
cracking
Contribution of adhesion to the bond is effective only
till occurrence of end slip or cracking
Mechanical Interlock
When concrete is cast around the 7 wire strand, it
forms the helical shape.
This helical shape prevents the prestressing strand from
twisting- mechanical interlock.
At the event of a crack, slip occurs and mechanical
interlock acts to prevent further slippage of the
prestressing strand.
Effective only if there is no twisting of strand
Hoyer's Effect or wedge action
Pretension causes reduction in diameter of strand,
proportional to Poisson’s ratio
Upon release, steel restrained from regaining its
originial dimension
Restraint causes the high radial pressure on the concrete
that in turn causes high frictional resistance along the
longitudinal axis of the prestressing strand.
This frictional resistance opposes any relative movement
between the prestressing strand and the concrete
Methods of
Pretensioning
Pretensioning with Individual mould
Tendons are anchored directly
to the individual steel moulds
in which the concrete is cast.
Moulds must be designed &
constructed to withstand the
additional forces induced by
the tendons
Mould Method
Pretensioning on stressing beds
The tendons are tensioned between and
subsequently anchored to the rigid vertical steel
anchor columns
Pretensioning on stressing beds
Long Line Pretensioning
Fixed Bed Rolling Bed
Flow line pretensioning
Sectional flow line
Simultaneous flow line
Long line or Hoyer Method
When several pre-tensioned concrete members are to
be manufactured.
Tendons are stretched between 2 bulk heads, a great
distance apart, so that a number of similar units may
be cast
Hoyer Method-Contd..
When concrete attains the required strength, jacks are
released and tendon wires in-between various units are
cut
Advantage of this method is that only two stretching
mechanisms (two jacks - one at each end or one jack at
one end and one anchoring device at the other end)
Fixed bed technologies
Elements remain in the same place during the entire
production and curing cycle
Different equipment (with or without operators) moves
along the line to perform the appropriate processes on
consecutive stations.
Bridge girders and double and single tee units are
manufactured most frequently in fixed beds with full
length fixed molds
Horizontal Shear
Horizontal shear imposed - large enough to cause
cracking
pull- off mold - molds are raised or lowered from
hardened concrete but before release of prestress
Slipping or rolling molds-before release of prestress,
the finger plates are removed, thus yielding a gap which
allows for mold to move longitudinally by means of low
friction rollers
Rolling bed technologies
Prestressed concrete members are manufactured in long
steel rolling beds which are capable of temporarily
holding the whole prestressing force
Hollow core floor slabs and sandwich wall panels are
manufactured by rolling bed method
Flow line Pretensioning method
Individual self stressing molds or pallets move through
the plant from one station to another
Moving of molds is performed by roller, chain or wheel
conveyors, sliding platforms or by crane
classified according to the movement of the molds
simultaneous (synchronous) movement
sectional (asynchronous) movement.
Flow line Methods
Simultaneous
Movement of the molds is
performed along the whole
conveyor simultaneously
Highly automated technology
can be used only for mass
production of standard
products
Sectional
Each mold or group of molds is
moved independently among
the processing areas of the plant
Allows simultaneous
production of different types of
elements on the same line
Stages and Devices
Stages
Anchoring of tendons against
the end abutments
Placing of jacks
Applying tension to the tendons
Casting of concrete
Cutting of the tendons.
Devices
Prestressing bed
End abutments
Shuttering / mould
Jack
Anchoring device
Harping device (optional)
Jacks
To apply tension to the
tendons
Individually stressing or
simultaneous stressing
(by special jacks) from
one end of the stressing
bed.
Anchoring device
Used to maintain tendons in tension.
Once tensioned to required level, jack is released
and the wedges lock against the sides of tendon and
barrel as tendon contracts
The barrels bear against an anchor block which
transmits the tensioning force via PS bed to the other
end
Anchoring Devices
After hardening,
prefabricated stool is
inserted between
anchor block and jack
Anchorage relieved,
jack pressure released
and PS force transferred
Release of Tendons
Tendons are released individually either by flame
cutting, sawing or by hydraulic cutters
Tendons are released simultaneously by making use of
hydraulic rams.
Upon release of the prestressing force, cracking at the
upper face near the ends can happen. To avoid
Draped or Harped tendons
Blanketed Tendons
Harped Tendons
Deflect some of the tendons to obtain the desired cable
profile
Held in their deflected position by special hold-down
devices at the lower deflection points (also called hold
down or draping points) and by hold-up devices at the
high positions
Blanketed Tendons
Concrete is prevented from bonding for some of the
pretensioned tendons
Plastic tubing which surrounds the ”blanketed” tendons
for certain lengths measured from the ends of the
elements.
Applications
Railway sleepers
Floor joists
Beams
Floor units,
Poles
Piles etc
Prestressed Piles
The prestressing strands are the only longitudinal
reinforcement in the piles.
The prestressing process introduces compression into
the pile, which counteracts the tensile stress resulting
from handling, driving eccentricity, and stress waves
generated during driving
Prestressed Piles
Square piles-fewer strands to obtain the same effective
prestress
Octagonal piles, Strands typically are positioned in a
circular pattern confined within a circular spiral
Railway Sleepers
(a)Travelling pre-tensioning stress bench (b)Anchoring of strands
(c)Stretching of strands
Railway Sleepers
(a)Stretching of strands (b)Pouring (c)Steam curing
(a) Cutting of strands (b)Demoulding (c)Stacking
Railway Sleepers
(a)Water curing (b)Storage and dispatching of sleepers